What are the heat treatments for stainless steel? Are different types of stainless steel heat treated differently?

What are the heat treatments for stainless steel? Are different types of stainless steel heat treated differently?

1 ferritic stainless steel

The main alloying element is Cr, or a small amount of stable ferrite elements are added, such as Al, Mo, etc., and the structure is ferrite. The strength is not high, and the properties cannot be adjusted by heat treatment. It has a certain plasticity and is relatively brittle. It has good corrosion resistance in oxidizing media (such as nitric acid) and poor corrosion resistance in reducing media.

2 Austenitic stainless steel

Contains high Cr, generally greater than 18%, and contains about 8% Ni. Some use Mn instead of Ni. In order to further improve corrosion resistance, elements such as Mo, Cu, Si, Ti, Nb, etc. must be added. It does not undergo phase change when heated and cooled, and cannot be strengthened by heat treatment. It has low strength, high plasticity and high toughness. It has strong corrosion resistance to oxidizing media, and after adding Ti and Nb, it has better resistance to intergranular corrosion.

3 martensitic stainless steel

Martensitic stainless steel mainly contains 12~18% Cr, and the amount of C is adjusted as needed, usually 0.1~0.4%. When making tools, C can reach 0.8~1.0%. Some to improve the anti-tempering stability, Add Mo, V, Nb, etc. After heating at high temperature and cooling at a certain speed, the structure is basically martensite. Depending on the difference in C and alloy elements, some may contain a small amount of ferrite, residual austenite or alloy carbides. Phase changes occur upon heating and cooling, so the tissue structure and morphology can be adjusted over a wide range, thereby changing properties. The corrosion resistance is not as good as austenite, ferrite and duplex stainless steel. It has good corrosion resistance in organic acids and poor corrosion resistance in media such as sulfuric acid and hydrochloric acid.

4 Ferritic-Austenitic Duplex Stainless Steel

Generally, the Cr content is 17~30%, and the Ni content is 3~13%. In addition, Mo, Cu, Nb, N, W and other alloying elements are added, and the C content is controlled to be very low. Depending on the proportion of alloying elements, some are ferrite. Mainly, some are mainly austenite, forming duplex stainless steel with two phases existing at the same time. Because it contains ferrite and strengthening elements, after heat treatment, its strength is slightly higher than that of austenitic stainless steel, and its plasticity and toughness are good. Basically, heat treatment cannot be used to adjust its properties. It has high corrosion resistance, especially in Cl-containing media and seawater, and has good resistance to pitting corrosion, crevice corrosion, and stress corrosion.

5 Precipitation hardened stainless steel

The composition is characterized by that in addition to elements such as C, Cr, and Ni, it also contains elements such as Cu, Al, and Ti that can precipitate over time. Mechanical properties can be adjusted through heat treatment, but its strengthening mechanism is different from martensitic stainless steel. Because it relies on precipitation and precipitation phase strengthening, C can be controlled very low, so its corrosion resistance is better than martensitic stainless steel and equivalent to Cr-Ni austenitic stainless steel.

Heat treatment of stainless steel

The composition characteristics of stainless steel, which is composed of a large number of alloy elements mainly Cr, are the basic conditions for its stainless steel and corrosion resistance. In order to give full play to the role of alloy elements and obtain ideal mechanical and corrosion resistance properties, it must also be achieved through heat treatment methods.

1 Heat treatment of ferritic stainless steel

Ferritic stainless steel is generally a stable single ferrite structure that does not undergo phase change when heated and cooled, so the mechanical properties cannot be adjusted by heat treatment. Its main purpose is to reduce brittleness and improve intergranular corrosion resistance.

①σ phase brittleness

Ferritic stainless steel is very easy to generate σ phase, which is a Cr-rich metal compound that is hard and brittle. It is especially easy to form between grains, making the steel brittle and increasing the susceptibility to intergranular corrosion. The formation of σ phase is related to the composition. In addition to 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 540~815°C range, which promotes the formation of σ phase. However, the σ phase formation is reversible, and reheating to a temperature higher than the σ phase formation temperature will redissolve it in the solid solution.

②Brittleness at 475℃

When ferritic stainless steel is heated for a long time in the 400~500°C range, it will show the characteristics of increased strength, decreased toughness, and increased brittleness. It is especially obvious at 475°C, which is called 475°C brittleness. This is because, at this temperature, the Cr atoms in the ferrite will rearrange to form a small Cr-rich area that is consistent with the parent phase, causing lattice distortion and internal stress, making the steel harder and more brittle. When the Cr-rich area is formed, a Cr-poor area must appear, which has a negative impact on corrosion resistance. When the steel is reheated to a temperature higher than 700°C, the distortion and internal stress will be eliminated, and the brittleness at 475°C will disappear.

