Study On Microstructure And Properties Of Alloy Steels After High Temperature Carburization

Stainless steel and alloy steel are important materials in the pump manufacturing. So it is an extremely important issue to improve hardness and wear resistance of stainless steel and alloy steel without reducing the corrosion resistance. Carburizing is a traditional surface heat treatment method. Carburizing parts formed have a well transitional microstructure of " hard in the outer and tough in the core", which ensuring the surface hardness and without affecting the internal toughness and having a good combination performance. Therefore, the carburizing surface heat treatment is widely used.In this paper, high-temperature carburizing and associated heat treatment process were applied to 40Cr steel and 2205 duplex stainless steel. Microstructure and properties after carburization were studied. Microstructure was observed using an optical microscope (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Phase analysis and chemical component analysis were detected using X-ray diffraction (XRD) and electron probe microanalysis (EPMA). Hardness and electrochemical corrosion test were detected by micro-hardness tester and CS300 electrochemical zeta potential scanner, respectively.Carbide precipitation types unchanged before and after carburizing by XRD analysis. After the two steels were quenched and tempered and solution and aging heat treated carbide types have not changed.After 950℃ high-temperature carburizing, the two steels have got gradient microstructures from the surface to the core. Microstructure of the carburized layer refined and soluts more carbon atoms in a solid solution. The core microstructure slightly roughed due to the high temperature during carburization. Microstructure size of the transitional zone was between the carburized layer and the core. The carbon concentration decreased and the microstructure size increased from the surface to the core. By TEM analysis, dislocation defects were intricate and messy and the number of dislocation and carbides increased after carburizing. Morphology of carbides precipitated (Fe3C、Cr7C3？Cr23C6) changed after carburization.Content of alloying elements such as C, Cr, Mo, etc. are different from the surface to the core in samples after carburizing. Chemical compositions of each phase between the carburized layer and the core in the carburized 2205 duplex stainless steel are different. The content of ferrite forming elements Cr, Mo in ferrites of carburized layer is higher than that of the core, and the content of austenite forming element Ni is lower.Hardness of the carburized layer increased due to increased carbon content after carburization. The decline of hardness is relatively flat from the surface to the core and hardness of the core tended to be the same level. The electrochemical corrosion resistance of both steels after carburizing in NaCl solution whose volume fraction is 3.5% has not reduced, but the corrosion rate increased slightly. The number of carbides precipitated of 2205 duplex stainless steel after carburization and solution and aging treatment increases. So its resistance to electrochemical corrosion lowered.