What is precipitation hardening stainless steel
What is precipitation hardening stainless steel?
Precipitation hardening stainless steel refers to a kind of high-strength stainless steel that adds different types and quantities of strengthening elements on the basis of the chemical composition of stainless steel, and precipitates different types and quantities of carbides, nitrides, carbonitrides and intermetallic compounds through the precipitation hardening process, so as to improve the strength of steel and maintain sufficient toughness, PH steel for short.
Classification of precipitation hardening stainless steel
The steel is divided into four categories according to the content of main alloying elements and hardening elements added:
- (1) Martensitic precipitation hardening stainless steel, carbon content is generally 0.1%. Strengthen by adding hardening elements (copper, aluminum, titanium and aluminum, etc.) to make up for insufficient strength. The content of chromium is generally higher than 17%, and an appropriate amount of nickel is added to improve the corrosion resistance;
- (2) Maraging stainless steel, with carbon content not higher than 0.03% to ensure the toughness, corrosion resistance, weldability and machinability of martensitic matrix, and with chromium content not less than 12% to ensure corrosion resistance. In addition, the alloying element cobalt is added to further improve the heat treatment effect of steel;
- (3) Semi austenitic type, i.e. transitional precipitation hardening stainless steel, with chromium content of no less than 12%. With low carbon content and aluminum as its main precipitation hardening element, this type of steel has better comprehensive properties than martensitic precipitation hardening stainless steel;
- (4) Austenitic precipitation hardening stainless steel is a stainless steel with stable austenitic structure in both quenched and Aged States. It contains high nickel (more than 25%) and manganese, and chromium is more than 13% to ensure good corrosion resistance and oxidation resistance. Titanium, aluminum, vanadium or phosphorus are usually added as precipitation hardening elements, and trace boron, vanadium, nitrogen and other elements are added to obtain excellent comprehensive properties.
Precipitation hardening (precipitation strengthening)
It refers to a heat treatment process in which the segregation region of solute atoms in supersaturated solid solution and/or desolved particles are dispersed in the matrix, resulting in hardening. For example, austenitic precipitated stainless steel can obtain high strength by precipitation hardening at 400 ~ 500 ℃ or 700 ~ 800 ℃ after solution treatment or cold working. That is, when the supersaturated solid solution of some alloys is placed at room temperature or heated to a certain temperature, the solute atoms will aggregate or form a second phase in a certain area of the solid solution lattice, resulting in the increase of the hardness of the alloy.
The matrix structure of precipitation hardening stainless steel can be martensite or austenite, depending on the composition and treatment process. Age hardening martensitic stainless steel has both good corrosion resistance and simple heat treatment. The most widely used age hardening martensitic stainless steel is 17-4PH (0Cr17Ni4Cu4Nb). Age hardening martensitic stainless steel is sensitive to chloride stress corrosion cracking. Sensitivity depends to some extent on the grade of steel and aging temperature.
17-4PH alloy is a stainless steel with precipitation, quenching and martensite. This grade has the characteristics of high strength, hardness (up to 3000 ℃/5000 ℃) and corrosion resistance. After heat treatment, the mechanical properties of the product are more perfect, and the compressive strength can reach 1100-1300Mpa (160-190 Ksi). This grade can not be used at temperatures higher than 3000 ℃ or very low. It has good corrosion resistance to atmosphere and diluted acid or salt. Its corrosion resistance is the same as that of 304 and 430.
Characteristics of precipitation hardening stainless steel
Precipitation hardening stainless steel is a kind of Fe Cr Ni alloy containing precipitation hardening elements (Cu, Al, Ti, Nb), which can be strengthened by heat treatment. This kind of steel has high strength, sufficient toughness and suitable corrosion resistance. It is mainly used in aerospace industry and some high-tech industries.
