Effect of the heat treatment on the hydrogen permeability of steel 40Kh

Abstract

Results of measurements of the rate of hydrogen permeation through specimens of steel 40Kh under condi. tions of their cathodic polarization in a I-N aqueous solution of sulfuric acid at room temperature and a cttrrenl density of 100 mA/cm 2 are presented. The role of diffusion and dissolution in the formation of the hydrogen flow through the specimen is considered.The experimental installation and the method of measurement have been described in detail in our previous work [1]. Tube specimens of steel 40Kh (0.4% C, 1% Cr) of length L = 27 ram, external diameter d~, = 7 mm, and wall thickness l=0.5mm were heat treated by various regimes, namely, 1 ) hardening from 850~ in oil (freshly hardened martensite with a hardness of 57 HRC), 2 ) 1 + tempering at 180~ for 30 min (low-tempered martensite with a hardness Figure 1 presents results of tests of three identical specimens of steel 40Kh heat-treated by regime 2. It can be seen that they lie on virtually the same curve. Scattered results that occurred were averaged.Analyzing Fig. 1 we will see that the flow of hydrogen J= dV/dx ~, AV/Ax, i.e., the slope of the dependence h =f(x) attains a steady (maximum) mode of permeation rather quickly. In longer (1.5-2 h) tests the rate of permeation in some specimens decreased; the slope of the curve h (x) decreased. In this case the surface of the specimen was covered by a dark oxide film. The constant P of the permeation rate (the flow density per unit thickness of the specimen or the socalled hydrogen permeability) was calculated using the value of the flow of hydrogen on a steady region -/st, namely, 1, = 4 tls, (21where l = 0.5 mm is the thickness of the wall of the specimen, S = r~di, L, mm 2, is the area of its internal surface, L = 27 mm is the height of the cylindrical part of the specimen. The diffusion equation for a steady-state regime of hy.