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Inconel Alloy 625

Inconel 625 is a high-performance nickel-chromium-molybdenum alloy known for its strength, temperature resistance, and corrosion resistance inconel 718 vs hastelloy c276. Nickel makes up 58 percent of this superalloy, followed by chrome, molybdenum, niobium, iron, tantalum, and cobalt, as well as manganese, silicon, aluminium, and titanium to a trace. In addition to Inconel 625, other names include Haynes 625, Altemp 625, Nickelvac 625, and Nicrofer 6020.

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Exceptional material strength

The strength of Inconel 625 lies not only in its nickel-chromium base but also in its hardening mechanism of niobium and molybdenum hastelloy vs inconel. Niobium and molybdenum interact to strengthen the alloy matrix, which provides high strength without precipitation-hardening treatment. This superalloy has a tensile strength of 690 MPa, and a yield strength of 275 MPa hastelloy c276 equivalent material.

High-temperature resistance

Featuring a melting point of about 1300°C and a thermal expansion coefficient of 1.28 x 10-5 1/K (at 20°C), Inconel 625 can withstand temperatures as low as cryogenic to extremely high.

The titanium and niobium compositions of the oxide film on Inconel 625 increase dramatically at high temperatures when oxidising agents are present.

Corrosion resistance

Moreover, due to its combination of elements, Inconel 625 is highly resistant to corrosion. This is why it works well under high saline seawater and even better in milder environments such as fresh water.

Upon exposure to oxidising agents, Inconel 625 is naturally protected by its nickel-chromium matrix. Molybdenum protects against pitting corrosion. In 15% sulfuric acid, the material corrodes at a rate of 0.188 mm per year.

High level of fabricability

As a result of its design, Inconel 625 alloy is easier to weld than earlier alloys, and it doesn't crack when exposed to changes in temperature and strain after welding. The high creep resistance and yield strength of this superalloy make it an ideal choice for welding tubes, piping, and plant equipment.

Production and processing

It is noteworthy that the production and composition of Inconel 625 vary c276 compositiondepending on the material's specifications hastelloy c276 mechanical properties. As an example, the nickel content of AMS 5666 or UNS N06625 variants may be around 58.2%, 22.2%, and 3.5%, respectively.

Material toughness can be increased by heat treating alloys. In the case of Inconel 625, these treatments can be applied:

  • As a result of high solution annealing, the alloy has excellent creep resistance. It involves soaking in 1095-1205°C for at most one hour, and then cooling it in ambient air or a cooling medium.
  • A low solution annealing is performed at temperatures of 925-1040°C, which is similar to high solution annealing. However, the alloy produced shows good tensile strength and ductility.
  • In this process, the material is soaked for up to four hours at 900°C, followed by cooling at ambient air temperature, resulting in an exceptional product with exceptional cryogenic performance.

If the temperature of the furnace is below 1010-1150°C, the material may become stiff, which will necessitate reheating.

Applications of Inconel 625

Marine applications

Because Inconel 625, c22 vs c276 has high corrosion resistance, especially pitting and crevice resistance, it is ideal for applications in high salty environments. Boat propeller blades and mooring lines are made from this material, submarine accessories and fixtures are made from this material, and oceanographic equipment is made from it.

Aerospace equipment

Among its applications are exhaust equipment, fuel lines, heat exchanger casings, and rocket components. Its properties enable it to withstand high temperatures and stresses even at exceptionally high temperatures.

Nuclear technology

In addition to its strength and corrosion resistance, Inconel 625 is an excellent choice for nuclear reactors, especially for control rods and cores. The strength of Inconel 625 is also able to withstand high temperatures above 650°C in nuclear systems.

Industrial processing

As an easy-to-fabricate alloy and a corrosion-resistant material, the alloy is a valuable ingredient for vessels, heat exchangers, valves, and fluid distribution systems. Due to its excellent weldability, it is also suited for pipes and tubes used in factories.

