I had a difficult time acquiring information on commercial superalloys in my own PhD research–often needing to read several books and scientific articles–so I decided to conglomerate this information in a series of articles. This article is about Hastelloy C-276, and you can find all my references at the end (although you may have trouble accessing some of them without a university subscription).
Hastelloy C-276 is one of the oldest, most corrosion-resistant alloys that is still used today. The alloy is primarily made of nickel with molybdenum and chromium additions, and its extremely low silicon content makes it suitable for welding.
Hastelloy C-276 is considered to be a “superalloy” because of its extreme corrosion resistance, but it does not have have the classic γ/γ’ microstructure, and it is not used for its creep resistance.
The Hastelloy series is Haynes International’s “corrosion resistant” line of alloys [1]. However, the patent has expired and now many other companies can produce alloy C-276, including SpecialMetals’ brand Inconel C-276 [2, 11].
Outline
Hastelloy C-276 Composition
Hastelloy is a nickel-based superalloy with more than 50% nickel and significant amounts of molybdenum and chromium. There are also small amounts of cobalt, tungsten and manganese in Hastelloy C-276. As with most industrial alloys, Hastelloy C-276 is defined in weight percent (wt%), with a minimum and maximum alloying tolerance.
For example, a small amount of sulfur does NOT improve the alloy properties, but companies expect to see a certain amount of sulfur contamination, below the maximum alloying tolerance.
I consulted 8 companies that sell Hastelloy C-276, as well as the trademark holder, and found the common composition given below [3-11].
| Element | Weight Content (wt%) | Atomic Content (at%) |
| Ni | 51 – 63.5 | 54.6 – 67.4 |
| Co | 0 – 2.5 | 0 – 2.7 |
| Cr | 14.5 – 16.5 | 17.4 – 20 |
| Mo | 15 – 17 | 10 – 11 |
| Fe | 4 – 7 | 4 – 8 |
| W | 3 – 4 | 1 – 1 ? |
| Mn | 0 – 1 | 0 – 1.14 |
| V | 0 – 0.35 | 0 – 0.43 |
| Si | 0 – 0.08 | 0 – 0.18 |
| C | 0 – 0.01 | 0 – 0.05 |
| Cu | 0 – 0.5 | 0 – 0.49 |
| P | 0 – 0.04 | 0 – 0.08 |
| S | 0 – 0.03 | 0 – 0.06 |
Note that not every company agrees on the exact composition. For example, the original developer Haynes International claimed that there was always 5 wt% iron [3], but other suppliers listed a range of 4-7 wt% iron. This table gives the widest compositional range I encountered, and also converts to an approximate atomic percent (at%).
One of the unique features of Hastelloy C-276 is its very low carbon and silicon contents, which made it one of the earliest nickel-chromium-molybdenum alloys engineered for weldability [3].
Hastelloy C-276 Processing
Hastelloy C-276 is a “wrought” alloy, which means it is forged into its final shape, rather than cast.
From the original patent, this alloy is an improvement on “alloy C” which must be processed by [2]:
- Annealing at 1200 ºC or higher
- Water quenching
If the alloy is heated in the range of 600-1100 ºC, brittle intergranular precipitates appear (that’s why water quenching is performed).
For the hard-core metallurgists, I’ll discuss the details of these precipitates at the end of the article in Hastelloy C-276 Miscellaneous Questions.
Because of these processing methods, the “heat affected zone” from welding results in embrittled material, especially at the grain boundaries. The primary improvement of alloy C-276 over alloy C is the extremely low silicon content, which suppresses the intergranular precipitates.
The inventor notes that silicon contamination often occurs because Si is used as a deoxidizing agent, and suggests deoxidizing with an alkaline-earth metal (preferably magnesium) or titanium.
With the low silicon content, brittle precipitates are suppressed even when welding. If extensive welding is performed on large samples, it is possible that such precipitates could form, but the inventor claims that “In the event solution annealing is necessary for some reason, this can be done preferably at 1,150 ºC and does not require a water quench as is the case with conventional alloys” [2].
