Two Acid-Resistant Bar Grades at a Glance
UNS N10276 and UNS N06455 are wrought nickel-chromium-molybdenum alloys supplied as round, square, hexagonal and flat bar, mostly in the cold drawn or hot finished and annealed condition. Both are solid-solution strengthened, both resist hydrochloric and sulphuric acid, and both outperform stainless steel in chloride-bearing service. They are nevertheless different alloys, and choosing between them depends on how the bar will be joined, what the final thermal history will be, and whether the corrosive duty is mainly reducing or oxidising.
In short, UNS N10276 carries more molybdenum and tungsten and is the stronger performer in reducing acids and chloride pitting. UNS N06455 is stabilised and kept very low in carbon and iron, which gives it better thermal stability and resistance to grain-boundary attack after welding or high temperature exposure. Neither grade can be substituted blindly for the other in a qualified design.
Chemical Composition Compared
| Element, % | UNS N10276 | UNS N06455 |
|---|---|---|
| Nickel (Ni) | Balance | Balance |
| Chromium (Cr) | 14.5 - 16.5 | 14.0 - 18.0 |
| Molybdenum (Mo) | 15.0 - 17.0 | 14.0 - 17.0 |
| Tungsten (W) | 3.0 - 4.5 | not specified |
| Iron (Fe) | 4.0 - 7.0 | 3.0 max |
| Cobalt (Co) | 2.5 max | 2.0 max |
| Carbon (C) | 0.010 max | 0.009 max |
| Titanium (Ti) | not specified | 0.70 max |
| Silicon (Si) | 0.08 max | 0.08 max |
| Manganese (Mn) | 1.00 max | 1.00 max |
| Vanadium (V) | 0.35 max | not specified |
Two differences drive most of the performance gap. First, the tungsten addition in UNS N10276 adds further resistance to reducing acids and raises the pitting resistance equivalent number, calculated as PREN = Cr + 3.3 x Mo + 16 x N, to roughly 68 for the nominal composition, compared with about 66 for UNS N06455. Second, the titanium addition and the very low carbon of UNS N06455 tie up carbon as stable titanium carbides instead of letting it precipitate as chromium-rich grain-boundary carbides during welding or intermediate temperature service.
Corrosion and Stress-Corrosion Behaviour
Reducing acids: Both grades handle hydrochloric acid and sulphuric acid across a wide concentration and temperature band. UNS N10276 has the wider envelope, particularly in hot concentrated sulphuric acid and in acid streams containing chlorides, because of its higher molybdenum plus tungsten level.
Oxidising and mixed media: Both alloys resist wet chlorine, chlorine dioxide, hypochlorite and nitric acid mixtures. Chromium provides the passive film, and the molybdenum content prevents the film from breaking down in acid-chloride mixtures.
Pitting and crevice corrosion: Both resist chloride pitting and crevice attack in seawater and neutral brine. UNS N10276 keeps the advantage, which matters for fasteners, valve stems and pump shafts where crevices cannot be avoided.
Stress corrosion cracking: Chloride stress corrosion cracking, the common failure mode of austenitic stainless bars in hot chloride solutions, does not occur in normal service for either grade. UNS N10276 is the usual selection where the assembly combines high stress, high temperature and chlorides.
Grain-boundary attack after welding: This is where UNS N06455 is preferred. Its stabilised, low carbon chemistry limits sensitisation, so welded and thermally cycled components keep their corrosion resistance without a solution anneal.
Mechanical Properties and Bar Product Forms
| Property, annealed bar | UNS N10276 | UNS N06455 |
|---|---|---|
| Tensile strength | 690 MPa (100 ksi) min | 690 MPa (100 ksi) min |
| Yield strength, 0.2% offset | 283 MPa (41 ksi) min | 283 MPa (41 ksi) min |
| Elongation in 50 mm | 40% min | 40% min |
| Melting range | approximately 1325 - 1370 °C | approximately 1335 - 1380 °C |
| Typical hardness | about 90 HRB | about 90 HRB |
Both grades are covered by ASTM B574 for rod and bar, with dimensions, straightness, surface finish and permissible deviations defined there and in the general requirements of ASTM B572. Cold drawn bar is produced to closer diameter tolerance and better straightness than hot finished bar, and it is the usual choice for machined shafts, fasteners and valve trim. Bars are normally supplied annealed and pickled or ground; a cold worked condition can be ordered when higher yield strength is needed, at the cost of ductility and corrosion margin.
Machining practice is similar for the two alloys: rigid set-ups, positive rake carbide tooling, low cutting speed and continuous feed to avoid work hardening at the surface. Hot working is performed at 1093 to 1232 °C and finished above about 954 °C, followed by a solution anneal at 1121 °C and rapid cooling. Joining is by gas tungsten arc or shielded metal arc welding with matching nickel-chromium-molybdenum filler; for UNS N06455 the stabilised chemistry means the as-welded condition is normally acceptable without post-weld annealing.
How the Two Grades Are Selected
Choose UNS N10276 for the most aggressive reducing acid duties, for chloride-rich streams where crevice corrosion risk is high, and for heavily loaded components in hot chloride service.
Choose UNS N06455 for welded fabrications, components that receive intermediate temperature exposure, and equipment where a final solution anneal is impractical and resistance to grain-boundary attack must be guaranteed.
Both grades are qualified for sour oil and gas service under NACE MR0175 / ISO 15156, and the selection there usually follows the specified partial pressure limits rather than a general preference.
For both grades, confirm the required condition of supply, whether hot finished, cold drawn or cold worked, because mechanical properties and machinability differ significantly between them.
Frequently Asked Questions
Q: Can UNS N06455 directly replace UNS N10276 in an existing design?
Not without review. The two alloys share similar minimum mechanical properties, but corrosion performance in the most aggressive reducing acids differs. A substitution must be checked against the actual medium, concentration and temperature.
Q: Which grade is better for welded assemblies?
UNS N06455 is generally the safer choice because titanium stabilisation and very low carbon restrict sensitisation in the heat affected zone. UNS N10276 joints are also usable as welded in most media, with a solution anneal reserved for severe duty.
Q: What is the largest bar size normally available?
Cold drawn bar is typically offered up to about 75 mm diameter, while hot finished and annealed bar extends to roughly 200 mm and beyond. Larger sections are usually supplied as forged bar or billet.
Q: Do the two grades need different machining parameters?
Both work harden quickly and are machined with similar settings. The practical rule for either grade is a sharp, positive rake insert, low surface speed, substantial feed per revolution and no dwell in the cut.
Q: How should bar surfaces be protected before fabrication?
Bars are pickled or ground and then kept free of carbon steel contact, moisture and chloride-bearing dust. Embedded iron and marking inks are removed before welding or heat treatment because they cause localised attack and can affect corrosion test results.
Q: Which documents should accompany a bar order?
An order normally calls for a mill test certificate showing heat analysis and mechanical results to the relevant product standard, plus any supplementary requirement such as intergranular corrosion testing or ultrasonic inspection agreed at the time of purchase.
