How to specify hardness and mechanical properties in steel RFQs

Separate design requirements, product acceptance, and informational checks

Before writing a value, decide what decision it controls. A design minimum may come from the component calculation; a product-standard range may qualify a delivered bar; an incoming check may protect machining; and an informational reading may support investigation without rejecting material. Label each requirement so a valid result answers the purchaser's intended acceptance question.

Start with the material identity and state: grade and product standard, edition, product form, ordered and ruling section, heat-treatment condition, surface route, and downstream processing. Hardness depends on composition, microstructure, section response, preparation, and test method. Tensile properties depend on the represented material, specimen, orientation, and test conditions. Neither should be copied from a generic grade sheet and applied to every diameter or delivery condition.

Use the property source hierarchy. The drawing or project material schedule sets component requirements; the product standard can set values for a named grade, condition, and section; the test-method standard explains how the result is obtained; the inspection plan defines sampling and release; and the accepted PO resolves conflicts and deviations. ISO 6506-1, ISO 6507-1, ISO 6508-1, and ISO 6892-1 are method standards. They do not select a universal acceptance range for the steel order.

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Choose the method by the material, section, and acceptance purpose

Brinell testing uses an indentation large enough to average a wider local area than common microhardness work and is often useful for bulk steel-bar controls, subject to the method's specimen and thickness rules. Vickers can cover a broad force range and supports localized or traverse work when the procedure, force, spacing, surface, and measurement are properly defined. Rockwell provides a direct depth-based scale but requires the correct scale, indenter, force, surface, thickness, support, and spacing. Procurement does not need to restate each method clause; it does need to name the controlled method and the product-specific choices left to the order.

Do not write HB, HV, or HRC as decoration. A complete Brinell result includes the accepted HBW designation basis; a Vickers requirement needs the HV test-force basis and any dwell variation required by the procedure; a Rockwell requirement needs the scale, such as HRC only where appropriate. The test laboratory should apply the current controlled method, while the RFQ defines material state, location, target, sample extent, and reporting.

Tensile testing provides properties that hardness cannot directly replace: yield or proof strength, tensile strength, elongation, and often reduction of area when the product route requires it. ISO 6892-1 defines room-temperature tensile testing. ASTM A370 supplies test methods and definitions for mechanical testing of steel products in ASTM routes. Choose the method required by the controlling product specification; do not ask a supplier to certify simultaneously to both merely because both are familiar.

Use the matrix below to make quotations comparable. It intentionally contains no generic steel hardness range or conversion table. Acceptance values must come from the controlling grade/condition/section requirement or the purchaser's approved schedule.

The methods above do not establish universal steel acceptance values. Verify current controlled editions, referenced product-standard clauses, equipment and specimen suitability, scale/force designation, preparation, locations, frequency, uncertainty and rounding treatment, retest, and document requirements. Only accepted order terms define the requirements for a specific supply.

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Fix surface preparation, location, curvature, and section representation

A hardness reading is local. Scale, decarburization, grinding damage, surface roughness, curvature, insufficient thickness, poor support, nearby impressions, temperature, and preparation can bias or invalidate it. The test-method standard controls technical suitability, while the purchase specification should identify the acceptance surface and preparation. State whether scale is removed, how much stock may be removed, whether a curved-surface correction or approved fixture is needed, and whether the tested surface remains on the final product.

For bars, define axial and radial location. Routine production checks might be taken at an agreed end distance and surface preparation, while qualification may require a transverse slice with surface, mid-radius, and center points. If a hardness traverse controls case depth or through-hardening response, provide a sketch, origin, spacing, force/scale, edge exclusions, number of traverses, target curve or points, and treatment of a single outlier. Do not allow a supplier to choose the easiest location after seeing the result.

Tensile specimen representation needs the same discipline. Link specimen to heat, product, diameter, heat-treatment lot, bar or prolongation, orientation, and radial/axial location. State whether it is full-size or machined and which standard/product clause controls. If a large production section is critical, engineering should specify the required core or section evidence rather than inferring it from a surface hardness-to-tensile conversion.

