Analysis of Degradation and Failure Mechanisms that Develop in Hot Forging Die
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TECHNICAL ARTICLE—PEER-REVIEWED
Analysis of Degradation and Failure Mechanisms that Develop in Hot Forging Die Fayc¸al Njeh • Mohamed Kchaou • Riadh Elleuch Foued El Halouani
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Submitted: 21 March 2011 / in revised form: 22 December 2011 / Published online: 22 March 2012 Ó ASM International 2012
Abstract A failed die, used to forge gas valves, was analyzed using macroscopic and microscopic equipment on some specimens. Fatigue cracks on the die were evidently observed on all the surfaces of the die. Many processes were found to be major contributors to have caused the die failure. Oxidation was observed in some areas, especially on plane surfaces and thin sections. Surface layers of tools at the tool-workpiece interface were not only exposed to high mechanical stresses but also to severe temperature cycles, which could have led to loss of strength and hardness. Metallographic analysis revealed that the oxide layers consisted of different diffusion layers which accelerated thermal fatigue mechanisms. Keywords Die
Degradation Forging Wear Fatigue
Introduction Hot-worked tool steels are widely used for many applications, particularly in hot forging applications because of its high temperature strength, impact toughness, heat checking, wear resistance, etc. Many signs of damage were observed on the surface of the die. Thermal fatigue failure is suggested as the most common mode of damage for many situations. Thermally induced surface damage, i.e., F. Njeh M. Kchaou R. Elleuch (&) F. E. Halouani Laboratoire des Syste`mes Electro-Me´caniques (LASEM), Ecole Nationale d’Inge´nieurs de Sfax, B. T. 1078, 3038 Sfax, Tunisia e-mail: [email protected] F. Njeh M. Kchaou R. Elleuch F. E. Halouani Universite´ de Sfax, Route de l’Ae´roport Km 0.5, 3029 Sfax, Tunisia
thermal fatigue, is believed to be controlled by the magnitude of the imposed cyclic strain [1, 2]. In fact, forging processes increase the density of metal, thus improving its strength, ductility, and thermal characteristics. Furthermore, forgings tend to conduct heat quickly. Different fatigue failures could occur if lubrication is also poor. Even though the adhesion may not be strong enough to tear out wear fragments, it may produce very large surface forces. The surface layers become extremely and heavily work hardened, and tensile stresses may develop below the surface. These will generate cracks on the surface, and at some point, these will emerge at the surface revealing a network of cracks [3]. Even though the stresses may be quite negligible, their cumulative effect after millions of stress cycles may lead to fatigue. The maximum shear stress occurs at a point below the free surface, and this is the region where a fatigue crack will first occur, particularly if there is some inhomogeneity in the steel such as a small, hard inclusion [4]. This phenomenon is followed by further subsurface crack growth which spreads to the surface and leads to the detachment of particles from the surface [2, 5]. In this analysis, a failed hot forging di
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