Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel

Development of duplex stainless steel

The development and application of duplex stainless steel began in 1930s, and three generations of duplex stainless steel have been developed up to now.

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First generation duplex stainless steel:

  • (1) AISI329 steel developed in the United States in the 1940s, as a representative, has high Cr and Mo content, good local corrosion resistance, but high carbon content (C ≤ 0.10%). After welding, its joint corrosion resistance and toughness are poor, which limits the application of the steel and is only suitable for casting and forging.
  • (2) Japan has reduced the carbon content of 329 steel in the United States and developed sus329j1steel, which can be used as welding steel.
  • (3) In the mid-1960s, Sweden developed the famous 3re60 steel, which is characterized by ultra-low carbon and Cr content of 18%. Good welding and forming properties make it the representative steel of the first generation duplex stainless steel.

Second generation duplex stainless steel:

  • (1) In the 1980s, Sweden first developed ultra-low carbon duplex stainless steel without mo. The representative steel is saf2304 steel.
  • (2) Then, based on the first generation of dual phase steel, ultra-low carbon dual phase stainless steel containing nitrogen was developed. The typical steel is SAF2205 developed in Sweden, which makes the dual phase steel widely used.

Third generation duplex stainless steel:

(1) Super duplex stainless steel was developed in the late 1950s, which is characterized by low carbon content (< 0.03%) and high Mo and N content (Mo is about 4%, n is about 0.3%); ferrite content of steel reaches 40-45%; it has excellent pitting resistance, and its pre value is more than 40%. The representative steel is SAF2507 steel.
As a special stainless steel material, duplex stainless steel is widely used in pressure vessels and other related equipment. Compared with austenitic stainless steel, duplex stainless steel has absolute advantages in resistance to intergranular corrosion, pitting corrosion, interstitial corrosion, especially stress corrosion cracking caused by chloride. The application prospect in the field of petroleum and chemical industry is very broad.

As shown in Table 1-1, the brand and chemical composition of some duplex stainless steels are shown.

Table.1-1 Brand and chemical composition of some duplex stainless steels

Grade

Countries

C

Cr

Ni

Mo

Mn

Si

N

The first generation

3RE60

Sweden

0.03

18.5

4.9

2.7

≤2.0

1.7

Uranus50

Finland

0.04

21.5

6.5

1.5

The second generation

SAF2205

Sweden

0.03

22.0

5.5

3.0

≤2.0

0.8

0.14

DP-3

Japan

0.03

25.0

6.5

3.5

0.4

0.20

08X21H6M2Ti

Russia

≤0.08

21.0

7.5

2.0

0Cr21Ni5Ti

China

0.06

22.0

5.8

≤0.8

0.8

The third generation

SAF2507

Sweden

0.03

25.5

7.0

4.5

0.30

DP-3W

Japan

0.03

25.5

7.6

3.0

2.5

0.32

0Cr21Ni5Mo3

China

≤0.08

26.5

5.0

3.0

≤1.5

1.0

Duplex stainless steel

Duplex stainless steel refers to the kind of steel with both austenite (α) and ferrite (γ) structure in stainless steel, and the two-phase structure should exist independently with large content. Generally, it is considered that if there is ≥ 15% ferrite on the austenitic matrix or ≥ 15% austenite on the ferritic matrix, it can be called as austenitic + ferritic duplex stainless steel, which is referred to as duplex stainless steel in this paper.

Development history of duplex stainless steel in China

Based on the development of duplex stainless steel abroad, duplex stainless steel has also been developed in China since the mid-1970s, mainly for the development of nitrogen-containing steel, focusing on the influence of nitrogen on the properties and process of steel. Up to now, there are five series of steel grades, but only the low chromium steel grades are included in the national standard, and the rest are produced according to the enterprise standard. At present, the national standard of steel plate is being revised, and the dual phase stainless steel of Cr22 and Cr25 will be considered to meet the standard. China’s duplex stainless steel is only at the development level of the second generation of duplex stainless steel abroad, with nitrogen content below 0.2%. As for the super duplex stainless steel with nitrogen content of 0.25% – 0.35% which has entered the market in foreign countries, China is still in the stage of laboratory development. The maturity of AOD refining process and duplex stainless steel continuous casting process will contribute to the success of industrial trial production of super duplex stainless steel plate.
At present, the main products of duplex stainless steel in China are duplex stainless steel pipe, plate and composite plate, as well as forgings and castings, with small output. The 219mm seamless pipe and 457mm thin-wall plasma welded pipe have been successfully developed. The 2205 and Q345C composite plates have also been used in the sand discharge pipe and lining of flood discharge deep hole of the Three Gorges Project of the Yangtze River. According to incomplete statistics, the domestic consumption of duplex stainless steel in 1999 was about 2000 tons, and the annual consumption in recent two years reached 4000 tons, doubling. With the development of application, domestic products can not meet the requirements, especially some engineering projects, mainly rely on imports.
At present, China has the strength and good level to manufacture duplex stainless steel and super duplex stainless steel equipment. Nearly ten large and medium-sized machinery manufacturing companies, including Nanjing Chemical Industry Co., Ltd., Shanghai Petrochemical Machinery Manufacturing Co., Ltd., and Jinzhou Heavy Machinery Co., Ltd., have manufactured nearly 100 sets of duplex stainless steel equipment. Nanjing Chemical Industry Co., Ltd. chemical machinery plant and Jinzhou Heavy Machinery Co., Ltd. can also manufacture more than one equipment according to foreign standards, which are exported to India and Malaysia respectively West Asia. The chemical machinery plant of Nanjing Chemical Co., Ltd. and Shanghai Petrochemical Machinery Manufacturing Co., Ltd. can not only manufacture ordinary duplex stainless steel equipment, but also manufacture super duplex stainless steel equipment. Of course, the pipe materials of UNS S32750 and UNS S32760 are imported. As for the pipeline steel needed for oil and gas and petrochemical industry, it has been manufactured and welded in the field in recent two years in China, and the experience in this field needs to be accumulated.

Main representative brand of duplex stainless steel

Duplex stainless steel can be generally divided into four categories:

  • Low alloy type — representative brand is UNS S32304 (23cr-4ni-0.1n), prEN value is 24-25;
  • Medium alloy type — the representative brand is UNS S31803 (22cr-5ni-3mo-0.15n), prEN value is 32-33;
  • High alloy type — standard brand is UNS S32550 (25cr-6ni-3mo-2cu-0.2n), prEN value is 38-39;
  • Super duplex stainless steel type — the standard brand is UNS S32750 (25cr-7ni-3.7mo-0.3n), prEN value > 40.

( prEN pitting resistance index prEN = CR% + 3.3 × Mo% + 16 × n%)
The low alloy UNS S32304 is Mo free and can be used instead of AISI304 or 316 in stress corrosion resistance.
The corrosion resistance of medium alloy UNS S31803 is between AISI 316L and 6% Mo + n austenitic stainless steel.
High alloy type, generally containing 25% Cr, molybdenum and nitrogen, and some containing copper and tungsten. The corrosion resistance of this kind of steel is higher than that of 22% Cr duplex stainless steel. Super duplex stainless steel type, containing high molybdenum and nitrogen, some also containing tungsten and copper, can be applied to harsh medium conditions, has good corrosion resistance and comprehensive mechanical properties, and can be compared with super austenitic stainless steel.
The main chemical composition of the representative brand is shown in table 2-1:
Table 2-1 Main chemical composition of representative brand

Type

UNS Grade

Chemical composition,%

C

Cr

Ni

Mo

Cu

N

Low alloy type

S32304

≤0.03

23

44

2

0.05

0.20

Low alloy type

S31803

≤0.03

22

5

3

0.08

0.20

Low alloy type

S32205

≤0.03

22

5

3

0.14

0.20

Low alloy type

S32550

0.04

25

6

3

0.10

0.25

Super DSS

S32750

≤0.03

25

7

4

0.24

0.32

Properties of duplex stainless steel

  • (1) The molybdenum containing duplex stainless steel has good chloride stress corrosion resistance under low stress.
  • (2The pitting resistance of duplex stainless steel is good, and the pitting resistance equivalen PRE=Cr%+3.3xMo%+16xN%.
  • (3) It has good corrosion fatigue and wear corrosion resistance.
  • (4) It has good comprehensive mechanical properties, high strength and fatigue strength.
  • (5) It has good weldability and small tendency of hot cracking. Generally, it can be welded with austenitic stainless steel or carbon steel without preheating before welding and heat treatment after welding.
  • (6) The duplex stainless steel with low chromium (18% Cr) has a wide range of hot working temperature and small resistance. It can be rolled directly to produce steel plate without forging. Compared with austenitic stainless steel, duplex stainless steel with high chromium content (25%) is a little more difficult in hot working. It can produce plate, tube and wire.
  • (7) Compared with 18-8 type austenitic stainless steel, the effect of work hardening is greater in cold working. In the initial stage of tube and plate bearing deformation, large stress is required to deform.
  • (8) Compared with austenitic stainless steel, it has large thermal conductivity and small linear expansion coefficient. It is suitable for lining of equipment and production of composite plate, and also suitable for making tube core of heat exchanger. Its heat exchange efficiency is higher than that of austenitic stainless steel.
  • (9) The brittle tendency of high chromium ferritic stainless steel is not suitable for working conditions higher than 300 ℃. The lower the chromium content in duplex stainless steel, the less harmful the brittle phase such as σ is.

