Key takeaways
- Keep the drawing grade unless an alternative has engineering approval. Clarify a bare CK45 designation before comparing quotations.
- Compare properties at the relevant section, condition, and test location. GB/T 3077 reference-specimen values and ISO size-banded values use different bases.
- Review keyways, shoulders, fits, surface finish, heat treatment, and machining allowance together; these affect the finished shaft as much as the grade name.
- Specify the sampling and inspection needed at delivery and after later processing. Keep incoming bar results distinct from finished-shaft acceptance.
Download the shaft material selection worksheet Fill in the drawing, treatment section, required properties, and any proposed grade deviation.
Shaft duty and the CK45 naming problem
For a shaft under steady torque, reversing load, bending, or shock, start with the drawing and duty record. Mark the shoulders, keyways, splines, cross-holes, threads, and small fillets where stress concentrates. Record the section that will actually be quenched and tempered. This gives the material comparison a specific part and process to address.
CK45 needs clarification when it appears without a standard. C45 and C45E under ISO 683-1 have specified product, delivery-condition, chemistry, and size-dependent property requirements. SAE J403 defines SAE 1045 chemistry and analysis rules; the order needs a product specification and property requirements as well. Ask the drawing owner which designation is required before inviting alternatives.
40Cr is a chromium alloy structural steel under GB/T 3077-2015. It is worth reviewing when the section and required heat-treatment response call for an alloy steel. Its prescribed reference-specimen values still need to be checked against the shaft diameter, treatment route, and agreed sampling. They do not describe the properties at every point in a finished shaft.
For each offer, put the required grade and standard beside the delivered condition, treatment section, test location, final machining route, and permitted deviations. Settle conflicts between the purchase order, drawing, customer specification, and material standard in writing. Otherwise, two apparently comparable prices may cover different material and inspection requirements.
Chemistry and property bases under each standard
The chemistry table explains why the routes can overlap in some applications but cannot be certified interchangeably by name. 40Cr has a chromium range of 0.80–1.10 mass %, while C45 and SAE 1045 are medium-carbon routes with residual-element limits rather than the same deliberate chromium addition. C45E keeps the C45 principal range but uses tighter phosphorus and sulfur limits; a drawing that requires the E suffix should not receive plain C45 without approval.
Mechanical-property wording differs just as much. GB/T 3077 publishes values for a prescribed heat-treated 40Cr reference specimen. ISO 683-1 publishes +QT values tied to the C45 product form and ruling-section band. SAE J403 doesn't provide one delivered-bar tensile, yield, impact, elongation, or hardness range for every SAE 1045 bar. A quote can therefore contain correct chemistry yet still be technically incomplete on condition, size, sampling, or properties.
Use ISO 6892-1 or the accepted alternative as the test method only after the product specification and order have fixed specimen geometry, orientation, location, condition, frequency, acceptance, and retest rules. A result from a separately heat-treated coupon or small prolongation should not be represented as a center property for a larger shaft blank unless that representation is expressly agreed.
