A625 Alloy

Alloy name: A625
Diagram No.: 1073
Type of diagram: TTT
Chemical composition in weight %: 20.5-22.5% Cr, 4.0-5.0% Fe, 8.1-8.9% Mo, 3.4-3.7% Nb+Ta, 0.01-0.40% C, 0.02-0.10% Mn, 0.05-0.10% Si, 0.02-0.04% Al, 0.1-0.25% Ti, balance is Ni
Alloy group: Nickel-based alloys
Note: Approximate Time-Temperature-Transformation Diagram for Phases Forming at High Temperatures in A625.
As indicated in Figure, a number of different carbides and intermetallic compounds can precipitate in A625 after thermal exposures, for times on the order of 0.1 to 100 hours. Still further changes, as discussed below, will occur with more prolonged exposures.
Table summarizes the crystal structures and typical compositions in atomic percent values of these phases. Note that the composition data are partitioned in this table by crystallographic position. Thus, in the case of the A,B Laves phase, the Cr, Fe, and Ni are assumed to occupy the A positions in the crystal structure, and Si, P, Nb, and MO occupy the B positions. As discussed before, there are two different NbC morphologies: the blocky shape that forms during solidification, and the dendritic form reported in weldments and also seen as a thin grain boundary film after heat treating wrought material.
Three different carbides can precipitate in wrought A625 grain boundaries. The positions of the noses of the various carbide curves in the T-T-T diagram are not well defined, but are on the order of 10 minutes. In practice, even fairly large sized cross-sections of nominal chemistry wrought Alloy 625 will not show grain boundary carbides after air cooling from solution annealing. Thus, if desired, grain boundary carbide precipitation can be easily suppressed during processing. The type of carbide formed during heat treating depends upon the temperature. At higher temperatures, roughly from 1600 to 19OO"F, the carbides are both NbC as thin grain boundary films, and M6C, where M is principally Ni, Cr, and MO. At temperatures in the 1300-1680F range, the grain boundary carbides are primarily M23C6, where M is almost entirely Cr. After intermediate temperature heat treatments, all three carbides usually can be found.
The M6C and M23C6 carbides commonly have blocky particles irregular shapes and form as series of separate, discrete grain boundary particles.
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