Research On Optimization Of Heat Treatment Process Of AF9628 Ultra-High Strength Stee

With the development trend of low-cost and high-performance materials in the aerospace field,a low-cost but with high strength,high impact toughness and high ductility AF9628ultra-high strength steel was developed internationally in 2016.This paper takes AF9628 ultra-high strength steel as the research object,and carries on the heat treatment process optimization and the corresponding mechanical properties and micro mechanism research.Through Thermo-Calc thermodynamic simulation software,optical microscope(OM),scanning electron microscope(SEM)and transmission electron microscope(TEM)observation,thermal expansion phase change instrument,X-ray diffractometer,microbeam X-ray fluorescence spectrometer and electron probe(EPMA)test characterization methods,etc.The influence of heat treatment process on AF9628 ultra-high strength steel was studied.The composition segregation of as-cast AF9628 ultra-high strength steel was reduced or eliminated by optimizing high temperature homogenization process,and the strength and toughness of forged AF9628 ultra-high strength steel was improved by optimizing quenching and tempering process.The results are as follows:In this paper,the composition segregation of as-cast AF9628 ultra-high strength steel has been studied and improved by high temperature homogenization.During the solidification process of AF9628 ultra-high strength steel,coarse dendrites will be produced.The existence of dendrites leads to the enrichment of alloying elements such as C,Cr,Mn,Mo,Ni,Si and V,forming microscopic segregation between the dendrites.With the increase of the high temperature homogenization temperature and the extension of the holding time,the alloying elements gradually diffused and the element segregation gradually eliminated.At the same time,it was found that increasing the homogenization temperature had a better effect on the diffusion of the elements than the holding time,but the high temperature would cause the grain to grow significantly.When the homogenization temperature is 1200 ℃ and the holding temperature is12 h,the alloying element diffusion is sufficient,the dendrite structure is basically eliminated,the element segregation is fully improved,and the excessive grain growth is avoided.It can be used as the recommended homogenization system for ingot at the corresponding cooling rate.In this paper,the influence of quenching temperature on mechanical properties of AF9628ultra-high strength steel has been studied,and the influence mechanism of strength and toughness has been revealed through microscopic characterization.With the same tempering temperature,the tensile strength and yield strength of AF9628 ultra-high strength steel show a downward trend with the increase of quenching temperature.The impact absorption energy,elongation and section shrinkage first increase and then decrease,and the plastic toughness reaches a peak at 970 ℃.In terms of microstructure,with the increase of quenching temperature,the grain size of AF9628 ultra-high strength steel gradually increases,the undissolved phase gradually dissolves,and the content and particle size of the undissolved phase gradually decreases.When the quenching temperature is 970 ℃,most of the undissolved phase dissolves,and only a trace of fine carbide remains.When the quenching temperature is 970 ℃,the test steel has good toughness and sufficient strength.The elongation,section shrinkage,impact absorption energy,tensile strength and yield strength are 15%,59%,100 J,1746 MPa and 1357 MPa,respectively.The influence of tempering temperature on mechanical properties of AF9628 ultra-high strength steel is further studied in this paper.With the quenching temperature optimized above(970 ℃),the tensile strength of AF9628 ultra-high strength steel shows a downward trend with the increase of tempering temperature,while the yield strength rises first and then decreases.The impact absorption energy fluctuates little at low temperature tempering,and decreases first and then increases with the increase of temperature.The elongation and section shrinkage begin to fluctuate little and then continue to rise.In microstructure,the nanoscale ε-carbide precipitates in the lath martensite of AF9628 ultra high strength steel at low temperature tempering.With the increase of tempering temperature,the recovery degree of lath martensite increases,carbon atoms continue to diffuse into carbide,ε-carbide gradually coarsened and grew,transformed into sheet cementite,lath martensite interface gradually blurred,at this time the plasticity toughness decreased;However,with the further increase of tempering temperature,the final development of spheroidal cementite size is larger,and the plastic toughness is greatly improved.Due to the need for ultra-high strength in the application field,AF9628 steel is tempered at 170～230 ℃ to obtain good strength and toughness.