Three Important Points that Relate to Improving the Hot Workability of Ledeburitic Tool Steels
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THE relatively low hot workability of ledeburitic tool steels, i.e., steels in which the solidification is completed by a eutectic transformation of the residual interdendritic liquid to austenite and various alloy carbides (ledeburitic carbides), is characterized by the production of surface and internal cracks during the hot deformation. This process reduces the profitability of the production process as well as the useful mechanical properties of the tool steels. In general, for hot deformation, the initial, inappropriate microstructure of the tool steels is a result of inappropriate processing. Alloying elements in these tool steels, like Cr, W, Mo, and V, form eutectic carbides that improve the hardness, wear resistance, cutting ability, and grain-growth control, etc., but they considerably decrease the hot workability and make the hot-working temperature range very narrow compared with conventional steels.[1] During solidification, heating, soaking, and hot deformation, various processes take place in the material: the formation of carbides by eutectic transformation and by precipitation, the decomposition of the carbide phases and the formation of new carbide phases, the dissolution of carbides and of the alloying elements, the growth of carbides, etc. Furthermore, the type, morphology,
TATJANA VECˇKO PIRTOVSˇEK, Researcher, and GORAN KUGLER and MILAN TERCˇELJ, Associate Professors, are with the Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana SI-1000, Slovenia. Contact e-mail: [email protected]. uni-lj.si MATJAZˇ GODEC, Director, is with the Institute of Metals and Technology, Ljubljana SI-1000, Slovenia. Manuscript submitted October 19, 2011. Article published online May 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
fraction, size, distribution, etc. of the carbides are strongly related to the process parameters that influence the behavior of the tool steels during hot deformation. Thus, during the hot working of steels, a large number of process parameters, which are in mutual dependence, influence the intrinsic material properties that make an investigation in this area very important.[2–18] Publications in the literature with regard to the hot workability of steels, i.e., of low-alloyed steels as well as of tool steels, are predominately of a partial nature because of the complexity of the problem of hot workability. Namely, they usually present the results of the influence of individual factors on the hot workability, i.e., the influence of the deformation parameters (temperature and strain rate),[2–10] a determination of the limit of the hot plasticity,[7,10] spheroidization, and the deformation conditions for the breakdown of the carbide network,[11,12] the influence of a small number of chemical elements on the hot workability,[14–16] etc. Hot-tensile,[4,13] hot-compression,[8,11,12] hot-torsion,[2,3,5,6,9,10,14] and laboratory hotrolling tests were most often applied, and the materials in the investigations so far were predominately in the wrought state. I
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