LASER NARROW GAP WELDING OF THICK SECTION DISSIMILAR METALS

Abstract

Dissimilar metal welding (DMW) between thick section low alloyed and stainless steels is essential in pressurised water reactor (PWR) construction. This study explored the potential of narrow gap laser welding (NGLW) to improve the quality and manufacturability of these welds. It concentrates on DMW joints located in the primary cooling circuit, where austenitic stainless steel pipes are connected to forged low alloy pressure vessel nozzles. Current welding processes for this application are manual metal arc (MMA) and narrow gap gas tungsten arc (NG-GTA) welding, which are slow and inefficient. They require large amounts of filler material to be deposited and generate considerable residual stresses. The residual stresses contribute to stress corrosion cracking (SCC), which has been found a major issue to the longevity and reliability of PWR’s. NGLW has the potential to reduce the amount of filler material required and has been shown to reduce the detrimental residual stresses. In this study NGLW was applied for welding SA508 Gr3 Cl2 low alloyed steel with AISI 316L austenitic stainless steel using Inconel Alloy 52 filler metal up to 40 mm thickness. This thesis is the first time that dissimilar metal NGLW has been reported. The process characteristics are discussed. The resulting welds were subjected to industry-standard radiographical approval according to ASME IX. Hardness mapping and microstructural analysis were carried out. Tensile and impact toughness tests were executed. Residual stresses were mapped using the contour method. Welding equipment was developed. The unusually narrow welding groove required special shielding gas and wire feed nozzles. Real-time weld monitoring systems using two different approaches were developed. An appropriate restraint system for the high distortion forces caused by the thick section welding was designed. The microstructural and hardness properties were found to be sound and impact toughness requirements were fulfilled in the as welded condition. This was caused by an effective multi-pass tempering. Lack of fusion (LoF) defects were found to limit the repeatability of the welds. Two main mechanisms were recognised: oxidation induced LoF and undercut related LoF. The filler material used was found to be prone to oxidation. Solutions to overcome the issues found are suggested.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Lin Li (Supervisor) & John Francis (Supervisor)