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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For comparison, both route materials were heat treated to obtain similar hardness range (hardness range of 42~44 HRc and 48~50 HRc). Microstructural analysis reveals that both materials possess tempered martensitic microstructure. Similar values of YS and UTS for both route materials may be due to similar hardness range as strength is proportional to hardness for similar type of microstructure. Fracture toughness and impact toughness properties are significantly influenced by microstructural parameters, such as grain size, type of phases & its morphology, volume fraction of inclusions, carbide size and their distribution etc. [Ravichandran and Vasudevan (1997)]. Among other parameters, impact toughness and fracture toughness properties at room temperature are greatly influenced by carbide particle size and its distribution. Both Impact test and Fracture Toughness test are a measure of the capability of the material to resist the growth of a crack. The crack growth is retarded when it encounters a carbide which makes it difficult for the crack to propagate. The crack has either to go through the carbide particle by breaking it (which is very difficult due to higher strength/hardness of the carbide) or change its course and circumnavigate the carbide particle. Thus, more the number of carbides being encountered by the crack, more will be the resistance to its propagation. Impact toughness test results show that CR material has better properties as compared to SF route. This may be due to finer carbide obtained in the former as against coarser carbides in the latter. Similarly, Fracture toughness (K IC ) value for CR material is higher as compared to that of SF route in both lower and higher hardness range. Finer carbide distribution in the former lead to better fracture toughness properties in both hardness ranges as compared to latter. In tempered martensitic microstructure, mechanism of fracture involves the nucleation and propagation of micro cracks from large carbides. Fine carbide microstructure results into higher local fracture stress which leads to higher fracture toughness [Cao et al. (2013), Takebayashi et al. (2013)]. Furthermore, distribution of carbide also affects the fracture toughness. When compared with increase in hardness, fracture toughness of both materials decreases. A similar type of observations was reported in the research work of [Karaaslan and Akca (2009), Souki et al. (2011)]. Due to the presence of uniformly distributed carbide, the crack gets deflected and thus leads to tortuous path which in turn increases fracture toughness. Alloying elements and heat treatment plays a vital role in determining the fracture toughness of a material [Ravichandran and Vasudevan (1997), Vishnevsky and Steigerwald (1968), Sitek and Trzaska (2011), Kumar et al. (2017)]. Among the alloying elements present, higher percentage of carbon in SF route reduces its fracture toughness although higher percentage of vanadium increases its toughness upto some extent. The extent of decrease in fracture toughness due to increase in carbon percentage is large as compared to other alloying elements. So, larger carbide size and higher content of carbon leads to lower fracture toughness value of SF route material as compared to that of CR. 7. Conclusion 1. Tensile properties like YS, UTS, %El and %RA obtained for both route materials are similar at both hardness levels. 2. Charpy impact strength obtained for SF route material is lower than that of CR at both hardness levels. 3. Fracture toughness of SF route material is lower than that of CR in both LS & TS orientations. 4. For both route materials, fracture toughness increases with decrease in hardness. 5. Larger carbide in case of SF route material leads to its lower fracture toughness as compared to that of CR material. Acknowledgments The authors gratefully acknowledge the extended support provided to this work by KCTI (Kalyani Centre for Technology & Innovation) for providing financial funding, laboratory and library facilities. The authors also acknowledge the support provided by Bharat Forge Ltd, Pune and DSIR (Department of Scientific and Industrial Research), Govt. of India. Finally, the authors would like to express special thanks and gratitude to review committee and top management of Bharat Forge Ltd for granting the permission to publish/present the research