Key takeaways
- Select from shaft geometry and load spectrum, not from nominal tensile strength. Fillets, keyways, splines, threads, oil holes, fits, and unsupported spans often control the failure review.
- ISO 683-2 mechanical requirements decline by ruling-section band. A result for a small specimen or surface location cannot be assigned to the center of a larger shaft without an agreed basis.
- 34CrNiMo6 offers a nickel-chromium-molybdenum route with higher listed strength and impact values in comparable ISO size bands; that is decision evidence, not a universal fatigue or service-life guarantee.
- The RFQ should identify heat-treatment blank size, final geometry, property locations, UT specification, certificate and traceability, alternative approval, and who owns final fitness-for-service acceptance.
Translate the shaft duty into a material decision
Build the comparison from a shaft load map. Record steady torque, reversing torque, bending from gears or overhung loads, axial load, start-stop cycles, overload and jam events, operating and minimum design temperatures, corrosion or fretting interfaces, rotational speed, balance requirement, and the consequence of fracture. Separate nominal stress from local demand at shoulders, keyway ends, spline runouts, cross-drilled holes, retaining-ring grooves, threads, weld repairs, and press fits. A material upgrade does not remove a stress concentration or an unstable support arrangement.
Define three geometries: purchased bar or forging size, heat-treatment blank, and finished part. The heat-treatment blank usually drives cooling distance and core response; the finished geometry drives local stress and inspection access. A hollow shaft, deep bore, or heavily stepped forging may not be represented by the original bar diameter. Give the supplier a drawing or a simplified section map and identify which dimension the property table should use. If engineering has not defined the ruling section, the bidder should not be expected to choose it silently.
42CrMo4 is often the efficient route when its size-banded properties, demonstrated heat treatment, and inspection plan satisfy the design. 34CrNiMo6 becomes a candidate when the required section, strength-toughness window, low-temperature or transient-load evidence, or qualification history calls for its nickel-chromium-molybdenum route. The correct result can also be 'neither as currently specified' if geometry, impact temperature, cleanliness, forging reduction, or verification has not been resolved.
Sources:[1]
Compare ISO chemistry and the same ruling-section bands
The table uses ISO 683-2:2016 as one coherent comparison route. EN 10083-3:2006 contains familiar legacy values but is withdrawn and replaced in the SIS catalog by EN ISO 683-2:2018. If an old project still names EN 10083-3, keep that edition visible and obtain approval before migrating. Do not combine chemistry from one edition with mechanics from another simply because the grade names match.
The mechanical rows shown are for quenched-and-tempered material and selected round ruling-section bands. They are not 'typical mill values,' and they are not guaranteed at every radial position of a purchased bar. ISO limits its mechanical requirements to the tabled sizes and permits special enquiry-and-order agreements. For a 145 mm heat-treatment section, the 100–160 mm row matters; a tensile coupon machined from a convenient smaller qualification piece answers a different question unless the order establishes equivalence.
| Grade and controlling standard | Cast-analysis chemical composition, mass % | Mechanical-property basis | Buyer interpretation |
|---|---|---|---|
| 34CrNiMo6 — ISO 683-2:2016 / EN ISO 683-2:2018 | C 0.30–0.38; Si 0.10–0.40; Mn 0.50–0.80; Cr 1.30–1.70; Ni 1.30–1.70; Mo 0.15–0.30; P ≤0.025; S ≤0.035. | +QT, round ruling section d >40–100 mm: Rp0.2 ≥800 MPa, Rm 1000–1200 MPa, A ≥11%, Z ≥50%, KV ≥45 J; d >100–160 mm: Rp0.2 ≥700 MPa, Rm 900–1100 MPa, A ≥12%, Z ≥55%, KV ≥45 J; d >160–250 mm: Rp0.2 ≥600 MPa, Rm 800–950 MPa, A ≥13%, Z ≥55%, KV ≥45 J. | A higher listed size-band route with nickel addition; still require production-section sampling and heat-treatment evidence that represents the ordered shaft. |
| 42CrMo4 — ISO 683-2:2016 / EN ISO 683-2:2018 | 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. | +QT, round ruling section d >40–100 mm: Rp0.2 ≥650 MPa, Rm 900–1100 MPa, A ≥12%, Z ≥50%, KV ≥35 J; d >100–160 mm: Rp0.2 ≥550 MPa, Rm 800–950 MPa, A ≥13%, Z ≥50%, KV ≥35 J; d >160–250 mm: Rp0.2 ≥500 MPa, Rm 750–900 MPa, A ≥14%, Z ≥55%, KV ≥35 J. | Often the lower-complexity approved route when its relevant band meets the design; do not borrow the 34CrNiMo6 values or a smaller-section row. |
Values are condensed from the named standard for RFQ comparison. Confirm licensed editions, product form, +QT condition, ruling-section definition, specimen orientation/location, impact method and temperature, and all order options. Only accepted order terms define the requirements for a specific supply.
