Effect of volume fraction of undissolved cementite on the high cycle fatigue properties of high carbon steels

A present study was performed to investigate the effect of volume fraction of undissolved cementite on the high-cycle fatigue (HCF) properties of high carbon steels. High carbon steels were heat treated to have three distinctive volume fractions of undissolved cementite phase in the martensite matrix. With increasing holding time at the austenitizing temperature of 800 °C, the volume fraction of undissolved cementite gradually decreased from 9% to 1%. A series of tensile and micro-hardness tests showed that the specimen with 1% volume fraction of undissolved cementite exhibited the highest strength, which was mainly attributed to the increased carbon content in the martensite matrix. Though of its highest tensile strength, however, the fatigue strength was the lowest in the specimen with 1% volume fraction of undissolved cementite, which was the reverse trend as compared to the general relationship between fatigue strength and tensile strength. This was attributed to the higher propensity of crack initiation in the specimen containing fewer amounts of undissolved cementite, where cracks were observed to nucleate at the interfaces between nonmetallic inclusions (Al2O3 and SiO2) and martensite matrix during HCF tests. A probabilistic model was developed to predict fatigue strength of heat treated high carbon steel and compared with the experimental data.