Effect Of PWHT On Microstructure Of Cemented Carbide/Stainless Steel Welded Joint

Post welding heat treatment(PWHT)is one of the most important methods to relieving the residual stress inside the cemented carbides for toughness purpose.In the present research,annealing was performed on dissimilar weldments composed of WC-Co to 316 L stainless steel with 1-mm-,1.5-mm-,and 2-mm-thick Invar interlayers by heating the samples to 1250°C for 2 hours,8 hours,and 16 hours respectively.Microstructures and chemical composition,the shape of WC grains and prismatic facets were characterized using optical microscopy,electron probe microanalysis,field emission scanning electron microscopy,field transmission electron microscopy,and energy dispersive spectroscopy.The results on original surface morphology and fracture mode after annealing indicated that there were four kinds fracture modes: carbide transgranular,carbide-carbide intergranular,fracture along the carbide-binder interfaces or through the binder phase.The dissolving behavior of prismatic facets was estimated by means of high-angle annular dark field microscopy.The influence of annealing process on the microstructures,shape of WC grains and dissolution was also discussed.The results indicated:(1)Because of the stress relieving,cracks initiated and propagated in the HAZ or in the fusion zone for all samples when being heat treated at 1250°C for 2 hours,8 hours,and 16 hours.The increase of the interlayer's thickness tends to partially inhibit the initiation and propagation of cracks.When the holding time is short(2 hours),the sample fractured in the HAZ.With the increase of holding time(8 hours,16 hours),the samples tended to fracture in the fusion zone.(2)TEM-EDS results showed strong plastic deformation with obvious elemental diffusion not only inside the WC grains but near the WC/Co/WC interface.Moiré fringes inside the WC and in the M matrix are found to be approximately 7.45 ? and 7.15 ?.(3)The shape of WC grains changed to faceted WC grains during the heat treatment,which exhibited abnormal grain growth in the matrix composing of ?-Co and ?-(Fe,Ni).In the fusion zone adjacent to the WC-Co,W-M-C carbides skeleton were formed with a character of continuous rods and isolated islands.The islands had lower tungsten and more binders than the rods.