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AbstractThis paper describes the estimation method of die service life based on wear and the plastic deformation of dies in hot forging processes.Die service life is considerably shortened due to the thermal softening of surface layer, caused by the high thermal load and long contacttime between the dies and the deforming material. Also, the die service life depended on wear and the plastic deformation of dies can be to alarge extent determined by finite element (FE) analysis, wear and thermal softening tests. These are some of the major limiting factors affectsdie accuracy and die service life, and forming velocity and initial die temperatures influence greatly wear and the plastic deformation of hotforging dies. In this study, two methods are suggested for estimating the service life of hot forging dies by plastic deformation and abrasivewear, and these applied to predict the product quantity according to two main process variables, forming velocity and initial die temperaturefor a spindle component. Through the applications of the suggested methods, the thermal softening of dies due to the local temperature riseled to the reduction of the service life of hot forging dies by plastic deformation more than by abrasive wear.© 2004 Elsevier B.V. All rights reserved.36397
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Keywords: Hot forging; Die service life; Wear; Plastic deformation;
Thermal softening; Tempering parameter 1. IntroductionHot forging is one of themost conventionalmetal-formingprocesses used in the production of critical parts in variousindustries [1].Actually, it iswidely used in themanufacturingof automobiles and industrial machine components. In par-ticular, this process can be effectively used to form materialswith the high flow stress. Die service life greatly influencesmanufacturing costs, productivity and product quality. Dur-ing hot forging process, die service life is dramatically short-ened by thermal cycle, excessive metal flow and a decreasein die hardness [2]. Nowadays, manufacturing costs depend on how die ser-vice life can be extended for sound products without anykinds of internal and external defects during hot forgingprocess. Subcontractors and suppliers are increasingly un-der pressure with regard to cost reduction and responsibilityfor the development of new components. These requirementsare more critical in the automotive industry. Therefore, it isimportant to improve the technical skills in the areas of ma-terial science and metallurgy as well as in the area of tooldesign.The knowledge of computer aided design (CAD) and nu-merical simulation also becomes very helpful. In the forgingindustry, tooling costs can reach up to about 50% of a com-ponent cost. Therefore, it is obvious that the reduction of D.H. Kim et al. / Journal of Materials Procomponent costs requires an optimization of tools, in par-ticular, an improvement in performance and service life [3].During hot forging process, forging tools are not only sub-jected to mechanical stresses, but also to thermo mechanicalstresses induced by the thermal cycling and successive forg-ing operations.Proper selection of the die material and of the die man-ufacturing technique determines, to a large extent, the use-ful life of forming dies. Dies may have to be replaced fora number of reasons, such as changes in dimensions due towear or plastic deformation, deterioration of the surface fin-ish, breakdown of lubrication, and cracking or breakage [4].Many researchers have been investigated the influences ofprocess conditions on die service life during metal formingprocess [5–7]. The surface hardness of a die decreases ow-ing to the thermal softening of hot forging dies. This ther-mal softening effect accelerates tool failures [8]. The lim-iting factors of die service life can occur simultaneouslyor separately during hot forging process. Due to the dif-ferent characteristics of processes or products, die servicelife can be decreased by wear or by the plastic deformation[9].This study developed two methods to estimate die servicelife in hot forging processes.