Precipitation hardening stainless steel can be divided into Martensitic, SEMI austenitic and austenitic precipitation hardening stainless steel. Ferritic precipitation hardening stainless steel has appeared in recent years. In martensitic precipitation stainless steel, based on the experience of maraging steel, maraging stainless steel has appeared in recent years. Compared with the former, the latter has lower carbon content, its strengthening means mainly depends on intermetallic compound precipitation, and the heat treatment is simple.
The characteristics of precipitation hardening stainless steel are as follows:
- ① The heat treatment process of precipitation hardening stainless steel is complex. The ideal performance can be obtained only by strictly implementing the regulations and strictly controlling the parameters such as temperature, time and cooling rate.
- ② Precipitation hardening phase in precipitation hardening stainless steel after aging treatment, the main precipitation hardening phase is face centered cubic structure copper rich phase. The type and existence of aging phase are related to alloy composition and heat treatment conditions.
The microstructure of precipitation hardening stainless steel is characterized by ferrite with austenite and volume fraction of 5% ~ 20% in solution or annealing state. After a series of heat treatment or mechanical deformation treatment, austenite is transformed into martensite, and then the required high strength is achieved by age precipitation hardening. This kind of steel has good formability and weldability. It can be used as ultra-high strength material in nuclear industry, aviation and aerospace industry.
Application and welding of precipitation hardening stainless steel
Precipitation hardening (PH) stainless steel is a kind of corrosion-resistant alloy. Some models can be heat treated and can provide 850Mpa-1700Mpa tensile strength and 520mpa-1500mpa yield strength, which is about three to four times that of 304 or 316 austenitic stainless steel.
This precipitation hardening stainless steel can be used in oil and gas, nuclear industry and aerospace industry to meet the requirements of high strength, high corrosion resistance and toughness materials in this industry. Precipitation hardening stainless steel can be achieved by adding copper, molybdenum, aluminum and titanium alone or in combination.
The most common 17/4PH martensitic stainless steel will be transformed into martensite at low temperature (usually about 250 ℃) and further strengthened during aging at 480-620 ℃. Austenitic martensitic precipitation hardening stainless steel is basically completely austenitic after solution treatment. It needs a second heating cycle to 750 ℃/2h, and then cooled to room temperature to form martensite. Some of these alloys require cooling for 8 hours (- 50/- 60 ℃) to ensure complete transformation to a stable austenite/martensite structure, although the two most commonly used alloys FV520 and 17/7PH do not require refrigeration to develop optimum performance.
The aging of these alloys occurs at a temperature of 500-600 ° C. The grade of austenitic stainless steel is stable at room temperature, and the increase of strength is due to the precipitates formed by aging at 650-750 ℃. These fully austenitic stainless steels can show good toughness, and some can be used at low temperatures.
In order to obtain the best weldability, it is recommended that all three types of precipitation hardening stainless steel be annealed or solution treated first. Precipitation hardening stainless steel in the form of sheet or strip can be cold worked, but its weldability will be seriously damaged. Like many precipitation hardening alloys, it is a problem to obtain mechanical properties matching with the base metal in welding and heat affected zone. Even if matching welding materials are used, the maximum strength of fully solution treated and age hardened joints in semi austenitic and austenitic alloys may be only about 90% of the base metal.
During aging heat treatment, 17/4PH precipitation hardening stainless steel may sometimes fail along the fusion line due to thermal cracks. In this case, the element should be welded in the over aged state, followed by solution heat treatment, followed by post weld heat treatment.
For example, austenitic filler metals such as 308L or the use of dual phase filler metals such as 2205 for higher weld metal strength can be used to tolerate lower strength joints or cracks due to high constraints. If dual phase filler metal is used or recommended for austenitic weld metal due to embrittlement, PWHT shall not be performed.
Due to the low carbon content, the martensitic phase in precipitation hardening stainless steel will be softer, so preheating is usually not required, although preheating of about 100 ℃ has been found to be used to reduce cracking for thicker (more than 25mm) highly constrained joints. Therefore, the transformation temperature of these stainless steels into martensite is low, and the maximum interlayer temperature of 200 ℃ is recommended.
