(V) Case study- A Failure Analysis of Semi Float Axle shaft- Incipient Melting Analysis

Primary function of Axle shaft is to transmit the power from Differentials to Wheels. Semi float axle shaft experience the Torsional Load in addition to bending load. In this case study –premature failure of Axles shafts were observed from field return with in 1000 kms run. The failure fashion is concentrated at the one location near shaft flange bearing diameter. Upon fracture analysis observed incipient melting at grain boundaries which occur due to repetitive heating at forging temperature & evidenced by MnS inclusion morphology change over from stringers to globular shape & precipitated at PAGB grain boundaries , Tensile Tearing at the core location also evidenced along with coarser Grain Size ASTM No. Zero. In order to generate the defects, the DOE is conducted. Batch-A -3 Samples are processed with Regular Forging Temperature at 1250 in MGM machine (Material Gathering Machine). Batch-B- 3 Samples were repeatedly heated 3 times at this forging temperature without allowing the part temperature drop to room temperature. Though the online pyrometer reads the correct set-up Surface temperature, the inside temperature of the shaft at core, could have gone reasonably much higher temperature. Upon metallography,the Batch B samples micro structure observed coarse grain size, incipient melting, MnS inclusion globular precipitated at PAGB. The Batch A samples found No deficiency in Metallurgy. So it was practically confirmed & evident that the axle shaft repeatedly heated mixed up with production lot, went to field, causing the premature field failure. As CAPA, the shafts used for Repetitive heating are identified separately with marks &put on separate scrap bins. The Robotic automation is also Implemented to minimize the manual intervention so that the once heated part will never come back to original process path & also operator do not have chance to re-heat the part.