Study of Thermal Aging Embrittlement in Low Alloy Steel Welds Containing Various Amounts of Ni and Cu

Abstract The present paper presents results of an investigation of the thermal aging embrittlement behavior of several weld metals (WM) of RCC-M code grade 18MND5 steel containing typically two amounts of Nickel (0, 90% and 1,50 %) and two amounts of Cu (0,045% and 0,10%). 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 no hardening phenomenon is detected neither through hardness, 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 P segregated at the grain boundaries in aged conditions were performed by Auger Electron Spectroscopy (AES). The results tend to show that in the higher Ni weld metals, P 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.