Abstract: A large number of cracks were found on the inner surface of SA-387Gr.91 steel spherical head after quenching and tempering processes which were used for improving the hardness. The chemical composition, microstructure, hardness and fracture appearance of the cracks and the nearby base metal were detected and analyzed. The results show that the cracks are quenching ones, and they are not related to the chemical composition and the metallurgical quality of the base metal. The prior austenite grain size of the base metal is too large and the microhardness is above standard value. It is revealed the by the experimental results that the cracks are mainly caused by coarse grains resulted from too high hot stamping temperature and the high cooling rate resulted from inappropriate quench medium. By lowering the hot stamping temperature, changing water cooling to forced air cooling and lowering the tempering temperature, no quenching cracks ever appear in the spherical head, and the surface hardness meet the expected requirements.

Key words: SA-387Gr.91 steel, spherical head, heat treatment, quenching crack