Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

At present, most of the process pipes in contact with seawater are nuclear grade z2cn1810 stainless steel or rubber lined nuclear grade carbon steel pipes. This kind of materials have weak corrosion resistance to seawater, which is prone to corrosion and leakage of pipes. The maintenance and replacement work during service is also very complex. The traditional way to solve this problem is to add cathodic protection or anti-corrosion coating, but either the cathodic protection system is too complex, the reliability is not high, or the coating falls off. Duplex stainless steel material is the most effective seawater corrosion-resistant material so far. If it is applied in the above system, it can avoid the above problems, improve the safety, reduce the auxiliary protection system and daily maintenance, and relatively reduce the cost.

Material characteristics of duplex stainless steel

The duplex stainless steel with two phases of austenite and ferrite at room temperature combines the good toughness and weldability of austenitic stainless steel with the chloride corrosion resistance of ferritic stainless steel.
Dual phase stainless steel has been developed for more than 80 years since it was applied to industrial production in 1930s. Three generations of dual phase stainless steel have been developed. The first generation is represented by s31500 containing Cr18%, which has good pitting resistance due to its chromium and molybdenum containing composition. Due to its ultra-low carbon content (C ≤ 0.03%), its welding performance has been improved greatly. The second generation is represented by S31803 containing Cr22% The third generation is represented by S32750. Because of its low carbon, high chromium, high molybdenum and high nitrogen, it has excellent pitting resistance (pitting resistance equivalent prEN > 40) and resistance to stress corrosion and crevice corrosion, also known as super duplex stainless steel.

The performance characteristics of duplex stainless steel are as follows:

  • 1) Molybdenum containing duplex stainless steel has good chloride stress corrosion resistance under low stress. Generally, 18-8 austenitic stainless steel is prone to stress corrosion cracking in neutral chloride solution above 60 ℃, while duplex stainless steel has good resistance.
  • 2) Duplex stainless steel containing molybdenum and nitrogen has good pitting resistance. The pitting resistance equivalent prEN (prEN = CR% + 3.3mo% + 16N%) is used to characterize the pitting resistance of materials. 2205 is more resistant to pitting and crevice corrosion than 316L because of its high content of chromium, molybdenum and nitrogen.
  • 3) It has high strength and fatigue strength, and the yield strength is twice that of 18-8 stainless steel.
  • 4) It has good corrosion fatigue and wear corrosion resistance.
  • 5) The dual phase stainless steel has good welding performance and little hot cracking tendency, so it can be welded with common 18-8 austenitic stainless steel or carbon steel.
  • 6) It has the tendency of precipitation of various brittle phases, such as 475 ℃ phase and σ equivalent brittle phase, so it is not suitable for use at temperatures higher than 300 ℃.

Chemical composition of 2205

2205 material has been put into commercial application since 1970s. After more than 40 years of application and improvement, especially the mature application of AOD and VOD and the development of continuous casting technology, the quality stability of 2205 material has been improved. It has been included in ASTM standard, seamless steel pipe standard astma789 and astma790, corresponding to China’s national standard GB / t21833, brand number is 022cr22ni5mo3n, detailed standard Composition requirements are shown in Table.1.

Table.1 Composition ranges for UNS S32205

Grade

 

