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Jul 3, 2024Keywords:
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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.
