Effects of hydrogen on the mixed mode I/III toughness of a high-purity rotor steel
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A . M . KUMAR, N.R. MOODY, J.P. HIRTH, and J.A. GORDON Recently, w e reported the effects of hydrogen on the mixed mode I/III toughness of a high-purity Ni-Cr-Mo-V rotor steel,l~j The steel was tested b o t h in the uncharged and hydrogen-charged conditions. It was shown that the presence of hydrogen degraded the overall J toughness incrementally by approximately 30 pct, with the degree of degradation increasing with the mode III component. Also, in the uncharged condition, the J toughness decreased to a minimum as the mode I / I I I ratio decreased and increased again for pure mode III loading. However, w e could not determine if a similar variation of toughness was displayed by the hydrogen-charged steel, because the pure mode III tests could not be completed at that time. The pure mode III tests are now complete, and these new results are presented in this communication. The chemical composition of the steel used in these studies was 0.25C-3.7Ni- 1.7Cr-0.4Mo-0.12V-0.05Mn0.02Si-0.003(P,Sn,As)-0.002Sb-0.0015 S (wt pct). An extensive heat treatment was g i v e n to the steel to produce a tempered lower bainitic microstructure. The details of the heat treatment and the resulting mechanical properties can be found in Reference 1 and are not repeated here. Pure mode [II specimens (illustrated in Figure 1) were machined out of the heat-treated stock. This specimen design was originally developed by Schroth et a l . 121 and later adopted by Raghavachary et al., t3~ Manoharan et a l . ,i41 and Kumar and Hirth,151a m o n g others. The specimen was bolted to a T-shaped base platethrough the two 4 . 8 - m m holes and the 6.35-mm hole in the center. Load was applied to the outer two legs of the specimen along the axes of the 6.35-mm holes. A doublecantilever beam extensometer was used to measure the displacements at the specimen e d g e , w h i c h were later converted to load-line displacements. Deep side grooves were provided t o constrain the crack t o extend in a pure mode III orientation and also t o produce crack extension with limited plastic bending of the legs. After machining, the specimens were charged with hydrogen at 373 K at a hydrogen pressure of 13.8 M P a for 4 days. This resulted in a hydrogen concentration of 2 at. ppm., equivalent to a hydrogen fugacity of 1.26 GPa at room temperature. The specimens were then copper-plated and stored at 78 K until they were tested to prevent hydrogen egress. The mode III J testing was carried out using the multiple specimen technique following the guidelines of the
A.M. KUMAR, Postdoctoral Fellow, and J.P. HIRTH, Professor, are with the Mechanical and Materials Engineering Department, Washington State University, Pullman, W A 99164. N.R. MOODY, Technical Staff Member, is with Sandia National Laboratories, Livermore, CA 94550. J.A. GORDON, Production Engineer, is with Advanced Silicon Materials Inc., Moses Lake, W A 98837. Manuscript submitted January 5 , 1993. 1450-- VOLUME 24A, JUNE 1993
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