C

Mn

Si

P

S

Cr

Mo

Ni

N

ASTM A789/790 UNS S31803

Min

Max

0.030

2.00

1.00

0.030

0.020

21.0

23.0

2.5

3.5

4.5

6.5

0.08

0.20

GB/T 21833 022Cr22Ni5Mo3N

Min

Max

0.030

2.00

1.00

0.030

0.020

22.0

23.0

3.0

3.5

4.5

6.5

0.08

0.20

Technical conditions 022cr22ni5mo3n

Min

Max

0.030

2.00

1.00

0.030

0.020

22.0

23.0

3.0

3.5

4.5

6.5

0.08

0.20

Because 2205 duplex steel seamless steel pipe is mainly used in pitting environment, stress corrosion environment and severe uniform corrosion conditions, especially in seawater corrosion resistant working environment, the pitting resistance equivalent prEN of the material must be considered. In order to ensure that the material has better pitting resistance in pitting environment, its calculation formula prEN =% Cr + 3.3% Mo + 16% n. N can improve the pitting resistance of austenite phase, achieve corrosion balance with ferrite phase rich in Cr and Mo, improve the overall pitting resistance of materials, and reduce selective corrosion.
At the same time, the balance of the two-phase ratio in the heat affected zone of the welding joint should be considered to avoid the increase of the ferrite phase ratio due to the high temperature in the heat affected zone of the welding. The composition of Ni and n should be controlled according to the upper limit to ensure that the high temperature ferrite reverses into enough secondary austenite phase in the heat affected zone of the welding to improve the corrosion resistance of the welding joint.
In order to control the smelting technology, 2205 material should be smelted in electric furnace and refined by AOD or VOD to ensure the cleanness of the molten steel. During the cooling and solidification process, the molten steel has the precipitation problems of CRC, CrN, χ phase, MC6, σ equivalent intermetallic compounds and harmful phases. Accelerating the cooling speed is beneficial to reducing the precipitation, so the continuous casting process is better than the mold casting process.
In order to ensure that 2205 seamless steel pipe has good corrosion resistance under severe corrosion conditions, inclusion control of tube blank shall be carried out. According to ASTM JK rating chart, inclusion of a, B, C and D shall not be greater than 1.5, and σ equivalent harmful phase shall not appear.

Characteristics of Physical properties

Table.2 lists the normal temperature density, specific heat reference values from 20 ℃ to 100 ℃, 200 ℃, 300 ℃ and 400 ℃, linear expansion coefficient and elastic modulus reference values, as well as the actual test linear expansion coefficient and elastic modulus of 2205 seamless steel pipe. Table 3 lists the mechanical properties of 2205 seamless steel pipe at room temperature and high temperature.

Density: 7.8 g/cm3, 0.28 lb/in3

Temperature, °C J/(kg °C) Temperature, °F Btu/(lb °F)
20 480 68 0.11
100 500 200 0.12
200 530 400 0.13
300 550 600 0.13
400 590 800 0.14

In the design of equipment and heat exchanger, attention should be paid to the linear expansion coefficient of materials. The linear expansion coefficient of 2205 duplex stainless steel is lower than that of ordinary austenitic stainless steel. It can reduce the stress caused by thermal expansion / contraction when the equipment and heat exchanger are raised / cooled, which is beneficial to improve the structural stability. At the same time, in the design of pipeline and exchanger tube side for the medium with corrosion characteristics and shell side for the general medium, the two-phase steel seamless steel pipe can be used, and the shell or tube plate of carbon steel or alloy steel can be used to reduce the manufacturing cost of the equipment. In the structural design, the expansion wave level pipe required when austenitic stainless steel pipe is not considered. Higher thermal conductivity than austenitic stainless steel can improve the heat rate of heat exchanger.

Characteristics of heat treatment 

In the heat treatment of 2205 material, the proportion of ferrite phase and austenite phase should be considered. Generally speaking, the processability, plasticity and corrosion resistance of 2205 material are the best when the proportion of austenite phase and ferrite phase is 50%. According to the actual chemical composition of the material, in the range of 1020-1100, select the suitable heat treatment system, and after rapid cooling, about 50% ferrite can be obtained Phase, the metallographic picture of 2205 tube after 1050 solid solution treatment is shown in Figure 1. As the temperature increases, the ferrite content will gradually increase. Therefore, the uniformity and stability of the temperature of the heat treatment furnace are very important to control the stability of the two-phase ratio. When the temperature of the heat treatment furnace is uneven or the temperature fluctuates greatly, the two-phase ratio of different parts of the same workpiece may be uneven and uneven The two-phase ratio of the same batch of materials fluctuates greatly, which may lead to selective corrosion of materials in service.

