Compared with ferritic stainless steel, austenitic stainless steel and duplex stainless steel, the most prominent feature of martensitic stainless steel is that the mechanical properties can be adjusted in a wide range by heat treatment to meet the needs of different service conditions. Different heat treatment methods also have different effects on corrosion resistance.
① Microstructure of Martensitic stainless steel after quenching
Depends on chemical composition
- 0Cr13, 1Cr13, 1Cr17Ni2 are martensite + a small amount of ferrite;
- 2Cr13, 3Cr13, 2Cr17Ni2 are basically martensitic structures;
- 4Cr13, 9Cr18 are alloyed carbides on the martensite matrix;
- 0Cr13Ni4Mo and 0Cr13Ni6Mo are martensite matrix with retained austenite.
② Corrosion resistance and heat treatment of Martensitic stainless steel
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 high corrosion resistance; medium temperature tempering at 400-550 °C has low corrosion resistance; high temperature tempering at 600-750 °C is used. , the corrosion resistance has been improved.
③ Heat treatment process method and function of Martensitic stainless steel
Annealing
According to the purpose and function to be achieved, different annealing methods can be used:
- Only required to reduce hardness, facilitate processing, and eliminate stress, low temperature annealing (some also called incomplete annealing) can be used for heating temperature of 740~780℃, and air cooling or furnace cooling hardness can guarantee 180~230HB;
- If it is required to improve the forging or casting structure, lower hardness and ensure low performance, it can be directly applied. It can be used for complete annealing, generally heated at 870~900℃, furnace cooled after heat preservation, or cooled to below 600℃ at a rate of ≤40℃/h out. The hardness can reach 150~180HB;
- Isothermal annealing, which can replace full annealing and achieve the purpose of full annealing. The heating temperature is 870~900℃. After heating and heat preservation, the furnace is cooled to 700~740℃ (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℃. 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 strengthening. The steel is heated to a temperature above the critical point and kept warm so that the carbides are fully dissolved into the austenite, and then cooled at an appropriate cooling rate to obtain a quenched martensite structure.
- Selection of heating temperature: The basic principle is to ensure the formation of austenite, and to fully dissolve the alloy carbide into the austenite for homogenization; it cannot make the austenite grains coarse or ferrite or residues exist in the structure after quenching Austenite. This requires that the quenching heating temperature should not be too low or too high. The quenching and heating temperature of martensitic stainless steel is slightly different from different materials and recommended ranges, and the temperature range is wide. According to our experience, generally choose heating in the range of 980~1020℃. Of course, for special steel grades, special composition control or special requirements, the heating temperature should be appropriately reduced or increased, but the heating principle should not be violated.
- Cooling method: Due to the composition characteristics of martensitic stainless steel, the austenite is relatively stable, the C curve is shifted to the right, and the critical cooling rate is small, so the effect of quenching martensite can be obtained by oil cooling and air cooling. However, oil cooling should be used for parts that require large hardening depth and high mechanical properties, especially impact toughness.
Tempering
After the martensitic stainless steel is quenched, the martensitic structure is obtained, which has high hardness, large brittleness and large internal stress, and must be tempered. Martensitic stainless steels are basically used at two tempering temperatures:
- Temper between 180~320℃. Obtain tempered martensite structure, maintain high hardness and strength, but have low plasticity and toughness, and have good corrosion resistance. Such as tools, bearings, wear parts, etc. can be tempered at low temperature.
- Temper at 600~750℃ to obtain tempered sorbite structure. It has certain good comprehensive mechanical properties such as strength, hardness, plasticity and toughness. It can be tempered at the lower limit or upper limit temperature according to the different requirements for strength, plasticity and toughness. This organization also has good corrosion resistance.
- Tempering at temperatures between 400 and 600°C is generally not used, because tempering in this temperature range will precipitate carbides with a high degree of dispersion from martensite, resulting in temper brittleness and reducing corrosion resistance. . However, springs, such as 3Cr13, 4Cr13 steel springs, can be tempered at this temperature, and the HRC can reach 40~45, which has good elasticity.
The cooling method after tempering can generally be air-cooled, but for steel grades with a tendency to temper brittleness, such as 1Cr17Ni2, 2Cr13, 0Cr13Ni4Mo, etc., it is best to use oil cooling after tempering. In addition, the problem that needs to be paid attention to is that it needs to be tempered in time after quenching. It should not exceed 24 hours in summer and 8 hours in winter. If it cannot be tempered according to the process temperature in time, measures should be taken to prevent static cracks.
4->Heat Treatment of Ferritic-Austenitic Duplex Stainless Steels
Duplex stainless steel is a young member of the stainless steel family with a relatively late development, but its characteristics have been widely recognized and valued. The composition characteristics (high Cr, low Ni, Mo, N) and structure 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 High resistance to pitting corrosion, crevice corrosion and stress corrosion damage in cl-media, seawater than any stainless steel.
