PSI - Issue 35

Vera Friederici et al. / Procedia Structural Integrity 35 (2022) 106–114 V. Friederici et. al / Structural Integrity Procedia 00 (2019) 000–000

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Table 1. Heat treatment parameters and resulting hardness and microstructure. condition Austenitization Temp. and time Quenching medium Annealing Temp. and time Hardness / HV1

Microstructure

Core

250-270

Annealed bainit with small amounts of annealed martensite Mixture of bainit and martensite

Transition 770, 2h

Oil

500, 1h

346±16

raceway

850, 2h

Oil

-

667±13

Martensite

The derived quasi-static material parameters are shown in Table 2. The material properties are related to the hardness of the material conditions. The raceway conditioned material as the hardest material shows the highest tensile strength R m and yield strength R p0.2 but the lowest fracture toughness K IC . The values of tensile strength and yield strength for transition and core condition decrease similarly as the hardness. Elongation A g and A follow the opposite trend. Fracture thoughness of core and transition condition on the other hand are very similar and much higher than the value of the raceway conditioned state.

Table 2. Results of quasi-static testing.

IC / MPa*m 0.5

R p0.2 / MPa R m / MPa

A g / %

A / % K

Sample

Core

677±37

871±25 1138±18 1863±158

6.8±0.6 3.99±0.5 1.8±0.8

14.7±1.5 10.8±1.1 2.3±1.2

138±11 139±10

transition 1069±19 Raceway 1273±31

34±1

The crack propagation curves of core and transition zone show a great similarity (Figure 1a). In comparison, crack propagation is significantly faster in the hardened specimens (Friederici, 2021). For all heat treatment conditions the curves shift to the left with increasing stress ratio. Fitting the linear part of the crack propagation curves by the Paris equations coefficients C and m can be calculated, which are displayed in Table 3. Figure 1b shows preliminary experiments with R = - 1 and the predicted values. Paris coefficients of prediction and results correlate also well as shown in Table 3.

Fig. 1. Crack propagation curves. (a) Crack propagation curves for stress ratios of 0.1, 0.3 and 0.5 of core and transition region. (b) Prediction and preliminary results of crack propagation for stress ratio of -1.