Effects of Alloying Concepts on Ferrite Morphology and Toughness of Lean Duplex Stainless Steel Weld Metals

Abstract

Alloying concepts for lean duplex weld metals have been explored. Focus was on compositions with 21.5–24 % Cr with different levels of Ni and Mn in combination with N producing a minimum PREn value of 26. Microstructures and properties of weld metals produced with experimental covered electrodes or metal-cored wires were evaluated. The electron backscattered diffraction (EBSD) technique combined with scanning electron microscopy (SEM) was used to explore relations between weld metal morphology and crystallographic orientation relationships between ferrite and austenite. A duplex microstructure with Widmanstatten type ferrite characteristic of a ferritic solidification was found throughout most weld metals. Regions with a morphology suggesting a mixed ferritic-austenitic solidification occurred particularly for higher Ni- and N-contents. EBSD studies showed that most of the austenite formed following a ferritic solidification was near either the Nishiyama-Wasserman (NW) or Kurjdumov-Sachs (KS) relationships with adjacent ferrite. More phase boundaries in the mixed mode solidification regions had a random orientation relationship. Strength, ductility and PREn requirements were readily matched but the combined Mn, Ni and N alloying levels and the ferrite morphology affected toughness significantly. Both a fully ferritic solidification and a sufficient Ni-content were necessary to produce acceptable and reproducible impact toughness at room and sub-zero temperatures. It was concluded that a composition of 23–24 % Cr, 7–8 % Ni and 0.12–0.16 % N was well suited to fulfil requirements.