Study On Toughening Mechanism Of Weld Metal In A 420 MPa Grade HSLA Offshore Engineering Steel

In this paper,the toughening mechanism of the 420 MPa grade submerged arc weld metal about Si-Mn,Si-Mn-Ni and Si-Mn-Ti-B alloys was studied by measuring the ductile-brittle transition temperature DBTT and the crack tip opening displacement CTOD.And their adaptability to the heat input was investigated by comparing their mechanical properties such as hardness and tensile properties.The results show that the Si-Mn alloy had the lowest DBTT temperature and the best toughness when the Mn content was 1.21%.When the Si content was between 0.1 and 0.4%,the increase of Mn content promoted the formation of weld metal inclusions,which stimulated a large number of acicular ferrite(AF)nucleation and improved impact toughness.However,as the Mn increased beyond 1.21%,the hardenability increased,and the fraction of the large-sized MA constituent in the reheated zone went up,and the toughness was deteriorated.The Si-Mn-Ni alloy system significantly reduced the DBTT when the Mn content was 0.9%and the Ni content was 2.6%.The ductile-brittle transition temperation curve did not show obvious low-temperature brittleness and the optimal toughness was obtained.The addition of Ni element toughed the matrix,consumed more crack initiation energy,and promoted the formation of granular bainite(GB),which arrested and distorted crack propagation.In the Si-Mn-Ti-B alloy system,B element was segregated at the prior austenite grain boundary,which inhibited the formation of pro-eutectoid ferrite(GBF),increased the solidification temperature,and refined the grain.Ti element was easily oxidized,and a large amount of inclusions containing Ti oxide were formed,which were mainly distributed in the prior austenite grains,which provided a large number of nucleation sites for AF and reduced nucleation work.At the same time,the addition of Ti also significantly reduced the grain size of the reheated zone.In the heat input adaptation range test,it was found that the welding heat input of the three alloy systems was too large,which would cause the toughness to deteriorate,and the Si-Mn-Ti-B alloy had the best adaptability to the large heat input.Moreover,in the heat input range of 15～45 k J/cm,the Si-Mn alloy system obtained the best toughness at 30 k J/cm.If the heat input was too high or too low,the DBTT would increase.As the heat input increasing,the size of the inclusions went up,and the amount and the areal density decreased,which in turn led to a decrease in the nucleation sites and the content of AF.In addition,the large heat input increased the depth of the reheated zone,the percent of the brittle second phase such as the MA component increased by about 3%,and the grain size also increased sharply,which contributed to the decrease in free energy of microcrack initiation and growth.In the CTOD three-point bending test of thick plate,it was found that both Mn and Ni played an important role in promoting the formation of AF and preventing the formation of GBF,while Ti-B was more effective than Mn.The inclusion density of Ti-B was increased from 9.6×10~3/mm~2 to 1.3×10~4/mm~2 in comparison with Mn.In addition,more than 20%of the high-angle grain boundaries were obtained and the average grain size was reduced by 1.3μm.Besides,the fine MA in the Ti-B weld is 22%more than that in the Si-Mn,which can deflect or even stop the crack propagation contributed to optimizing the CTOD.