Performance comparison of duplex stainless steel

Due to the characteristics of two-phase structure of duplex stainless steel, the duplex stainless steel has the characteristics of both ferritic stainless steel and austenitic stainless steel through proper control of chemical composition and heat treatment process.

Compared with austenitic stainless steel, the advantages of duplex stainless steel are as follows:

  • (1) The yield strength of austenitic stainless steel is more than twice that of ordinary stainless steel, and it has enough plastic toughness for forming. The wall thickness of the tank or pressure vessel made of duplex stainless steel is 30-50% less than that of austenite, which is helpful to reduce the cost.
  • (2) Even duplex stainless steel with the lowest alloy content has a higher ability of stress corrosion cracking resistance than austenitic stainless steel, especially in chloride containing environment. Stress corrosion is an outstanding problem that is difficult to solve for ordinary austenitic stainless steel.
  • (3) The corrosion resistance of 2205 duplex stainless steel, which is widely used in many media, is better than 316L austenitic stainless steel, while super duplex stainless steel has very high corrosion resistance. In some media, such as acetic acid and formic acid, it can even replace high alloy austenitic stainless steel and even corrosion resistant alloy.
  • (4) Compared with austenitic stainless steel with the same alloy content, it has better wear resistance and fatigue corrosion resistance than austenitic stainless steel.
  • (5) Compared with austenitic stainless steel, the coefficient of linear expansion is lower, close to carbon steel, suitable for connection with carbon steel, which has important engineering significance, such as production of composite plates or linings.
  • (6) No matter in the condition of dynamic or static load, it has higher energy absorption capacity than austenitic stainless steel, which has obvious advantages and practical application value for structural members to deal with sudden accidents such as collision, explosion, etc.

Compared with austenitic stainless steel, the disadvantages of duplex stainless steel are as follows:

  • (1) The universality and versatility of the application are not as good as austenitic stainless steel, for example, its service temperature must be controlled below 250 ℃.
  • (2) Its plastic toughness is lower than that of austenitic stainless steel, and its cold and hot working processes and forming properties are not as good as austenitic stainless steel.
  • (3) In order to avoid the appearance of harmful phase and damage the performance, the process system of heat treatment and welding should be strictly controlled.

Compared with ferritic stainless steel, the advantages of duplex stainless steel are as follows:

  • (1) The comprehensive mechanical properties of ferritic stainless steel are better than that of ferritic stainless steel, especially the plastic toughness, which is not as sensitive to brittleness as ferritic stainless steel.
  • (2) In addition to stress corrosion resistance, other local corrosion resistance are better than ferritic stainless steel.
  • (3) The properties of cold working process and cold forming are much better than that of ferritic stainless steel.
  • (4) The welding performance is also far better than that of ferritic stainless steel. Generally, preheating is not required before welding and heat treatment is not required after welding.
  • (5) The application range is wider than that of ferritic stainless steel.

Compared with ferritic stainless steel, the disadvantages of duplex stainless steel are as follows:

Duplex stainless steel has high element content and relatively high price. Generally, ferrite does not contain nickel.
To sum up, we can see the general situation of the service performance and process performance of duplex stainless steel. It has won the favor of users with its superior mechanical and corrosion-resistant comprehensive performance, and has become an excellent corrosion-resistant engineering material that not only saves weight but also saves investment.

Application of duplex stainless steel

Application and scope of duplex stainless steel in industry

Refining industry (chloride and hydrogen sulfide)

  • A. Heat exchanger and pipeline in crude oil desalination unit.
  • B. FCCU. Catalytic absorption desorption tower lining, tray plate, pipe, etc., gasoline reheater.
  • C. Hydrogenation degradation unit. Heat exchanger of air cooler, desulfurization reactor and water cooler.

Petrochemical Industry (neutral chloride)

  • A. PVC stripper and heat exchanger.
  • B. Coil type oxychlorination reactor and vinyl chloride reboiler in vinyl chloride unit.
  • C. Catalyst tube bundle, material / effluent flow sprinkler and condenser on the top of medium pressure flash tank of methanol synthesis reactor.
  • D. A tubular loop reactor for carbonylation of alcohols.
  • E. Acetic acid and other organic acids (formaldehyde, formic acid) production equipment and pipeline.

Onshore and offshore oil and gas industries (sour well pipes and pipelines).

  • A. Transmission pipeline and gas collection pipe.
  • B. Heat exchanger, water treatment and water supply system, fire fighting system, water spray system and water stabilization system of offshore oil platform.

Pulp and paper industry
Continuous sulfuric acid cooking or intermittent cooking device. Such as: sawdust preheater, digester, condenser, etc.
Chemical fertilizer industry

  • A. Urea industry. CO2 compressor cooler line. Carbamate pump body. The stripping pipe of the stripping tower, the condenser pipe of the high-pressure condenser, the decomposition pipe and the transmission pipe of the high-pressure decomposition tower.
  • B. Phosphate fertilizer industry. Reaction tank, slurry circulation and slurry transmission pipeline.

High strength structure
The tension system of derrick and submarine pipeline are adopted. Steam passes through flat blades, barges, tankers, etc.