Maintaining a very high interpass temperature will lead to the transformation of the entire weld to martensite when cooled to room temperature, and the volume change when this occurs may lead to the formation of quenching cracks. It has been found that the stress rise effect of fillet welds and notches partially penetrating butt welds leads to cracking. If the reduction in strength is acceptable, the TP308l root channel can be used to solve this problem. It is also found that HAZ hot cracks will be formed during welding of 17/4PH precipitation hardening stainless steel castings. For cast articles, the copper content is limited to a maximum of 3%.
Postweld heat treatment usually includes a temperature of 750 ℃ and cooling to room temperature to ensure that the metallography of stainless steel is 100% martensite, and then aging at 550 ℃. According to the alloy composition and aging heat treatment temperature, this should reach UTS of 900-1000mpa, yield strength of 800-900mpa and ductility of about 15%.
Semi austenitic stainless steel is usually supplied in solution treated condition. This means that the stainless steel is completely austenitic and usually does not need preheating, although preheating of about 100 ℃ is found to be helpful for the welding of thick and highly limited joints. In fact, all common arc welding processes can be used, but TIG (GTAW) will provide the best performance.
For precipitation hardening stainless steel containing aluminum, such as 17/7PH, MMA and submerged arc welding should be avoided, because the proportion of aluminum loss in the welding process will be relatively high, so it is necessary to apply inert gas protection process. The molten pool has less fluid than non aluminum alloy. The matching composition filler metal for FV520 is easy to obtain, but it is difficult to obtain 17/7PH precipitation hardening stainless steel vulnerable parts. Therefore, the base metal cut from the strip is often used for TIG welding. Alternatively, 17/4PH or FV520 filler can be used. If 17/4PH filler is used, preheating at 100 ° C is recommended. Post weld heat treatment is similar to martensitic stainless steel, but without complete solution heat treatment and matching filler metal, it is impossible to achieve the strength matching with the base metal.
It is recommended that fully austenitic precipitation hardened stainless steel be welded in solution treatment and quenched with water or oil at about 980 ° C. The aging process is very slow, and it takes 15 hours to reach the maximum strength at 720 ° C, which means that HAZ has almost no change with the base metal. Therefore, the best strength can be produced during post weld aging treatment. Like austenitic stainless steel, these steels are insensitive to cold cracking and do not need preheating. However, because they are completely Austenitic, they are very sensitive to hot cracks. This makes the welding of thick wall part a problem, and requires that the welding conditions be strictly controlled, low heat input, and the temperature of small weld bead and intermediate layer be controlled below 150 ℃.
At present, aerospace alloys equivalent to a286 have been produced, such as AMS5858, which has improved weldability. 17/10P grade is particularly sensitive and cannot be welded with matching filler; Although thermal cracks in the heat affected zone may still occur, the 312 (29Cr/9Ni) filler still has the best chance of success.
Since aluminum or titanium is present in many precipitation hardened stainless steels, inert gas shielded arc welding should be used. Some matching component filler metals are available, and in aerospace grades, such as AMS 5804, these can be aged to provide strength close to the parent metal. Alternatively, Austenitic, duplex stainless steel or nickel based weld filler metals may be used.
Heat treatment of precipitation hardening stainless steel
Precipitation hardening stainless steel developed relatively late. It is a kind of stainless steel that has been tested, summarized and innovated in human practice. Among the early stainless steels, ferritic stainless steel and austenitic stainless steel have good corrosion resistance, but their mechanical properties can not be adjusted by heat treatment, which limits their role. Martensitic stainless steel can use heat treatment method to adjust the mechanical properties in a large range, but the corrosion resistance is poor.
