The development of stainless steel is due to its own characteristics, and the characteristics meet the needs. The important characteristic of stainless steel is corrosion resistance, but it is by no means just corrosion resistance, but also has unique mechanical properties (yield strength, tensile strength, creep strength, high temperature strength, low temperature strength, etc.), physical properties ( Density, specific heat capacity, linear expansion coefficient, thermal conductivity, resistivity, magnetic permeability, elastic coefficient, etc.), process performance (forming performance, welding performance, cutting performance, etc.) and metallography (phase composition, organizational structure, etc.), etc. These properties constitute the characteristics of stainless steel. Only some of the basic characteristics are briefly introduced below.


Strength (tensile strength, yield strength)

The strength of stainless steel is determined by various factors, but the important and basic factor is the different chemical factors 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, so 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 nickel added to martensitic chromium-based stainless steel 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 and give the steel a high 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 high.

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, φ phase and x 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 various types of stainless steel, but its resistance to thermal fatigue is strong.


  (2)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.


  (3)Duplex stainless steel

Research on the mechanical properties of duplex stainless steel with a chromium content of approximately 25% shows that the r phase also increases when the nickel content increases in the α+r duplex zone. When the chromium content in the steel is 5%, the yield strength of the steel reaches a high value; when the nickel content is 10%, the strength of the steel reaches a maximum value.


Creep strength

The phenomenon of deformation due to the increase of external force over time is called creep. At a certain temperature, especially at high temperatures, the greater the load, the faster the creep will occur; under a certain load, the higher the temperature and the longer the time, the greater the possibility of creep. On the contrary, the lower the temperature, the slower the creep rate. Creep is not a problem down to a certain temperature. This low temperature varies depending on the type of steel. Generally speaking, pure iron is around 330°C, while stainless steel has been strengthened through various measures, so the temperature is above 550°C.

Like other steels, the smelting method, deoxidation method, solidification method, heat treatment and processing have a great impact on the creep characteristics of stainless steel. According to reports, creep strength tests on 18-8 stainless steel conducted in the United States showed that the standard deviation of the creep rupture time of samples taken from the same part of the same steel ingot was about 11% of the average value, while those taken from different steel ingots The standard deviation of the samples at different parts of the upper, middle and lower parts differs from the average value by as much as twice. According to the test results conducted in Germany, the strength of 0Cr18Ni11Nb steel under the time of 10 to the 5th power h is less than 49MPa to 118MPa, and the dispersion is very large.