ABSTRACT The effect of hydrogen contents on crack nucleation time was conducted by using compact tension specimens of 473K tempered AISI 4340 steel in three kinds of environmental solutions under various electrode potentials. The crack nucleation at the notch root is determined by the electrical potential method. When the crack initiates, the voltage differences between the notch mouth starts to increase. The crack initiation times depend on hydrogen contents and the apparent stress intensity factors. These dependencies are successfully explained by a dislocation pile-up model with the hydrogen atom interaction.
1. INTRODUCTION Considerable works have been done to clarify the crack nucleation mechanisms of hydrogen assisted cracking (Troiano, 1760, Beachem, 1972, Leeuwen, 1975, Oriani, 1974, Page, 1982 and Hirose, 1977). In the present paper, the effect of hydrogen contents on crack nucleation time was investigated for 473K tempered AISI 4340 steel in three kinds of environmental solutions; distilled water, 3.5% NaCl solution and 0.1N H2S04 solution. A new dislocation model is proposed to explain the crack nucleation phenomenon. Experiments are performed to compare with the present theory.
2.EXPERIMENTAL PROCEDURE The material used was AISI 4340 steel (wt.%; 0.39C, 0.74Mn, 1.38Ni, O.78Cr, O.23Mo)" Compact tension specimens (see Fig. 1) with notch radius, ρ = 0.5mm and thickness, B = 5.5mm were machined from cutting slice at the right angle to the axis from hot rolled round bar with 100mm diameter. The specimens were normalized at 1153K for 1 hour and austenized at 1123K for 1 hour and then oil-quenched and tempered at 473K for 2 hours. After the surface layer was removed by grinding, the notch was made carefully by an electrical discharge machine to minimize the worked layer. The specimens were finally finished by electro-polishing. The mechanical properties of the material after heat treatment are given in Table 1.