Hastelloy  Alloy C-276

It was difficult for me to acquire information about commercial superalloys during my Ph.D. research - often reading multiple books and scientific articles - so I decided to aggregate this information into a series of articles. Despite the fact that some references may not be accessible without a university subscription, you can find all my references at the end of this article about Hastelloy C-276.

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In the past, Hastelloy C-276 was made of nickel and molybdenum with chromium additions, and was one of the most corrosion-resistant alloys ever made. Welding is possible due to its extremely low silicon content.

Since Hastelloy C-276 exhibits extreme corrosion resistance, it is classified as a "superalloy," but it has no classic microstructure, nor is it used for creep resistance.

Hastelloy is Haynes International's corrosion-resistant alloy line. Now that the patent has expired, many other companies can produce alloy C-276, such as SpecialMetals' Inconel C-276.

Hastelloy C-276 Composition

In addition to nickel, Hastelloy C-276 contains significant amounts of molybdenum and chrome, and it contains small amounts of cobalt, tungsten, and manganese, hastelloy c276 chemical composition as well as small amounts of cobalt. As with most industrial alloys, Hastelloy C-276 has a minimum and maximum alloying tolerance, defined in weight percent (wt%).

Sulphur, for instance, does NOT improve alloy properties, but companies are expecting to see some sulphur contamination, below the maximum alloying tolerance.

Upon consulting eight companies that sell Hastelloy C-276, as well as the trademark holder, I found the following common composition: 

In fact, not every company agrees on the composition. For example, Haynes International stated that the iron content was always 5 wt% [3], while other suppliers reported iron content ranging from 4-7 wt%. In addition to providing the widest range of compositional ranges I found, this table also converts to an approximate atomic percent (at%).

Due to its low carbon and silicon content, Hastelloy C-276 is one of the earliest nickel-chromium-molybdenum alloys designed for welding.

Hastelloy C-276 Processing

Cast alloys, like Hastelloy C-276, are forged into their final shape, whereas wrought alloys are forged.

Based on the original patent, this alloy is an improvement over "alloy C", which requires 

  • At least 1200 oC for annealing
  • Water quenching

In the range of 600-1100 oC, brittle intergranular precipitates form (for this reason, water quenching is performed). 

The details of these precipitates will be discussed in Hastelloy C-276 Miscellaneous Questions at the end of the article.

Welding results in embrittled material because of the "heat affected zone", particularly at grain boundaries, as a result of these processing methods. As compared to alloy C, alloy C-276 has a very low silicon content, which suppresses intergranular precipitates.

According to the inventor, silicon contamination is often caused by Si being used as a deoxidizing agent, and he suggests deoxidizing with magnesium or titanium instead.

Although welding is possible, the silicon content suppresses brittle precipitates. In the event that extensive welding is performed on large samples, such precipitates may form, but the inventor claims that “if, for some reason, solution annealing is necessary, it can be done preferably at 1,150 oC and does not require a water quench as with conventional alloys”.

As well as hot-working and cold-working, Hastelloy C-276 can also be machined by electrical discharge machining (EDM).

For maximum corrosion resistance, alloy C-276 should be hot worked between 870 and 1230 degrees Celsius, followed by a water quench.

Cold working alloy C-276 on annealed material is recommended since it hardens more rapidly than stainless steel. If the alloy undergoes more than 15% deformation, it should be annealed again before cold working. 

It is recommended to machine alloy C-276 on annealed material, and at a low cutting speed, due to the alloy's work hardening rate.

As a welding alloy, alloy C-276 can be welded with PLASMA, gas metal arc welding (GMAW) such as MIG and MAG, shielded metal arc welding (SMAW or MMA), electron beam welding, gas tungsten arc welding (GTAW or TIG), laser welding (LW), and wire arc additive manufacturing (GT-WAAM).

Hastelloy C-276 Properties

In addition to its corrosion resistance, Hastelloy C-276 has good mechanical and thermal properties as well.

Mechanical Properties

Even though Hastelloy C-276 is primarily used for corrosive applications rather than structural applications, it still offers good mechanical properties. It is about as stiff and strong as martensitic steel, and it can be used at relatively high temperatures.

