Key takeaways
- Replace the phrase 'hardened CK45' with a measurable requirement: process responsibility, target zone, hardness scale and range, depth definition, test positions, core requirement, and acceptance method.
- C45/C45E to ISO 683-1 and SAE 1045 to SAE J403 overlap in carbon content but are not one contract. CK45 is often legacy language and needs its controlling document identified.
- A hard surface reading does not establish core strength or effective hardened depth. Section geometry, quench route, prior structure, decarburization, and test location must be connected to the result.
- 40Cr or 42CrMo4 can enter an engineering review when section response or strength margin is insufficient, but alloy content alone does not authorize a drawing change or guarantee a production result.
Define what hardening is supposed to achieve
Buyers often receive a drawing note such as 'CK45 hardened' without enough information to quote or inspect the part. That phrase could mean bar supplied in a quenched-and-tempered condition, induction hardening after machining, flame hardening on a local wear surface, or merely an incoming hardness range for machinability. These routes create different stock allowances, distortion risks, test locations, certificates, and responsibilities. Resolve the process stage and functional target before comparing materials.
For a surface-hardened shaft, identify the functional surface, excluded zones, finished diameter, grinding allowance, hardness scale and accepted range, effective or total hardened-depth definition, reference hardness used to measure depth, traverse positions, core hardness or mechanical requirement, and whether the supplier or customer performs final hardening. Also specify crack inspection, decarburization limits, transition-zone concerns, straightness after treatment, and whether a destructive qualification sample can represent the production lot.
For through hardening or quenched-and-tempered bar, start with the heat-treatment ruling section and the location that matters in service. A tensile specimen near the surface of a smaller test bar does not prove the center response of a larger shaft blank. ISO 683-1 ties C45/C45E mechanical requirements to condition and size bands. If the actual geometry or ordered size falls outside the cited row, engineering must establish a suitable property and evidence route rather than extending the values by assumption.
Compare the base and alloy routes on the same basis
The table separates chemical identity from mechanical or hardening evidence. ISO 683-1 publishes size-banded C45/C45E properties for specified delivery conditions; SAE J403 is a chemistry standard for SAE carbon steels and does not create a universal 1045 bar-property range. GB/T 3077 gives 40Cr and 42CrMo reference-specimen mechanical values under its prescribed heat-treatment route. Those reference results must not be presented as guaranteed core properties for any production diameter.
C45 and C45E share the same principal element ranges in ISO 683-1, while their phosphorus and sulfur limits differ. SAE 1045 uses another manganese range and its own residual-element framework. A heat described informally as CK45 should therefore be tied to a recognized standard or purchaser specification before acceptance. A name match in an ERP system cannot show that cast analysis, permissible variation, delivery condition, size-banded properties, surface state, and documentation all conform.
The alloy alternatives add chromium, and 42CrMo4 also adds molybdenum, which can support a different heat-treatment response. That is a reason to investigate—not proof of suitability. Compare the actual section, process capability, distortion and cracking risk, machinability sequence, property targets, impact requirement, test location, availability, and total processing cost. An alloy upgrade that still uses an unrepresentative coupon or undefined hardening note leaves the central procurement risk unresolved.
| Route / controlling standard | Chemical composition range, mass % | Mechanical / hardening basis | When it belongs in the review |
|---|---|---|---|
| C45 — ISO 683-1:2016 | C 0.42–0.50; Si 0.10–0.40; Mn 0.50–0.80; Cr ≤0.40; Mo ≤0.10; Ni ≤0.40; Cu ≤0.30; P ≤0.045; S ≤0.045; Cr+Mo+Ni ≤0.63. | +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. | Use only inside the specified ISO product, condition, size, sampling, and test scope. Surface-hardening depth still needs a separate drawing requirement. |
| C45E — ISO 683-1:2016 | Principal ranges as C45; P ≤0.025 and S ≤0.035 under the C45E route. | ISO size- and condition-banded requirements apply under the relevant C45E tables and clauses; do not infer a result from the cleaner designation alone. | Review when the drawing or purchaser specification requires C45E. Do not substitute C45 or SAE 1045 solely because nominal carbon overlaps. |
| 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 limits and analysis rules, not one delivered-bar strength, impact, hardness, or hardening-depth range. | Use with an applicable product specification and explicit condition, section-based properties, surface-hardening, inspection, and certificate clauses. |
| 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 reference heat-treated specimen: Rm ≥980 MPa; yield strength ≥785 MPa; A ≥9%; Z ≥45%; KV2 ≥47 J. This is not a blanket full-bar result. | Review as an alloy route when approved section response or strength needs exceed the validated medium-carbon route; retain the GB/T specimen basis. |
| 42CrMo4 — ISO 683-2:2016 | 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 >16–40 mm: Rp0.2 ≥750 MPa, Rm 1000–1200 MPa, A ≥11%, Z ≥45%, KV ≥35 J; d >40–100 mm: Rp0.2 ≥650 MPa, Rm 900–1100 MPa, A ≥12%, Z ≥50%, KV ≥35 J. | Review when a purchaser-approved Cr-Mo, size-banded QT route fits the duty. It is not an automatic upgrade from CK45 or SAE 1045. |
This condensed table is for RFQ screening. Confirm the purchaser's licensed standards, editions, product forms, conditions, size bands, heat/product analysis rules, sampling, tests, and any surface-hardening specification. Only accepted order terms define the requirements for a specific supply.
