INCONEL 625 Properties

Engineering variability

• Strength changes with cold work and product form; 625 is not primarily precipitation hardened like 718.
• Thermal conductivity is much lower than carbon steel or copper.
• The approximate Poisson ratio is a typical engineering reference rather than a guaranteed product property.
• Corrosion performance depends strongly on environment, weld condition and surface state.

Selection note

Used in chemical processing, seawater, pollution-control equipment, aerospace ducting, bellows and high-corrosion piping or components.

Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.

Where it is specified

625 is the alloy for seawater and sour service hardware, flue-gas desulphurisation ducting and liners, bellows and expansion joints, subsea umbilicals and control lines, chemical process piping, and as a weld overlay (cladding) on carbon steel vessels where the base metal supplies strength and the overlay supplies corrosion resistance.

Its niobium and molybdenum give solid-solution strength without a precipitation heat treatment, so it arrives strong in the annealed condition and stays ductile — the property that makes it the default for formed and welded corrosion-resistant components.

Chosen over what

Against 316L: 625 is many times the price but resists chloride pitting and stress-corrosion cracking where austenitic stainless simply fails. The step is usually taken when 316 has already been tried and pitted.

Against Inconel 718: 625 is weaker but far easier to weld and needs no age hardening, which is why 718 goes into rotating and bolted hardware while 625 goes into ducting, bellows and overlays.

Against Hastelloy C-276: broadly comparable in seawater; C-276 is better in reducing acids (hydrochloric, sulphuric), while 625 is better in oxidising conditions and at high temperature, and is more widely stocked.

How it fails

Thermal embrittlement is the trap. Prolonged exposure between roughly 600 and 800 °C precipitates the brittle delta phase and carbides at grain boundaries, and room-temperature ductility can fall sharply. This limits sustained-service temperature well below what short-term strength data suggests.

Crevice corrosion in hot seawater, where even 625 has limits — under gaskets and deposits above about 80 °C it can be attacked.

Hot cracking in weld metal from niobium segregation to interdendritic regions, and ductility-dip cracking in multipass welds.

Erosion-corrosion where high-velocity or particulate-laden flow removes the passive film faster than it reforms.

Fabrication notes

Machinability is poor, perhaps 15–20% of B1112. The alloy work-hardens aggressively and has low thermal conductivity, so heat concentrates at the cutting edge. Rigid setups, sharp positive-rake carbide or ceramic tooling, heavy positive feed and generous flood coolant are needed; dwelling or rubbing glazes the surface and destroys the next cut.

Weldability is excellent by nickel-alloy standards and is the alloy's main selling point — no post-weld heat treatment is normally required, and 625 filler is also widely used to weld dissimilar joints and to overlay steel.

Interpass temperature and cleanliness still matter: sulphur, lead and other low-melting contaminants from marker pens, grease or shop dirt cause severe hot cracking, so surfaces must be degreased before welding.