1. Heat treatment of ferritic stainless steel: Ferritic stainless steel is generally a stable single ferrite structure. When heated or cooled, there is no phase change. Therefore, the mechanical properties cannot be adjusted by heat treatment. Its main purpose is to reduce brittleness and improve resistance to intergranular corrosion.
①σ phase brittleness: Ferritic stainless steel is very easy to generate σ phase, which is a Cr-rich metal compound. It is hard and brittle, and is particularly easy to form between grains, making the steel brittle and increasing the sensitivity to intergranular corrosion. The formation of the σ phase is related to the composition. In addition, Cr, Si, Mn, Mo, etc. all promote the formation of σ phase; it is also related to the processing process, especially heating and staying in the range of 540~815℃, which further promotes the formation of σ phase. However, the formation of the σ phase is reversible. Reheating to a temperature higher than the σ phase formation temperature will redissolve in the solid solution.
②475℃ brittleness: When ferritic stainless steel is heated for a long time in the range of 400~500℃, it will show the characteristics of increased strength and decreased toughness, that is, increased brittleness, which is most obvious at 475℃, which is called 475℃ brittleness. This is because, at this temperature, the Cr atoms in the ferrite will rearrange to form a small Cr-rich area, which is coherent with the parent phase, causing lattice distortion, generating internal stress, increasing the hardness of the steel, and increasing its brittleness. At the same time as the Cr-rich area is formed, there must be a Cr-poor area, which has an adverse effect on corrosion resistance. When the steel is reheated to a temperature higher than 700℃, the distortion and internal stress will be eliminated, and the 475℃ brittleness will disappear.
③High temperature brittleness: When heated to above 925℃ and cooled down rapidly, Cr, C, N, etc. form compounds that precipitate in the grains and grain boundaries, causing increased brittleness and the occurrence of intergranular corrosion. This compound can be eliminated by heating at 750~850℃ and then cooling rapidly.
Heat treatment process:
① Annealing: In order to eliminate the σ phase, 475℃ brittleness, and high-temperature brittleness, annealing can be used, heating at 780~830℃, keeping warm, and then air cooling or furnace cooling. For ultra-pure ferritic stainless steel (containing C≤0.01%, strictly controlling Si, Mn, S, P), the annealing heating temperature can be increased.
② Stress relief treatment: After welding and cold processing, parts may produce stress. If annealing is not suitable for specific circumstances, heating, keeping warm, and air cooling can be used in the range of 230~370℃ to eliminate some internal stress and improve plasticity.
2. Heat treatment of austenitic stainless steel: The effect of alloying elements such as Cr and Ni in austenitic stainless steel causes the Ms point to drop below room temperature (-30 to -70℃). To ensure the stability of the austenitic structure, no phase change occurs above room temperature during heating and cooling. Therefore, the main purpose of heat treatment of austenitic stainless steel is not to change the mechanical properties, but to improve corrosion resistance.
A. Solution treatment of austenitic stainless steel
Function:
① Precipitation and dissolution of alloy carbides in steel: C in steel is one of the alloying elements. In addition to playing a certain strengthening role, it is not conducive to corrosion resistance, especially when C forms carbides with Cr, the effect is even worse, and efforts should be made to reduce its presence. For this reason, according to the characteristics of C in austenite that changes with temperature, that is, the solubility is large at high temperature and small at low temperature. According to data, the solubility of C in austenite is 0.34% at 1200℃, 0.18% at 1000℃, and 0.02% at 600℃, and even less at room temperature. Therefore, the steel is heated to a high temperature to fully dissolve the C-Cr compound and then cooled quickly so that it has no time to precipitate, so as to ensure the corrosion resistance of the steel, especially the resistance to intergranular corrosion.
②σ phase: If austenitic steel is heated for a long time in the range of 500-900℃, or elements such as Ti, Nb, and Mo are added to the steel, the precipitation of σ phase will be promoted, making the steel more brittle and reducing corrosion resistance. The means to eliminate the σ phase is to dissolve it at a temperature higher than its possible precipitation temperature, and then cool it quickly to prevent re-precipitation.
