MBE study was carried out at a magnetizing frequency of 120 Hz. This was adequate since the shot peening effect was found by microstructural studies on a failed piece to have extended upto 0.1-0.2 mm of depth. The rms voltage, V_rms, of MBE signal was measured at four circumferential positions (12 O'clock, 3 O'clock, 6 O'clock and 9 O'clock with 12 O'clock position representing outer diameter of the spring section) and also at four different positions along the length. MBE results indicated a variation in rms voltage along the circumferential direction with m aximum value at 12 O'clock position (V_rms = 240 mv) and a minimum at 9 O'clock (V_rms = 45 mv). As the compressive stresses were introduced due shot peening, MBE results demonstrated that stress relaxation occurred at 12 O'clock position of the s p ring during service. XRD study was made at the corresponding points to determine the absolute values of stresses The XRD technique showed typical values of stresses as follows - a compressive stress of 380 MPa at 12 O'clock position and that of 650 MPa at 9 O'clock position. It was thus found that the residual stresses at the inner surface (3-6-9 O'clock position) of the spring section had much more compressive stresses as compared to the outer surface (9-12-3 O'clock position). It was found that highest residual stress (~650 MPa) was less than half of the yield stress (~1600 MPa) of the material, obtained from hardness data.

To get a better understanding of the cause of failure, residual stresses were measured by XRD technique over some cross sectional surfaces. It was found that there exist a highly tensile stressed region (700 MPa) at the subsurface close to the inner surface of the spring section. Microstructural analysis also revealed high inclusion content in the materials. Fractographical analysis indicated that the failure was not due to fatigue fracture. Hence, it could be inferred from the analysis that the presence of inclusions facilitated the crack initiations in the presence of the sub-surface tensile stress which also helped further crack growth. In fact, it was observed that most of the failure started at the regions of intense stress gradient.