Failure mechanisms of H13 die on relation to the forging process – A case study of brass gas valves Mohamed Kchaou a,b , Riadh Elleuch a, * , Yannick Desplanques b , Xavier Boidin b , Gérard Degallaix b a Laboratoire des Systèmes Electro-Mécaniques (LASEM), Ecole Nationale d’Ingénieurs de Sfax, B.P. 1173-3038 Sfax, Tunisia b Laboratoire de Mécanique de Lille (LML UMR CNRS 8107), Ecole Centrale de Lille, BP 48, 59651 Villeneuve d’Ascq Cedex, France article info Article history: Received 18 July 2009 Accepted 31 August 2009 Available online 6 September 2009 Keywords: Hot forging die Damage analysis Failure mechanisms Coupling fatigue-wear abstract In the hot forging industry, die life is an important process factor because of the cost involved in lost production, replacement of die blocks and operative handling of the dies. There is still no consensus, however, on the type of wear affecting dies or the dominant mechanisms for die failure, which varies from one situation to another. This metallographic study of a failed industrial hot forging die used to forge gas cylinder valves has indicated various failure modes. Although plastic deformation and thermal fatigue are usually quoted as the main causes of damage, oxidative and abrasive wear, fatigue cracking and chipping appeared to bet he most important in this study. Feedback coupling of fatigue and wear effects are detected. Detailed scanning electron microscopy observations and energy-dispersive X-ray and optical profilometry analysis suggest that these failures might very depending on their localisation on the die surfaces and show a complex mechanisms related to the variation of process parameters. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Forging is a metal-forming process in which hot metal, in the form of bar stock or cut pieces, is shaped by forging between impressioned dies. As a process, forging can be characterised by good mechanical properties of the workpiece, optimal mate- rial utilisation, a short production time, high productivity. These advantages are achieved normally for rather large produc- tion quantities because of the high costs of tooling as well as the long set-up times for production lines. The process is ideally suited to the repetitive production of large numbers of identical components, such as those required by the mass production industries, like gas valves. Working life of the dies is an important consideration from the point of view of cost reduction associated with tool replacement and maintenance and improvement in productivity and in product quality. Among forging process, the life of the tools is affected by a complex combination of high mechanical and thermal stresses [1,2]. The former originates from repeated impact and high-pressure flow of the metal. Thermal stresses are generated due to cyclic contact between die and workpiece at a temperature that is generally between 200 and 300 °C for brass workpiece and may be even higher for other material [3]. Hot working tools undergo severe thermal and mechanical shocks during each forming blow. These lead to damage of the stressed tool’s surfaces [4]. The majority of studies insisted that damage mechanism responsible for reduction in life of hot forging die is on the order thermal fatigue, mechanical fatigue, fatigue wear and plastic deformation [5–7]. So, they are concentrated to thermo-me- chanic approaches to understand die degradation. Bernhart et al. [8] observed that when working at high temperature work- pieces, the surface of the tool undergoes further thermal cycling which is the result of successive hot forging, waiting periods 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.08.015 * Corresponding author. Address: Laboratoire des Systèmes Electro-Mécaniques (LASEM), Ecole Nationale d’Ingénieurs de Sfax, BP N 599, B.P. 1173-3038 Sfax, Tunisia. Tel.: +216 74655189; fax: +216 74655192. E-mail address: riadh.elleuch@gnet.tn (R. Elleuch). Engineering Failure Analysis 17 (2010) 403–415 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal