Apart from use of Austenitic Stainless Steels in fabrication of Reactor Pressure vessel (RPV) of Nuclear Power Plants, for most of the austenitic weld metals like 308, 308L, 308H, 309,309L, 310,316,316H, 316L, 317L & 347, the accidental inclusion of copper by some means of abrasion of Stainless steel plates with copper plates, contact tips, fixtures etc lead to a well –known phenomena called Copper Contamination Cracking (CCC).
It is characterized by Liquid metal embrittlement, in which molten copper wets and penetrates the austenitic grain boundaries close to temperature 1100 °C due to its lower melting / freezing points (approximately 1083 °C) than the Steel. Besides this residual and external stress causing the CCC to propagate through austenitic grain boundaries. CCC is generally found near fusion boundaries.
(For More Detail Pl. Refer:- Welding Metallurgy & Weldability of Stainless Steels, John C Lippod/ D.J.Kotecki, Wiley Pub, Page No. 199 Figure 6.46).
The Concern for Copper & Nickel content in weld metal of Austenitic Stainless Steels used for fabrication of Reactor Pressure vessel (RPV) of Nuclear Power Plants is Neutron induced irradiation embrittlement, which is rather different than the phenomena explained above.
As presence of Copper, Nickel (And also Phosphorous) irradiation reduces the Charpy impact toughness (CVN Values) by shifting the ductile–brittle transition to higher temperatures and reducing the upper shelf Energy (USE). The back ground of the History from the literatures 1 & 2 is found as below:-
The welding processes used are Most of the time, SAW and SMAW welding processes are used for fabrication of RPVs. So, Before, 1970s year, copper-coated weld wire was in use to enhance the electrical contact in the welding process and to provide protection from corrosion during storage of the weld wire, Subsequently, When it was discovered that copper, Nickel and phosphorus are increasing the sensitivity to Neutron induced-radiation embrittlement, RPV fabricators have put on strict limits on the percentage of copper and phosphorus in the welds as well as in plates. As a result, the use of copper coated weld wire was gradually eliminated in the market due to the strict limits on the percentage of copper in the weld. Moreover, weld metal chemistry (copper and nickel content) may vary through the Variations in the weld wire used in fabrication. As a precautionary measures, the weld wire or stick electrodes are properly stored inside plastic bags and/or kept inside low temperature furnaces to prevent formation of moisture on the weld wire and electrodes.
As per the reference 1 & 2 from the year 1970s, the total copper content in RPV steels and their welds has been limited to a max value of 0.1%, and to even lower values in most Western countries (0.07% in France).
From Metallurgical point of View, As the solubility limit of copper at the irradiation is substantially low (approximately 0.007% in pure iron), as a result copper atoms have a tendency to form micro or nano scopic size precipitates or clusters in RPV steels during operation. Many research studies shows that Nickel can integrate with Copper precipitates and also along with Mn to form MnNi Phases.
These phases are contributing sensitivity towards neutron induced embrittlement. The response of RPV Steels to various SAW / SMAW Fillers and electrodes, Flux chemistry deserve huge scope of research.
References:-
Welding Knowledge 