A strain-based fracture model for stress corrosion cracking of low-alloy steels
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I.
INTRODUCTION
A companion articlem has described the stress corrosion cracking (SCC) behavior of 4135 steel under different heat treatments, performed in order to analyze the influence of microstructure on the SCC susceptibility of this highstrength low-alloy (HSLA) steel. The effect of tempering temperature was especially studied. Very impressive improvements in SCC resistance, associated with changes from intergranular (IG) to transgranular (TG) fracture paths, were observed in a very short range of tempering temperature, from 425 ~ to 450 ~ where transmission electron microscopy (TEM) techniques showed no apparent microstructural changes. Several microstructural effects were also analyzed: content and type of bainite in the bainitic-martensitic microstructures; quantity of ferrite present in mixed microstructures; and grain size and grain boundary precipitation for microstructures with an IG cracking path. Variation in strength was evaluated in all cases. The much qualitative and quantitative information obtained should be helpful to optimize the heat treatment to be used on this kind of steel in applications where SCC phenomena may be present. However, the data provided are essentially empirical results which, in order to get a better understanding of the improvements seen in the SCC resistance of the 4135 steel and to extend the knowledge gained to other materials, must be explained on the basis of a fracture theory. Therefore, a further analysis was necessary to explain (1) which variables control the different fracture mechanisms corresponding to the SCC behavior of this steel; (2) how these variables control such mechanisms; and (3) which are
F. GUTII~RREZ-SOLANA and J.M. VARONA, Professors, and J. GONZ?~LEZ, Associate Professor, are with the Department of Materials Science, E.T.S. Ingenieros de Caminos, Canales y Puertos, University of Cantabria, 39005 Santander, Spain. A. VALIENTE, Professor, is with the Department of Materials Science, E.T.S. lngenieros de Caminos, Canales y Puertos, Polytechnic University of Madrid, 28040 Madrid, Spain. Manuscript submitted August 19, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
the critical conditions achieved that promote the changes from IG to TG types, where the most impressive increase in SCC resistance was observed. For SCC situations, as the one analyzed here, previous works t2-1~] have shown the important role that local hydrogen embrittlement at the crack tip plays in the control of crack propagation. Consequently, an important number of articles have been dedicated to modeling the cracking processes, [2'9'12-22] including threshold conditions,t9,~9,23-321as hydrogen induced phenomena, thus offering an important base for analyzing the SCC data of 4135 steel. Most of them suggest models based on hydrogen transportf12,33-491 to and embrittlement mechanismstt~,23,31,a2,49-66]at the crack tip region, where hydrogen preferentially concentrates. Transport mechanisms such as adsorption,t34,351absorption, t35.36.37] and diffusion[7,9.26,38~q as well as dislocati
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