Corrosive Properties

In reducing environments, Hastelloy C-276 performs well and can handle all but the harshest oxidising environments.

C-276 is resistant to pitting and stress corrosion cracking, as well as sulfuric, hydrochloric, formic, acetic, and phosphoric acids; it is one of few grades that can withstand wet chlorine gas, hypochlorite, or chlorine dioxide. 

C-276 can withstand hydrogen fluoride gas at 500-600oC for over 50 times longer than comparison alloys, such as 70Cu-30Ni, and has a PRCN (Pitting Resistance Equivalent Number) of 45.2, nearly double the PRCN of stainless steel 316.

Different corrosion-resistant alloys are graphed in the following two figures with corrosion lines of 0.1 mm/year. A 12% concentrated hydrochloric acid, for instance, must be heated to about 30 oC for 0.1 mm of Hastelloy C-276 to corrode within a year. The same acid, however, will corrode Inconel 625 at less than 20 oC for the same amount of time.

In spite of its corrosion-resistant properties, C-276 is unable to withstand harsh oxidizing environments, such as hot concentrated nitric acid.

Properties of thermal, electrical, and magnetic energy

The material Hastelloy C-276 is resistant to oxidation at temperatures up to 1100 oC and to pitting, corrosion, and cracking at temperatures up to 1040 oC.

Hastelloy C-276 Applications

This corrosion-resistant alloy has a long, proven track record, and is a "safe" choice for general corrosive environments.

Alloy C-276 is used in a number of industries

  • Containment of pollution
  • Processes involving chlorides or halides, especially in chemical processing
  • Wells that produce sour gas, especially oil and gas
  • Production of pharmaceuticals
  • The food processing industry 
  • Treatment of waste
  • A sweater and a marine
  • Investing in mining

Selected Applications

  • Alloy C-276 is used for many parts of flue gas desulfurization systems, including scrubbers, ducting, and stack liners. As part of these systems, electrical plants are controlled against pollution through aggressive chloride scrubbing. Even under harsh scrubbing conditions, alloy C-276 is highly resistant to chlorides.
  • As a result of its ability to handle hydrogen sulphide and chloride-containing “sour” natural gas in oil fields, C-276 is excellent. Even at high temperatures, C-276 resists corrosion from this environment, even when hydrogen embrittlement (sulphide stress cracking) and stress corrosion destroy steels.
  • As a result of its corrosion resistance, C-276 is also popular with chemical processing plants that produce soap, paint, fertiliser, adhesives, and other products. Parts made of the alloy include heat exchangers, tanks, evaporators, pipes, fittings, pumps, and valves.
  • In the paper industry, C-276 plays a unique role because the paper is bleached (bleach is a chloride).
  • Even if the alloy did react with foods, its main alloying elements (Ni, Cr, Mo) are not particularly toxic to humans. The food industry uses C-276 because it avoids reacting with foods.

Hastelloy C-276 Miscellaneous Questions

On the internet, there is much misinformation about Hastelloy C-276. For instance, some sites refer to it as stainless steel, but this is completely false. Although Hastelloy C-276 shares some properties with stainless steel (high strength and corrosion resistance), the two alloy families have very different compositions. Superalloy Hastelloy C-276 is nickel-based, whereas stainless steel is iron-based.

Is Hastelloy C-276 Magnetic?

There is no alloy C-276 that is not ferromagnetic. Lu et. al. have plotted the magnetization vs applied magnetic field, and calculated a magnetic susceptibility for it. 

Hastelloy C-276 is only paramagnetic.

There is an article here that explains everything about magnetism, including the difference between paramagnetic and ferromagnetic materials.

In this article, we will cover the specific details of reading the M-H plot if you already know the basics of magnetism.

Is Hastelloy C-276 a Superalloy?

It is marketed as a superalloy, but I would not consider it a "true" superalloy because it does not display an anomalous yield strength or a hollow microstructure.