Hastelloy C-276 can be processed by conventional hot working and cold working methods, as well as electrical discharge machining (EDM).
Hot working for alloy C-276 should be performed in the temperature range 870-1230ºC. Ideally, this should be followed by a water quench to maintain maximum corrosion resistance [6].
Cold working for alloy C-276 should be performed on annealed material, and it has a higher work hardening rate than stainless steels. If the alloy undergoes more than 15% deformation, it is recommended to anneal again before continuing [6].
Due to alloy C-276’s work hardening rate, machining should be performed in on annealed material, and with low cutting speeds [6].
Alloy C-276 is explicitly made for welding, and can be welded with PLASMA [6], Gas Metal Arc Welding (GMAW) such as MIG and MAG [6], Shielded Metal Arc Welding (SMAW or MMA) [6], electron beam welding [12], gas tungsten arc welding (GTAW or TIG) [13], laser welding (LW) [14], and wire arc additive manufacturing (GT-WAAM) [15].
Hastelloy C-276 Properties
Hastelloy C-276 is known for its corrosion resistance, but the alloy also has good mechanical and thermal properties.
Mechanical Properties
Although Hastelloy C-276 is primarily used for corrosive applications, rather than structural applications, the alloy still boasts good mechanical properties. Alloy C-276 is about as stiff and strong as martensitic steel, and can be used at relatively high temperatures.
| Property | Value |
| Density | 8.89 g/cm3 |
| Poisson’s ratio | 0.31 |
| Charpy V-notch 10mm plate (Toughness) | 472 J |
| Young’s modulus | 205 GPa |
| Shear modulus | 79 GPa |
| Yield strength | 355-365 MPa |
| Ultimate Tensile Strength | 785-792 MPa |
| Elongation at failure | 60% |
| Hardness | 88 HRB (Rockwell B) |
Corrosive Properties
Hastelloy C-276 has excellent corrosion resistance in reducing environments, and can handle all-but the harshest oxidizing environments.
Alloy C-276 has exceptional resistance to pitting and stress corrosion cracking, as well as sulfuric, hydrochloric, formic, acetic and phosphoric acids; it is one of few grades that can operate under wet chlorine gas, hypochlorite, or chlorine dioxide environments [6]
Alloy 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 Equivalency Number) of 45.2, which is more than double the PRCN of 316 stainless steel [6].
The following two figures graph the 0.1 mm/year corrosion lines for different corrosion-resistant alloys. For example, you can see that a 12% concentrated hydrochloric acid needs to be heated to about 30 ºC to corrode 0.1 mm of Hastelloy C-276 in one year. In contrast, the same acid for the same amount of time will corrode 0.1 mm of Inconel 625 at less than 20 ºC.
Alloy C-276 is one of the most universally corrosion-resistant alloys, but its largest weakness is oxidizing environments. It is not suitable for extremely harsh oxidizing environments, such as hot concentrated nitric acid.
Thermal, Electrical, and Magnetic Properties
| Property | Value |
| Thermal conductivity | 10.5 W/(mºC) |
| CTE 11.2 | μm/(mºC) |
| Specific heat | 427 J/kg ºC |
| Melting range | 1323-1371 ºC |
| Magnetic permeability (relative) at 15.9 kA/m | 1.002 |
| Electrical resistivity | 1.23 μΩm |
Hastelloy C-276 maintains oxidation resistance at temperatures up to 1100 ºC and resistance to pitting, corrosion, and cracking at temperatures up to 1040 ºC [16].
Hastelloy C-276 Applications
Hastelloy C-276 is a corrosion-resistant alloy with a long, proven track record. It is a “safe” alloy to choose for general corrosive environments, and has been popular with manufacturers in different industries for decades.
A few industries that rely on alloy C-276 include [6, 17]
- Pollution control
- Chemical processing plants, especially those involving chlorides or halides
- Oil and gas, especially sour gas wells
- Pharmaceutical production
- Food processing
- Waste treatment
- Marine and sweater
- Mining
Selected Applications
- Many parts of flue gas desulfurization systems are made with alloy C-276, including scrubbers, ducting, and stack liners. These systems control pollution from electrical plants, and usually involve aggressive scrubbing with chlorides. Alloy C-276 is especially resistant to chlorides, even under scrubbing conditions [11].