20200217082800 75298 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Fig.1 2205 steel pipe metallography

The chemical composition of 2205 material includes 21% ~ 23% Cr, 2.5% ~ 3.5% Mo and 0.14% ~ 0.20% n. Figure 2 shows TTC (temperature time precipitation) curve of 2205. It can be seen from the figure that in the range of 850-950, 2205 has produced intermetallic compounds and complicated precipitation problems, such as Cr2N, χ phase, M23C6, and σ equivalence, due to the high content of Mo and Cr elements and the action of C, N and other elements.

20200217083309 73658 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Fig.2 TTC (temperature time precipitation) curve of 2205

σ phase is one of the most harmful precipitates in duplex stainless steel. It is hard and brittle, which will significantly reduce the plasticity and toughness of the steel. Because of its rich chromium, it will appear chromium poor area around it, resulting in the degradation of corrosion resistance. The morphology and composition of σ phase under SEM are shown in Fig.3.

20200217084046 44755 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Fig.3 σ phase under SEM

The formation of the intermetallic compounds and the precipitation of the precipitated phase can reduce the corrosion resistance and toughness of the material, and produce “475 brittleness” transformation in the range of 350-525. Therefore, in the heat treatment of 2205, rapid cooling should be adopted to avoid staying in the range of 850-950 and 350-525 for a long time.
When local heat treatment is adopted, except that the part requiring heat treatment reaches the specified temperature of heat treatment, it may be in the σ phase precipitation sensitive area and “475 brittleness” transition area in the transition area without heat treatment. If the heating time is too long in 850-950, σ phase and other intermetallic compounds may appear, which will reduce the corrosion resistance of the material and generate selective corrosion in service. If the residence time is too long in the “475 brittleness” transition zone, the material may crack. The local heat treatment shall be carried out by means of rapid heating, heat preservation and rapid cooling.
For local heat treatment or overall equipment heat treatment, heat treatment process shall be evaluated, and mechanical properties, precipitates, phase ratio and corrosion performance shall be evaluated.

Technological process

According to the R&D plan, three specifications of 2205 duplex steel seamless pipe are proposed.

  1. Specification 1: 19.05 × 1.65mm round bar (Φ 65mm) → incoming inspection → tube blank processing → hot piercing (68 × 6mm) → cold rolling (33.4 × 3mm) → cold rolling (19 × 1.65mm) → heat treatment → pickling and passivation → inspection → warehousing
  2. Specification 2: 219.08 × 12.7mm round bar (Φ 210mm) → incoming inspection → tube blank processing → hot piercing (214 × 18mm) → cold expansion (246 × 17mm) * → cold drawing (219.08 × 12.7mm) * → heat treatment → pickling and passivation → inspection → warehousing
  3. Specification 3: 355.6 × 12.7mm round bar (Φ 310mm) → incoming inspection → tube blank processing → hot piercing (310 × 27mm) → cold expansion (409 × 22.5MM) * → cold drawing (355.6 × 12.7mm) * → heat treatment → pickling and passivation → inspection → warehousing

Note: * the intermediate cold drawing process is circulated many times, that is, cold expansion – degreasing – heat treatment – straightening – pickling.

