PSI - Issue 68

Julie Papin et al. / Procedia Structural Integrity 68 (2025) 727–733 Julie Papin et al. / Structural Integrity Procedia 00 (2025) 000–000

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4. Discussion 4.1. Effect of the quenching rate on fracture toughness

By comparing the Master curves obtained at different quenching rates and for the two tempering durations at 640°C (Fig.3), one can first observe that T 0 decreases (and the fracture toughness increases) with increasing the quenching rate. Comparing Fig. 3a and b shows that the decrease in T 0 as a function of the quenching rate is lower for a longer tempering duration (20h vs. 6h).

Fig. 3. Master curves for the four quenching rates and for two tempering durations (6h and 20h) at 640°C.

4.2. Effect of the tempering conditions on fracture toughness By comparing median stress intensity factors obtained with the Master curve method for the 2000°C/h quenching rate (Fig. 4), at different tempering conditions, variations of reference temperature T 0 as high as 55°C can be observed, which is higher than the standard deviation on T 0 . The curves appeared clustered in two sets, namely, a set at ‘low’ tempering parameters TP < 20.00 (with T 0 close to -110°C), and a set at ‘high’ tempering parameters TP > 20.00 (with T 0 close to - 60°C). The fracture toughness was optimal for a tempering parameter around 19.25 (taking the standard deviation on T 0 into account) for this quenching rate. 4.3. Cross-effects of quenching rate and tempering time at 640°C

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Fig. 4. Master curves of five tempering conditions for a 2000°C/h quenching rate.

Comparing the previous two tempering conditions at all quenching rates, the shifts of T 0 were similar for quenching rates between 800°C/h and 8000°C/h, whereas the shift at 150°C/h was 2.4 times smaller. Furthermore, the decrease in T 0 was linear with respect to the logarithm of the quenching rate for the 6h tempering duration, but this was only true from 800 °C/h to 8000°C/h for the 20h tempering duration (Fig. 5). A possible explanation to this observation is that the microstructure quenched at 150°C/h is the only one that contained proeutectoid ferrite (which is insensitive to tempering), while other microstructures were composed of lower and upper bainite. Another plausible explanation for, this very low quenching rate would be that extensive auto-tempering during quenching may have lowered the further influence of the different subsequent tempering conditions.