| Although iron alloys are widely applied,its strength is still insufficient.Therefore,to strengthen the surface of Fe-based alloys and obtain the comprehensive mechanical properties,its surface is modified by carburizing by means of the higher carbon content of iron substrate.The aim of the present work is to strengthen and toughen the surface of iron alloys.In this paper,tungsten cemented carbide layers on the surface of iron-based alloys were produced by using isothermal annealing process via a diffusion-controlled in situ reaction in solid state at 1000 ?,1050 0C and 1100 ? for 1 h,2 h,3 h,4 h and 5 h,respectively.The microstructure,physical and chemical properties of carbide layers were characterized by using scanning electron microscope(SEM),energy dispersive spectral(EDS),X-ray photoelectron spectroscopy(XPS),electron backscattered diffraction(EBSD)and transmission electron microscope(TEM)techniques,respectively.Based on Gibbs free energy and diffusion theories,the growth kinetics of W2C and WC-Fe layers and phase transformation were discussed.The nucleation,growth and morphology evolution of tungsten carbide grains were described by means of classical theory and model.The load-bearing capacity of the layer was evaluated by indentation method.Based on tribological theory and experimental results,the wear behavior of the carbide layer and substrate was analysed in condition of dry friction.The purpose of this paper is to reveal the formation mechanism of tungsten carbide hardmetal layer and to cevaluate accurately its mechanical properties.The obtained conclusions are shown as following:Three groups of samples obtained,the microstructure of cemented carbide layers prepared at 1100 ? is optimized with a few defects(pores and microcracks).The total thickness of the layer ranges from 52.20±1.94 ?m to 197.86±2.23 ?m.Compared with the similar preparation technology of tungsten carbide coatings/iron or steel,the adopted process possesses a number of characteristics in the present work,such as the simple and cost saving.The phase composition of the hard layer consists of W2C,Fe2W2C,Fe3W3C,WC and a-Fe,and WC is a dominant phase.A small amount of a-Fe exists between grain boundaries of WC/WC,W2C is adjacent to W,and the region between W2C and WC-Fe is Fe2W2C(or Fe3W3C).Due to the effect of variation in temperature,a transition of Fe2W2C to Fe3W3C occurs.The size of carbides(W2C?Fe3W3C?WC)increases with the time,and the increasing rate of WC size is fastest among them.The grain boundary deflection angle(<15°)is caused by the special growth orientation of carbide grains.The lattice plane orientation of both W2C and WC is {0001},and that of Fe3W3C is {0001} or {1010}.The crystal orientation of carbide grains occurs the transformation:WC[0110]?[1210],W2C[0001]?[0110],Fe3W3C[001]?[101],which indicates that the present process parameters have a significant effect on the preferential growth orientation of carbide grains.W2C forms initially,WC appears as a second phase.The WC grows rapidly,but the W2C is consumed gradually,resulting in a great quantity of WC and a small amount of W2C that exist simultaneously and the formation of double carbide layer.A possible reason for the transformation of W2C to WC can be attributed to a eutectoid decomposition of the W2C.The WC-Fe layer grows by C diffusion,where the growth kinetics of the layer exhibits completely a parabolic growth law.However,the growth of W2C layer obeys incompletely a parabolic law,because that the growth and consumption of W2C occur simultaneously.The diffusion mechanism of carbon in the W2C layer and WC-Fe layer is dominant grain boundary diffusion.Variation in diffusion coefficient depends strongly on the temperature(DWC-Fc>DW2C).The growth activation energy of WC-Fe layer with a value of 286.71 kJ/mol is 3 times of W2C layer(81.62 kJ/mol),indicating that the formation of the former by in-situ reaction needs to overcome the higher energy barrier than the latter.The obtained function relationship between the thicknesses of W2C layer and WC-Fe layer,annealing time and temperature can be used to predict the growth law of both and to guide the preparation and application of controllability.XRD results of tungsten carbide particles exhibit that both of the W and WZC transform to the WC,and that the sequence of phase transformation is W?W2C?WC.The average values of prism length L and base radius r of tungsten carbide grains increase simultaneously with the extended time.Additionally,there are a number of precipitates on {0001} and {0110} lattice planes of WC grains to be found at 1100 ?.The nucleation of precipitated phases in the WC-Fe layer contains three modes:(a)one-dimensional nucleation along the<0001>direction on {0110} lattice plane,(b)two-dimensional flat plane nucleation on {0001} and(c)two-dimensional terrace plane nucleation of the multilayer structure on {0001}.The cooperation among these nucleation modes considerably promotes the formation of tungsten carbide and accelerates the thickening of WC-Fe layer.Furthermore,the three-dimensional morphology of grains consists of the hexagonal prism,triangular prism,cuboid,plate-like and multilayer structure.Due to morphology evolution stemming from crystal growth,the competitive growth between different crystal faces leads to a change in morphology.The coarsening kinetics of in-situ tungsten carbide grains grown shows that the coarsening rates are K1000?=9.98(?m3/h),K105?=42.56(?m3/h),K1100?=442.07(?m3/h),respectively.Furthermore,the obtained coarsening activation energy by fitting has a higher value of 554.64 kJ/mol,because that the drag effect of precipitates reduces the driving force of the parent phase coarsening.The diffusion-controlled coarsening mechanism causes the mutual mosaic and symbiotic growth of WC grains,which differs the Ostwald ripening growth mechanism.Nanoindentation results of the cemented carbide layer produced at 1050 ? for 4 h provide values of 18.66±1.57-29.84±8.17 GPa and 524.20±45.23?662.23±126.26 GPa,respectively,for the nanohardness and Young' modulus.Its critical load for the fracture is 450 mN.Thus,the calculated fracture toughness has a mean value of 3.08 MPa·ma1/2.Under Vickers indentation testing,due to indentation size effect,the values of fracture toughness of WC-Fe layers produced at 1050 ? for 4 h and 1100 ? for 5 h with various loads are determined to be within the ranges of 1.85?3.44 MPa·m1/2 and 4.88?6.16 MPa·m1/2,respectively.Toughening mechanism consists of the crack deflection and bridging,which are external factors.The toughness of the sample at 1100 ? annealed for 5 h is improved significantly in comparation with the sample at 1050 ? annealed for 4 h,which can be attributed mainly to the optimization of microstructure,mutual mosaic between WC/WC grains and precipitated phase on the crystal face of {0001}and {0110}.These internal factors can hinder the crack initiation and propagation and play a toughening role.Tribological results indicate that the friction coefficients of the WC-Fe layer and substrate increase during the initial state,then decrease and achieve a steady state finally.The wear rate of WC-Fe layer is 18.5%?34.3%of the substrate,which indicates that the layer effectively improved the wear resistance of the substrate surface.The main damage modes of the layer are particle deformation,pull-out,crushing and fatigue fracture.Nevertheless,the fracture mode of the substrate takes on the plastic deformation,ploughing,delamination and material removal.The wear mechanism of the WC-Fe layer is dominant by abrasive wear.Low stress two-body wear is caused by surface micro-bulge at low load.When the load is largest,abrasive dusts lead to the occurrence of high stress three-body wear.The wear mechanism of the substrate is dominated by adhesive wear and accompanied by abrasive wear. |
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