It can be seen that with increasing the nitriding temperature, the fatigue limit ([sigma]-1) of nitrided samples grows at the beginning from 260 MPa (nonnitrided steel) to the maximal value of fatigue limit (510 MPa) achieved at nitriding temperature 630[degrees]C (Fig.
The further increase in nitriding temperature from 550[degrees]C to 700[degrees]C gives a decreased limited durability of nitrided steel.
The fractured surface of fatigue nitrided and nonnitrided samples was examined with a scanning electron microscope to assess the effect of nitriding on fatigue crack initiation and its propagation.
The fatigue failure of nitrided samples in the small load region begins inside the sample in the boundary, which separates the nitrided (hardened) and unhardened metal (Fig.
It should be noted that the same fracture mechanism for nitrided specimens has been also reported elsewhere [7, 10, 18-21].
Figures 4 (a,b) show the X-ray diffraction profiles of (a) as received and (b) laser surface nitrided (with a power of 600W) Ti-6Al-4V.
Microhardness of the top surface and cross section of the laser surface nitrided Ti-6Al-4V was carefully measured to observe the influence of laser surface nitriding on microhardness and its distribution.
Figure 5 shows the variation of microhardness with depth from the surface of laser surface nitrided Ti-6Al-4V lased under different processing conditions.
Microhardness of the surface nitrided layer was improved (to a maximum of 900 VHN) as compared to 260 VHN of as-received substrate.