Cleavage initiation in the intercritically reheated coarse-grained heat affected zone: Part II. Failure criteria and sta
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I.
INTRODUCTION
DURINGthe welding process, the base plate plate microstructure adjacent to the weld bead is altered by the welding heat input, producing a heat affected zone (HAZ). The HAZ generated during multipass welding is complex and comprises many different regions, each with distinctive microstructural and mechanical properties. The HAZ for a two-pass weld is shown schematically in Figure 1 and is related to that produced during multipass welding. For high-strength low-alloy (HSLA) steels, it has been found that the untempered coarse-grained HAZ (CG HAZ) and the intercritically reheated coarse-grained HAZ (IC CG HAZ) show the worst toughness propertiesJ ~1 Part I of this article described the IC CG HAZ microstructures seen in three different steels under typical welding conditions, m] It was found that the IC CG HAZ microstructure that showed the worst toughness properties during crack tip opening displacement (CTOD) and Charpy impact testing consisted of upper bainite with a large prior austenite grain size and blocky MA (high carbon martensite with some retained austenite) particles located at the prior austenite grain boundaries. The MA particles have significantly higher hardness than the bulk bainitic microstructure. Fractographic examination of samples produced during CTOD, tensile, blunt notch, and Charpy impact testing revealed that cleavage initiation occurred between two closely spaced blocky MA particles. The micromechanism of cleavage initiation in this structure was determined to be from a combination of an overlap of residual transformational induced stress fields between two closely spaced particles and stress concentration effects resulting from debonding of the particles. The residual transformational induced stress fields are generated C.L. DAVIS, formerly Graduate student with the Department of Materials Science and Metallurgy, University of Cambridge, is University Lecturer, Department of Metallurgy and Materials Science, University of Birmingham, Birmingham, B15 2TT, United Kingdom, J.E. KING, Head of Materials, is with the Aerospace Division, Rolls Royce plc, Derby, DE24 8BJ, United Kingdom. Manuscript submitted December 15, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
by the volume increase that occurs during the formation of the MA particles. The purpose of this article is to determine the precise stress and strain requirements for failure to occur by the micromechanism proposed in part I and to investigate the statistical competition between potential initiation sites and mechanisms. This has been achieved through a detailed study involving a number of test techniques to provide different stress and strain conditions ahead of the notch/crack. This has enabled the fracture criteria for this micromechanism of crack initiation to be determined. II.
MATERIALS AND MICROSTRUCTURE
Three steels were used during this investigation; their compositions are given in Table I. Steels 1 and 2 were used in part I of this article, rsl Steel 1 is a commercial reheated, quenched, and tempered (
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