Study on the Weldability of Ni Base Superally (Part 6)

Metallurgical Investigation on White Constrituents

Abstract

In this report, metallurgical consideration was made on white constituents which had been confirmed tobe the main cause of hot crackings in the heat affected zone of Inconel 713C in the previous paper (Part 5).
An assumption that white constituents in Inconel 713C are crystalized by the eutectic reaction between Ni solid solution and γ' phase was set up on the basis of studies on the white phases (similar to white constituents in Inconel 713C) observed in the other cast Ni-base superalloys.
To verify the assumption, the formation process of white constituents was investigated and a metallurgical examination of white constituents was done.
Moreover, the effects of γ' phase forming elements on the formation of white constituents were researched.
The conclusions obtained in this study are as follows;
(1) White constituents are crystallize from liquid by the eutectic reaction between Ni solid solution and γ' phase.
(2) In Inconel 713C, massive white constituents are formed at the nodes of dendrite boundaries and fine white constituents at the grain boundaries.
These white constituents are different from one another in morphorgy but same crystallographically. (3) The amounts of white constituents increase with an increase in γ' phase forming elements such as Al, Ti and Nb.
This experimental result supports conclusion (1).