Microstructure of a safe-end dissimilar metal weld joint (SA508-52-316L) prepared by narrow-gap GTAW

Highlights

• Residual strain and GBCD change as a function of the distance from FB in 316L HAZ.

• Neither type II boundary nor obvious carbon-depleted zone is found in SA508 HAZ.

• No carbon concentration is found at the SA508-52 interface.

• The middle part of the DMWJ has the highest residual strain.

Abstract

The microstructure, residual strain and interfacial chemical composition distribution of a safe-end dissimilar metal weld joint (DMWJ, SA508-52-316L) prepared by narrow-gap gas-tungsten arc welding (NG-GTAW) were studied by optical microscope (OM) and scanning electron microscope equipped with an energy dispersive X-ray microanalysis (SEM/EDX) and an electron back scattering diffraction (EBSD) system. Complex microstructure and chemical composition distribution are found, especially at the SA508-52 interface and the 52-316L interface. In brief, a complicated microstructure transition exists within the SA508 heat affected zone (HAZ); the residual strain, the fraction of high angle random grain boundaries and low angle boundaries decrease with increasing the distance from the fusion boundary in 316L HAZ; neither typical type II boundary nor obvious carbon-depleted zone is found near the SA508-52 interface; dramatic and complicated changes of the contents of the main elements, Fe, Cr and Ni, are observed at the distinct interfaces, especially at the SA508-52 interface. No carbon concentration is found at the SA508-52 interface.

Introduction

Dissimilar metal weld joints (DMWJs) are widely used to connect the stainless-steel safe-end to the low-alloy-steel nozzles of pressure vessels in primary loops of nuclear power plants [1], [2], [3], [4], [5], [6]. And operating experience and laboratory tests have demonstrated that the DMWJ can jeopardize the safety operation of plants as premature failures, mainly stress corrosion cracking (SCC), always occurred at this place [7].

To improve the resistance to corrosion and SCC of the DMWJs in primary water, choosing Alloy 52 and 52M as filler metals to fabricate the newly designed safe-end DMWJ and for repair and replacement of the failed ones has become the trend. Usually, there are two steps to fabricate a safe-end DMWJ. A buttering layer should be firstly buttered to the end of the low-alloy-steel vessel nozzle and should be post-weld heat treated to release the residual stress. Then the safe-end will be welded to the buttering layer, and this weld joint will not be heat treated again to avoid the sensitization of the stainless steel safe-end pipe [8], [9].

Narrow gap welding (NGW) is a powerful method for the fabrication of weld joints with very high thickness. In many engineering cases, a groove with a bevel angle of 37.5^o is required for traditional welding method, however, when NGW method is applied, the angle can be reduce to even 3^o. In general, the NGW is primarily used in combination with two methods, submerged arc welding (for high deposition rate) and gas-tungsten arc welding (for high quality). Nowadays, advanced narrow-gap gas-tungsten arc welding (NG-GTAW) technique has been applied for the fabrication of safe-end DMWJ in nuclear power plants (NPPs) without any buttering layer, e.g. in the Olkiluoto 3 European Pressurized Reactor (OL3 EPR) [10]. NG-GTAW is much more economical than the conventional V-grooved and double V-grooved welds as less welding consumables, shorter welding time and reduced volume of inspection is required. In addition, the reduced amount of weld metal and lower heat input will lead to less shrinkage, distortion and smaller residual stress/strain [11].

In our previous studies and other papers, the microstructure, local properties and corrosion behavior of DMWJs with 308L/309L and 52M filler metals have been investigated in detail [8], [9], [12], [13], [14], [15], [16], [17], [18], [19]. Some common phenomena were found in all of the these DMWJs, for example, the microstructure transition in SA508 HAZ, the chemical compositions across the interfaces, the changes of the residual strain and the number fractions of CSL boundaries and the RGBs in the 316L HAZ, and so forth. However, some novel results are found in the DMWJ prepared by NG-GTAW as the welding method is different from others. In addition, researches of the microstructure and local properties of the DMWJ prepared by NG-GTAW are limited although this method has many advantages than traditional welding techniques [10], [11].

In this research, the microstructure, residual strain and interfacial chemical composition distribution of a safe-end DMWJ prepared by NG-GTAW were studied as these aspects are significant to understand a weld joint and are necessary for the integrity assessment of the DMWJ and the safe operation of the nuclear power plants.