The dependence of some tensile and fatigue properties of a dual-phase steel on its microstructure
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INTRODUCTION
O F the many papers that have appeared in recent years dealing with structure-property relationships in dual-phase steels, few have been concerned solely with the effects on mechanical properties of changes in the morphology of the microconstituents. Kim and Thomas ~ studied the effects of changes in the microstructure on the initiation and growth of fracture in a tensile specimen tested at room temperature. They found that, in a steel containing 0.1 pct C and 2 pct Si, specimens with a coarse dual-phase structure, formed by step quenching from the austenite into the two-phase temperature range, had a smaller elongation, but a slightly higher strength, than specimens with a fine structure, produced by introducing an intermediate quench. They deduced from their microscopic observations that fracture in specimens with a coarse structure is initiated by cleavage in the ferrite phase and in the fine structure by the nucleation and coalescence of voids. Suzuki and McEvily 2 studied fatigue crack growth rates in an AISI 1018 steel. One set of specimens was step quenched to give a coarse structure with a ferrite matrix, while another was intercritically annealed to give a structure in which the martensite encapsulated islands of ferrite. They found that the latter treatment resulted in a significant increase in the fatigue threshold and an increase in the yield strength. Ishihara3 also studied fatigue crack growth rates in steels with a "continuous martensitic network" but their steel had twice the carbon content of the steel studied by Suzuki and McEvily. They report that the mean stress level has an
XUE-LING CAI, formerly Visiting Scientist at Massachusetts Institute of Technology, is now Research Scientist at the Baotou Institute of Metallic Materials. People's Republic of China. J. FENG is Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology. W.S. OWEN is Professor of Physical Metallurgy. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted September 10, 1984. METALLURGICAL TRANSACTIONS A
important effect on the fatigue crack growth characteristics and the fracture mode. Under some conditions, cleavage of the ferrite occurred; in other circumstances, no cleavage could be seen. Wan et al. 4 used metallographic techniques to study the initiation of fatigue cracks in a steel containing 0.1 pct C, 2 pct Mn, 2 pct Si which had been given an intermediate quench to produce a fine dual-phase structure on annealing. They found that in such steels many fatigue cracks are initiated at the martensite-ferrite interface due to the inhomogeneous strain at the grain boundaries and fatigue hardening of the ferrite. It is difficult to compare the results from these few investigations because each used a different steel and, perhaps more importantly, there are many differences in ferrite and austenite grain sizes, carbon content of the martensite, volume fraction of retained austenite, an
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