Research On Low Dilution Spray Transfer Surfacing In Vertical Direction

The boiler water wall could be protected by surfacing on its surface,to improve the service life and reduce the production cost.In order to meet the requirements of surfacing work pieces like water wall in vertical direction with lower dilution rate,a new surfacing method,low dilution spray transfer surfacing in vertical direction,was put forward.The surfacing layers of carbon steel and stainless steel were deposited on Q235 substrates by this method.The surfacing process was systematically studied,and the microstructure,mechanical properties and dilution rate of surfacing layer were also discussed.The main research contents were as follows:The vertical direction surfacing system was designed.A MIG welding torch and a TIG welding torch equipped with a water cooling device were connected to the anode and cathode of the MIG power supply,respectively.Two torches was connected by the connector and connected to the operating machine.Shielding gas was fed to the torches in the surfacing process.This method could be used in vertical direction.Good quality surfacing layer could be obtained and the dilution rate of surfacing layer could be reduced.Q235 steel plate of 4 mm thick was selected for surfacing test and ER50-6 carbon steel welding wire was selected as surfacing material.Optimized process parameters were chosen as:Current 220A,welding voltage 22V,welding speed 2.7mm/s,distance of tungsten electrode to work-piece 20mm,angle between tungsten electrode and welding wire 60°,gas flow 10-12L/min,swing speed about 1000mm/min,swing width9mm and wire extension 15mm.With these parameters,the weld was well formed.The microstructures consisted of pro-eutectoid ferrite,side-plate ferrite and acicular ferrite.The overlay mainly consisted of Fe element.The overlay and substrate were?-Fe.The microhardness of overlay was 180HV-220HV,higher than that of the substrate.The tensile test results showed that,compared with the base metal,the yield strength and tensile strength of the surfaced substrate increased,the contraction of area and elongation after fracture decreased.The impact energy of overlay and heat affected zone were 36J and 35J,respectively.The corresponding impact toughness was 0.73J/mm~2and 0.71J/mm~2,respectively.Slightly lower than those of the base metal.ER304 stainless steel welding wire was used for surfacing.It was found that the change of surfacing parameters would lead to the change of dilution rate.With the increase of welding current and distance of tungsten electrode to work-piece and the decrease of welding speed,the dilution rate of overlay was increased.The microstructure of the overlay consisted of plane crystal,parallel bundle dendrite and cellular dendrite.Content of Fe in overlay was lower than that in the substrate,and the content of Cr was higher.Content of Ni,Mn and C had no difference.Fe oxide might form in the interface.There was a narrow transition zone of Fe and Cr.Microhardness of the overlay was 180HV-220HV,higher than that of the substrate.When the dilution rate was higer,more base metal was mixed into the overlay,so the corrosion resistance of the overlay was lower.