Growth Morphologies and Primary Solidification Modes in a Dissimilar Weld between a Low-Alloy Steel and an Austenitic Stainless Steel

Fanny Mas,Miguel Yescas,Patrick Todeschini,Yves Bréchet,Catherine Tassin,François Roch

doi:10.3390/met8040284

Abstract

Dissimilar welds close to the fusion boundary exhibit a variety of solidification microstructures that strongly impact their service behavior. Investigations were therefore undertaken to clarify the origins of the morphological and microstructural evolutions encountered in a 18MND5/309L dissimilar joint produced by submerged arc welding, using a combination of microstructural characterizations, thermodynamic computations, and solidification modelling. An unexpected evolution was observed in the solidification mode, from primary austenite towards primary ferrite with increasing growth rate. Solidification of austenite at the fusion boundary was assigned to its epitaxial growth on the metastable austenitic structure of the base metal resulting from an incipient melting mechanism. The evolution of the solidification mode toward primary ferrite was explained based on computations of the solute built up between austenite cells followed using the so-called “interface response function model”. Analyzing macro- and microstructural characteristic lengths with the published solidification model and data enabled evaluation of local values of the solidification rate, thermal gradient, and cooling rate close to the fusion boundary, thus providing useful data for numerical modelling of the submerged arc-welding process.

Highlights

Dissimilar metal welds (DMWs) are widely used in nuclear power plant and oil refinery components for joining/cladding ferritic low-alloy steels to/with corrosion-resistant austenitic stainless steels
A “transition layer” or “partially mixed zone” PMZ is always present in which the chemical composition varies continuously from that of the solid base metal to that of the bulk weld metal characterized by a homogeneous composition resulting from the mixing in the liquid pool of the filler metal with part of the base metal
For most of the authors [19,20,21,22,23] the origin of this partially mixed zone is related to fluid flow and the existence in the molten pool of a stagnant fluid layer poorly stirred by convection in contact with the solid substrate, where solute transport would occur mainly by diffusion in the liquid

Summary

Introduction

Dissimilar metal welds (DMWs) are widely used in nuclear power plant and oil refinery components for joining/cladding ferritic low-alloy steels to/with corrosion-resistant austenitic stainless steels. With ferritic low-alloy steel as base metal, the most frequently used electrode materials, i.e., filler metals for welding or overlaying materials for surfacing, are austenitic stainless steels AISI. Close to the fusion line in this transition layer the rapid decrease of the thermal gradient and increase of the solidification rate produce morphological transitions of the liquid-solid interface, from planar front to cells and dendrites, and eventually a change in the primary solidification phase [24,25,26]. For Fe-C-Al-Mn steels solidifying into primary δ-ferrite in conditions close to equilibrium, an increase of the solidification rate was observed to promote the primary solidification of non-equilibrium γ-austenite using time-resolved X-ray diffraction [11,27]

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