Application performance of duplex stainless steel in China

Duplex stainless steel has been widely used in various industries abroad, such as pulp and paper industry, oil and gas industry on land and at sea, chemical processing industry, transportation industry (chemical ship and tank car), pharmaceutical and food industry and construction industry. Most of them are used to make reaction vessels, various industrial equipment and pipelines. In most applications, duplex stainless steel is considered to be a cost-effective material, which fills the gap between ordinary austenitic stainless steel such as 316 and high alloy austenitic stainless steel.
The following is a typical example of the application of duplex stainless steel in related industries in China.
Oil and gas industry
This is one of the main fields of the application of duplex stainless steel in foreign countries. At present, 1000 km of oil and gas transmission pipeline has been laid. Only a small amount of Nanhai oil field is used in China, all of which are imported. In addition, dual phase stainless steel welded pipe is considered for the gas gathering pipeline from Tarim Basin in the west of West to east gas transmission project, which has been produced and manufactured in China.
The oil refining industry is the first department to use domestic duplex stainless steel. In Nanjing, Zhenhai, Tianjin, Jinan and other refining and chemical companies, it is mainly used in the overhead lining (or composite plate), tower internals, air coolers and water coolers of atmospheric and vacuum distillation tower. The longest one has been used for 20 years. Zhenhai Refining & Chemical Co., Ltd. is the largest refining base in China, with a processing capacity of 16 million tons. It is one of the top 100 companies in the world. Multiple sets of equipment in the condensing cooling system are made of duplex stainless steel.
Chemical and petrochemical processing industry
This field involves a wide range, complex working conditions and various media. It is also the earlier and more fields where duplex stainless steel is used. Methanol is an important raw material for energy and chemical industry. In 2002, the domestic output was 2.1 million tons, and the import volume was equivalent to this. There was a large gap in China. Of course, a small amount (thousands of tons) of methanol was exported to South Korea. At present, the catalyst tubes of 200000 tons of large-scale and multiple sets of small and medium-sized methanol synthesis reactors with 100000 tons or less are made of duplex stainless steel, and 2205 steel tubes are used for large and medium-sized units, with many imported tubes and small ones 18-5Mo type domestic steel pipe is mostly used in the type-I device.
The medium condition (HCI, water vapor) of the cooling coil in the oxychlorination reactor of the vinyl chloride unit of Qilu Petrochemical Company is very severe. At present, imported 2205 duplex stainless steel has been used, and the results need to be observed. The catalyst regeneration cooler in the ethylene plant of Shanghai Petrochemical Company is made of 00c25ni7mo3wcun duplex stainless steel similar to DP3 steel. The outlet temperature of sea water is 40 ℃, which has been used intermittently for 15 years. The effect is very good. Several coolers of the pulverized coal gasification plant in Henan coal chemical plant are made of imported 2205 steel pipes.
Chemical fertilizer industry
The urea industry is also the first department to use domestic duplex stainless steel. The heat exchange equipment containing chloride ion water is widely used in the unit. For example, 304L Austenitic stainless steel tube bundle was originally used in the three-stage cooler of CO2 compressor in urea unit, which was leaked due to stress corrosion cracking one month later. Duplex stainless steel can be used for more than five years, and then 18-5Mo or 2205 duplex coolers were also used in the first and second stage coolers Phase stainless steel.
Because duplex stainless steel has good anti-corrosion fatigue performance in urea medium, it is very suitable for manufacturing the key equipment of urea production, i.e. pump body. The 0cr25ni6mo2n steel made in China can pass the intergranular corrosion tendency test of Huey method, and has been used in Heilongjiang chemical fertilizer plant, Dongting nitrogen fertilizer plant (five plunger type) and other large-scale chemical fertilizer plants. The pump body of a press pump in small and medium-sized chemical fertilizer plants in China is basically made of 18-5Mo steel, and dozens of them are made of high chromium and lead containing duplex stainless steel. In addition, the forging of this steel has passed the inspection of JIS G0573, G0591 nitric acid method and sulfuric acid method in Japan. It is exported to Japan in batches, and the price is cheaper than that produced in Japan.
In addition, the domestic 0cr25ni6mo3cun aging strengthening duplex stainless steel is used for the internals of the high pressure stop valve of various specifications of the main process pipeline of the urea plant with its wear resistance and corrosion resistance, and the effect is good.
Transportation
In recent years, the marine chemical carrier industry is the largest two-phase stainless steel user in foreign countries, and the consumption accounts for about 50% of the hot-rolled plate. There are many kinds of liquid cargoes loaded on the chemical ship, including chemical and petrochemical products, food, etc., which require that the cabin materials not only be corrosion-resistant, but also have high strength. Nowadays, 2205 duplex stainless steel has replaced 316L and 317L austenitic stainless steel and become the standard material for offshore chemical ships.
Qingshan shipyard of China Yangtze River Shipping Group has just started in this respect in China. It adopts European construction standards, uses imported 2205 steel plates, and makes the first 18500t chemical ship by itself. The steel plate consumption is about 1200t, which has been exported to Belgium. The breakthrough of building chemical vessel with duplex stainless steel in China has been realized, and the factory has formed large-scale production capacity.
Papermaking and salt light industry
As duplex stainless steel has good resistance to pitting and crevice corrosion in neutral chloride solution, its application in vacuum salt making and salt and nitrate co production unit has been developed in China. The large-scale salt water and mirabilite evaporation tank of 200000-300000 tons salt plant adopts duplex stainless steel lining and composite plate, which solves the problem of equipment scaling and corrosion. The longest one has been It has been used for 10 years. Biphasic stainless steel is used in large-scale vacuum salt plant, and there is mature experience in China.
In the field of pulp and paper industry, there is almost a blank in China. The kraft cooking method still mostly uses low-carbon steel cooking pots, while the two-phase stainless steel cooking and bleaching equipment has been widely used in foreign countries. At present, there are also introduced in China, but the number is very small.
To sum up, it can be seen that the use of duplex stainless steel in China has certain limitations. For example, the large use of duplex stainless steel in foreign countries, such as pulp and paper industry, oil and gas industry, transportation industry, and even construction industry, we are not involved in many areas, some of which are just at the beginning. According to the national conditions and the performance advantages of duplex stainless steel, in the future, in addition to expanding the application in the fields of chemical industry and petrochemical industry, combined with the technical transformation of pulp and paper industry, it is necessary to develop the application in this field. As for the oil and gas pipeline, it is difficult to promote it at present. The price of duplex stainless steel is too high, but it is necessary to manufacture the devices that are resistant to chloride ion and hydrogen sulfide for oil and gas fields The gas gathering pipeline and heat exchange equipment can be made of duplex stainless steel or even super duplex stainless steel.
With the development of marine transportation industry, the chemical ship manufacturing industry is in the ascendant, which needs a large number of steel plates. This gap needs to be filled. As for the application in the construction industry, it has not been involved so far. In fact, the coastal city sculpture landscape and the development of 2304 steel for civil water heaters can completely replace 304 and 316 austenitic stainless steel.

2205 duplex stainless steel

2205 duplex stainless steel was first developed in Sweden in 1970s. The material grade is SAF2205, which belongs to the second generation duplex stainless steel. Duplex stainless steel 2205 alloy is a typical duplex stainless steel with N, ultra-low carbon and duplex ferrite austenite, which is composed of 22% chromium, 2.5% molybdenum and 4.5% nickel nitrogen alloy.
Its chemical composition is: w (c) < 0.03%, w (n) < 2.0%, w (P) < 0.03%, w (s) < 0.02%, w (SI) < 1.0%, w (Ni) = 4.5% ~ 6.5%, w (CR) = 21% ~ 23%, w (MO) = 2.5% ~ 3.5%, w (n) = 0.08% ~ 0.2%. It has high strength, good impact toughness and good overall and local stress corrosion resistance. Compared with 316L and 317L austenitic stainless steel, 2205 alloy has better resistance to spot corrosion and gap corrosion. Compared with austenitic stainless steel, 2205 alloy has lower coefficient of thermal expansion, higher thermal conductivity and better weldability.
At present, the domestic 2205 stainless steel products include 2205 stainless steel pipe, 2205 duplex stainless steel seamless pipe, 2205 duplex stainless steel plate, 2205 stainless steel bar, forging material, pipe fitting, strip, etc. The early duplex stainless steel can resist medium strength uniform corrosion and chloride stress corrosion cracking, but its performance will be greatly reduced when it is used under welding conditions. In order to improve this situation, nitrogen was added to the duplex stainless steel 2205, which not only improved the corrosion resistance, but also obtained good welding performance. Because of its special properties, 2205 dual phase steel has a wide range of applications, and it is a brand of dual phase stainless steel widely used up to now.

Properties of 2205 duplex stainless steel

Uniform corrosivity
Because of chromium content (22%), molybdenum content (3%) and nitrogen content (0.18%), 2205 is superior to 316L and 317L in most environments.
Local corrosion resistance
The content of chromium, molybdenum and nitrogen in 2205 duplex stainless steel makes it highly resistant to point corrosion and gap corrosion in oxidizing and acidic solutions.
Stress corrosion resistance
The two-phase microstructure of stainless steel is helpful to improve the stress corrosion cracking resistance of stainless steel. Chloride stress corrosion occurs in austenitic stainless steel under certain temperature, stress, oxygen and chloride. Because these conditions are not easy to control, the use of 304L, 316L and 317L is limited in this regard.
Corrosion fatigue resistance
The high strength and corrosion resistance of 2205 dual phase steel make it have high corrosion fatigue strength. Processing equipment is easy to be affected by corrosion environment and loading cycle. 2205 is very suitable for such applications.
Pressure resistance
Compared with austenitic stainless steel, the compressive strength of duplex stainless steel 2205 alloy is twice that of austenitic stainless steel. Compared with 316L and 317L, the designer can reduce its weight. Alloy 2205 is particularly suitable for temperatures in the range of – 50 ° f / + 600 ° F and can also be used for lower temperatures under strict restrictions, especially for welded structures.
Thermoforming
During the heat forming treatment, the whole workpiece shall be heated as a whole, which shall be carried out in the temperature range of 1750 ° f to 2250 ° F. 2205 alloy is very soft at this temperature. If the temperature is too high, 2205 alloy is easy to tear. If it is lower than this temperature, austenite will fracture. Below 1700 ° F, due to the influence of temperature and deformation, the intermetallic phase will form quickly. After hot forming, it should be annealed at a minimum temperature of 1900 ° F and quenched to restore its phase balance, toughness and corrosion resistance. We do not recommend stress relief, but if this is necessary, the material should be solution annealed at a minimum temperature of 1900 ° F, then rapidly cooled and water quenched.
Cold forming
2205 alloy can be cut and cold formed. However, because of its high strength and hardness, 2205 alloy needs more cold forming than austenitic steel, and because of its high strength, the springback factor should be fully considered.
Heat treatment
Alloy 2205 shall be annealed at a minimum temperature of 1900 ° f followed by rapid cooling and water quenching. This treatment should be used for remelting annealing and stress relief. If the stress relieving treatment is carried out at a temperature lower than 1900 ° F, harmful metal or non-metal phases will be easily precipitated.
Machinability
In high-speed machine tools, the feed rate and cutting speed of 2205 alloy are the same as 316L. If the carbonization knife is used, the cutting speed is reduced by about 20% compared with 316L. The performance of machine equipment and its components plays a key role here.
Welding performance
The weldability of 2205 alloy is very good. The performance of 2205 alloy is that the weld metal and the heat modified part still maintain the same corrosion resistance, strength and toughness as the base metal. It is not difficult to weld 2205, but its welding procedure should be designed so as to keep a good phase balance state after welding and avoid harmful metal phase or non-metal phase precipitation. 2205 can be welded in the following equipment: GTAW (TIG); GMAW (MIG); SMAW (stick “electrode); saw; FCW; and paw.
Performance comparison
Compared with 316L and 317L austenitic stainless steel, 2205 alloy is superior in resistance to pitting corrosion and crack corrosion. It has high corrosion resistance. Compared with austenite, 2205 alloy has lower coefficient of thermal expansion and higher thermal conductivity.
Compared with austenitic stainless steel, the compressive strength of duplex stainless steel 2205 alloy is twice that of austenitic stainless steel. Compared with 316L and 317L, the designer can reduce its weight. Alloy 2205 is particularly suitable for temperatures in the range of – 50 ° f / + 600 ° F and can also be used for lower temperatures under strict restrictions, especially for welded structures.

