Hastelloy C276

The alloys based on nickel content comprised of a significant category of materials, which have high applications in the demanding conditions of high temperature and high corrosion resistance strength. There exist a number of alloys of this type because of the capability of nickel matrix to contain a heavy amount of elements, for example iron, copper, molybdenum chromium, etc., with the maintaining fundamentally a structure of single phase face centered cube. The alloys of Ni-Cr-Mo-W elements are known as Hastelloys, and are well-known for their excellent physical and mechanical and corrosion resistance to many different corrosive environments. These alloys have a number of applications such as in aviation, nuclear, space and chemical processing industries due to such excellent properties. The Hastelloy C-276, one of the Hastelloy alloys of C category, is a Ni–Mo–Cr superalloy which has high corrosion resistance strength. It possesses the face centered cubic structure and the lattice parameter of a =3.620 Å with the room temperature density of 8.89 g/cm 3 . This superalloy specifically has wide range of high temperature applications chemical processing, nuclear, and aviation industries. The most recent
and advanced application of this superalloy is Hastelloy C-276 tapes which use as a substrate of superconducting coated conductor for example MgB 2 and YBCO. This alloy belongs to the category of Alloy C which has very low carbon content of maximum 0.01% and silicon content of maximum 0.08%. The development of Hastelloy C-276 was due to prevail over the corrosion issues related to that occurs during the Alloy C welding. The Alloy C was vulnerable to the severe attacks of intergranular corrosion during the as-welded condition in several environments containing chlorides and oxides. The low content of silicon and carbon in Alloy C-276 helps in preventing the precipitation at continuous grain boundaries in the heat affected zones of welds. Therefore, the Hastelloy C-276 in as-welded condition has a number of applications without being suffered in serious intergranular attacks. The alloy C-276 has an exceptional resistance to corrosion in both reducing and oxidizing solutions which also include those contaminated with halogen ions. Resistance to crevice and pitting corrosion of Alloy C-276 is excellent when in contact with acid chloride salts, during the processing and in simply treating hot seawater in heat exchangers.

Alloy C-276 is a very favorable material because of the exceptional resistance to corrosion in strongly oxidizing and reducing media, however it is a compromising material is some conditions such as during handling 98% sulfuric acid in coolers from absorption towers. Hastelloy C-276 is not favorable material for the process-side corrosion, however good for the water-side corrosion allowing the usage of seawater or brackish water. Hastelloy C-276 alloys have various applications in the pollution control industries, mainly in the scrubbing system. The performance of Alloy C-276 is being satisfactory for the years predominantly in the aggressive conditions of wet or dry environment. It has also been an excellent material for the construction of scrubbers in which the issues of occurrence of localized attack are common with other material due to the chloride fraction, PH or temperature.

History of Hastealloy C-276

The inventor of Hastelloys, Haynes International Inc was started initially their company with the name of the company of Haynes Stellite Works. During the early years of the company, the experimentation was continued with the cobalt based alloys and in 1913 was granted a patent for the cobalt-chromium molybdenum-tungsten-carbon alloy which now called as Haynes® alloy 6B, and still in production since its discovery. Along with that other patents were also granted for more cobalt alloys. In 1920s, the research on nickel molybdenum alloys was also conducted particularly for the application of corrosion resistant materials. In 1921, the patent for the nickel-molybdenum alloy composition range was obtained. And Hastelloy® A alloy came from this patent and after the two years the Hastelloy B alloy was introduced. The composition of Hastelloy B was very unique and because of that it is still being produced. In the 1926s, the research on nickel-molybdenum-chromium alloys was started for the better corrosion resistance properties particularly in the oxidizing environments and from that research the Hastelloy C was introduced in the market. In the 1960s, the advancement in the superalloy metallurgy was being the main focus, predominantly the introduction of Argon Oxygen Decarburization and Electro Slag Remelting. Argon Oxygen Decarburization processing was supportive for the production of alloys with very low content of carbon. This method is same as the decarburizing of steel with the oxygen lance. The Electro Slag Remelting process is the other melting operation which treats the cast structure and eliminates the impurities. These developments helped in the production of a new category of superalloys for both corrosive and high temperature applications. Among them one of the most prominent was Hastelloy C-276 alloy. This alloy at the present is an industry standard alloy for
the application in the chemical and petrochemical processing.

Properties of Hastelloy C-276

Mechanical and Physical Properties of Hastelloy C-276 The addition of substantial amount of alloying elements for example molybdenum, tungsten and chromium are required for applications at higher temperature range. Addition of chromium provides high mechanical properties and resistance to oxidizing environments and molybdenum increases the reducing environments resistance along with high strength and toughness. Tungsten also helps in increasing the mechanical properties and localized resistance to corrosion. The presence of Ni with high level of fraction of molybdenum and chromium helps in maintaining the stable austenitic single-phase structure. This is essential to obtain the adequate corrosion resistance in the alloy capable of being produced economically and excellent fabrication.

