Heat Treatment Optimization and Properties Correlation for H11-Type Hot-Work Tool Steel
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INTRODUCTION
IN hot metal-forming applications, like forging, stamping, rolling, or die casting, the service life of the tool is limited by the extreme working conditions, including thermal, mechanical and impact loading, the high contact pressures, and the abrasive flow of the work material.[1,2] Under such complex working conditions, which may vary significantly across the tool, the tool surface is deteriorated and damaged through different wear mechanisms and thermo-mechanical fatigue processes.[3–5] In recent years, the requirements for tool resistance and performance have become increasingly demanding, which is strongly related to tool design, material selection, and heat treatment optimization.[6] On one hand, an increase in energy costs,[7] the competitiveness of companies from emerging countries, and investments in new technologies dictate the increased productivity, efficiency, and performance of existing tools.[8] As die costs represent between 15 and 30 pct of the total costs of the forging process, producing more parts leads to a sizeable cost reduction and increased profitability.[1] Another major factor is coming from the automotive sector. The automotive industry is continuously working on reducing the weight of vehicles in order to lower fuel consumption and CO2 emissions, while maintaining or B. PODGORNIK, G. PUSˇ, B. ZˇUZˇEK, V. LESKOVSˇEK, and M. GODEC are with the Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia. Contact e-mail: [email protected] Manuscript submitted June 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
even improving the strength of the components in accordance with increasing safety demands.[9] This requires the introduction of low-weight high-strength materials, which are very difficult and demanding to form.[10,11] A high-strength work material combined with the increased complexity of the formed parts results in tools working under extreme stresses. Increased tool demands mean tougher property requirements for the tool material, including temper resistance, yield and ultimate tensile strengths, wear resistance, fatigue, and shock resistance, but mainly ductility and fracture toughness, which are essential for high-temperature forming applications.[12,13] The properties of tool steels depend on a balanced chemical composition and the processing route, but mainly on the heat treatment process, which defines the final microstructure. In general, the required properties, mainly the hardness and toughness are generated by a controlled heat treatment process consisting of an austenitization treatment with a subsequent hardening and a multiple tempering procedure.[12,14] Traditionally, a trade-off between high hardness and sufficient toughness is required, where the hardening and tempering parameters have to be chosen from a relatively narrow range.[15] On the other hand, through an optimized vacuum-heat-treatment procedure, a fine-grained microstructure with a homogeneous distribution of fine, secondary carbides and a reduced retained austenite content can be o
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