Process:
In the GB1200 standard, the recommended heating temperature range is relatively wide: 1000~1150℃, usually 1020-1080℃. Considering the specific grade composition, whether it is a casting or a forging, etc., the heating temperature should be appropriately adjusted within the allowable range. If the heating temperature is low, C-Cr carbides cannot be fully dissolved. If the temperature is too high, there will also be problems with grain growth and reduced corrosion resistance.
Cooling method: Cooling should be done at a faster speed to prevent carbides from re-precipitating. In the standards of my country and some other countries, “fast cooling” after solution treatment is indicated. Combining different literature and practical experience, the scale of “fast” can be mastered as follows:
C content ≥ 0.08%; Cr content > 22%, Ni content is relatively high; C content < 0.08%, but the effective size > 3mm, should be water-cooled;
C content < 0.08%, size < 3mm, can be air-cooled;
Effective size ≤ 0.5mm can be air-cooled.
B. Stabilization heat treatment of austenitic stainless steel
Stabilization heat treatment is limited to austenitic stainless steel containing stabilizing elements Ti or Nb, such as 1Cr18Ni9Ti, 0Cr18Ni11Nb, etc.
Function:
As mentioned above, Cr combines with C to form Cr23C6-type compounds and precipitates at the grain boundaries, which is the reason for the decrease in corrosion resistance of austenitic stainless steel. Cr is a strong carbide-forming element. As long as there is a chance, it will combine with C and precipitate. Therefore, elements Ti and Nb with stronger affinity than Cr and C are added to the steel, and conditions are created so that C preferentially combines with Ti and Nb, reducing the chance of C combining with Cr, so that Cr is stably retained in austenite, thus ensuring the corrosion resistance of the steel. Stabilization heat treatment plays the role of combining Ti, Nb with C and stabilizing Cr in austenite.
Process:
Heating temperature: This temperature should be higher than the dissolution temperature of Cr23C6 (400-825℃), lower than or slightly higher than the starting dissolution temperature of TiC or NbC (such as the dissolution temperature range of TiC is 750-1120℃), and the stabilization heating temperature is generally selected at 850-930℃, which will fully dissolve Cr23C6 so that Ti or Nb will be combined with C, while Cr will continue to remain in austenite.
Cooling method: Generally, air cooling is used, and water cooling or furnace cooling can also be used, which should be determined according to the specific conditions of the parts. The cooling rate has no significant effect on the stabilization effect. From our experimental research results, when cooling from the stabilization temperature of 900℃ to 200℃, the cooling rate is 0.9℃/min and 15.6℃/min. In comparison, the metallographic structure, hardness, and intergranular corrosion resistance are basically the same.
C. Stress relief treatment of austenitic stainless steel
Purpose: Parts made of austenitic stainless steel inevitably have stress, such as processing stress and welding stress during cold working. The existence of these stresses will bring adverse effects, such as the impact on dimensional stability; stress corrosion cracking will occur when parts with stress are used in Cl-containing media, H2S, NaOH, and other media. This is a sudden damage that occurs locally without precursors and is very harmful. Therefore, austenitic stainless steel parts used under certain working conditions should minimize stress, which can be achieved through stress relief methods.
Process: When conditions permit, solution treatment and stabilization treatment can better eliminate stress (solid solution water cooling will also produce certain stress), but sometimes this method is not allowed, such as pipes in the circuit, complete workpieces without margins, and parts with particularly complex shapes that are easy to deform. At this time, the stress relief method of heating at a temperature below 450°C can be used to eliminate some stress. If the workpiece is used in a strong-stress corrosion environment and the stress must be completely eliminated, it should be considered when selecting materials, such as steel containing stabilizing elements or ultra-low carbon austenitic stainless steel.
D. Heat treatment of martensitic stainless steel
The most prominent feature of martensitic stainless steel compared to ferritic stainless steel, austenitic stainless steel, and duplex stainless steel is that the mechanical properties can be adjusted in a wide range through heat treatment methods to meet the needs of different use conditions. Different heat treatment methods also have different effects on corrosion resistance.
① The organizational state of martensitic stainless steel after quenching
Depending on the chemical composition
0Cr13, 1Cr13, 1Cr17Ni2 are martensite + a small amount of ferrite;
2Cr13, 3Cr13, 2Cr17Ni2 are basically martensitic organizations;
4Cr13, and 9Cr18 are alloy carbides on the martensitic matrix;
0Cr13Ni4Mo, and 0Cr13Ni6Mo are residual austenite on the martensitic matrix.
