Key takeaways
- ISO 683-2 supplies 42CrMo4 +QT mechanical values by ruling-section band; SAE J404 supplies SAE 4140 chemistry, not a universal delivered-bar property range.
- The same 42CrMo4 grade has different +QT yield and tensile requirements in the >16–40 mm and >40–100 mm round ruling-section bands. Never quote the smaller-section row for a larger bar.
- Hardness, tensile, and impact results answer different questions. Fix the method, specimen, location, temperature, lot covered, frequency, and acceptance rule for each ordered test.
- A certificate type controls document form, not the missing technical requirements. List every actual result and traceability link the purchaser needs before release.
How to specify 42CrMo4 / 4140 mechanical properties
A complete RFQ identifies the material designation, product form, governing standard and edition, delivery condition, heat-treatment section, and acceptance evidence. The grade name identifies chemistry; the product standard and order set the delivered properties.
The first RFQ decision is not the tensile number; it is the route that owns that number. 42CrMo4 is covered by ISO 683-2 for specified alloy-steel product forms and conditions. SAE 4140 is a chemical designation in SAE J404.
ASTM A434/A434M can be invoked as a quenched-and-tempered alloy-bar product specification with strength classes and size controls, but 'ASTM 4140' does not identify a complete order. If a drawing cites EN 10083-3, retain the controlled edition stated by the purchaser or submit a clear standards-migration proposal to the design engineer; do not replace it silently.
Write the specification hierarchy in the inquiry: accepted purchase order and deviations; drawing and project material schedule; product/material standard and edition; test methods; inspection plan; then supplier procedures. Equivalent-grade tables may support sourcing research, but they cannot authorize substitution. A SAE 4140 offer against 42CrMo4 must state the proposed chemistry route, product specification, QT class or agreed property values, size limits, sampling, surface, inspection, certificate, and every variance.
Define the material state represented by acceptance. A bar may be hot rolled, forged, annealed, normalized, quenched and tempered, peeled, turned, ground, or rough machined. Thermal condition and surface route are separate fields. If the buyer will reforge or re-heat-treat after delivery, the incoming QT certificate does not guarantee the final component. If the supplier owns final QT, its qualification and production evidence must represent the ordered section and lot.
Start from the drawing's load case and failure consequence, but do not ask the bar supplier to infer design values. Procurement should obtain the required property schedule from engineering, then ask suppliers whether that schedule is achievable within the named standard route. Values on a website or historical certificate remain context until incorporated into accepted order language.
Chemistry and section-banded properties
The chemistry ranges explain why 42CrMo4 and SAE 4140 are often reviewed together, but they aren't identical. ISO 42CrMo4 permits a different carbon maximum, manganese band, chromium maximum, molybdenum maximum, and phosphorus/sulfur limits from the cited SAE 4140 row. An actual heat can meet one standard and miss another. Dual certification requires verification against both complete standards and product routes, not a seller's family-name statement.
ISO +QT mechanics are clearly size-sensitive. The smaller round ruling-section row shown below has higher minimum yield and tensile range than the >40–100 mm row. Ruling section is the standard's basis for choosing a mechanical-property band; it isn't necessarily the final shaft's fatigue-critical diameter. Identify the heat-treatment section, purchased size, finished geometry, and which section controls product acceptance.
SAE J404 does not supply one tensile, yield, elongation, impact, or hardness range for all 4140 bars. Pair it with a product specification and property route. ASTM A434/A434M is one possible QT bar route, but its class and applicable size must be named from the purchaser's licensed edition. Do not paste ISO 683-2 values beside SAE J404 chemistry and call the combination ASTM compliance unless engineering clearly defines and approves that hybrid requirement.
| Route / controlling standard | Chemical composition range, mass % | Mechanical-property basis | RFQ control |
|---|---|---|---|
| 42CrMo4 +QT — ISO 683-2:2016, round ruling section d >16–40 mm | C 0.38–0.45; Si 0.10–0.40; Mn 0.60–0.90; Cr 0.90–1.20; Mo 0.15–0.30; P ≤0.025; S ≤0.035. | Rp0.2 ≥750 MPa; Rm 1000–1200 MPa; A ≥11%; Z ≥45%; KV ≥35 J for the stated +QT round band. | State the exact product, +QT condition, ruling section, specimen location/orientation, impact basis, frequency, surface and certificate. |
| 42CrMo4 +QT — ISO 683-2:2016, round ruling section d >40–100 mm | The same ISO 42CrMo4 cast-analysis ranges apply; product-analysis variation and all other licensed clauses still control. | Rp0.2 ≥650 MPa; Rm 900–1100 MPa; A ≥12%; Z ≥50%; KV ≥35 J for the stated +QT round band. | Do not apply the >16–40 mm row. Identify the actual section represented and add core evidence only where the project requires it. |
| SAE 4140 — SAE J404_200901 | C 0.38–0.43; Si 0.15–0.35; Mn 0.75–1.00; Cr 0.80–1.10; Mo 0.15–0.25; P ≤0.035; S ≤0.040. | SAE J404 defines alloy-steel chemistry; it doesn't define the delivered-bar strength, ductility, impact, hardness, size, sampling, or heat-treatment result. | Add the applicable product specification, condition, size-banded property basis, sampling, dimensions, surface, inspection, traceability, and document requirements. |
| SAE 4140 QT bar — ASTM A434/A434M-24 plus SAE J404 chemistry | Use the SAE 4140 chemistry only when the order invokes it and the selected ASTM route accepts that grade/product combination; verify heat and product analysis clauses in both licensed standards. | ASTM A434/A434M provides named QT strength classes with size limits. No class value is established by writing '4140 QT' or 'A434' alone. | State class, applicable size, required properties, test locations/frequency, heat treatment, general requirements, certificate, and any project additions. |
The specimen and the section it represents
A tensile report is interpretable only when the specimen's identity and representation are known. ISO 6892-1 defines the room-temperature tensile method and the properties that can be determined; the product standard and order must define specimen geometry, location, orientation, preparation, test rate option where relevant, number of tests, acceptance, and retest. Ask whether a result comes from the production bar, a prolongation, a sacrificial bar, or a separately heat-treated coupon. Record the heat and heat-treatment lot it represents.
