Weldability, Thermal Aging and PWSCC Behavior of Nickel Weld Metals Containing 15 to 30% Chromium

In 1991, a vessel head penetration was found leaking at Bugey 3 plant during the hydrotest. Metallurgical investigations confirmed that this problem was again related to primary water stress corrosion cracking of alloy 600. Moreover, the main crack initiated in the base metal of the penetration (alloy 600) has also propagated in the weld metal in alloy 182. More recently, stress corrosion cracking in alloy 182 has been found on welds of U.S. plants. SCC susceptibility of alloy 182 has been evidenced by several laboratories. In France, all original vessel heads using alloy 600 have been or will be replaced with penetrations in alloy 690 (with 30% chromium). With respect to substitution materials, ELECTRICITE´ DE FRANCE has undertaken a large R&D study focusing on the development of new weld metals. The aim of this study was to identify new materials that will be able to weld alloy 690. Weld metals containing 15 to 30% Chromium have been studied. This paper presents an overview of the main results obtained on 19% Cr, 26% Cr and 30% Cr alloys with respect to alloy 182 (15% Cr). Firstly, the weldability of weld metals has been studied focusing on the susceptibility to hot cracking. Secondly, the resistance to thermal ageing has been investigated in order to detect any long term ordering of the solid solution Ni-Cr that could induce embrittlement. Hardness tests, Charpy tests and resistivity measurements did not show any effect of ageing up to 60,000 hours at 360°C. Thirdly, stress corrosion cracking susceptibility in primary water at 360°C has been evaluated during constant load tests, RUB tests, slow strain rate tests. No cracking was observed on material containing more than 26% Cr for both initiation and propagation. Finally, a life assessment was performed for all weld materials with respect to alloy 182.