Alekseenko T. Structure and mechanical properties of metal of heat affected zone of welded joints of high-strength low-alloyed steels

The thesis work is devoted to investigation of changes in structure composition of metal of welded joints in HAZ at the following conditions: change of cooling rate (W6/5 = 2,5; 10; 20oC/s), using different types of welding wires, for welding conditions at different rigidity of fastening b=150mm, b=50mm and also changes of structure parameters under the influence of external load (static and cyclic). The work presents the results of investigation of microstructure of welded joints applying light microscopy, scanning and transmission electron microscopy. The method of analytical evaluation of properties of welded joints was described based on evaluation of contribution of structures and their parameters which allow determining the optimal structural composition in HAZ metal of welded joints of high-strength low-alloyed steels. It was established that maximal value in general value of yield strength is influenced by the structures of lower bainite and tempering martensite. The calculating-experimental method showed that after application of external load the maximal level of local inner stresses is concentrated along the boundaries of cementite plates in the structures of upper bainite which results in initiation of microcracks in these zones. In the structures of lower baininte and martensite of tempering the fragmentation is occurred which expands the capabilities of plastic relaxation of stresses due to linking of rotation mechanisms of inner stresses relieving to dislocation mechanisms. It was established that using welding wire Sv-10KhN2GSMFTYu (bainite-martensite weld) in welded joints of steel 17Kh2M the high complex of mechanical properties is provided.