Paper Investigates Carbon Behavior in the Heat-Affected Zone of Grade 91 Steel using DICTRA

The efficiency of fossil-fired and nuclear power plants has caused raised operating temperatures, which requires use of creep-resistant stainless steels in the hottest regions of the plant. Grade 91 steels are used in the lower-temperature heat recovery steam generators. To be able to join the high- and low-temperature sections, dissimilar metal welds (DMWs) are necessary. The problem with using DMWs is that it often results in extensive carbon diffusion near the fusion boundary which creates brittle and large carbides that make the welds weaker.

Creep strength-enhanced ferritic (CSEF) steels are today welded with Ni-based filler metals to reduce the carbon diffusion between the dissimilar steels, which reduces the formation of hard and soft zones that negatively affects the creep strength. However, the high concentration of carbide forming elements in Ni-based alloys still creates a driving force for carbon diffusion toward the filler metal.

In this paper, Retention of delta ferrite in the heat-affected zone of Grade 91 steel dissimilar metal welds, the case of Grade 91 steel DMWs with Ni-based filler metals is studied. The higher chromium content of the steel reduces the carbon diffusion, resulting in acceptable welds for industrial use. Nevertheless, failures near the phase boundary have been reported. A phenomenon that may be related to the unexpected failures is retention of δ-ferrite in the heat-affected zone (HAZ).

Thermodynamic and kinetic simulations were performed to investigate what effect the filler metal composition has on the behavior of carbon in the HAZ and the partially mixed zone. Thermo-Calc with the TCFE9 and TCNI8 databases was used both to estimate the solidification temperature ranges and to compute chemical potentials of carbon. The CALPHAD method was used to explain the observed δ-ferrite in the coarse-grained heat-affected zone of DMWs and a prediction of the compositional profile across a DMW was calculated with the Diffusion module (DICTRA).

Results from the simulations were compared with experimental results and the authors concluded: “The developed computational model of carbon diffusion in Grade 91 steel DMWs can be applied in studies of carbon behavior during postweld heat treatment and service, and of the potential effect of carbide precipitation and coarsening on the failure mechanism in the partially mixed zone.”

This paper is written by Michael W. Kuper and Boian T. Alexadrov.

Abstract

This study aimed to determine the mechanism of δ-ferrite retention in the coarse-grained HAZ (CGHAZ) of Grade 91 steel dissimilar metal welds (DMWs) with Ni-based filler metals. This phenomenon was investigated in four DMWs made with cold-wire gas tungsten arc process using Alloys 625, 617, 82, and P87 filler metals. A narrow band of d-ferrite grains was identified in the CGHAZ in all welds. It was hypothesized that δ-ferrite retention was caused by local carbon depletion in the CGHAZ, which was validated through extensive thermodynamic and kinetic simulations and metallurgical characterization. Carbon diffusion across the fusion boundary was driven by the carbon chemical potential gradient between Grade 91 steel and the Ni-based filler metals, which was facilitated by long high-temperature dwell times resulting from a difference in heat capacity and thermal conductivity between the base and filler metals. A linear relationship was established between the amounts of retained δ-ferrite and the predicted carbon depletion in the CGHAZ of each DMW. Alloy 625 filler metal generated the largest extent of carbon depletion and the most retained δ-ferrite, followed by Alloys 617, 82, and P87. The carbon depletion resulted in local softening of the CGHAZ martensite.