Qualify heat treatment for the production section
Hardenability controls how the microstructural response changes with cooling depth; it is not the same as a hardness value. ISO 642 standardizes the Jominy end-quench method and permits an agreed calculated curve within a defined application field. Jominy evidence can help compare heats or set a band, but it does not reproduce the geometry, quench severity, residual stress, or tempering history of a heavy shaft. Historical NIST work on slack-quenched alloy steels likewise illustrates that hardenability, quench condition, and section response must be considered together rather than reduced to a grade label.
For a critical large shaft, agree a qualification path before production. Options include a representative heat-treatment qualification blank, prolongation material integral with a forging, a test ring located at an agreed axial and radial position, or sacrificial production material. State which piece follows the production furnace and quench load, which tests are taken, and whether the evidence qualifies only the tested heat/lot or a validated process family. Do not let a small coupon receive a faster cooling route than the shaft unless the difference is explicitly accepted.
Ask the heat treater to declare the route used to meet results: furnace uniformity controls, loading and spacing, austenitizing window, transfer timing, quenchant state and agitation, tempering cycle, straightening, and any stress relief. Procurement does not need to prescribe proprietary recipes, but it needs enough route definition to identify a material change. A different heat-treatment subcontractor or substantially different section may require renewed approval even when the grade is unchanged.
Compare total manufacturing and verification risk
Material price is only one line in the shaft-route cost. Compare available melt and casting route, minimum heat or forging quantity, forging reduction and discard plan, heat-treatment batch size, destructive-test allowance, straightening, rough machining, UT access, expected yield from purchased to finished weight, qualification lead time, and the disposition of failed results. 34CrNiMo6 may add alloy and sourcing complexity while reducing technical risk in an approved demanding section; 42CrMo4 may offer a broader, simpler route where its demonstrated property window is sufficient. Quantify the trade rather than treating one grade as insurance.
Define ultrasonic inspection independently of the grade standard. State whether the product is bar or forging, examination standard and edition, manual or mechanized method, scanning surfaces and coverage, reference or calibration basis, acceptance class or written criteria, near-surface and end-zone treatment, test stage before or after rough machining, personnel qualification, report content, and heat/part traceability. A high-alloy grade with undefined UT is not a controlled internal-quality requirement.
The certificate and marking plan should let the buyer connect each test result to the delivered shaft blank after cutting and processing. Require heat number, forging or bar identity where needed, condition, dimensions, actual chemistry, actual mechanical values, test locations, heat-treatment lot, NDT report references, and approved deviations. Plan third-party witness points before work starts; a request made after heat treatment cannot recreate an unobserved hold point.
Issue a shaft RFQ with a defined decision path
The worked wording below asks for 34CrNiMo6 while allowing 42CrMo4 only as a fully described alternative. Reverse the grades when 42CrMo4 is the drawing requirement. The important control is that the bidder does not choose the standard, ruling section, property location, or inspection class on the purchaser's behalf. Replace every bracketed or illustrative field with the approved drawing and specification.
For bid evaluation, use a compliance matrix with engineering, quality, and commercial ownership. Engineering reviews grade, section, properties, heat treatment, and deviation; quality reviews sampling, UT, traceability, and documents; procurement reviews yield, quantity, processing scope, delivery, and price. A technically incomplete low quote should remain incomplete until its missing scope is priced and approved.
At order acceptance, freeze the drawing revision, standard edition, grade, heat-treatment blank, property and NDT clauses, certificate type, approval points, and alternative status. If the supplier's acknowledgment changes any item, resolve it before production. Only accepted order terms define the requirements for a specific supply.
