PSI - Issue 14

Santosh Kumar et al. / Procedia Structural Integrity 14 (2019) 872–882 Santosh / Structural Integrity Procedia 00 (2018) 000–000

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The high cooling rate and fast solidification of atomized molten particles leads to the formation of fine grained and more homogeneous microstructure without macro-segregation as compared to conventionally made products [Schruff et al. (2002), Speigelhauer et al. (1999), Yang and Hannula (2008)]. Spray forming process can be used to manufacture alloy compositions which are problematic when produced using conventional process [Grant (2007)]. Due to its good temper resistance and ability to maintain high hardness and strength at elevated temperature, AISI H13 steel is one of the most widely used hot work die steels. It is a chromium, molybdenum and vanadium alloyed steel with high hardenability. Higher content of vanadium leads to higher dispersion of vanadium carbides and thus higher wear resistance [Philip and McCaffrey (2005)].

Fig. 1. Schematics of spray forming process [Lawley and Doherty (1998)]

In hot forging dies, the presence of high temperature and high stresses during its operation can result in heavy surface damage. This damage can arise due to wear, plastic deformation, thermal fatigue and mechanical fatigue. Among the different failure mechanisms, wear and mechanical fatigue are predominant modes of die failure during forging [Caliskanoglu et al. (2002), Zurine et al. (2015)]. Fatigue related with forging dies is predominantly low cycle fatigue associated with high stresses and temperature [Shivpuri and Babu (2005), Mellouli et al. (2014), Patil et al. (2015), Patil et al. (2016)]. Any fatigue failure consists of three stages namely, crack initiation, crack propagation and final failure. Generally, in low cycle fatigue, larger proportion of the ‘total number of cycles to failure’ is associated with the crack propagation [Dieter (1988)]. Crack propagation is measured through material properties like fracture toughness and fatigue crack growth rate (FCGR). Fracture toughness is the material’s ability to resist growth of a crack. It basically relates with stress and crack size & shape. It is the measure of critical stress required for growth of a particular size of crack. Thus, study of fracture toughness becomes important for hot work tool steels. The objective of present work is to evaluate and compare the tensile, Charpy impact and fracture toughness properties of H13 tool steel, produced through spray forming and conventional routes. 2. Experimental work For the present study, H13 material produced through spray forming (SF) route followed by rolling and conventional route (CR) is considered. Conventional route material is produced through casting followed by cogging/rolling process. In SF route casting process is replaced by spray forming process. Both route materials were heat treated to obtain hardness range of 42~44 HRc (lower hardness range) and 48~50 HRc (higher hardness range). The measured chemical composition of considered steels is given in Table 1.