Heat treatment of stainless steel under JIS G 4303 standard
Stainless steel is Cr content greater than 10.5%, C content of less than 1.2% of the alloy steel with high corrosion resistance. According to the steel’s room temperature organization, stainless steel is divided into five categories: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, austenitic-ferritic stainless steel, and precipitation-hardening stainless steel. The following 5 categories of stainless steel heat treatment according to the Japanese JIS G 4303 is introduced.
Heat treatment of austenitic stainless steel
Solution heat treatment
Solution heat treatment is the elimination of martensite in steel and steel strain, the solid solution of carbide, so that the steel organization becomes a single-phase austenite organization, improve the corrosion resistance and processability of steel heat treatment method. Table 1 is the JIS G 4303 heat treatment conditions for austenitic stainless steel. Heat treatment temperature is roughly divided into 1010-1150 ℃, greater than 900 ℃, 1030-1180 ℃ three categories.
Table.1 Heat treatment of austenitic stainless steel
|Grade||Solution heat treatment||Grade||Solution heat treatment|
|SUS201||1010-1120 ℃, quench||SUS316L||1010-1150 ℃, quench|
|SUS301||1010-1150 ℃, quench||SUS316LN|
|SUS302||SUS316Ti .||920 – 1150 ℃, quench|
|SUS303||SUS316J1||1010-1150 ℃, quench|
|SUS304N2||SUS317J1.||1030-1180 ℃, quench|
|SUS305||SUS321||920-1150 ℃, quench|
|SUS309S||1030 – 1150 ℃, quench||SUS347||980-1150 ℃, quench|
|SUS310S||1030 – 1180 ℃, quench||SUSXM7||1010-1150 ℃, quench|
|SUS316||1010-1150 ℃, quench|
Austenitic carbides are mainly Cr carbides, which need to be decomposed by solution heat treatment. The higher the temperature, the greater the amount of solid solution C in the steel, the easier the carbide decomposition. Therefore, the higher the C content of the steel, the more you need to increase the temperature of the solution heat treatment. In addition, Ti, Nb and C have a strong affinity for the formation of stable carbides with C. Therefore, the amount of solid solution C of steel containing Ti and Nb is much smaller than the C content of steel. Therefore, the addition of stabilizing elements Ti, Nb SUS316I, SUS321, SUS347 and other grades of austenitic stainless steel, can be solid solution heat treatment at about 900 ℃. On the other hand, the amount of solid solution C containing more Cr, Ni steel is low and requires a higher temperature solution heat treatment. So SUS310S, SUS321L, SUS836L and other grades of austenitic stainless steel to high temperature solution heat treatment. All grades of austenitic stainless steel solution heat treatment after the acute cooling. If the cooling rate is small in the temperature range of 550-800 ℃, a large number of Cr carbide precipitation in the grain boundary, Cr carbide around the formation of Cr concentration is very small poor Cr layer, poor Cr layer will become the starting point of corrosion, so that the steel corrosion, resulting in a significant decrease in corrosion resistance of steel. Therefore, in austenitic stainless steel solution heat treatment, when the C solid solution to be sharply cooled, not to make the Cr carbide precipitation.
Stabilization heat treatment
Add Ti, Nb SUS316I, SUS321, SUS347 and other grades of austenitic stainless steel, sometimes need to be stabilized heat treatment. Stabilization heat treatment is to make the affinity with C than Cr Ti, Nb and solid solution C reaction, the generation of precipitates, reduce the amount of solid solution C, to prevent Cr carbide grain boundary precipitation, thereby improving the resistance to grain boundary corrosion heat treatment method. The stabilization heat treatment temperature is in the temperature range of 850-930°C where Cr carbides are difficult to precipitate and TiC and NbC can precipitate sufficiently. Since the amount of solid solution C has been reduced, the cooling after the stabilization heat treatment does not require rapid cooling.
Heat treatment of austenitic-ferritic stainless steel
Austenitic-ferritic stainless steel in the ratio of austenitic phase and ferritic phase (phase ratio) affects the corrosion resistance and mechanical properties of the steel. The phase ratio varies depending on the heat treatment temperature. Therefore, the solid solution treatment of austenitic-ferritic stainless steel should not only eliminate the precipitates and strains generated by the previous process, but also obtain the appropriate phase ratio. Table 2 is JIS G 4303 heat treatment conditions of austenitic-ferritic stainless steel. In order to prevent the precipitation of σ-phase during the cooling process after solution treatment, reduce the corrosion resistance and mechanical properties of steel, austenitic-ferritic stainless steel solution treatment should be rapid cooling.
