What are the heat treatment methods of stainless steel? Different types of stainless steel heat treatment methods are different?

What are the heat treatment methods of stainless steel Different types of stainless steel heat treatment methods are different

 

1 ferritic stainless steel

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

2 austenitic stainless steel

Contains a higher Cr, generally more than 18%, and contains about 8% of Ni, some to Mn instead of Ni, in order to further improve corrosion resistance, there are Mo, Cu, Si, Ti, Nb and other elements. No phase change occurs during heating and cooling, can not be strengthened by heat treatment, and has low strength, high plasticity and high toughness. It has strong corrosion resistance to oxidizing media, and has good intergranular corrosion resistance after adding Ti and Nb.

3 martensitic stainless steel

Martensitic stainless steel mainly contains 12~ 18% Cr, and according to the need to adjust the amount of C, generally in 0.1~ 0.4%, for the production of tools, C can reach 0.8~ 1.0%, some to improve tempering stability, add Mo, V, Nb and so on. After high temperature heating and cooling at a certain speed, the structure is basically martensite, depending on the difference of C and alloying elements, some may contain a small amount of ferrite, residual austenite or alloy carbide. Phase transitions occur when heated and cooled, so the structure and morphology of the organization can be adjusted over a wide range, thereby changing the performance. The corrosion resistance is not as good as austenite, ferrite and duplex stainless steel, and has good corrosion resistance in organic acids, and poor corrosion resistance in sulfuric acid, hydrochloric acid and other media.

4 ferritic-austenitic duplex stainless steel

Generally containing Cr is 17~ 30%, Ni content 3~ 13%, in addition to Mo, Cu, Nb, N, W and other alloying elements, C content control is very low, according to the proportion of alloying elements are different, some to ferrite based, some to austenite based, constituting two phases exist at the same time dual-phase stainless steel. Because it contains ferritic and strengthening elements, after heat treatment, the strength is slightly higher than that of austenitic stainless steel, and the plasticity and toughness are good, and basically the performance can not be adjusted by heat treatment means. It has high corrosion resistance, especially in the Cl-containing medium and seawater, and has good resistance to pitting corrosion, crevice corrosion and stress corrosion.

5 Precipitation hardening stainless steel

In addition to containing C, Cr, Ni and other elements, it also contains Cu, Al, Ti and other elements that can be aged precipitated. The mechanical properties can be adjusted by heat treatment, but the strengthening mechanism is different from that of martensitic stainless steel. Because it relies on precipitation phase strengthening, so C can be controlled very low, so its corrosion resistance is better than martensitic stainless steel, and Cr-Ni austenitic stainless steel.

Heat treatment of stainless steel

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

1 Heat treatment of ferritic stainless steel

Ferritic stainless steel is generally a stable single ferritic structure heating, cooling does not undergo phase change, so it can not be used to adjust the mechanical properties of heat treatment method, its main purpose is to reduce brittleness and improve resistance to intergranular corrosion.

①σ phase is brittle

Ferritic stainless steel is very easy to form the σ phase, which is a Cr-rich metal compound, hard and brittle, especially easy to form in the intergranular, making the steel brittle and increasing the intergranular corrosion sensitivity. The formation of σ phase is related to the composition. Except Cr, Si, Mn and Mo 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 ° C, which promotes the formation of σ phase. However, σ-phase formation is reversible, and reheating to a temperature higher than σ-phase formation will redissolve in the solid solution.

② Brittle at 475℃

Ferritic stainless steel is heated for a long time in the range of 400~500 ° C, which will show the characteristics of increased strength and decreased toughness, that is, increased brittleness, especially at 475 ° C, which is most obvious, called 475 ° C brittleness. This is because, at this temperature, the Cr atoms in the ferrite will be rearranged to form a Cr-rich small region, which is in common with the parent phase, causing lattice distortion, generating internal stress, and increasing the hardness and brittleness of the steel. At the same time of the formation of rich Cr zone, there must be poor Cr zone, which has an adverse effect on corrosion resistance. When the steel is reheated above 700 ° C, the distortion and internal stress will be eliminated, and the brittleness will disappear at 475 ° C.

③ High temperature brittleness

When heated to more than 925℃ and cooled down rapidly, Cr, C, N and other compounds precipitate in the crystal and the grain boundary, resulting in increased brittleness and intergranular corrosion. This compound can be eliminated by rapid cooling after heating at 750~850 ° C.