How to Specify Superalloy Bolts: A Procurement Engineer's Checklist
RAS Materials Engineering Team
Fastener & Procurement
Most RFQs for superalloy bolts arrive with three pieces of information missing: the heat treatment condition, the thread class, and the certification level. The first two determine whether the bolt functions at 700 degrees Celsius or fails at ambient. The third determines whether your receiving inspector accepts the shipment or puts your project on hold for two weeks. Here is what they are, why they matter, and how a complete specification gets you a firm quote in 24 hours instead of an email thread that consumes your calendar.
TL;DR — A complete superalloy bolt RFQ requires six items: (1) grade plus ASTM/AMS/DIN standard number, (2) heat treatment condition code, (3) EN 10204 certification level, (4) thread class and dimensional tolerance callouts, (5) realistic quantities with stated lead-time flexibility, and (6) a sample spec block that leaves nothing to interpretation. Suppliers who quote against incomplete RFQs are pricing the cheapest interpretation of your ambiguity.
1. Grade and Standard: Say the Number, Not Just the Name
Specifying “Inconel 718 bolts” is a conversation starter, not a purchase specification. Inconel 718 can be sourced to AMS 5662 (solution treated and aged bar), AMS 5663 (solution treated only), ASTM B637 (aged), and API 6ACRA (oilfield fasteners). Each standard imposes different chemistry windows, mechanical property minima, and testing frequency. A bolt made to ASTM B637 is not the same product as one made to AMS 5662, even when both carry the same UNS N07718 designation.
The ASTM number defines the minimum 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 end-use spans jurisdictions.
What to specify:
| Parameter | Example | Why It Matters |
|---|---|---|
| Trade name | Inconel 718 | Identifies the alloy family |
| UNS number | UNS N07718 | Eliminates grade ambiguity across suppliers |
| Applicable standard | AMS 5662 | Defines chemistry, mechanicals, and testing |
| Alternative standard | ASTM B637 | Fallback for non-aerospace supply chain |
| Product form | Hex head bolt, M12 x 1.75 x 60 mm | Complete dimensional description |
How to verify the standard is actually met: Request the supplier’s internal control plan for the specified standard. A supplier running AMS 5662 should be able to show you their process control document within one working day. If they cannot name the standard revision year they are certifying to, they are not qualified.
The most 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
2. Heat Treatment Condition: The Most-Ignored Spec Line
A superalloy bolt in the wrong heat treatment condition is functionally a different product. Inconel 718 in the solution-treated-only condition (AMS 5663) delivers roughly 60% of the tensile strength of the fully aged condition (AMS 5662). This is not a nuance. It is the difference between a bolted flange that seals and one that leaks at design pressure.
Despite this, heat treatment condition is the specification line most frequently left blank on RFQs. Suppliers default to the cheapest deliverable condition unless you constrain them. Do not let them.
Solution treated vs solution treated + aged:
The solution treatment dissolves strengthening phases back into the matrix. The subsequent aging step precipitates them in a controlled size and distribution — gamma-prime in nickel-iron-chromium alloys, gamma-double-prime in niobium-bearing grades like 718. Without the aging step, the bolt never develops the mechanical properties that justified selecting the alloy in the first place.
What to put on the RFQ:
- Inconel 718: Solution treated 980 degrees Celsius, 1 hour + aged 720 degrees Celsius, 8 hours, furnace cool to 620 degrees Celsius, hold 8 hours, air cool. Or simply: “AMS 5662 condition.”
- A286: Solution treated 980 degrees Celsius, oil quench + aged 720 degrees Celsius, 16 hours, air cool. Or: “AMS 5731 condition, aged per AMS 5737.”
- Monel K-500: Hot-finished + aged 580-590 degrees Celsius, 16 hours, furnace cool or air cool.
A note on hardness: For NACE MR0175 sour-service fasteners, the maximum hardness is a spec requirement, not a nice-to-have. Inconel 718 fasteners for sour service are limited to 40 HRC maximum. Specify “NACE MR0175 compliant, 40 HRC max” as a separate line item on the RFQ.
3. Certification Requirements: EN 10204 3.1 vs 3.2
Every superalloy bolt shipment comes with documentation. The question is whether that documentation is self-certified by the manufacturer or independently verified by a third party.
EN 10204 Type 3.1 (standard): The manufacturer declares compliance based on their own testing. The test results are signed by a representative of the manufacturer’s quality department — someone on the manufacturer’s payroll, independent of production. This is acceptable for the vast majority of industrial applications, and it is what most suppliers quote by default.
EN 10204 Type 3.2 (third-party witnessed): An independent inspector — SGS, Bureau Veritas, TUV, or a customer-nominated agency — witnesses the testing and co-signs the certificate. This adds cost (typically 5-15% of order value) and lead time (1-2 additional weeks). Require 3.2 when:
- The bolts go into a pressure vessel subject to PED 2014/68/EU Category IV
- The end-user contractually mandates third-party inspection
- You are qualifying a new supplier from whom you have never purchased
- The application is safety-critical with no redundancy in the bolted joint
What a complete MTC looks like for superalloy bolts:
Each test certificate should document, at minimum: heat number, heat-specific chemistry from OES or wet analysis, tensile test results (yield, UTS, elongation, reduction of area at room temperature and, if applicable, at the design temperature), hardness values, grain size (ASTM E112), and any supplemental tests your standard requires (stress-rupture for AMS 5662, intergranular corrosion for ASTM A262 Practice E on C276, etc.).