Application field of 2205 duplex stainless steel

Because 2205 duplex stainless steel has very strong corrosion resistance (including stress corrosion, spot corrosion, crevice corrosion and intergranular corrosion), as well as high strength and toughness. It can be widely used in the following fields.

  • (1) Pressure vessel, high pressure storage tank, high pressure pipeline, heat exchanger (chemical processing industry).
  • (2) Oil and gas pipeline, heat exchanger pipe fittings.
  • (3) Sewage treatment system.
  • (4) Pulp and paper industry classifiers, bleaching equipment, storage and processing systems.
  • (5) Rotary shaft, press roll, blade, impeller, etc. in high strength and corrosion resistant environment.
  • (6) The cargo box of a ship or truck.
  • (7) Food processing equipment.

Study on the weldability and process of 2205 duplex stainless steel

Welding mechanism of 2205 duplex stainless steel

Welding is a metallurgical process. The welding quality of 2205 duplex stainless steel depends on the balance of ferrite and austenite content in its weld and heat affected zone and the homogeneity of two-phase structure. In the process of welding, from melting to cooling, from freezing point to 1200 ℃, the metal is ferrite structure; from 1200 ℃ to 800 ℃, austenite is precipitated from ferrite; from 800 to 475 ℃, intermediate phase (σ phase, carbide, nitride) may be precipitated. Therefore, the input of welding line energy directly affects the content of ferrite in the weld and heat affected zone. Too small line energy is not conducive to austenite precipitation; too large line energy will cause burning loss of alloy elements Cr, Ni and Mo, which will lead to the deterioration of corrosion resistance and mechanical properties of the material, unable to get good metallographic structure, and easier to precipitate intermediate phase.
The highest temperature and rapid cooling of welding thermal cycle can promote the ferrite formation of duplex stainless steel. The increase of δ ferrite content leads to the decrease of impact toughness and corrosion resistance. Therefore, it is very important to choose appropriate welding parameters.
Compared with austenitic stainless steel, 2205 duplex stainless steel is more sensitive to contamination, especially moisture and moisture. Any kind of oil, grease, moisture and other pollutants will affect the corrosion resistance and mechanical properties of the material, so the material should be cleaned strictly before welding. The welding joint form of duplex stainless steel shall be fully prepared in advance. If the groove is cut by hot processing method, the groove surface shall be polished to expose the metallic luster, and the groove surface shall be subject to penetrant testing. The welding groove is preferably machined. In general, preheating is not used in the welding of duplex stainless steel, because preheating will reduce the cooling speed of the HAZ. Of course, preheating is beneficial to reduce the moisture on the steel surface. When preheating is used to reduce the moisture, the welding surface must be cleaned first, and then heated to 95 ℃ evenly. If the cooling rate of the weld is too fast and the ferrite content in the HAZ increases too much, preheating is meaningful. For example, the cooling speed is very fast when the thin plate is welded to the thick plate and the container is welded with the liner, so preheating can be considered.
Compared with austenitic steel, duplex stainless steel has the characteristics of good thermal conductivity and low coefficient of thermal expansion, so it will not produce large residual stress and has higher ability to resist hot cracks. Therefore, duplex stainless steel can be welded with large linear energy, with the maximum interlayer temperature of 150 ℃. In actual welding, it must be ensured that the interpass temperature is not higher than the interpass temperature set in the process test. When the amount of welding is very large, the welding sequence should be arranged reasonably to ensure that there is enough cooling time between weld layers, which can not only ensure the temperature between layers, but also improve the labor productivity. The test plate size of welding procedure qualification will affect the cooling rate and interpass temperature. It should be noted that the interpass temperature determined by welding procedure qualification should be lower than the actual welding. Therefore, the process evaluation can not predict the degree of cooling rate reduction due to the use of higher interlayer temperature in practice.
After welding, there is no need for heat treatment. Duplex stainless steel is very sensitive to temperature at 300-1000 ℃. Stress relief treatment at 300-700 ℃ will lead to σ phase precipitation and 475 ℃ embrittlement, which will reduce the toughness and corrosion resistance. Stress relief treatment at 700-1000 ℃ will lead to the precipitation of intermetallic compounds, as well as the reduction of toughness and corrosion resistance.

Basic requirements of SAF2205 dual phase steel joint and measures to meet the requirements

Welding requirements:
There is no defect beyond the quality standard of the welded joint, and the mechanical properties meet the expected service performance requirements of the welded structure. There are no welding hot cracks and cold cracks, stress corrosion, pitting corrosion and δ phase embrittlement.
Precautions:

  • (1) Duplex stainless steel has good weldability. Generally, welding materials with the same or similar composition to the base metal are selected. Due to the great influence of carbon content on corrosion resistance, the carbon content of the deposited metal should not be higher than that of the base metal. For equipment with weak corrosion or only to avoid corrosion pollution, stable elements such as Ti or Nb or ultra-low carbon welding materials can be selected. For workpieces with high acid corrosion resistance, welding materials containing Mo are often used. Suitable welding materials shall be selected to avoid welding hot crack and cold crack, such as e309mol-16, a042 argon arc welding wire h00cr18ni14mo2.
  • (2) Design welding joint reasonably. In order to avoid the accumulation of corrosive medium in the welding joint, reduce or eliminate the stress concentration, and eliminate or reduce the residual stress of the welding joint, the ideal stress relief effect can be obtained only when the heating temperature is between 800-900 ℃ with common process measures; the butt joint shall be used as far as possible in the structural design to avoid the cross weld, and the single V-groove shall be Y-groove instead.
  • (3) Adopt small heat input, i.e. small current, large welding speed, reduce transverse swing. After the previous weld is cooled to preheating temperature, weld the next weld. Conduct annealing treatment at 750-800 ℃ after welding. After annealing, it shall be cooled quickly to prevent δ phase and 475 ℃ embrittlement.

2205 duplex stainless steel welding technology

Some key technologies of 2205 duplex stainless steel welding are as follows:

Selection of welding methods and materials

Generally, the welding methods for austenitic stainless steel, such as manual arc welding, tungsten inert gas arc welding and gas metal arc welding, can be used for the welding of duplex stainless steel. The welding material shall be duplex steel with higher nickel content than the base metal. Ensure that the austenitic phase is dominant in the weld. The content of ferrite in the weld should be controlled at 30% ~ 45%.

Selection of welding process parameters

Too much or too little welding line energy is not good. It is generally controlled in the range of 0.5-2.5 kJ / cm. The specific size shall be selected according to the thickness of the weldment. Generally, preheating is not required during welding, but if the wall thickness of the weldment is too large or the ambient temperature is too low, preheating measures shall be taken if necessary to prevent the content of ferrite in the weld joint and thermal effect caused by too fast cooling rate. In order to avoid the formation of precipitates due to low cooling rate, the interlayer temperature of multi-layer / multi pass welding should be controlled.

Weld pool and back protection

Adding nitrogen into the shielding gas can improve the corrosion resistance of the weld. Effective back gas protection is the premise of welding quality. The purity of the shielding gas shall meet the process requirements, and effective back protection tooling shall be adopted. When welding is started, the oxygen content at the back of the weld shall be tested, and the welding can be started only after meeting the process requirements.

Tack weld

If the length of the tack weld is too short, the welding process will end without establishing a balance, and the weld will be cooled quickly, which may lead to high ferrite content, low toughness and corrosion resistance due to nitride precipitation. Therefore, if tack welding is used, the minimum length of tack welding shall be specified and larger heat input specification parameters shall be used.

Protection of welding process materials

The arc striking and arcing on the surface of the material is an instantaneous high temperature process, and the cooling speed is very fast. The ferrite content in the surface microstructure is very high, which is very sensitive to cracks and corrosion. It should be avoided as much as possible. If it occurs, it must be removed by grinding with a fine grinding wheel. The protection of materials is very important in the field welding process. The pollution of stainless steel welding materials caused by carbon steel, copper, low melting point metal or other impurities should be avoided. Where possible, stainless steel plates and carbon steel plates shall be stored and welded separately. During welding and cutting, measures shall be taken to prevent splashing, arc striking, carburizing, local overheating, etc.

