Some special features of transformations in heat treatment of steels with different initial structural and concentration inhomogeneities

Abstract

The paper concerns transformations occurring in various operations of heat treatment and surface impregnation of well-known structural steels. It is established that the degrees of structural inhomogeneity before and after heat treatment of the steels are related by inheritance. Possible mechanisms of formation of structural inhomogeneity by inheritance are described.In accordance with the generally accepted technological standards the structure of steel in actual parts or the quality of heat treatment is often evaluated from results obtained by analyzing control specimens treated together with the parts. Destructive testing of the parts themselves is used much more rarely, only if needed for a special purpose. At the same time, it is well known from practice that the structure of control specimens does not always correspond completely to the structure of the material of the actual parts. This is illustrated by Fig. 1, which presents the structures of two cross sections of a rolling-contact bearing of a roller bit that have different thicknesses and the structure of the corresponding control specimen after carburizing at 930°C (the thickness of the layer is 2.0 mm) and direct cooling in oil from the temperature of preliminary cooling (870°C). In all three cases the photographs of the structures were made at a distance of 0.3 mm from the surface. The parts and the control specimen were carburized on one tray in a universal chamber furnace with an installed quenching tank.The control specimen was not subjected to any kind of treatment (except for turning) before carburizing. On the other hand, the part had a long technological history before carburizing, namely, hot multiple-pass stamping with cooling of the preform in air, normalizing (in a container), and mechanical treatment. The control specimen had the same cylindrical cross section (20 ram) over the entire length (100 ram); the thinnest cross section of the part was 5 mm and the thickest was over 50 ram. It can be seen from Fig. 1 that there are Fig. 1. Structure of the carburized layer at a distance of 0.3 mm from the surface in a control specimen (a) and in the neck of a rollingcontact bearing of a roller bit of steel 14Kh2N3MA (b, c) (x 400): b) massive cross section of the part (raceway); c) thinnest cross section.