AMS 5599 AMS 5666 ASTM B446 ASTM B443 – High Temperature (UNS N06625)

Common trade names

Inconel 625® (® Special Metals), Alloy 625, Chronin® 625, Altemp® 625, Haynes® 625 (® Haynes International), Nickelvac® 625, Nicrofer® 6020, Nickel 625, Chronic 625

A brief history of Inconel® 625

The patent for Inconel 625 was issued on December 8th, 1964 after long years of research into a Ni-Cr-Mo-Nb alloy (Nickel Chromium Molybdenum Niobium). It is a so-called “superalloy”, because of its ability to withstand high temperatures, stress and corrosion.

Originally developed for high pressure steam lines in power plants, it quickly became apparent that alloy 625 could handle extreme corrosion and oxidation from harsh environments.

Molybdenum, chromium, and niobium give this alloy additional creep strength from stressors like high temperatures (maintaining its resistance to oxidation at temperatures up to 1800°F) and other harsh conditions that could deform less resistant alloys over time.

Thermal strengthening through heat treatment improves the yield strength, but due to embrittlement at high temperatures over extended time periods, this alloy is best used in lower temperature applications where its corrosion resistance shines.

Product Description

Inconel® 625 is a nickel-based superalloy with excellent resistance to oxidation and corrosion, in conditions ranging from jet engine propulsion systems to chemical processing of oxidizing and reducing acids.

Nickel-chromium-molybdenum alloy 625 is a material with excellent resistance to pitting, crevice and corrosion cracking. Highly resistant in a wide range of organic and mineral acids. Good high temperature strength.

The nickel-chromium matrix of Inconel 625 is reinforced by the addition of molybdenum and niobium, which is alloyed through solid solution strengthening, and this allows it to maintain high strength and toughness at temperatures ranging from cryogenic up to 2000°F (1093°C).

It is non-magnetic, austenitic, and displays high tensile strength, fabricability, and brazeability.

Due to its high nickel content, this alloy is nearly immune to chloride ion stress-corrosion cracking and pitting, which is commonly found in metals in seawater applications like heat exchangers, fasteners, and cable sheathing.

Characteristics

• Excellent mechanical properties at both extremely low and extremely high temperatures.
• Outstanding resistance to pitting, crevice corrosion and intercrystalline corrosion.
• Almost complete freedom from chloride induced stress corrosion cracking.
• High resistance to oxidation at elevated temperatures up to 1050C.
• Good resistance to acids, such as nitric, phosphoric, sulfuric and hydrochloric, as well as to alkalis makes possible the construction of thin structural parts of high heat transfer.

Composition of Nickel Alloy Inconel 625

The composition range for Nickel Alloy 625 is provided in table below :

Chemical Analysis

Physical Properties of Alloy 625

• Density: 0.303 lb/in³ (8.44 g/cm³
• Specific Gravity: 8.44
• Melting Range: 2350 – 2460°F (1280 – 1350°C)
• Specific Heat: 0.098 Btu/lb x °F (410 Joules/kg x °K)
• Magnetic Permeability (75°F, 200 oersted): 1.0006

Mechanical Properties of Alloy 625

Applications

• Components where exposure to sea water and high mechanical stresses are required.
• Oil and gas production where hydrogen sulfide and elementary sulfur exist at temperature in excess of 150C.
• Components exposed to flue gas or in flue gas desulfurization plants.
• Flare stacks on offshore oil platforms.
• Hydrocarbon processing from tar-sand and oil-shale recovery projects.

Common Applications

• Propeller blades
• Submarine propulsion motors
• Utility boat exhaust ducts
• Steam-line bellows
• Base plates

Even though researchers initially touted its creep strength at high temperatures, it was also shown that alloy 625 could remain nearly corrosion free at ambient to low temperature elevations, like seawater environments or chemical processing of acids and salts. Marine heat exchangers commonly use 625, to isolate corrosive seawater to materials that can endure them, such as 625 based plate and shell and tube heat exchangers.

It is nearly immune to chloride-ion induced stress cracking, by virtue of its high nickel content, and has been used in propellers and propulsion systems as well as wires used in cable sheathing in marine environments. In addition to saltwater corrosion resistance, the high ductility of 625 makes it ideal for fasteners like hex bolts in underwater environments.

Because of its ease of weldability, Inconel 625 has been used in weld overlays (weld overlay cladding) to improve the strength and corrosion resistance of base metals, such as those found in boiler tubes or petrochemical equipment like wellheads.

Cheaper, thick layers of base materials like steel alloys can be weld cladded with alloy 625 even at economical dilutions with the right technique, giving much needed strength and protection against corrosion to these parts.

It is also used in waste-to-energy boilers, where refuse-derived-fuel is used to power steam generators with refuse boilers. Inconel 625 replaced heat resistant materials like ceramic tiles for corrosion protection, primarily as welded cladding and composite tubes, which significantly lowered the cost of maintenance on corroded refractory. By the late 1990s, this alloy was widely seen as the most corrosion resistant alloy at conditions caused by waste combustion.

Heat Treatment

Alloy 625 / inconel 625 has three basic heat treatments:

(1)High Solution Anneal – 2000/2200°F (1093/1204°C), air quench or faster.
(2)Low Solution Anneal – 1700/1900°F (927/1038°C), air quench or faster.
(3)Stress Relieve – 1650°F (899°C), air quench.

The time at the above temperatures depends on volume and section thickness. Strip, for example, would require shorter times than large sections.

Temperatures for treatments No. 1 and 2 are generally held for 1/2 to 1 hour, 1 to 4 hours for treatment No. 3.

Treatment No. 1 is not commonly used for applications below 1500°F (816°C). It is generally used above 1500°F and where resistance to creep is important. The high solution anneal is also used to develop the maximum softness for mild processing operations such as cold rolling or drawing.

Treatment No. 2 is the used treatment and develops an optimum combination of tensile and rupture properties from ambient temperatures to 1900°F (1038°C). Ductility and toughness at cryogenic temperatures are also very good.

Treatment No. 3 is recommended for application below 1200°F (649°C) when maximum fatigue, hardness, tensile and yield strength properties are desired. Ductility and toughness at cryogenic temperatures are excellent. When a fine grain size is desired for fatigue, tensile and yield strengths up to 1500°F (816°C), treatment No. 3 is sometimes used.

Workability

Hot Working

Hot working may done at 2100°F (1149°C) maximum furnace temperature. Care should be exercised to avoid frictional heat build-up which can result in overheating, exceeding 2100°F (1149°C). Alloy 625 becomes very stiff at temperatures below 1850°F (1010°C). Work pieces that fall below this temperature should be reheated. Uniform reductions are recommended to avoid the formation of a duplex grain structure. Approximately 15/20% reduction is recommended for finishing.

Cold Forming

Alloy 625 can be cold formed by standards methods. When the material becomes too stiff from cold working, ductility can be restored by process anneal.

Machineability

Low cutting speeds, rigid tools and work piece, heavy equipment, ample coolant and positive feeds are general recommendations

This is an example of a mill’s certificate of inconel 625 or alloy 625

You can learn more about inconel 625 / alloy 625 here:

http://www.specialmetals.com/assets/smc/documents/alloys/inconel/inconel-alloy-625.pdf

References:

• http://www.specialmetals.com
• https://www.hpalloy.com
• https://www.azom.com
• https://www.upmet.com
• https://www.hightempmetals.com
• https://en.wikipedia.org/wiki/Inconel_625
• https://www.valbruna-stainless-steel.com