- C-276 is excellent at handling “sour” natural gas in oilfields, which usually contains hydrogen sulfide and chlorides. This gas destroys steels by hydrogen embrittlement (sulfide stress cracking) and stress corrosion, but C-276 resists corrosion from this environment, even at high temperatures [11].
- C-276’s corrosion resistance also makes it the material of choice for many chemical processing plants that make products such as soap, paint, fertilizer, adhesives, and more. The alloy may be used in parts such as heat exchangers, pressure vessels or tanks, evaporators, pipes, fittings, pumps, and valves [6, 17].
- The paper industry also has a unique role for C-276, because paper is bleached (bleach is a chloride). C-276 is the material of choice for bleaching vessels and digesters [6, 17].
- The food industry uses C-276 because it can avoid reacting with foods. Even if the alloy did react with foods, the main alloying elements (Ni, Cr, Mo) are not especially toxic to humans.
Hastelloy C-276 Miscellaneous Questions
It turns out, there is a lot of misinformation on the internet about Hastelloy C-276. For instance, you can see some websites list this alloy as a “stainless steel,” which is completely false. While Hastelloy C-276 shares some properties with stainless steel (high strength and corrosion resistance), the two alloy families have a very different composition. Hastelloy C-276 is a nickel-based superalloy, while stainless steel is iron-based.
Is Hastelloy C-276 Magnetic?
No alloy C-276 is not ferromagnetic. Lu et. all have plotted the magnetization vs applied magnetic field, and calculated a magnetic susceptibility of 
.
Hastelloy C-276 is only paramagnetic.
If you want an article that explains everything about magnetism, including the difference between paramagnetic and ferromagnetic materials, you can find that here.
If you already know the essentials of magnetism but aren’t sure how to read the M-H plot, this article will explain those specific details.
Is Hastelloy C-276 a Superalloy?
Although Hastelloy C-276 is often marketed as a superalloy, I would not consider it to be a “true” superalloy because it does not exhibit an anomalous yield strength, nor a γ/γ’ microstructure.
However, the term “superalloy” is not strictly defined and often means “an alloy that’s good in harsh conditions;” since Hastelloy C-276 has excellent corrosion resistance and has a relatively high operating temperature, it could be considered a superalloy by this weaker definition. Hastelloy is not used in turbine blades.
What Microstructural Features Does Hastelloy C-276 Have?
- Unaged is single phase FCC [19]
- μ (A6B7 type) is a hard and brittle TCP, HCP structure, precipitates in the 600-900 ºC range [19]
- M6C, typically M = W or C, diamond cubic structure, precipitates in the 600-900 ºC range [19]
- P phases, tetragonal TCP, precipitates in the 600-900 ºC range [19]
- MoC, lamellar, forms during welding (1300 ºC) [19]
The table below presents compositions of different second phase particles in Hastelloy C-276 given in atomic percentage. If you want to know the weight percentage, click here to view the original paper [20].
| Element | µ (at.%) Atomic Content | M6C (at.%) Atomic Content | P (at.%) Atomic Content |
| Ni | 36.0 | 29.8 | 34.7 |
| Mo | 39.4 | 32.1 | 36.5 |
| Cr | 16.2 | 17.8 | 18.7 |
| W | 3.1 | 1.8 | 1.6 |
| Fe | 2.7 | 2.4 | 6.4 |
| Co | 1.9 | 1.6 | 2.1 |
| C | 0.013* | 14.0** | – |
| V | 0.15 | 0.4 | – |
*Alloy carbon content, **Calculated for M6C
Why Use Hastelloy C-276 Instead of Stainless Steel?
Stainless steel is cheaper and generally has better mechanical properties than Hastelloy C-276. However, alloy C-276 has much better corrosion resistance than stainless steel, especially in halide-containing atmospheres such as chlorine or hydrofluoric acid.
Why Is Hastelloy C-276 Called “Austenitic?”
