Corrosion Resistant Nickel Chromium Molybdenum Alloy

Alloy 625 is a nickel-chromium-molybdenum alloy containing niobium, iron, and tantalum to improve its corrosion resistance and mechanical properties. Alloy 625 is a solid solution-strengthened alloy that retains a large amount of its mechanical strength through a range of temperatures from cryogenic values up to elevated temperatures.

It also provides high corrosion resistance in a wide range of media and is used in power generation, chemical process industries, offshore sour oil and gas production, waste incineration, phosphoric acid production, nuclear fuel processing, high-temperature environments, and aerospace. The annealed state of Grade 1 material alloy 625 as stocked by Columbia Metals provides an excellent combination of hardness, corrosion resistance, yield, fatigue and tensile strength at high temperatures up to 815°C.

It also features high resistance to oxidation and scaling up to 980°C under cyclic heating conditions. The use of Alloy 625 is restricted to a maximum temperature of 595°C for long term exposure.

Nominal Composition (%)

The composition of Alloy 625 is shown in the table below:

Ni Cr Mo Nb Fe Ti Al
58 22 9 3.5 2 0.2 0.2

Properties of Alloy 625

Alloy 625 has excellent properties for resisting corrosion erosion and corrosion fatigue in a wide variety of media including freshwater, seawater, neutral salts and also alkalis.

It provides increased resistance to oxidizing chemicals and non-oxidizing environments including organic and mineral acids and high-temperature sulfides and chlorides. Alloy 625 provides high resistance to crevice and pitting corrosion that can be attributed to its high molybdenum content, while freedom from chloride stress corrosion cracking is mainly due to the nickel content.

Good hot and cold formability is offered by Alloy 625. The material’s high strength implies that powerful equipment must be used and the alloy should be annealed prior to forming. Hot forming of alloy 625 is probably a challenge when compared to other materials due to the natural retention of mechanical properties at elevated temperatures but heating to a temperature of around 1175°C should enable heavy forging.

Mechanical Properties (Specification Minima))

The mechanical properties of Alloy 625 are shown in the table below:

  As delivered <100mm dia Cold worked annealed condition
Ultimate Tensile Strength (N/mm2) 830
0.2% Proof Strength (N/mm2) 415
Elongation (%) 30

Typical Physical Properties

  Alloy 625 Annealed condition
Density (g/cm3) 8.44
Melting Range (°C) 1290 - 1350
Young's Modulus (GPa) 207
Thermal conductivity (20°C - W/m°K) 9.8
Coeff. Thermal Exp. (20-100°C - m/m°K x 10-6) 12.8
Electrical Resistivity (microhm/cm) 132
Magnetic Permeability <1.001

Machining Properties of Alloy 625

The machining properties of Alloy 625 are classified as moderate but in the annealed condition this grade can be successfully machined using conventional methods and proper precaution. The key issues arise from a rapid work hardening and the generation of high heat during cutting. It is thus important that the workpiece, the machine, and the tool are rigid with minimal tool overhang. For most applications, carbide-tipped tools are suggested and tooling must be sharp at all times. For most machining operations a positive rake angle must be used with negative rake angle tools being considered only for intermittent cuts and heavy stock removal. Heavy constant feeds should be used to maintain a positive cutting action combined with high lubrication. Alloy 625 can also be welded readily using conventional processes and matching filler metal.

Applications of Alloy 625

The versatile corrosion resistance and high strength even at high temperatures enable Alloy 625 to be used for components such as exhaust systems, flanges, fuel, and hydraulic tubing, spray bars, bellows, turbine shroud rings, heat exchanger tubing, bubble caps, reaction vessels, valve and valve trim, pumps, marine propeller blades, submarine transducer controls, flare stacks, fasteners, springs, seals and nuclear control rod components.

This information has been sourced, reviewed and adapted from materials provided by Columbia Metals.

For more information on this source, please visit Columbia Metals.


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