From shaft duty to 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.
South African supplier pages commonly use the buying names 817M40 and EN24. Ovako lists them among designations similar to 34CrNiMo6, while Macsteel lists 817M40 (EN24) alloy round bar in South Africa. Use these names to interpret an enquiry, not to assume automatic substitution. Where the RFQ says EN24, keep its specified BS 970 edition and condition, then compare any proposed 34CrNiMo6 or 4340 route against the required chemistry, properties, and approval basis.
When a South African drawing or supplier list uses EN24 round bar or 817M40, keep that buyer term visible while reviewing the 34CrNiMo6 / 4340 round-bar route ; keep the drawing's controlling designation, edition, condition, section, and approval rule.
ISO chemistry and matching ruling sections
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 aren't 'typical mill values,' and they aren't 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. |
Heat treatment for the production section
Hardenability controls how the microstructural response changes with cooling depth; it isn't 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. [3]
Jominy evidence can help compare heats or set a band, but it doesn't 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. [4]
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 clearly 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.
Manufacturing and verification risk
Material price is one part of shaft cost. Compare the available melt and casting route, minimum heat or forging quantity, and forging reduction and discard plan. Account for heat-treatment batch size, destructive-test allowance, straightening, rough machining, and UT access. Compare 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.
Specify ultrasonic inspection separately from the grade standard. Identify bar or forging, examination standard and edition, and manual or mechanized method. Define scanning surfaces and coverage, reference or calibration basis, and acceptance class or written criteria. Address near-surface and end zones and the test stage before or after rough machining. Include personnel qualification, report content, and heat/part traceability. A high-alloy grade still needs a defined UT plan to control internal quality.
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.
RFQ wording and the 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 contactship. 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, lock 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.
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]Questions buyers ask
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.
How should an EN24 / 817M40 requirement be compared with 34CrNiMo6 or 4340?
Treat it as a comparison, not an automatic match. Supplier cross-references can identify a candidate route, but the RFQ should retain its controlling designation, standard edition, delivery condition, section, chemistry and mechanical-property basis, inspection, certificate, and substitution rule. Confirm conformance before accepting a 34CrNiMo6 or 4340 proposal against an EN24 / 817M40 requirement.
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 approver
- 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/rule for failed resultss
- 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 approval
- Technical-deviation matrix for alternatives, engineering/customer approval, and no-substitution wording
- Packing/preservation, piece protection, destination, delivery term, and shipment documentation
References
Inquiry support
Working from a part drawing?
Send the drawing or RFQ with the grade, size, condition, processing, inspection, quantity, and destination. We can flag open points before quotation.