Fatigue crack propagation in intercritically tempered Fe-9Ni-0.1C and Fe-4Mn-0.15C
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I.
INTRODUCTION
THE increased
use of liquefied natural gas and liquid nitrogen in the last few decades has spurred efforts to improve low temperature structural steels. The primary aim in developing alloys and heat treatments for subzero temperature applications has been to overcome low temperature brittleness. Acceptable values of strength and toughness have been attained in steels such as Fe-9Ni (ASTM-A553). The key heat treating step in these cryogenic steels, called "intercritical tempering" or "intercritical annealing", consists of tempering above the A1 temperature, in the twophase a + ~/field. Given enough time for the diffusion of substitutional elements, a fine duplex microstructure of and 31 phases is developed. The subsequent quench transforms some of the 3' to untempered martensite, while some solute rich 3/is retained as austenite. The exact mechanism by which this heat treatment lowers the ductile to brittle transition temperature is not firmly established; however, dramatic improvement in low temperature Charpy energy has been correlated with increasing retained austenite concentration, 1'2'3 The presence of retained austenite is viewed as beneficial although its direct link with the fracture mechanism is less clear. It is, however, generally accepted that the precipitated 3~ acts as a sink for detrimental interstitial atoms, increasing the toughness of the tempered martensite matrix. ~'3 In addition to improved fracture toughness, resistance to fatigue crack propagation is a desirable goal for cryogenic steels; however, the study of the influence of intercritical tempering on near threshold crack propagation rate has not H.J. CHOI is with Instituto de Pesquisas Technologicas, Divisao de Metalurgia, Sao Paulo, Brazil. L.H. SCHWARTZ is Professor of Materials Science and Engineering and Director, Materials Research Center, Northwestern University, Evanston, IL 60201. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.
METALLURGICALTRANSACTIONS A
received much attention. The intercritical tempering produces a rather complex mixture of hard and soft phases on a very fine scale (typically of the order of 0.5 /zm). Using the Fe-9Ni-0.1C systems as example, the microstructure consists of tempered martensite (approaching 7 pct nickel content for long tempering times) and untempered martensite and the untransformed austenite (approaching 18 pct nickel and =0.3 pct carbon for long tempering times). The morphology and the relative amounts of these phases may be expected to influence near threshold crack growth rates. 4 Due to the high cost and dwindling resources of nickel, an alternative alloying element for cryogenic steels has been sought in recent years. Manganese appears to deserve attention due to the metallurgical similarities with Ni when alloyed with Fe and also due to its
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