Research On The Friction Welding Of Ultra-fine Grained Steel

Ultra-fine grained steel dramatically improves the strengthening and toughening for itsultra-fine grain, which has great development prospects and application value. But thewelding problems limit its application. Friction welding is an advanced solid-phase bondingtechnique, which has achieved reliable connection of a variety of materials for the weldedjoint has the characters of structure refinement, high strength an so on. In this research,friction welding was applied to weld ultra-fine grained steel weld for the first time, theprocess and mechanism of ultra-fine grained steel friction welding are also both studied.Ultra-fine grained steel was welded by using orthogonal test method, studying themicrostructure and properties of ultra-fine grained steel friction welding joints by means ofmacro-morphology, microstructure, microhardness testing and mechanical testing. The resultsshow that ultra-fine grained steel has good friction weldability. With the process parameters ofspindle speed1500r/min, friction pressure64MPa, friction time3s, upsetting force128MPaand upsetting time1s, the weld zone and heat-force-affected zone grain refinement, compactstructure, although the grain size of heat-affected zone has grown up and a certain degree ofsoftening relative to base metal, but due to the softening zone is very narrow (0.2mm) and thedegree of softening is not serious (down4%), the strength and toughness of ultra-fine grainedsteel friction welding joint are not less than base metal, achieving strong and even supermatch. Studying the tempering stability of ultra-fine grained steel friction welding jointsmicrostructure and performance, the test results show that the tempering stability of itsmicrostructure is poor, and the hardness of the weld zone，heat-force-affected zone and basemetal deereases with inereasing tempering temperature, but the affect on heat affected zonedoes not follow the law, if need to release the residual stress by tempering, it is recommendedto use300℃×1h low-temperature tempering. By the―emergent stop‖experiment, usingmetallographic observation method to study the metal flow behavior and the expansion ofdeformation layer and high temperature area in friction welding, and to explore the ultra-fine grained steel friction welding joint formation process.The results of this study indicate that the friction welding technique is suitable for thewelding of ultra-fine grained steel,achieving its reliable connection without additional processand follow-up treatment as long as the process parameters are proper.