Precautions for 2205 duplex stainless steel welding:

  • (1) 2205 duplex stainless steel has high strength and its elongation is lower than that of austenitic stainless steel. Therefore, when assembling the weld, the quality of assembling should be strictly controlled. If the powerful assembling is completed, it is very difficult to rectify after welding. Even if it can be corrected, it will also produce great residual stress, which will degrade the mechanical properties and reduce the corrosion resistance.
  • (2) The welding of 2205 duplex stainless steel is more sensitive to pollution. Any type of oil, grease and moisture will affect the corrosion resistance and mechanical properties of the material. Therefore, before welding, the welding area shall be cleaned with acetone to thoroughly remove oil, soil, dust and water pollution.
  • (3) It is very important to choose a reasonable welding line energy. When a very small welding line energy is used, the high ferrite content in the heat affected zone and weld due to rapid cooling is unfavorable to the corrosion resistance and mechanical properties; however, the high welding line energy will burn the alloy elements and slow cooling, resulting in the precipitation of intermediate phase, resulting in the imbalance of phase structure, which is also unfavorable to the corrosion resistance and mechanical properties. Therefore, the best performance of weld and HAZ can be obtained by adjusting the energy input of welding line.
  • (4) After welding, ferrite inspection shall be carried out on the weld and heat affected zone of the weld to check the rationality of the welding process. When the ferrite content is less than 30%, 2205 duplex stainless steel will produce embrittlement.

Experimental materials and welding process

Test materials and electrodes

The raw materials used in this test are from Sandvik company of Sweden. The brand is SAF2205 (ASTM 240-00, UNS S31803). The processing size of welding sample is 150 mm × 80 mm × 6 mm, and the welding rod is ER2209 (φ = 2.0 mm). See table 4-1 and Table 4-2 for chemical composition of test plate and welding wire.
Table.4-1 Chemical composition (mass fraction%) of 2205 duplex stainless steel base metal and ER2209 electrode

C

S

P

Si

Mn

Cr

Ni

Mo

N

Cu

Base metal

0.0166

0.001

0.024

0.36

0.82

22.48

5.46

3.12

0.16

Electrode

0.013

0.007

0.018

0.49

1.54

22.92

8.61

3.18

0.18

0.012

Table.4-2 Mechanical properties of 2205 test plate base metal

Yield strength/Mpa

Tensile strength/Mpa

Elongation after fracture/%

Hardness/HRB

660

815

33.0

40.1

Welding process instruction

According to the technology mastered in the research, the process specification is drawn up for welding procedure qualification. Table 4-3 is the welding process instruction of SAF2205 duplex stainless steel plate.
Table.4-3 Welding process instruction for 2205 duplex stainless steel pipe

Date: May 4                                              Welding procedure qualification report No.: 001

Welding method: manual arc welding                  Mechanization degree: semi-automatic

Welding joint form and groove size:
Joint type: as shown in the right figure
Groove type: Y groove
Clearance e: 1.5mm
Blunt edge P: 2mm
Groove angle α: 75 °
Gasket (material and specification):——
Others:——

 Diagram

20200218053228 99608 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel

Welding base metal:
Welding material: 22052pw, produced by Avesta, Sweden
Specification of base metal: 150 mm × 80 mm × 8 mm plate

Welding materials:

Type of welding material

ER2209 duplex stainless steel electrode

Standard of welding materials

Easy to weld, good fusibility

Dimensions of filler metal

2.00mm

Specification of welding material

φ3.2×350mm

Others

—–

Electrode Description:
ER2209 is a stainless steel electrode with titanium calcium coating. It has excellent mechanical properties and intergranular corrosion resistance, and can be used in both AC and DC. Excellent performance. It is used for welding stainless steel structure with working temperature lower than 300 ℃.
Matters needing attention:
Before welding, the electrode should be baked at 250 ℃ for 1 hour, and DC power supply should be used as much as possible. When AC welding is used, the penetration is shallow, and the current should not be too large, so as to avoid redness of the electrode.

Welding position:
Butt weld position: flat welding
Welding direction: (up, down)—-
Welding temperature: 25 ℃ (room temperature), interlayer temperature less than 150 ℃
Fillet weld position:—
Welding direction: (up, down)—-
Weld metal width (range): 8-12 mm
Seam metal thickness (range): 2-4 mm
Post weld heat treatment:
Temperature range: (℃)—-
Holding time: (H)—-

Welding equipment:
Model of electric welding machine: Panasonic DC pulse TIG / manual arc welding machine sa-300tp-3
Technical parameters of electric welding machine:
1. Output voltage: 380V
2. Number of phases: single phase
3. Frequency: 50 / 60 Hz
4. Rated input capacity: KVA dc20.2 ac30.8
5. Output current (a) DC arc starting current argon arc welding: 5-300
Manual welding: 5-300
Welding current: 5-300
Arc stopping current: 5-300
6. Argon arc welding: 10-300
Manual welding: 10-300
Welding current: 10-300
Arc stopping current: 10-300
7. Output voltage (V) DC argon arc welding: 10.2 ~ 22.6
Manual welding: 20-32
AC:10.2~22.6
No load voltage (V): DC 100
8. Rated temporary load rate: cycle 10 minutes
9. Rectifying mode: single phase full wave rectifying mode of thyristor
10. External characteristics: constant current characteristics
11. Pulse frequency: 0.5-15hz
12. Pulse width: 15-85%
13. Incremental time: 0.5-5.0sec continuous regulation
14. Attenuation time: 0.5-5.0sec continuous adjustment
15. Gas pre flow time: 0.2sec regulated in PCB
16. Gas lag time: 2-20sec continuous regulation
17. Spot welding timing: 0.5-5sec continuous adjustment
18. Overall dimension (WxDxH): 415x625x853 (mm)
19. Weight: 193kg

Electrical characteristics:
Current type: DC polarity: reverse connection
Welding current range: (a) 65-80 arc striking voltage (V): 150-220

Welding process parameters:

Weld bead/Layer

Method of welding

Filled material

Welding power source

Electric arc
Voltage

Speed of welding/(cm/min)

Line energy/(kJ/cm)

Brand name

Thickness

Polarity

Electric current(A

1

Electrode arc welding

A042

3.2mm

Reverse connection

70

30

45

12

2

Electrode arc welding

A042

3.2mm

Reverse connection

70

30

45

12

Welding requirements:
1. After groove processing and plate cutting, use a grinder to grind out the groove. The groove angle is 30 ° ± 2.5 ° on one side and 0.5-1.5mm on the blunt side. The processing groove shall not cause overheating and discoloration of the base metal.
2. Groove and welding wire cleaning: the groove and the inner and outer surfaces within 25 mm on both sides of the groove shall be cleaned. The cleaning procedures are as follows: grinding by a grinder – cleaning by acetone (or anhydrous ethanol). After cleaning, welding cannot be carried out directly, and the operation can only be carried out after the groove end surface is dried. The welding wire shall also be wiped clean with a sponge moistened with acetone (or anhydrous ethanol).
3. Before the plate welding construction, the welding operation instruction shall be prepared according to the welding procedure qualification, and the welder shall weld according to the specified welding operation instruction.
4. For small current, fast welding, the input heat should be controlled within 0.2-2.5kj/mm (each process parameter will not exceed the range strictly according to the welding process). The maximum inter layer temperature is 100 ℃. For multi layer welding, the arc energy of filling weld bead should not be higher than that of backing. The heat affected zone of the outermost weld is difficult to achieve a balanced two-phase structure due to the lack of heat treatment effect of the outer weld, so the last weld should be located on the non working medium surface as far as possible when developing the welding process. The arc striking of circumferential weld shall be carried out in the weld bead, and the arc striking and ending of longitudinal weld shall be carried out on the arc striking plate and arc extinguishing plate.
5. Avoid too much lateral swing of electrode and too wide molten pool to avoid too much current and high residual stress.
6. After manual arc welding, the coating shall be knocked out after the weld cools down to cool down under the protection of coating,
Prevent oxidation.
7. When using electrode arc welding, position welding shall be carried out according to the appropriate length and interval. The root pass shall not be started from tack welding. In order to avoid cracks in the root weld bead caused by tack welds, the welder shall interrupt the root welding before tack welds, and continue the root welding after the tack welds are completely polished off.
8. The welding rod shall be kept dry and stored in the heat preservation barrel at any time, and it is not allowed to litter the welding rod.
9. Repair welding is not allowed.

