Study On The Strengthening-toughening Treatment And Fatigue-Wear Behavior Of 18Cr2Ni4W Carburized Layer

In this thesis,the strengthening-toughening treatment and fatigue-wear behavior of carburized layer of the low-carbon alloy steel 18Cr2Ni4 W was studied,which is typically applied for manufacturing heavy-duty gear.By means of the multistep heat treatment composed of carburizing,high temperature tempering,quenching(cryogenic treatment)and low temperature tempering,cryogenic treatment and different low temperature tempering treatment were utilized to promote phase transformation and generate carbon redistribution and carbide precipitation,via adjusting the subsequent treatment process after quenching,through which the strength and toughness of the carburized layer were optimized.The effect of different strengthening-toughening treatment on microstructure,mechanical property and fatigue-wear resistance on the carburized layer was investigated and analyzed,via optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),electron back scattering diffraction(EBSD)and fatigue-wear resistance test.The corresponding mechanism of strengthening-toughening effect was discussed.On the basis of the fatigue-wear condition and the wore morphology of contact pair,the fatigue damage mechanism for rolling contact and sliding-rolling contact conditions was analyzed and proposed.This thesis elaborated in depth the relationship between the strengthening-toughening treatment,microstructure,mechanical property and fatigue-wear resistance of the 18Cr2Ni4 W carburized layer,and mainly investigated the fatigue-wear morphology and corresponding damage mechanism under rolling contact and sliding-rolling contact conditions of the carburized layer after different strengthening-toughening treatment,which could provide the theory reference for optimizing the strength and toughness of heavy-duty gear and is of positive significance for improving its heat treatment process,fatigue-wear resistance and service life.The main conclusions are as follows:Gradient distributed and strong-in-surface and tough-in-core microstructure and mechanical property were obtained from surface to core of 18Cr2Ni4 W via carburizing-quenching-tempering treatment.Different low temperature tempering treatment generated the mixed microstructure composed of the relatively high carbonα’ matrix and finely-densely distributed carbide precipitates.In favor of effect of solid solution strengthening,martensite intrinsic strengthening and precipitation strengthening,the lattice and matrix microstructure with high solute carbon and carbide precipitation generated large obstruction for deformation,due to which high strength was obtained by the surface of the carburized layer.Cryogenic treatment promoted the effect of martensite transformation strengthening with evident decrease of retained austenite,due to which the hardness and strength of surface of the carburized layer were improved and its strength-toughness was optimized.In comparison,optimal microstructure and mechanical property were obtained by surface of the carburized layer after quenching with cryogenic treatment and tempering at 175°C for 12 h,with the yield strength and ultimate tensile strength of about 2160 MPa and 2270 MPa respectively.In combination with the solid solution strengthening and martensite intrinsic strengthening,finely and densely distributed favorable carbide precipitation provided the surface of carburized layer with high hardness,yield limit and tensile strength with appropriate toughness,matching with the core matrix material,due to which the carburized layer obtained favorable strength-toughness and fatigue-wear resistance.Various fatigue-wear behaviors were generated in the carburized layer after different strengthening-toughening treatment.Rolling/ sliding-rolling contact conditions and contact pair with inconsistent material and property also have evident influence on the fatigue-wear behavior.In terms of rolling contact condition,contact fatigue and wear damage induced by micro-plastic strain accumulation were continuously generated by the long duration cyclic normal contact stress.The friction coefficient and tangential stress between the contact area of contact pair is relatively small,due to which the microstructure damage accumulation is relatively slow.While under the sliding-rolling contact condition,the contact area of contact pair contains both rolling friction and sliding friction,causing larger tangential shear stress and severe abrasive wear,due to which the contact fatigue and wear damage are severer.Under the sliding-rolling contact state with quenched-tempered 30 Cr Mn Si,of which the hardness,yield limit and tensile strength are lower,it is easy to prematurely form larger numbers of spalling with larger size.And under the effect of normal extrusion and tangential friction force,severer abrasive wear with large dust was generated between the contact pair,causing serious fatigue-wear damage.Cryogenic treatment significantly optimized the fatigue-wear behavior and fatigue-wear resistance of the carburized layer after strengthening-toughening treatment.Higher hardness and strength were obtained by surface of the carburized layer with cryogenic treatment after quenching,possessing large resistance for deformation dislocation migration,due to which more micro-plastic strain accumulation formed and the energy of normal and tangential force was consumed.Hence,the contact fatigue and wear damage were delayed,and the wore surface has better integrity and less contact fatigue and wear damage under sliding-rolling contact condition.The contact fatigue-wear damage presents uniformly accumulated feature and non-Gaussian and heterogeneous distribution of failure.The primary cause for initiation of fatigue-wear is the maximum contact stress located in the subsurface of the contact pair and the induced strain accumulation.Alteration of contact state has direct influence on the fatigue stress and strain accumulation induced fatigue damage feature.Contact fatigue and wear damage originate from the long duration of cyclic contact stress,during which micro-plastic strain continuously accumulate from surface to subsurface and stress concentrated region is induced around the precipitation and heterogeneous micro-area.Upon approaching the threshold for formation of unrecoverable plastic deformation,fatigue crack and micro-defects used to initiate with the effect of further contact stress.With continuous action of the cyclic contact stress and accumulation of micro-plastic strain,the micro-cracks interact,converge and expand to the surface,inducing macroscopic fatigue-wear damage.