Effect Of Heat Treatment On Microstructure And Properties Of GH3625 Alloy Tubes

Heat treatment is a indispensable step in the preparation process of GH3625alloy seamless tubes,which can optimize the microstructure and improve the mechanical properties,so as to ensure the structure stability and operational reliability of the alloy tubes during service.In this paper,the effects of solution treatment on the microstructure and mechanical properties of GH3625 alloy tubes were systematically studied by means of OM,SEM,XRD,EDS,Vickers hardness tester and Universal material testing machine and other analytical methods.In addition,the changes of microstructure and mechanical properties of GH3625alloy after solution treatment in different states were further studied.Meanwhile,the behavior of grain growth,the change of mechanical properties and the problem of microstructure stability of GH3625 alloy under the condition of over-temperature service are discussed in detail by aging treatment to simulate the evolution of GH3625 alloy tube during service.The effects of solution treatment parameters?solution temperature,holding time and cooling method?on the microstructure and properties of GH3625 alloy tube were studied.The results that with the increase of the solid solution temperature,the carbides in the matrix gradually dissolve.When the temperature is higher than 1130°C,the carbides in the matrix almost completely dissolve and the grains begin to rapidly grow.When the temperature was at the ranges of950°C to 1250°C,grain growth activation energy reached at 208.60 k J/mol.Hardness declined during the process of heat treatment temperature increased,and its values and the average grain size of the alloy kept in line with Hall-Petch relation.With the change of holding time,the grain growth rate first rapidly increases and then gradually becomes stable.When the holding time is 20 min,static recrystallization was basically completed.The change of hardness of the alloy is not obvious in this process.After air cooling and water cooling,the average grain size of the alloy remains unchanged and the hardness increases with the increase of cooling rate.The influence of solution treatment temperature on microstructure evolution and mechanical properties of the GH3625 alloy tubes in different states?hot extrusion,cold rolled and annealed?were investigated.The results show that the critical temperature of the transition from mixed crystal structure to homogeneous one is considered to be around 1000°C,1050°C,and 950°C,respectively,after solution treatment of hot-extruded,cold-rolled and annealed GH3625 alloy samples.With the increase of solution temperature,the hardness of GH3625 alloy in three states is gradually decreasing.At the same solution temperature,comparing the hardness values of three states,it is found that the hardness value of hot extrusion and cold rolled is much higher than that the annealed state.On the whole,GH3625 alloy will emerge dimples at different degrees after fracture at room temperature.The grain growth behavior and mechanical properties of the annealed GH3625 alloy aged isothermally at temperatures between 600°C and 800°C for times up to 120 h were studied.The results indicate that,the grain structure of GH3625 alloy tube is still equiaxed grains after aging treatment.Grains continue to grow after aging treatment at 600°C.After grain aging at 700°C and 800°C,the grains begin to gradually grow up,followed by grain refinement?the degree of refinement is not obvious?.The tensile strength of the alloy has been improved to varying degrees,but the tensile strength at 600°C is significantly higher than the tensile strength at 700°C and 800°C.At the beginning of aging,the yield strength of the alloy is rapidly increases.With the further prolongation of the aging time,the change of yield strength tends to be gentle.The elongation of the alloy shows a trend of continuous decline,but the decline of elongation at 800°C is the most serious,and the decrease is 34.37%.The problem of microstructure stability of GH3625 alloy tubes under the condition of over-temperature service was studied.The results show that the MC,M₆C,M₂₃C₆,??and?phase are mainly precipitated in the GH3625 alloy at 750°C for times up to 1000 h.With the prolongation of the aging time,the grain size shows a slowly increasing trend.The grain boundaries width of the GH3625 alloy with increases initially and then decreases during the long-term heat preservation at 750°C.Before and after aging,the change of tensile strength of the alloy is not obvious,and the yield strength of alloy shows a trend of continuously increasing.However,the elongation of the alloy has lost by 62.55%,and the fracture mode changes from ductile fracture to brittle fracture.