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Hastelloy alloys have varying amounts of Nickel, Chromium and Molybdenum added to them to create specific capabilities making them acid resistant metals. As well as high strength at temperatures these alloys have exceptional corrosion resistance, so they have become the material of choice for applications in chemical process plants, particularly in the petrochemical industry.
Hastelloy – which alloys are used in industry?
Hastelloy B
Hastelloy B-2 and B-3 alloys are resistant to acid, reducing environments so tend to be selected for piping and valves where Hydrochloric and Sulphuric acids are chemicals that are used in the process plant. The acids can be handled in liquid or gaseous form.
According to Titan Metal Fabricators Hastelloy B-3 has the “Same excellent resistance to hydrochloric acid and other strongly reducing chemicals as B-2 alloy, but with significantly better thermal stability, fabricability and stress corrosion cracking resistance.”
In a further advancement Hybrid Hastelloy BC-1 is a new “Ni-Cr-Mo alloy with superior resistance to hydrochloric and sulfuric acids, without the drawbacks of the Ni-Mo materials. Its resistance to these key chemicals is much greater than that of the popular Ni-Cr-Mo (C-type Hastelloy) alloys”.
Hastelloy C-4
The C-4 alloy has “High-temperature stability in the 1200-1900°F (650-1040°C) range as evidenced by good ductility and corrosion resistance. Virtually the same corrosion resistance as alloy C-276.”
Hastelloy C-22
C-22 has better overall corrosion resistance to oxidizing corrosives than C-4, C-276 and 625 alloys. This alloy is also considered to be the best choice as a universal weld filler metal to resist corrosion at any weld.
Hastelloy C-276
C-276 is a versatile, corrosion-resistant alloy, which makes it a frequently selected fabrication material in the petrochemical and other industries, as all common methods of welding can be used due to the low carbon and silicon content. It has exceptional stress corrosion cracking resistance with high resistance to localized sulfide attack.
Hastelloy’s main advantage over the other nickel superalloys such as Inconel and Incoloy, which are also corrosion-resistant, is that Hastelloy is easier to weld. Therefore, any process plant construction project involving heat exchangers and pressure vessels where stress cracking in a corrosive environment could occur is most likely to select Hastelloy material, particularly Hastelloy C-276.
Hastelloy welding
Nickel and its alloys are like austenitic stainless steels so welding procedures are likewise also similar. Nickel, however, has a coefficient of thermal expansion less than that of stainless steel so distortion and distortion control measures are similar to those used for carbon steel.
Nickel-molybdenum alloys (Hastelloy range) are identified with the prefix B eg B1, B2, C, etc.. Alloy B2 is the most frequently encountered alloy but Hastelloy C-276 is frequently used in fabrication, all with matching filler metals when welded. Recently the C-22 alloy has been more commonly used as the weld filler material for all welding applications.
Hastelloy is usually welded using the Gas Metal Arc (GMA/MIG), Gas Tungsten Arc (GTA/TIG), and Shielded Metal Arc (SMA/Stick) welding processes. Matching filler metals (i.e., solid wires and coated electrodes) are available.
Additionally, for welding preparation, such as beveling, leveling of welds, opening weld roots, and more, you can TFT Milling Discs. These discs are ideal to work with Hastelloy and other alloys and are a better alternative to traditional abrasive wheel grinding and cutting. Their main characteristics include No Kickback, their lifespan extends into the hundreds of hours per unit.
Machining, Cutting and Grinding of Hastelloy
One of the features of working with nickel-based alloys is that work hardening due to temperature increases can occur when tools used in machining or cutting operations are not able to remove material from the work face efficiently. This can be due to the tool losing sharpness which in machining operations significantly affects machining time and tool replacement costs if turning equipment is not set up correctly regarding speed and rigidity.
In grinding and cutting operations that use hand tools at a worksite, composite discs or ceramic materials are favored so that the tool tip condition is not a concern. The lubricity of the cutting disc material is a benefit in keeping the temperature at the work face as low as possible which reduces work hardening of the alloy. The slower cutting speed that is possible with these advanced materials also helps in keeping the temperature down so that sparks of molten material are not projected far away, which would be a greater fire hazard.
Gaseous environment hazard potential during Petrochemical shutdown or mining maintenance
In the petrochemical industry where the process equipment is handling highly flammable fluids and gases, the risk of explosion should leakage occur makes plant integrity and reducing spark potential even more critical. Nickel-based alloys such as Hastelloy are non-ferrous so are considered to be less hazardous materials than carbon steels from a spark generation point of view however lower energy red sparks are still produced during cutting and grinding operations. The use of Hastelloy for piping and all petrochemical process equipment such as heat exchangers does reduce spark potential during construction and maintenance operations.
During shutdown maintenance where repairs become necessary the selection of non-sparking pneumatic tools is a pre-requisite. Using this type of tool Cold work is possible rather than work being carried out under a hot work permit system which is a big time and consequently cost saving. Companies such as DNV GL have been used as third-party assessors of whether so called cold work tools can be certified for use in gaseous and combustible dust environments. This gives end user’s assurance that risks at the worksite to personnel and plant are as low as reasonably practicable.
Further information on tools that are DNV certified, compliant with ATEX gas zone guidelines and are safe to use in Class 1 Division 1 & 2 classified areas can be obtained at:
