Behavior of Precipitates in Ni-Base Superalloy, Waspaloy

Abstract

In Ni-base superalloys, mechanical and chemical properties of welded joints depend on the behavior of precipitates such as γ' phase, M₂₃C₆ etc. The susceptibilities of solidification cracking, liquation cracking, reheat cracking etc. are also influenced by the behavior of precipitates.
In the present study, the behavior of precipitates in Waspaloy has been researched to get fundamental information about the behavior of minor phases during welding in Ni-base superalloys.
Main results obtained in this study are as follows;
1) Intergranular M₂₃C₆ carbides, intragranular r' phases and massive TiC were observed in base metal (1080°C×4 hr, W.Q.).
2) Precipitated zone heated up to 700-900°C was formed in HAZ. Precipitation of γ' phase and M₂₃C₆ carbide was promoted in this region.
3) During isothermal heat treatment;
a) Heat treatment at 900-1000°C resulted in most rapid precipitation of M₂₃C₆.
b) γ' phase most rapidly precipitated at about 900°C and γ' phase coarsening was found to obey Lifschitz-Wagner theory.
c) It was found that increment of hardness by γ' phase precipitation, ΔHv was a linear function of (aging time, t)^1/6 within experimental error.
4) During weld thermal cycle;
a) M₂₃C₆ carbide precipitated at 750-950°C during welding.
b) Incipient melting of TiC was observed at the temperature more than 1270°C.
c) γ' phase precipitated at 900-980°C during welding.
d) γ' phase coarsened in the temperature range from 1000°C to 900°C.
e) Hardness was a function of cooling time from 1000°C to 700°C.
5) It was possible to estimate hardness of solution treated material during post weld heat treatment by using the relation between ΔHv and t^1/6.