Heat Treatment of Stainless Steel ( Part 1 )

Stainless steel has a variety of classification methods, such as chemical composition, functional characteristics, metallographic structure and heat treatment characteristics. From the point of view of heat treatment, it is more practical to divide according to the metallographic structure and heat treatment characteristics.

1、Ferritic Stainless Steel

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

It contains high Cr, generally greater than 18%, and contains about 8% Ni, and some replace Ni with Mn. In order to further improve the corrosion resistance, elements such as Mo, Cu, Si, Ti, Nb must be added. It does not undergo phase transformation during heating and cooling, cannot be strengthened by heat treatment, and has low strength, high plasticity and high toughness. It has strong corrosion resistance to oxidizing media, and has better resistance to intergranular corrosion after adding Ti and Nb.

Martensitic stainless steel mainly contains 12~18% Cr, and the amount of C is adjusted according to the needs, generally 0.1~0.4%, for making tools, C can reach 0.8~1.0%, and some are to improve the tempering resistance stability, Add Mo, V, Nb, etc. After heating at high temperature and cooling at a certain rate, the structure is basically martensite. Depending on the difference in C and alloying elements, some may contain a small amount of ferrite, retained austenite or alloy carbides. Phase transitions occur on heating and cooling, so tissue structure and morphology can be tuned over a wide range, thereby changing properties. The corrosion resistance is not as good as that of austenite, ferrite and duplex stainless steel. It has good corrosion resistance in organic acids, but poor corrosion resistance in sulfuric acid, hydrochloric acid and other media.

Generally, the content of Cr is 17~30%, and the content of Ni is 3~13%. In addition, alloying elements such as Mo, Cu, Nb, N, and W are added, and the content of C is controlled very low. Mainly, some are dominated by austenite, which constitutes a duplex stainless steel with two phases existing at the same time. Because it contains ferrite and strengthening elements, after heat treatment, the strength is slightly higher than that of austenitic stainless steel, and the plasticity and toughness are good, so it is basically impossible to adjust the performance by heat treatment. It has high corrosion resistance, especially in the medium containing Cl- and seawater, and has the characteristics of good resistance to pitting corrosion, crevice corrosion and stress corrosion.

The characteristics of the composition are that in addition to C, Cr, Ni and other elements, it also contains Cu, Al, Ti and other elements that can precipitate precipitates with aging. The mechanical properties can be adjusted by means of heat treatment, but the strengthening mechanism is different from that of martensitic stainless steel. Because it relies on precipitation to strengthen, so C can be controlled very low, so its corrosion resistance is better than martensitic stainless steel, and comparable to Cr-Ni austenitic stainless steel.

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

1->Heat Treatment of Ferritic Stainless Steel

1) Ferritic stainless steel is generally a stable single ferrite structure without phase transformation when heated and cooled, so the mechanical properties cannot be adjusted by heat treatment. The main purpose is to reduce brittleness and improve intergranular corrosion resistance.

② 475℃ 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, decreased toughness, that is, increased brittleness, especially at 475℃, which is called 475℃ brittleness. This is because, at this temperature, the Cr atoms in the ferrite will be rearranged to form a small Cr-rich region, which is coherent with the parent phase, causing lattice distortion and internal stress, which increases the hardness and brittleness of the steel. When the Cr-rich area is formed, there must be a Cr-depleted area, which has an adverse effect on the corrosion resistance. When the steel is reheated above 700°C, the distortion and internal stress will be eliminated, and the brittleness at 475°C will disappear.

When heated to above 925 °C and cooled down rapidly, Cr, C, N and other forming compounds are precipitated in the grains and grain boundaries, causing the increase of brittleness and the occurrence of intergranular corrosion. This compound can be eliminated by rapid cooling after heating at 750~850℃.

2) Heat treatment process

① Annealing

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

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

② Stress relief treatment

After welding and cold working, the parts may generate stress. If annealing treatment is not suitable for specific conditions, heating, heat preservation and air cooling can be performed in the range of 230~370℃, which can eliminate part of the internal stress and improve the plasticity.

2->Heat Treatment of Austenitic Stainless Steel

The effect of alloying elements such as Cr and Ni in austenitic stainless steel reduces the Ms point to below room temperature (-30 to -70 °C). To ensure the stability of the austenite structure, 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

Craft: Wide heating temperature range, 1000~1150℃, usually 1020-1080℃. Considering the specific brand composition, whether it is a casting or a forging, etc., within the allowable range, adjust the heating temperature appropriately. If the heating temperature is low, the C-Cr carbide cannot be fully dissolved, and if the temperature is too high, the grains will grow and the corrosion resistance will be reduced.

Cooling method: It should be cooled at a relatively fast speed to prevent the carbides from re-precipitating. In many national standards, it is indicated that "quick cooling" after solid solutionization, combining different literature and practical experience, the scale of "quick" can be grasped according to the following conditions，

- C content ≥ 0.08%; Cr content > 22%, high Ni content; C content < 0.08%, but effective size > 3mm, should be water-cooled;

- C content <0.08%, size <3mm, can be air-cooled;

- Air-cooled with effective size ≤0.5mm.

② Stabilization Heat Treatment of Austenitic Stainless Steel

Stabilization heat treatment is limited to austenitic stainless steels containing stabilizing elements Ti or Nb, such as 1Cr18Ni9Ti, 0Cr18Ni11Nb, etc.

Heating temperature: This temperature should be higher than the dissolution temperature of Cr23C6 (400-825℃), lower than or slightly higher than the initial dissolution temperature of TiC or NbC (for example, the dissolution temperature of TiC is in the range of 750-1120℃), and the stabilization heating temperature It is generally selected at 850-930 ° C, which will fully dissolve Cr23C6, so that Ti or Nb can be combined with C, while Cr continues to remain in austenite.

Cooling method: Air cooling is generally used, 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 little effect on the stabilization effect. 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. Compared with these, the metallographic structure, hardness and intergranular corrosion resistance are basically the same.

③ Stress Relief Treatment of Austenitic Stainless Steel

When conditions permit, the use of solid solution treatment and stabilization treatment can better eliminate stress (solid solution water cooling will also generate certain stress), but sometimes this method is not allowed, such as pipe fittings in the loop, no For finished parts with surplus, easily deformable parts with particularly complex shapes, etc., the stress relief method of heating at a temperature below 450 °C can be used at this time, and part of the stress can also be eliminated. 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 the use of steel containing stable elements, or the use of ultra-low carbon austenitic stainless steel.