Study of the effects of Thermal Aging coupled with Shot-peening Treatment of Carbonitrided DIN 1.6587 Low Alloy Steel on Retained Austenite and Residual Stresses

Abstract

This work investigated the effects of thermal aging and shot-peening of carbonitrided 1.6587 steel grade on retained austenite and residual stresses. After carbonitriding samples contained maximum of 0.87% carbon, 0.34% nitrogen and 50 mass. -% retained austenite. Thermal aging conditions investigated were -30°C, 90°C and 150°C while the aging period were 14, 96 and 720 hours. After thermal aging, some of the samples were subjected to shot-peening treatment using standard cut wire shots StD – G3 - 0.6 mm VDFI 8001 with intensity in the range of 0.25 to 0.30 mmA and coverage of 1.00 to 1.24 x 98%. It was found that the quantity and size of finely disseminated precipitates increase with increasing aging temperature and time accompanied by darkening of martensite plates. The magnitude and distribution of hardness in the case layer was highly influenced by aging conditions with an increase of up to 50 HV for peak hardness and up to 120 HV at a depth of 50 µm on aging at -30°C. Aging at 90°C, hardness distribution remains relatively unaffected while at 150°C for 720 hours the magnitude of hardness is slightly improved mainly to decomposition of retained austenite to bainite. Aging at 90°C, retained austenite remains relatively stable and readily transforms on aging at - 30°C and 150°C for 720 hours; transformation of up to 20% of the initial retained austenite can be reached which enhances compressive residual stresses of up to -120 MPa in retained austenite and up to - 250 MPa in martensite phase. Aging coupled with shot-peening treatment induces transformation of up to 48% of the initial retained austenite and enhances compressive residual stresses of up to -1200 MPa in both phases. It can be concluded that thermally aging coupled with shot-peening treatment reduces retained austenite and significantly enhances residual stresses which is essential to improved ductility and fatigue life of carbonitrided parts.