Effects of microstructure on fracture toughness of a high-strength low-alloy steel
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I.
INTRODUCTION
HIGH-strength low-alloy (HSLA) steels can be processed carefully to produce a fine-grain polygonal-ferrite microstructure with an attractive combination of strength and fracture toughness. Microalloying additions of strong carbide and nitride formers provide precipitation hardening and assist in grain refinement; copper also can be added to provide an age-hardening response. The resultant microstructure has a high resistance to cleavage, a low transition temperature, good cold formability, 1 and excellent resistance to both opening-mode and shear-mode ductile fracture. The purpose of the research described in this paper was to determine the effects of variations in ferrite grain size and in the sizes of copper precipitates and niobium carbonitride precipitates on fracture toughness of a copper-bearing HSLA steel similar to ASTM A710, Grade A (Armco NI-COP steel). Above the ductile/brittle transition temperature, this steel has an unusually high toughness/yield strength ratio in comparison with those of other low-alloy steels. A previous paper 2 reported the effect of heat treatment on ductile-fracture resistance for mixed Mode I/Mode III crack growth. Both fracture toughness and the tearing modulus were found to decrease monotonically with increasing yield strength. Opening-mode toughness follows a similar trend, 3 suggesting that microstructure influences the ductile failure of this steel only indirectly through its influence on strength. The present work is concerned with the effects of heat treatment on cleavage and ductile failure and the ductile-brittle transition. In the case of ductile failure, microstructure appears to affect toughness through its influence on strength and work hardening. Most of the fracture-toughness testing of ferrite-pearlite HSLA steels to date has involved the resistance of the steels to brittle fracture at low temperatures, because of their use for gas pipelines in arctic environments. Thus, most of the available toughness data are from Charpy tests, drop-weight tear tests, and instrumented Charpy tests, 4 and most of the knowledge of microstructural effects concerns the DBTT ductile-brittle transition temperature) and dynamic toughM. T. MIGLIN, formerly Graduate Student, The Ohio State University, is now Research Engineer, Babcock and Wilcox, Alliance, OH 44601. J. P. HIRTH is Professor, Metallurgical Engineering Department, The Ohio State University, Columbus, OH 43210. A. R. ROSENFIELD is Research Leader, Battelle's Columbus Laboratories, Columbus, OH 43201. Manuscript submitted December 31, 1982. METALLURGICALTRANSACTIONSA
ness. One exception is the work of Cutler and Krishnadev5 on the same copper-bearing steel used in this study. They investigated the effects of aging on as-rolled plate. Below the DBTT, Kic was independent of aging treatment, while on the upper shelf there was an indication that the overaged condition was less tough than either the as-rolled or underaged conditions. The DBTT was highest for the peak-aged condition and lower for underaged than
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