Hastelloy C-276: machining the alloy that hardens while you watch
C-276 exists because some chemistry destroys everything else. Roughly 16 % molybdenum, 15.5 % chromium and 3.75 % tungsten in a nickel matrix give it resistance to wet chlorine, hypochlorite, ferric chloride and sulphuric acid that stainless cannot approach.
The same solid-solution strengthening that buys that corrosion resistance makes it one of the most aggressively work-hardening alloys a shop will see.
What makes it different from Inconel
Machinists who have run 625 expect C-276 to behave similarly. It mostly does, with two differences that matter.
It hardens faster. The layer forms sooner and deeper, which means the rules about never dwelling and never taking light finishing passes are not guidelines here — they are the process.
It is gummier. High nickel plus high molybdenum produces long stringy chips that weld to the cutting edge. Chip control is the first problem to solve, not an afterthought.
Annealed C-276 is around 87 HRB, softer than 316 stainless. The hardness is not the problem. The behaviour is.
Starting parameters
Coated carbide, rigid setup, high-pressure coolant.
| Operation | Surface speed | Feed | Depth of cut |
|---|---|---|---|
| Turning, roughing | 18–30 m/min | 0.15–0.25 mm/rev | 2–4 mm |
| Turning, finishing | 25–40 m/min | 0.10–0.18 mm/rev | 0.5–1 mm |
| Milling, roughing | 18–28 m/min | 0.08–0.15 mm/tooth | 15–25 % radial |
| Milling, finishing | 25–35 m/min | 0.06–0.10 mm/tooth | 0.5–1 mm axial |
| Drilling | 6–12 m/min | 0.05–0.10 mm/rev | peck, never dwell |
Notice the finishing depth of cut. On steel you would finish at 0.2 mm. Here 0.5 mm minimum, because anything lighter rides on the hardened skin instead of getting underneath it. That single number is the most common mistake on this alloy.
Setup rules
Rigidity first. Cutting forces run well above carbon steel and any deflection turns a cut into a rub. Short tools, full-contact workholding, minimum overhang. A setup that produces acceptable parts in stainless will chatter here.
Sharp positive geometry. Ground positive inserts with a polished rake face. Heavy edge preparation designed for steel pushes material rather than shearing it.
Coolant at pressure, aimed at the edge. 70 bar through-tool is the standard in shops that run this material routinely. It breaks the stringy chip and stops the built-up edge that otherwise ruins the finish.
Replace tools on schedule, not on failure. A worn edge in C-276 does not degrade gracefully; it hardens the workpiece, then fails, often taking the surface with it. Tool life here is predictable enough to schedule.
The general mechanism is in work hardening in superalloys.
Where it is used, and where it is over-specified
C-276 turns up in flue gas desulphurisation, chemical process valves and fittings, pulp and paper bleach plant, sour gas wellhead components, pharmaceutical reactors and waste treatment.
It is also over-specified more often than any other alloy on this list. It is significantly more expensive than 625 and dramatically more than super duplex, and plenty of parts carrying a C-276 callout would survive in something cheaper.
The question worth asking before the drawing is frozen: is the governing condition reducing acids and chlorides, or is it general seawater and moderate chemistry? For the second case, 2507 super duplex often does the job at a fraction of the cost — duplex stainless machining covers where the line sits. If the load is mechanical rather than chemical, 625 or 718 is the comparison you want.
On the drawing
- Specify the standard. ASTM B574 for bar, B575 for plate and sheet. Not just “Hastelloy”.
- State the condition. Solution annealed is the normal supply condition and the only one most shops will quote.
- Say what the part sees. Media, concentration, temperature. It lets a supplier tell you whether the alloy is right, which is worth more than a slightly better price on the wrong material.
- Expect material lead time. C-276 is not a stock item in most sizes. Distributor stock exists in common bar and plate; anything else is a mill order. Plan for it — see exotic alloy lead times.
Background: Hastelloy.
Send a C-276 drawing and we will tell you whether the alloy is doing work on your part or just costing money.