Microstructure And Properties Of Modified Layer Produced By Duplex Treatment Of Nitrocarbuieizing And Laser Quenching Of38CrMoAl Steel

38CrMoAl steel is a kind of widely utilized steel in industries. It is of greatsignificance to optimize its microstructure and properties by means of surfaceengineering. In this paper, major phase transitions during the nitrocarburizing arestudied based on the thermodynamic calculation, according to which the cyclicnitrocarburizing is conducted. Laser quenching technique is employed to furtherimprove the case depth of the cyclic nitrocarburized layers. The microstructure andmorphologies of both the nitrocarburized and duplex treated layers are characterized byX-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with an energydispersive X-ray analyzer (EDS) facilities. The properties such as microhardness, wearand corrosion resistances of the duplex treated layers are analyzed and compared withthe nitrocarburized counterparts.The transitions between-Fe and Fe4N, as well as Fe4N and Fe3N will occur at thetemperature range of500~800K according to the thermodynamic calculation results. Itis found that Fe4N is more stable at low temperature (500C). When the temperature isup to the region of500~800K, the Fe3N is observed to be more stable than Fe4N. Thetransition from nitrides such as Fe4N and Fe3N to-Fe can occur at the temperatureover800K. The nitrocarburizing process with La addition at a lower O partial pressureis favorable to the transitions occurrence.Cyclic plasma nitrocaruburizing under high nitrogen containing atmosphere at lowtemperature for a long duration usually results in the formation of nitrides and densecompound layer on the treated surfaces. The phase composition of the nitrocarburizedlayer consists of-Fe, Fe4N and Fe2-3N. The layer depth increases with thenitrocarburizing duration.Laser quenching of the substrates with different scanning rates,600mm/min,1200mm/min and1800mm/min results in the hardened layers with thickness of722μm,467μm and272μm, respectively. In comparision with the nitrocarburized specimens, theduplex treated specimens show lower surface hardness but exhibit about83%~120%improvement in case depth. There is a large region with a hardness over600HV0.1canbe observed. The phase composition of duplex treated layers consists of Fe2O3, Fe3O4,α′-Fe and Fe4N. The vanish of Fe2-3N indicates the decompostition of the nitrides.Tribology and wear tests show that the duplex treatment optimize coefficient offriction of the surface layers. Compared with the nitrocarburized layers, the weight lossfor the duplex treated layers reduces significantly, demonstrating its better wearresistance. The dominant wear mechanisms are severe abrasion for nitrocarburizedspecimen, but milder abrasion and oxidation for duplex treated one. Electrochemicalpotentiodynamic corrosion tests demonstrate the superior corrosion resistance of the duplex treated layer compared with that of the nitrocarburized layer, characterized byhigher corrosion potential and lower current density. The morphologies for the corrodedsurfaces explored in air condition shows the severe general corrosion occurs on theduplex treated layer with large amount of corrosion products adhering to the surface. Onthe contrary, the corrosion occurs on the nitrocarburized specimen is much milder.