Effects of Cr-W Balance and C Content on Structure and Creep Rupture Strength in Ni-Cr-W Alloy System

Abstract

In order to deterrnine the practically optimum composition in an Ni-Cr-W austenitic alloy system for an intermediate heat exchanger of a high temperature gas cooled reactor, the effects of Cr-W balance and carbon content on the struture and creep rupture strength in the alloy system have been investigated.
Austenite single phase region determined by experiment is much smaller than that calculated by Barrows method in Ni-Cr-W system at 1000°C. Outside the austenite single phase region α-W and at least one of M₆C and M₂₃C₆ carbides precipitate depending on the Cr-W balance. The precipitation of α-W and M₂₃C₆ carbide improves creep rupture strength, but that of M₆C carbide rather degrades it, although solid solution strengthening is maximized at a Cr-W balance in the M₆C predominant region. Among the alloys investigated in this study the 0.05% C-23% Cr-18% W-Ni alloy which is strengthened through the precipitation of M₂₃C₆ carbide and about 3% of α-W is the best from the point of view of creep rupture strength. For the carbon content of the above mentioned alloy the range from 0.040% to 0.065% is practically aptimum, because excessive carbon causes M₆C carbide precipitatian and degrades creep rupture strength.