An approach to developing an alternative hot work die steel
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INTRODUCTION
T H E maximum temperature at which a hot work steel can be used for prolonged periods is determined, among other factors, by its strength and resistance to softening at elevated temperatures, cll The maximum possible strength at the working temperature is required to both resist plastic deformation and to minimize heat checking which is associated with thermal fatigue. I~lMaim and Norstrom have shown that increasing the yield strength at the working temperature will result in greater resistance to both crack initiation and propagation by thermal fatigue.~2] While a variety of hot work die steels is available which maintain high strength after tempering up to 600 ~ there are few which can retain the same strength levels when tempered above 600 ~ The hot work die steels which achieve the highest hardness when tempered above 600 ~ are those of the tungsten type. [3] As the tempering behavior of steels of this class is sensitive to the temperature of austenitization and the rate of quenching after austenitizing, it is difficult to identify one steel of this class as having the best resistance to tempering. However, of the steels of this class, one of the most resistant to softening on tempering is the steel H-20. The tempering curve of this steel is compared to that of a more commonly used hot work die steel, H-13, in Figure 1. The tempering curves of the two steels are similar until they are tempered above 550 ~ Above that temperature, the hardness of the H-13 drops rapidly with increasing temperature, while the hardness of the H-20 remains essentially constant until tempered above 650 ~ Even though H-20 is one of the leaner tungsten hot work die steels, it contains over 9 wt pct tungsten. The objective of this work has been to evaluate an approach to developing a steel which combines a resistance to softening on tempering comparable to that of H-20 with a more economical use of alloying elements. W. M. GARRISON, Jr. is Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. M . S . BHAT is Manager, Quality Assurance, Product and Performance Analysis, General Products Division, IBM Corporation, 5600 Cottle Road, San Jose, CA 95193. Manuscript submitted August 15, 1986. METALLURGICALTRANSACTIONS A
COMPOSITIONS
To obtain the desired tempering behavior, compositions have been evaluated which exploit the ability of non-carbideforming elements to enhance the secondary hardening reaction and the tendency for vanadium additions to reduce the coarsening rate of Mo2C.[41 Previous studies of the influence of silicon additions of 1, 2, and 3 wt pct on the tempering response of a 0.4C/2Mo/1Cr/3Ni base steel found that silicon additions of 2 wt pct or more significantly enhanced the secondary hardening reponse of the base steel.ISl Two of the compositions considered here are the base steel of that study and that base steel modified by the addition of 2 wt pct silicon (base + 2Si). However, the hardness of the base + 2Si steel dec
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