Session: MF-06-02 Materials and Technologies for Nuclear Power Plants-2

Paper Number: 153628

153628 - Study of Thermal Aging Embrittlement in Weld Metals of 18mnd5 Steel (Type Asme A508 Gr 3), Containing Various Amounts of Nickel and Copper. 

Abstract: 

 

Thermal aging embrittlement is a degradation mechanism that can affect Low Alloy Steels of Pressurized Water Reactors (PWR) large components. Among the components of a PWR that are the most concerned by this mechanism, is the Pressurizer (PZR) which is operated at the highest temperature of the Main Coolant Loop (MCL), around 345°C. The embrittlement is generally attributed to segregation of impurities, like Phosphorus, at grain boundaries. A previous study conducted on parts of a PWR Pressurizer decommissioned after ~ 27 years of operation has shown unexpectedly important shifts of notch impact transition temperature, in some SAW welds, which had the peculiarity of containing relatively high contents of Nickel (~ 1,5 %), along with “normal” content of impurities such as P (~ 0,010%), and Cu (~ 0,10 %). The present paper presents results of an investigation of the thermal aging behavior of several weld metals (WM) containing typically two amounts of Nickel (0, 90% and 1,50 %) and two amounts of Cu (0,045% and 0,10%). Two of these welds, with the higher Ni content were manufactured with the same Wire-Flux couple, one having its wire submitted to a “mechanical” removal (by a “brushing” process) of the copper layer used by welding products vendors to protect it. Accelerated thermal aging treatments were performed in laboratory furnaces, at 450°C and 400°C. After aging, the welds were submitted to various mechanical tests to investigate the evolution of the mechanical properties as affected by thermal aging. The results show that for the same aging conditions, the weld metal leading to the most important transition temperature shift is the one with both the highest Ni and Cu contents. In parallel tensile properties and hardness profiles show that no hardening phenomenon is detected neither through hardness evolution, nor by tensile properties evolutions, in the WM. This tends to show that embrittlement induced by the thermal aging treatment is not due to any hardening mechanism. Fractographic observations clearly confirmed for some welds that intergranular fracture is present in the most severely aged conditions. Some measurements of the amount of Phosphorus segregated at the grain boundaries in aged conditions were performed by Auger electron spectroscopy. The results tend to show that in the higher Ni weld metals, Phosphorus segregation at grain boundary is higher. The effect of copper appears to be of secondary importance. Nevertheless, previous observations with Atom probe tomography (APT) of microstructural evolutions during aging, tended to confirm that although no significant hardening is observed copper precipitates as nanoparticles along dislocations. A discussion is developed on the relative beneficial effect of Ni in the “as PWHT” transition temperature with respect to the detrimental higher transition shift for "typical" impurities contents (P and Cu), concluding that Ni additions up to 1,50 % in the weld metal is not desirable when long operating time (> 40 years) at 345°C are considered.

Presenting Author: Lingtao Sun FRAMATOME

Presenting Author Biography: PhD, in Materials Science and Technology, Ecole Nationale Supérieure des Mines de Paris (2012)

With FRAMATOME, Mechanical Engineering Divison, and Materials Department of FRAMATOME since 2014,

Authors:

Lingtao Sun FRAMATOME
Pierre Joly FRAMATOME