Effect of plasma nitriding on fatigue crack growth on AISI 4140 steel under variable amplitude loading

AISI 4140 steel was subjected to a plasma nitrification process using 50/50 gas mixture of N2 and H2 at 500 °C for 2 h. The structural and mechanical properties of the plasma-nitrided and -unnitrided specimens were investigated using microhardness test device, servo-hydraulic universal testing machine, scanning electron microscope, and optical imaging devices. The influence of the plasma nitrification on fatigue crack growth (FCG) was investigated by applying block overload cycles on test specimens. The rate of FCG was accurately determined from the sequence of image frames taken from the specimens during testing. Results of analyses indicated that the application of a 10-cycle tensile block overload results in the retardation of FCG for both unnitrided and nitrided specimens, however, the period of crack retardation is more prolonged for the unnitrided specimens. Besides, it was also found that in a constant amplitude fatigue test the plasma nitrification leads to an increase in total FCG life.