Author: RAS Thermal Applications Team · Updated 2026-10-10
TL;DR Every grade looks competitive at 20 °C. Read the 600 °C and 700 °C columns before the ambient one.
One of the most common errors in high-temperature bolt specification is selecting a material on its room-temperature strength rather than its strength at operating temperature. A-286 looks competitive with Inconel 718 at 20 °C; by 650 °C the gap has widened dramatically, and above 700 °C only the strongest grades remain viable.
The table below gives tensile strength at five temperatures for each of the six grades we stock, with the metallurgical reason each curve turns down where it does.
| Grade | 20 °C | 400 °C | 600 °C | 700 °C | 815 °C | Why the curve turns |
|---|---|---|---|---|---|---|
| A-286 (Alloy 660) | 1000 | 910 | 700 | 420 | 250 | Sharp drop above 650 °C as the gamma-prime Ni3(Al,Ti) over-ages. Not recommended for load-bearing service above 650 °C. |
| Inconel 718 (UNS N07718) | 1275 | 1160 | 1080 | 900 | 550 | Highest room-temperature strength. Gamma-double-prime begins transforming to the stable delta phase above 650 °C, but the grade retains useful load to 700 °C. |
| Nimonic 80A (UNS N07080) | 1100 | 1000 | 930 | 780 | 400 | Thermally stable gamma-prime to 815 °C. Lower peak strength, but the best high-temperature retention of the six grades. |
| Inconel 625 (UNS N06625) | 930 | 810 | 700 | 550 | 420 | Solid-solution alloy. Modest strength with no ageing degradation, which suits intermediate temperatures combined with corrosion. |
| Hastelloy C-276 (UNS N10276) | 790 | 660 | 600 | 550 | 450 | Low strength; select it for corrosion, not load. Intermetallic precipitation above 425 °C limits long-term high-temperature use. |
| Monel K-500 (UNS N05500) | 1100 | 900 | 750 | 600 | 350 | Good strength to 300 °C, then gamma-prime over-ageing sets in. Select it for seawater and marine environments rather than for temperature. |
All figures are tensile strength in MPa. Yield strength at the same temperatures is tabulated on the temperature-strength curves page.
How to work through this topic
- 01 Convert the design temperature into a retained strength. Read tensile and yield at the design metal temperature for every candidate grade. A-286 reads 1000 MPa at 20 °C and 700 MPa at 600 °C, and every alloy on the rack flatters itself the same way on a cold datasheet.
- 02 Size preload against yield at temperature. Use the yield figure at the operating temperature, not at ambient, and keep the gap up to tensile in reserve. A 15% torque overshoot on a preload sized to ambient yield can land above yield once the joint is hot.
- 03 Check where the curve turns down. Each grade has a temperature where its strengthening mechanism stops working: gamma-prime over-ageing for A-286 above 650 °C, the gamma-double-prime to delta transformation for Inconel 718 above 650 °C, and gamma-prime over-ageing for Monel K-500 above 300 °C.
- 04 Confirm the condition and the certificate. Retained strength depends on the heat-treat condition, so name it on the line item along with the standard, the UNS number and the EN 10204 certificate grade.
Frequently Asked Questions
Why does A-286 lose strength so sharply above 650 °C?
Its gamma-prime Ni3(Al,Ti) precipitate over-ages, so the strengthening phase coarsens and stops blocking dislocation motion. Tensile falls from 700 MPa at 600 °C to 420 MPa at 700 °C, which is why A-286 is not recommended for load-bearing service above 650 °C.
Which grade retains the most strength at 815 °C?
Inconel 718 still reads 550 MPa tensile at 815 °C and Nimonic 80A reads 400 MPa, against 250 MPa for A-286 and 350 MPa for Monel K-500. Nimonic 80A has the best high-temperature retention of the six even though its room-temperature strength is lower than Inconel 718.
Can strength retention be read off a room-temperature datasheet?
No. A-286 looks competitive with Inconel 718 at 20 °C at 1000 MPa against 1275 MPa, but the gap widens dramatically by 650 °C. Always read tensile and yield at the design temperature, never at ambient.
Does Hastelloy C-276 hold its strength at temperature?
It is a low-strength grade selected for corrosion rather than load: 790 MPa at 20 °C falling to 450 MPa at 815 °C. Intermetallic precipitation above 425 °C limits long-term high-temperature use.
Bottom line: Ambient strength ranks the grades wrongly for hot service. Between 20 °C and 815 °C the ordering changes three times, so the only safe comparison is at the design metal temperature.