| The4Cr13martensitic stainless steel is usually used in oil pumps, valve bearings, medical devices, springs and other parts due to its good hardness, corrosion resistance and high temperature oxidation resistance. But the wear resistance of the4Crl3steel is deficient compared with the high strength alloy steels and a heavy oxidation occurs when the temperature is over900℃.It greatly limits its application in the wear components or ones for high temperature.A Nb-alloyed layer is prepared on the4Cr13stainless steel by using double glow plasma surface alloying technique in the paper. Effects of the temperature on surface morphology, microstructure, chemical composition, phase structure, hardness as well as the formation mechanism of the Nb-alloyed layer are analyzed. At the same time, the tribological properties, high temperature oxidation behaviour and their mechanisms for the Nb-alloyed layers are also studied. The experimental results show as follows.(1) A continuous and compact Nb-alloyed layer formed on the4Cr13steel surface is metallurgically bonded to the4Cr13substrate by the double glow plasma surface alloying. The Nb-alloyed layer mainly consists of Nb2C, NbC, Fe2Nb, Cr2Nb and Nb. The thickness of the alloyed layer formed at900℃,950℃,1000℃is about7μm,13μm and7μm, respectively. The surface roughness increases with the increase of the alloying temperature. The increase of the alloying temperature which is dependent on the voltages of the Nb target and sample cathode results on accelerating the Nb diffusion into the substrate and enhancing the sputtering for both the Nb target and the sample cathode. The microstructures of Nb-alloyed layers formed at different temperatures are related to the interaction between the adsorption and reverse sputtering of Nb elements on the alloyed surface.(2) The surface hardness of the Nb-alloyed layers formed at900℃,950℃and1000℃is approximate758HV0.025,698HV0.025and985HV0.025, respectively, which greatly increase more than390HV0.025compared with the untreated4Cr13substrate. The essential factor of increasing the surface hardness is the formation of niobium carbide in the alloyed layer..(3) The coefficient of friction for the Nb-alloyed layers formed at different temperatures is0.35-0.6, which is lower than that of the untreated substrate (about0.8) The specific wear rate of the Nb-alloyed layers formed at900℃,950℃and1000℃is1/14.1/2and1/60of the4Cr13substrate, respectively. The lower coefficient of friction is attributed to the precipitation of hard carbides in the alloy layer and solid lubricating of oxides generated during the sliding process. The carbides and second phase strengthening are the main reason for the significant increase of the wear resistance.(4) The mass gains for samples alloyed at900℃,950℃and1000℃after exposure in air for100h at900℃,which are lower that that of the untreated substrate are15.32mg·cm-2,16.32mg·cm2and7.53mg·cm-2, respectively. The surface of the oxided steel treated by Nb-alloying is compact and smooth. The oxidation scales of the Nb-alloyed steel treated at900℃and950℃consist of Fe2O3layer/Fe-Nb oxide/FeO·Cr2O3/Cr-rich oxide/substrate. For the samples alloyed at1000℃, it is composed of the mixture of Fe2O3+FeNbO4+(Fe0.6Cr0.4)2O3+Cr oxide. The Nb-Fe and rich Cr oxide layers in the oxide scale impede the Fe ions diffusing to the surface and the O anions migrating into the substrate, causing the improvement of the high temperature oxidation resistance for the Nb-alloyed steel. |
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