GCr15 steel ring part decarburization and decarburization

GCr15 steel ring decarbonizing whole and part of the 2018 - 12 - 12 bearing ring surface decarburization is one of the important reasons for the premature failure of the rolling bearing, bearing steel GCr15 ring local decarbonizing surface including the spheroidizing annealing process and partial decarburization, the surface hardness of the quenched or inadequate quenching soft point, reduce the bearing abrasion resistance and contact fatigue strength, leading to early failure, as shown in figure 1. Bearing steel decarburization, surface layer of certain thickness by interface reaction volume carbon oxidation into gaseous CO escape material, reaction speed is controlled by the carbon potential of furnace; At the same time, the decrease of surface carbon concentration, formed in the surface carbon concentration gradient process, controlled by carbon atom diffusion velocity. From the point of view of chemical reaction on the analysis, the spheroidizing annealing temperatures, while iron and carbon oxide, but the former reaction product is tight to prevent further oxidation of iron oxide membranes, and the oxidation of carbon with carbon atoms were generated oxide CO escape surface results in the decrease of surface carbon content, is the foundation of steel reinforcement for carbon element, particularly in the metallography to express steel decarburization on the surface of the carbon oxidation process, to show and oxidation process of iron and other alloying elements. Decarbonization results in the decrease of microstructure change, strength index, worsen the performance and service life of workpiece's role is very big. Fig. 2 illustrates the schematic GCr15 ring spheroidizing annealing surface decarburization process, the A1 point is 760 ℃, Acm point is 900 ℃. According to the bearing steel carbon content is 0. 95-1. 05%, set the ring spheroidizing annealing temperature as 795 ℃, the heated to point A spheroidizing annealing heat preservation. Lamellar pearlite spheroidization driving force from the cementite spheroidizing to lower surface energy, because the volume of cementite before and after spheroidizing unchanged, under the precondition of constant volume, with the sphere surface area to a minimum. Atoms in the lattice diffusion capacity at room temperature is not enough, lamellar cementite exist in metastable, when heated to set spheroidizing temperature, crystal lattice atom vibration frequency and amplitude, can get rid of the constraint of the lattice potential barrier to diffusion, happen spheroidizing the lattice atom and the dissolution of cementite phase reconstruction. Depending on the lamellar cementite spheroidization process is a spontaneous process thermodynamics, but can only rely on the surface of differential drive, spheroidizing driving force is small, is a time consuming process. When the lamellar pearlite in heated to Ac1 ( 760℃） Above, temperature occurs first ferrite to austenite transformation. Compared with ferrite, austenite of dissolved carbon ability far more, obviously promote the migration process of carbon atoms, is advantageous to the spheroidizing; Spheroidizing heat preservation cementite first began to local dissolved, the lamellar cementite disconnect for a number of intermittent lamellar cementite, carbon atoms into austenitic matrix, austenitic matrix by migration to the lamellar cementite deposited in small curvature, realizing the ball; At the same time due to the two-phase zone temperature, material in the uneven residual cementite and composition austenitic state. In cooling process, the austenite to ferrite eduction supersaturated carbon atoms at the same time, because different residual cementite surface curvature, carburizing length large curvature small parts of the carbon deposition, less curvature big deposition, which tend to sphere way to grow up, eventually forming at residual carburizing constitution as the core, uniform and fine granular carbides. This process can be made of figure 2 in and slow cooling spheroidizing point A heat preservation; The microstructure was normal bearing steel ball annealing organization. If furnace gas low carbon potential surface decarburization, surface carbon content into the scope of DC, caused by residual cementite completely dissolved, the surface form of single-phase austenitic certain depth. In the slow cooling process, because of the high chromium content in steel, makes the supercooled austenite high stability without proeutectoid precipitation; Continue to cooled to figure 2 gray 'false eutectoid region, supercooled austenite false eutectoid transformation, form a lamellar pearlite, is characteristic of bearing steel ball annealing part decarburization microstructure. If near decarburization in D, part surface area into single-phase austenitic area, other parts are still in the austenite plus residual cementite two-phase zone, cooling after the former happen pseudo eutectoid transformation generates lamellar pearlite, the latter complete cementite spheroidizing process, forming parts in the surface layers of decarburization. From figure 1 ( c) And ( d） Visible at low field no formation on the surface of the sample thickness of the decarburization layer of the uniform or partial decarburization layer, decarburization only occurs on the surface of the local areas; These partial decarburization area in the field of view at high magnification lamellar pearlite good-quality, deep layer of about 100 microns, show that the volume of single-phase austenitic from high temperature and cooling to the A1 temperature below the eutectoid transformation, so in the spheroidizing annealing process of heating and heat preservation, ring surface single-phase austenitic these local regions, its cooling transformation product only generate pearlite without proeutectoid phase generated, prove that these surface austenitic carbon content in the local area near eutectoid composition in the scope of DC. If furnace gas carbon potential is lower than C, entered the second phase of ferrite and austenite zone, which generate more or less at high temperature ferrite phase. If carbon potential close to point B and heat preservation time is longer, the surface will be generated under the ferrite namely complete decarburization layer shown in figure 1 ( 一) , high power under the lateral matrix is generated by austenite transformation is of a small amount of pearlite as shown in figure 1 ( b） 。 The lower the carbon potential, the longer the time of heat preservation, the decarburization layer depth is larger.