③High temperature brittleness

When heated to above 925°C and rapidly cooled down, compounds formed by Cr, C, N, etc. precipitate within the grains and grain boundaries, causing increased brittleness and intergranular corrosion. This compound can be eliminated by rapid cooling after heating at a temperature of 750~850℃.

Heat treatment process:

①Annealing

In order to eliminate the σ phase, 475°C brittleness and high temperature brittleness, annealing treatment can be used, heating and insulation at 780~830°C, and then air cooling or furnace cooling.

For ultra-pure ferritic stainless steel (containing C ≤ 0.01%, strictly controlling Si, Mn, S, and P), the annealing heating temperature can be increased.

②Stress relief treatment

After welding and cold working, parts may produce stress. If annealing is not suitable under specific circumstances, heating, insulation, and air cooling can be performed in the range of 230~370°C to eliminate part of the internal stress and improve plasticity.

2 Austenitic stainless steel heat treatment

The effect of Cr, Ni and other alloying elements in austenitic stainless steel causes the Ms point to drop below room temperature (-30 to -70°C). The austenite structure is ensured to be stable, so no phase transformation 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 the corrosion resistance.

1 Solution treatment of austenitic stainless steel

effect:

①Precipitation and dissolution of alloy carbides in steel

C is one of the alloying elements contained in steel. In addition to its strengthening effect, it is detrimental to corrosion resistance. Especially when C and Cr form carbides, the effect is even worse, so efforts should be made to reduce its presence. . For this reason, based on the characteristics of C that changes with temperature in austenite, that is, the solubility is large at high temperatures and the solubility is small at low temperatures. It is reported that the solubility of C in austenite is 0.34% at 1200°C; 0.18% at 1000°C; 0.02% at 600°C; 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 does not have time to precipitate, ensuring 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°C, or when 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 method of eliminating σ phase is also Dissolve it at a temperature higher than its possible precipitation, and then cool it quickly to prevent further precipitation.

Craftsmanship:

In the GB1200 standard, the recommended heating temperature range is wide: 1000~1150℃, and 1020-1080℃ is usually used. Consider the specific grade composition, whether it is a casting or a forging, etc., and adjust the heating temperature appropriately within the allowable range. If the heating temperature is low, the C-Cr carbide cannot be fully dissolved. If the temperature is too high, the grains will grow and the corrosion resistance will be reduced.

Cooling method: Cool quickly to prevent carbide from re-precipitating. In our country and some other national standards, "quick cooling" after solid solution is indicated. Based on different literature and practical experience, the scale of "quick" can be grasped as follows:

Those with C content ≥ 0.08%; those with Cr content > 22% and higher Ni content; those with C content < 0.08% but 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.

2 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.

effect:

As mentioned before, Cr combines with C to form Cr23C6 compounds and precipitates at the grain boundaries, which is the cause of the decrease in corrosion resistance of austenitic stainless steel. Cr is a strong carbide-forming element and will combine with C and precipitate as long as there is an opportunity. Therefore, elements Ti and Nb with stronger affinity than Cr and C are added to the steel, and conditions are created so that C can combine with Ti and Nb preferentially. , Reduce the chance of C and Cr combining, so that Cr can be stably retained in austenite, thus ensuring the corrosion resistance of steel. The stabilization heat treatment plays the role of combining Ti, Nb and C to stabilize Cr in austenite.

Craftsmanship:

Heating temperature: This temperature should be higher than the dissolution temperature of Cr23C6 (400-825℃), lower or slightly higher than the starting dissolution temperature of TiC or NbC (such as the dissolution temperature range of TiC is 750-1120℃), stabilizing heating temperature Generally selected at 850-930℃, which will fully dissolve Cr23C6, allowing Ti or Nb to combine with C, while Cr will continue to remain in the austenite.

Cooling method: Air cooling is generally used, but water cooling or furnace cooling can also be used. This should be determined according to the specific conditions of the parts. The cooling rate has no significant impact on the stabilization effect. Judging from the results of our experimental research, when cooling from the stabilization temperature of 900°C to 200°C, the cooling rates are 0.9°C/min and 15.6°C/min. In comparison, the metallographic structure, hardness, and intergranular corrosion resistance are basically the same.