Effect of Tempering Temperature on the Structure and Mechanical Properties of Medium-Carbon Steel with Elevated Silicon Content

The effect of quenching and tempering on the microstructure, phase composition and mechanical properties of steel 0.33C – 1.8Si – 1.44Mn – 0.58Cr is investigated. The structure is studied using scanning and transmission electron microscopy, dilatometry and differential scanning calorimetry. Tensile and impact strength tests are performed. The Rockwell hardness is measured. The processes of formation of carbides in the steel at different temperatures are analyzed, as well as the influence of the structure on the mechanical properties. Quenching yields laths of martensite with carbon-saturated dislocation boundaries due to formation of Cottrell atmospheres. The internal stresses reach 40% of the yield strength σ_0.2. Tempering to a temperature of 280°C causes precipitation of transition η-carbides (Fe₂C) inside the laths, which depletes the martensite matrix of carbon by about 90%. This increases the yield strength by 16% to 1130 MPa despite the lowering of the internal stresses and growth of the width of the laths. Precipitation of cementite chains over the lath and block boundaries occurs in tempering at 500°C, which is accompanied by lowering of the yield strength to 1130 MPa. Calculations of the proportion of carbon atoms in the martensite shows that almost all of the carbon present in the steel goes to formation of cementite. The decomposition of martensite during tempering increases the toughness but has a minor effect on the strength-ductility parameter σ_r × δ (MPa ∙ %).