Mechanical property variation within Inconel 82/182 dissimilar metal weld between low alloy steel and 316 stainless steel

Abstract

In several locations of pressurized water reactors, dissimilar metal welds using Inconel welding wires are used to join the low alloy steel components to stainless-steel pipes. Because of the existence of different materials and chemistry variation within welds, mechanical properties, such as tensile and fracture properties, are expected to show spatial variation. For design and integrity assessment of the dissimilar welds, these variations should be evaluated.
In this study, dissimilar metal welds composed of low alloy steel, Inconel 82/182 weld, and stainless steel were prepared by gas tungsten arc welding and shielded metal arc welding techniques. Microstructures were observed using optical and electron microscopes. Typical dendrite structures were observed in Inconel 82/182 welds.
Tensile tests using standard and mini-sized specimens and micro-hardness tests were conducted to measure the variation in strength along the thickness of the weld as well as across the weld. In addition, fracture toughness specimens were taken at the bottom, middle, and top of the welds and tested to evaluate the spatial variation along the thickness. It was found that while the strength is about 50–70MPa greater at the bottom of the weld than at the top of the weld, fracture toughness values at the top of the weld are about 70% greater than those at the bottom of the weld.

Preparation procedure of the 316l h3 & h4 weld mock-up

Recent studies suggest that material hardening should cause enhancement in stress corrosion cracking (SCC) growth rate of stainless steels (SS).

BWR plants, stress corrosion cracking (SCC) in components such as the reactor core shroud and primary loop re-circulation (PLR) piping made of Lgrade stainless steel (SS) has been an issue since 2003.

Investigation on SCC in the core shroud has revealed the following,

1. Nearly 3601 circumferential discrete cracks existed in the ring along the weld between the shroud shell and the ring. Most of the cracks were several mm away from the fusion line.
2. Several radial cracks were found in the heat-affected zone (HAZ) of H4 weld, the highest fluence position in the shroud, some of which propagated into the weld metal

Recently, it is concerned that the SS of high yield strength or hardness tends to provide high SCC growth rate. The da/dt-K diagrams that were prepared based on L-grade SS (heat treatment with 620 1C_24 h) SCC data might not envelope the condition of radiation-hardened and/or cold worked shroud. Incidentally, in the process of investigating PLR piping SCC, the hardness of weld HAZ was found to be around 250 HV and many SCC growth data with the PLR piping weld joint mock-ups were acquired to prepare a new diagram for hardened SS [3].

Since the reactor core shroud suffers from neutron irradiation, it would provide different SCC behaviors. Main factors for increasing crack growth rates seem to be the increased yield strength due to the cyclic weld heat input and the neutron irradiation effect.

Fig. 1. Reactor core shroud:


Fig. 2. Preparation procedure of the 316L H3 weld mock-up:

Fig. 3. Geometry of 0.5T-CT specimen made from the shroud mockups (Dimensions are in mm):

Fig. 4 Geometry of 1T-CT specimen made from the actually used shroud:

Fig. 5. Result of micro-vickers hardness measurement (H3):

Fig. 6. Result of micro-vickers hardness measurement (H4):

See this Experimental procedure at the following url,
http://www.4shared.com/file/153740839/ef18c0d/SCC_growth_behavior_of_BWR_core_shroud_materials.html