PSI - Issue 18

Nikolas Baak et al. / Procedia Structural Integrity 18 (2019) 274–279 Author name / Structural Integrity Procedia 00 (2019) 000–000

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performance, these parts usually undergo subsequent processing steps like deep rolling or autofrettage to induce compressive residual stresses. Oevermann et al. (2017) and Saalfeld et al. (2018) investigated the influence of hot deep-rolling on the fatigue performance of AISI 4140 and showed a residual stress induced change in the slope of the Basquin curve, what was represented in a better fatigue performance for lower stresses at the expense of the fatigue life for higher amplitudes. Baak et al. (2018) showed, that optimised deep-drilling processing steps allow the aimed induction of the wanted residual stresses and therefore have the ability to replace the subsequent processing steps. Micro-magnetic measurements were used to determine the material parameters and microstructural properties by numerous researchers. For example, Santa-aho et al. (2012) utilized Barkhausen noise measurements to determine the case-depth of hardened steels and Morthy et al. (2006) used high frequency Barkhausen noise to assess residual stresses in case-carburized EN36 steel. Micro-magnetic measurements can also be applied to characterize component characteristics. Baak et al. (2017) showed the suitability of micro-magnetic testing methods to determine the spring characteristic of incremental formed austenitic disc springs.

2. Materials and Methods 2.1. Materials

For this study two different melts of AISI 4140 (42CrMo4+QT, 1.7225) with different sulphur content were investigated. S110 contains 0.011 wt.-% S and S280 0.028 wt.-% S. Both melts meet the standards of 42CrMo4, respectively 42CrMoS4 defined in EN 10083-3 and otherwise do not differ significantly. Table 1 shows the chemical composition of the investigated melts.

Table 1. Chemical composition (wt.-%). C Si

Mn 0.85 0.83

P

S

Cr

Mo 0.18 0.17

Fe bal. bal.

S110 S280

0.41 0.39

0.18 0.17

0.011 0.014

1.01 0.97

0.011 0.028

Both melts were quenched and tempered (QT) resulting in a comparable tensile strength of 965 MPa (S110) and 969 MPa (S280) and yield strength of 848 MPa (S110) and 861 MPa (S280), respectively. 2.2. Methods Single-lip deep drilling results in high quality bores, qualitative parameters like surface roughness, straightness deviation, as well as roundness and diameter deviations of the bore meet excellent standards. The asymmetrical tool design (Fig. 1), requires guide pads, which on the one hand allow the tool to guide itself in the bore; on the other hand, transmit the cutting and passive forces into the bore wall (Pfleghar 1974).

Fig. 1 Comparison of the autofrettage process with the single-lip deep drilling. (Greuling et al. 2006)