High-temperature fatigue refers to the process in which materials are damaged to fracture due to the action of periodically changing stress at high temperatures. The results of the research conducted on it show that at a certain high temperature, the high-temperature fatigue strength of 10 to the 8th power is 1/2 of the high-temperature tensile strength at that temperature.

Thermal fatigue refers to the process of heating (expansion) and cooling (contraction). When the temperature changes and is subject to external restraints, stress is generated inside the material corresponding to its own expansion and contraction deformation, and causes Material damage occurs. When heated and cooled repeatedly rapidly, the stress becomes impactful, and the stress generated is greater than usual. At this time, some materials show brittle failure. This phenomenon is called a shock. Thermal fatigue and thermal shock are similar phenomena, but the former is mainly accompanied by large plastic strains, while the damage of the latter is mainly brittle failure.

The composition and heat treatment conditions of stainless steel have an impact on high temperature fatigue strength. Especially when the carbon content increases, the high-temperature fatigue strength increases significantly, and the solution heat treatment temperature also has a significant impact. Generally speaking, ferritic stainless steel has good thermal fatigue properties. Among austenitic stainless steels, grades that are high in silicon and have good ductility at high temperatures have good thermal fatigue properties.

The smaller the thermal expansion coefficient, the smaller the strain under the same thermal cycle, the smaller the deformation resistance and the higher the fracture strength, the longer the life. It can be said that the fatigue life of martensitic stainless steel 1Cr17 is long, while the fatigue life of austenitic stainless steel such as 0Cr19Ni9, 0Cr23Ni13 and 2Cr25Ni20 is short. In addition, castings are more susceptible to damage caused by thermal fatigue than forgings. At room temperature, the fatigue strength of 10 to the 7th power is 1/2 the tensile strength. Compared with the fatigue strength at high temperatures, it can be seen that there is not much difference in fatigue strength in the temperature range from room temperature to high temperature.


Impact toughness

When a material is subjected to impact load, the area included in the load deformation curve is called impact toughness. For cast maraging stainless steel, the impact toughness is lower when the nickel content is 5%. As the nickel content increases, the strength and toughness of steel can be improved, but when the nickel content is greater than 8%, the strength and toughness values decrease again. Adding molybdenum to martensitic chromium-nickel stainless steel can increase the strength of the steel while maintaining its toughness.

Although increasing the molybdenum content in ferritic stainless steel can increase the strength, the notch sensitivity is also increased and the toughness is reduced.

Austenitic stainless steel has a stable austenite structure and the toughness (toughness at room temperature and low temperature toughness) of chromium-nickel austenitic stainless steel is very excellent, so it is suitable for use in various environments at room temperature and low temperature. . For stable austenite structure and chromium-manganese austenitic stainless steel. Adding nickel further improves its toughness.

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