What impact does the nickel element have on stainless steel pipes

Nickel is the main alloying element in austenitic stainless steel. Its main function is to stabilize austenite, enabling the steel to obtain a fully austenitic structure, thereby giving the steel excellent strength, plasticity, and toughness in combination, as well as excellent cold and hot processability, cold forming properties, welding, low-temperature and non-magnetic properties, and improving the thermodynamic stability of austenitic stainless steel, making it not only have better corrosion resistance and resistance to oxidizing media than the ferritic, martensitic and other types of stainless steel with the same chromium and molybdenum content, but also enhance the stability of the surface film, thus making the steel have even more excellent resistance to some reducing media. 

1. The influence of nickel on the structure

Nickel is an element that strongly stabilizes austenite and expands the austenite phase region. To obtain a single austenite structure, the minimum nickel content required when the steel contains 0.1% carbon and 18% chromium is approximately 8%. This is the basic composition of the most famous 18-8 chromium-nickel austenitic stainless steel. In austenitic stainless steel, as the nickel content increases, the residual ferrite can be completely eliminated and the tendency for σ phase formation is significantly reduced; at the same time, the transformation temperature of martensite decreases, and even the λ→M phase transformation may not occur. However, an increase in nickel content will reduce the solubility of carbon in austenitic stainless steel, thereby enhancing the tendency for carbide precipitation. 

2. The influence of nickel on performance

The influence of nickel on the mechanical properties of austenitic stainless steel, especially chromium-nickel austenitic stainless steel. This is mainly determined by the effect of nickel on the stability of austenite. Within the nickel content range where martensitic transformation may occur in the steel, as the nickel content increases, the strength of the steel decreases while the plasticity increases. The toughness (including extremely low-temperature toughness) of chromium-nickel austenitic stainless steel with stable austenite structure is very excellent, and thus it can be used as a low-temperature steel. This 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, mainly due to the increase in austenite stability, which reduces or eliminates the martensitic transformation during the cold processing. At the same time, the cold work hardening effect on the austenite itself is not very obvious. The effect of nickel is to reduce the cold work hardening rate of austenitic stainless steel and to reduce the room temperature and low-temperature strength of the steel, while enhancing the plasticity. This determines that increasing the nickel content is beneficial to the cold working forming performance of austenitic stainless steel, and increasing the nickel content can also reduce or eliminate δ ferrite in 18-8 and 17-14-2 type chromium-nickel austenitic stainless steel, thereby improving its hot working performance. However, the reduction of δ ferrite is detrimental to the weldability of these steel types and increases the tendency of welding cracks. In addition, nickel can significantly improve the hot working performance of chromium-manganese-nitrogen (chromium-manganese-nitrogen) austenitic stainless steel, thereby significantly increasing the steel's 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 corrosion resistance to many medium intergranular stress corrosion and better resistance to oxidizing media. And with the increase in nickel content, the performance in reducing-reductive media is further improved. It is worth noting that nickel is the only important element for improving the corrosion resistance of austenitic stainless steel to many media. 

The influence of nickel on the corrosion resistance of austenitic stainless steel in various acidic media needs to be pointed out. Under certain conditions in high-temperature and high-pressure water, an increase in nickel content leads to an increase in intergranular stress corrosion sensitivity of the steel and alloy. However, this adverse effect can be alleviated or suppressed due to the increase in chromium content in the steel and alloy. As the nickel content in magnetic ferritic austenitic stainless steel increases, the critical carbon content for intergranular corrosion decreases, meaning the intergranular corrosion sensitivity of the steel increases. As for the performance of austenitic stainless steel in resisting pitting corrosion and crevice corrosion, the effect of nickel is not significant. Moreover, nickel also enhances the high-temperature oxidation resistance of austenitic stainless steel, which is mainly due to the fact that nickel improves the composition, structure and performance of the chromium oxide film, and the higher the nickel content, the more harmful it is. This is mainly because the 1 million low-melting-point nickel sulfide at the grain boundaries in the steel is the cause.