Session: 10-03: Advanced Manufacturing 3

Paper Number: 135920

135920 - Sigma Embrittlement Evaluation Test for Dissimilar Welding Between F6nm and Fxm-19 

Abstract: 

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 to evaluate for various welding processes that there are no problems with the joint performance.

Submerged arc welding (SAW), Gas Metal Arc Welding (GMAW) with Pure Ar gas shield and Gas Tungsten Arc Welding (GTAW) were tested using ER209 welding material. F6NM and XM-19 base metal were used. PWHT was applied at 580℃ for 7hours after buttering weld. Tensile test, side bend test, Charpy impact test were used to assess mechanical properties. Transverse macro cross section was taken from weld joint to identify the weld boundary, and hardness test was conducted to obtain hardness map.

SAW and GMAW satisfied the mechanical requirements, and no specific hardened area were observed throughout the weld. GTAW failed the Charpy impact test at the buttering weld area. Buttering weld metal, joint weld metal and heat affected zone (HAZ) of XM-19 were harder than other welding process.

Metallography (etchant to highlight Sigma) and scanning electron microscopy with electron backscatter diffraction (EBSD) were carried out to identify any sigma and quantify the volume fraction of all phases in order to determine the cause of the differences in mechanical properties.

Presenting Author: Ryoji Osafune IHI Corporation

Presenting Author Biography: Ryoji Osafune
Prodction engineering department of IHI Yokohama Works
Welding Engineer

Authors:

Ryoji Osafune IHI Corporation
Yoshihiro Tanabe IHI Corporation
Daisuke Yagi IHI Corporation
Daisuke Koike IHI Corporation