Surface Microstructure Control of Cr-Mo Steels by High-Pressure Gas Cooling after Vacuum Carbonitriding

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We have investigated surface microstructures of high silicon containing Cr-Mo steel carbonitrided to high nitrogen content and quenched using cooling power-controlled N2 gas. Pitting resistance of the steel DLC(Diamond-like-carbon)-coated by ion plating process after the carbonitriding and quenching have been investigated under the conditions of a high Hertzian pressure(Pmax) of up to 4.5GPa with slipping of-60% and a 95℃oil-lubricating using roller-pitting tester. XRD analysis shows that the surface hardness and softening resistance during tempering are improved by precipitation ofεand/orγ' nitrides from nitrogen-solute martensite at the carbonitrided layer. Pitting lifetime of the steel DLC-coated after the carobonitriding is approximately 6 times longer than that of non-DLC coated one when Pmax is 4.2GPa. This excellent pitting endurance can be explained from both the DLC adhesion and the friction coefficient decreased by approximately 17% due to the DLC coating. Thus, this hybrid treatment will be expected to be able to applied to the several super-high pressure and/or high revolution speed gears for next generation automobiles.