How to Specify Superalloy Bolts: A Procurement Engineer's Checklist

You know what’s missing on most superalloy bolt RFQs? Three things. Heat treat condition. Thread class. Certification level. The first two decide if your bolt holds at 700°C or snaps at room temp. The third decides if your receiving inspector signs off or parks your project for two weeks. I’ve seen this a hundred times in the Wuxi office. Last month, a buyer in Hamburg sent over “Inconel 718 bolts, M16.” No standard, no condition. The supplier quoted AMS 5663 (solution treated only, weaker) at a rock‑bottom price. Had the buyer not caught it, those bolts would’ve gone in and likely leaked. That’s what we’re going to fix here. Make the spec tight, and you’ll get a firm quote inside a day instead of an email chain that eats your calendar.

TL;DR — A complete superalloy bolt RFQ needs six things: (1) grade plus the ASTM/AMS/DIN number, (2) heat treatment condition code, (3) EN 10204 certification level, (4) thread class and dimensional tolerances, (5) realistic quantities with stated lead‑time flexibility, and (6) a sample spec block that leaves nothing to interpretation. If you hand a supplier an incomplete RFQ, they’re pricing the cheapest reading of your ambiguity.

1. Grade and Standard: Give Me the Number

Writing “Inconel 718 bolts” is a conversation starter. It’s not a purchase spec. That same alloy can be sourced to AMS 5662 (solution treated and aged bar), AMS 5663 (solution treated only), ASTM B637 (aged), or API 6ACRA (oilfield fasteners). Each standard sets different chemistry ceilings, different mechanical minimums, different testing frequency. A bolt made to ASTM B637 isn’t the same product as one to AMS 5662, even though both wear the UNS N07718 badge. I’ve seen a heat treated to B637 come in 30‑50 MPa under the AMS 5662 tensile floor and still be “compliant” to its own spec. That’s a trap.

The ASTM number is your baseline bar. The AMS number tightens it for aerospace. The DIN or ISO number adapts it for European pressure equipment. Write all three on the RFQ if your job crosses jurisdictions. And while you’re at it, ask the supplier which revision year they’re certifying to. I once caught a mill still using AMS 5662K from 2005. The current revision is M. That mismatch can change the melt-source requirements and the solution‑anneal temperature window. If they can’t name the revision, they aren’t qualified.

What to specify:

Parameter	Example	Why It Matters
Trade name	Inconel 718	Points to the alloy family
UNS number	UNS N07718	Kills grade confusion across suppliers
Applicable standard	AMS 5662	Defines chemistry, mechanicals, testing
Alternative standard	ASTM B637	Fallback for non‑aerospace supply chain
Product form	Hex head bolt, M12 × 1.75 × 60 mm	Complete dimensional description

Common grades and their governing standards:

• Inconel 718 (UNS N07718): AMS 5662 (solution treated + aged bar), ASTM B637, API 6ACRA
• A286 (UNS S66286): AMS 5731 (solution treated bar), ASTM A453 Grade 660
• Inconel 625 (UNS N06625): AMS 5666, ASTM B446 Grade 1
• Hastelloy C276 (UNS N10276): ASTM B574, ASME SB‑574
• Monel K‑500 (UNS N05500): AMS 4676, QQ‑N‑286

Pro tip: request the supplier’s internal control plan for the standard you’ve picked. A shop running AMS 5662 should be able to hand you their process control document in a working day. If not, walk away.

2. Heat Treatment Condition: The Line Everyone Leaves Blank

This is the spec line that’s missing more often than any other. And it’s the one that turns a superalloy bolt into a very expensive paperweight. Inconel 718 in the solution‑treated‑only condition (AMS 5663) manages about 60% of the tensile strength of the fully aged condition (AMS 5662). That’s not a nuance. It’s the difference between a flange that seals and one that sprays steam at design pressure.

The solution treatment dissolves the strengthening phases back into the matrix. The subsequent aging step precipitates them back out in a controlled size and distribution — gamma‑prime in nickel‑iron‑chromium alloys, gamma‑double‑prime in niobium‑bearing grades like 718. Without that aging step, the bolt never develops the properties that made you pick this alloy in the first place. Suppliers, left to themselves, will ship whatever is easiest and cheapest in the warehouse. Usually that’s solution‑treated stock. So if you don’t write “solution treated + aged per AMS 5662” on the PO, you’ll get the soft stuff.

Here’s a floor insight that saves headaches. For smaller diameter bolts (M12 and below), many workshops cut the threads before final aging. Cheaper tooling, less wear. But aging the bolt afterward can cause a little distortion, so they chase the threads with a die. In Inconel 718, that re‑chasing creates microscopic tears at the thread root. Later, under tension at 600°C, those tears propagate. I’ve seen M10 studs snap at 70% of design load because of this. So for critical joints, specify that threads must be rolled after final heat treatment, or at least insist that chasing is followed by a fluorescent penetrant inspection of every thread. That’s the kind of detail that only comes from watching failures on the test bench.

If you’re ordering large‑diameter bolts (M36 and up), tensile testing a full‑size bolt is a pain. Many mills will test a machined specimen from a prolongation. Fine. But demand that they also proof‑load test every single bolt in the lot. That’s the cheapest insurance you can write onto the PO.

3. Certification Level: 3.1 or 3.2? Don’t Guess This After the Order

EN 10204 certification is Europe’s language for inspection documents. Type 3.1 means the manufacturer’