Effect of a thin hard chromium coating on fatigue behaviour of 4140 steel

摘要

The fatigue properties of a quenched and tempered (Q&T) 4140 (NiCrMo) steel coated with a hard Cr deposit approx 7 mu m thick were investigated. The fatigue properties of the coated specimens were evaluated in the as deposited condition. The results obtained showed that both fatigue life and fatigue limit are substantially reduced in comparison with the uncoated material. The reduction in fatigue life was computed from the Basquin parameters of the materials tested under different alternating stresses. It was shown that the fatigue life of the steel substrate can be reduced by up to 88 percent, whereas the fatigue limit can be reduced by -21 percent, as a consequence of the coating. SEM observations showed that the fatigue fracture of the coated specimens initiates at the substrate-deposit interface and that the coating remains well adhered to the substrate during fatigue testing. The fracture surfaces of the specimens tested showed multiple crack initiation sites which led to the conclusion that the coating acts as a crack source for the steel substrate.