Separate surface hardness, hardening depth, and core response
A surface hardness value answers only one question at one prepared location. It can miss a shallow hardened layer, a soft transition, excessive brittleness, local overheating, decarburization, or an unsuitable core. When effective case or hardened depth controls wear life, define the applicable measurement method and the hardness threshold that marks the depth. State whether results are taken on a sacrificial extension, a representative coupon, or a sectioned production part, and how that item is linked to the lot.
ISO 642:2024 defines the Jominy end-quench method for comparing steel hardenability. Jominy data can help materials engineers assess heat response, but it does not directly certify induction-hardened depth or center strength in a finished shaft. If Jominy limits are required, name the band source, distances, heat frequency, reporting, and permitted calculation route. Pair that evidence with a qualified production process and section-based verification when the functional result depends on geometry.
Plan machining and inspection around distortion. Leave a controlled grinding allowance if final sizing follows hardening, but avoid an allowance that removes the required hardened zone. Define straightness before and after heat treatment, permitted correction method, crack inspection stage, and whether repaired or reprocessed parts are allowed. If the order is for raw bar rather than finished components, state clearly which downstream hardening results are buyer design requirements and which incoming characteristics the steel supplier is actually responsible for.
Approve an alloy change as an engineering deviation
Trigger the upgrade review with a documented gap: insufficient validated hardening depth, inadequate core property at the ruling section, impact or fatigue concern, distortion experience, repeated process nonconformance, or a customer standard that requires alloy steel. Avoid a vague instruction to quote 'CK45 or better.' Better is not a measurable designation, and a higher nominal strength can introduce machining, welding, heat-treatment, cost, and approval consequences.
Ask each alternative offer for a clause-by-clause comparison: required and proposed grade, controlling documents and editions, actual or target chemistry, product form, delivery condition, section size, heat-treatment responsibility, mechanical and hardness basis, hardenability evidence, specimen locations, surface route, NDT, inspection document, marking, traceability, lead time, and price. Materials or design engineering decides functional suitability; quality accepts the evidence plan; procurement records the commercial and approval status.
ISO 404 provides general technical delivery requirements for steel and states that requirements in the order or product standard prevail where they differ. Use a clear document hierarchy and a deviation log so a supplier qualification note cannot silently override the drawing. When dual compliance is intended, require evidence against both complete routes. When it is not intended, preserve the accepted controlling designation on the order, marking, inspection record, and material certificate.
Turn the hardening note into an inspectable RFQ line
The example below treats C45E as the base incoming bar and makes local induction hardening a defined downstream requirement. If the steel supplier is not responsible for that operation, split the purchase into incoming-bar acceptance and finished-component process qualification. Do not ask the bar certificate to guarantee a result created later by another company. Instead, preserve heat identity through cutting and machining so downstream results can be traced back to the supplied heat and lot.
For early sourcing, invite 40Cr or 42CrMo4 as alternative proposals but require the engineering case and commercial effect. The supplier should not rewrite the grade line or mix a GB/T reference-specimen value with an ISO product designation. Any proposed route must use one coherent standard basis, unless the purchaser deliberately defines and verifies dual compliance. Reject unsupported claims such as 'same as CK45 but stronger' from the technical comparison.
Before PO release, close hardness, depth, core, test location, lot, destructive-sample allowance, dimensional stage, straightness, crack inspection, certificate, traceability, and deviation authority. Record who owns process qualification and who may approve rework. A concise order with attachments is acceptable; an ambiguous order is not. Only accepted order terms define the requirements for a specific supply.
Item: Machined hydraulic-rod blanks for subsequent local induction hardening
Base route: C45E to ISO 683-1:2016, Ø75 mm × 3,250 mm, 60 pieces, normalized condition; heat-treatment ruling section and machining allowance to be confirmed
Finished geometry: Drawing [number/revision], functional journal zones and no-harden zones identified
Hardening requirement: Induction hardened after rough machining to [HR scale and range]; effective hardened depth [range] measured at [threshold/method]; core requirement [value/basis]
Process responsibility: [supplier/customer/subcontractor]; qualified procedure, lot definition, destructive qualification sample, and heat-to-part traceability required
Alternative: 40Cr to GB/T 3077-2015 or 42CrMo4 to ISO 683-2:2016 may be proposed only with a clause comparison and written purchaser approval
Inspection: Incoming heat analysis and condition; after hardening, hardness map, depth traverse, straightness, dimensions, decarburization review and magnetic-particle inspection to attached criteria
Certificate: Actual incoming and downstream results linked to heat, process lot, sample, and drawing revision
Destination: [city, country and Incoterm]CK45 / 1045 hardening review and alloy-upgrade questions buyer questions
When is CK45 / 1045 suitable for machined rods or shafts?
