Transformation-toughening in cemented carbides: Part II. Thermomechanical treatments
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I.
INTRODUCTION
THEconcept of "transformation-toughening" and its application to WC-(Fe, Ni, C) were discussed in an earlier publication where the degree of metastability of the binder phase was altered through composition control. ~ It was expected that an additional degree of control on the metastability of the binder and its propensity to transform under stress could be achieved by combining mechanical working with a suitable heat treatment. This practice, called thermomechanical treatment, is used in many alloy systems to provide improvements in toughness at a given strength level and/or resistance to environmental degradation. At first sight, application of such treatments to cemented carbides appeared very difficult, if not impossible, since the binder cannot be selectively deformed by an externally applied stress without extensive cracking of the carbide grains. 2 Even if cracking of the carbide grains could be tolerated, the total strain that can be obtained by external deformation is rather small. Application of thermomechanical treatments to the binders of WC-(Fe, Ni,C) cemented carbides appeared very remote until it was recognized that transformation of the binder from austenite to martensite in this class of materials involves considerable deformation. 3 The large volume increase and the shapestrain associated with the transformation induce considerable strain in the binder while the carbide network is essentially left intact with no cracking. If the phase transformation is then followed by a suitable heat treatment, the binder is in effect thermomechanically treated. The phase transformation thus makes it possible to subject the binder to selective thermomechanical treatment with little or no effect on the carbide grains. Such treatments have a large positive effect on the fracture toughness and crack opening R.K VISWANADHAM, formerly Manager, Research-Development, Reed Tool Company, Houston, TX, is Staff Sclenust, Martin Marietta Laboratories. 1450 South Rolling Road, Baltimore, MD 21227-3898, P. G. LINDQUIST is a Research Assistant in the Department of Materials Science and Engineering, University of Illinois at Champaign-Urbana, Urbana, IL 61801. Manuscript submitted November 6, 1986 METALLURGICALTRANSACTIONS A
displacement of WC-(Fe, Ni, C) cermets. The results of these investigations are reported here.
II.
MATERIALS AND M E T H O D S
All the materials used in this investigation were made by standard powder metallurgy methods and densified by liquid phase sintering and hot isostatic pressing at 1320 ~ and 103 MPa (15 ksi) for 60 minutes. For further details on materials preparation and the specific procedures used to measure and control the carbon content of the binder, the reader is referred to Reference 1. The following procedure was used for thermomechanical treatment of the WC-(Fe, Ni, C) cermets. Samples in the as-hipped condition were cooled in a bath of liquid nitrogen and left in the bath until temperature equilibration occurred. The samples were then removed, washed, dried, and trans
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