Quenching cracks in medium carbon steel initiated at manganese sulfide inclusions

Abstract

Abstract Quench crack propensity has been investigated for two identical medium carbon steels (50CrMo4) which only differed in sulfur content. The standard variant of the material was affected by manganese sulfide inclusions whereas the low sulfur material was essentially free of sulfides. 80 mm, cylindrical test specimens were used. The test specimens had been cut out of cross rolled steel plates in which the manganese sulfide were flattened to discoid shape. The specimen orientation was in direction of principal deformation where the flattened sulfides were aligned with the test specimen axis. The largest sulfides reached lengths of 150 μm. Both materials were austenitized at 850 °C for 60 minutes, quenched in a salt-bath at 180 °C and immediately annealed at 425 °C for one hour. It turned out that the standard material developed quench cracks which initiated at manganese sulfide. No cracks could be found in the low sulfur material. Thermal and transformational stresses, which develop during quenching, aggravate the stress situation around the manganese sulfide and cause the cracking. The results are interpreted in fracture mechanical terms.