Weldability Investigation And Microstructure Prediction Of HAZ For Q-T Q690D Steel

Welding plays an important role during the high strength low alloy steel, such as Q690D is processed into the engineering structure whose service life is closely related with the microstructure and property of the welding joints. To control the weldment quality，therefore, the microstructure and property prediction of the welding joints becomes a hot study topic.This thesis was concentrated on the weldability investigation and microstructure and property prediction by numerical simulation of10mm thickness Q690D plates. The Gleeble-1500D thermal-mechanical tester and L78-RITA quenching dilatometer were employed to finish the thermal simulation tests of Q690D steel, meanwhile, the SH-CCT diagram was obtained. After multi-passes welding by GMAW method, the microstructure of the weldment was observed by OM and SEM. The hardness, tensile and impact tests were implemented to investigate the mechanical properties; The temperature distribution of welding process for Q690D steel was simulated by ANSYS.For the simulated continuous cooling transformation of coarse grain, the critical formation t8/5of bainite, proeutectoid ferrite and pearlite were4s,500s,1200s respectively; When the t8/5was at the range of4.4~30s, the microstructure were lath martensite and bainite, and100%bainite was obtained when t8/5was ranged at60~200s; The grains of parent metal were refined at a peak temperature range of880~1150℃, while the grains were coarsening when peak temperature is up to1150℃; The fraction of ferrite at880℃was high enough to decrease the hardness; When the peak temperature of second thermal cycle was800℃, the grains were partly refined at the boundary and bainite was obtained.The microstructure in welding zone was mainly acicular ferrite, and lath martensite mixed by bainite were obtained in the heat affected zone (HAZ) of Q690D weldment; There also existed a soften zone in the HAZ; The-20℃impact value of welding zone was over to100J, when value of HAZ was approximately50J; The increasing current decreased the impact value of fusion zone; The tensile strength of all weldments was over780MPa which meet performance requirements.On the basis of welding thermal cycle simulation results and SH-CCT diagram, the width and microstructure distribution on HAZ of all welding passes were accurately predicted through the temperature distribution calculated by finite element method, which were verified by the welding tests results; Consequently, numerical simulation combined with the SH-CCT diagram was successfully implemented to microstructure prediction, by which the working efficiency was improved and cost was reduced.