Alloy C-276 is a nickel-based alloy, and is not related to steel. However, you may hear someone call C-276 an “austenitic alloy,” which is a term commonly associated with steel.
The term “austenitic” means an alloy with a single phase FCC crystal structure. This structure is commonly associated with high temperature carbon steel, but also describes many room temperature stainless steels and nickel-based alloys such as Hastelloy C-276.
What Is Inconel C-276?
Wait, I thought Haynes owned the patent to Hastelloy C-27?. How can SpecialMetals offer an identical Inconel C-276?
The alloy C-276 is a modification on the original Hastelloy “C” alloy. I hunted down the patent, which appears to be expired. You can find it here.
Since the patent is expired, anyone can produce this alloy, as long as they conform to the proper standards. Since alloy C-276 was a modification of an original Hastelloy alloy, most people refer to it as “Hastelloy C-276.”
If you’re curious about the history of alloy C and the Hastelloy line of corrosion-resistance alloys, you can check out this article [17] or scroll down for my summary of that article.
Are There Any Other Materials Equivalent to Hastelloy C-276?
Yes, there are several ways to refer to an alloy with the same composition as Hastelloy C-276, which I’ve given below. Many of these ASTM standards specify not only a composition, but also a processing method and shape. For example, ASTM B574 specifies alloy C-276 in a rod form [22], but ASTM B575 specifies the same composition in plate, sheet, and strip forms [23].
Here is a list of equivalent materials to alloy C-276, in terms of standards, courtesy of Azom [5].
- ATSM B366
- ASTM B574
- ASTM B575
- ASTM B619
- ASTM B622
- ASTM B626
- ASTM F467
- ASTM F468
- DIN 2.4819
Other “C” Alloys, and a Brief Hastelloy History
Alloy “C” was one of the first corrosion-resistant alloys that could compete with stainless steel.
The original Hastelloy alloy “C” was developed in the 1930s, with C-276 following in the 1960s and C-4 in the 1970s. C-22 and 622 were developed in the 1980s, and alloys 59, 686, and C-2000 followed in the 1990s.
Alloy C-276 was developed by the German company BASF by using the novel AOD (Argon-Oxygen Decarburization) process. This alloy is still used today, even more than it’s supposed improvement, alloy C-22. C-22 is better in oxidizing environments, but C-276 is better in reducing environments, and is available from more manufacturers.
All the information from this sub-section came from this article, which provides excellent historical context and property comparisons between the alloys in the C family.
At the end of this paper, which was published in 1997, the authors claim that alloy 59 is the new “best” alloy from the C series, and it’s major roadblock was that companies preferred alloys with a longer track record. Today, I don’t see “alloy 59” in the Hastelloy suite of alloys. However, C-276 remains the “workhorse” of corrosion-resistant Ni-Cr-Mo alloys.
Final Thoughts
Alloy C-276 is a corrosion-resistant Ni-Cr-Mo alloy that was designed with extremely low levels of silicon to improve weldability. The alloy has been used since the 1960s and–due to its extreme corrosion resistance, especially to chlorides and other halides– is still popular in many industries such as petroleum, chemical processing, and paper.
References and Further Reading
If you’re interested in superalloys, you be appreciate this article that explains the basics about superalloys.
If you want a better understanding about why the original alloy C couldn’t be welded without a special heat treatment and quenching (and why C-276 can), you might be interested in the article I wrote about quenching.
[1] Trademarks. Hanyes International.
[2] Margarete, S., Immanuel, C., & Hubert, G. (1965). HIGHLY CORROSION RESISTANT NICKEL-CHROMIUM-MOLYBDENUM ALLOY WITH IMPROVED RESISTANCE TO INTERGRANULAR CORROSION (US3203792A). United States Patent Office.
[3] Haynes International. HASTELLOY® C-276 alloy.
[4] High Performance Alloys, Inc. – Superalloy Producer. HASTELLOY C276 (UNS N10276).
[5] AZoM. (2013, June 12). Super Alloy HASTELLOY(r) C276 (UNS N10276). AZoM.Com.
[6] Sandmeyer Steel Company. Specification Sheet: Alloy C276.
