1. Strength (tensile strength, yield strength)
The strength of stainless steel is determined by various factors, but the most important and fundamental factor is the different chemical elements added to it, mainly metallic elements. Different types of stainless steel have different strength properties due to differences in their chemical compositions.
(1) Martensitic stainless steel
Martensitic stainless steel has the same characteristics of hardening through quenching as ordinary alloy steel, and has the same mechanical properties as stainless steel channel steel. Therefore, a wide range of different mechanical properties can be obtained by selecting the grade and heat treatment conditions.
Martensitic stainless steel is classified from a large perspective and belongs to the iron-chromium-carbon series stainless steel. It can be further divided into martensitic chromium series stainless steel and martensitic chromium-nickel series stainless steel. The change trend in strength when elements such as chromium, carbon, and molybdenum are added to martensitic chromium-based stainless steel and the strength characteristics of martensitic chromium-nickel-based stainless steel when nickel is added are as follows.
Under quenching-tempering conditions of martensitic chromium stainless steel, increasing the chromium content can increase the ferrite content, thus reducing the hardness and tensile strength. Under annealing conditions, the hardness of low-carbon martensitic chromium stainless steel increases when the chromium content increases, while the elongation decreases slightly. Under the condition of a certain chromium content, the increase in carbon content will increase the hardness of the steel after quenching, while the plasticity will decrease. The main purpose of adding molybdenum is to improve the strength, hardness and secondary hardening effect of steel. After low-temperature quenching, the effect of molybdenum addition is very obvious. Content is usually less than 1%.
In martensitic chromium-nickel stainless steel, a certain amount of nickel can reduce the delta ferrite content in the steel, allowing the steel to obtain the maximum hardness value.
The chemical composition of martensitic stainless steel is characterized by adding elements such as molybdenum, tungsten, vanadium and niobium based on different composition combinations of 0.1%-1.0%C and 12%-27%Cr. Since the tissue structure is a body-centered cubic structure, its strength drops sharply at high temperatures. Below 600°C, the high temperature strength is the highest among all types of stainless steel, and the creep strength is also the highest.
(2) Ferritic stainless steel
According to research results, when the chromium content is less than 25%, the ferrite structure will inhibit the formation of the martensite structure, so the strength decreases as the chromium content increases; when it is higher than 25%, the strength is slightly lower due to the solid solution strengthening effect of the alloy. improve. The increase in molybdenum content can make it easier to obtain a ferrite structure, promote the precipitation of α' phase, σ phase and χ phase, and improve its strength after solid solution strengthening. But it also increases notch sensitivity, thereby reducing toughness. Molybdenum improves the strength of ferritic stainless steel more than chromium.
The chemical composition of ferritic stainless steel is characterized by containing 11%-30% Cr, with niobium and titanium added. Its high-temperature strength is the lowest among all types of stainless steel, but its resistance to thermal fatigue is the strongest.
(3) Austenitic stainless steel
After increasing the carbon content in austenitic stainless steel, the strength is improved due to its solid solution strengthening effect.
The chemical composition characteristics of austenitic stainless steel are based on chromium and nickel with elements such as molybdenum, tungsten, niobium and titanium added. Because its structure is a face-centered cubic structure, it has high strength and creep strength at high temperatures. Also due to the large linear expansion coefficient, the thermal fatigue strength is worse than that of ferritic stainless steel.
(4) Duplex stainless steel
Research on the mechanical properties of duplex stainless steel with a chromium content of approximately 25% shows that the γ phase also increases when the nickel content increases in the α+γ duplex zone. When the chromium content in the steel is 5%, the yield strength of the steel reaches its maximum value; when the nickel content is 10%, the strength of the steel reaches its maximum value.
