Influence of microstructural parameters on static strain ageing in C–Mn steel welds

C-Mn steel welds from the Main Steam Lines (MSLs) of pressurised water reactors (PWRs) are sensitive to static strain ageing (SSA). This study investigates the influence of the initial nitrogen content in solid solution, the amount of prestrain, and the ageing conditions on the segregation of interstitial atoms to dislocations throughout SSA. Microstructural characterisations were conducted, based on thermoelectric power (TEP) and internal friction (IF) measurements. Preliminary studies on as-welded (AW) samples treated above 250 °C emphasised a dissolution mechanism of iron carbonitrides formed during welding, resulting in the release of carbon and nitrogen into solution particularly at temperatures equal to or above 300 °C. This phenomenon was found to occur simultaneously with segregation when AW samples were prestrained and aged in this temperature range, but it was considered negligible at 250 °C. Based on the decrease in the height of the IF spectra between the strained and strain aged states at 250 °C, the amount of nitrogen atoms segregated to dislocations and that of nitrogen atoms remaining in solution after ageing could be qualitatively assessed for different amounts of prestrain and initial nitrogen contents in solution. This led us to analyse the effect of both parameters on SSA and, notably, to highlight a cross-influence, which may explain the contradictory results reported in the literature regarding their individual effects. In the considered material, the amount of segregated nitrogen was found to increase with the amount of prestrain and the initial free nitrogen content, up to a critical threshold, beyond which saturation occurred.