PREMATURE FAILURE OF A LEAF SPRING DUE TO IMPROPER MATERIALS PROCESSING

Failure analysis of a leaf spring which failed prematurely during serv ice was carried out using optical and scanning electron microscopy, ha rdness and tensile testing, and residual stress evaluation by X-ray di ffraction. Quench cracks, which are aligned normal to the length of th e leaf, have been identified in shot peened and polished surfaces. The se cracks have been attributed to an improper quenching process. It is established by theoretical analysis that the leaf thickness is smalle r than the critical plate thickness required for this composition of s teel, and that leads to an increase in quench severity. It appears tha t some of these quench cracks have propagated by a fatigue mode which is confirmed by the presence of beach marks on the fracture surface. O bservation of intergranular cracking and the presence of FeS inclusion s at the prior austenite grain boundaries implies that some sort of gr ain boundary embrittlement might have facilitated crack growth and led to failure. It has been suggested that quenching should be carried ou t by recommended procedures guided by the thickness of the component a nd chemical composition of the steel. Careful inspection of the surfac e after quenching must also be carried out to maintain quality assuran ce in order to avoid premature failure. (C) 1997 Elsevier Science Ltd.