Table.2 Austenitic-ferritic stainless steel heat treatment
|Grade||Solution heat treatment|
|SUS329J1||950-1100 ℃, quench|
Heat treatment of ferritic stainless steel
The purpose of annealing ferritic stainless steel is to eliminate the stress and strain generated by the previous process, so that the steel has good corrosion resistance and processability. Table 3 is the JIS G 4303 heat treatment conditions for ferritic stainless steel.
Table.3 Heat treatment of ferritic stainless steel
|SUS405||780-830 ℃, air cooling or slow cooling|
|SUS410L||720-820 ℃, air cooling or slow cooling|
|SUS430||780-850 ℃, air cooling or slow cooling|
|SUS43OF||680-820 ℃, air cooling or slow cooling|
|SUS434||780-850 ℃, air cooling or slow cooling|
|SUS447J1||900-1050 ℃, quench|
The heat treatment temperature of ferritic stainless steel has two categories. One category is SUS430 and other steel heat treatment temperature limit is less than 850 ℃. The other category is SUS447J1, SUSXM27 and other steels heat treatment temperature of 900-1050 ℃.
SUS430 and other steels generate austenitic phase at high temperatures, so heat treatment is performed in the ferrite region. Since the solid solution of C is small in the heat treatment temperature region, the steel is organized as a ferritic phase containing a large amount of Cr carbides. Therefore ferritic stainless steel can be slow cooled after annealing. In the slow cooling process, even if there is carbide precipitation, the formation of a Cr-poor layer, due to the rapid diffusion of Cr in the ferrite, the recovery phenomenon (the disappearance of the Cr-poor layer phenomenon) will occur, so the corrosion resistance of the steel will not decline.
SUS447J1, SUSXM27 and other steels are steels that do not form austenite at high temperatures. Since the Cr content in the steel is large, the recrystallization temperature is high, so the annealing temperature is also increased. However, the upper temperature limit is set in order to prevent coarsening of grains.
The annealing cooling of both of the above-mentioned steel grades is rapid to prevent embrittlement and reduced corrosion resistance due to σ-phase precipitation and 475°C embrittlement. High annealing treatment temperature, TiC and NbC in the steel begins to decompose, causing grain coarsening and an increase in the amount of solid solution C, leading to embrittlement and corrosion resistance of the steel. So SUS447J1, SUSXM27 and other steel annealing temperature is specified as the upper limit of 1050 ℃. As TiC, NbC precipitation, the amount of solid solution C in the steel is very small, so the annealing can be sharply cold.
Heat treatment of martensitic stainless steel
The purpose of annealing heat treatment of martensitic stainless steel is to eliminate the strain and organizational heterogeneity generated by the previous process, softening the steel, making it suitable for cold working and machining. Table 4 is JIS G 4303 heat treatment conditions for martensitic stainless steel.
Table.4 Heat treatment of martensitic stainless steel
|SUS403||800-900 ℃, slow cooling. Or about 750 ℃, quench||950 – 1000 ℃, oil cooled||700-750 ℃, quench|
|SUS410J1||830-900 ℃, slow cooling. Or about 750 ℃, quench||950 – 1020 ℃, oil cooled||650-750 ℃, quench|
|SUS410F2||800-900 ℃, slow cooling. Or about 7509c, quench||950-1000 ℃, oil cooled||700-750 ℃, quench|
|SUS420J1||920 – 980 ℃, oil cooled||600-750 ℃, quench|
|SUS420F2||630-700 ℃, quench|
Once about 750 ℃, quench
Once about 650 ℃, quench
|1000-050 ℃, oil cooled|
|SUS440A||800-920 ℃, slow cooling||1100-070 ℃, oil cooled||100-180 ℃, quench|
From SUS430 to SUS420F2 a total of two annealing conditions. One is complete annealing, one is low-temperature annealing. Complete annealing is the steel is heated to austenite transformation temperature (A1) above 50-100 ℃ high temperature zone to make steel austenite, reduce solidification segregation by diffusion, so that the organization of uniform heat treatment method. Low-temperature annealing is to heat the steel to the A1 point below the ferrite carbide region, so that the steel recrystallization, the heat treatment method to return to processability. Low-temperature annealing is suitable for softening treatment in cold working. Complete annealing has no effect on SUS431 containing Ni. Quenching is followed by annealing below two levels of phase change point.