Request a sample MTC before you issue the purchase order. A supplier who hesitates to provide one is telling you something about their quality system.
4. Dimensional Tolerances: Threads, Roots, and Fillets
The metallurgy gets all the attention. The geometry causes most of the failures.
Thread class: For unified threads, specify 2A/2B for general industrial, 3A/3B for aerospace and high-vibration applications. The 3A/3B tolerance band is roughly half the width of 2A/2B. A 3A stud in a 2B nut may not assemble. A 2A stud in a 3B nut will have excessive clearance. Both combinations produce unpredictable preload at a given torque.
For metric threads, the equivalent distinction is tolerance grade 6g/6H (general) versus 4g/5H (precision). Specify the tolerance explicitly. “M12 x 1.75” without a tolerance grade invites the supplier to deliver 6g when your assembly requires 4g.
Thread root radius: Fatigue life in a threaded fastener is dominated by the stress concentration at the thread root. A rolled thread with a controlled root radius (per ASME B1.1 or ISO 965) can outlast a cut thread by a factor of 3-5 in high-cycle fatigue. Specify “rolled threads, radius root per ASME B1.1” for any application above 10,000 cycles.
Under-head fillet radius: The transition from shank to head is the second-highest stress concentration in a bolt, and the one most often overlooked on drawings. A sharp corner at the under-head fillet is a crack initiation site waiting to happen. Specify a minimum fillet radius (or reference the head standard — DIN EN 1665 for hex flange bolts already defines an acceptable fillet). For custom or non-standard heads, dimension the fillet radius explicitly.
Additional geometry to specify for elevated-temperature service:
- Thread engagement length: At least 1.5x the nominal diameter for superalloy bolts in similar-material tapped holes
- Preload method: Torque, torque-plus-angle, or hydraulic tensioning — the supplier needs to know which
- Surface finish: Specify Ra maximum for the bearing surface under the head and in the thread flanks when galling is a concern
5. Quantity and Lead Time: What Is Actually Realistic
Minimum order quantities are real, and they are larger than you think. A mill producing Inconel 718 bar stock does not change over a melt furnace for 50 kg of finished bolts. Typical MOQ realities:
| Product | Typical MOQ | Why |
|---|---|---|
| Standard hex bolts, M6-M20 | 100-500 pieces | Stock material, limited by setup cost |
| Custom/oversize bolts, M24+ | 20-50 pieces | Forged individually, lower MOQ possible |
| Stud bolts with rolled threads | 200 pieces | Thread rolling setup is the cost driver |
| Nuts, washers (mating components) | Match bolt quantity | Always order together from same heat |
The melt-to-bolt timeline: Bar stock production: 2-4 weeks. Forging or heading: 1-2 weeks. Heat treatment: 3-5 days including testing. Thread rolling and finishing: 1-2 weeks. Final inspection and MTC preparation: 3-5 days. Total realistic lead time for new production: 6-10 weeks from order acknowledgment to shipment. Expedited production on stock material can compress this to 3-4 weeks at a premium of 20-30%.
What you can do to shorten lead time: Accept random mill lengths instead of exact cut-to-length. Accept the supplier’s standard heat treatment batch schedule instead of requiring a dedicated furnace run. Accept combined shipments for multi-line orders. Each of these concessions removes a bottleneck from the production schedule.
6. What a Complete RFQ Looks Like
Stop describing bolts in prose. Deliver a spec block. Here is what a complete, quotable RFQ looks like:
Item: Hex Head Bolt
Grade: Inconel 718 / UNS N07718
Standard: AMS 5662 (solution treated + aged)
Condition: Aged per AMS 5662, hardness 36-44 HRC
Dimensions: M12 x 1.75 x 60 mm, thread to within 10 mm of head
Thread Class: 6g per ISO 965, rolled threads, radius root
Head: Hex head per ISO 4014, under-head fillet radius R0.6 min
Surface: As-rolled, free from scale and burrs
Certification: EN 10204 Type 3.1 MTC including:
- Heat-specific chemistry (OES)
- Room-temperature tensile (ASTM E8)
- Hardness (ASTM E18, Rockwell C)
- Grain size (ASTM E112, ASTM 4 or finer)
- Ultrasonic inspection (AMS-STD-2154, Class A)
Quantity: 200 pieces (1 lot, single heat preferred)
Packaging: Individually tagged with heat number, packed in sealed bags
Lead Time: Please quote production plus air freight option
A spec block like this eliminates the interpretation game. Every supplier you send it to is quoting the same bolt. The quotes you receive are comparable on price, lead time, and compliance — which is the entire point of a procurement specification.
One additional note on washers and nuts: If your application uses superalloy bolts, do not pair them with carbon steel nuts. Specify the nut material to match or exceed the bolt material. Specify washers in the same alloy or, for galling-prone pairings (Inconel 718 on Inconel 718), consider a silver-plated or dry-film-lubricated washer. Galling is the leading cause of superalloy fastener assembly failure, and it is almost entirely preventable with material selection at the RFQ stage.
Bottom line: A complete RFQ gets a firm quote in 24 hours. An incomplete one starts an email thread that takes two weeks. The difference is not whether you know the alloy. It is whether you write down the standard number, the heat treatment condition, the thread class, and the certification level. Those four lines are what separate a procurement specification from a request for conversation. Write them down. The supplier will thank you, and your project schedule will thank you.
For a sample MTC, capability statement, or a quotation against a complete specification, contact the RAS Materials fastener team: sales@rasalloy.com
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