The Study Of Gas Nitrocarburizing And Post-quenching On A Low Carbon Steel

Nitrocarburizing is a normal surface chemical heat treatment process.After nitrocarburizing,the compound layers composed of of?-Fe_2³N and??-Fe₄N,usually form in the surface of steels,which improve the surface hardness,wear resistance,corrosion resistance and so on.For pure iron and carbon steel,the maximum hardness of the nitrocarburized compound layer is only about 600HV,which has the disadvantages of thin thickness and poor toughness.In the paper,the microstructure and properties of the gas nitrocarburized layers on a low steel were controlled by adjusting the nitrocarburizing time,the flows of NH₃ and CO,the heating-up temperature,holding time and aging.By means of X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?and microhardness tests,the phase constitutions,the microstructures and the properties of the strengthened layers on the nitrocarburized and quenched samples were investigated.The main conclusions are as follows:?1?After low-carbon steel is treated by gas nitrocarburized and post-quenching,a multiphase strengthened layer composed of retained austenite and martensite is formed on the surface,and the maximum hardness is as high as 1000 HV _0.05.In order to reduce the pores which formed on the surface layer of the quenched sample,the atmosphere flow in the nitrocarburized and post-quenched parameters were adjusted.The results show that low NH₃ and high CO flow can reduce the thickness of the pore layer.With the increase of austenitizing temperature and the prolongation of the holding time,the austenite which in the multiphase strengthened layer gradually decreases and the pores increase gradually.Finally,it was confirmed that when the flow of NH₃,austenitizing temperature and holding time were 70sccm,680? and0.5h,respectively,the multiphase strengthened layer had the best compactness and highest hardness.Compared with nitrocarburized samples without austenitizing,the multiphase strengthened layers forming in the nitrocarburized and post-quenching process have the higher hardness,toughness and wear resistance.?2?The cooling rate has huge influences on the microstructure and properties of the nitrocarburized layer with austenitizing.If quench mediums are liquid nitrogen,water or oil,austenite will transform into martensite during quenchin g,and the microstructure consists of martensite and retained austenite.During air cooling or furnace cooling,austenite decomposes into or pearlite.As the cooling rate slows down,the retained austenite?phase in the surface strengthened layer gradually decreases.When austenite turns into martensite pearlite,the hardness of the strengthened layer decreases,so that of the furnace cooled samples drops to about400HV_0.05.?3?After the austenitized samples were aged at low or middle temperature for 14h,the retained austenite must have transformed into bainite,companying with transformation of the quenched martensite into tempered martensite.When the tempering temperature was not more than 120?,the hardness of the multiphase strengthened layer is improved a little as compared with that of quenched one.When the tempering temperature was in the range of 140²00?,the hardness of the multiphase strengthened layer keeps a value of 800⁹00HV_0.05 although there are massive indecomposed austenite.When the tempering temperature was more than200?,the multiphase layer become the lamellar pearlite,companying with decrease of the hardness.Besides,after aging at 120?,the abrasive resistance of the multiphase strengthened layer is better than that of the nitrocarburized and tempered compound layer with the same thickness.