Influence of martensite content and morphology on the toughness and fatigue behavior of high-martensite dual-phase steel
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INTRODUCTION
THE superior combination of strength, ductility, continuous yielding, and high work-hardening rate of dual-phase (DP) steels has been exploited by the sheet metal industry for several years.[1–4] The understanding gained on various aspects of DP steels over the last three decades is limited to microstructures containing volume fractions of martensite (Vm) of approximately within 0.25 in the thin section. The major cause for this limitation is the fact that although the strength gradually increases, the ductility and impact toughness of DP steels degrade rapidly beyond such a level of martensite content.[5] But in a recent report,[6] the authors have demonstrated that high-martensite dual-phase (HMDP) steels with an excellent combination of strength, ductility, and impact toughness can be prepared by an intermediate quench (IQ) heat treatment, which results in finely dispersed precipitate-free ferrite and martensite. But the information presented in the earlier report is insufficient to ensure structural integrity of these developed steels, especially if these are to find any structural applications in thicker sections. The toughness characterization of structural materials for quality control purposes is popularly carried out using impact-toughness assessments. But this property, unlike fracture toughness, cannot establish defect acceptance and inspection criteria for quality control.[7] It was, therefore, ASIM BAG, Senior Materials Engineer, is with the Materials and Corrosion Assessment and Testing, Det Norske Veritas Pte Ltd., DNV Technology Centre, Singapore 118 224. K.K. RAY, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721 302, India. E.S. DWARAKADASA, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore - 560 012, India. Manuscript submitted October 4, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
contended that the fracture-toughness behavior of the developed HMDP steels needs to be understood prior its consideration for applications. But, in an earlier investigation,[8] one of the present authors had indicated that standard ASTM procedures[9–12] are either difficult or impossible to employ for the characterization of the fracture toughness of highstrength low-alloy steels and has shown the usefulness of evaluating quasi-static fracture-toughness using chevronnotched specimens (KICV) for the inherent distinguished merits[7,8,13–17] of this procedure. The present investigation incorporates consideration of evaluating KICV and estimating the plain-strain (KID), equivalent-energy (KD), and J-integral (KJD) dynamic fracture-toughness parameters[18,19] using precracked Charpy specimens. Analyses of KICV , KID , KD , and KJD are expected to bring out the influence of Vm on the fracture-toughness behavior of the developed HMDP steels[6] prepared by an IQ heat-treatment. Studies on fatigue-crack growth (FCG) in ferritic, pearlitic, bainitic, and martensitic steels indicate that improved res
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