Item: Alloy steel bar/forging stock for heavy-duty gearbox output shaft
Required grade: 34CrNiMo6
Standard and edition: ISO 683-2:2016 / purchaser-required national adoption
Drawing: GS-4821 revision F; supplier to confirm heat-treatment blank and ruling section
Size and quantity: Rough Ø210 mm × 2,850 mm, 12 pieces; machining allowance and forging route per attached specification
Delivery condition: +QT after final forging/rough-machining stage stated in the process plan
Mechanical requirements: Applicable ISO size band plus drawing values; tests at stated axial/radial location and orientation
Inspection: UT to attached standard, class, coverage, calibration, stage and report requirements; dimensions, surface and marking
Certificate: EN 10204 type 3.1 with actual chemistry, heat-treatment lot, mechanical results, sampling sketch and NDT report references
Alternatives: 42CrMo4 only as a separate technical deviation with section-relevant evidence and written purchaser approval
Hold points: Process plan approval before heat treatment; witness points per inspection and test plan
Packing/marking: Heat and piece identity preserved through delivery
Destination: [city, country and delivery term]34CrNiMo6 vs 42CrMo4 for heavy-duty shafts buyer questions
Is 34CrNiMo6 always better than 42CrMo4 for shafts?
No. 34CrNiMo6 has a different chemistry and higher listed values in comparable ISO +QT size bands, but suitability still depends on geometry, load spectrum, temperature, heat-treatment capability, test location, surface and NDT controls, regulatory/customer approval, and total manufacturing risk.
What details help compare 34CrNiMo6 and 42CrMo4 for supplier review?
Send the drawing and load/failure summary, standard edition, required and alternative grades, purchased and heat-treatment geometry, ruling section, +QT property and sampling basis, impact temperature, heat-treatment responsibility, machining allowance, UT specification, certificate/traceability, hold points, quantity, packing, and destination.
When should buyers review 34CrNiMo6 instead of 42CrMo4?
Review 34CrNiMo6 when the approved design and production-section evidence require its higher ISO size-band strength/impact route or established qualification history. Do not upgrade on grade reputation alone; compare a demonstrably capable 42CrMo4 route, the actual failure risks, and lifecycle cost of qualification.
Can a Jominy curve guarantee the center properties of a heavy shaft?
No. Jominy evidence characterizes hardenability under a standardized end-quench or agreed calculation route. Finished-section response also depends on geometry, chemistry, austenitizing, quench conditions, tempering, and sampling. Use representative production-section verification when the center condition is acceptance-critical.
34CrNiMo6 vs 42CrMo4 for heavy-duty shafts RFQ checklist
- Drawing/revision, shaft function, load spectrum, speed, temperatures, stress raisers, and failure consequence
- Required ISO/EN designation and edition; legacy EN disposition and alternative approval authority
- Purchased stock, forging and heat-treatment blank geometry, final geometry, ruling section, length, and quantity
- Melt/forging route, reduction or discard requirements, +QT stage, straightening, stress relief, and rough machining
- Yield/tensile range, elongation, reduction, impact method/temperature, hardness, and any hardenability requirement
- Test-piece relationship to production, axial and radial location, orientation, frequency, and retest/disposition rules
- Surface quality, decarburization, machining allowance, dimensional tolerance, and balance-related controls
- UT standard, method, coverage, calibration/reference, acceptance, test stage, personnel, and report
- Certificate type, heat/part/lot traceability, actual results, sampling sketch, marking, and record retention
- Inspection and test plan, purchaser or third-party hold/witness points, and release authority
- Technical-deviation matrix for alternatives, engineering/customer approval, and no-substitution wording
- Packing/preservation, piece protection, destination, delivery term, and shipment documentation
References
- ISO 683-2:2016 Alloy steels for quenching and tempering
Supports: Provides the controlling ISO scope and size-restricted chemistry and mechanical-property route for 34CrNiMo6 and 42CrMo4 comparison.
Limitation: The public abstract does not replace the licensed tables, sampling clauses, impact definitions, condition symbols, optional requirements, or purchaser agreements.
- SS-EN 10083-3:2006 Technical delivery conditions for alloy steels
Supports: Confirms the withdrawn legacy EN 10083-3 status and replacement by EN ISO 683-2:2018 in the SIS catalog.
Limitation: Catalog replacement status does not automatically amend a legacy drawing, customer approval, or contract that names an older edition.
- ISO 642:2024 Steel hardenability test by end quenching
Supports: Defines the current standardized Jominy end-quench method and an agreed calculation option within a defined field of application.
Limitation: Jominy or calculated-curve evidence does not itself guarantee strength, hardness, toughness, residual stress, or service performance in a finished heavy shaft.
- Impact properties of slack-quenched alloy steels
Supports: Provides historical experimental context connecting alloy-steel hardenability, quench condition, and impact-property response rather than grade name alone.
Limitation: The historical research materials and treatments are not a modern production specification and cannot set acceptance values for a current shaft order.
Revision note: Expanded on 18 July 2026 with ISO size-band comparisons, hardenability and production-section boundaries, heavy-shaft verification controls, and worked RFQ language.
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