Effect of Carbon on Stress-Relief Cracking Susceptibility of T23 Steel

The coarse-grained heat-affected zone (CGHAZ) samples were prepared from T23 steel with three different carbon contents on a thermal simulator, then isothermal slow strain rate tensile tests were carried out at 500° ~ 750°C, and the effect of carbon content on the stress-relief cracking (SRC) sensitivity of T23 steel was evaluated according to fracture ductility. The microstructure evolution of the CGHAZs in the process of SRC was observed by optical microscopy, structural equation modeling energy-dispersive spectroscopy, and transmission electron microscopy energy-dispersive spectroscopy/selected area electron diffraction to reveal the mechanism of SRC. Finally, the feasibility of adjusting carbon content to prevent SRC was discussed. The results showed the classical theory of intragranular precipitation hardening cannot explain the cause of SRC, and a new viewpoint was put forward that the precipitation of M23C6 at the grain boundary was the main reason for SRC. The precipitation of M23C6 carbide cannot only provide a preferred site for void nucleation but also leads to the depletion of alloy elements in the matrix near the grain boundary. The interaction of these two aspects leads to the direct weakening of grain boundaries. Moreover, the SRC of T23 steel could be inhibited when the carbon content was determined to be close to the ASME standard lower limit of 0.04 wt-%. In summary, reducing the carbon content can inhibit the SRC sensitivity of T23 steel, which is not due to reducing the relative weakening of the grain boundary by reducing the precipitation hardening in the grain interior but to inhibiting the direct weakening of the grain boundary by reducing the precipitation of M23C6 at the grain boundary.