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Effect of nickel in stainless steel

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Effect of nickel in stainless steel

 

Nickel is the main alloy element in austenitic stainless steel. Its main function is to strengthen and stabilize austenite, so that the steel can obtain complete austenite structure, so that the steel has a good combination of strength, plasticity and toughness, and has excellent cold and hot workability. At the same time, it can improve the thermodynamic stability of austenitic stainless steel, which is not only higher than ferrite and martensite with the same content of chromium and molybdenum Class stainless steel has better rust resistance and oxidation resistance of medium Performance. Moreover, due to the stability of the surface film, the steel has better resistance to some reducing media.

 

In order to obtain a single austenite structure, when the steel contains 0.1% carbon and 18% chromium, the ultra-low nickel content is about 8%, which is 18-8 chromium nickel austenitic stainless steel. In austenitic stainless steel, with the increase of nickel content, the residual ferrite can be completely eliminated and decreased significantly. However, the solubility of carbon in austenitic stainless steel will decrease with the increase of nickel content, and the Precipitation Tendency of carbides will be enhanced.

 

The effect of nickel on the mechanical properties of austenitic stainless steel, especially chromium nickel austenitic stainless steel, is mainly determined by the influence of nickel on the stability of austenite. Within the range of nickel content in steel where martensitic transformation may occur, the strength and plasticity of steel decrease with the increase of nickel content. The toughness of chromium nickel austenitic stainless steel with stable austenite structure (including extremely low temperature toughness )It is very good and can be used as low temperature steel It is well known that the addition of nickel can further improve the toughness of chromium manganese austenitic stainless steel with stable austenite structure.

 

 

Nickel can also significantly reduce the cold work hardening tendency of austenitic stainless steel, which is mainly due to the increase of Austenite Stability, reduction or even elimination of martensitic transformation during cold working. At the same time, the cold work hardening effect of austenite itself is not obvious, and the influence of cold work hardening tendency of stainless steel is that nickel reduces the cold work hardening rate of austenitic stainless steel and reduces the room temperature of steel And low temperature strength, improve plasticity, determine the nickel content Increasing the content of nickel can reduce or even eliminate 8 ferrite in 18-8 and 17-14-2 type chromium nickel austenitic stainless steel, so as to improve its hot working performance. However, the decrease of 8 ferrite is unfavorable to the weldability of these steels, which will increase the tendency of welding hot crack wire. In addition, nickel can significantly improve the Cr Mn n (Cr Mn Ni n) )The hot working properties of austenitic stainless steel, and The yield of steel is increased significantly.

 

In austenitic stainless steel, the addition of nickel and the increase of nickel content lead to the increase of thermodynamic stability of steel. Therefore, austenitic stainless steel has better corrosion resistance and oxidation resistance medium performance, and with the increase of nickel content, the performance of reducing medium is further improved. It is worth pointing out that nickel is still a rare and important element to improve austenitic stainless steel's resistance to transgranular stress corrosion in many media.

 

The effect of nickel on the corrosion resistance of austenitic stainless steel in various acid media needs to be pointed out that under some conditions of high temperature and high pressure water, the increase of nickel content leads to the increase of intergranular stress corrosion sensitivity of steel and alloy, but this adverse effect will be reduced or restrained due to the increase of chromium content in steel and alloy. With the increase of nickel content in austenitic stainless steel, the critical carbon content for intergranular corrosion of austenitic stainless steel decreases, that is, the sensitivity of steel to intergranular corrosion increases. As for the pitting corrosion resistance and crevice corrosion resistance of austenitic stainless steel, nickel has no significant effect.

 

In addition, nickel also improves the high temperature oxidation resistance of austenitic stainless steel, which is mainly due to the improvement of the composition, structure and properties of chromium oxide film. The higher the nickel content, the more harmful it is. This is mainly due to the low melting point nickel sulfide at the grain boundary in steel. Generally speaking, simple chromium nickel (and chromium manganese nitrogen) austenitic stainless steel is only used for medium requiring rust resistance and oxidation resistance( Such as nitric acid).

 

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