Research on main production process

Hot piercing

Hot working properties of duplex steel

Due to its unique two-phase structure, duplex stainless steel has lower high temperature plasticity than single-phase austenitic stainless steel, and it is easy to crack in the process of hot working (especially hot piercing). Moreover, due to the high alloy content of duplex stainless steel, it is easy to form all kinds of precipitates, such as σ phase, χ phase, Cr2N and other precipitates. These precipitates are hard and brittle, which will reduce the plastic toughness of the steel.
The hot working performance of duplex stainless steel 2205 is studied. It is found that the ferrite phase in duplex steel recovers dynamically when hot working at high temperature, forming a sub crystal with flat grain boundary, while the austenite phase recrystallizes dynamically after large deformation, and the two phases soften and recover in their own way, making the two-phase boundary round passivated, reducing the stress concentration caused by deformation. However, when the deformation or strain rate of the material increases at low temperature, it is difficult to coordinate the deformation of ferrite and austenite, which presents a sharp area at the phase boundary, so that the stress concentration caused by the deformation cannot be softened, thus the crack source is generated. Therefore, the deformation temperature and strain rate are the main factors affecting the hot working of duplex steel.
We have carried out Gleeble test and Research on duplex steel 2205, and its stress-strain curve is shown in Figure 4. When the temperature deformation is over 1100, the stress-strain curve after the peak stress appears a steady-state platform, that is, the stress remains unchanged while the strain increases, and the specimen produces steady-state plastic deformation, which shows that the softening rate and hardening rate of the material reach a balance state during the thermal deformation. The longer the steady-state region is, the better the thermoplasticity of the material is. Therefore, 2205 duplex stainless steel has better thermoplasticity above 1100.
20200217084623 82581 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Fig.4 Stress strain curves of 2205 duplex steel at different temperatures

Hot piercing process test of duplex steel

The hot piercing performance of 2205 dual phase seamless steel tube is determined by the temperature of tube blank and the deformation rate of hot piercing. The temperature of tube blank is controlled by the tapping temperature of inclined bottom heating furnace in the process of cross rolling and piercing.
In the first step of this test, four different tapping temperatures are set: 1120, 1150, 1170, 1200, and other perforation conditions are fixed. The pipe blank used in the test is S31803, and its composition meets the requirements of ASTM A789 standard. The specification is Φ 63mm, the specification of perforated blank pipe is 68 × 6mm, the setting feed angle is 10 °, the roll distance is 56 ~ 57mm, the guide plate distance is 62 ~ 63mm, the head forward extension is 45 ~ 55mm, and the roll rotation is 62 ~ 63mm Speed 190rpm. A group of 3 branch billets is used to control the tapping temperature. The results are as shown in Table 4. When the tapping temperature is 1170, the inner and outer surface of the raw tube get good quality.
Table.4 Hot piercing test results at different temperatures

Test sequence number Heating temperature/℃ Surface quality of raw pipe
1 1120 Head internal crack (2 pieces), external thread (1 piece)
2 1150 External thread (1 piece), good (2 pieces)
3 1170 Good (3 pieces)
4 1200 Pits (3 pieces)

In the second step, in order to test the influence of the piercing deformation rate, the deformation rate is changed by changing the roll rolling speed based on the tapping temperature (1170) obtained in the first step. In this test, three rolling speeds of 190, 200 and 210 RPM are set, each speed is tested with 5 branch billets, and other conditions are the same as those in the first step. The results are shown in Table 5, the relatively low speed 190 and 200 rpm blank tube table Generally speaking, the surface quality of the raw tube is almost the same when the rolling speed is 190 ~ 210 rpm.

Table.5 Hot piercing test results at different rolling speeds

Test sequence number Rolling speed/rpm Surface quality of raw pipe
1 190 Good (5 pieces)
2 200 Good (5 pieces)
3 210 Good (4 pieces)Slight internal fissure of head (1 piece)

Through batch test, under the control of 1170 tapping temperature and 190rpm rolling speed, the surface quality of 2205 duplex steel pipe is better, and the qualified rate is 95%.

