When a German Tier-1 turbocharger supplier sent us their RFQ last March, all 12 hot-side bolting positions specified Inconel 718. The bill of materials came to €47,000 per production batch. Our application engineer asked one question: “What’s the measured flange temperature at each position?”
The answer changed everything.
The data they hadn’t looked at
The customer had thermocouple data from 18 months of engine dyno testing. Positions 1-3 (turbine inlet flange) consistently measured 660-690°C. Positions 4-9 (bearing housing and compressor backplate) measured 420-550°C. Positions 10-12 (compressor outlet) never exceeded 380°C.
Only three positions, the turbine inlet flange studs, operated above A-286’s 650°C limit. The other nine positions were being over-specified by 100-300°C of temperature margin, at 2-3x the material cost.
The fix: a split bill of materials
We proposed:
- Positions 1-3: Inconel 718 studs (AMS 5662), as originally specified
- Positions 4-9: A-286 studs (AMS 5732), 650°C rated, 60% lower material cost
- Positions 10-12: A-286 hex bolts (ASTM A453 Grade 660 Class D), 380°C max, standard cost
Total BOM: €28,000, a 40% reduction. Same performance. Same certification level (EN 10204 3.1 across all positions). Same lead time.
The catch: don’t blindly copy the split
The split works because this customer’s temperature distribution is uneven. A different turbocharger with a different turbine housing design might have different hot spots. The principle (measure before you specify) applies universally. The specific split does not.
We’ve since applied the same methodology to three other turbocharger programs. The savings range from 25% to 55%, depending on how conservatively the original specification was written. The common thread: nobody had cross-referenced the bolt specification against actual measured service temperatures before.
Frequently Asked Questions
How much of Inconel 718's strength does A-286 deliver?
About 80% of it, from 1000 MPa tensile and 690 MPa yield at AMS 5732 against 1275 and 1030 for Inconel 718 at AMS 5662. That gap only matters above A-286's 650°C ceiling, which is why three of the twelve positions kept the 718 studs.
Which bolting positions actually need Inconel 718?
Only the ones that run above 650°C. On this engine that meant the three turbine inlet flange studs at 660-690°C. The nine positions from the bearing housing back sat at 380-550°C and never needed the extra cost.
What did the split bill of materials save?
The batch fell from €47,000 to €28,000, a 40% cut, with certification held at EN 10204 3.1 across every position and no change to lead time.
Can I copy this split onto another turbocharger?
Not as a template. A different turbine housing puts the hot spots somewhere else, so measure the flange temperature per position first. Across the three programs we have done since, the saving ranged from 25% to 55%.
Does A-286 need a different heat treatment from Inconel 718?
Yes. A-286 is solution treated at 980°C and aged at 720°C for 16 hours, while Inconel 718 double ages at 718°C and 621°C. Threads are rolled after aging on both grades so the compressive stress stays in the thread root.
Bottom line: "Inconel 718 for everything hot" is an expensive habit, not an engineering requirement. Measure your actual flange temperatures. You might be over-specifying 75% of your bolting positions.