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Reactor material Inconel 625

By Thomas Huang December 9th, 2023 60 views

I, What is Inconel 625:

Inconel 625 alloy is an austenitic superalloy with nickel as the main component.
The strength of Inconel 625 comes from the solid solution strengthening effects of molybdenum and niobium contained in the nickel-chromium alloy. These elements also give the alloy excellent corrosion resistance properties.
Inconel 625 was developed in the 1950s to meet the need for high-strength main steam piping materials. Its patent application was filed on January 24, 1962. Patent #3,160,500 was granted to H.L. Eiselstein and J. Gadbut on December 8, 1964.

II, Inconel 625 composition

2.1 Incone l625 Chemical Composition:

Ni Cr Fe Mo Nb Co C Mn Si S Cu Al Ti P
remain 20.0-23.0 ≤ 5.0 8.0-10.0 3.15-4.15 ≤1.0 ≤0.10 ≤0.50 ≤0.50 ≤0.015 ≤0.50 ≤0.40 ≤0.40 ≤0.015

2.2 Metallographic structure

Inconel 625 is a solid solution lattice-hardened face-centered cubic alloy. The alloy may contain carbides that are inherent in this alloy. The carbides that have been found are MC and M6C (abundant in nickel, niobium, molybdenum, and carbon). In addition, M23C6, a chromium-rich carbide, is found in the solid solution treated materials at low temperatures.
A hardening effect occurs when the material is held at a center range of 649°C(around 1200°F). It is because of the slow, uniform precipitation of a nickel-niobium-rich phase, dominated by gama. This phase gradually transforms to the rhombohedral Ni3Nb when the alloy is heated in the medium temperature range for extended periods.
Extensive stability studies have shown the absence of embrittlement and intermediate metallic phases such as the SIGMA phase, when Inconel 625 alloy is held at temperatures in the 538 °C (1000°F) to 982°C(1800°F) range for a long time.

III. Manufacturing

3.1 Manufacturing methods:

Alloy 625 may be produced by the method of Vacuum Induction Melting or Argon Oxygen Decarburization (AOD). Further refining can be done by the Self-consuming Electrode Redissolution method.

3.2 Inconel 625 Approximate Grades

NS3306 N06625 W.Nr.2.4856NiCr22Mo9Nb NA21 NC 22 DNb

IV, Particular properties of Inconel625 


Excellent corrosion resistance to various corrosive media in oxidizing and reducing environments.


Excellent resistance to pitting corrosion and crevice corrosion, high nickel content to enhance the resistance to chloride stress corrosion cracking.


Excellent resistance to inorganic acid corrosion, such as nitric acid, phosphoric acid, sulfuric acid, hydrochloric acid and sulfuric acid and hydrochloric acid mixed acid, etc.


Excellent resistance to corrosion of mixed solutions of various inorganic acids


It has good corrosion resistance in various concentrations of hydrochloric acid solution when the temperature reaches 40℃. 
Simulated pipeline gas desulphurization tests have shown that Inconel 625 has better corrosion resistance than stainless Steel 316 and that Inconel 625 almost with the same corrosion resistance as Hastelloy C-276.
This is illustrated by the following data. Typical corrosion rates are in mils/year (mm/a).
Boiling solution of organic acids
45% formic acid 10% oxalic acid 88% formic acid 99% acetic acid
 Inconel 625 5.0 (0.13) 6.0 (0.15) 9.0 (0.23) 0.4 (0.01)
 SUS 316 11 (0.28) 40 (1.02) 9 .0 (0.23) 2.0 (0.05)

Dilution to reduce acid content - boiling solution
1% sulfuric 5% sulfuric 10% sulfuric 1% HCl
 Inconel 625 2.2 (0.06) 8.9 (0.23) 25.3 (0.64) 36.3 (0.92)
 SUS 316 25.8 (0.65)  107 (2.72) 344 (8.73) 200 (5)

Hybrid Environments
 Environment  Inconel 625  SUS 316
 20% phosphoric acid  0.36 (<0.01)  6.96 (0.18)
 10% sulfanilic acid  4.80 (0.12)  63.6 (1.61)
 10% sodium bisulfate  3.96 (0.10)  41.6 (1.06)

Resistance to chloride stress corrosion fission test
 Test  Inconel 625  SUS 316
 Magnesium chloride
 No fission  fission 
 1000 hours  <24 hours 
 Sodium chloride
 Non-fissile  fissile 
 1000 hrs  600 hrs 


Good workability and weldability, no susceptibility to post-weld cracking


High strength below 650°C, good molding, easy to weld; The maximum operating temperature is 1175°C in the atmosphere.


