Effect of Selected Cooling and Deformation Parameters on the Structure and Properties of AISI 4340 Steel

Abstract

The paper is focused on investigation of the high-strength AISI 4340 steel at various temperature and deformation conditions. The article is divided into two specific analyses. The first is to examine the dilatation behavior of the steel at eight different cooling rates, namely, 100, 10, 5, 1, 0.5, 0.1, 0.05 and 0.01 °C·s−1. The mapping of the phase transformations due to varying cooling rates from the austenitizing temperature of 850 °C allows the construction of the CCT diagram for a given high-strength steel. These dilatation curves were also compared with the metallography of the selected samples for the proper construction of the CCT diagram. A further analysis of the high temperature deformation of high strength steel AISI 4340 was performed in the range of temperature 900–1200 °C, and the strain rate was in the range from 0.001 to 10 s−1 with maximum value of the true strain 0.9. Changes in the microstructure were observed using light optical microscopy (LOM). The effect of hot deformation temperature on true stress, peak stress and true strain was investigated. The hardness of all deformed samples, depending on the temperature, the deformation rate and the peak stress σp overall together related with hardness, has also been evaluated.

Hot deformation analysis of lean medium-manganese 0.2C3Mn1.5Si steel suitable for quenching et partitioning process

“…To predict the flow stress over a wide range of temperatures and strains, it is necessary to interpolate the obtained values by a specific continuous function. Strain-dependent material parameters are often translated by a polynomial curve of a certain degree [34][35][36]. To select the polynomial degree correctly, we fitted obtained points by the polynomials from the 2 nd to 7 th order, and based on the quality of fitting, represented by the coefficient of determination R 2 , the final decision for an appropriate equation selection could be made.…”