Technical measures:
Swing welding or non swing welding: non swing welding
Swing parameters:—
Cleaning before welding and cleaning between layers: mechanical polishing
Cleaning method: mechanical polishing
Single pass or multi pass welding (each side): double-sided single pass weld
Single wire welding or multi wire welding: single wire welding
Distance between conductive nozzle and workpiece: (mm) 3-5
Others:—

Process standard:
Visual inspection shall be carried out after welding, and there shall be no undercut, air hole, crack, slag inclusion and other defects. Weld reinforcement 0.8mm,
According to the standard of JB47302205, the photo quality is class AB and class II.

Plate sample after welding: (as shown in Figure 4-1 (a), (b))

20200218054542 92508 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel 20200218054519 83924 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
(a) (b)

Fig.4-1 Plate sample after 2205 duplex stainless steel welding

Effect of heat treatment process on properties of 2205 duplex stainless steel

Preparation method of metallographic sample

If the above two-phase stainless steel forming welding plate meets the requirements of welding and welding, the welding slag and weld burr around the welding plate shall be polished to the required specification by hand grinder. The polished plate is cut into small pieces of 10 mm × 10 mm × 8 mm by numerical control wire cutting machine for future test.
The preparation of metallographic specimen includes five steps: sampling, inlaying, grinding, polishing and etching. Equipment and materials used in the test:

  1. Metallographic microscope; 
  2. Grinding wheel machine; 
  3. Polishing machine; 
  4. Blower; 
  5. Sample; 
  6. Sandpaper of different models; 
  7. Glass plate; 
  8. Polishing powder suspension; 
  9. Alcohol; 
  10. Aqua regia solution mixed by 1 volume of concentrated nitric acid and 3 volumes of concentrated hydrochloric acid; 
  11. Cotton; 
  12. Clip.

(1) Sampling
Sampling is a very important step in metallographic microanalysis. The selection of microsamples should be based on the purpose of research and take the representative parts. In this experiment, the section with better weld conditions in the welding plate is selected and chamfered when grinding the surface. In the process of sample cutting, the temperature of sample should not be too high, so as not to cause the change of metal structure and affect the analysis results.
(2) Embedding
For the samples with too small size, such as irregular shape of wire, sheet, strip and tube, and with special needs (observing the surface structure), they can be embedded in many ways, such as low melting point alloy, plastic, epoxy resin and clamp.
At present, plastic inlays are generally used. There are two kinds of plastics used in the sample: thermosetting and thermoplastic. The former is bakelite powder or bakelite powder, opaque, with various colors (generally black). This kind of plastic is relatively hard, but its corrosion resistance is poor, such as acid and alkali. The latter is translucent or transparent, with good acid and alkali resistance, but soft. When the two kinds of plastics are embedded in the sample, they must be put into the film on the film machine for heating. The heating temperature of the two kinds of plastics is 110-150 ℃ for thermosetting plastics and 140-160 ℃ for thermoplastic plastics. At the same time, the pressure is 200kg / cm2. After a certain period of time, the embedded sample will be ejected from the film by removing the pressure. This method is fast, but it has to be heated and pressurized, which may change some microstructure. For example, quenched martensite is tempered. When the sample needs to observe several sides, it cannot be embedded.
(3) Grinding
The grinding of samples is generally divided into rough grinding and fine grinding. The purpose is to obtain a smooth surface. Prepare for polishing.
Rough grinding
Before grinding the sample, grind it with grinding wheel or file it with file. During grinding, keep the grinding surface of the sample parallel to the side of the grinding wheel, slowly contact with the grinding wheel, and evenly apply appropriate pressure to the sample. In the process of grinding, the sample shall contact slowly back and forth along the grinding wheel, and apply appropriate pressure to the sample evenly. During the grinding process, the sample should move slowly back and forth along the radial direction of the grinding wheel to avoid the unevenness of the sample caused by the groove of the grinding wheel. In addition, care should be taken not to heat the specimen due to grinding. Therefore, it is necessary to cool the sample in cold water from time to time to ensure that the structure of the grinding surface will not change due to the temperature rise during the grinding process.
Generally, the grinding surface of the sample shall be chamfered and the edges and corners of the grinding surface shall be removed (edges and corners shall be reserved, except for the inspection land of carburizing layer), so as to avoid tearing the sandpaper or scraping the polishing cloth during fine grinding and polishing. Even cause the specimen to fly out of the polishing machine and hurt people. When the surface of the sample is flat, the rough grinding is completed, and then the sample is washed and dried with water.
Fine grinding
After rough grinding, there are still relatively thick and deep grinding marks on the sample. In order to prepare for polishing, fine grinding is required. Fine grinding is to polish the sample after rough grinding on the abrasive paper from coarse to fine.
Fine grinding is divided into manual grinding and mechanical grinding. Manual grinding is to place the abrasive paper on the glass plate, first grind the sample from the coarser abrasive paper, and the grinding direction shall be perpendicular to the grinding mark direction of the grinding wheel until the original grinding mark is eliminated. Only after all the grinding marks of the current sandpaper have been removed can the next finer sandpaper be replaced. When replacing the sandpaper, the sample must be cleaned to avoid taking the sand particles onto the sandpaper, so that the sample will be marked with deep slip marks. After changing the sandpaper every time, the grinding direction of the sample should also be changed accordingly (generally turn 90 ℃). In this way, the fine grinding is to lay the killed sample on the glass plate, press the grinding surface of the sample lightly on the sandpaper with one hand, and retire it forward for grinding. It is not allowed to push back and forth, the module to the head, take it back and then push. During the grinding process, the grinding surface should be pressed evenly, and the pressure should be moderate to ensure that the surface of the sample will not be heated without too deep slip marks. Until there is only one direction on the grinding surface. The grinding surface can reach the ground cleanliness before polishing.
Fine grinding is carried out in order from coarse to fine on a set of metallographic sandpaper with different thickness measurement.
Sandpaper No.: 180, 280, 320, 0, 01, 03, 04, 05
(4) Polishing
The polishing of the sample is the best grinding process. The purpose is to take out the grinding marks on the surface of the sample to achieve a bright mirror surface without grinding marks.
The polishing of samples is generally divided into mechanical polishing, electrolytic polishing, electrolytic polishing and chemical polishing. In this test, the sample is polished by mechanical polishing method.
Mechanical polishing
The mechanical polishing of the sample is carried out on a special polishing machine. The main structure of the polishing machine is composed of a motor and a tearing disc, with a rotating speed of 300-500 rpm. Polishing fabrics such as fine canvas, cloth, velvet and silk are laid on the polishing disc. During polishing, the polishing solution is continuously dripped on the polishing disc. The polishing solution is usually made of fine powder such as Al2O3, MgO or Cr2O3 (particle size is about 0.3-1 μ), suspension in water (5-10 g of Al2O3 is added to each liter of water) or paste polishing agent made of very fine diamond powder. Al2O3, also known as corundum, is white and transparent, which is used for rough and fine polishing. MgO white is suitable for final polishing of soft materials such as aluminum, magnesium and their alloys. Cr2O3 is green and has high hardness. It is suitable for polishing alloy steel, high speed steel and titanium alloy after quenching.
Mechanical polishing is to eliminate grinding marks by the relative grinding and rolling effect of very fine polishing powder on the grinding surface. During the operation, the grinding surface of the sample shall be evenly pressed on the rotating polishing disc (it can be light at first and then heavy) and continuously move radially from the edge of the disc to the center. The polishing time is generally about 3-5 minutes. The polished sample surface shall be bright without any grinding marks. It should be pointed out that the polishing time is not easy to be too long, and the pressure fluid should not be too large, otherwise the disorder layer will be produced, which will lead to the wrong conclusion of tissue analysis.
After polishing, wash the sample with water and dry it with cotton or blower. If you only need to observe various inclusions in the metal or the shape of graphite in the cast iron, you can directly observe the sample under the metallographic microscope.
(5) Etching
When the polished surface of the sample is observed directly under the microscope, all it can see is a bright light. Except for some inclusions or grinders, it is impossible to distinguish the machine morphological characteristics of the components of various tissues. Therefore, the specimen surface must be “etched” with an etchant to clearly show the microstructure.
The most commonly used method of metallographic structure display is chemical etching. The main principle of the chemical etching method is to show the structure of the metal by the chemical dissolution or electrochemical action (i.e. the principle of micro cell) caused by the etchant on the surface of the sample. The etching mode depends on the nature and quantity of the constituent phases in the structure.
For pure metals and single-phase alloys, etching is still a pure chemical dissolution process. Due to the disordered arrangement of atoms on the grain boundary of metals and alloys and the high energy, the grain boundary is easy to be etched and presents concave grooves. At the same time, due to the different arrangement of atoms in each grain, the dissolution rates of each grain are different, resulting in the depth of the etched area Do not, in the vertical light will show different light and shade of the grain.
For the alloy structure with more than two phases, the etching is mainly an electrochemical corrosion process. Due to the different composition of each component phase, each component has different electrode potential. When the sample is immersed in the etchant with the effect of electrolyte, countless pairs of “micro batteries” are formed between the two phases. The negative potential phase becomes the cathode, which will not be eroded under the normal electrochemical action And keep the original smooth surface. When the light hits the uneven surface of the sample, different tissues and phases can be observed under the microscope due to the different degree of reflection on the light.
The etching method is usually to immerse the grinding surface of the sample in the etching agent, or to wipe the surface of the sample with cotton. The etching time should be appropriate. Generally, when the grinding surface of the sample is dark, it can be stopped. If the etching is insufficient, it can be repeatedly etched. After the etching, wash it with water immediately, and then wipe the grinding surface with cotton stained with alcohol and blow it dry. So far, the preparation of the metallographic sample is completely completed The tissue can be observed and analyzed under the microscope.

