Key takeaways
- Size the rod and review buckling, fatigue, thread/fillet stress, side load, and alignment before treating alloy grade as the solution. ISO 3320 dimensions are not functional approval.
- C45/C45E and 42CrMo4 mechanical values in ISO 683 are condition- and ruling-section-specific; SAE J403/J404 chemistry names do not supply equivalent universal bar properties.
- Base material, heat treatment, straightening, machining allowance, grinding/polishing, and plating preparation form one process chain. Inspect at the stage where each requirement matters.
- A useful RFQ separates rough bar, pre-plating size, and final plated rod; names the controlling standards; and defines test locations, certificate, traceability, alternatives, and surface responsibility.
Define the rod load case before comparing grades
First state what the bar becomes: piston rod carrying compression and tension, guide rod, tie element, plated linear shaft, or another cylinder component. For a piston rod, record maximum extension, unsupported length, mount and end constraints, bore/rod geometry, maximum pressure and force in both directions, dynamic side load, acceleration and stops, duty cycle, operating temperature, alignment tolerance, corrosion exposure, and whether failure is governed by yielding, column instability, fatigue, wear, seal damage, or surface breakdown. A grade comparison without those inputs can only be preliminary.
ISO 3320:2013 provides a metric series of bore and piston-rod diameters and area ratios. Its official scope explicitly says it addresses dimensional criteria and not functional characteristics. A standard diameter therefore does not demonstrate buckling safety, stiffness, fatigue life, thread adequacy, or material suitability. Engineering must evaluate the actual unsupported length and boundary conditions, and procurement should attach the approved diameter rather than ask the steel supplier to infer it from pressure alone.
Local geometry may control before the smooth rod body. Capture thread form and runout, shoulder and fillet radii, wrench flats, cross holes, welded attachments, induction-hardened zones, chrome termination, and repair limits. Higher-strength 42CrMo4 may increase the available design-property route, but it cannot remove a sharp transition or misalignment. C45/C45E may be entirely appropriate for an approved medium-duty design when the condition and process chain are controlled.
Sources:[5]
Keep chemistry and rod-property routes distinct
The table compares current ISO C45/C45E and 42CrMo4 routes with SAE 1045 and 4140 chemistry designations. CK45 is not given a separate invented row because legacy or informal CK45 language must first be resolved to a controlling document. If an existing drawing truly names an older standard, preserve and review that exact edition rather than assuming current C45 values.
ISO mechanical values shown are for +QT material in selected round ruling-section bands. A hydraulic rod supplied normalized, induction hardened, cold finished, or for customer heat treatment does not inherit those numbers. SAE J403 and J404 define chemistry ranges; the purchaser must add an applicable product specification and order-specific condition and test requirements. This distinction is especially important when quotations say 'CK45/1045 normalized' and '4140 QT' without identifying the actual mechanical range or sampling location.
| Grade and controlling standard | Cast-analysis chemical composition, mass % | Mechanical-property basis | Buyer interpretation |
|---|---|---|---|
| C45 / C45E — ISO 683-1:2016 | Both: 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; Cr+Mo+Ni ≤0.63. C45: P/S ≤0.045. C45E: P ≤0.025, S ≤0.035. | +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. | A current route for an engineering-approved medium-carbon rod; keep C45E's suffix, condition, section, and property location explicit. |
| 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. | A higher size-banded strength route when design and processing require it; it is not proof of buckling, fatigue, plating, or full-section performance. |
| 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; reported residual limits include Cu ≤0.20, Ni ≤0.25, Cr ≤0.20, Mo ≤0.06 for the standard 1045 route. | SAE J403 defines chemistry and product-analysis rules; no universal delivered-bar mechanical-property range is established by J403. | Add the applicable hot-wrought or cold-finished bar specification, condition, properties, sampling, straightness, surface, and inspection. |
| 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; no universal delivered-bar QT tensile, yield, impact, elongation, or hardness range is established by J404. | Do not substitute ISO 42CrMo4 mechanical rows. Select a 4140 product/condition specification and state section-relevant test requirements. |
This table condenses chemistry and selected ISO +QT size bands for RFQ screening. Confirm licensed editions, product scope, condition, ruling section, sampling, impact test details, and product specification. Only accepted order terms define the requirements for a specific supply.
Specify the complete bar-to-plated-rod process chain
Separate at least three diameters: incoming bar, pre-plating machined/ground rod, and final plated diameter. State allowance for scale and decarburization removal, turning or peeling, heat treatment and straightening, stress relief if required by the approved route, grinding and polishing, plating build, and final finishing. Without those stages, bidders may quote different scopes under the same final diameter. Also distinguish straightness of the supplied base bar from straightness of the final rod; heat treatment, straightening, machining, and plating can occur between those inspections.
