Tool Steel Heat Treatment Optimization Using Neural Network Modeling
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EVER increasing demands, especially in automotive sector on using light-weight high-strength or even ultrahigh-strength materials[1,2] put tremendous loads and pressure on tool’s design, but even more on tool material properties.[3,4] Forming of high-strength materials results in a very complex combination of thermal, mechanical, chemical, and tribological loading of the tool. Thus, in hot metal-forming applications, like stamping, rolling, forging, or die casting, the service life of the tool is limited due to the extreme working conditions, which results in thermal cracking, plastic deformation, fatigue, and wear.[5,6] In this respect tool material needs to fulfill many different requirements in terms of heat resistance, strength, toughness, ductility, hardness, and wear resistance. Unfortunately these properties, at least to a certain extent are not mutually compatible. However, the most important and basic properties that govern the performance of the forming tool are the toughness, which prevents instantaneous fracture and controls crack initiation and propagation, and the hardness responsible for resistance to wear and plastic deformation.[7] BOJAN PODGORNIK, Head of Department, IGOR BELICˆ, VOJTEH LESKOVSˆEK, Researchers, and MATJAZ GODEC, Director, are with the Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia. Contact e-mail: [email protected] Manuscript submitted May 2, 2016. Article published online August 18, 2016 5650—VOLUME 47A, NOVEMBER 2016
Tool material properties including hardness and fracture toughness depend on the microstructure, which is mainly defined by heat treatment conditions. Heat treatment technique and parameters play a crucial role in changing microstructure components, such as precipitation, retained austenite content, and characteristics of the martensite,[8] and by varying the steel composition it is possible to obtain microstructure providing required properties.[9] Traditionally trade-off between high fracture toughness and sufficient hardness and wear resistance is required. However, in the case of vacuum heat treatment proper combination of austenitizing and tempering time and temperature allows optimization of microstructure, resulting in improved fracture toughness while maintaining high hardness.[10] Response of tool steel in terms of fracture toughness and hardness on vacuum heat treatment conditions depends on tool steel type and processing route, but mainly on the steel composition and alloying elements involved in the precipitation of secondary carbides.[11] These alloying elements can be divided into two different categories depending on their effect on precipitation: (a) carbide-forming elements including Cr, Mo, V, W, Nb, etc. and (b) elements modifying the tempering kinetics such as Si, Mn, Co, etc.[12] AISI H11- and H13-type hot work tool steels are used in tooling and forming industry for making hot-forging dies, casting dies, and extruding tools. Their use is based on a combination of high hardness, hot strength, and excellent toughness.
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