Quenching of martensitic stainless steel is a heat treatment method that heats and austenitizes the steel to undergo martensitic transformation and obtain the required high strength.
In the quenching of martensitic stainless steel, the steel is heated to above the beginning of the phase transformation temperature to austenitize the steel and homogenize the steel composition. Then oil cooling is performed and martensitic transformation occurs. The quenching and holding time of martensitic stainless steel should be long enough. This is because the steel is heated in the process of decomposition of a large number of Cr carbides, diffusion of slow Cr to move, so that the steel composition homogenization takes a longer time. The higher the quenching temperature of martensitic stainless steel, the more carbide decomposition, the more solid solution C in austenite, the greater the hardness of the steel after quenching. However, the quenching temperature is too high, the amount of solid solution C in austenite is too much, which will cause the martensite transformation temperature to begin to decline, the result is an increase in the amount of residual austenite, hardness decline. Residual austenite is unstable tissue, at room temperature will gradually transform into martensite, resulting in the generation of placement cracks and a decrease in toughness. Therefore, in order to reduce the amount of residual austenite, cold treatment is sometimes necessary. The general temperature of cold treatment is -73°C.
Quenched state of martensitic stainless steel is hard, but very brittle, so to tempering treatment, sacrificing some hardness to obtain the toughness required for use. Martensitic stainless steel low-temperature tempering is mainly 100-250 ℃ tempering, through low-temperature tempering to relieve the internal stress in the steel, reduce brittleness. Low temperature tempering of martensitic stainless steel is suitable for items such as knives that emphasize hardness.
The high temperature tempering of martensitic stainless steel is mainly 600-750°C tempering, which is suitable for structural parts that emphasize toughness. When martensitic stainless steel is tempered at an intermediate temperature of 400-500°C, the toughness and corrosion resistance of the steel decreases. Therefore, in principle, the tempering is not carried out in the temperature range.
Heat treatment of precipitation-hardening stainless steel
Dissolution-hardening stainless steel is solid solution treatment after precipitation hardening treatment, so that the microscopic second phase in the matrix uniform diffusion precipitation, to obtain the required hardness of stainless steel. Table 5 is the JIS G 4303 heat treatment conditions for precipitation-hardening stainless steel.
Table.5 heat treatment of precipitation-hardening stainless steel
|Type||Symbol||Heat treatment conditions|
|SUS630||Solution treatment||S||1020-1060 ℃, quench.|
|Precipitation hardening treatment||H900||470 – 490 ℃, air cooling.|
|H1025||540 – 560 ℃, air cooling.|
|H1075||570 – 590 ℃, air cooling.|
|H1150||610-630 ℃, air cooling.|
|SUS631||Solution treatment||S||1000-1100 ℃, quench.|
|Precipitation hardening treatment||R H950||955 ± 109 ℃ for 10min, air cooled to room temperature, cooled to – 73 + 6 ℃ within 24h and kept for 8h, then kept at 510 ± 10 ℃ for 60min and then air cooled.|
|TH1050||760 ± 15 ° C for 90min, cool it to below 15C within 1h and keep it warm for 30min, then keep it at 565 soil temperature for 90min and then air cool it.|
SUS630 steel by solid solution treatment to form a low C soft martensite, through the precipitation hardening treatment, the steel precipitation of Cu-rich phase, improve the hardness of the steel. The lowest treatment temperature, H900 precipitation hardening treatment, allows the steel to obtain maximum strength. The treatment temperature increases, the strength of the steel decreases and the toughness becomes better. SUS631 steel is solid solution treated to generate sub-stable austenite, and the precipitation hardening treatment adjusts the Ms point of austenite to martensitize the steel and precipitates Ni-Al intermetallic compounds at the final treatment stage of 510°C or 565°C to harden the steel.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.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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