What effect does nickel have on austenitic stainless steel?

               
Update: 15-06-2020
               
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The influence of nickel on the mechanical properties of […]

The influence 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 nickel content range in which martensite transformation may occur in steel, With the increase of nickel content, the strength of steel decreases and the plasticity increases. The toughness (including extremely low temperature toughness) of chromium nickel austenitic stainless steel with stable austenite structure is very good, so it can be used as low temperature steel, which is well known. For chromium manganese austenitic stainless steel with stable austenite structure, the addition of nickel can further improve its toughness. Nickel can also significantly reduce the cold work hardening tendency of austenitic stainless steel. This is mainly due to the increased stability of austenite, which reduces or even eliminates the martensite transformation during cold work, and does not affect the cold work hardening of austenite itself. Too obvious, the effect of cold work hardening tendency of stainless steel, nickel reduces the cold work hardening rate of austenitic stainless steel, and reduces the room temperature and low temperature strength of steel, and improves the plasticity, which determines that the increase in nickel content is beneficial to the cold forming of austenitic stainless Performance, improving nickel content can also reduce or eliminate δ ferrite in 18-8 and 17-14-2 chromium-nickel austenitic stainless steels, thereby improving its hot workability.

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However, the reduction of delta ferrite is detrimental to the weldability of these steel types and will increase the tendency of welding hot cracked wire. In addition, nickel can significantly improve the hot workability of chromium manganese nitrogen (chromium manganese nickel nitrogen) austenitic stainless steel , Which significantly improves the steel yield rate. In austenitic stainless steel, the addition of nickel and the increase in nickel content lead to an increase in the thermodynamic stability of the steel. Therefore, austenitic stainless steel has better stainlessness and oxidation resistance Performance, and with the increase of nickel content, the performance of reduction resistance media is further improved. It is worth pointing out that nickel is the only important element that improves the resistance of austenitic stainless to many media through crystal stress corrosion. The influence of nickel in the medium on the corrosion resistance of austenitic stainless steels needs to be pointed out that under some conditions of high temperature and high pressure water, the increase in nickel content leads to increased sensitivity to intergranular stress corrosion of steel and alloys, but this adverse effect will Due to the increase in the content of chromium in steel and alloys, it is alleviated or suppressed.

With the increase in the content of nickel in magnetic austenitic stainless steel, the critical carbon content of intergranular corrosion decreases, that is, the sensitivity of intergranular corrosion of steel increases, as The effect of nickel on pitting and crevice corrosion resistance of austenitic stainless steel is not significant. In addition, nickel also improves the high temperature oxidation resistance of austenitic stainless steel. This is mainly due to the improvement of the composition and structure of the chromium oxide film of nickel. And the performance is reduced, and the higher the nickel content, the more harmful it is. This is mainly due to the low melting point nickel sulfide at the grain boundaries in the steel. In general, simple chromium nickel (and chromium manganese nitrogen) austenitic stainless steels are only used for requirements Under the conditions of use of stainless and oxidizing resistance media (such as nitric acid, etc.), molybdenum is added to the steel as an important alloying element in austenitic stainless steel to further expand the scope of use. The role of molybdenum is mainly to improve the reducibility of steel. medium.

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