Experimental and Thermokinetic Simulation Studies on the Formation of Deleterious Zones in Dissimilar Ferritic Steel Weldments

The methods to predict and prevent the formation of hard and soft zones in dissimilar weldments of 9Cr-1Mo and 2¼Cr-1Mo ferritic steels during high-temperature exposure are examined in this article. The computational studies have been carried out using multicomponent diffusion model incorporated in Dictra and validated by experimental methods using EPMA and TEM. Carbon concentration profiles across the interface of the weld joint between the two ferritic steels were simulated in the temperatures ranging from 823 K to 1023 K (from 550 °C to 750 °C) for various time durations using “diffusion in dispersed phase model” in Dictra. When precipitation and diffusion were incorporated into the calculations simultaneously, the agreement was better between the calculated and the experimentally measured values of carbon concentration profiles, type, and volume fractions of carbides in the hard zone and diffusion zone, width, and the activation energy. Calculation results of thermodynamic potentials of carbon in 2¼Cr-1Mo and 9Cr-1Mo steels suggested that the diffusion is driven by the activity gradient of carbon across the joint. The effectiveness of nickel-based diffusion barrier in suppressing the formation of hard and soft zones is demonstrated using calculations based on the cell model incorporated in Dictra.