Impact of Carbon Macrosegregation on the Mechanical Properties of Low-Alloy Steel Forgings

Abstract

Macrosegregation in large steel ingots is a known phenomenon: it corresponds to an uneven distribution of alloying elements in the bulk of the as-solidified ingot, over macroscopic scales, caused by the difference in solubility of these elements in the liquid and solid phases. As the ingot solidifies, these elements (mainly carbon, sulfur and phosphorus) concentrate in the liquid phase and are carried to the top of the ingot by convective currents. This process produces large regions of heterogeneity that can negatively impact the mechanical properties of the resulting steel. Westinghouse is in charge of the manufacture of twelve steam generators (SG) for Electricité de France (EDF), for which the main forged parts are made of 20MND5 low alloy steel. Due to the context and concerns in France, and more broadly in the nuclear industry, on carbon macrosegregation in large scale forgings, Westinghouse led studies with the forgemaster The Japan Steel Works (JSW) in order to assess the influence of carbon content on mechanical properties of such forgings. The present paper aims at introducing the investigation work carried out by Westinghouse, in partnership with JSW, in order to analyze the effect of increasing carbon on the 20MND5 low-alloy steel tensile and impact toughness properties. This work was performed through: - A thorough review of the existing literature; - The manufacture of forged plates mock-ups, metallurgically representative of the SG channel head, containing various carbon contents (namely 0.18wt.%, 0.21wt.%, 0.26wt.% and 0.29wt.%); - The testing of above plates to assess the evolution of tensile (tensile strength Rm, 0.2% yield strength Rp0.2 and percentage elongation after fracture) and impact properties (absorbed energy and transition temperature).