[7] Corrosion Materials. Alloy C-276 / UNS N10276 / W.Nr. 2.4819.
[8] Stardust Impex Pvt Ltd. (2020, November 9). Hastelloy C276 | N10276 | 2.4819 | NW 0276.
[9] Haynes International. Alloy Nominal Compositions.
[10] Mega Mex. Hastelloy C-276.
[11] Special Metals. INCONEL ® alloy C-276.
[12] Bal, K.S., Dutta Majumdar, J. & Roy Choudhury, A. Optimization of Melt Zone Area for Electron Beam Welded Hastelloy C-276 Sheet and Study of Corrosion Resistance of the Optimized Melt Zone in 3.5 wt% NaCl Aqueous Solution. Arab J Sci Eng 44, 1617–1630 (2019).
[13] M. Manikandan, N. Arivazhagan, M. Nageswara Rao, G.M. Reddy. Microstructure and mechanical properties of alloy C-276 weldments fabricated by continuous and pulsed current gas tungsten arc welding techniques. Journal of Manufacturing Processes, Volume 16, Issue 4, 2014, Pages 563-572.
[14] Dongsheng Chai, Guangyi Ma, Siyu Zhou, Zhuji Jin, Dongjiang Wu. Cavitation erosion behavior of Hastelloy™ C-276 weld by laser welding. Wear. Volumes 420–421, 2019, Pages 226-234.
[15] Zhijun Qiu, Bintao Wu, Hanliang Zhu, Zhiyang Wang, Alan Hellier, Yan Ma, Huijun Li, Ondrej Muransky, David Wexler. Microstructure and mechanical properties of wire arc additively manufactured Hastelloy C276 alloy. Materials & Design. Volume 195, 2020, 109007.
[16] Enerquip. (2020, February 6). Hastelloy C-276 resists corrosion in some of the harshest environments.
[17] Agarwal, D.C. and Herda, W.R. (1997), The “C” family of Ni-Cr-Mo alloys’ partnership with the chemical process industry: The last 70 years. Materials and Corrosion, 48: 542-548.
[18] Lu, Jun & Choi, E. & Zhou, H.. (2008). Physical properties of Hastelloy (R) C-276 (TM) at cryogenic temperatures. Journal of Applied Physics. 103. 064908-064908. 10.1063/1.2899058.
[19] Jianguo Yang, Yanming He, Chunjie Qin, Wenjun Zhao, Shuangjian Chen, Zengliang Gao. Microstructure evolution in a Ni–Mo–Cr superalloy subjected to simulated heat-affected zone thermal cycle with high peak temperature. Materials & Design. Volume 86, 2015, Pages 230-236.
[20] Raghavan, M., Berkowitz, B.J. & Scanlon, J.C. Electron Microscopic Analysis of Heterogeneous Precipitates in Hastelloy C-276. Metall Mater Trans A 13, 979–984 (1982).
[21] Cieslak, M.J., Headley, T.J. & Romig, A.D. The welding metallurgy of HASTELLOY alloys C-4, C-22, and C-276. Metall Mater Trans A 17, 2035–2047 (1986).
[22] ASTM B574-17, Standard Specification for Low-Carbon Nickel-Chromium-Molybdenum, Low-Carbon Nickel-Molybdenum-Chromium, Low-Carbon Nickel-Molybdenum-Chromium-Tantalum, Low-Carbon Nickel-Chromium-Molybdenum-Copper, and Low-Carbon Nickel-Chromium-Molybdenum-Tungsten Alloy Rod, ASTM International, West Conshohocken, PA, 2017.
[23] ASTM B575-17, Standard Specification for Low-Carbon Nickel-Chromium-Molybdenum, Low-Carbon Nickel-Chromium-Molybdenum-Copper, Low-Carbon Nickel-Chromium-Molybdenum-Tantalum, Low-Carbon Nickel-Chromium-Molybdenum-Tungsten, and Low-Carbon Nickel-Molybdenum-Chromium Alloy Plate, Sheet, and Strip, ASTM International, West Conshohocken, PA, 2017.