Inconel 625 Alloy

Alloy name: Inconel 625
Diagram No.: 228
Type of diagram: TTP
DIN designation: NiCr22Mo9Nb
Mat. No. (Wr. Nr.) designation: 2.4856
Chemical composition in weight %: max. 0.10% C, max. 0.50% Si, max. 0.50% Mn, max. 0.015% P, max. 0.40% Al, 20.00-23.00% Cr, 8.00-10.00% Mo, max. 5.00% Fe, max. 0.40% Ti, 3.15-4.15% Nb/Ta, balance is Ni
Alloy group: Nickel-based alloys
Note: Fig. shows the time-temperature-precipitation diagram of the alloy after the solution treatment 1150 C 3h/water). On aging the precipitation begins at temperatures below 1000 C with the carbide M23C6. It precipitates at first only on the grain boundaries, where it forms net-like layers. However, this phase is not stable and redissolves after longer aging time.

As a further precipitate phase the niobium-titanium carbonitride MX occurs in the form of fine, thin platelets within the grains. At higher temperatures, this phase also precipitates as a network on the grain boundaries and as needles within the grains.

In the precipitation sequence the last carbide phase is M6C. Its formation is coupled with the dissolution of M23C6. Presumably an in situ transformation takes place, at least in part. At the start of the M6C formation both carbides show the same distribution and can be distinguished only by electron diffraction. M6C coagulates very rapidly. Even after the appearance of the stable phase M6C, new M23C6 particles are still precipitated for a limited time near the grain boundaries in the form of needles from which fine needles are seen to grow. These precipitates also dissolve later.

Below 1000 C the niobium carbonitride M2X occurs after somewhat longer holding times. This phase is probably stable also. In addition to the carbides and carbonitrides mentioned above, several intermetallic phases are precipitated. Below 800 C, coherent particles of the gamma primer-phase were first detected. This phase reaches its highest precipitation rate at 700 C. After longer holding times the particles loose their coherence, coarsen and take on the form of plates or rods. After longer heat treatments the gamma primer-phase transforms to the stable phase (Mo,Nb)Ni3. In the temperature range from 800 to 950 C (Mo,Nb)Ni3 is precipitated directly from the matrix. This phase may be recognized easily even in the light microscope on account of its characteristic plate shape.
Reference: Atlas of Precipitates in Steels, Verlag Stahleisen GmbH, Germany, December 1983, pp. 512-523.

Transformation Diagram

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