AN INVESTIGATION OF THE CARBON MIGRATION PROCESS IN THE WELDED JOINT OF A 12AlMoV STEEL

An investigation of the first kind migration process of carbon has been carriedout on the welded joints of a 12AlMoV steel by means of microscopy, EPMA anddilatometry. It is shown that this carbon migration is caused by the existance ofa significant activity gradient of carbon between the two sides of fusion line. Thismay take place as a result of the earlier precipitation of pro-eutectoid ferrite onone side of the fusion line within a temperature range that rapid diffusion ofcarbon atoms occurs. This confirms the activity reasoning proposed by the authorfor carbon migration. The difference of the activity coefficient of carbon caused by different crystalstructures is larger than that caused by different compositions of the alloys forthese welded joints. Hence, it is the main thermodynamic condition accountingfor this kind of carbon migration. From kinetic point of view, the degree ofcarbon migration increases with the duration under certain temperatures. Changingthe composition as well as modification of welding technology have been proposedso as to retard or suppress the carbon migration in the welded joints of the steel.