Influence on the general tempering process The alloying element silicon can delay the nucleation and growth of carbides, and effectively retard the transformation of ε-carbides into cementite; adding about 2% silicon to steel can keep ε-carbides in place. to 400°C. In carbon steel, the squareness of martensite basically disappears at 300°C, while steels containing elements such as Cr, Mo, W, V, Ti and Si can still maintain a certain squareness after tempering at 450°C or even 500°C Spend. It shows that these elements can delay the decomposition of iron-carbon supersaturated solid solution. Conversely, Mn and Ni promote this decomposition process (see alloy steels).
Alloying elements also have a great influence on the amount of retained austenite after quenching. The retained austenite forms a fine network around the martensitic laths; these austenites decompose after tempering at 300°C, resulting in a cementite film at the lath boundary. When the retained austenite content is high, this continuous film is likely to be one of the reasons for the brittleness of tempered martensite (300-350 °C). Alloying elements, especially Cr, Si, W, Mo, etc., enter the cementite structure, and increase the cementite particle coarsening temperature from 350 to 400 ° C to 500 to 550 ° C, thereby inhibiting the tempering softening process, and at the same time. It hinders the grain growth of ferrite.
Special carbides and secondary hardening When a sufficiently high concentration of strong carbide-forming elements is present in the steel, they can replace cementite to form their own special carbides in the temperature range of 450-650 °C. Diffusion and redistribution of alloying elements are required to form special carbides, and the diffusion coefficients of these elements in iron are orders of magnitude lower than those of elements such as C and N. Therefore, certain temperature conditions are required before nucleation and growth. For the same reason, the growth rate of these special carbides is very low. The highly dispersed special carbides formed at 450-650 °C maintain their dispersibility even after long-term tempering. Figure 4 shows that the formation of alloy carbides between 450 and 650 °C has a strengthening effect on the matrix, so that the hardness of the steel increases again, and a peak appears. This phenomenon is called secondary hardening.
