Inconel 718 has no single hardness value that applies to every delivered condition. A useful result identifies the heat-treatment condition, hardness scale and material or part tested. For a bolt order, it must also connect the test to the finished lot and the acceptance requirement.
The selected aged-bar observations below are 45 HRC and 46 HRC. They help explain published material data; the required bolt hardness comes from the specified final condition and acceptance requirement. Compare a handbook value, bar certificate and finished-part report on that basis.
Published 718 hardness values need their conditions
Aging is a controlled heat treatment, not time spent in storage. In 718, solution annealing prepares the material for a later aging treatment that forms strengthening particles within the metal. The two stages therefore produce different material conditions and hardness results. [1, p.2]
Special Metals' Table 8 reports these room-temperature observations in its first 0.625-inch hot-rolled bar group. [1, p.6]
| Reported condition | Hardness | State |
|---|---|---|
| 1750°F, 1-hour anneal | 99 HRB | Annealed |
| 1950°F, 1-hour anneal | 85 HRB | Annealed |
| 1750°F, 1-hour anneal plus associated aging | 46 HRC | Aged |
| 1950°F, 1-hour anneal plus associated aging | 45 HRC | Aged |
Associated aging: after the 1750°F anneal, 1325°F/8 hours, furnace cool to 1150°F, 18 hours total aging time; after the 1950°F anneal, 1400°F/10 hours, furnace cool to 1200°F, 20 hours total aging time. The wider table covers five heats.
Compare each value together with its treatment condition and scale.
HRC, HRB, HBW and HV describe different tests
HRC and HRB are Rockwell scales. HBW identifies a Brinell result; HV identifies a Vickers result. A report should retain the scale and the test conditions needed by the applicable method. A bare entry such as “hardness 350” is incomplete. [5] [6]
When comparing reports, the method and test conditions are part of the result. NIST explains how controlled testing and reference standards make Rockwell results comparable. ASTM E18 covers the Rockwell method; it does not set a finished 718 bolt's required hardness. [2] [4]
Converting a value is a separate decision. ASTM E140 explains that conversions are approximate and depend on the material and its condition. Its nickel-alloy table has a stated material scope; finding “nickel” in a chart heading does not establish that every 718 condition is covered. Agree the applicable conversion basis before using a converted result for acceptance, and retain the original measured value. [3, public scope and significance]
Check where and when the bolt was tested
A room-temperature hardness reading can help check a specified final condition, but the test still needs a suitable location. A small curved shank, a thread flank and a prepared flat are different measurement surfaces.
NIST's Rockwell practice guide discusses surface preparation, specimen support and curvature. Preparation should not change the material by overheating or cold working. Curved-surface corrections have geometry and material limitations; they are not a general correction factor for any bolt. [2, sections 3.2 and 3.4.8]
Before testing, agree a location and preparation compatible with the method and the part's requirements. The inspection record should identify the part or sample, test stage, method and scale, measurements, sampling scope and acceptance criterion. If a representative coupon is permitted, record how it represents the finished lot. A raw-bar result is evidence about that bar at its recorded stage.
The 718 fastener specification guide explains how to carry the final condition and required product tests through manufacture. A hardness result cannot replace a separately required tensile, proof-load or other test merely because it looks consistent with a handbook.
Example: a converted result on a finished-bolt report
Consider a hypothetical order that requires a stated HRC range on finished, aged 718 bolts. The supplier's report gives a Vickers measurement on a heat-treated bar coupon and an HRC value calculated from a chart. The calculated value falls inside the ordered range.
That numerical match leaves two questions open: does the agreed acceptance basis permit this measurement and conversion, and does the coupon represent the finished bolts after the relevant operations? Ask for the original scale and test conditions, the conversion reference and its applicability, and the coupon-to-lot connection. If the requirement calls for direct finished-part HRC testing, agree compliant testing or an approved alternative before release.
This is a document-review example, not evidence that the material is defective. It shows why a matching number alone cannot close an acceptance requirement. The MTC, PMI and inspection-report guide explains the wider evidence chain.
Put the hardness requirement in the order
Start with the specified final condition and finished-fastener requirements. Make the inspection basis easy to check:
- Test object: the finished part or permitted representative specimen, its test location and the lot it represents.
- Test stage: the manufacturing or heat-treatment stage and the condition at measurement.
- Test method: method, scale, preparation and sampling; identify any permitted conversion.
- Acceptance basis: the applicable drawing or specification revision, hardness criterion and approval of any alternative method.
Include the mating parts and service conditions when asking for a material proposal.
If the order is for sour service, keep the selected material route and environmental qualification attached to the hardness requirement. A familiar hardness ceiling alone does not establish conformity; use the sour-service fastener guide for those additional checks.
Review the Inconel 718 material page and 718 hex-bolt enquiry page, or send your drawing or written requirement. Without a drawing or specified hardness range, start with the part description, sizes, quantity and known service conditions. Identify the open questions so the material condition and inspection basis can be agreed before production; do not select an acceptance range from the reference table.
Every BYBOLT delivery includes an MTC / EN 10204 3.1 certificate, heat-number traceability and mechanical testing. The agreed inspection basis identifies what material, specimen or finished part is tested, the lot represented, and the methods, sampling and acceptance criteria. Threaded products include sampled GO/NO-GO inspection. PMI, third-party inspection and additional requirements are agreed separately.