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Define the lot, frequency, rounding, retest, and document decision path

Sampling language should connect test frequency to the process that can vary. Define a lot using heat, diameter or section, delivery condition, and heat-treatment batch or continuous-furnace campaign as appropriate to the product route. 'One test per shipment' can conceal multiple heats and thermal lots. Conversely, testing every bar may add cost without improving a decision if the property is qualified on a properly controlled batch. Ask quality and engineering to choose the risk-based unit before soliciting bids.

Write how results are judged. State whether every reading must fall within a range, whether an average is allowed, how results are rounded under the controlling method or product standard, and what happens after one nonconforming result. Retest, resampling, re-heat-treatment, sorting, concession, rejection, and reporting are different dispositions. Do not let a supplier invent the rule after testing. Any concession should identify affected heat/lot/bar identity and receive the required purchaser or customer approval.

ISO 10474 defines inspection-document types, but the PO must list content. Require actual values rather than 'pass' where the result matters: method and scale; specimen or indentation location; material condition; heat and lot identity; individual readings where required; averages only if authorized; tensile specimen data and results; equipment or laboratory evidence when specified; deviations and retests; and links to separate reports. Make bundle or piece marking reconcile with the certificate and packing list.

Document pre-review is not automatically release. If the buyer needs a hold point, state the submission deadline, reviewer, allowed response time, closure of comments and nonconformances, and written shipment authority. For third-party inspection, define witness points and notice. These controls prevent a technically detailed certificate from arriving only after the material is already in transit.

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Write one worked RFQ line and force every bidder onto the same basis

Use a single mandatory property schedule and ask suppliers to identify comply, deviation, or not applicable for every line. The quotation should state the exact test-method edition, proposed sample plan, material state, location, included reports, subcontracted testing, exclusions, and any price or lead-time effect. If a supplier proposes another hardness scale, sampling frequency, or certificate route, require the technical comparison and written purchaser approval before manufacture.

The sample below keeps targets in brackets because the correct values must come from the approved product standard, drawing, or project schedule. It is safer to leave a visible engineering input open during RFQ preparation than to paste a typical web value into a purchase order. Once approved, remove the brackets and freeze the revision and deviation status.

At bid review, reject ambiguous phrases such as 'standard hardness,' 'high strength,' 'as per mill practice,' or 'MTC included' unless the offer maps them to the requested method, value, lot, sampling and evidence. Check that the certificate will identify the same heat, condition and section as the shipment. Only accepted order terms define the requirements for a specific supply.

Item: Heat-treated alloy-steel round bars for [component / drawing revision], Ø[diameter] × [length], [quantity]
Material: [grade] to [product/material standard and edition]; no unapproved equivalent designation or cross-standard property substitution
Delivery condition: [thermal condition] and [hot rolled / peeled / ground / machined surface]; ruling section [definition]; supplier to declare heat-treatment and straightening sequence
Hardness: [HBW / HV force / HR scale] [approved range], method [ISO 6506-1:2014 / ISO 6507-1:2023 / ISO 6508-1:2023], preparation [state], locations [sketch], [readings] per [heat-treatment lot], individual/average and retest rule [state]
Tensile: [Rp0.2, Rm, A, Z as applicable] to [ISO 6892-1:2019 or ASTM A370-26 only as required by the controlling product route]; specimen geometry, orientation, radial/axial location and represented section/lot per attached schedule
Sampling: Lot = [heat + diameter + condition + furnace batch]; [frequency]; no pooling of unlinked heats or thermal lots; nonconformance and retest disposition per [clause]
Inspection: Dimensions, straightness, surface and marking plus the ordered hardness/mechanical tests; third-party witness or hold points [state if required]
Certificate: ISO 10474 type [state type] with actual individual/approved average results, methods, specimen/indentation locations, heat, process-lot, bar/bundle traceability, retests, deviations and linked reports
Supplier response: State proposed methods, locations, frequency, included/subcontracted scope, exclusions, price and lead-time effects, and every deviation
Packing / destination: [preservation, supports, bundle mass, port, Incoterm]
Only accepted order terms define the requirements for a specific supply.

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