Stainless steel is often divided into martensitic stainless steel, ferrite stainless steel, austenitic stainless steel, austenitic ferrite duplex stainless steel and precipitated hardened stainless steel. Whether as stainless steel plate or heat-resistant steel plate, austenitic steel plate has the best comprehensive performance. It has sufficient strength, and excellent plasticity. This is one of the reasons why austenitic stainless steel is so widely adopted. Similar to most other metal materials, the tensile strength, yield strength and hardness of austenitic stainless steel increase with decreasing temperature. And plasticity decreases with decreasing temperature. The tensile strength increases uniformly in the range of 15 to 80°C. More importantly, the impact toughness decreases slowly with the decrease of temperature, and there is no brittle transition temperature. So stainless steel at low temperature can maintain enough plasticity and toughness. The heat resistance of stainless steel refers to the oxidation resistance or corrosion resistance of gas medium at high temperature. That is thermal stability. Austenitic stainless steel contains more than 18% chromium, but also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements.
Nickel is an important alloying element in austenitic stainless steel. Nickel has great influence on the microstructure and properties of austenitic stainless steel. Its main function is to stabilize austenite and make steel obtain complete austenite structure. This makes the steel get good strength, plasticity, hot and cold workability, cold formability, welding and no magnetic properties. At the same time, nickel can improve the thermodynamic stability of austenitic stainless steel. Compared with the chromium and molybdenum content of ferrite, martensitic and other stainless steel, nickel austenitic stainless steel has better rust resistance and oxidation resistance. And it can improve the stability of the surface film. This also makes the steel have more excellent resistance to reducing media properties.
1. Characteristics of Nickel
- Physical properties. Nickel is a silver-white metal with magnetic properties and good plasticity. Nickel is a malleable and ferromagnetic metallic element that is highly polished and resistant to corrosion. Dissolved in nitric acid, it appears green. It is used chiefly in alloys (e.g., nickel steel and nickel silver) and as a catalyst (e.g., laney nickel, especially as a catalyst for hydrogenation).
- Chemical properties: Nickel is insoluble in water, forming a dense oxide film on the surface of the humid air at room temperature, which can prevent the continued oxidation of the main metal. It can dissolve slowly in dilute acid and release hydrogen gas to produce green Ni-divalent ion Ni (strong alkali resistance). Nickel can be burned in pure oxygen, producing a brilliant white glow. Nickel can also be burned in chlorine and fluorine. The oxidizing solution, including nitric acid, does not react. Nickel is a moderate-strength reducing agent. Nickel is slowly corroded by hydrochloric acid, sulfuric acid, organic acid and alkaline solution. Nickel dissolves slowly in dilute nitric acid. Fuming nitric acid can passivate nickel surface and has corrosion resistance. Nickel, like platinum and palladium, can absorb a large amount of hydrogen during passivation. The smaller the particle size is, the greater the absorption will be.
2. Influence of Nickel on Stainless Steel Structure
Nickel is an element that strongly stabilizes austenite and expands the austenitic phase region. 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%. This is the most famous 18-8 chromium nickel austenitic stainless steel basic fraction. In austenitic stainless steel, with the increase of nickel content, the residual ferrite can be completely eliminated. With the increase of nickel content, the solubility of carbon in austenitic stainless steel will be reduced and the tendency of carbide precipitation will be enhanced.
3. Influence of Nickel on the Properties of Stainless Steel
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 austenitic stability. In the range of nickel content in steel where martensitic transformation may occur, the strength of steel decreases and the plasticity increases with the increase of nickel content. Chromium-nickel austenite has stable austenitic structure, and its stainless steel toughness (including extremely low temperature toughness) is very good, so it can be used as low temperature steel. This is well known. The toughness of chromium manganese austenitic stainless steel with stable austenitic structure can be further improved by adding nickel. Nickel can also significantly reduce the cold working hardening tendency of austenitic stainless steel. The stability of austenite increases, reduces or even eliminates the martensitic transformation in the cold working process. And the cold working hardening of austenite itself is not obvious. When talking about the influence of cold working hardening tendency of stainless steel, nickel reduces the cold working hardening rate of austenitic stainless steel, and reduces the strength of steel at room temperature and low temperature. It also improves the role of plasticity. This determines that the increase of nickel content is conducive to the cold working formability of austenitic stainless steel. The increase of nickel content can also reduce or eliminate the δ ferrite in 18-8 and 17-14-2 chromium nickel austenitic stainless steel, thus improving its hot working performance. However, the decrease of δ ferrite is detrimental to the weld ability of these steels and increases the tendency to weld hot crack wires. In addition, nickel can significantly improve the hot working properties of chromium-manganese-nickel-nitrogen austenitic stainless steel, thus significantly increasing the yield of steel. In austenitic stainless steel, the addition of nickel and the increase of its content lead to the increase of thermodynamic stability. Therefore, austenitic stainless steel has better rust resistance and oxidation resistance of the medium performance. With the increase of nickel content, the resistance of austenitic stainless steel to reducing medium is further improved. It is worth pointing out that nickel is also the only important element to improve the resistance of austenitic stainless steel to trans-granular stress corrosion in many media. It should also be pointed out that under some conditions of high temperature and high pressure water, the increase of nickel content leads to the increase of the inter-granular stress corrosion sensitivity of steels and alloys. However, this adverse effect can be alleviated or inhibited with the increase of chromium content in steel and alloy. With the increase of nickel content in austenitic stainless steel, the critical carbon content of inter-granular corrosion decreases. That is the sensitivity of the steel to inter-granular corrosion increases. Nickel has no significant effect on the resistance of austenitic stainless steel to spot corrosion and slot corrosion. In addition, nickel can improve the oxidation resistance of austenitic stainless steel at high temperature. This mainly improved the composition of the chromium oxide film with nickel. In general, simple chromium nickel (and chromium manganese nitrogen) austenitic stainless steel is only used under conditions requiring non-rust and oxidation resistant media (such as nitric acid).
