Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels
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I.
INTRODUCTION
IT is well established that small surface cracks can grow at an accelerated growth rate in the near-threshold regime compared to longer, through-thickness cracks, t~-5] Further, surface cracks often show a dip or minimum in crack growth rate.[~'3-7] Tanaka[S] recently interpreted this behavior as due to a decreasing effective fraction of the stress intensity factor range, AKeu, that reaches a steady value as the crack length increases. The steady value of AKaf is often approached near the threshold value of AK for long cracks, tl] which, in turn, means that the minimum growth rate for short cracks also occurs near the long-crack threshold, AKth. An integrated understanding of fatigue life behavior evidently requires knowledge of crack initiation, shortcrack growth, and long-crack growth. The fact that surface cracks behave differently from long cracks, coupled with the lack of surface fatigue crack data, has made it difficult, for example, to interpret fatigue life behavior (S-N curves) in pearlitic steel, even though crack propagation data for long cracks existed, t9,~~ Studies of surface crack initiation, surface crack propagation, and fatigue life were accordingly undertaken on such steels. The fully pearlitic eutectoid steel AISI 1080 is of particular interest for such studies, not only because it is known to exhibit a marked dependence of long-crack propagation rates on microstructuret9'l~ but also because of its known independence of yield strength and M.A. DAEUBLER, formerly with Carnegie Mellon University, is with MTU-Munich, Posffach 500640, 8000 Munich 50, West Germany. A.W. THOMPSON, Professor and Department Head, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. I.M. BERNSTEIN, formerly with Carnegie Mellon University, is Provost and Academic Vice President, Illinois Institute of Technology, Chicago, IL 60616. Manuscript submitted June 8, 1987. METALLURGICALTRANSACTIONSA
fracture toughness, t11,12,131That independence occurs because yield strength is controlled by pearlite spacing, Sp, while toughness depends on prior austenite grain size. Thus, metallurgical treatments to control independently the grain size and Sp, through austenitization temperature and transformation temperature, respectively, can provide independence of yield strength and toughness. One purpose of the present experiments was an examination of the role of these same microstructural variables in the behavior of short surface cracks. II.
EXPERIMENTAL PROCEDURES
For this investigation, two steels were used, a hot-rolled rail steel made by United States Steel, conforming to AISI 1080, and a weldable Cr-Mo rail steel produced by Colorado Fuel and Iron. The chemical compositions of these alloys are given in Table I. Cylindrical tension and hourglass fatigue specimens with a gage or midsection diameter, respectively, of 5 mm were cut from the rail heads in the longitudinal or rolling direction. After machining, the samples were austenitized in a
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