Hastelloy C276, also designated UNS N10276 and W.Nr 2.4819, is a nickel-molybdenum-chromium alloy with a tungsten addition and very low carbon. The high molybdenum and tungsten content gives it outstanding resistance to both oxidising and reducing media, and the low carbon content suppresses grain-boundary carbide precipitation after welding. Hot rolled seamless tube is the workhorse product form for reactor internals, heat exchanger shells, scrubber piping and flue gas desulfurisation systems where chlorides and mixed acids are present.
The Hot Rolled Route
Hot rolled seamless tube starts as a pierced hollow that is elongated over a mandrel at temperatures between about 1100 °C and 1230 °C, then sized and straightened. Compared with cold drawn tube, the hot rolled product has a coarser grain structure, slightly wider dimensional tolerances and a heavier minimum wall, but it requires fewer operations and offers lower residual stress. Tube is normally delivered in the hot worked condition, in the annealed condition, or annealed and pickled, and the delivery condition must be stated on the purchase order because it affects both corrosion performance and subsequent forming.
Chemical Composition
| Element | Requirement, % |
|---|---|
| Nickel, Ni | balance |
| Molybdenum, Mo | 15.0-17.0 |
| Chromium, Cr | 14.5-16.5 |
| Iron, Fe | 4.00-7.00 |
| Tungsten, W | 3.00-4.50 |
| Cobalt, Co | 2.50 max |
| Manganese, Mn | 1.00 max |
| Vanadium, V | 0.35 max |
| Silicon, Si | 0.08 max |
| Carbon, C | 0.010 max |
| Sulfur, S | 0.030 max |
| Phosphorus, P | 0.040 max |
The combination of 16 % molybdenum and 4 % tungsten is the reason the alloy resists pitting and crevice corrosion in chloride solutions at much higher temperatures than a molybdenum-free nickel alloy, while the chromium level maintains a passive film in oxidising conditions.
Specification Requirements
Seamless tube of UNS N10276 is ordered to ASTM B622, which covers nickel-chromium-molybdenum alloys in the form of cold worked or hot worked seamless pipe and tube. Annealed product is required to meet a minimum tensile strength of 690 MPa (100 ksi), a minimum 0.2 % offset yield strength of 283 MPa (41 ksi) and a minimum elongation of 40 %. Either a hydrostatic test or a nondestructive electric examination is applied, and the permissible variations in outside diameter and wall thickness are tabulated in the specification. For pressure vessel work, the corresponding ASME specification is used with the allowable stresses tabulated in the code, which fall well below the room-temperature yield strength at elevated design temperatures.
Corrosion Performance
The alloy is resistant to sulfuric, hydrochloric and phosphoric acids over wide concentration and temperature ranges, to wet chlorine and hypochlorite, and to seawater. Its pitting resistance equivalent number is above 60, which explains why it resists crevice attack in aerated chloride solutions where austenitic stainless steels fail quickly. It is not resistant to strong oxidising media that break down the passive film, and it is not suitable for hot concentrated nitric acid. Where the process stream also carries fluorides or high-temperature alkaline salts, the corrosion rate should be established from test data for the specific chemistry rather than from general corrosion tables.
Fabrication, Heat Treatment and Inspection
The alloy is readily welded by gas tungsten arc and shielded metal arc processes using matching filler, and no post-weld heat treatment is needed in most services because the carbon content is low enough to prevent sensitisation. Hot forming should be carried out above about 950 °C and followed by a solution anneal at roughly 1100 to 1170 °C with rapid cooling, because deformation below that range leaves carbides at the grain boundaries. Cold forming requires allowance for high springback. Inspection of the finished tube normally combines dimensional checks, hydrostatic or eddy current testing and, where specified, ultrasonic examination of the wall. Dimensional tolerances on hot rolled tube are wider than on cold drawn tube, so the design should be checked against the actual tolerance class before ordering.
Frequently Asked Questions
Q: What is the difference between hot rolled and cold drawn C276 tube?
A: Hot rolled tube has a coarser grain, wider dimensional tolerance and lower residual stress than cold drawn tube, and it is usually the more economical option for heavy walls and large diameters.
Q: What is the density and melting range of the alloy?
A: Density is about 8.89 g/cm3 and the melting range is approximately 1325 to 1370 °C.
Q: Is post-weld heat treatment required?
A: Generally no. The carbon content of 0.010 % maximum limits carbide precipitation, so welded joints can be placed in service without a solution anneal in most chemical process duties.
Q: What strength can be guaranteed in the annealed condition?
A: ASTM B622 requires a minimum tensile strength of 690 MPa and a minimum yield strength of 283 MPa with at least 40 % elongation for annealed material.
Q: Where should C276 not be used?
A: It should be avoided in strongly oxidising media such as hot concentrated nitric acid, where the passive film is destroyed and corrosion rates rise sharply.
Q: How should the tube be ordered to avoid tolerance disputes?
A: State the outside diameter and wall thickness with the applicable tolerance class, the delivery condition, the test requirements and the length range, since all of these are options within ASTM B622.