② Corrosion resistance and heat treatment of martensitic stainless steel
The heat treatment of martensitic stainless steel can not only change the mechanical properties but also have different effects on corrosion resistance. Take tempering after quenching as an example: after quenching into martensite, low-temperature tempering is used, which has higher corrosion resistance; medium-temperature tempering at 400-550℃ is used, and the corrosion resistance is lower; high-temperature tempering at 600-750℃ is used, and the corrosion resistance is improved.
③ Heat treatment process and function of martensitic stainless steel
Annealing: Different annealing methods can be used according to the purpose and function to be achieved: only required to reduce hardness, facilitate processing, and eliminate stress, low-temperature annealing (some are also called incomplete annealing) can be used. The heating temperature can be selected from 740~780℃, and the hardness can be guaranteed to be 180~230HB by air cooling or furnace cooling;
The requirement to improve forging or casting structure, lower hardness and ensure low performance for direct application, can use complete annealing, generally heated to 870~900℃, furnace cooled after insulation, or cooled to below 600℃ at a rate of ≤40℃/h. The hardness can reach 150~180HB;
Isothermal annealing can replace full annealing to achieve the purpose of full annealing. The heating temperature is 870~900℃, and the furnace is cooled to 700~740℃ after heating and heat preservation (refer to the transformation curve), and the temperature is kept for a long time (refer to the transformation curve), and then the furnace is cooled to below 550℃ and taken out of the furnace. The hardness can reach 150-180HB. This isothermal annealing is also an effective way to improve the poor structure after forging and improve the mechanical properties after quenching and tempering, especially the impact toughness.
Quenching: The main purpose of quenching martensitic stainless steel is to strengthen. Heat the steel to above the critical point temperature, keep it warm, so that the carbide is fully dissolved into the austenite, and then cool it at an appropriate cooling rate to obtain the quenched martensite structure.
Selection of heating temperature: The basic principle is to ensure the formation of austenite, and to make the alloy carbides fully dissolved into the austenite and homogenized; it is also not possible to make the austenite grains coarse or to have ferrite or residual austenite in the structure after quenching. This requires that the quenching heating temperature should not be too low or too high. The quenching heating temperature of martensitic stainless steel varies slightly in different materials and the recommended range is wide. According to our experience, it is generally sufficient to heat in the range of 980~1020℃. Of course, for special steel grades, special component control, or special requirements, the heating temperature should be appropriately lowered or increased, but the heating principle should not be violated.
Cooling method: Due to the composition characteristics of martensitic stainless steel, austenite is relatively stable, the C curve shifts to the right, and the critical cooling rate is relatively low, so oil cooling and air cooling can be used to obtain the effect of quenching martensite. However, for parts that require a large quenching depth, mechanical properties, especially high impact toughness, oil cooling should be used.
Tempering: After quenching, martensitic stainless steel obtains a martensitic structure with high hardness, high brittleness, and high internal stress, and must be tempered. Martensitic stainless steel is basically used at two tempering temperatures:
Tempering between 180~320℃. The tempered martensite structure is obtained, maintaining high hardness and strength, but low plasticity and toughness, and has good corrosion resistance. For example, low-temperature tempering can be used for tools, bearings, wear-resistant parts, etc.
Tempering between 600~750℃ to obtain tempered martensite structure. It has good comprehensive mechanical properties such as certain strength, hardness, plasticity, and toughness. It can be tempered at the lower or upper limit temperature according to the different requirements for strength, plasticity, and toughness. This structure also has good corrosion resistance.
Tempering between 400~600℃ is generally not used, because tempering in this temperature range will precipitate highly dispersed carbides from the martensite, produce temper brittleness, and reduce corrosion resistance. However, springs, such as 3Cr13 and 4Cr13 steel springs, can be tempered at this temperature, and HRC can reach 40~45, with good elasticity.
The cooling method after tempering can generally be air cooling, but for steel grades with temper brittleness tendency, such as 1Cr17Ni2, 2Cr13, 0Cr13Ni4Mo, etc., it is best to use oil cooling after tempering. In addition, it should be noted that tempering should be carried out in time after quenching, not more than 24 hours in summer and not more than 8 hours in winter. If tempering cannot be carried out in time according to the process temperature, measures should also be taken to prevent the generation of static cracks.
