Research On Microstructure And Properties Of HAZ Of 40CrNiMo Quenched And Tempered Steel Designed For Gear By CMT Cladding

With the advantages of high strength,good toughness and thermal stability,40CrNiMo quenched and tempered steel has been widely used to manufacture mechanical parts with high strength and large cross-sections,such as steam turbine shafts,large gears and shearer cutting teeth.Surfacing welding is a common method for surface repairing and strengthening of mechanical parts.Due to the high carbon content of 40CrNiMo steel,the unsuitable welding process could result in a propertiesdeteriorated welding heat affected zone(HAZ),causing the surfacing welded steel cannot meet the requirements for use.Therefore,it is of great significance to develop a suitable surfacing welding process for 40CrNiMo quenched and tempered steel.In this paper,40CrNiMo quenched and tempered steel was surfacing welded by cold metal transfer(CMT)technology.Firstly,the effects of wire feeding speed,welding speed and swing width on weld forming were studied by means of optical stereo microscope(OSM).Then,the combination of wire feeding speed,welding speed and swing width was optimized through the orthogonal test,and good weld forming was obtained under the above welding process parameters.Finally,the effects of preheating temperature,post-weld tempering temperature and the number of surfacing layers on the microstructure and properties of the HAZ were studied through the characterization methods such as optical microscope(OM),scanning electron microscope(SEM),energy dispersive spectrometer(EDS),microhardness test,impact toughness test,tensile test,etc.The results showed that the wire feeding speed,welding speed and swing width all had significant influence on the weld forming.Wire feeding speed,welding speed and swing width needed to be matched to obtain a well-formed welding bead.Through the orthogonal test,the welding process parameters with good weld forming were obtained: wire feeding speed of 4m/min,welding speed of 3mm/s,swing width of 12 mm.Various preheating temperatures were used for surfacing welding,respectively.With the increase of preheating temperature,the width of coarse-grained HAZ and the size of proto-austenite grains inside it gradually increased.The microhardness of the HAZ decreased gradually.The impact toughness increased first and then decreased,reaching the a maximum at the preheating temperature of 250?.The tensile strength of the HAZ decreased gradually,and the elongation of the HAZ increased gradually.Different tempering temperatures were used for post welding heat treatment,respectively.When tempered at 400? and 500?,the precipitated carbides were distributed on the bar boundary,beam boundary and grain boundary of martensite,which worsened the impact toughness of the HAZ.When tempered at 600?,the impact toughness of the HAZ was impoved,which met the requirements of the standard value.The reason for this was that the carbides aggregated and spheroidized,and were distributed more dispersive.With the increase of post-weld tempering temperature,the microhardness and tensile strength of the HAZ decreased,and the elongation increased gradually.Different number of surfacing layers were used for surfacing welding,respectively.With the increase of the number of surfacing layers,the quenched martensite in the HAZ gradually decreased,which was produced from the first layer welding.When the three layers were surfacing welded,the quenched martensite in the HAZ inherited from the first layer welding was almost completely eliminated.With the increase of the number of surfacing layers,the microhardness of the micro-zone in the HAZ decreased.The impact toughness of the HAZ was improved obviously,which matched the standard value when two and three layers were surfacing welded.Moreover,the tensile strength of the HAZ decreased gradually,and the elongation of the HAZ increased.