Define the surface numerically where the drawing controls it: pre-plating diameter and tolerance, final diameter and tolerance, straightness over stated length and inspection support, surface roughness and measurement method, allowable surface discontinuities, runout datum, plating specification and thickness distribution, hardness method and location if required, corrosion test if required, and protected areas. The steel grade does not establish any of these coating or finished-rod requirements.
Clarify responsibility at every handoff. If JOTAIN or another steel supplier provides base bar only, the purchaser or downstream processor owns later machining and plating results unless the order says otherwise. If the quotation includes plating-prepared or finished plated rods, require a process and inspection plan that identifies subcontractors, traceability through cutting and coating lots, pre-plating release checks, final inspection, and nonconformance disposition. Do not use a base-material MTC as evidence that the coating system passed.
Sources:[5]
Match testing and commercial scope to the rod risk
For a rod purchased +QT, specify the property range and what the test represents: heat, heat-treatment lot, delivered bar, or a sample at an agreed orientation and radial position. Surface hardness is not a substitute for core evidence. For a rod that will be heat treated after machining, incoming annealed or normalized controls may focus on chemistry, machinability, dimensions, and traceability while final properties are verified later. Avoid paying twice for tests that do not follow the accepted process, but do not leave final responsibility unassigned.
Ultrasonic testing should be consequence-driven and separately specified. If required, identify the bar/product examination standard or written procedure, diameter and condition, scan surfaces and coverage, calibration/reference basis, acceptance criteria, near-surface or end-zone treatment, test stage, personnel qualification, report fields, and heat/bar identity. 'UT according to international standard' lets suppliers choose materially different scopes. Surface inspection before and after grinding may also answer different risks.
Compare total route cost: material yield from rough to final diameter, heat-treatment lot and destructive testing, straightening and rework, grinding/polishing stock, plating subcontract scope, inspection fixtures, rejected coating disposition, packing that protects the finished surface, and the cost of mixed responsibility at interfaces. C45/C45E may be the more efficient approved route for many rods; 42CrMo4 may be justified by the calculated load and verified property needs. Grade price alone does not settle the decision.
Write the RFQ around supply stage and acceptance
The worked example is for a plating-prepared 42CrMo4 rod blank, not a finished chrome-plated rod. That boundary is deliberately repeated in the item, surface, and certificate lines. If the order covers final plating, add the approved coating specification, thickness, hardness or corrosion tests where required, masking, finish, inspection, packaging, and coating-lot documents. If the design calls for C45E, rebuild the condition and mechanical clauses from ISO 683-1 rather than only changing the grade name.
Ask for alternatives in a separate deviation matrix. A C45E or SAE route may be commercially attractive, but the bidder must show standard edition, chemistry, product form, condition, section/property evidence, process stages, inspection, certificate, dimensional consequences, and price/lead-time effect. Engineering approves structural suitability; quality approves verification and documents; procurement accepts only the aligned commercial scope.
Before production, reconcile the supplier acknowledgment to the drawing and process responsibility matrix. Confirm which party owns heat treatment, straightening, rough and final machining, polishing, plating, final dimensional acceptance, and system validation. Only accepted order terms define the requirements for a specific supply.
Item: Plating-prepared piston rod blanks (chrome plating excluded from this item)
Required grade: 42CrMo4
Standard and edition: ISO 683-2:2016
Cylinder/rod reference: Approved drawing HC-771 revision D; final rod diameter and unsupported length shown
Size and quantity: Rough Ø75 mm × 3,250 mm, 40 pieces; pre-plating diameter per drawing
Delivery condition: +QT, peeled/turned and ground to pre-plating condition; heat-treatment and straightening sequence declared
Mechanical requirements: Applicable ISO ruling-section band plus drawing requirements; actual results at agreed sampling location
Surface/dimensions: Pre-plating diameter, tolerance, straightness, runout and roughness per drawing; plating build and final plated size excluded
Inspection: Chemistry, mechanics, dimensions, straightness, surface, marking; UT only per attached method, coverage and acceptance
Certificate: EN 10204 type 3.1 with heat and heat-treatment-lot traceability and actual results
Alternatives: C45E, SAE 1045 or SAE 4140 only as separate technical deviations with written purchaser approval
Packing: Non-contact protection for prepared surfaces; piece and bundle identity retained
Destination: [city, country and delivery term]42CrMo4 vs CK45 for hydraulic cylinder rods buyer questions
Is 42CrMo4 always better than CK45 for hydraulic rods?
