| The 21-4N and the 4Cr9Si2 are two types of high-temperature resistant alloy steels,which are usually used in engine valves.However,the 21-4N,as a nickel saving austenitic heat-resistant steel,may fail due to layered precipitates at grain boundaries during prolonged use at high temperatures.Therefore,two kinds of metals are usually welded together in application.In recent years,a new welding technology inertial friction welding has been widely used on pipes,shafts,disks and other workpieces.In this paper,the inertial friction welding joint of the 21-4N and the4Cr9Si2 and its post weld heat treatment technology are studied.The 21-4N and the 4Cr9Si2,were successfully welded under welding process parameters of maximum torque,forging pressure,rotational speed,and holding time of 70 N·m,430 MPa,5500 r/min,and 3 s,respectively.The metallographic structure shows that the welded joint is divided into three zones: weld center zone(WZ),shear deformation zone(SZ)and Heat-affected zone(HAZ).The grain structure in the center area of the 21-4N weld seam has undergone complete recrystallization,the results show that the temperature at the welding interface has reached 1473 K(1200 ℃).Severe shear behavior can be observed in the 21-4N shear deformation zone,and some grain boundaries are serrated.On the side of the 4Cr9Si2,the precipitates in the matrix of the weld center area flow with the ferrite,causing the precipitates in the matrix to be arranged parallel to the weld.The temperature and composition differences at the welding interface can lead to element diffusion.The diffusion of Fe,Si,Cr and other elements can be directly observed through EDS,and the observed diffusion distance is about 30 μm.Some of the matrix structure of the 4Cr9Si2 will be embedded in the 21-4N matrix,forming a "bite" phenomenon,promoting metallurgical bonding at the joint.The microhardness test showed that the maximum hardness appeared in the shear deformation zone on the 21-4N side,about 473 Hv.Its microhardness is related to microstrain,grain size,and precipitated phases.The maximum tensile strength of the joint reached 1057 MPa,which is equivalent to the 21-4N base material.The fracture mode is brittle fracture,with a few equiaxed shallow dimples at the fracture surface.The large dimples contain fragmented precipitates,indicating that the precipitates directly bear stress.The welded workpiece is subjected to heat treatment at four temperatures: 600 ℃,650 ℃,700 ℃,and 800 ℃ for 4 hours.Under the conditions of 600 ℃ and 650 ℃,the microstructure of the joint did not undergo significant changes.The microhardness indicates that the microhardness at the joint remains relatively high,and the tensile performance test results show that the tensile strength of the welded workpiece has been significantly improved compared to before heat treatment,resulting in a significant increase in the toughness of the material;After heat treatment at 700 ℃,a large number of annealing twins appeared in the 21-4N shear deformation zone,which maximized the elongation of the treated workpiece at this temperature;When the temperature reaches 800 ℃,the matrix structure of the 4Cr9Si2 has become completely granular pearlite structure,causing a sharp decrease in hardness on one side of the 4Cr9Si2.Through mechanical performance testing,it was found that the welded workpiece exhibits the best performance under the condition of 650 ℃ heat treatment.The tensile fracture appears on one side of 4Cr9Si2,with a maximum tensile strength of 1755 MPa.The fracture morphology is a mixed ductile brittle fracture. |
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