Has pressure vessel manufacturing certification for wall temperature of -196 ~ 450 ℃. 


Certified by the American Association of Corrosion Engineers NACE standards (MR-01-75) in line with the highest standard class VII for use in acid gas environments.

V, Other Properties

5.1 Physical Properties:

 Density g/cm3  8.44  Melting point ℃  1290-1350
 Thermal conductivity λ/(W/m•°C)  12.1(100°C)    Specific heat capacity J/kg•°C  430
 Modulus of elasticity GPa   205  Shear modulus GPa  79
 Resistivity μΩ•m  1.28  Poisson's ratio  308
 Coefficient of linear expansion a/10-6℃-1  12.3 (20~100℃)

5.2 Mechanical properties of inconel625:

Minimum value of mechanical properties tested at 20℃.
Heat treatment Solid solution treatment
 Tensile strength σb/MPa   Yield strength σp0.2/MPa  Elongation σ5 /%  Brinell hardness HBS
 830  410  30  ≤290

VI, Process performance of inconel625 

6.1 Hot processing for Inconel625


The cooling method is water quenching or other rapid cooling methods when the hot working temperature is from 1150°C to 900°C. 


To get the best performance and corrosion resistance, annealing treatment should be carried out after hot working.


When heated, the Inconel625 material can be fed directly into the furnace that has been warmed up to the maximum working temperature, and quickly withdraw the material from the furnace after holding for a sufficient time (60 minutes per 100 mm thickness). The hot working shall be down in the specified temperature range of the high-temperature section. If the material temperature drops below the hot working temperature,  it shall be reheated.
Inconel 625 hot working process photo

6.2 Cold working of Inconel625


The Inconel 625 materials shall be in the annealed state with a greater work-hardening rate than austenitic chromium-nickel stainless steels.


Intermediate annealing is required for cold working.


When the machining quantity is greater than 15%, annealing is to be carried out after hot working.

6.3 Machining


Inconel625 has good cold and hot forming properties, forging heating temperature is 1120 ℃.


The average grain size of Inconel625 is closely related to the degree of deformation of the forgings and the final forging temperature.


Inconel625 welding property is good.
Welding methods
Inconel625 is not suitable for oxygen-acetylene welding but can be welded in most other welding processes. Weldability is the same as SUS304. The sensitivity to weld solidification cracking is higher than that of SUS304 and lower than that of SUS310.
Inconel625 should be completely decontaminated and cleaned under annealing conditions. Preheating and post-welding treatments are not required, so can maintain or restore the corrosion resistance of the alloy.
Composition of 625 welding wire
Inconel 625 wires and Inconel 112 rods are a nickel-chromium-molybdenum family of products, specifically designed to perform the welding of 625 alloys by itself and with other materials. With both electrodes, no post-weld heat treatment is required to maintain their high strength and ductility.
Both products can accommodate high dilution rates and maintain performance when welding Inconel 625 and other metals.
Inconel 625 wire and Inconel 112 rods are also used as overmatching compositions for welding iron-nickel-chromium-molybdenum corrosion-resistant alloys (including 316 and 317 stainless steels, 6% molybdenum super stainless steels, Incoloy 825 and 020 alloys, and Inconel G3 alloys). The higher alloy content of Inconel 625 alloys creates a susceptibility to elemental segregation in weld areas, leading to selective corrosion. segregation at the weld site resulting in selective corrosion.

Inconel 625 welding wire photo Inconel 625 wire is designed for use in tungsten gas-shielded welding and various metal gas-shielded welding processes. Operating characteristics are similar to other nickel-chromium wires.
Inconel112 electrodes for shielded metal arc welding have excellent operability and produce a hard slag that breaks up into large flakes, leaving a very clean weld metal.