No. 42CrMo4 offers a higher ISO +QT size-band property route, but the approved choice depends on rod stability, stiffness, fatigue geometry, duty cycle, environment, heat treatment, process capability, surface/coating system, inspection, and total cost. Higher grade cannot correct an inadequate rod design.
What should buyers send when comparing hydraulic rod materials?
Send the cylinder/rod drawing, function, loads, unsupported length and end constraints, grade/standard edition, rough/pre-plating/final sizes, condition, process responsibility, properties and sampling, straightness/runout/roughness, plating specification if in scope, NDT, certificate/traceability, quantity, packing, and destination.
What is the practical difference between 42CrMo4 and CK45 for rods?
CK45 should first be resolved to C45/C45E or another controlling route. ISO C45/C45E is a medium-carbon option with lower +QT size-band values; 42CrMo4 is a chromium-molybdenum route with higher listed bands. The design and verified process chain—not the label—decide suitability.
Does ISO 3320 prove a standard piston rod diameter is suitable?
No. ISO 3320 establishes metric bore/rod diameter series and area ratios, and its scope explicitly excludes functional characteristics. Rod stability, stress, fatigue, end geometry, stiffness, surface system, and safety factors still require application-specific engineering approval.
42CrMo4 vs CK45 for hydraulic cylinder rods RFQ checklist
- Rod function, cylinder drawing/revision, maximum extension, unsupported length, end constraints, alignment, load directions, duty cycle, and environment
- Approved diameter and design verification; ISO 3320 dimensional use distinguished from functional approval
- Required designation and standard edition; CK45 legacy disposition; permitted alternative and approval authority
- Incoming bar, heat-treatment section, pre-plating and final plated diameters, length, quantity, allowances, and tolerances
- Normalized, annealed, +QT, induction-hardened, peeled, turned, ground, polished, plating-prepared, or finished-plated scope
- Yield/tensile/impact/hardness requirements, ruling section, test orientation/location, lot frequency, and retest rule
- Straightness, runout, roughness, surface-discontinuity, decarburization, and inspection-stage requirements
- Plating specification, thickness distribution, masking, finish, final tests, and responsibility when coating is in scope
- UT or other NDT standard, coverage, calibration/reference, acceptance, stage, personnel, and report
- Certificate type, actual results, heat/bar/heat-treatment/coating-lot traceability, marking, and deviation record
- Packing that protects prepared or plated surfaces, bundle/piece identity, destination, delivery term, and shipment documents
References
- ISO 683-1:2016 Non-alloy steels for quenching and tempering
Supports: Provides the current C45/C45E chemistry and size-bounded quenched-and-tempered product/property route used in the rod comparison.
Limitation: The catalog abstract does not replace the licensed tables, product scope, sampling rules, condition symbols, impact definitions, or purchaser agreements.
- ISO 683-2:2016 Alloy steels for quenching and tempering
Supports: Provides the current 42CrMo4 chemistry and size-bounded quenched-and-tempered product/property route used in the rod comparison.
Limitation: ISO 683-2 does not establish hydraulic-cylinder design, buckling, fatigue, coating, straightness, or finished-rod functional acceptance.
- SAE J403_202402 Chemical Compositions of SAE Carbon Steels
Supports: Defines the current SAE 1045 chemistry, heat-analysis, product-analysis variation, and reporting route relevant to an alternative offer.
Limitation: SAE J403 does not define a complete bar product, delivery condition, mechanical-property, surface, dimensional, inspection, or cylinder requirement.
- SAE J404_200901 Chemical Compositions of SAE Alloy Steels
Supports: Defines the SAE alloy-steel chemistry route for SAE 4140 when it is proposed as a separately specified rod-base alternative.
Limitation: SAE J404 does not provide universal quenched-and-tempered bar properties or authorize substitution for ISO 42CrMo4.
- ISO 3320:2013 Cylinder bores and piston rod diameters and area ratios
Supports: Confirms the standard metric bore/rod diameter series and the explicit boundary that the document does not cover functional characteristics.
Limitation: The dimensional series does not approve rod material, stability, stress, fatigue, surface treatment, sealing performance, or fitness for service.
Revision note: Expanded on 18 July 2026 with ISO/SAE chemistry and property boundaries, hydraulic load and surface-process controls, responsibility interfaces, and worked RFQ wording.
Inquiry support
Send the required grade standard, allowed alternatives, size, and condition.
JOTAIN can review the stated grade, size, delivery condition, processing route, inspection requirements, and export details against the project requirement.