Heat treatment process and microstructure observation of metallographic specimen

The sample is heated in srjx-4-3 box resistance furnace for heat treatment. One group of samples were dissolved at 950 ℃, 1050 ℃ and 1150 ℃ for 2h, then cooled; the other group of samples were aged at 650 ℃, 750 ℃ and 850 ℃ for different times, then cooled. The holding time was 10 min, 30 min, 1 h and 2 h at 650 ℃ and 2 h, 4 h, 10 h and 20 h at 750 and 850 ℃. After heat treatment, the samples were polished by 200 × 3, 280 × 3, 320 × 3, 400 × 3 and 600 × 3 metallographic sandpaper, and then polished by 1 μ m Al2O3, and corroded in 5mg FeCl3 + 15ml HNO3 + 50ml HCl + 100ml H2O solution. The temperature is 82-89 ℃, and the time is 2-6 min. Microstructure observation and quantitative analysis were carried out on qantab metallographic analysis system.
The microstructure of 2205 stainless steel after aging treatment at different temperatures is shown in Figure 5-1. It can be confirmed that δ → γ + σ phase transition does occur in ferrite during aging. When aging temperature is high, σ phase nucleates and grows in δ / γ phase boundary and δ phase at the same time; when aging temperature is low, σ phase nucleates in δ / γ phase boundary and then grows in δ phase. The morphology of σ phase will vary with the temperature. When the temperature is high, σ phase is easy to form massive structure; when the temperature is low, σ phase is easy to form lamellar structure. The phenomenon that the nucleation position and morphology of σ phase change with the temperature can be explained in this way. The precipitation of σ phase is controlled by the diffusion behavior of Cr and mo. With the increase of temperature, Cr and Mo are easy to diffuse and accumulate, so it is easy to nucleate in the phase boundary and phase, and σ phase is also easy to aggregate and grow into massive. When the temperature is low, the diffusion coefficients of Cr and Mo decrease, but because the diffusion coefficient of the grain boundary is higher than that in the crystal, and the ratio of Cr and Mo at the δ / γ phase interface is conducive to the formation of σ phase, so it is mainly the precipitation of phase boundary and the formation of lamellar structure.

20200218060124 89300 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel 20200218060156 99611 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel 20200218060221 49185 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
(a) 650 ℃ for 2h; (b) 750 ℃ for 2h; (c) 850 ℃ for 2h

Fig.5-1 Microstructure of 2205 duplex stainless steel after aging at different temperatures

The microstructure of 2205 stainless steel aged at 600 ℃ for different time is shown in Figure 5-2. It can be seen from Figure 5-2 that the content of σ phase increases with the increase of aging time. The σ phase is produced by the decomposition of δ phase in the biphasic structure. When the aging time is long enough, the δ phase will almost disappear. That is to say, the solid solution structure (δ + γ) will eventually change into the two-phase structure (γ + σ) due to aging. At the same time, σ phase coarsens and agglomerates in the long time aging.

20200218060531 16107 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel 20200218060553 98841 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel 20200218060611 80234 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
(a) 2 h at 750 ℃;  (b) 4 h at 750 ℃; (c) 10 h at 750 ℃

Fig.5-2 Microstructure changes of 2205 duplex stainless steel after aging for different times

Effect of heat treatment process on mechanical properties of samples

It can be seen from Table 5-1 that compared with solution treatment at 1050 ℃, the strength of the material after aging treatment at 650 ℃ and 750 ℃ is significantly improved, but the toughness is reduced. When the aging time is fixed, the strength of the material decreases with the increase of the aging temperature, and the elongation (δ) and the area shrinkage (ψ) also decrease; when the aging temperature is fixed, the strength of the material increases obviously with the increase of the aging time, but the plasticity decreases obviously. It shows that the aging treatment has a certain strengthening effect on the material. After aging treatment, σ phase appeared in the structure, and the content of precipitated phase increased with aging time. σ phase belongs to brittle phase with high hardness and high brittleness. When plastic deformation occurs, the brittle phase precipitated from the grain boundary blocks the dislocation movement of ferrite phase and austenite phase, resulting in plane plug group, which makes it difficult to move dislocation. At the same time, the distance of dislocation slip becomes shorter, which can improve the deformation resistance of the material, produce certain strengthening effect, and reduce the plasticity at the same time. The strengthening effect of brittle phase increases with the increase of precipitate content.
The comparison of the tensile properties of the materials aged at 650 ℃, 750 ℃ and 850 ℃ and solution treated at 1050 ℃ is shown in Table 5-1. Aging treatment at 650 ℃ * 1H significantly improved the strength of the material, σ s and σ B increased by 17.7% and 18.2% respectively, plasticity slightly decreased, σ and ψ decreased by 12.6% and 15.8%, indicating that the material had a strong strengthening effect at 650 ℃.
Table 5-1 mechanical properties of 2205 stainless steel after different aging treatment

Sample number

Yield strength σs/MPa

Tensile strength σb/MPa

 Elongation δ/%

Reduction of area/%

Original plate

660

815

33.0

75.0

1050℃ Solid solution

705

695

25.5

76.0

650*1h

746

820

22.4

27.9

750*2h

750

830

19.8

18.3

750*10h

771

835

15.5

11.3

850*10h

710

820

13.1

11.1

When the duplex stainless steel is aged at 650 ℃, the α phase in the two-phase structure will undergo amplitude modulation decomposition, and the α phase will be decomposed into dispersion distribution chromium rich α ‘and iron rich α phase. Chromium rich α ‘precipitates on the surface of α phase in nanometer scale, and forms a coherent relationship with the parent phase α phase, resulting in serious coherent distortion, significantly improving the friction resistance of dislocation movement, hindering dislocation movement, making it difficult to slide, thus producing strong strengthening effect and increasing the strength of the material. The chromium rich α ‘precipitated from the α phase has little effect on the plastic deformation of the γ phase in the structure, but has little effect on the plasticity of the material.

Effect of heat treatment process on Vickers hardness of duplex stainless steel

Principle of Vickers hardness measurement

Vickers hardness test is one of the indentation hardness tests, and its measured value is represented by HV.
Vickers hardness test was first proposed in the early 1920s. Compared with other hardness tests, it has the following advantages: hardness value has nothing to do with the size and load value of the indenter; it does not need to change the indenter according to the material’s hardness and softness; the edge of the square indentation is clear and easy to measure. Vickers hardness is used in all metals and is one of the most widely used hardness standards. As long as the texture of the tested material is uniform, the Vickers hardness test can obtain reliable hardness value with low load and small indentation, which can reduce material damage, or be used for thin and small test materials. At this point, Vickers hardness is better than Brinell hardness.
The principle of Vickers hardness test is basically the same as that of Brinell hardness test, but the difference is that diamond positive pyramid indenter is used as indenter. The angle between the two opposite sides of the regular pyramid is 136 ° and the bottom is square, as shown in Figure 5-3. The basic principle of Vickers hardness test is to press a diamond positive pyramid indenter with an included angle of 136 ° (148 ° 6’42 “) between two opposite sides into the surface of the sample under a certain test force, after maintaining a certain time, remove the test force, and measure the diagonal length of the indentation, as shown in Figure 5-4. The quotient of the test force divided by the conical surface area of the indentation is used to express Vickers hardness Value.

20200218061137 81781 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
Figure.5-3 Vickers diamond pyramid indenter

20200218061157 32672 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
Figure.5-4 Basic principle diagram of Vickers hardness test

Measuring method

In the Vickers hardness test, the diamond indenter with regular pyramid shape is used, and the angle between the two sides is 136 °. Because of its high hardness, diamond indenter can be used to press almost all materials, and the shape of the pyramid makes the indentation independent of the size of the indenter itself. Press the indenter into the surface of the tested material with a certain load (test force). After holding the load for a certain time, remove the load and measure the diagonal length of the square indentation on the surface of the material. Take the arithmetic mean of the length of the two perpendicular diagonals (L1 and L2).
20200218061348 65050 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel

Figure.5-5 Shows the projection of the indentation on the material surface on the left, and L1 and L2 show the diagonal length of the indentation; on the right is the side view of the diamond indenter, with an included angle of 136 °.

