Chemical composition has a great influence on the microstructure, mechanical properties, physical properties and corrosion resistance of steel. Chromium, molybdenum, nickel and other alloying elements can replace the vertex Angle of the austenite lattice and the center of the six sides of the cube iron, carbon and nitrogen are located in the gap between the lattice atoms (gap position) due to small volume, produce huge strain in the lattice, so become effective hardening elements. Different alloying elements have different effects on the properties of steel, sometimes beneficial and sometimes harmful. The main alloying elements of Austenitic stainless steel have the following effects:
Chromium is an alloying element that makes stainless steel “rust free”. At least 10.5% chromium is required to form the surface passivation film characteristic of stainless steel. The passivation film can make stainless steel effectively resist corrosive water, a variety of acid solutions and even strong oxidation of high-temperature gas corrosion. When the chromium content exceeds 10.5%, the corrosion resistance of stainless steel is enhanced. The chromium content of 304 stainless steel is 18%, and some high-grade Austenitic stainless steels have chromium content as high as 20% to 28%.
Nickel can form and stabilize the Austenitic phase. 8%Ni makes 304 stainless steel, giving it the mechanical properties, strength and toughness required by austenite. High-performance austenitic stainless steels contain high concentrations of chromium and molybdenum, and nickel is added to maintain the austenitic structure when more chromium or other ferrite forming elements are added to the steel. The austenite structure can be guaranteed by about 20% nickel content, and the stress corrosion fracture resistance of stainless steel can be greatly improved.
Nickel can also reduce the work hardening rate during cold deformation, so the alloys used for deep drawing, spinning and cold heading generally have a high nickel content.
Molybdenum improves the pitting and crevice corrosion resistance of stainless steel in a chloride environment. The combination of molybdenum and chromium, especially nitrogen, makes the high-performance austenitic stainless steel have strong resistance to pitting and crevice corrosion. Mo can also improve the corrosion resistance of stainless steel in reductive environments such as hydrochloric acid and dilute sulfuric acid. The minimum molybdenum content of Austenitic stainless steel is about 2%, such as 316 stainless steel. High-performance Austenitic stainless steels with the highest alloy content contain up to 7.5% molybdenum. Molybdenum contributes to the formation of the Ferrite phase and affects the phase equilibrium. It is involved in the formation of several harmful secondary phases and will form unstable high-temperature oxides, have a negative impact on high-temperature oxidation resistance, the use of molybdenum-containing stainless steel must be taken into account.
Carbon stabilizes and strengthens the Austenitic phase. Carbon is a beneficial element for stainless steel used in high temperature environments such as boiler tubes, but in some cases can have a detrimental effect on corrosion resistance. The carbon content of most Austenitic stainless steel is usually limited to the lowest practicable level. The carbon content of welding grades (304L, 201L and 316L) is limited to 0.030%. The carbon content of some high alloy high-performance grades is even limited to 0.020%.
Nitrogen stabilizes and strengthens the Austenite phase, and slows down carbide sensitization and secondary phase formation. Both standard austenitic stainless steels and high performance austenitic stainless steels contain nitrogen. In low carbon grade (L), a small amount of nitrogen (up to 0.1%) can compensate for the loss of strength due to low carbon content. Nitrogen also helps improve resistance to chloride pitting and crevice corrosion, so some of the best corrosion-resistant high-performance austenitic stainless steels have nitrogen content as high as 0.5%.
Steel mills use manganese to deoxidize molten steel, so a small amount of manganese remains in all stainless steel. Manganese can also stabilize the Austenitic phase and improve the solubility of nitrogen in stainless steel. Therefore, in 200 series stainless steel, manganese can be used to replace part of the nickel to increase the nitrogen content, improve the strength and corrosion resistance. Manganese is added to some high-performance Austenitic stainless steels to achieve the same effect.
Copper can improve the corrosion resistance of stainless steel in reducing acids, such as some mixed solutions of sulfuric and phosphoric acid.
In general, silicon is a beneficial element in Austenitic stainless steel because it can improve the corrosion resistance of steel in concentrated acid and a high oxidation environments. It is reported that UNS S30600 and other high silicon special stainless steels have high pitting corrosion resistance. Silicon, like manganese, can also be used to deoxidize molten steel, so small oxide inclusions containing silicon, manganese and other deoxidizing elements always remain in steel. But too many inclusions will affect the surface quality of the product.
Nb and Ti
These two elements are strong carbide-forming elements and can be used in place of low carbon grades to mitigate sensitization. Niobium carbide and titanium carbide can improve the high-temperature strength. 347 and 321 stainless steels containing Nb and Ti are commonly used in boilers and refining equipment to meet high temperature strength and weldability requirements. They are also used in some deoxidation processes as residual elements in high performance Austenitic stainless steels.
S and P
Sulfur is both good and bad for stainless steel. It can improve the machining performance, the harm is to reduce the thermal workability, increase the number of manganese sulfide inclusion, resulting in stainless steel pitting corrosion resistance reduced. High-grade Austenitic stainless steel is not easy to heat process, so the sulfur content should be controlled at the lowest level as far as possible, about 0.001%. Sulfur is not normally added as an alloying element to high-performance austenitic stainless steels. However, the sulfur content of standard grade stainless steel is often high (0.005% ~ 0.017%), in order to improve the weld penetration depth of self-fusion welding, improve cutting performance.
Phosphorus is a harmful element and can adversely affect the hot working properties of forging and hot rolling. In the cooling process after welding, it will also promote the occurrence of thermal cracking. Therefore, phosphorus content should be controlled at a minimum level.