Corrosion-fatigue properties of a 4340 steel coated with Colmonoy 88 alloy, applied by HVOF thermal spray

The corrosion-fatigue behavior of a quenched and tempered AISI 4340 steel h as been evaluated under two different conditions: (a) uncoated; and (b) gri t-blasted with alumina and coated with a thermal-sprayed Colmonoy 88 alloy (220 mum in thickness), employing a high-velocity oxygen fuel (HVOF) gun. T he tests were conducted under rotating bending conditions employing a 4-wt. % NaCl solution. The results indicated that the fatigue behavior of the coa ted material under this condition is very similar to that previously report ed after testing in air. The fatigue cracks were nucleated at the alumina p articles deposited in the matrix of the substrate steel during blasting rat her than at corrosion pits formed during testing. Therefore, the corrosion- fatigue strength of the coated substrate has been found to be controlled by the same mechanism that governs the fatigue behavior of the material in ai r. The fatigue strength of the uncoated substrate tested in the NaCl soluti on has also been found to be significantly less than that in air and that, if the substrate steel is coated with the Colmonoy alloy, its corrosion-fat igue life increases substantially. The microscopic observation of the fract ure surfaces also showed that under some alternating stress conditions, the substrate-deposit interface can be severely cracked giving rise to the det achment of the deposit from the substrate steel. The fatigue performance of the material under the two conditions analyzed has been quantified by dete rmining the Basquin parameters from the fatigue life curves obtained. (C) 2 001 Elsevier Science B.V. All rights reserved.