What is Hastelloy C-276?
Hastelloy C-276, also known as UNS N10276 or C276, is one of the most common nickel-based corrosion resistant alloys. It is suitable for use in the chemical industry with various oxidizing and reducing media. Its high molybdenum and chromium content allows the alloy to resist corrosion by chloride ions, and the addition of tungsten further increases its corrosion resistance.
Hastelloy C-276 is one of the few materials that can resist corrosion caused by wet solutions of chlorine gas, hypochlorite and chlorine dioxide. It also exhibits significant resistance to highly concentrated chloride saline solutions such as ferric chloride and cupric chloride.
Applications where the Hastelloy C-276 can be used include:
- Pollution control stack liners, pipes, dampers, scrubbers, flue gas reheaters, fans and fan housings.
- Flue gas desulfurization systems.
- Chemical processing components such as heat exchangers, reaction vessels, evaporators and transport tubes.
- Acid gas wells.
- Pulp and paper production.
- Sewage treatment.
- Pharmaceutical and food processing equipment.
Precautions for Welding Hastelloy C-276
There are 9 main considerations to remember when welding Hastelloy C-276 alloy, which can be summarized as follows:
1. Weld preparation and cleaning:
Since the surface of the Hastelloy alloy is likely to contain impurities and oxides, the welding area must be cleaned before welding. Manual cleaning can be done with angle grinders to clean the area until the metal shine is visible.
It is recommended that the cleaning width is at least 100 mm to ensure that impurities do not enter the weld zone.
2. Welding method:
Direct current positive connection is generally used for welding. When using positive DC connection, the tungsten electrode temperature is low, the allowable current is large, and the tungsten electrode loss is small.
The end of the tungsten electrode is ground at 30° and the head is ground slightly flat.
3. Gas shielding:
Measures must be taken to minimize degradation of the corrosion resistance of the weld and heat-affected zone, such as tungsten gas inert gas welding (GTAW), metal inert gas welding (GMAW), submerged arc welding, or other welding methods. welding that can minimize the degradation of the corrosion resistance of the weld and the heat-affected zone.
Special Steel 100 Seconds” suggests that the shielding effect of argon gas is significant: good shielding effects, concentrated heat, good weld quality, small heat-affected zone and small weld distortion, making the weld and the affected zone by heat have minimum resistance to corrosion degradation.
4. Practical training:
Mechanical processing is the preferred method for welding grooves, and cold processing is preferable to ensure that the shape, size and roughness of the processing surface meet the standard requirements or welding process specifications.
Mechanical processing of the pre-welding groove will cause work hardening, so it is necessary to grind the groove before welding. The welding groove must not have defects such as delamination, bending, cracks or tears.
The metal surface of the welding groove and the width of 50 mm on both sides must be polished to remove oxide colors. Solvents such as ethanol, acetone or propanol should be used to clean pollutants such as grease, water and chalk marks.
The solvent brush should be a leather or cellulose sponge that will not come loose during cleaning. Special Steel 100 Seconds” suggests that unnecessary welding materials and harmful substances on workers' clothes and shoes should not come into contact with the workpiece to avoid polluting the workpiece.
5. Selection of welding material:
It is recommended to use ERNiCrMo-4 welding wire and ENiCrMo-4 welding rod. This welding wire has excellent corrosion resistance and process performance. Its chemical composition is similar to that of the base metal and contains more manganese than the base metal.
By welding, it can improve crack resistance and control porosity. Super low carbon content provides protection against intergranular corrosion.
Welding Materials Table
| Base material | American Welding Society (AWS) Standard | |
| SMAW welding electrode | GTAW Welding Wire | |
| C276 and C276 | ENiCrMo-4 | RENiCrMo-4 |
| C276 and carbon steel or low alloy steel | ENiCrMo-3 ENiCrMo-4 |
ERNiCrMo-3 ERNiCrMo-4 |
6. Preheating and temperature between passes
Preheating is generally not necessary when welding Hastelloy alloys at room temperature unless the air temperature is below freezing or moisture has accumulated. If preheating is necessary, the temperature should only be raised to 30-40°C.
During welding, if the weld metal is exposed to high temperatures (375-875°C) for a long period of time, it will form Fe-Cr metallic compounds, known as the sigma phase. The sigma phase is extremely hard and brittle and is distributed along the grain boundaries, causing a decrease in impact toughness and weakening of the weld metal.
When using multilayer welding, the inter-pass temperature must be maintained below 90°C to prevent the formation of the sigma phase due to prolonged exposure to temperatures between 375-875°C.
7. Welding Precautions
To reduce heat input when welding, use the lowest possible welding current and a fast welding method. Furthermore, due to the tendency of Hastelloy alloys to crack early in the arc, the arc crater must be completely filled.
Before restarting the arc, the previous arc crater should be sanded and then cleaned with a soft bristle brush before proceeding with subsequent welding. These two treatment methods can suppress the generation of thermal cracks.
The welded joint is prone to intergranular corrosion, including intergranular corrosion of the weld seam, “knife corrosion” near the fusion line, and intergranular corrosion of the sensitization temperature in the heat-affected zone.
8. Post-welding heat treatment
However, in extremely aggressive environments, C-276 materials and welded parts need to be solution heat treated to achieve the best corrosion resistance.
Solution heat treatment of Hastelloy C-276 alloy material involves two processes, according to “Special Steel 100 Seconds”:
(1) Heating to 1040-1150°C;
(2) Rapidly cooling to a black state (about 400°C) in two minutes.
The material treated in this way has excellent resistance to corrosion. Therefore, performing stress relief heat treatment on Hastelloy C-276 alloy alone is ineffective. Any dirt that may produce carbon elements during the heat treatment process, such as oil stains on the alloy surface, must be cleaned before heat treatment.
During welding or heat treatment of Hastelloy C-276 alloys, oxides may be produced on the surface of the alloy, causing a decrease in the Cr content and affecting its corrosion resistance.
Therefore, the surface must be cleaned. Stainless steel brushes or stainless steel grinding wheels can be used, followed by immersion in a mixed solution of nitric acid and hydrofluoric acid in the appropriate proportion for pickling, and finally rinsed with clean water.
9. Precautions with welding tools
Processing tools must be specific cleaning tools for nickel alloys. These tools should be stored separately and clearly marked to avoid confusion with other tools.
Avoid contact between the workpiece and low-melting metals to avoid embrittlement caused by the increase in carbon or sulfur forming unstable metals. The use of chalk, paint and grease for temperature measurement should be limited during the manufacturing process.
The grinding wheel used to grind parts must not contain iron ions, and the adhesive must not be an organic resin.
For compression welded joints, the same process as the officially qualified welding process must be used for positioning the pre-welding equipment, and the positioning weld must finally merge into the permanent weld. Forced assembly of welded parts must not cause local hardening of the welded parts.
