Effect Of Welding Process On Microstructure And Properties Of Micro-alloyed Steel Joints

After decades of development, micro-alloyed steel has become one of the high performance steel which is the most widely used in various industrial fields. The study on the welding problem of micro-alloyed also was attracted people's widespread attention. The traditional welding methods used in welding Micro-alloyed steel have lots of disadvantages, such as high heat input, welding joint deformation, low efficiency and so on. As a new developing welding technology, laser welding and laser-arc hybrid welding as a new welding technology can effectively solve this problem. In this paper, Nb-Ti-Mo micro alloyed high strength steel was studied. Thermal simulation technology was used to study the change of microstructure and properties of heat affected zone under various t_8/5 conditions?3s⁶0s?. Three different heat inputs were selected to test the actual laser welding and laser-arc hybrid welding, and the influence of heat input on the microstructure and properties of the welded joint was analyzed. The main work and conclusions of this paper are as follows:?1? The effects of t_8/5 on the microstructure and properties of coarse grain heat affected zone?CGHAZ? and fine grain heat affected zone?FGHAZ? were studied in the welding thermal simulation test. With the increase of t_8/5, grain size of CGHAZ and FGHAZ were increased, and the martensite in CGHAZ was gradually transformed into granular bainite?GB?. For FGHAZ, the microstructure is mainly composed of equiaxed ferrite and M-A, and a small amount of martensite is formed under the condition of lower t_8/5. Due to the change of microstructure and grain coarsening, the hardness of the two regions decreased with the increase of t_8/5. The low temperature toughness of FGHAZ is much better than that of CGHAZ, and CGHAZ is mainly brittle fracture, FGHAZ is mainly ductile fracture. The low temperature impact energy in FGHAZ decreased more obviously when the t_8/5 was increased to 20 s, while the low temperature impact energy in CGHAZ decreased to a lower level at the beginning. It shows that the overall toughness of heat affected zone?HAZ? can reach the best level only if the t_8/5 is in a low level.?2? In the laser welding tests, it is found that the laser power is 3.5 k W, the amount defocus is-2 mm and the heat input in the 1.17-1.75 kJ/cm, we can realize the full penetration welding in 5 mm thick micro alloyed steel. The microstructure of the weld seam?WS? is mainly of martensite, and a small amount of bainite and ferrite can be formed in the case of high heat input. CGHAZ was organized as lath martensite?LM?, and the original austenite grain size increases with increasing heat input. The microstructure of FGHAZ and mixed grain heat affected zone?MGHAZ? is composed of ferrite and M-A constituent, and the change of heat input has little effect on it. The hardness of welded joint is higher than that of base metal, and the hardness of WS and CGHAZ increases with the increase of heat input, and the peak hardness appears in the coarse grain zone. The tensile fracture appear in base metal welding, laser welding joint hardness were higher than the base metal. Welding joint impact toughness is good and impact energy is greater than or equal to the base metal. The fracture morphology is mainly dimple fracture and heat input has little effect on it.?3? In the laser-arc welding tests, it is found that the laser power is 3.5 kW, the amount defocus is 0mm, welding current is 170 A, arc voltage is 22 V and the heat input in the 3.87-7.74 kJ/cm, we can realize the full penetration welding in 8 mm thick micro alloyed steel. The microstructure of the weld zone is mainly of martensite, but in the high heat input?7.74 kJ/cm?, the upper, middle part and the root part is LM, GB and acicular ferrite?AF?. When the heat input is 3.87 kJ/cm, CGHAZ is full of martensite, when the heat input is up to 5.16 kJ/cm, a small amount of GB is formed. As the heat input increases, GB content increases, and the original austenite grain size of CGHAZ is gradually increasing. FGHAZ and MGHAZ is ferrite and M-A constituent, and the peak hardness of the welded joint is CGHAZ and decreases with the increase of the heat input. In the high heat input?7.74 kJ/cm?, the weld hardness is decreased from top to bottom along the center line. Tensile fracture occurred in the base metal. WS and HAZ of impact energy was slightly higher than that of base metal, the overall impact toughness is good. WS impact fracture is dimple fracture and quasi cleavage fracture, and HAZ was dimple fracture.