Effects of hydrogen on the mixed mode I/III toughness of a high-purity rotor steel
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I.
INTRODUCTION
T R A D I T I O N A L L Y , mode I fracture has been used to study elastic plastic fracture mechanics. However, in recent years, mixed mode fracture has become the focus of many studies [1-91 because many observed failures include shear components. In our prior research, E4-9] fracture characteristics have been found to differ, depending on microstructure, strength, and toughness level. In lowtoughness high-strength alloys, mode Ill additions to mode I loading had little or no effect on the overall value of Jic, the critical value for the mode I component of the J integral for crack initiation, and tended to increase Jrc, the total J integral for mixed mode crack initiation. In the tougher materials, which failed primarily by a microvoid nucleation and growth mechanism, mode III additions lowered the overall J initiation values considerably from their pure mode I values. One particular set of experiments t6,91 on a high-purity Ni-Cr-Mo-V steel showed that the overall J initiation values passed through a minimum at a position between pure mode I and pure mode III on a plot of Jrc v s crack inclination angle. The objective of the present study was to further characterize mixed mode I / I l l fracture toughness for the same Ni-Cr-Mo-V steel after gaseous charging with hydrogen. Hydrogen had been shown earlier to enhance the tendency for shear localization in mixed mode I / I I fracture of plane strain tension specimens of several steels II~ and, hence, was expected to influence the magnitude of the mode III loading effect in mixed mode I / I I I fracture. II.
EXPERIMENTAL PROCEDURE
The material used in these tests was a high-purity rotor steel with composition 3.70Ni, 1.70Cr, 0.40Mo, 0.25C, 0.12V, 0.05Mn, 0.02Si, 0.003 for P, Sn, and As, 0.002Sb, and 0.0015S, in weight percent. The steel was forged by the Japanese Steel W o r k s - T o s h i b a plant. The heat treatment of the steel was as follows: 900 ~
J.A. G O R D O N , Production Engineer, is with Advanced Silicon Materials Inc., Moses Lake, W A 98837. J.P. HIRTH, Professor, and A.M. K U M A R , Postdoctorate Fellow, are with the Department of Mechanical and Materials Engineering, Washington State University, Pullman, W A 99164-2920. N.E. M O O D Y , Jr., Technical Staff Member, is with Sandia National Laboratories, Livermore, CA 94550. Manuscript submitted July 15, 1991. METALLURGICAL TRANSACTIONS A
19 hours, air cool; 870 ~ 19 hours, air cool; 650 ~ 22 hours, furnace cool; 840 ~ 42 hours, water spray quench; 595 ~ 54 hours, furnace cool; and 590 ~ 50 hours, furnace cool. This produced a tempered lower bainitic microstructure with the tensile properties given in Table I. A schematic of the compact tension samples used in this study is given in Figure 1. This design was originally developed by Abou-Sayed e t al. [13] In this scheme, ~b = 0 deg represents pure mode I. Increasing the mode III component is accomplished by increasing ~b. The values of ~b used in this study were 0, 15, 25, 35, 45, and 55 deg. All values of ~b were used for the pr
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