20200218061606 61924 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel 20200218061619 83211 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
 (a) (b)

Figure.5-6 Pyramid indentation during Vickers hardness measurement

The calculation formula of Vickers hardness value is:

20200218061938 34871 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel( 5-1 )

  • F = load (Newtonian force)
  • S = indentation surface area (mm2)
  • α = angle between the relative faces of the indenter = 136 °
  • D = average indentation diagonal length (mm)

Experimental operation steps

  • (1) According to the nature of the material, select the test force to test;
  • (2) According to the selected test force, select the appropriate standard block for calibration;
  • (3) Adjust the height of the sample so that the surface of the sample can be imaged clearly in the eyepiece;
  • (4) Rotate the eyepiece so that the pressure head is directly above the imaging;
  • (5) Press down the loading switch, and under the action of the test force, the diamond will load the sample surface;
  • (6) Unloading after a certain period of time;
  • (7) Turn the eyepiece to observe the clear diagonal, and read the diagonal length on the eyepiece;
  • (8) Check the table GB / t4340.1-1999 and read out the corresponding Vickers hardness value according to the test force selected by the average value of indentation diagonal L and a, and make records. The experimental results are shown in table 5-2 and figure 5-7.

Table 5-2 Vickers hardness of test plate before and after solution treatment

Test ordinal

Hardness of base metal(HV

Hardness of weld edge area(HV

Hardness of weld area(HV

Before heat treatment

After heat treatment

Before heat treatment

After heat treatment

Before heat treatment

After heat treatment

1

130

121

140

121

160

143

2

137

113

135

143

159

147

3

135

106

133

122

168

156

4

129

127

141

126

163

142

5

132

122

138

106

155

157

Fig.5-7 Change curve of Vickers hardness before and after solution treatment of test plate

20200218063151 13328 - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel

It can be seen from table 5-2 and figure 5-7 that the hardness of weld area and weld edge area of test plate after solution treatment at 1050 ℃ is significantly lower than that before solution treatment. The solution treatment makes all kinds of phases in the alloy fully dissolved, strengthens the solid solution, improves the toughness of the plate, effectively eliminates the welding stress of the plate, so the hardness of the material tends to decline. Through the analysis of the Vickers hardness value of each part of the sample plate measured in the test, it can be seen that the hardness value of the weld edge area is lower than that of the weld area. In theory, the mechanical properties of plates after welding should not be lower than the level before welding, so as to ensure the effectiveness of welding process. The observation data in the test prove this point well. The hardness value of the welding area is slightly higher than that of the weld edge area, which shows the validity of the welding process and the reliability of the welding process of the test plate. Er2209 stainless steel electrode can well meet the welding requirements of 2205 duplex stainless steel, and the experimental welding process can be effectively applied in the field of industrial welding.

Experimental environment requirements

  • (1) Generally, the experiment is carried out at room temperature of 10 ℃ ~ 35 ℃, and the laboratory with strict temperature requirements is 23 ℃;
  • (2) Install the Vickers hardness tester horizontally on a stable basis;
  • (3) There is no vibration and corrosive medium in the whole environment, and the relative humidity is less than 65%.

Conclusion

In this paper, taking SAF2205 duplex stainless steel as an example, the welding properties, joint mechanical properties, heat treatment process, microstructure and hardness of duplex stainless steel and SAF2205 duplex stainless steel plate are analyzed and tested in detail. It can be seen from the experimental study that the welding process of duplex stainless steel plate is very important, and good welding process is often the key factor to ensure the mechanical properties of the joint. Before and after sample welding, the welding requirements of duplex stainless steel must be strictly followed, and clear welding process specification shall be prepared before welding.
The change of hardness before and after heat treatment is related to the change of metallography during solution and artificial aging. In the aging process, the transformation of ferrite is δ → γ + σ. When aging temperature is high, σ phase nucleates and grows in δ / γ phase boundary and δ phase at the same time; when aging temperature is low, σ phase nucleates in δ / γ phase boundary and then grows in δ phase. The morphology of σ phase will vary with the temperature. When the temperature is high, the σ phase is easy to form massive structure, and the hardness of the material increases; when the temperature is low, the σ phase is easy to form lamellar structure, and the hardness of the material decreases. When the temperature is low, the diffusion coefficients of Cr and Mo decrease, but because the diffusion coefficient of the grain boundary is higher than that in the crystal, and the ratio of Cr and Mo at the δ / γ phase interface is conducive to the formation of σ phase, so it is mainly the precipitation of phase boundary and the formation of lamellar structure.
Compared with the solution treatment at 1050 ℃, the strength of the material after aging treatment is obviously improved, but the toughness is reduced. When the aging time is fixed, the strength of the material decreases with the increase of the aging temperature, and the elongation (δ) and the area shrinkage (ψ) also decrease; when the aging temperature is fixed, the strength of the material increases obviously with the increase of the aging time, but the plasticity decreases obviously. It shows that the aging treatment has a certain strengthening effect on the material. At the same time, after solution treatment at 1050 ℃, the hardness of the sample tends to decrease. It shows that the solution treatment process can eliminate the influence of thermal stress during welding, strengthen the solution and improve the toughness of the plate. These provide good guidance for the application of duplex stainless steel in different fields of industrial production.
In the process of the test, through the research and analysis of the sample plates, it is found that some of the sample plates also have the following problems: hot cracks and cold cracks, stress corrosion, pitting corrosion in welding, and δ phase embrittlement after solution heat treatment. The above welding and heat treatment defects can be explained by combining the experimental data and metallographic pictures.

  • (1) The reason of welding crack is that the content of Si, C, P and other elements in welding base metal is too high. Due to the existence of these impurities, the ability of weld metal to resist crack at the later stage of crystallization decreases. At the same time, under the tensile stress of thermal deformation, the weld metal cracks at the later stage of crystallization. These cracks can be eliminated by different heat treatment after welding.
  • (2) The occurrence of stress corrosion and pitting corrosion is mainly related to the selection of electrode. In stainless steel welding, it is generally required that the electrode is close to the base metal element, but there is always a certain difference between the electrode and the base metal, so there will be a certain degree of stress corrosion and pitting at the weld edge due to the new distribution of different elements. Reasonable selection of electrode and the consistency between electrode and base metal elements can reduce the occurrence of corrosion.
  • (3) The embrittlement of δ phase should also be avoided in industrial application, which is often the primary factor that endangers the safety of duplex stainless steel. When the duplex stainless steel is heated at 400-850 ℃ for a long time, it will form a Fe Cr intermetallic compound, which is a brittle and hard phase, which will reduce the strength of stainless steel. The embrittlement of δ phase is mainly related to the Cr content in the base metal. When the Cr content is high, it is easy to produce. However, the Cr content of duplex stainless steel is generally high, so it is easy to embrittle in the temperature range of 400-850 ℃. In order to reduce the embrittlement of δ phase, we should try our best to avoid heat treatment in the embrittlement temperature range or shorten the heat treatment time in the temperature range.

Compared with austenitic stainless steel, duplex stainless steel has an absolute advantage in resistance to intergranular corrosion, pitting corrosion, interstitial corrosion, especially stress corrosion cracking caused by chloride, and has good mechanical properties and wide application prospects. It can be seen from many researches at home and abroad that duplex stainless steel is widely used in many fields of industrial production. With the continuous growth of the application of duplex stainless steel, the production process, welding technology, heat treatment and other process technologies of duplex stainless steel are also in continuous development. In the development trend of duplex stainless steel, the development of low alloy and manganese containing duplex stainless steel is worthy of attention. In recent ten years, some countries, such as the United States and South Africa, have studied the development of dual phase stainless steel with manganese instead of nickel. In recent years, the low manganese and low nickel dual phase stainless steel developed in Sweden is relatively mature, with clear objectives. In order to save nickel and replace 304, which is widely used, it can even replace the price of 304, but use 2304 dual phase stainless steel, which is not widely used, has the value of practical promotion. With the development and application of duplex stainless steel and the development of new duplex stainless steel, we believe that the value of duplex stainless steel will become more and more prominent, which will also provide a strong guarantee for industrial production.

Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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welding heat treatment and metallographic analysis of 2205 duplex stainless steel - Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
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Welding, heat treatment and metallographic analysis of 2205 duplex stainless steel
Description
Generally, the welding methods for austenitic stainless steel, such as manual arc welding, tungsten inert gas arc welding and gas metal arc welding, can be used for the welding of duplex stainless steel.
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