Microstructure and Origin of Hot-Work Tool Steel Fracture Toughness Deviation
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NTRODUCTION
TOOLS and dies used in hot metal forming (extrusion, forging, rolling, etc.) are exposed to elevated temperatures and high contact pressures, and therefore subjected to simultaneous action of thermal, mechanical, chemical, and tribological loads. This complex loading leads to different types of tool damage such as wear, plastic deformation, and thermal and mechanical cracking.[1–3] However, the three main failure modes of in-service hammer forging dies were found to be plastic deformation, abrasive wear, and mechanical fatigue[2–6] with the fatigue fracture being among the most critical ones. It leads to unexpected premature failure of the die, thus directly affecting production economy. Although die costs normally account for less than 10 pct of the product’s cost, unexpected die failure can increase production costs by over 30 pct.[7] In general, type of die failure mode and its progression depend on the tool steel material and heat treatment used, die shape design and manufacturing, forging parameters and process applied, as well as forging stock properties.[6–11] However, the biggest impact comes from the tool material and its microstructure, with the properties’ profile of the tool material greatly influencing the die lifetime.[11,12] Basic properties of the tool material that govern the performance of the die are the toughness and ductility, which prevent instantaneous fracture and control crack initiation and propagation, and the hardness, which must be sufficiently high to avoid local plastic deformation.[11,13,14] Although the prevention of instantaneous BOJAN PODGORNIK and VOJTEH LESKOVSˇEK, Researchers, are with the Institute of Metals and Technology, Lepi Pot 11, 1000 Ljubljana, Slovenia. Contact e-mail: [email protected] Manuscript submitted April 21, 2013. Article published online August 13, 2013 5694—VOLUME 44A, DECEMBER 2013
tool failure is often connected to a critical hardness level that must not be exceeded for a specific forming application, toughness reveals full potential of the material.[11,12] The most reliable measure of toughness is the plain-strain fracture toughness. The same value of fracture toughness should be found for specimens of the same material but with different geometries and with a critical combination of crack size and shape and fracture stress. Within certain limits, this is indeed the case, and information about the fracture toughness can be used to predict failure for different combinations of stress and crack size and for different geometries.[15] Therefore, beside hardness level, information on the fracture toughness achieved under selected heat treatment procedure needs to be provided. As the automotive industry increasingly focus toward the use of advanced high-strength steels requirements on the wear and fatigue properties of tools and dies are constantly growing. This requires high-strength highhardness tool materials with high fracture toughness vs hardness ratio.[6,16] High fracture toughness means that tool will be more resistant to shock loadings as well
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