For a large bar, radial location can matter as much as diameter. Near-surface, mid-radius, and center material experience different cooling histories. If the design requires center performance, the drawing or inspection plan should state a section-relevant verification route. A surface hardness check or small coupon cannot stand in for full-bar core properties. Destructive core testing on every lot may also be unnecessary unless its purpose, specimen location, and effect on acceptance are clear.
Impact requirements need their own complete clause. ISO 148-1 defines the Charpy pendulum impact test method, but the product/order basis must still set specimen size, V-notch configuration, orientation, location, test temperature, number of specimens, minimum/average rule, and retest disposition. The ISO 683-2 values shown in the table must remain attached to their standard impact conditions. Do not compare a room-temperature result, a subsize specimen, and a low-temperature project requirement as if they were the same evidence.
Sampling frequency should reflect the process lot. Define a lot by heat, furnace charge or continuous-furnace batch, diameter, condition, and relevant process variables. If mixed heats or heat-treatment batches share a certificate, the buyer cannot tell what a failing or passing result represents. Require controlled notification when the supplier changes the qualified section, furnace route, quench medium, processing site, or specimen representation where the accepted order makes that change material.
Hardness as a defined control
Hardness may support process consistency, incoming machining control, or a project acceptance requirement, but it isn't a stand-in for tensile strength, toughness, fatigue, or core response. State the method and scale, target or range, surface preparation, indentation location, distance from ends, number and spacing of readings, product state, frequency, and treatment of individual versus averaged values. If a traverse is required, define the section and radial points. Do not rely on an unapproved scale conversion for acceptance.
Coordinate hardness with the process sequence. A reading taken before final tempering, grinding, stress relief, straightening, induction hardening, or coating preparation may not describe the delivered or finished state. Identify which operation establishes acceptance and which later operations can alter or obscure the result. If the buyer performs final heat treatment, incoming hardness should be labeled as an incoming control rather than a finished-part guarantee.
ISO 10474 defines inspection-document types supplied in accordance with the order. The order must identify the tests. Specify the certificate contents: grade and standard edition, heat analysis, product form, delivery condition, heat, and heat-treatment lot. Include ordered tensile, impact, and hardness results, with specimen identity and location. Record dimensions, straightness, surface, and marking. Add any separately required NDT report, deviations, and heat-to-bar or heat-to-bundle traceability. [6]
If purchaser or third-party review is required, distinguish document review from witness and hold points. State notice period, records available before the event, who may waive attendance, who releases a nonconformance, and whether shipment waits for written release. A generic 'TPI accepted' line cannot substitute for an executable inspection plan.
An HRC result is easier to investigate when the report preserves the setup. NIST’s Rockwell practice guide describes errors associated with specimen movement, surface condition and equipment checks. Review these measurement controls before attributing an unexpected value to the steel grade or heat treatment. [7]
From property schedule to RFQ
Give bidders one mandatory route and a separate place for alternatives. The mandatory line should fix the grade and product standard, edition, purchased and ruling section, length and quantity, delivery and surface condition, applicable property band, added project values, test representation, frequency, certificate, traceability, dimensions, surface, packing, and destination. The alternative schedule should show every chemistry, condition, property, sampling, inspection, price, and lead-time difference and require written approval before manufacture.
The worked example uses bracketed fields where the drawing owner must make a project-specific decision. It doesn't add a generic hardness range because neither ISO 683-2 nor SAE J404 creates one finished-component target for every 42CrMo4/4140 use. If the buyer chooses ASTM A434/A434M, replace the ISO route with the exact ASTM class, size and test basis from the controlled edition rather than mixing both casually.
At bid review, compare compliance line by line. Check whether each result represents the same state and section, whether excluded tests are visible, and whether deviations are priced rather than buried in notes. Resolve contradictions before PO release and require the supplier acknowledgment to repeat the accepted standard, condition, property route, sampling, certificate, and deviations.
