Investigation Into The Mechanism For The Effect Of Nb And Ti On Ductility Dip Cracking Susceptibility

The ductility dip cracking(DDC) susceptiblity of six kinds self-designedInconel 690 welding filler wire were evaluated by usingSTF(Strain-to-Fracture) and hot ductility experiment, lt is the first time toevaluate the DDC susceptibility of Inconel 690 welding wire filler metal inour country. The result of these experiments will have a function that guidingthe development of Inconel 690 welding filler wire.In this paper, the main aim is the effect of alloy elements Nb and Ti inInconel 690 welding filler wire on ductility dip craking susceptibility offiller metal. The STF results show that the DDC susceptibility of Inconel 690welding wire filler metal is improved by increasing the content of Nb and Tiin welding filler wire. The results of analysis show that alloy elements Nband Ti can form some complex MC precipitates which contain both Nb and Ti alonggrain boundary. These MC precipitates can improve grain boundary strength,impede grain boundary to slide, and these precipitates can also pin grainboundaries and avoid forming straight boundaries that are very susceptive tocracking during solidification, all of these show that MC precipitates alonggrain boundary can improve DDC susceptibility of welding filler metal.There is a best value in content of Nb in Inconel 690 welding filler wire, itis 1.1%, the liquid cracking susceptibility of Inconel 690 welding wire fillermetal will be increased if the content of Nb exceed this value, therecommendable ratio of Nb/Ti is from 1.0 to 2.0. Alloy elements Nb and Ti showa cooperative effect on improving DDC susceptibility. Niobium carbide willnucleate on pre-existing Titanitum carbide and grow up, form complexprecipitate (Nb, Ti)c. The shape of precipitate (Nb, Ti)C is influenced by the ratio of Nb/Ti in atoms number, so the precipitates have varied morphologiesincluding irregular spheres and cubes. The nunber and size of precipitatesobviously increase with enhancing content of Nb and Ti.The DDC of the Inconel690 welding wire filler metal is a kind of hightemperature solid cracking phenomenon, the mechanism of DDC is that relativelyweakness of grain boundary strength at high temperature, under the outsideforce, grain boundaries slip and cause the strain concentration at grainboundaries, then cracking will appear when local strain exceeds the extremeplasticity of material...