890MPa Grade High Strength Steel Welding Of TMCP

According to the weldability of lower-level strength TMCP high strength steel, a systematic study of the weldability of a890MPa grade TMCP high strength steel which was newly developed by domestic factory was carried out in this paper. Adopting thermal simulation test, SH-CCT curve of test steel was determinated, and the microstructure and properties of simulated HAZ were analyzed; By theoretical estimate method, the maximum hardness of HAZ, and inclined Y groove cracking method, this paper studied the crack resistance of the test steel; Finally, the actual welding test of test steel was studied with three different heat inputs, and the microstructure and properties of weld joint were analyzed.The critical cooling rate of completely martensite which could be obtained by CGHAZ of test steel was16.7℃/s; the critical cooling rate of completely bainite was2.5℃/s; the microstructure of CGHAZ was the hybrid organization which consisted by lath martensite and bainite, when the cooling rate was in the range of2.5～16.7℃/s.The local embrittlement would be occurred in the CGHAZ of test steel, whent8/5was bigger or smaller; the hybrid organization of lath martensite and bainite could enable CGHAZ has good toughness, when t8/5was in the range of30--60s.The softening zone of HAZ is in the ICHAZ, this was mainly because of part phase change recrystallization and dislocations disappearance or polygonization.890MPa grade TMCP high strength steel had a certain tendency of hardening, but the degree was not large, higher preheating temperature could reduce the degree of hardenability of the HAZ, and the welding cold crack could be avoided when the preheating temperature was80℃.In the case of three different inputs, the weld metal was mainly composed of lath bainite, as well as a certain amount of lath martensite and granular bainite, in the same time, there are some residual austenite between the bainite lath or the martensitic lath. With the increasing of heat input, the bainite lath and martensitic lath were coarsed, in the same time, the content of lath martensite was reduced, while the granular bainite was increased. Under the three different heat inputs, the HAZ can be divided into CGHAZ、FGHAZ and ICHAZ. With the increasing of heat input, the size of original austenite grain in CGHAZ had a tendency to increase,and the microstructure of CGHAZ was transformed from lath martensite to the hybrid organization of lath martensite and bainite. However, heat input had little effect on the FGHAZ and ICHAZ. The microstructure of FGHAZ mainly consisted of lath martensite, and the microstructure of IGHAZ was composed of lath marstensite and tempered bainite.With the increasing of heat input, the tensile strength of the weld metals and joints showed a downward trend, but were higher than90%of the strength of the base metal; The impact toughness of weld metal decreased with the increasing of heat input, while the impact toughness of HAZ appeared a trendency of decrease after the first increase. The softening of ICHAZ in the joints had no effect on its strength, and the weak link of joints was the fusion line or weld metal near the fusion line.Under the three kinds of heat input, the fracture microtopography of fibrous zones of weld metal and HAZ was ductile fracture morphology, and crack propagation zones were quasi-cleavage fracture. There were some dimple type ductile ridges in the quasi-cleavage plane when the toughness was well, while the size of quasi-cleavage plane was larger, and larger river pattern and fan-shaped pattern were formed when the toughness was poor.