When the mandatory route and allowed alternatives are complete, move the reviewed fields into the RFQ form so bidders receive the same technical and commercial basis.
Item: 42CrMo4 / 4140 alloy-steel round bars for shaft blanks, drawing [number / revision], [quantity]
Required route: 42CrMo4 to ISO 683-2:2016 [or purchaser-controlled adoption]; SAE 4140 is an alternative only when paired with SAE J404_200901 and an approved product/property specification
Size: Ø[ordered diameter] × [length / tolerance]; ruling section [definition]; heat-treatment blank and finished envelope shown on drawing
Delivery condition: +QT, [hot rolled / peeled / rough machined]; supplier to declare furnace-lot and straightening/stress-relief sequence
Mechanical requirements: [applicable size-band Rp0.2, Rm, A, Z and impact basis plus project additions]; no transfer from another section band or standard
Hardness: [method, scale, range if genuinely required, preparation, surface/radial locations, number of readings, frequency and nonconformance rule]
Sampling: [specimen geometry, axial/radial location, orientation, represented heat-treatment lot, tests per lot, impact temperature/averaging, retest rule]
Inspection: Heat analysis, ordered mechanics/hardness, dimensions, straightness, surface and marking; NDT only to the separately stated method, coverage, calibration and acceptance
Certificate: ISO 10474 type [state type] with actual results, specimen identity, heat, process-lot, bar/bundle traceability, deviations and linked reports
Alternatives: Clause-by-clause schedule with technical, price and lead-time effects; written purchaser approval required before manufacture
Packing / destination: [preservation, supports, bundle mass, port, Incoterm]Questions buyers ask
What mechanical properties should buyers state for 42CrMo4 / 4140 round bar?
State one controlling product/material route and edition, ordered and ruling section, delivery and surface condition, applicable strength/ductility/impact basis, any project hardness requirement, specimen location and orientation, frequency, retest rule, inspection document, actual results, and traceability.
Why do diameter and heat-treatment condition matter for 42CrMo4 / 4140 properties?
Cooling rate and resulting section response change with the heat-treatment section, and product standards assign different +QT property rows to different ruling-section bands. Thermal condition also determines which row can apply; peeled or ground wording is a separate surface route and does not replace it.
How should hardness targets be written in a 42CrMo4 / 4140 RFQ?
State the hardness method and scale, range only where engineering requires it, preparation, end and radial locations, number and spacing of readings, delivery state, lot frequency, individual/average rule, retest or rejection action, and actual result required on the certificate.
Can ISO 683-2 mechanical-property values be applied directly to SAE 4140?
No. ISO 683-2 assigns 42CrMo4 requirements within its own product, delivery-condition, ruling-section, specimen, and test framework, while SAE J404 defines SAE 4140 chemistry. If a buyer wants ISO-derived property values with an SAE 4140 chemistry route, the RFQ must state that hybrid requirement explicitly and require engineering and purchaser approval before manufacture.
RFQ checklist
- Drawing, project specification, revision, property schedule, application, load context, and failure consequence
- Required grade, product/material standard, national adoption where relevant, and exact edition
- Ordered diameter, heat-treatment/ruling section, length, quantity, tolerance, and finished geometry relationship
- Thermal delivery condition and separate hot-rolled, forged, peeled, turned, ground, or machined surface route
- Applicable size-banded Rp0.2, Rm, elongation, reduction, and impact basis without cross-standard mixing
- Hardness method, scale, range, preparation, locations, number of readings, frequency, and rejection rule if required
- Tensile specimen geometry, orientation, axial/radial location, section covered and heat-treatment lot
- Impact specimen, notch, orientation, location, test temperature, quantity, individual/average rule, and retest
- Heat-treatment lot definition, production-route qualification, straightening sequence, and controlled-change notification
- Surface quality, decarburization, machining allowance, minimum clean-up size, roughness, and repair limits
- Dimensions, ovality, straightness method/stage, marking, and heat-to-bar or heat-to-bundle identity
- NDT method, edition, coverage, calibration/reference, sensitivity, acceptance, stage, report, and end zones if required
- Inspection-document type, actual ordered results, specimen identity, linked reports, deviations, and release approval
- Separate alternative-route matrix, engineering/customer approver, and no-manufacture-before-approval rule
- Packing supports, corrosion protection, maximum bundle mass, destination, Incoterm, and shipment window
References
- ISO 683-2:2016 Alloy steels for quenching and tempering
- SAE J404_200901 Chemical Compositions of SAE Alloy Steels
- ASTM A434/A434M-24 Quenched and Tempered Alloy Steel Bars
- ISO 6892-1:2019 Metallic materials — Tensile testing at room temperature
- ISO 148-1:2016 Metallic materials — Charpy pendulum impact test
- ISO 10474:2013 Steel and steel products — Inspection documents
- Low, S. R. III — NIST SP 960-5: Rockwell Hardness Measurement of Metallic Materials
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