Hot Forming

The temperature of hot forming for Hastelloy C-276 is in the range from 870°C to 1230°C (1600 to 2250°F). It is necessary to water quench the alloy after the hot forming. Heat treatments are advised after the hot forming to
increase the resistance to corrosion and mechanical properties. Hastelloy C-276 alloys are recommended for the annealing temperature of 1121°C, and after that it is recommended to quench which is the rapid air cooling and is
practical with thinner structure of less than 10 mm. The holding time of 10 to 30mins at the annealing temperature is recommended; can be changed dependent on structure thickness such that the thicker structure requires 30mins holding time. Although, the Hastelloy C-276 alloys are much more sensitive than the austenitic stainless steels to strain and strain rates, and have relatively narrower range of temperature for hot working, such as, the advised starting temperature for the hot forming is 1232°C with finishing temperature of 954°C.

Cold Forming

For cold working of Alloy C-276, it is necessary to be in the annealed condition. The fact should be taken in consideration that these alloys have a higher rate of work hardening than the austenitic stainless steels. When the alloy is undergoing a higher degree of cold working, it may be necessary to have in-process annealing. The solution anneal may become essential, if the alloy is undergoing deformation greater than 15% while cold working. The reason of this solution annealing practice is designed for the optimum corrosion resistance and ductility of the alloy. The material is recommended to re-anneal after all the hot forming procedures; the material for the restoration of the optimum properties of alloy. It is also advised that the alloy should undergo re-annealing process after the cold working procedure which can lead to an elongation of outer fibers with 7% or more percent. Hastelloy C-276 alloy has high stiffness than the austenitic stainless steel, and thus it is essential to have more energy for cold forming of the products. It should also be noted that Hastelloy C-276 are work hardened more eagerly than the austenitic stainless steel, and can be required to have many stages of cold working along with the intermediate annealing. Whereas, the resistance to localized corrosion, crevice attack and chloride-induced pitting of the alloy C-276 is not usually become affected by the cold working, but can affect the stress corrosion cracking resistance. Hence, the re-annealing of cold worked components after the elongation of outer fiber of 7% or more is essential for the optimum performance of corrosion properties.

Machining

It is preferred for alloy C-276 to be machined in the as-annealed condition. The low cutting speed should only be utilized and the cutting tools must be engaged every time because the Alloy C276 is vulnerable for work hardening. The sufficient cut depth is required in order to avoid contact with the previously created work-hardened region.
Fabrication and Heat Treatment As it is mentioned above that the alloy C-276 can be produced by using both hot working and cold working methods. However, Hastelloy C-276 is likely to be work hardened; it can be cold worked by utilizing the more violent processes for example punching, press forming or deep drawing. The temperature range for the hot working to be carried out should lie between 870 to 1230°C (1600°F and 2250°F) with pre-heating of heavier parts before working to the lowest of 2000°F temperature. The recommended annealing temperatures of material are 2050°F and 2150°F to be performed after working after the rapid quenching in an agitated reducing quench bath or in a protective environment. It is advised to add 2% of propyl or ethyl alcohol by volume
to water for obtaining the reducing media.

Thermal Stability

Hastelloy C-276 alloys in the mill-annealed condition are thermally stable and known to have the long range atomic order in which the disordered face centered cube matrix transformed into the ordered orthorhombic superlattice structure. The alloy experiences this phenomenon usually only after minimum at the some hundred hours of temperature range 600⁰C and therefore is not significant in the concern of the microstructure of the fusion zone formed by the cooling rates which is a feature of arc welding. The intermetallic phases such as μ, P, precipitation and carbides also exist in alloy C-276. The exposure of few minutes at the temperature of 875⁰C can precipitate the P phase in alloy C-276. It is considered as the P phase transformed in to the μ phase as a long time result. The topologically-close-packed phases also occur in the alloy C-276 weld metal and the alloy can contain both the phases P and μ The Ni based alloys usually have applications in the aqueous environments at temperature more than around 500°C; although, the material can experience the high temperature during heat treatment or welding procedures of parts or vessels where the intergranular attack (IGA) resistance in service can be a significant selection problem. The high temperature phase formation after the welding process or inaccurate heat treatment can result in to the accelerated attack, mainly along the grain boundaries, and to be known as the sensitization.

Welding

Special corrosion consideration can be required to the produced components having welds and the weld metal can repeatedly be a limiting element as compared to the base metal with the concern of resistance to rigorously corrosive environments. The welded sample in the as-welded condition and surface finish were tested. The data is taken from the literature [1]. The maximum depth of attack in the base metal or weld metal is shown in the assessment of welds which were made on solid alloy sheet (around the thickness of 0.062″ thickness or plate around 0.250″ thick).

Thermal Stability of Welds

The intergranular corrosion of sensitized base metals or weld can be resulted in the highly oxidizing environment. In table 1, in the ASTM G-28A, only alloy C-276 showed the confirmation of attack of heat affected zone in these as-welded samples. The in general higher corrosion rate for the alloy C-276 in this result was usual, and is because of the moderately low content of chromium in the alloy. In the ASTM G-28B test, serious attack of intergranular weld has observed in regions in the alloy C-276 when welded with matching filler metals.