Effect the hot deformation on austenite structure of the carbon steel

Abstract

Purpose. Determination influence temperature and degree of hot plastic deformation carbon steel on the development secondary recrystallization of the austenite. M ethodology. The carbon steels with a concentration of carbon by 0.55 and 0.65%, which correspond to the maximum and minimum values within the brand composition for all-rolled railway wheels, were used as material for the research. The number of other chemical elements met requirements of the documentation for railway wheels. The samples for research had the form of cylinders with a diameter of 20 and a height of 40 mm. Different degrees of plastic deformation were obtained during compression with a deformation rate of 10-3 s-1. Chamber-type electric heating furnaces were used to heat the samples to temperatures of the hot pressing. When heating samples, measures are taken to prevent oxidation and a local decrease concentration of the carbon atoms on their surfaces. After heat treatment of the samples and mechanical preparation of the surface, detection of the structure of austenite was carried out using of the etching based on a mixture of solutions hydrochloric and picric acids. The structure of the steels was investigated under a light microscope. The size of the austenite grain was determined by the methods of quantitative metallographic. Findings. The study structure of the carbon steel determined that under conditions of increasing the annealing temperature, the occurrence of structural heterogeneity of the austenite is caused by a change at mechanism of the development secondary recrystallization. Starting from 10% of hot pressing, in proportion to the degree of plastic deformation, development secondary recrystallization by the mechanism of movement of the large-angle boundaries leads to grinding of the austenite grain and the formation of a homogeneous structure. At degrees of hot deformation less than 10%, the density of dislocations is not enough to form a nucleus for the development secondary recrystallization by the mechanism of movement of the large-angle boundaries. As a result, recombination of the dislocations in grains of hot deformed austenite will occur much faster, which will lead to the separation of grains into fragments. Their size will be determined by the density of dislocations introduced during hot pressing. Disorientation angles between fragments have intermediate values between boundaries with large disorientation angles and subboundaries, which determines their low mobility. For such a structure, development of the secondary recrystallization in austenite will occur by the mechanism of grain boundary dissolution. As a result, there is significant heterogeneity structure of the austenite, which is determined by the difference in sizes between neighboring grains up to several times. Originality. Depending on the conditions of hot pressing of the carbon steel, the value of plastic deformation is determined, which separates development the secondary recrystallization of the austenite by qualitatively different mechanisms. At a low value of hot deformation, when the density of dislocations is not sufficient for the formation of a nucleus of the secondary recrystallization by the mechanism movement of the large-angle boundaries, the growth austenite grains occurs by the mechanism of dissolution of the grain boundaries. Practical value. Determining effect temperature and degree of hot plastic deformation on the mechanism of the development secondary recrystallization will allow to the optimize tenology crimping the blank of the all-rolled railway wheel.