Carbides and Carbon Control in MC – Reinforced Superalloys

Abstract

Polycrystalline alloys based on cobalt and elaborated by classical foundry are known since the middle of the last century. They are currently still used, notably for geometrically complex components working at high temperature. Beyond the oldest ones reinforced by chromium carbides, new principles of carbides–strengthened cobalt–based alloys have recently appeared. MC–type refractory mono–carbides allow maintaining the melting start temperature at a high level when they are present as single carbide phase. Their high temperature stabilities and script–like morphologies also favor high mechanical properties at elevated temperatures. Optimized MC–carbides fractions can be obtained with carbon contents, close to 0.4 wt.%C, for achieving significant strengthening without threat for the low and high temperature toughness. Controlling the carbon content is thus of prior importance. Unfortunately the most common metallographic apparatus used for measuring the chemical composition of alloys – the Energy Dispersion Spectrometers (EDS) attached to Scanning Electrons Microscopes (SEM) – are not able to analyze, with sufficient accuracy, carbon with so low contents. A simple indirect method using both EDS and SEM is proposed here to get some information about the carbon content in several MC–reinforced Co–based superalloys.