Factors influencing the thermal stabilisation of alloy 600 tubing against intergranular corrosion

Abstract

An evaluation has been made of the efficiency of thermal treatment (15 h, 700°C) in conferring resistance to carbide precipitation and intergranular attack, following subsequent heat treatment. The resulting carbide precipitation and resistance to intergranular corrosion has been determined after treatments carried out over the temperature range 300–800°C for up to 1 h. Marked differences between nominally equivalent commercial materials have been found, but these differences are evident only after extended corrosion testing. The ability of the thermal treatment to eliminate chromium depletion and provide good corrosion resistance is critically dependent on the grain size and carbon content of the alloy. Thermal treatments which fall short of the required period appear to induce a susceptibility to sensitisation to intergranular corrosion at temperatures met in steam generator service. A thermal treatment index is derived, which allows calculation of the period at 700°C necessary to complete the equilibrating reactions.