Quick answer: When ordering Inconel 718 round bar for machining, do not specify only the finished-part diameter. The RFQ should separate the finished dimensions from the raw-bar dimensions and state the required machining allowance, diameter tolerance, straightness, length allowance, surface condition and delivery condition. These details allow the supplier to review whether hot-worked, peeled, turned, ground or another bar condition is appropriate before quotation.
This guide explains the information buyers should send when sourcing Inconel 718 round bar for shafts, fasteners, rings, valve parts and other machined components. It does not prescribe one universal allowance: the correct value depends on the drawing, starting diameter, bar condition, machining route, heat-treatment sequence and final inspection requirements.
Why finished size and raw bar size must be separated
A drawing normally defines the finished component, while a material quotation defines the supplied bar. Those are not the same dimensional requirement. A bar may need enough extra material for scale removal, surface cleanup, straightening effects, workholding, saw-cut cleanup, heat-treatment movement and final grinding.
If a buyer requests a finished shaft diameter as the bar diameter without stating an allowance, the supplier cannot know whether the quoted material leaves enough stock for the planned operations. Conversely, an unnecessarily large starting diameter increases material removal, machining time and scrap. The purchasing document should therefore show both the target part size and the requested raw-stock size or minimum machining envelope.
What ASTM B637 defines—and what the RFQ must still define
ASTM B637 covers precipitation-hardening and cold-worked nickel-alloy rod, bar, forgings and forging stock for moderate- or high-temperature service, including UNS N07718. Its published scope addresses material identity, processing and required testing for the covered product forms. It does not replace the buyer's component drawing or automatically select a commercial surface finish and machining allowance for every part.
A complete order should identify the required standard and revision, Inconel 718 / UNS N07718, bar form, delivery condition, dimensions and supplementary inspection requirements. If an aerospace drawing calls for an AMS specification or a customer-controlled material document, that governing requirement must be stated rather than silently replaced with ASTM B637.
Six dimensional items to include in an Inconel 718 bar RFQ
| RFQ item | What the buyer should state | Why it affects the quotation |
|---|---|---|
| Finished component size | Drawing dimensions, critical features and datum scheme | Shows the machining objective but does not by itself define the supplied bar |
| Raw bar diameter | Nominal diameter plus the required tolerance or minimum cleanup diameter | Controls available stock for turning, grinding and surface cleanup |
| Machining allowance | Radial or diametral allowance, clearly labelled, with any minimum after cleanup | Prevents confusion between allowance per side and total diameter difference |
| Length and cut allowance | Supplied length, finished length, end cleanup and saw-cut tolerance | Affects cutting method, yield, handling and final facing operations |
| Straightness | Maximum permitted deviation, measurement span and test method | Important for long shafts, deep-hole work, turning and centreless grinding |
| Surface condition | Hot-worked, peeled, turned, ground, polished or drawing-specific requirement | Changes the starting surface, achievable dimensional control and processing cost |
Do not confuse machining allowance with dimensional tolerance
Machining allowance is intentional extra material reserved for later removal. Dimensional tolerance is the permitted variation in the supplied or finished dimension. They must be written separately. For example, “leave machining stock” is incomplete unless the order states whether the allowance is radial, diametral or tied to a minimum finished cleanup diameter.
The same principle applies to length. A cut bar can be ordered with extra length for facing, workholding or test sampling, but that extra length is not the same as the permitted variation on the supplied cut length. Clear terminology prevents suppliers and machine shops from interpreting one number in different ways.
Delivery condition affects the machining plan
Inconel 718 is precipitation hardenable. Manufacturer reference information notes that material intended for machining, forming or welding is commonly purchased in a mill-annealed or stress-relieved condition, followed by specification-controlled processing as required. This is not a universal instruction for every component: the controlling drawing and material specification determine the acceptable sequence.
The RFQ should say whether the bar must be supplied solution treated, solution treated and aged, or in another explicitly defined condition. It should also identify who is responsible for final heat treatment. If tight final dimensions are required after heat treatment, the manufacturing plan should reserve appropriate stock and inspection stages instead of assuming that the part will remain dimensionally unchanged.
Surface condition should match the next operation
- Hot-worked or forged surface: may be suitable when substantial machining or further forging is planned; state the minimum usable section after cleanup.
- Peeled or turned bar: can remove surface scale and provide a more defined starting diameter for general machining.
- Ground bar: may be requested where tighter starting dimensions, straightness or surface requirements support the downstream operation.
- Cut blanks: should identify cut length, end condition, squareness requirement and any gripping or facing allowance.
These descriptions are purchasing options, not statements of current inventory. The available route, diameter, length, condition and inspection scope must be confirmed for the individual enquiry.
Inspection and documents to define before ordering
Dimensional requirements should be linked to an inspection plan. State which dimensions are to be reported, the measurement temperature or method where relevant, the sampling frequency and whether a full dimensional report is required. For long bars or critical rotating parts, define the straightness measurement rather than requesting “good straightness.”
The purchase order should also identify the required material certificate, heat-number traceability, chemical and mechanical results, heat-treatment records and any ordered nondestructive examination. PMI, hardness or ultrasonic testing must be tied to an agreed method, coverage and acceptance criterion. One satisfactory check does not prove compliance with every requirement in the product specification.
Common RFQ mistakes
- Sending only the finished-part drawing and expecting the supplier to guess the starting diameter.
- Writing “extra material” without stating whether the allowance is per side or on diameter.
- Requesting ASTM B637 but omitting the required revision and delivery condition.
- Using “black,” “bright” or “machined” without a measurable diameter, tolerance or surface requirement.
- Leaving straightness undefined for a long shaft or bar that will be centre-supported during machining.
- Assuming catalogue dimensions, typical properties or example test reports guarantee a specific order.
Copy-ready Inconel 718 round bar RFQ checklist
- Material: Inconel 718 / UNS N07718.
- Governing standard and required revision.
- Delivery and heat-treatment condition.
- Raw bar diameter and diameter tolerance.
- Finished component dimensions or drawing reference.
- Machining allowance, clearly identified as radial or diametral.
- Bar length, cut-length tolerance and end condition.
- Straightness requirement and measurement basis.
- Surface condition and any minimum cleanup diameter.
- Quantity, certificate, traceability, inspection and destination requirements.
Conclusion
An effective Inconel 718 round bar enquiry connects the material specification to the actual machining route. State the finished component size, raw-bar dimensions, machining allowance, tolerance, straightness, surface and delivery condition as separate requirements. This gives the supplier enough information to review feasibility and quote the intended scope without guessing.
Review the Inconel 718 bar product page, then send SASAALLOY your drawing or RFQ with the items above. Availability, manufacturing route, testing and delivery details will be confirmed against the specific enquiry.
Post time: Oct-22-2026