Dynamic In-situ Study On Microstructure Transformation In Welding Heat Affected Zone Of Low Alloy High Strength Steel

Low alloy high strength steel is widely used in engineering machinery,which has high weldability requirements.How to improve and enhance the comprehensive performance of welding heat affected zone has become the core problem to meet its application.At present,the use of high temperature laser scanning confocal microscope（HT-LSCM）to study the dynamic microstructure transformation at high temperature has attracted more and more attention.In this paper,single-pass and multi-pass welding thermal simulation experiments were carried out on two kinds of high strength steels by HT-LSCM.Two kinds of low alloy high strength steels with different strengths were used for welding thermal simulation experiments by HT-LSCM and Gleeble welding thermal simulation experiments machine respectively.The method of in-situ observation of dynamic microstructure transformation was used to study the microstructure transformation of heat affected zone under different welding heat input conditions and the difference of microstructure properties.The two thermal simulation methods were compared and analyzed,and the weldability of high strength steel was studied by dynamic observation of the microstructure transformation.This method can quickly find out the process parameters that meet the weldability conditions and narrow the process window of welding evaluation experiment.The experimental results are as follows:The welding thermal simulation experiment by Gleeble was carried out for the two kinds of high strength steels under the same t_8/5conditions.The microhardness goes up gradually with the decrease in welding heat inputs of the experimental steel,and the impact toughness firstly increased and then decreased.When the welding heat input is 11.5k J·cm^-1,the impact absorbing energy of 900MPa high strength steel reaches 86.6J,which meets the weldability requirements.When the welding heat input is 21k J·cm^-1,the impact absorbing energy of 1100MPa high strength steel reaches 114J,which meets the weldability requirements and has the best performance.Under this welding heat input,after a single pass welding heat simulation,the microstructure of the steel is lath bainite with interlocking structure,and the bainite lath through the whole grain can separate and refine the original austenite grain.When HT-LSCM was used for welding thermal simulation experiment,the in-situ observation of dynamic microstructure at high temperature shows that austenite grains coarsen at 1100℃.From peak temperature to 900℃,the austenite will still grow larger but the velocity will slow down.With the decrease of welding heat input(or t_8/5),the microstructure of thermal simulation coarse-grained heat affected zone of 1100MPa high strength steel transforms from granular bainite plus lath bainite to full bainite and then to lath bainite and lath martensite mixed microstructure.With the decrease of welding heat input,the microstructure of coarse-grained zone of 900MPa high strength steel transforms from proeutectoid ferrite and granular bainite to lath bainite and lath martensite mixed microstructure.The transformation microstructure is consistent under the different welding heat inputs under the two different welding thermal simulation methods.The results show that HT-LSCM can be used in t_8/5experiment to study the welding process of materials.Compared with the traditional Gleeble thermal simulation,this process takes less time and is simpler to operate.At the same time,HT-LSCM was used to conduct two-pass welding thermal simulation with heat input of 6.9k J·cm^-1for 1100MPa high strength steel.The results show that the mixed microstructure of lath bainite and martensite is dominant in the unaltered reheated coarse-grained heat affected zone,and the grain size of austenite is more bulky than that of single-pass.The grain refinement structure of the supercritically reheated coarse-grained heat affected zone is bainite with smaller lath size.Incomplete austenitizing occurs in the intercritically reheated coarse-grained heat affected zone,and some regions after single-pass welding thermal simulation at the end of bainite lath formed by microstructure transformation and original austenite grain boundary are refined and transformed into martensite and M-A components,while the remaining regions maintain A bainite shape in the sub-coarse-grained region.The microstructure of subcritically coarse-grained heat affected zone is similar to that of single-pass coarse-grained zone,and some lath is decomposed and tempered to a certain extent.