| Route / controlling standard | Chemical composition range, mass % | Mechanical-property basis | Shaft buyer interpretation |
|---|---|---|---|
| 40Cr — GB/T 3077-2015 | C 0.37–0.44; Si 0.17–0.37; Mn 0.50–0.80; Cr 0.80–1.10; P ≤0.030; S ≤0.030; Ni ≤0.30; Cu ≤0.30. | Prescribed heat-treated reference specimen: Rm ≥980 MPa; yield strength ≥785 MPa; A ≥9%; Z ≥45%; KV2 ≥47 J. This isn't a blanket full-bar or finished-shaft guarantee. | Use when the approved GB route and section response fit the shaft duty. Define delivered condition, actual test representation, and downstream heat treatment. |
| C45 / C45E — ISO 683-1:2016 | C45: C 0.42–0.50; Si 0.10–0.40; Mn 0.50–0.80; P ≤0.045; S ≤0.045; Cr ≤0.40; Mo ≤0.10; Ni ≤0.40; Cu ≤0.30; Cr+Mo+Ni ≤0.63. C45E uses P ≤0.025 and S ≤0.035 with the same principal ranges. | +QT round ruling section d >16–40 mm: Rp0.2 ≥430 MPa, Rm 650–800 MPa, A ≥16%, Z ≥40%, KV ≥15 J; d >40–100 mm: Rp0.2 ≥370 MPa, Rm 630–780 MPa, A ≥17%, Z ≥45%, KV ≥15 J. | Apply only to the matching ISO product, +QT condition, ruling section, sampling and impact basis. Preserve the E suffix when required. |
| SAE 1045 — SAE J403_202402 | C 0.43–0.50; Mn 0.60–0.90; Si 0.15–0.35; P ≤0.030; S ≤0.035; residual limits Cu ≤0.20, Ni ≤0.25, Cr ≤0.20, Mo ≤0.06. | SAE J403 defines chemical composition and analysis rules; it cannot add a single delivered-bar tensile, yield, elongation, impact, hardness, or shaft-property range. | Add the controlling product specification, condition, section properties, sampling, surface, dimensions, inspection and certificate requirements. |
Section size and hardenability in context
The relevant diameter is the section during heat treatment. A small coupon cools differently from the centre of a large shaft blank. Confirm the ruling section, radial and axial specimen locations, and whether the sample comes from the delivered bar, a prolongation, or a separately heat-treated piece. If rough machining precedes final quench and temper, show that intermediate size on the process plan.
ISO 642:2024 defines the Jominy end-quench hardenability test. Jominy data can help compare heats and estimate relative depth response, but it doesn't directly predict a production shaft's full thermal history, core properties, distortion, residual stress, or fatigue strength. If hardenability is a design concern, define the acceptable Jominy band or another way to verify the section and then require production-section validation at the ordered diameter.
A higher tensile strength does not settle fatigue performance. Surface finish, decarburization, inclusions, grinding damage, straightening marks, thread or spline manufacture, keyway radii, press fits, fretting, and residual stress can affect crack initiation. Carry the drawing's surface and geometry requirements into the purchase order and into the checks after final processing.
When impact or toughness matters, state the method, specimen type, notch, orientation, location, test temperature, frequency, minimum individual/average rule, and retest disposition. Never compare a GB reference KV2 value with an ISO result unless the specimens, temperature, and acceptance definitions are aligned.
Maximov and co-authors studied heat-treated 42CrMo4 specimens finished by dry turning and diamond burnishing. Their fatigue results show why the final surface process belongs in a shaft review. This is evidence about a different alloy and a defined specimen route; it does not rank 40Cr against CK45 or supply an allowable shaft stress. [7]
Machining, heat treatment, surface removal, and straightness
A shaft RFQ needs both purchased and finished dimensions. Show forging or rolling size, rough-machined size, final bearing and seal diameters, length, stock allowance, and the section at heat treatment. If a supplier quotes peeled or ground bar, define whether that surface remains on the component or will be removed. Surface route affects decarburization exposure, defect detection, machining allowance, straightness, and the evidence available before final finishing.
Sequence matters. Quench and temper, stress relief, induction hardening, nitriding, chrome plating, grinding, polishing, and straightening can change hardness, residual stress, dimensions, and surface integrity. Identify who owns each operation, which state is covered by each test, and whether the material certificate describes the delivered bar or the final shaft. If the buyer heat treats after delivery, incoming hardness is a processing control—not proof of final properties.
Specify straightness as a measurement, not an adjective. State total-indicator-reading or gap method, support/span arrangement, measurement temperature, surface used, allowable value, and whether localized bends are assessed. Define the straightening stage and whether post-straightening stress relief or reinspection is required. A long slender shaft can pass chemistry and tensile requirements yet still be unusable because the straightness basis was never shared.
Inspection should follow risk: heat and product analysis where required; mechanical tests at defined locations; hardness map or surface-hardening traverse; decarburization and microstructure when relevant; dimensional and surface inspection; ultrasonic or magnetic-particle testing to a named method and acceptance; and heat-to-piece or heat-to-lot traceability. ISO 10474 can identify the inspection-document type, but only the order defines which actual results must appear.
Sources:[6]
RFQ wording for the required route and alternatives
Write the required designation exactly as it should appear on the certificate. Where an alternative is permitted, request a comparison of chemistry, product scope, condition, section properties, sampling, dimensions, surface, and NDT requirements. Show price and lead-time differences separately. The named engineering or customer representative should approve the deviation before manufacture.
