Influence Of Welding Thermal Cycle Parameters On Low Temperature Toughness Of EH460 Steel In Heat Affected Zone

In recent years,the demand for container ships has soared,and tends to large-scale,intelligent and high-speed development.In order to ensure the stability of hull structure,most structures need to use ultra-high strength steel plate with excellent toughness.However,the mechanical properties of the welded joint of ship plate steel,especially the heat affected zone（HAZ）,will be reduced by high wire energy welding.EH460 produced by Nb,V and Ti microalloying combined with TMCP process was studied in this paper.The effects of welding line energy,peak temperature and residence time of peak temperature on microstructure and properties of HAZ were studied by Gleeble simulation welding to expand the width of heat affected zone.After thermal simulation,the sample was subjected to-20℃impact test and HAZ microhardness test.Combined with OM,SEM,EDS and other characterization methods,the influence of microstructure change on the low-temperature impact properties and the morphology and composition change law of inclusions were studied.The following conclusions are drawn:（1）With the increase of welding wire energy,the HAZ low temperature impact energy decreases at 1300℃,1350℃and 1400℃,respectively.When the peak temperature is 1400℃,the impact energy of 100k J/cm and 200k J/cm at-20℃does not meet the requirements of national standard.The HAZ microhardness increases first and then decreases with the increase of welding wire energy,and reaches the peak value at 80k J/cm.The results of fracture analysis of different welding line energy samples at 1400℃show that the test steel exhibits brittle cleavage fracture.The microstructure analysis results show that,with the increase of welding wire energy,the transition from lath bainite to grain boundary ferrite and side lath ferrite is gradually transformed.At 100k J/cm,a large number of grain boundary ferrite is distributed in a network.The analysis results of inclusions show that most inclusions are Al₂O₃-Mg O-Ca S-Mn S-Ti₂O₃composite inclusions.The analysis shows that the increase of welding wire energy,grain coarsening and brittle structure content are the main reasons for the decrease of impact toughness.（2）The impact toughness of HAZ decreases with the increase of peak temperature,and decreases about 40%in the temperature range from 1350℃to1400℃.The hardness values are generally higher than the base metal and reach the maximum value at 1300℃.The results of fracture analysis show that at different peak temperatures with line energy of 50k J/cm,each section has a certain proportion of dimples.At 1300℃,the proportion of shear lip is larger,and a lot of plastic deformation occurs at the macroscale,and the proportion of dimples is the largest,and the toughness is the best.The microstructure analysis results show that with the increase of peak temperature,the intra-grain structure changes from bainite to intra-grain boundary ferrite.The results of grain size analysis show that the austenite grain size has a positive relationship with residence time above 1000℃.（3）With the extension of residence time at peak temperature,the low temperature toughness of the test steel decreases slightly at 50k J/cm and 1300℃,but still far exceeds the requirements of the national standard.The hardness value reached the peak value at 1s,and then decreased slightly,but it was basically the same as the base metal.Austenite grain size increases and tends to homogenize gradually.The intra grain structure changed from lath bainite to granular bainite and preeutectoid polygonal ferrite,and the M/A component changed from granular to massive or long strip.（4）The results of the above three groups of parameters simulated welding thermal cycle show that welding line energy is the most critical factor affecting the impact toughness,followed by peak temperature,and the residence time of peak temperature has little influence on the toughness.