What Are Alloy 625 Plates?
Alloy 625 is a solid-solution strengthened nickel-chromium-molybdenum-niobium alloy with UNS number N06625. In plate form it is supplied solution annealed, with a fully austenitic structure and no precipitation-hardening treatment, which is why its physical properties are stable across a wide temperature range and why the plate can be welded, formed and machined without a post-weld aging step. Plates are used where high mechanical loads and aggressive chemistry meet: heat exchanger tube sheets, pressure vessel internals, flue-gas ducting, marine hardware and chemical process equipment.
Because the grade is chosen as much for corrosion resistance as for strength, buyers need reliable physical data rather than the mixed figures that circulate in trade literature. The values below follow the relevant ASTM and AMS product specifications for plate, sheet and strip.
Physical Properties at a Glance
Alloy 625 has a high nickel content, roughly 58 % minimum, which raises density and melting range slightly above common stainless steels and keeps the material non-magnetic down to cryogenic temperature.
| Property | Typical value |
|---|---|
| Density | 8.44 g/cm³ |
| Melting range | 1290–1350 °C |
| Elastic modulus | about 208 GPa |
| Poisson's ratio | 0.31 |
| Thermal conductivity (20 °C) | 9.8 W/(m·K) |
| Specific heat | 410 J/(kg·K) |
| Electrical resistivity | 1.29 µΩ·m |
| Mean thermal expansion, 20–100 °C | 12.8 µm/(m·°C) |
| Curie temperature | about −196 °C |
Low thermal conductivity combined with a relatively high thermal expansion coefficient is the practical point to remember during welding and thermal cycling: heat concentrates at the arc, distortion is greater than in carbon steel, and preheat is generally avoided while interpass temperature is controlled.
Mechanical Properties and Governing Standards
Published ranges for this grade are sometimes confused because solution-annealed plate and heavily cold-worked strip have very different strength levels. For plate supplied to ASTM B443 or AMS 5599, the minimum room-temperature properties apply, and values far above them only appear in cold-reduced product.
| Property / requirement | Value | Reference |
|---|---|---|
| Tensile strength, solution annealed | ≥827 MPa (120 ksi) | ASTM B443 |
| Yield strength, solution annealed | ≥414 MPa (60 ksi) | ASTM B443 |
| Elongation in 50 mm | ≥30 % | ASTM B443 |
| Hardness, annealed plate | typically 145–240 HB | product form dependent |
| Plate, sheet and strip | AMS 5599 | aerospace material spec |
| Bar and rod | ASTM B446 | product spec |
| Seamless pipe and tube | ASTM B444 | product spec |
| Welded tube | ASTM B704 / B705 | product spec |
| Forgings | ASTM B564 | product spec |
Elevated-temperature performance is retained to about 650 °C for load-bearing service, and the material keeps useful creep and rupture strength well beyond that point in low-stress applications. Stress relief after cold forming is carried out around 870–980 °C followed by a rapid cool, which also restores corrosion performance in the heat-affected zones of welds.
Corrosion and Oxidation Behaviour
The combination of about 21 % chromium, 9 % molybdenum and 3.6 % niobium gives the plate exceptional resistance to chloride pitting and crevice attack, to seawater and to many mineral acids. The niobium addition also stabilises the alloy against sensitisation, so welded plate does not suffer the grain-boundary chromium depletion seen in unstabilised stainless grades.
Sulphuric and phosphoric acid service, including contaminated streams.
Seawater, brine and marine splash-zone exposure without pitting.
Oxidation resistance in air up to about 980 °C in continuous service.
Resistance to chloride-induced stress corrosion cracking in wet process environments.
Where reducing acids, hot concentrated caustic or strong oxidisers are involved, hydrogen pickup and general corrosion should be reviewed against the actual operating chemistry before the grade is specified.
Selection and Processing Notes
Three recurring mistakes appear in specification work. First, the plate is assumed to be interchangeable with ordinary stainless steel or high-carbon steel on the basis of strength alone; the nickel and molybdenum content, not the tensile figure, is what delivers stability in hot corrosive service. Second, the grade is applied at temperature extremes without review, when in some very high-temperature duties a solid-solution alloy with higher chromium or an oxide-dispersion material performs better. Third, the machining behaviour is underestimated: alloy 625 work-hardens rapidly, so it needs rigid tooling, positive-rake carbide or ceramic tools, low surface speed, heavy feeds and continuous coolant.
Forming is normally done cold with generous bend radii and interstage annealing for severe shapes. Welding uses matching nickel-based filler and controlled heat input; plate thicker than about 12 mm benefits from a slight preheat only when ambient conditions are cold and damp.
Ordering and Delivery Conditions
Plate is normally supplied solution annealed and pickled or bright annealed, with sheared, plasma-cut or waterjet-cut edges, in thicknesses from about 0.5 mm to 100 mm and widths up to 3000 mm depending on the mill. Buyers should specify:
Product specification and revision, for example ASTM B443 or AMS 5599.
Delivery condition and surface finish, including whether a ground or polished finish is required.
Thickness tolerance class, flatness limit and edge condition.
Non-destructive testing level and whether mill test certificates to EN 10204 3.1 are needed.
Whether the plate will be welded or cold formed before final installation, since this affects the required heat treatment condition.
Frequently Asked Questions
Q: What is the density of alloy 625 plate?
8.44 g/cm³, which is slightly higher than 316L stainless steel because of the high nickel and molybdenum content.
Q: What tensile strength can solution-annealed alloy 625 plate reach?
A minimum of 827 MPa to ASTM B443 at room temperature. Figures of 1350 MPa and above refer to cold-reduced strip or wire, not to annealed plate.
Q: Is alloy 625 plate magnetic?
It is essentially non-magnetic, with a Curie temperature near −196 °C, so it stays non-magnetic through room temperature and most cryogenic service.
Q: Which standard covers alloy 625 plate?
ASTM B443 covers plate, sheet and strip, with AMS 5599 used for aerospace plate supply. Bar, pipe, tube and forgings follow ASTM B446, B444 and B564 respectively.
Q: Can alloy 625 plate be welded?
Yes. It is readily welded by gas tungsten arc and gas metal arc with matching filler; the weld zone keeps its corrosion resistance because the niobium content prevents sensitisation.
Q: What is the maximum service temperature of alloy 625 plate?
About 980 °C in continuous oxidising service, although structural loads should be limited to roughly 650 °C where strength governs the design.