It can be suitable when the approved medium-carbon route, section, incoming condition, machining sequence, final hardening process, core and surface requirements, distortion controls, and inspection plan have been validated for the part. The informal name CK45 should be tied to a controlling standard or purchaser specification before the order is accepted.
When should buyers review 40Cr or 42CrMo4 instead of CK45 / 1045?
Open an engineering review when validated medium-carbon processing cannot meet required depth, core response, impact, fatigue, distortion, or customer-standard needs at the actual section. Compare a coherent 40Cr or 42CrMo4 standard route and require design, quality, and customer approval when the contract calls for it; do not treat alloy content as automatic authorization.
What hardening details should be included in a CK45 / 1045 RFQ?
Include the controlling grade and edition, product form, condition, ruling section, finished drawing, process owner, hardening method and zones, scale and hardness range, depth definition and threshold, core requirement, sample location, lot and frequency, grinding allowance, straightness, crack inspection, certificate, traceability, and alternative-approval rules.
CK45 / 1045 hardening review and alloy-upgrade questions RFQ checklist
- Finished component function, wear zones, no-harden zones, credible failures, and service temperature
- Required CK45, C45, C45E, or SAE 1045 designation with controlling standard and edition
- Product form, ordered diameter, heat-treatment ruling section, length, quantity, and machining allowance
- Incoming as-rolled, normalized, annealed, quenched-and-tempered, peeled, turned, or ground condition
- Whether supplier, customer, or named subcontractor owns final hardening and process qualification
- Induction, flame, through-hardening, or QT route and the exact drawing zones to which it applies
- Hardness scale, range, surface preparation, number and position of readings, and permitted variation
- Effective or total depth definition, threshold hardness, traverse method, section location, and sample allowance
- Core hardness or mechanical property, specimen orientation and location, test temperature, and represented lot
- Jominy band, distances, frequency, reporting, and calculation permission only when design-controlled
- Grinding allowance, final dimensional stage, straightness limits, correction method, and distortion plan
- Decarburization, crack inspection, surface quality, NDT method, acceptance criteria, and rework rules
- Heat analysis, inspection document, actual results, heat-to-part traceability, marking, and record retention
- 40Cr or 42CrMo4 alternative matrix, engineering authority, customer approval, lead-time and price effect
- Packing, preservation, destination, Incoterm, shipment documents, and receiving-inspection reconciliation
References
- ISO 683-1:2016 Non-alloy steels for quenching and tempering
Supports: Defines the C45 and C45E product route, scope, and size-banded mechanical-property framework used in the comparison.
Limitation: The catalog record does not replace the licensed chemistry, condition, size, sampling, dimensional, and optional-requirement tables.
- SAE J403_202402 Chemical Compositions of SAE Carbon Steels
Supports: Provides the SAE 1045 cast-analysis chemistry and product-analysis framework used for the medium-carbon route.
Limitation: It does not define one delivered-bar mechanical range, surface-hardening depth, process qualification, or inspection plan.
- GB/T 3077-2015 Alloy structure steels
Supports: Confirms the current Chinese standard identity used for 40Cr chemistry and its prescribed reference-specimen property route.
Limitation: The public record does not replace licensed specimen heat treatment, sampling, variation, retest, and delivery-acceptance clauses.
- ISO 683-2:2016 Alloy steels for quenching and tempering
Supports: Defines the 42CrMo4 alloy-steel scope and size-banded QT mechanical-property framework used for upgrade screening.
Limitation: Its requirements apply only within the stated product, condition, section, sampling, and test scope and do not authorize substitution.
- ISO 642:2024 Steel — Hardenability test by end quenching
Supports: Defines the standardized Jominy end-quench route used to explain bounded hardenability evidence and calculation by agreement.
Limitation: Jominy results do not directly establish induction-hardened depth or core properties in a production shaft or bar.
- ISO 404:2013 Steel and steel products — General technical delivery requirements
Supports: Provides general delivery context and confirms the precedence of order or product-standard requirements where they differ.
Limitation: General delivery rules do not create purchaser-specific grade, hardening, property, test, deviation, or certificate requirements.
Revision note: Expanded on 2026-07-19 with C45/C45E and SAE 1045 distinctions, five-route comparison data, surface-versus-core hardening controls, alloy-upgrade governance, and worked RFQ wording.
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