4685 - IMPROVEMENT OF DEGRADATION OF TOOL STEELS DUE TO PLATING

In order to evaluate the amount of degradation due to plating, SERT (slow extension rate test), delayed fracture test and fatigue test were conducted on JIS-SCM440 chromium molybdenum steel bars. Delayed fracture was observed on the plated specimen and the strength and the ductility were decreased by plating. After investigation of optimum condition for baking, baking at 473 K for 24 hrs was found to recover nearly from the degradation due to plating even at the low extension rate of 1.68x10 mm/min. Comparing the ductility and strength of fully plated specimen with those of partly plated specimen with two layers of nickel and without chromium, the degradation was found to be mainly took place at the final stage of plating. Although the diffusible hydrogen content of plated specimen was sufficient for hydrogen embrittlement, fracture was not originated at the inner subsurface but at around the outer surface. Microscopic observation by SEM indicated that the fracture was originated at around the final plating layer of chromium. These results suggested that the strength of plated material is affected by hydrogen embrittlement and the condition of the final plating process of chromium.