1. What Determines the Strength of a Ni-Mo-Cr Corrosion-Resistant Alloy?
The C-type family is not a single material but a group of nickel-based grades in which molybdenum and chromium are the principal additions. Molybdenum provides resistance to pitting and crevice attack and contributes strong solid-solution hardening; chromium provides oxidising resistance and a protective surface oxide. A nickel-chromium-iron-molybdenum grade trades some of that corrosion resistance for good oxidation resistance and fabricability at very high temperature, while a nickel-molybdenum grade without chromium is reserved for hydrochloric acid service.
Because these grades are not age-hardenable, their strength comes from solid-solution hardening plus carbides formed with the small carbon addition - not from a precipitating intermetallic phase. That difference explains both their lower room-temperature yield strength compared with gamma-prime superalloys and their outstanding ability to keep a large fraction of that strength at elevated temperature.
2. Typical Strength Values by Grade
| Grade | Yield strength at room temperature, MPa | Tensile strength at room temperature, MPa | Yield strength at 800 °C, MPa | Tensile strength at 800 °C, MPa |
|---|---|---|---|---|
| C276 (Ni-Mo-Cr) | 275–310 | 690–730 | About 120 | About 280 |
| C22 (Ni-Cr-Mo) | 280–320 | 700–750 | About 130 | About 300 |
| X (Ni-Cr-Fe-Mo) | 250–280 | 650–690 | About 180 | About 350 |
| N (Ni-Mo-Cr) | 240–270 | 620–660 | About 150 | About 320 |
The figures are typical annealed values for the stated temperatures and will vary with product form, section thickness and test direction, so they should be treated as a design guide rather than a guaranteed minimum. The pattern they show is consistent: roughly a third of room-temperature yield strength is still available at 800 °C in the C-type grades, and close to three-quarters in the oxidation-resistant nickel-chromium-iron-molybdenum grade.
3. Temperature Limits: Short-Term and Continuous Service
| Grade | Short-term maximum, oxidising atmosphere | Recommended continuous range | Basis of the limit |
|---|---|---|---|
| C276 | About 1093 °C | 650–815 °C | Oxidation of chromium and molybdenum above the short-term maximum |
| C22 | About 1093 °C | 650–815 °C | Same oxidation limit; better pitting resistance in mixed acids |
| X | About 1260 °C | 870–1093 °C | Optimised for oxidation resistance and creep in combustion environments |
| N | About 1150 °C | Set by molten-salt service conditions | Designed for molten fluoride salt environments |
| Nickel-molybdenum B-type | Not applicable in oxidising service | 600–750 °C | Limited by acid reactivity rather than by melting |
Melting ranges for the family lie between about 1290 and 1450 °C; the nickel-molybdenum-chromium C276 grade melts at roughly 1325 °C and the oxidation-resistant nickel-chromium-iron-molybdenum grade at roughly 1370 °C. Long-term limits are deliberately set well below these figures to avoid creep and the precipitation of brittle intermetallic phases such as mu phase, which reduces ductility.
4. Ductility, Toughness and Strength Retention in Corrosive Media
High strength is only useful if it does not come at the expense of toughness, and these grades are notably ductile. Elongation at break is typically 40–60 % at room temperature, and the C276 grade records Charpy impact energy above 100 J even at −196 °C, so brittle failure is not a concern in low-temperature or dynamic-load service.
The more distinctive property is that mechanical strength is retained while corrosion resistance is working. A C276 component retains more than 90 % of its room-temperature strength after long exposure to 5 % sulphuric acid at 100 °C, and resists pitting, crevice corrosion and stress corrosion cracking in chloride-rich media without losing load-bearing capacity. This is not a coincidence: the molybdenum, chromium and tungsten additions that resist corrosion also provide the solid-solution strengthening that carries the load.
5. Typical Industrial Applications
Chemical processing: reactors and vessels for sulphuric, hydrochloric and acetic acid service, chlorine handling, pipework, valves and heat exchangers.
Aerospace and aviation: combustion chambers, afterburners and turbine components that must endure exhaust gas temperatures above 1200 °C with cyclic thermal loading.
Energy: molten-salt reactor internals, downhole and wellhead components for sour and high-pressure-high-temperature wells, and waste-to-energy furnace liners and flue-gas ducts exposed to acidic gases at 800–900 °C.
Pharmaceutical and medical: reaction and mixing vessels for drug synthesis, plus small components in high-temperature sterilisation autoclaves.
Metallurgical processing: crucibles and thermocouple sheaths for melting high-purity metals, where molten-metal attack and 1200 °C service coincide.
Pollution control: scrubber internals that handle alkaline or acidic washing solutions.
6. Frequently Asked Questions
Q: Is high strength maintained at 800 °C?
Partially. The C-type grades retain roughly 120–130 MPa yield strength at 800 °C against 275–320 MPa at room temperature, while the oxidation-resistant nickel-chromium-iron-molybdenum grade retains about 180 MPa. That is far better retention than austenitic stainless steels achieve at the same temperature.
Q: Which grade is the strongest at high temperature?
Among the four families listed, the nickel-chromium-iron-molybdenum grade has the highest retained strength at 800 °C, at roughly 350 MPa tensile against 650–690 MPa at room temperature. It is the usual choice for combustion-zone components, while the C-type grades are preferred where pitting and crevice corrosion dominate.
Q: What limits the long-term service temperature?
Creep and microstructural ageing, not melting. Continuous exposure above roughly 815 °C in the C-type grades can precipitate brittle intermetallic phases, which reduces ductility even though the alloy remains sound.
Q: Are these alloys ductile at cryogenic temperature?
Yes. The C276 grade records Charpy impact energy above 100 J at −196 °C, and room-temperature elongation is typically 40–60 %, so it is suitable for both cryogenic and dynamic-load duty.
Q: Do the grades keep their strength in acid service?
The C276 grade retains over 90 % of its room-temperature strength after prolonged exposure to 5 % sulphuric acid at 100 °C. The corrosion-resistant alloying elements do double duty, providing both the passive surface and the solid-solution strengthening.
Q: Why is a nickel-molybdenum grade limited to acid service?
Without chromium it cannot form a protective oxide, so it performs poorly in oxidising or high-temperature oxidising atmospheres. Its advantage is hydrochloric acid resistance, and its service range of roughly 600–750 °C is set by acid reactivity rather than by the melting point of the alloy.
