The Mechanical Properties of Stainless Steel

The mechanical properties of materials refer to the mechanical characteristics that materials exhibit under different environmental conditions (temperature, medium, humidity) when subjected to various external loads (tensile, compressive, bending, torsional, impact, alternating stress, etc.).

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 metal elements. Different types of stainless steel have different strength characteristics due to their differences in chemical composition.

01 Strength (tensile strength, yield strength)

Martensitic stainless steel

Like common alloy steel, martensitic stainless steel has the characteristic of hardening through quenching, so it can have a wide range of different mechanical properties by selecting the grade and heat treatment conditions.

In terms of broad classification, martensitic stainless steels belong to the iron-chromium-carbon stainless steel system. They can be further divided into martensitic chromium stainless steels and martensitic chromium-nickel stainless steels. The trend of strength changes when chromium, carbon, and molybdenum elements are added to the martensitic chromium stainless steels, and the strength characteristics when nickel is added to the martensitic chromium-nickel stainless steels are as follows.

In the quenching-tempering condition of martensitic chromium stainless steel, increasing the chromium content can increase the amount of ferrite, thus lowering the hardness and tensile strength. In the annealed condition of low-carbon martensitic chromium stainless steel, when the chromium content is increased, the hardness is improved slightly, while the elongation decreases slightly. In the condition where the chromium content is constant, increasing the carbon content makes the steel harder after quenching, while reducing its plasticity. The main purpose of adding molybdenum is to improve the steel's strength, hardness, and secondary hardening effect. The addition effect is very obvious after low-temperature quenching. The content is usually less than 1%.

In martensitic chromium-nickel stainless steels, a certain amount of nickel can reduce the amount of delta ferrite in the steel, resulting in the highest hardness value.

The chemical composition feature of martensitic stainless steel is that it is based on different composition combinations of 0.1%-1.0%C, 12%-27%Cr, and additions of elements such as molybdenum, tungsten, vanadium, and niobium. Due to its body-centered cubic structure, its strength drops sharply at high temperatures. However, its high-temperature strength is the highest among all stainless steels below 600℃, and its creep strength is also the highest.

Ferritic stainless steel

According to research results, when the chromium content is less than 25%, the ferrite structure will inhibit the formation of martensite structure, and therefore its strength decreases with the increase of chromium content; When it exceeds 25%, the strength slightly increases due to the solid solution strengthening effect of the alloy. The increase in molybdenum content can make it easier to obtain ferrite structure, promote the precipitation of α 'phase, σ phase and χ phase, and improve its strength after solid solution strengthening. But at the same time, it also increases the sensitivity to gaps, thereby reducing toughness. The effect of molybdenum on improving the strength of ferritic stainless steel is greater than that of chromium.

The chemical composition of ferritic stainless steel is characterized by containing 11% -30% Cr, with the addition of niobium and titanium. Its high-temperature strength is the lowest among all types of stainless steel, but it has the strongest resistance to thermal fatigue.

Austenitic stainless steel

By increasing the carbon content in austenitic stainless steel, its strength is improved due to its solubility strengthening effect.

The chemical composition characteristics of austenitic stainless steel are based on chromium and nickel, with the addition of molybdenum, tungsten, niobium, and titanium, etc. Due to its face-centered cubic structure, it has high strength and creep strength at high temperatures. However, it has poor thermal fatigue strength compared to ferritic stainless steel due to its large coefficient of linear expansion.

The study of the mechanical properties of dual-phase stainless steel with a chromium content of approximately 25% shows that as the nickel content increases in the α+γ dual-phase region, the γ phase also increases. When the chromium content 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.

Duplex stainless steel

The study on the mechanical properties of duplex stainless steel with a chromium content of about 25% shows that as the nickel content increases in the α+γ duplex region, the γ phase also increases. When the chromium content in steel is 5%, the yield strength of steel reaches its highest value; When the nickel content is 10%, the strength of the steel reaches its maximum value.

