Abstract

Low cycle fatigue tests were performed at room temperature (RT) and at 673 K for l%Cr-0.5%Mo steel comparing the specimens coated with chromium carbide by gas spraying and the ordinary uncoated specimens, and the mechanism of fatigue crack formation was investigated. Following observations and conclusions were made: (1) When sprayed with ceramic, the fatigue life suffers reduction at either temperature, but at 673 K, the degradation was so much smaller than that at RT that the fatigue life was actually, though slightly, longer than that at RT. (2) The cracks are initiated in the ceramic layer very early in the whole fatigue life, the crack initiation lifetime becoming the longer, the smaller the strain range. (3) The fatigue failure process can be viewed as comprising following steps: first, early initiation of fatigue crack at the surface of the ceramic coating, rapid propagation through it to the substrate metal, and initiation of crack in the metal, the initial rate of propagation of such a crack being a number of times (perhaps as much as one full order of magnitude) faster than that in uncoated steel.

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