| Low-alloy high-strength(HSLA)steel is an important structural material,which is widely used in various fields of human social life due to its excellent comprehensive properties and good economic applicability.Welding is an important production process for HSLA steel,and the primary goal is to obtain high-quality welded joints.However,HSLA steel welding microstructure is various in phase transformation and has many types of phase transformation.The welding microstructures are usually composed of two or more types of microstructures,which makes the welding performance show huge performance differences with the changes of microstructure type and volume fraction.Therefore,it is important to determine the welding structure type and volume fraction of HSLA steel,which is an important reference for judging its welding performance.Aiming at the limitation of the current method to calculate the volume fraction of HSLA steel welding structure,this paper is based on the principle of phase transformation dynamics,by establishing the phase transformation kinetic equations of various welding structures of HSLA steel,to carry out research on a new method for calculating phase volume fraction,and to solve the problem of HSLA steel welding.The calculation of tissue volume fraction is of great significance.The conclusion of the full text is as follows:(1)The welding microstructure types of low-alloy high-strength steels involve proeutectoid ferrite(PF),pearlite(P),acicular ferrite(AF),granular bainite(GB),lath bainite(Ll B)and lath martensite(LM).The theoretical study of phase transformation kinetics combined with the in-situ observation of the phase transformation process of various structures by laser scan confocal microscopy shows that all types of welded structures are suitable for the interface controlled growth model,and the PF and P nucleation processes are suitable for the pre-existing nucleation model.The nucleation process of AF and Ll B is suitable for the continuous nucleation model,and the growth process is suitable for the 1-dimensional growth model;the GB nucleation process is suitable for the continuous nucleation model,and the growth process tends to the 2-dimensional growth model.The LM nucleation process is suitable for the continuous nucleation model,and the growth process is suitable for the 0-dimensional growth model.(2)Phase transformation kinetic equations are constructed through various phase transformation models.The characteristics of the equations are: the activation energy/free energy parameters in the equations can be approximated,and the equations only contain one unknown parameter.The experimental data of four welding cooling processes(including phase transformation kinetic data and phase transformation rate data)were used to fit and verify the kinetic equations of each type of microstructure.The results show that each phase transformation kinetic equation can describe its phase well.The change process shows the accuracy of the equation.(3)Based on the principle of phase transformation kinetics and the constructed phase transformation kinetic equations of various welding structures of HSLA steel,the extended lever rule is improved,and a kinetic fitting rule is established.The improved extended lever rule has simpler operation process and smaller calculation error,and can solve the calculation problem of the volume fraction of each phase when the phase transformation temperature range overlaps a little.The established kinetic fitting rule makes up for the shortcomings of the current method and the extended lever rule,and can solve the calculation problem of the volume fraction of each phase when the phase transformation temperature ranges overlap.The comparative analysis of the calculation results of various methods shows that both the improved extended lever rule and the kinetic fitting rule can obtain accurate phase volume fraction results.The innovations of this paper are: constructing the continuous cooling kinetic equations of various welding structures of HSLA steel;improving the extended lever rule;establishing the kinetic fitting rule. |
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