It has low C content (generally ≤ 0.09%), high Cr content (generally ≥ 14%), and Mo, Cu and other elements, which makes it have high corrosion resistance, even equivalent to austenitic stainless steel. The precipitation hardening phase precipitated on the martensite matrix can be obtained through solid solution and aging treatment, so it has high strength, and the strength, plasticity and toughness can be adjusted within a certain range according to the adjustment of aging temperature. In addition, the heat treatment method of first solid solution and then precipitation strengthening according to the precipitate phase can be basically formed under the condition of low hardness after solid solution treatment, and then aging strengthening, which reduces the processing cost and is better than martensitic steel.
① Martensitic precipitation hardening stainless steel and its heat treatment
Martensitic precipitation hardening stainless steel is characterized in that the starting temperature ms of austenite to martensite transformation is above room temperature. After heating and austenitizing and cooling at a faster rate, a plate-like martensite matrix is obtained. After aging, fine particles of Cu precipitate from the lath martensite matrix to strengthen.
The heating temperature is 1020-1060 ℃. After heat preservation, it is water-cooled or oil-cooled. The structure is plate martensite with a hardness of about 320hb. The heating temperature should not be too high. If it is greater than 1100 ℃, the amount of ferrite will increase, the MS point will decrease, the residual austenite will increase, the hardness will decrease, and the heat treatment effect will be poor.
According to different aging temperature, the dispersion and particle size of precipitates are different, but they have different mechanical properties.
② Heat treatment of SEMI austenitic stainless steel
The MS point of this steel is generally slightly lower than the room temperature, so after the solid solution treatment is cooled to the room temperature, the austenitic structure is obtained with very low strength. In order to improve the strength and hardness of the matrix, it needs to be heated to 750-950 ℃ again for heat preservation. At this stage, carbides will precipitate in the austenite, the stability of austenite will be reduced, the MS point will be increased to above the room temperature, and the martensitic structure will be obtained when it is cooled again. Some can also add cold treatment (sub zero treatment), and then aging to make the steel finally obtain the strengthened steel with precipitates on the martensitic matrix.
Solution + adjustment + aging treatment
- The solid solution heating temperature is 1040 ℃, after heating and heat preservation, water cooling or oil cooling to obtain austenite, with a hardness of about 150HB;
- After cooling to about 290 ℃, the hardness of austenite can be reduced to about 90 ℃ and martensite can be obtained after cooling to about 90 ℃;
- After aging at 560 ℃, Al and compounds precipitate, and the steel is strengthened, and the hardness can reach about 340hb.
Solution + adjustment + cold treatment + aging
- Austenite structure was obtained by solution treatment at 1040 ℃ and water cooling;
- Adjust the treatment temperature to 955 ℃, increase the MS point, and obtain lath martensite after cooling;
- Cold treatment – 73 ℃ × 8h, reduce retained austenite in the structure and obtain maximum martensite;
- The aging treatment temperature is 510-560 ℃ to precipitate al. After strengthening treatment, the hardness can reach 336hb.
Solid solution + cold deformation + aging
- The solution treatment temperature was 1040 ℃, and the austenite structure was obtained by water cooling;
- Cold deformation, using the cold working deformation strengthening principle, austenite is transformed into martensite at MD point, and the cold working deformation is greater than 30-50%;
- Aging treatment: heat aging at about 490 ℃ to precipitate and harden al.
It is reported that the hardness of solid solution austenite reaches 430hb after 57% cold rolling deformation, σ B up to 1372 n/mm2, aging at 490 ℃, hardness up to 485HB, σ B up to 1850 n/mm2.
It can be seen that after proper treatment, the mechanical properties of precipitation hardening martensitic stainless steel can fully reach the properties of martensitic stainless steel, while the corrosion resistance is equivalent to that of austenitic stainless steel. It should be pointed out that although martensitic stainless steel and precipitation hardening stainless steel can be strengthened by heat treatment, the strengthening mechanism is different. Due to the characteristics of precipitation hardening stainless steel, it has been paid attention to and widely used.
Source: China 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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