Effect of Aging Treatment on Toughness and Hardness Behavior in Custom 450 PH Steel

Custom 450 precipitation-hardenable martensitic stainless steel was solutionized and aged to achieve the best possible combination of hardness and impact toughness for cryogenic application. As-received material was solution-treated at 1040 degrees C followed by aging at 565 degrees C for different holding times, 2-16 h. On aging, the austenite phase is reverted by the memory-effect phenomenon, and its phase fraction was found to increase linearly with aging time. Austenite reversion (gamma) occurred with acicular morphology at lath, subblock, and block boundaries and globular morphology at the packet and prior austenite boundary. It was observed that acicular morphology of reverted-gamma converted to globular with the aging time, and it was found to influence the hardness, and Charpy V-notch (CVN) impact toughness significantly influences. On the aging treatment of 2 h, the hardness was maximum, and toughness was least because of the possible precipitation effect. However, an enormous increase in toughness was observed for 4 h of aging with a small drop in hardness. A higher toughness is due to the presence of well-developed continuous reverted acicular-gamma. On aging beyond 4 h, the hardness increases, causing a decrease in the toughness to a similar level as solution-treated samples. Moreover, acicular austenite-gamma thickness and globular- y fraction increases with aging beyond 4 h, which may have accelerated the crack propagation. The dislocation density determined by the modified Williamson-Hall and modified Warren-Averbach method also increased on aging beyond 4 h, which may have attributed to higher hardness. The irregular trend in mechanical properties with aging time is due to the competition effect between the precipitation, phase transformation, austenite morphology, and recovery process with the aging time. Thus, careful aging for 4 h results in the best combination of hardness and toughness in Custom 450 stainless steel.

Automated summary

Custom 450 precipitation-hardenable martensitic stainless steel was solutionized and aged to achieve the best combination of hardness and impact toughness for cryogenic application. The aging time significantly influenced the phase transformation, hardness, and toughness of the material.