6.3.4 Surface treatment process: 

To remove Inconel625 surface oxide skin, first use an alkaline wash, and then in the nitric acid - hydrofluoric acid - water solution pickling.

6.3.5 Inconel625 cutting and grinding performance 

It should use carbide or high-speed steel tools. The cutting tool shall have a positive front angle. Water-based coolant is recommended for high-speed cutting, and viscous coolant shall be used during low-speed cutting.

VII, Scope of application

7.1 Seawater

The properties of Inconel 625 make it an excellent choice for freedom from localized corrosion (pitting and crevice corrosion), high corrosion fatigue strength, high tensile strength, and resistance to chloride ion stress corrosion cracking in seawater. It is used in mooring cables, motor gunboat propeller blades, submarine auxiliary propulsion motors, submarine quick-disconnect couplings, exhaust piping for naval vessels, protective jackets for submarine communication cables, submarine sensing controllers, and steam line expansion joints. Potential applications include springs, seals, expansion joints for underwater control, cable glands, fasteners, flexible devices, and instrumentation components for oceanography.
inconel 625 application in seawater photo

7.2 Aerospace Launch Vehicle Components

Inconel 625 alloy's high tensile, creep, and fracture strength, excellent fatigue and thermal fatigue strength, oxidation resistance, and excellent welding and brazing properties make it also very useful in the aerospace field.
Applications include:
ᆞAircraft ducting systems,
ᆞEngine exhaust systems,
ᆞThrust conversion systems,
ᆞWeld-resistant honeycomb structures in engine mounting structures,
ᆞFuel and hydraulic system piping,
ᆞInjection rods,
ᆞExpansion joints,
ᆞSwirl-clad rings
ᆞHeat exchanger tubes in environmental control systems.
It is also suitable for use in conversion liners in combustion sections, turbine seals and compressors, blades, and thrust chamber piping in rocket engines.

Inconel 625 is used in the manufacture of engine receivers, guide vanes, mounting edges and barrels, and other components,  with a maximum service temperature of 950°C.

7.3 Chemical industry:

7.3.1 The excellent and all-purpose corrosion resistance of Inconel 625 alloy over a wide range of temperatures and pressures is the main reason for its wide acceptance in the chemical industry. Due to its ease of processing, it is utilized in various plant components. Its high strength enables it to be made into equipment that would not be possible with other materials, such as thin-walled vessels or heat exchanger tubes to improve heat transfer and reduce weight. 
Soft annealed low-carbon Inconel625 is widely used in the chemical process industry, where better corrosion resistance and high strength enable it to be used as a thin structural component. 
7.3.2 Some applications require a combination of the strength and corrosion resistance of Inconel 625, such as bubble caps, tubes, reactors, distillation towers, heat exchangers, transmission piping, valves, and chemical plants.

7.3.3 Inconel 625 alloy is being considered for advanced reactor concepts due to its high allowable design strength at elevated temperatures, especially at 1200°-1400°F (649-760°C).
7.3.4 Highly alloyed Inconel 625 alloy is capable of withstanding a wide range of strongly corrosive environments. The material is recommended for the manufacture of storage and handling equipment for chemical waste products, including hydrochloric and nitric acids - two acids that directly represent corrosion problems in two opposite directions; materials that are normally resistant to one of the two will be severely corroded by the other.
7.3.5 Components of organic chemical processes containing chlorides, especially where acidic chloride catalysts are used
7.3.6 Digesters and bleach tanks for the pulp and paper industry
7.3.7 Absorption towers, reheaters, flue gas inlet baffles, fans (wet), agitators, deflectors, and flues in flue gas desulphurization systems.
photo of inconel 625 in chemical equipment application
7.3.8 Equipment and components for manufacturing applications in acid gas environments
7.3.9 Acetic acid and acetic anhydride reaction generators
7.3.10 Sulfuric acid condenser

7.4 Nuclear: 

Due to its high strength in 500°-600°F (260-316°C) water, excellent and uniform corrosion resistance, stress cracking resistance, and outstanding pitting resistance, Inconel 625 is used in reaction cores and control rods of nuclear water reactors.

7.5 Environmental Equipment

Nuclear wastewater equipment
Waste incineration equipment
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