Effect of nickel

1 Effect of nickel on tissue

Nickel is an element that strongly stabilizes austenite and enlarges the austenite phase. In order to obtain a single austenitic structure, the minimum nickel content required for steel containing 0.1% carbon and 18% chromium is about 8%. It is the basic score of the most famous 18-8 chromium-nickel austenitic stainless steel. In the austenitic stainless steel, with the increase of nickel content, the residual ferrite can be completely eliminated.

2 Effect of nickel on performance

The effect of nickel on the mechanical properties of austenitic stainless steels, especially chromium-nickel austenitic stainless steels, is mainly determined by the effect of nickel on the austenite stability, within the range of nickel content in which martensite transformation may occur in steel. As the nickel content increases, the strength of the steel decreases and the plasticity increases. The toughness (including the extremely low temperature toughness) of the chromium-nickel austenitic stainless steel having a stable austenite structure is excellent, and thus it can be used as a low-temperature steel, which is well known.

For chromium-manganese austenitic stainless steels with a stable austenitic structure, the addition of nickel further improves their toughness. Nickel can also significantly reduce the cold work hardening tendency of austenitic stainless steel, mainly due to the increased austenite stability, reducing or even eliminating the martensite transformation during cold working, and not the cold work hardening effect on austenite itself. Too obvious, the effect of cold working hardening tendency of stainless steel, nickel reduces the cold work hardening rate of austenitic stainless steel, lowers the room temperature and low temperature strength of steel, and improves the plasticity, which determines that the increase of nickel content is beneficial to the cold forming of austenitic stainless steel.

Performance, increased nickel content can also reduce or even eliminate delta ferrite in 18-8 and 17-14-2 type chromium-nickel austenitic stainless steel, thereby improving its hot workability, but the reduction of delta ferrite for these The weldability of steel grades is unfavorable, which increases the tendency of welding hot cracked filaments. In addition, nickel can significantly improve the hot workability of chromium manganese nitrogen (chromium manganese nickel nitrogen) austenitic stainless steel, thereby significantly increasing the yield of steel. In austenitic stainless steel, the addition of nickel and the increase of nickel content lead to an increase in the thermodynamic stability of steel, so austenitic stainless steel has better rust and resistance.

The performance of the medium, and with the increase of nickel content, the performance of the reduction-resistant medium is further improved. It is worth noting that nickel is the only important element to improve the austenitic stainless resistance to many media through-type stress corrosion. The effect of nickel on the corrosion resistance of austenitic stainless steel in acid medium, it should be pointed out that under some conditions of high temperature and high pressure water, the increase of nickel content leads to an increase in intergranular stress corrosion sensitivity of steel and alloy, but this adverse effect It will be alleviated or inhibited due to the increase of chromium content in steel and alloy. With the increase of nickel content in magnetic austenitic stainless steel, the critical carbon content of intergranular corrosion is reduced, that is, the intergranular corrosion sensitivity of steel increases. As for the pitting corrosion resistance and crevice corrosion resistance of austenitic stainless steel, the effect of nickel is not significant.

In addition, nickel also improves the high-temperature oxidation resistance of austenitic stainless steel, which mainly improves the composition of the chromium oxide film with nickel. The structure and properties are reduced, and the higher the nickel content, the more harmful, mainly due to the low melting point nickel sulfide at the grain boundary in steel. Generally, simple chromium Nickel (and chrome-manganese-nitrogen) austenitic stainless steel is only used in applications requiring rust and oxidation resistant media (such as nitric acid). Molybdenum is added to steel as an important alloying element in austenitic stainless steel. The scope of use is further expanded, and the role of molybdenum is mainly to improve steel in reducing media.