Effect of rapid solidification on stainless steel weld metal microstructures and its implications on the Schaeffler diagram

Abstract

An investigation was carried out to determine the effect of rapid solidification on the weld metal microstructure of austenitic stainless steels and its implication on the ferrite constitution diagram. A wide variety of stainless steels were laser welded at different welding speeds and laser power levels. The results indicate that both weld pool cooling rate and the postsolidification solid state cooling rates have a profound effect on the microstructures. For the steels investigated, the microstructures ranged from duplex austenite (7") + ferrite (6) to fully austenitic or fully ferritic. These microstructures were found to be sensitive to both cooling rates and composition. The observed results are rationalized based on rapid solidification theory. Observations of this investigation indicate that solidification rates and postsolidification cooling rates have a profound effect on the observed microstructures, thus making it impossible to predict the microstructures of rapidly cooled weld metal from the conventional constitution diagrams. The influence of the observations made in this investigation on the Schaeffler diagram is demonstrated and possible corrections to the constitution diagram incorporating the cooling rate effects are proposed.

Inclusion Formation and Microstructure Evolution in Low Alloy Steel Welds.

“…This increased velocity may lead to changes in solidification morphology, partitioning, and primary phase selections. [44][45][46][47][48][49] In welds, epitaxial solidification occurs from the heat-affected-zone, the nucleation of solid phase occurs readily.…”

The effect of cladding speed on phase constitution and properties of AISI 431 stainless steel laser deposited coatings

“…In the case of stainless steel coatings, this seems to be necessary because it has been shown before that the high cooling rate during solidification alters the microstructures and phases of stainless steel weld deposits [9][10][11]. In other words, rapid solidification of stainless steels may produce deposits with properties different from those observed during conventional solidification which can strongly affect the final mechanical, chemical and technological characteristics of stainless steel deposited coatings [12,13].…”

Transformaciones Microestructurales en Soldaduras Disímiles de Acero Inoxidable Austenítico con Acero Inoxidable Ferrítico

“…Los métodos predictivos como los diagramas de Schaeffler y WRC-92 no toman en cuenta las velocidades de enfriamiento que son un factor determinante en la cantidad de ferrita δ presente en los depósitos de soldadura [15][16][17][18][19]28,31,32,[34][35][36][37], además, el diagrama de Schaeffler no tiene en cuenta el contenido de Nitrógeno en su cálculo del Níquel equivalente, que para el electrodo dúplex es de 0,2%. La técnica de permeabilidad magnética (Ferritoscopio Fischer) presenta algunas limitaciones prácticas relacionadas con las dimensiones de la muestra, por ejemplo, al usar muestras de tamaño reducido pueden disminuir los valores de ferrita δ medidos debido al efecto de borde.…”

“…Autores como Shinozaki et al [38], Lilijas y Qvartof [39] y Gill et al [40], reportan relaciones entre el FN y el porcentaje de ferrita, y aunque concuerdan que FN exagera el porcentaje de ferrita, no coinciden con la relación entre los dos [34].…”