Fatigue of Metals: Part Four

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Comparison with Carbon Steels

The fatigue strength of austenitic and duplex stainless steels were compared with those of high strength ferritic-martensitic carbon steels, DP steel, at the same yield strength levels.

Figure 1: Comparison of fatigue properties between stainless and carbon steels at different yield strength levels

The result of this comparison is given in Figure 1. On all strength levels studied the stainless steel showed better and often substantially better, fatigue properties than a corresponding carbon steel.

In the work of B.Šurštašić et al. results of fatigue strength determination of a selected spring steel is presented. The investigated spring steel is designated as standard DIN 51CrV4 (W.Nr. 1.8159, EN 10089). As can be seen, fatigue strength (S-N curves) of the selected steel is determined in two different loading modes, i.e. tension-compression (T/C) and rotating-bending (R/B) for two different treatment conditions (HT1 and HT2).

Heat treatment HT1 (austenizing at 870°C for 10 min., fast cooling in N2 and tempering at 425°C for 1 hour) gives higher Rockwell hardness (HRc), tensile strength (Rm) and yield point (Rp_0.2) but lower ductility (A and Z) compared to HT2 (austenizing at 870°C for 10 min., fast cooling in N2 and tempering at 475°C for 1 hour).

Figures 2 and 3 show S-N curves obtained in T/C fatigue regime on notched cylindrical samples for two different heat treatment conditions. It can be seen that the fatigue strength of longitudinally oriented material is higher. HT1 also gives a little higher fatigue limit compared to HT2. Interestingly, in both cases the slopes of S-N curves are almost the same for the same heat treatment, independent of segregation orientation.

Figure 2: S-N curves of HT1 material for T/C fatigue mode

Figure 3: S-N curves of HT2 material for T/C fatigue mode

Figure 4 shows the S-N curve obtained in the R/B regime on smooth cylindrical samples for two different heat treatment conditions and longitudinal segregation orientation. As one can see, the fatigue strength of the HT1 material is significantly higher. Again, the slopes for both heat treatment conditions are similar to the slopes obtained in T/C fatigue mode.

Figure 4: S-N curves of HT1 and HT2 material for R/B fatigue mode

Fatigue strength obtained on smooth samples in the R/B regime is approximately two times higher compared to the fatigue strength in the T/C regime on notched samples. It is in good agreement with the theoretical notch sensitivity factor of used specimens (K_t=1 and K_t=2) neglecting the mode of fatigue.

As a conclusion B.Šurštašić et al. noted that appropriate heat treatment can lead to the increase of the steel’s fatigue strength. Steel with higher hardness, higher tensile strength and yield point has higher (approx. 8%) fatigue strength.

The notched cylindrical specimens (K_t=2) have an adequately lower fatigue limit compared to the smooth cylindrical samples (K_t=1) considering the selected fatigue testing mode and the type of specimens. The T/C mode fatigue testing on smooth specimens still has to be performed in order to obtain a more complete figure of the dynamic properties of investigated steel.