Fractography survey on high cycle fatigue failure: Crack origin characterisation and correlations between mechanical tests and microstructure in Fe–C–Cr–Mo–X alloys Je ´ro ˆ me Tchoufang Tchuindjang * , Jacqueline Lecomte-Beckers Universite ´ de Lie `ge (ULg), De ´partement, Ae ´rospatiale & Me ´canique, Service de Me ´tallurgie et Science des Mate ´riaux, Unite ´, Mate ´riaux Me ´talliques Spe ´ciaux IMGC, Ba ˆt. B52-1, Chemin des chevreuils, 4000 Lie `ge, Belgium Received 22 November 2005; received in revised form 22 May 2006; accepted 21 June 2006 Available online 6 September 2006 Abstract Raw materials were cast from different specific processes in order to produce cleaner steels with a reduced amount of inclusions. Stud- ied materials that are HSS hardened alloys belonging to the Fe–Cr–C–X system were shared out in four groups depending on the tem- pering temperature and the presence of secondary and primary carbides. Both inclusions and carbides were roughly assessed by means of Image Analysis. Forging was done on all studied materials with different reduction ratios in order to highlight the texture influence on mechanical properties. High cycle fatigue tests were made using the boundary method to allow a quick evaluation of results. Fractographic analyses carried out on broken samples led to the definition of four failure modes depending on the nature and the location of the point from which the crack was initiated in one hand, and the roughness of striations in the propagating area over and around the initiation point. Internal and surface crack initiation points were found, the latter being more harmful than the first ones. Though oxides appeared to be more detrimental than other inclusions, primary carbides were also found to be both crack initiation can- didates and crack propagation enhancers. Various parameters likely to influence high cycle fatigue failures were finally defined, the most significant one dealing with the nature and location of embedded precipitates and the forging reduction ratio. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Tool steels; High cycle fatigue; Forging; Inclusions; Carbides; Boundary method; Fractography; Crack initiation; Soakness 1. Introduction Complex Fe–C–Cr–Mo–X systems, where X consists of V and/or W elements, are custom alloys for tool steels or high speed steels (HSS) as they exhibit excellent hardness and good wear resistance at higher temperatures [1]. The final microstructure is directly influenced not only by the solidification rates at the time of casting, but also by the later heat or thermo mechanical treatments. Fe–C–Cr–Mo–X alloys can thus contain in their hard- ened matrix desired precipitates such as carbides, and also more or less exogenous and endogenous inclusions, depending on the way casting process was performed [1–4]. Inclusions are present in all commercial materials as a result of deoxidising additions, impurities or entrained exogenous material. Inclusions are common sites for fati- gue crack nucleation and are known to be particularly del- eterious in high strength steel [5]. Considerable studies have been done on weakening the influence of inclusions on fatigue behaviour, especially on its nature and size features [6–9]. Reducing inclusions size leads to increased fatigue behaviour [7–9] and a methodology 0142-1123/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijfatigue.2006.06.012 * Corresponding author. Tel.: +32 4 366 91 62; fax: +32 4 366 91 13. URL: http://www/ulg.ac.be/metaux (J. Tchoufang Tchuindjang). www.elsevier.com/locate/ijfatigue International Journal of Fatigue 29 (2007) 713–728 International Journalof Fatigue