Hot workability of three grades of tool steel
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I.
INTRODUCTION
P L A I N carbon steels can generally be worked without great difficulty, since they can easily be preheated to single phase austenite and worked through the required stages of reduction before any transformation or precipitation takes place] a'3 The C dissolved in the austenite usually increases the workability since it increases the self-diffusion of Fe in the lattice, thus enhancing dynamic recovery, and does not impede boundary migration, thus not interfering with dynamic recrystallization] '4 On the other hand, in most alloy steels solute strengthening occurs because the preheat normally takes the alloying elements into solution. The effects of solute increase with the total metallic alloying concentration which reaches the high level in austenitic stainless steels of about 30 pct. 5 In practice, solidification segregation can result in regions with higher than normal solute levels and with undissolved carbides or nitrides, which both reduce the workability, particularly the latter. In addition, as the temperature falls in the final passes, precipitation of carbonitrides can begin, thereby diminishing ductility and raising the strength, e.g., Nb (CN) in HSLA steels. 1-3'6'7Stainless steels which undergo precipitation become as hard to work as the Ni-base superalloys in which ~' precipitates raise strength and lower ductility. 8'9 In certain tool steels the level and nature of metallic elements and the carbon content are such that the carbides are not dissolved in the common working range so that they are more difficult to work. Good examples are the coldwork (A), hot-work (H), and high-speed (M) tool steels. The A steels are usually austenitized for heat treating in the range 920 to 980 ~ in which the carbides are dissolved and hence pose no problem when the working temperature is above that. The H grade steels, although annealed at 840 to 900 ~ are austenitized at 995 to 1040 ~ to dissolve the carbides, except VC, so that when the temperature falls below the latter level, Cr23C6 carbides precipitate reducing C. IMBERT is Assistant Professor, Industrial Engineering, University of the West Indies, St. Augustine, Trinidad. N.D. RYAN, Research Assistant, and H.J. McQUEEN, Professor, are with the Department of Mechanical Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada. Manuscript submitted October 19, 1983. METALLURGICALTRANSACTIONS A
workability. In the M grade steels even when austenitized at 1190 to 1230 ~ the particles of M6 C [Fe3(W, M o ) 3 C Fea(W, Mo)2C] have not been completely dissolved, and their quantity which rises as the temperature declines through the normal working range limits the workability compared to carbon steels] ~ Although the hot working of highly alloyed tool steels is of great interest from both fundamental and industrial viewpoints, there has been comparatively little research reported in the scientific literature. The objective of this research was to investigate the hot working characteristics of three representative grades of tool steel, namely
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