Premature failure of a leaf spring due to improper materials processing

Failure analysis of a leaf spring which failed prematurely during service was carried out using optical and scanning electron microscopy, hardness and tensile testing, and residual stress evaluation by X-ray diffraction. Quench cracks, which are aligned normal to the length of the leaf, have been identified in shot peened and polished surfaces. These cracks have been attributed to an improper quenching process. It is established by theoretical analysis that the leaf thickness is smaller than the critical plate thickness required for this composition of steel, and that leads to an increase in quench severity. It appears that some of these quench cracks have propagated by a fatigue mode which is confirmed by the presence of beach marks on the fractures surface. Observation of intergranular cracking and the presence of FeS inclusions at the prior austenite rain boundaries implies that some sort of grain boundary embrittlement might have facilitated crack growth and led to failure. It has been suggested that quenching should be carried out by recommended procedures guided by the thickness of the component and chemical composition of the steel. Careful inspection of the surface after quenching must also be carried out to maintain quality assurance in order to avoid premature failure.