Study On Mechanical Behavior Of Low Alloy Steel Transformation During Welding

As low-alloy steel is more widely used in many fields such as pressure vessel, construction machinery, bridge, ship and so on, the influence of transformation on welding has caught more and more attention. Because of the complex mixture of alloying elements, a variety of welded joints with distinctive property are obtained deriving from different transformation at certain temperature while low-alloy is undergoing a thermal cycle. It will not only affect the life of welded structure, but also has effects on welding deformation and residual stress due to the changes of stress-strain relationship resulting from transformation strain.In this paper, 945 steel is taking as the object to study on the mechanical behavior of transformation during welding. At the first, a lot of expansion curves were tested on Gleeble-3500 with thermal cycle parameters similar to welding process, to investigate two primary factors on transformation during welding that were cooling rate and grain size. Then the availability of scheil-law for calculation of diffusion transformation was verified by experimental results. Grain size was taken into consideration while describing the TTT diagrams that would be used in this method.Secondly, several welding experiments with different cooling conditions were done to get the temperature curves at some points by thermocouples. And heat transfer analysis was done by inversion techniques based on modified pattern search method, to make the temperature field and history coincide with the experimental results. By comparing to the metallographs, the method mentioned last paragraph is also proved to be available for describing the transforming process on every region of welded joint that has a steep temperature gradient.Finally, the stress-strain behavior of material during the heating and cooling process under unidirectional constraints was studied quantitatively. FEM was used to show the effects of transformation on residual stress qualitatively by simulating the satoh experiment. At the same time, the relationship of transformation and welding distortion was also talked about, based on the experimental results by the distortion measuring system.