Diffusion of chromium and silicon in steel under cyclic heat treatment with a temperature gradient

Abstract

It is known from published data that the processes of mass transfer in metals are intensified in periodic heating and cooling and upon the appearance of a temperature gradient in the specimen. The aim of the present work is to investigate the diffusion of elements in a steel specimen during cyclic heat treatment in the presence of a temperature gradient.Specimens 10 mm in diameter and 100 mm long were prepared from low-carbon steel (---0.2% C). Metallic plates 3 mm thick were fused onto one side of a specimen by the induction method. The surfacing metal was metallic chromium or ferrosilicon FS-50 or FS-75.It has been shown in [1,2] that cyclic heat treatment (CHT) accelerates diffusion processes in metals. Cyclic heat treatment with a temperature gradient over the length of the specimen was conducted using a special end heater that allowed us to change the temperature of the specimens in time by the regime presented in Fig. I. The temperature at different points of the specimens was measured by four ChromelAlumel thermocouples caulked in the specimens at a distance of 2 -3 mm from the surface.After 50 heat cycles the specimens were cut along the axis and the preforms obtained were used to prepare polished specimens for a metailographic investigation. The steel part of the specimens was etched in a 4% solution of HNO 3 in alcohol, and the fused zone and the chromium facing were etched electrolytically in a solution of chromium anhydride and a mixture of HCI and HNO 3 . The structure was investigated on an MMR-4 microscope.We established that the structure of the facing is represented by an ct-solid solution of chromium. The fusion line is well defined and at some places it has small (up to I00 p.m) chromium tongues penetrating into the base metal. the base metal. Under the diffusion zone we observed a ferrite zone that passed into a well-defined pearlite zone. The total length of this zone was about 900 ~tm (with the tongues). Then followed a region consisting of pure ferrite grains that gradually (for 3 -5 ram) passed into the base metal.An x-ray spectral analysis 2 was conducted on an MAR-2 microanalyzer. The micrographs were obtained under an accelerating voltage of 50 kV (for Cr and Fe) in a vacuum spectrometer with the use of a quartz crystal analyzer. The reflection angles were 32030 ' and 2704 ' for Cr and Fe respectively.In Fig. 2 we present curves describing the distribution of chromium and iron over the length of a specimen. A mathematical analysis of the curves was conducted using the theory of atomic diffusion of elements. For one-dimensional diffusion of elements, when the entire diffusing substance is con-