Heat treatment

Pitting resistance, ferrite ratio and mechanical properties are three important properties of 2205 duplex steel, which are greatly affected by heat treatment. Therefore, the research on heat treatment is an important project in the production of duplex steel.
The results show that the suitable heat treatment temperature of 2205 duplex steel is 1020 ~ 1100. The technical conditions of duplex steel for nuclear power also put forward the requirements of heat treatment and quench in 1020 ~ 1100 for 2205. On the basis of this range, the further experiment of 2205 duplex steel is carried out in this project. 1020, 1040, 1060, 1080, 1100 were set and kept for 15 minutes to ensure that the sample was fully heated and cooled quickly after being discharged. Point corrosion test, ferrite proportion test and mechanical property test were carried out on the samples after the experiment. The results are as follows:

Point corrosion inspection

The pitting test was carried out for the sample according to ASTM A923c, the test temperature was 25 ± 1, and the corrosion rate was detected after 24 hours corrosion in 6% ferric chloride solution. The results are shown in Figure 5. The pitting corrosion rate of 2205 is not high in the range of 1020-1100, which is less than 1 MDD, especially in the range of 1040-1060.

20200217092817 18211 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Fig.5 Corrosion rate of 2205 at different temperatures

Ferrite ratio

After the heat treatment experiment, polish the transverse surface of the sample and use 10% NaOH solution for electrolytic corrosion. Observe the metallographic structure with optical microscope, as shown in Fig. 6. Using image analysis software to calculate the ferrite ratio, the results are shown in Figure 7. The higher the temperature is, the higher the ferrite ratio is. In the experimental temperature range, the proportion of 2205 ferrite is in the range of 48% ~ 53%.
20200217093105 63833 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant
Fig.6 2205 metallographic structure at different temperatures

20200217093426 81674 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Fig.7 2205 ferrite ratio at different temperatures

Tensile test and hardness test at room temperature

After heat treatment test, tensile test and hardness test at room temperature are carried out for mechanical property samples, and the results are shown in Figure 8.

20200217095009 51072 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant

Figure.8 2205 mechanical properties at different heat treatment temperatures

High plasticity is undoubtedly needed. The intermediate heat treatment temperature can be set at 1080.
In conclusion, the best pitting resistance can be obtained when the heat treatment temperature of 2205 duplex steel is 1050. The ratio of ferrite is close to 50%, and the mechanical properties are good. The higher the heat treatment temperature of the intermediate product is 1080, the lower the strength hardness and the better the plasticity can be obtained, which is more conducive to the subsequent cold working.

Cold working

Avesta has studied the cold working characteristics of 2205 duplex steel, as shown in Figure 9. They believe that 2205 has good cold working performance and can use the same cold working deformation mode as ordinary stainless steel, such as cold rolling, cold drawing and cold expansion. However, its initial strength and hardness are higher than those of ordinary austenitic stainless steel, so it needs cold working equipment and mould to have higher strength. Generally, after more than 10% cold working deformation, it must undergo solution heat treatment to restore its plasticity.

20200217095418 72708 - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant
Figure.9 2205 cold working performance under different cold working variables

The following introduces several cold working methods and points for attention of 2205 duplex steel pipe.

Cold drawing

Due to the high strength and hardness of 2205, more attention should be paid to the quality of cold drawing die. In the process of cold drawing, the internal and external surface quality must be inspected one by one. If there is any galling, the die must be inspected and polished.
For 2205, the deformation of cold drawing should be strictly controlled. The deformation of Soviet cold drawing should not be more than 15%, the deformation of expanding cold drawing should not be more than 10%, and the deformation of Chinese cold drawing should not be more than 20%.

Cold rolling

Because 2205 has high strength and hardness and poor plasticity, the cold-rolled compression stress condition is more conducive to cold deformation. Therefore, the cold-rolled deformation of 2205 duplex steel is better when conditions permit.
In the process of cold rolling, it is necessary to control the smaller speed and feed rate, and the cold rolling deformation shall not be more than 70%. Otherwise, rolling crack will occur.

Source: China Duplex Stainless Steel Seamless Pipe 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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seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant - Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant
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Seawater corrosion resistant nuclear grade duplex stainless steel seamless pipe for nuclear power plant
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Duplex stainless steel material is the most effective seawater corrosion-resistant material so far.
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