Thèse de doctorat en Sciences et techniques
Sous la direction de Jean-Pierre Chevalier.
Soutenue en 2001
à Paris, CNAM .
Ferritic grain growth during the welding of interstitial free steels
Pas de résumé disponible.
This study concerns understanding of the ferritic grain growth phenommenon in heat affected zone (HAZ) during the welding of interstitial free steels. Firstly, we have examined in which conditions the grain growth occurs in wedded joints. We have performed thermal simulation test (Dilatometry, Gleeble) in order to reproduce the formation conditions of large grains. We have observed that the large grain in the order of 100 um in HAZ has a very elongated shape as a result of the heat flow and occurs in certain grades of IF steels at a certain distance from the fusion zone slightly over the A temperature Then we have modelled the thermel phenomena during the different welding processes in IF steels and performed an experimental study to verify the proposed mechanism. We have determined the thermal gradient value G (c/mm) as an essential factor for the grain growth phenomenon just as the (austenite ferrite) transformation and then we have explained this phenomenon by means of the proposed model in two stages : nucleation and growth. The growing of these ferrite nucleus depend strongly on the G parameter just as the equations) and also the composition (by transformation temperature Ar3). We have constructed the diagrams allowing to predict the value of G parameter and the corresponding grain size. We have realised the welding limit method show the principal factors allowing to obtain a small ferritic grain size (energy, thickness, transformation, temperature, composition, etc. . . ) and give pratical and simple solutions both of the steel markers and welding shpo. And then, the inetractions between the grain growth phenomenon and precipitation state play an important role and the TiC particles are numerous and suitable to sissolve at the level of the grain growth appearance temperature. At this stage the influence of the precipitation is reinforced by thermal gradient. It may be modified by the composition (changing the transformation temperature) and/or by the welding parameter. Finally, impact tensile tests performed on the welded joints well estimate the fine interest for obtaining the best thoughness value to the fracture