Fatigue strength improvement of a hard chromium plated AISI 4140 steel using a plasma nitriding pre-treatment

The effect of a plasma nitriding (PN) pre-treatment on the fatigue performance of hard chromium (HC) plated AISI 4140 steel has been investigated by conducting a series of rotary bending fatigue tests at a frequency of 95 Hz. hourglass shaped test specimens of 4-mm diameter had been plasma nitrided at 510 degrees C for 4, 8 and 12 h. It was found that HC-plated specimens with a coating layer of 23 +/- 2 mu m thickness showed approximately 33% reduction in fatigue strength when compared to quenched and tempered (Q&T) specimens. An application of the PN pre-treatment before the plating process was effective in improving the fatigue performance of HC-coated steel. An improvement of 71% in the fatigue strength of pre-treated specimens was recorded as compared with the specimens, which were HC plated only. The results also indicated that prolonged nitriding time did not cause better improvement in the fatigue performance.