Evolution of the Structural State and Properties of 32CrB4 Steel during the Production of High-Strength Fasteners

Abstract

For the production of high-strength fasteners, bar sections are used, which must withstand significant cold plastic deformation and, therefore, are subjected to a special heat treatment. In the present work, the dependences of the microstructure and mechanical properties of round bars with diameters of 17 mm and 21 mm made of 32CrB4 steel on the heat treatment modes are established. During the study, the methods of optical and scanning electron microscopy, mechanical tests, and hardness measurements were used. Dependences of the degree of spheroidization of the ferrite–pearlite structure and mechanical properties on the spheroidizing annealing mode have been established. It is shown that preliminary obtainment of a bainitic or martensitic structure of rolled products after hot-rolling of steel accelerates the process of pearlite spheroidization. Additional heat treatment, including austenitizing, quenching, and tempering, allows obtaining fasteners of various strength classes: 8.8, 9.8, and 10.9. However, with a guaranteed receipt of strength characteristics for all strength classes, including 12.9, the problem arises of achieving the required values of the relative elongation of steel.