PSI - Issue 13

Avanish Kumar et al. / Procedia Structural Integrity 13 (2018) 548–553 Avanish et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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In order to determine the plane-strain fracture toughness, K IC of steels, test data in the form of load (P) versus crack opening displacement were plotted for all the specimens. The values of P Q and P max were identified for all the specimens as per the theory outlined in section 9 of the ASTM E399-12 Ɛ 3 (ASTM 2013). The ratio P max /P Q did not exceed 1.10. Plastic zone size was calculated for all the specimens at the maximum load and B, a and (W-a) were all found to be greater than 25 times the plastic zone size. We assessed the test parameters and results and found that all the conditions to qualify K Q (candidate fracture toughness) as K IC were met. Fracture toughness test results are shown in Table 2. It has been found that the decrease in isothermal transformation temperature leads to worsening of fracture toughness of bainitic steels. With a decrease of 100°C in transformation temperature, fracture toughness reduced by ~ 40%. It is well documented that the ductility and toughness of carbide free bainitic steels are governed by the volume fraction of ductile phase austenite and its stability towards stress or strain induced transformation to brittle phase martensite (Bhadeshia & Edmonds 1979; Bhadeshia & Christian 1990; Bhadeshia 2010; Avishan et al. 2013; Morales-Rivas et al. 2016). The phase transformation of metastable austenite to brittle phase martensite provides transformation toughening, crack blunting and crack closure due to the compression caused by dilatation due to phase transformation (Mei & Morris 1991; Antolovich & Chanani 1972; Sudhakar & Dwarakadasa 2000). It has also been reported that energy dissipation in martensitic transformation reduces the net energy available for the crack growth (Wu et al. 2013). This leads to delayed void nucleation and enhanced toughness of steels. The SEM micrographs (Fig. 2) showed increase in austenite plate thickness with increase in austempering temperature. However, island-type retained austenite blocks were rarely seen in even NB350 steel. Thus, based on the available literature and obtained results, it is expected that the volume fraction of softer phase austenite is dominating factor than the morphology in controlling the fracture toughness of produced bainitic steels. The variation in Charpy impact energy with austempering temperature is given in Table 2. Impact energy increases from 7 J to 15 J when austempering temperature increases from 250 to 350°C. Since impact test is a high strain rate test, it is likely that the materials behave in a less ductile manner. NB250 steel contains very fine bainitic ferrite plates with high dislocation density, so very less deformation by dislocation movement in bainitic ferrite is expected. Moreover, the volume fraction of retained austenite, which is the primary carrier of ductility, is also the least. It was reported earlier that the increasing the volume fraction of film type retained austenite is a major factor in improving the impact energy (Avishan et al. 2012; Yang et al. 2012; Caballero & Bhadeshia 2004; Garbarz & Niżnik – Harańczyk 2015) , but the present results show that the steels which were transformed at higher transformation temperatures show better impact toughness properties despite of lower volume fraction of film type retained austenite. Ductile phase retained austenite causes stress relief and blunting of the crack tip during its growth and hence volume fraction of this phase present within the microstructure is a critical factor to improve the impact toughness properties of bainitic steels (Wu et al. 2013). Therefore, it appears that higher volume fraction of retained austenite at higher austempering temperature could be a more important factor than its morphology in controlling the toughness variation in the present study. This study shows for the first time that an increase in austempering temperature for bainitic transformation enhances fracture as well as impact toughness. Higher amount of plastic work absorbed due to larger ductility is partly responsible for this. Moreover, greater volume fraction of austenite also implies higher energy consumed during austenite to martensite transformation. Thus, strength and toughness show opposite trends with a change in austempering temperature. Hence, an optimum operating window for heat treatment for bainitic transformation needs to be decided based on the engineering application. 4. Conclusions

Acknowledgement

The authors would like to acknowledge the Centre of Excellence in Steel Technology (CoEST) and Funding for Infrastructure in Science and Technology (SR/FST/ETII-023/2012(C)) for provision of laboratory facilities. The financial support from Industrial Research and Consultancy Centre (IRCC), Indian Institute of Technology, Bombay is also appreciated.