Sigma embrittlement evaluation test for dissimilar welding between F6NM and FXM-19

Martensitic stainless steel Code Case N-774, SA-336 Grade F6NM (F6NM) and nitrogen-strengthened austenitic stainless steel SA-965 Grade FXM-19 (FXM-19) are permitted materials in ASME pressure vessels. F6NM and FXM-19 are expected to use as pressure retaining materials because of its high strength, good toughness, and corrosion resistance.

There are several challenges when welding these two materials (dissimilar welds).

Ni-base alloy welding materials are often used in dissimilar welds between low alloy steel and austenitic stainless steel. However, in dissimilar weld between F6NM and FXM-19, it is necessary to use type 209 austenitic welding materials that match the mechanical properties and chemical composition of FXM-19, because Ni-base alloy welding material would be undermatched. The welds of F6NM must be subjected to Post-Weld Heat Treatment (PWHT) to restore ductility and toughness. The temperature of PWHT is usually around 600℃.

Generally, austenitic stainless steel welding materials are designed to have a minimum of 5% delta ferrite to prevent weld cracking. In the case of type 308 and 316 welding materials, PWHT is known to cause the delta ferrite to transform to sigma phase, resulting in reduced ductility. Type 209 welding materials is also in the danger zone of embrittlement on the Schaeffler diagram, and there is concern about sigma embrittlement.

The primary objective of this paper is to evaluate the degree of sigma embrittlement of dissimilar welds and evaluate that there are no problems with the joint performance. Welding test will be followed by mechanical testing such as side bend test and Charpy impact test to evaluate toughness of welds, and electron back scatter diffraction (EBSD) will be carried out to identify any sigma and quantify the volume fraction of all phases.