Microstructure Evolution During 700l Tempering And Its Effect On Residual Stress

700L high-strength steel is an indispensable and important basic material for economic construction,which is widely used in automobiles,bridges,energy,engineering machinery and other fields.Residual stress is an important basic problem restricting the application of high-strength steel,which seriously affects the shape,strength and resistance to stress corrosion of the product.In industrial production,tempering heat treatment method is widely used to adjust the residual stress inside high-strength steel.Even so,at present,the evolution law and adjustment mechanism of residual stress during tempering of high-strength steel are still unclear,and the following three problems still exist in practical applications:1)The existing residual stress testing methods mainly focus on surface stress testing The analysis of the evolution law of residual stress in the heat treatment process focuses on the surface stress,and there is a lack of understanding of the evolution law of the overall residual stress of the material;2)The analysis of the existing residual stress evolution mechanism mainly starts from the mechanical properties of the material The main reason is that the yield strength and elastic modulus of the material are reduced;the main reason for the residual stress relaxation at high temperature stage is the high temperature creep of the material,and the microscopic mechanism of the evolution of residual stress is not clear;3)The transformation of the material structure has not been established Correspondence between the evolution of residual stress.In this paper,700L low-carbon microalloy high-strength steel is taken as the research object,and the key and common problems of the evolution law of residual stress and its control mechanism during tempering are adopted.The residual stress test of the crack compliance method（CCM）and SEM,TEM,XRD microstructure analysis and other experimental methods are systematically studied from the aspects of residual stress testing before and after tempering,microstructure evolution during tempering,and the effect of tissue transformation on residual stress evolution.The main conclusions are as follows:（1）The 700L tempering process includes five stages:precipitation of cementite I（50-200℃）;decomposition of residual austenite（300-350℃）;precipitation of cementite II（250-350℃）;alloy Carbide precipitation（500-600℃）;Mn distribution（580-650℃）,in which carbide precipitation is the main reaction during 700L tempering.（2）There are two obvious residual stress adjustment stages in 700L during tempering,which is related to the transformation of microstructure.The first stage is the precipitation phase of cementite II（250-350℃）,the absolute value of the residual stress is reduced from 571MPa to 208MPa,but the elastic strain energy remains basically unchanged;The second stage is the alloy carbide precipitation stage（450-650℃）,the absolute value of the residual stress is reduced from 208MPa to174MPa,and the elastic strain energy is reduced from 1695MPa·mm to 441MPa·mm,a decrease of 73.98%.（3）Carbide precipitation induces an increase in dislocation density.In the tempering process,the dislocation density does not continue to decrease as the tempering temperature increases,but is closely related to the precipitation of carbides during the tempering process.During the precipitation of carbides in low-carbon microalloyed steel 700L,the dislocation density in the structure increased rapidly from7.44x10¹³m^-22 to 3.69x10¹⁴m^-2,and carbide precipitation induced the proliferation of dislocation density.（4）The control mechanism of residual stress during low temperature tempering is precipitation-induced plasticity mechanism.The driving force for the precipitation of plasticity during tempering is the initial residual stress in the material.Therefore,the direction of the precipitation plastic strain is consistent with the direction of the initial residual stress.The effect of precipitation plasticity is to convert the elastic strain generated by the residual stress in the material into precipitated plastic strain,thereby reducing the residual stress level in the material.Since the temperature of the core of the material lags behind the surface temperature during tempering,the effect of precipitation plasticity on the residual stress of the surface layer of the material is more significant.（5）The control mechanism of residual stress during high temperature tempering is precipitation-induced creep mechanism.During the precipitation of alloy carbides,solute atoms are separated from vacancies,and a large number of vacancies are aggregated into vacant disks.After the vacant disks collapse,dislocation loops are formed,which leads to the increase of dislocations,induces creep strain,and then induces residual stress relaxation in the tissue.（6）While the tempering residual stress is adjusted,the mechanical properties of the material are further improved.At the tempering stage of 100-300℃and450-600℃,the yield strength,tensile strength and elongation of the material were further improved.Compared with before tempering,the yield strength of high-strength steel after tempering at 300℃increased from 584MPa to 707MPa,the tensile strength increased from 700MPa to 792MPa,and the elongation from 20.5%to 22.68%;After tempering at 600℃,the yield strength increased from 584MPa to 719MPa,the tensile strength from 700MPa to 786MPa,and the elongation from 20.5%to 23.95%.In this paper,through the residual stress test of the crack compliance method,the evolution law of the residual stress before and after the tempering process of the low-carbon microalloyed steel is obtained,which provides experimental data for industrial production.On this basis,the stress evolution is related to the transformation of the structure.Through the analysis of the structure,the driving mechanism of the residual stress evolution of the high-strength steel during the tempering process based on carbide precipitation is revealed,which provides a theoretical basis for the residual stress research during the heat treatment process.