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Vacuum heat treatment process for stainless steel precision parts

Compared with ferritic stainless steel, austenitic stainless steel and duplex stainless steel, the most prominent feature of martensitic stainless steel is that it can adjust the mechanical properties in a wide range through vacuum heat treatment to meet the needs of different service conditions. Different vacuum heat treatment methods also have different effects on corrosion resistance.

① Microstructure of martensitic stainless steel after vacuum quenching

According to different chemical composition

0Cr13, 1Cr13, 1Cr17Ni2 are martensite + a small amount of ferrite;

2Cr13, 3Cr13, 2Cr17Ni2 are basically martensite;

4Cr13 and 9Cr18 have alloy carbides on the martensite matrix;

0Cr13Ni4Mo and 0Cr13Ni6Mo are martensite matrix with retained austenite.

②Corrosion Resistance and Vacuum Heat Treatment of Martensitic Stainless Steel

Vacuum heat treatment of martensitic stainless steel can not only change the mechanical properties, but also have different effects on corrosion resistance. Take vacuum tempering after vacuum quenching as an example: after vacuum quenching into martensite, low-temperature vacuum tempering has high corrosion resistance; medium-temperature vacuum tempering at 400-550°C has low corrosion resistance; 600- 750 ℃ high temperature vacuum tempering, the corrosion resistance is improved again.

③ Martensitic stainless steel heat treatment process and function

Stainless steel vacuum annealing

According to the purpose and function to be achieved, different vacuum annealing methods can be used:

Low-temperature vacuum annealing (some are also called incomplete vacuum annealing) can be used for low-temperature vacuum annealing (some are also called incomplete vacuum annealing) if it is only required to reduce hardness, facilitate processing, and eliminate stress. The heating temperature can be selected from 740 to 780 ° C, and the hardness can be guaranteed to be 180 to 230 HB by air cooling or furnace cooling;

It is required to improve forging or casting structure, lower hardness and ensure low performance for direct application. Complete vacuum annealing can be used for general heating at 870~900°C, then furnace cooling after heat preservation, or cooling to below 600°C at a rate of ≤40°C/h out of the oven. Hardness up to 150~180HB;

Isothermal vacuum annealing can replace complete vacuum annealing to achieve the purpose of complete vacuum annealing. The heating temperature is 870~900°C, after heating and holding, the furnace is cooled to 700~740°C (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°C. The hardness can reach 150-180HB. This isothermal vacuum annealing is also an effective way to improve the bad structure after forging and improve the mechanical properties after vacuum quenching and vacuum tempering, especially the impact toughness.

Stainless steel vacuum quenching

The main purpose of vacuum quenching of martensitic stainless steel is strengthening. Heating the steel above the critical point temperature and keeping it warm to fully dissolve the carbides into the austenite, then cooling at an appropriate cooling rate to obtain a vacuum quenched martensite structure.

Heating temperature selection: The basic principle is to ensure the formation of austenite, and to fully dissolve the alloy carbide into the austenite and homogenize it; it is also not possible to make the austenite grain coarse or ferrite or residual in the structure after vacuum quenching austenitic. This requires that the vacuum quenching heating temperature should not be too low or too high. The heating temperature for vacuum quenching of martensitic stainless steel is slightly different in the introduction and recommended range of different materials, and the temperature range is wider. According to our experience, generally choose to heat 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 shifts to the right, and the critical cooling rate is small, so the effect of vacuum quenching martensite can be obtained by oil cooling or air cooling. However, oil cooling should be used for parts that require large hardening depth and high mechanical properties, especially impact toughness.

Stainless steel vacuum tempering

After vacuum quenching, martensitic stainless steel will obtain martensite structure, which has high hardness, high brittleness and large internal stress, so it must be treated by vacuum tempering. Martensitic stainless steels are basically used at two vacuum tempering temperatures:

Vacuum tempering between 180~320℃. Vacuum tempered martensite structure is obtained, which maintains high hardness and strength, but has low plasticity and toughness, and has good corrosion resistance. Such as knives, bearings, wear-resistant parts, etc., can be tempered in low temperature vacuum.

Vacuum tempering between 600~750°C to obtain vacuum tempered sorbite structure. It has good comprehensive mechanical properties such as strength, hardness, plasticity and toughness. According to different requirements for strength, plasticity and toughness, it can be tempered in vacuum with a lower or upper limit temperature. This organization also has good corrosion resistance.

Vacuum tempering at a temperature between 400 and 600 °C is generally not used, because vacuum tempering in this temperature range will precipitate carbides with a high dispersion from the martensite, resulting in vacuum tempering brittleness and reducing the resistance. corrosion. However, springs, such as 3Cr13 and 4Cr13 steel springs, can be tempered in vacuum at this temperature, and the HRC can reach 40~45, which has good elasticity.

The cooling method after vacuum tempering can generally be air cooling, but for steel grades that tend to be brittle due to vacuum tempering, such as 1Cr17Ni2, 2Cr13, 0Cr13Ni4Mo, etc., it is best to use oil cooling after vacuum tempering. In addition, the problem that needs to be paid attention to is that vacuum tempering should be done in time after vacuum quenching, not more than 24 hours in summer, and not more than 8 hours in winter. If vacuum tempering cannot be timely according to the process temperature, measures should also be taken to prevent static cracks.