Thermal stability of internal stress in drawn pearlitic steel

Anisotropy found at the difference in yield stress between uniaxial tensile and compressive tests of the drawn pearlitic steel was studied at the perspective of thermal stability of internal stress. The 0.84 wt.% C steel was drawn to a drawing strain of 1.26, followed by the heat treatment at various temperature ranging from 200 °C to 500 °C. These samples were examined by both the tensile and compression tests in the direction parallel to the drawing direction. The as-drawn specimen shows that the tensile yield stress is higher than that in the compression, indicating the back stress parallel to the drawing direction exists to bring the anisotropy. The difference in the yield stress decreased with increasing the post heat treatment temperature, while the compressive yield stress showed significant increase. These indicates the less thermal stability of compressive internal stress. The microstructure observation showed that the elongated morphology of the pearlite and the texture of ferrite showed no significant change in the heat treatment at the temperature less than 500 °C. This thermal instability of internal stress was confirmed by the X-ray diffraction which shows the elastic extent of the {110} spacing of ferrite without the recovery of dislocations indicated by the half width of diffraction peak. These results suggests that the internal stress is governed by constraint effect by interphase boundaries between ferrite and cementite.