As "superalloy" is not strictly defined and often means "an alloy that performs well in harsh conditions," Hastelloy C-276 could be considered a superalloy by this weaker definition since it has excellent corrosion resistance and operates at a relatively high temperature. Turbine blades are not made from Hastelloy.

How does Hastelloy C-276 differ from other alloys in terms of its microstructure?

  • There is only one phase of FCC for unaged
  • A 6 B 7 type is a hard and brittle type of TCP, HCP that precipitates between 600 and 900°C
  • Typically, M = W or C, diamond cubic structure, precipitates between 600 and 900 degrees Celsius
  • In the range of 600-900 oC, P phases, tetragonal TCP, precipitate
  • Lamellar MoC forms during welding (1300 oC)

Below is a table illustrating different second-phase particle compositions in Hastelloy C-276, expressed as an atomic percentage. For weight percentages, please refer to the following table.

When should Hastelloy C-276 be used instead of stainless steel?

Hastelloy C-276 has poor mechanical properties and is more expensive than stainless steel. However, alloy C-276 has better corrosion resistance, especially in halide-containing environments like chlorine or hydrofluoric acid.

What Does "Austenitic" Mean for Hastelloy C-276?

The alloy is nickel-based and has nothing to do with steel, but it may be referred to as an "austenitic alloy," which is a term commonly associated with steel.

Austenitic alloys are those that have a single-phase FCC crystal structure. This structure is often associated with high-temperature carbon steels, but can also describe stainless steels and nickel-based alloys like Hastelloy C-276.

What Is Inconel C-276?

Haynes owns the patent to Hastelloy C-27. How can SpecialMetals offer Inconel C-276 that is identical?

The alloy C-276 is a modification of the original Hastelloy C alloy. I located the patent, which appears to have expired.

Due to the expired patent, anyone can produce alloy C-276, as long as they meet the proper standards. Since alloy C-276 is a modification of an original Hastelloy alloy, it is usually called Hastelloy C-276.

You can read more about alloy C and the Hastelloy line of corrosion-resistant alloys in this article or scroll down for my summary.

Is there any other material that is comparable to Hastelloy C-276?

Yes, there are several ways to refer to an alloy with the same composition as Hastelloy C-276, which I have given below. Many ASTM standards specify not only a composition, but also a processing method and shape in addition to the composition. For example, ASTM B574 specifies alloy C-276 for rod, while ASTM B575 specifies the same composition for plate, sheet, and strip.

As a courtesy of Azom, here is a list of equivalent materials to alloy C-276.

  • ASTM B366
  • ASTM B574
  • ASTM B575
  • ASTM B619
  • ASTM B622
  • ASTM B626
  • ASTM F467
  • ASTM F468
  • DIN 2.4819

An Overview of Other "C" Alloys and Hastelloy's History

A corrosion-resistant alloy called alloy "C" was one of the first alloys capable of competing with stainless steel.

C-22 and 622 were developed in the 1980s, followed by alloys 59, 686, and C-2000 in the 1990s. The original Hastelloy alloy "C" was developed in the 1930s, and C-276 and C-4 followed in the 1960s and 1970s.

As a result of the AOD (Argon Oxygen Decarburization) process, BASF developed alloy C-276. The alloy is still widely used today, even more so than its supposed improvement, alloy C-22. It is better in an oxidising environment, but it is better in a reducing environment and is available from a greater number of manufacturers than C-22.

In this subsection, all information comes from this article, which provides excellent historical context and property comparisons between the C family alloys.

This paper, published in 1997, claims alloy 59 is the newest "best" alloy from the C series, but companies prefer alloys with a longer history of use. 

The Hastelloy suite of alloys no longer includes alloy 59, but C-276 remains a corrosion-resistant Ni-Cr-Mo alloy of choice.

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Final Thoughts

C-276 is a corrosion-resistant NiCr-Mo alloy designed with extremely low silicon levels to enhance weldability, making it corrosion-resistant. It has been used since the 1960s and is still widely used in many industries, including petroleum, chemical processing, and paper, because of its excellent corrosion resistance, especially to chlorides and other halides.