Effect of Multiple Quenching and Tempering on SCM440 Steel Properties

Effect of Multiple Quenching and Tempering on SCM440 Steel Properties

Due to the harsh work environment of marine machinery, it is often used at low temperatures (≤ -20 °C), and at the same time it is subjected to very large loads, which imposes certain requirements on the low-temperature impact properties of material. When the total surface area of the grain boundary increases, the concentration of impurity segregation at the grain boundary will decrease, the harmful effect will decrease, and the brittle transition temperature may decrease. Since the grain size is inversely proportional to the total grain boundary area, refining the grains can improve the low temperature impact toughness of the material. The main purpose of the quenching and tempering heat treatment is to adjust the structure, reduce the carbides, obtain the Sorbite or Sorbite structure  and the granular carbide structure with uniform distribution to improve the overall performance. Studies have shown that multiple quenching and tempering can obtain fine equiaxed grains. However, multiple heat treatments of the steel material may cause oxidation of the grain boundaries and affect the mechanical properties of the material. Therefore, researches on multiple quenching and tempering heat treatments on SCM440 alloy steel commonly used in marine machinery are very important.

Test results show that after the secondary quenching and tempering of the SCM440 alloy steel, not only the low-temperature impact value was significantly improved, but also the yield strength and tensile strength were significantly improved. After three times of quenching and four times of quenching and tempering, there is no obvious change in low temperature impact, yield strength and tensile strength, indicating that secondary quenching and tempering has obvious effect on improving low temperature impact value, and extra quenching and tempering heat treatmets have no obvious effect. It can be seen from the photomicrograph that after the first quenching and tempering, the microstructure is tempered sorbite and retained austenite, and the grain size is 7.0, and there are clusters of flocculent carbides. After the second quenching and tempering, the retained austenite details are reduced, and the grain size level is increased to 7.5 without agglomerated carbides, and the granular carbides are replaced. When the third and fourth quenchings were carried out, almost no retained austenite was observed, the grains were further refined, and the carbides remained in the form of particles. Tempered sorbite is a mixture of ferrite and granular cementite. The tempered sorbite reduces the brittleness of the steel and improves the impact performance. The precipitated fine particulate carbide as the second phase particle can hinder the migration of the grain boundary, so that the grain refines as the number of times of quenching and tempering increases. In addition, the quenching cooling process has a process of nucleation and core growth. The quenching cooling time is controlled, the new crystal grains which are not grown up to the original grain size will be heated and cooled again, and the nucleation and core growth will be performed again, and a plurality of fine crystal grains are formed in the original crystal grains, and the crystal grains are obtained and refined. The impact toughness of steel is related to the grain size, and the ductile-brittle transition temperature is linear with the grain diameter. Therefore, refining the grains can improve the impact properties of the steel. From the perspective of yield and tensile strength, after the second quenching and tempering, the yield and tensile strength of SCM440 steel are significantly improved, which may be directly related to the grain refinement. During the plastic deformation process, the grains directly caused by the additional shear stress are only a minority. The plastic deformation of most grains is caused by the stress concentration of large dislocation clusters in the preceding grains. Only the plastic deformation is carried out for all grains, macroscopic effect of plastic deformation will be caused, and its effect is related to the grain size. It can be seen that refining the crystal grains can improve the yield strength and the tensile strength.

We can draw a conclusion as followings. After the secondary quenching and tempering of SCM440 alloy steel, the low-temperature impact performance is obviously improved, and the yield and tensile strength are also obviously improved. After three times of quenching and four times of quenching and tempering, the low temperature impact, yield strength and tensile strength are not significantly changed compared with the secondary quenching. After secondary quenching and tempering of SCM440 steel, the retained austenite in the microstructure is reduced, the carbides become granular, and the crystal grains are refined. After the third quenching and tempering of 42CrMo4 steel, the retained austenite disappears and the grains are further refined.