Very high cycle fatigue behaviour of 42CrMo4 steel with plate-like alumina inclusions

Abstract

Recent progress in steel refining shows significant reduction of non-metallic inclusions (NMIs) of which alumina (Al2O3) is one of the most problematic. Among other refining methods, metal melt filtration by ceramic foam filters shows promising results in steel cleaning. In the present work the influence of alumina inclusions on the fatigue behavior is investigated after the reaction of steel melt with filters. Different batches are compared where carbon-bonded alumina foam filters with different coatings were introduced into the steel melt of 42CrMo4 for 10 s (so called “finger test”). Fatigue tests were performed using ultrasonic fatigue testing (USFT) up to 109 cycles. Specimens were nitrided in order to prevent crack initiation from the surface and to study internal failure on NMIs. Surface hardening of quenched steel increased fatigue limit significantly. Metallographic sections were analyzed using optical and scanning electron microscopy (SEM) for the estimation of NMIs distribution properties. NMI size distribution analysis based on maximum Feret diameter (instead of area) is found to be an effective method for detecting plate-shaped inclusions. Fracture surfaces after fatigue tests were investigated by methods of SEM and confocal laser scanning microscopy (CLSM), revealing that plate-like NMIs initiate crack with all their area even being inclined to the crack plane. Properties of crack initiating NMIs – alumina plates and MnS dendrites – are compared and analyzed. Formation of alumina NMI as plate lead to significant enlargement of its stress-concentrating area in comparison to the spherical shape of the same volume. Thus, total NMI content reduction in steel could give no fatigue limit improvement if NMI morphology changes from spherical to plate-like.