Effect:
① Eliminate secondary austenite
Under higher temperature conditions (such as casting or forging), the amount of ferrite increases, and when it is above 1300 °C, it can become single-phase ferrite. Aging, there will be austenite precipitation, 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 source of corrosion, so it should be eliminated by heat treatment.
② Eliminate Cr23C6 carbides
Duplex steel will precipitate Cr23C6 below 950 °C to increase brittleness and reduce corrosion resistance, which should be eliminated.
③ Elimination of nitrides Cr2N, CrN
Due to the N element in the steel, it can form nitrides with Cr, which affects the mechanical and corrosion resistance properties and should be eliminated.
④ Eliminate intermetallic phase
The compositional characteristics of dual-phase 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.
Craft:
Similar to austenitic steel, solid solution treatment is adopted, the heating temperature is 980~1100 °C, and then it is quickly cooled, generally using water cooling.
5->Heat Treatment of Precipitation Hardening Stainless Steel
Precipitation hardening stainless steel developed relatively late, and it is a 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 cannot be adjusted by heat treatment, which limits their role. The martensitic stainless steel can use the heat treatment method to adjust the mechanical properties in a large range, but the corrosion resistance is poor.
Features:
It has a low C content (generally ≤ 0.09%), a high Cr content (generally ≥ 14% or more), plus Mo, Cu and other elements, which make it have high corrosion resistance, and even the same Austenitic stainless steel is comparable. Through solid solution and aging treatment, the structure in which the precipitation hardening phase is precipitated on the martensite 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 first solid solution, and then precipitation strengthening according to the precipitation, can be processed after solid solution treatment, under the condition of low hardness, and then processed by aging to reduce the processing cost, which is better than martensitic steel.
Classification:
① Martensitic 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 for austenitization and cooling at a relatively fast rate, a lath martensite matrix is obtained, and after aging, Cu fine particles are precipitated from the lath martensite matrix to strengthen.
-> Solution treatment:
The heating temperature is 1020-1060 ℃, water cooling or oil cooling after heat preservation, the structure is lath martensite, and the hardness is about 320HB. The heating temperature should not be too high. If it is higher than 1100 °C, the amount of ferrite in the structure will increase, the Ms point will decrease, the retained austenite will increase, and the hardness will decrease, and the heat treatment effect will be poor.
-> Aging treatment:
Depending on the aging temperature, the dispersity and particle size of the precipitates are different, and they have different mechanical properties.
② Semi-austenitic stainless steel heat treatment
The Ms point of this kind of steel is generally slightly lower than the room temperature, so after the solution treatment is cooled to room temperature, the austenite structure is obtained, and the strength is very low. At this stage, carbides will be precipitated in the austenite, the stability of the austenite will be reduced, the Ms point will be raised to above room temperature, and the martensitic structure will be obtained when it is cooled again. Some can also add cold treatment (subzero treatment), and then re-age the steel to finally obtain a strengthened steel with precipitates on the martensitic matrix.
Solid solution + adjustment + aging treatment
- The solution heating temperature is 1040℃. After heating and heat preservation, water cooling or oil cooling can obtain austenite, and the hardness is about 150HB;
- Adjust the treatment temperature to 760°C, and air-cool after heat preservation to precipitate alloy carbides in the austenite, reduce the stability of the austenite, increase the Ms point to about 50-90°C, and obtain lath martensite after cooling. The hardness can reach about 290HB;
- After aging at 560℃, Al and compounds are precipitated, and the steel is strengthened, and the hardness can reach about 340HB.
Solid solution + adjustment + cold treatment + aging
- Solution heat treatment at 1040℃, water cooling to obtain austenite structure;
- Adjust the treatment temperature to 955℃, increase the Ms point, and obtain lath martensite after cooling;
- Cold treatment -73℃×8h to reduce the retained austenite in the structure and obtain the maximum martensite;
- The aging treatment temperature is 510-560℃, so that Al is precipitated. After strengthening treatment, the hardness can reach 336HB
Solid solution + cold deformation + aging
- Solution treatment temperature is 1040℃, water cooling to obtain austenite structure;
- Cold deformation, using the principle of cold working deformation strengthening, the austenite is transformed into martensite at the Md point, and the cold working deformation is greater than 30-50%;
- Aging treatment: heating and aging at about 490℃ to make Al precipitation hardening.
- It is reported that the solid solution austenite is deformed by 57% cold rolling, the hardness reaches 430HB, σb reaches 1372 N/mm2, and then aged at 490℃, the hardness reaches 485HB, σb reaches 1850 N/mm2.
It can be seen that after correct treatment of precipitation hardening martensitic stainless steel, the mechanical properties can fully reach the performance of martensitic stainless steel, while the corrosion resistance is comparable to that of austenitic stainless steel. It should be pointed out here that although both 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 is valued and widely used.
