Properties of long-term operated welded joints

The causes are investigated of cracks under overlays in the heat affected zone of medium alloy Cr-Ni-Mo-V steels. Cracks develop in submerged are welding with austenitic strip electrodes at a high heat input. At high temperatures 1250 - 1430 degC, grains in the overheated zone adjoining the austenitic overlay become coarse. Cracks are intergranular, along the original austenitic grains, with smooth intergranular facets. A test was made of simulating cycles of welding and annealing. The samples were heated to a temperature of 1350 degC for 30 s and subsequently cooled for 30 s to a temperature of 100 - 20 degC. At constant stress lower than threshold stress and at critical temperature of 500 - 800 degC no overlay cracks appeared. Also simulated was the consequence of a 5000 h dwell in an operating temperature of 325 degC. Under cyclic stress the service life of material in the overlay and in the joints with basic material decreased by 5 - 15%. The austenitic overlay showed greater resistance to plastic deformation. Owing to carburization of the boundary of the overlay and basic material, crack propagation in that zone slowed down, in some instances ceased completely. (Pu)