Effect of non-metallic inclusions (NMI) on crack formation in forged steel

Abstract

Effect of non-metallic inclusions on crack formation in forged steel components is presented in this work. Ultrasonic phased array system was used to locate and identify the defects in the forged steel samples. X-Ray Computed Tomography (XCT), Scanning Electron Microscopy (SEM), optical metallographic and image analysis, Optical Emission Spectrometry (OES) techniques were used for defect analysis and material characterization. Few subsurface cracks were found in the rejected sample of quench tempered steel acquired after the forging process. Defects were clustered near the core of the specimen. Ultrasonic phased array transducer system and XCT confirmed the presence of approximately 100 µm wide and 10 mm long cracks. Optical and SEM images suggested that the cracks were discontinuous and revealed a predominant presence of MnS and Al2O3 inclusions along with traces of SiO2 and SiC inclusions. Further, alumina inclusions were found to be clustered near the core of specimen, which is the crack zone. Cracks could have been initiated due to quench stress and augmented by the clustered alumina inclusions and propagated through the grain boundaries. Strain analysis using EBSD was performed in a sample near the crack to study the distortion of crystal lattice around the alumina inclusions. Considerable strain distribution could be observed in the matrix containing alumina inclusion, but strain seems to be relieved near the crack. It was presumed that alumina inclusions in the matrix acted as a stress concentration factor and led to the crack initiation mechanism.