The example below leaves property values for the drawing owner to complete. Attach the shaft drawing and the selection worksheet, including the section during heat treatment and the final bearing or seal diameters. Ask the supplier to identify any requirement it cannot meet in the proposed delivery condition.
Item: Rough-machined shaft blank, [drawing / revision], [quantity]
Required route: 40Cr to GB/T 3077-2015 or C45E to ISO 683-1:2016 [select one required route and edition]; informal CK45 wording alone is not acceptable
Alternative: SAE 1045 to SAE J403_202402 plus [product specification] may be offered only in a clause-by-clause deviation schedule and requires written purchaser approval before manufacture
Finished shaft: [final diameters / length], shoulders, fillets, keyways, splines, threads, surface finish, straightness method and acceptance per drawing
Heat treatment: [delivered condition / supplier QT / buyer downstream treatment], ruling section [definition], property values [state], impact basis [state if required], induction-hardening or plating sequence [state]
Machining allowance: [radial / end allowance], defect-removal rule, decarburization limit, surface route and stage represented by inspection
Inspection: Heat analysis, agreed mechanical tests and locations, hardness [method / map], dimensions, straightness, surface, [UT / MT method and acceptance], heat-to-blank traceability
Certificate: ISO 10474 type [state type] with actual ordered results, specimen identity, condition and drawing revision
Packing / destination: [protection, supports, bundle limits, port, Incoterm]Questions buyers ask
When should buyers compare 40Cr and CK45 for shafts?
Compare them when the drawing permits a material review and the shaft duty could be met by either a medium-carbon or chromium alloy steel. Check the heat-treatment section, required properties, test locations, and finished surface. Keep the specified grade until the drawing owner approves any change.
What should buyers send before choosing between 40Cr and CK45?
Send the drawing and revision, shaft duty, purchased and finished dimensions, section during heat treatment, delivery condition, machining sequence, specified mechanical or hardness values, and inspection requirements. Clarify which standard the CK45 designation refers to.
What RFQ details help compare 40Cr and CK45 for shafts?
Use the same required standard, size, condition, test locations, machining allowance, and inspection-document requirements for both offers. List an alternative grade as a deviation with its technical differences, price, and lead time so engineering can review it before manufacture.
RFQ checklist
- Shaft function, load spectrum, torque direction, bending, axial load, shock, temperature, and failure consequence
- Drawing revision, customer specification, required grade, standard, edition, and specification hierarchy
- Resolution of informal CK45 wording to C45, C45E, SAE 1045, or another approved route
- Rolled/forged size, rough-machined size, final dimensions, quantity, tolerance, and governing ruling section
- Shoulders, fillets, holes, keyways, splines, threads, fits, grooves, and other fatigue-sensitive geometry
- Delivered condition, heat treater, furnace/lot basis, and downstream thermal-processing sequence
- Tensile, yield, elongation, reduction, impact, hardness and hardenability requirements only where specified
- Specimen geometry, axial/radial location, orientation, test frequency, retest rule, and section represented
- Machining allowance, peeled/turned/ground surface route, decarburization, roughness, and defect-removal rule
- Straightness method, supports/span, measurement surface, stage, acceptance, and straightening controls
- UT or MT method, coverage, calibration/reference basis, acceptance, stage, and end-zone disposition
- Inspection-document type, actual reported results, heat-to-blank traceability, marking, and release approval
- Alternative-grade deviation matrix, engineering/customer approver, price effect, and lead-time effect
- Third-party witness or document pre-review points and hold/release responsibilities
- Packing support, corrosion protection, bundle mass, destination, Incoterm, and shipment window
References
- GB/T 3077-2015 Alloy structure steels
- ISO 683-1:2016 Non-alloy steels for quenching and tempering
- SAE J403_202402 Chemical Compositions of SAE Carbon Steels
- ISO 642:2024 Steel — Hardenability test by end quenching (Jominy test)
- ISO 6892-1:2019 Metallic materials — Tensile testing at room temperature
- ISO 10474:2013 Steel and steel products — Inspection documents
- Maximov et al.: dry turning and diamond burnishing of 42CrMo4
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.