02 Creep Strength

The phenomenon in which a force deforms over time due to the action of an external force is called creep. In particular, the greater the load, the faster the creep occurs at a certain temperature, especially at high temperatures. At a certain load, the higher the temperature and the longer the time, the greater the likelihood of creep occurring. Conversely, the slower the creep occurs at lower temperatures, and creep is not a problem at a certain lower temperature. The lowest temperature varies depending on the steel grade, and in general, pure iron is around 330℃, while stainless steel has been strengthened through various measures, so the temperature is above 550℃.

Like other steels, the melting method, deoxidation method, solidification method, heat treatment, and processing all have a significant impact on the creep characteristics of stainless steel. According to reports, in the creep strength test conducted on an 18-8 stainless steel in the United States, the standard deviation of the creep rupture time of the test specimens from the same billet and the same position was about 11% of the average value, while the standard deviation of the test specimens from different billets and from different upper, middle, and lower positions was more than twice the average value. According to the test results in Germany, the strength of 0Cr18Ni11Nb steel at 105h is less than 49MPa to 118MPa, with a large scatter.

03Fatigue Strength

High-temperature fatigue refers to the process in which a material is damaged and eventually ruptures due to the cyclic and alternating stress at high temperatures. The results of research into this phenomenon show that at a certain high temperature, the 108-cycle high-temperature fatigue strength is half of the high-temperature tensile strength at that temperature.

Thermal fatigue refers to the process wherein when a material is subjected to heating (expansion) and cooling (contraction) cycles, internal stresses corresponding to its own expansion and contraction deformation arise and cause damage to the material. When rapid repeated heating and cooling occurs, the stresses are of an impact nature and are greater than those in normal conditions. In such cases, some materials may exhibit brittle failure. This phenomenon is referred to as thermal shock. Thermal fatigue and thermal shock are phenomena with similarities, but the former is primarily associated with large plastic deformation, while the latter's failure is primarily brittle.

The composition and heat treatment conditions of stainless steel have an impact on its high-temperature fatigue strength. In particular, when the carbon content increases, the high-temperature fatigue strength improves significantly. The solid solution heat treatment temperature also has a significant impact. Generally, austenitic stainless steels have good thermal fatigue properties. In the austenitic stainless steels, the grades with high silicon content and good tensile properties at high temperatures have good thermal fatigue properties.

The longer the service life, the smaller the coefficient of thermal expansion, the smaller the strain under the same thermal cycle, the lower the deformation resistance, and the higher the fracture strength. It can be said that the fatigue life of martensitic stainless steel 1Cr17 is the longest, while the fatigue life of austenitic stainless steels such as 0Cr19Ni9, 0Cr23Ni13, and 2Cr25Ni20 is the shortest.

Additionally, castings are more susceptible to thermal fatigue-induced failure than forged parts.

At room temperature, the fatigue strength is 1/2 of the tensile strength. Compared with the fatigue strength at high temperatures, it can be seen that the fatigue strength from room temperature to high temperature has little difference.

04 Impact Toughness

The area under the load deformation curve under impact load is called impact toughness. For cast martensitic age-hardening stainless steel, its impact toughness is low when the nickel content is 5%. With the increase of nickel content, the strength and toughness of the steel can be improved, but when the nickel content exceeds 8%, the strength and toughness values decrease again. Adding molybdenum to the martensitic chromium nickel stainless steel can improve the strength of the steel while maintaining the toughness unchanged.

In ferritic stainless steels, increasing the content of molybdenum can improve strength, but the sensitivity to notching is also increased, resulting in a decrease in toughness.

In austenitic stainless steels, the toughness (toughness at room temperature and toughness at low temperatures) of chromium-nickel-based austenitic stainless steels is very good, making them suitable for use in various environments at room temperature and low temperatures. For chromium-manganese-based austenitic stainless steels with stable austenitic structure, adding nickel can further improve their toughness.

The impact toughness of duplex stainless steel increases with the increase of nickel content. Generally, its impact toughness is stable within the range of 160-200J in the a+r two-phase region.