Transformation of retained austenite in carburized 4320 steel |
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Authors: | R W Neu Huseyin Sehitoglu |
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Affiliation: | (1) Department of Mechanical and Industrial Engineering, University of Illinois, 61801 Urbana, IL |
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Abstract: | A systematic study of stress-induced and thermal-induced transformation of retained austenite in carburized 4320 steel with
an initial retained austenite of 35 pct has been conducted. The transformation was monitored by recording the change in volume
of smooth fatigue specimens. Stress-induced transformation was studied by conducting monotonic and cyclic tests at temperatures
in the range from 22 °C to 150 °C. The volumetric transformation strain was as large as 0.006 at 22 °C. The anisotropy of
the transformation was such that the axial transformation strain component exceeded the diametral transformation strain component
by a factor of 1.4. Thermal-induced transformation was investigated with temperature stepup tests in the range from 150 °C
to 255 °C at constant stress (-500 MPa, 0 MPa, and 500 MPa) and with static tests where temperature was held constant at zero
load. The maximum thermal-induced volumetric transformation strain of 0.006 was independent of stress. However, the anisotropy
of the transformation strain components was dependent on stress direction and magnitude. An axial tensile stress increased
the axial transformation strain relative to the diametral transformation strain. The influence of low-temperature creep(T = 150 °C) on the anisotropy of strains is noted. The differences between stress-induced and thermal-induced transformation
mechanisms are discussed. Thermal-induced transformation primarily occurred at temperatures between 100 °C and 200 °C, with
the rate of transformation increasing with temperature, while the stress-induced transformation primarily occurred at 22 °C,
with the rate of transformation decreasing with increasing temperature. There was no stress-induced transformation above 60
°C. |
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