E. Heat treatment of ferrite-austenite duplex stainless steel
Duplex stainless steel is a young member of the stainless steel family and developed later, but its characteristics are widely recognized and valued. The composition characteristics (high Cr, low Ni, Mo, N) and organizational characteristics of duplex stainless steel make it have higher strength and plasticity than austenitic stainless steel and ferritic stainless steel; equivalent to the corrosion resistance of austenitic stainless steel; higher resistance to pitting, crevice corrosion, and stress corrosion damage than any stainless steel in cl- medium and seawater.
Function:
① Eliminate secondary austenite: Under higher temperature conditions (such as casting or forging), the amount of ferrite increases. When it is above 1300℃, it can form a single-phase ferrite. This high-temperature ferrite is unstable. When it is aged at a lower temperature in the future, austenite will precipitate. This austenite is called secondary austenite. The amount of Cr and N in this austenite is less than that of normal austenite, so it may become a corrosion source, so it should be eliminated by heat treatment.
② Eliminate Cr23C6 type carbide: Dual-phase steel will precipitate Cr23C6 below 950℃, which increases brittleness and reduces corrosion resistance, and should be eliminated.
③ Eliminate nitrides Cr2N and CrN: Because there is an N element in steel, it can generate nitrides with Cr, which affects the mechanical and corrosion resistance, and should be eliminated.
④ Eliminate intermetallic phases: The composition characteristics of duplex steel will promote the formation of some intermetallic phases, such as σ phase and γ phase, which reduce corrosion resistance and increase brittleness, and should be eliminated.
Process: Similar to austenitic steel, it adopts solution treatment, heating temperature 980~1100℃, and then rapid cooling, generally water cooling.
F. Heat treatment of precipitation-hardening stainless steel
Precipitation hardening stainless steel is relatively late in development. It is a type of stainless steel that has been tested, summarized, and innovated in human practice. Among the stainless steels that appeared earlier, ferritic stainless steel and austenitic stainless steel have good corrosion resistance, but the mechanical properties cannot be adjusted by heat treatment methods, which limits their role. Martensitic stainless steel can use heat treatment methods to adjust the mechanical properties within a larger range, but its corrosion resistance is poor.
Features:
It has a lower C content (generally ≤0.09%), a higher Cr content (generally ≥14%), and Mo, Cu, and other elements, which makes it have higher corrosion resistance, even comparable to austenitic stainless steel. Through solution and aging treatment, a structure with a precipitation hardening phase precipitated on the martensitic matrix can be obtained, so it has higher strength, and the strength, plasticity, and toughness can be adjusted within a certain range according to the adjustment of the aging temperature. In addition, the heat treatment method of solid solution first and then precipitation strengthening can be processed into basic shape under low hardness after solid solution treatment, and then strengthened by aging, which reduces the processing cost and is better than martensitic steel.
Classification:
①Martensitic precipitation hardening stainless steel and its heat treatment: The characteristics of martensitic precipitation hardening stainless steel are: the starting temperature Ms of austenite to martensite transformation is above room temperature. After heating austenitization and cooling at a faster rate, a lath-shaped martensitic matrix is obtained. After aging, fine particles of Cu are precipitated from the lath martensitic matrix to strengthen.
②Heat treatment of semi-austenitic stainless steel: The Ms point of this steel is generally slightly lower than room temperature, so after solid solution treatment and cooling to room temperature, an 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 and kept warm. At this stage, carbides will precipitate in austenite, the stability of austenite is reduced, and the Ms point is increased to above room temperature. When cooled again, a martensitic structure is obtained. Some can also add cold treatment (sub-zero treatment), and then age the steel to finally obtain a strengthened steel with precipitates on the martensite matrix.
It can be seen that after the precipitation hardening martensitic stainless steel is properly treated, the mechanical properties can fully reach the performance of martensitic stainless steel, while the corrosion resistance is equivalent to that of austenitic stainless steel. It should be pointed out here that although martensitic stainless steel and precipitation-hardening stainless steel can be strengthened by heat treatment methods, the strengthening mechanism is different. Due to the characteristics of precipitation-hardening stainless steel, it has been valued and widely used.
Post time: Feb-06-2025