Rod load case before grade
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, and bore/rod geometry.
Add maximum pressure and force in both directions, dynamic side load, acceleration and stops, and duty cycle. State operating temperature, alignment tolerance, and corrosion exposure. Identify whether yielding, column instability, fatigue, wear, seal damage, or surface breakdown governs failure. 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 clearly 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]
Chemistry and delivered properties are different questions
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 clear. |
| 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 isn't 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 single 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 single 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. |
Bar-to-plated-rod process sequence
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.
Where the drawing controls the surface, give numerical requirements. Specify pre-plating and final diameters and tolerances. Define straightness over a stated length and inspection support, surface roughness and measurement method, allowable surface discontinuities, and runout datum. State the plating specification, thickness distribution, and protected areas. Add hardness method and location, and corrosion testing, where required. Steel grade alone cannot confirm 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 action. Do not use a base-material MTC as evidence that the coating system passed.
A roughness result needs its measurement conditions. NIST’s calibration work identifies filtering, evaluation length and stylus geometry as relevant to the measured profile. Agree those conditions for the seal-travel surface, and identify whether the report concerns the ground base rod or the final plated surface. [6]
Sources:[5]
Testing and commercial basis
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 the cost of the complete process. Include material yield from rough to final diameter, heat-treatment lot and destructive testing, and straightening and rework. Account for grinding/polishing stock, plating subcontract scope, inspection fixtures, and rejected coating disposition. Include packing that protects the finished surface and the cost of unclear responsibility between process stages. C45/C45E may be the more efficient approved route for many rods. Calculated load and verified property needs may justify 42CrMo4. Grade price alone does not settle the decision.
RFQ wording by supply stage
The worked example is for a plating-prepared 42CrMo4 rod blank, not a finished chrome-plated rod. That boundary appears 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 basis.
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.
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]Questions buyers ask
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 clearly excludes functional characteristics. Rod stability, stress, fatigue, end geometry, stiffness, surface system, and safety factors still require application-specific engineering approval.
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 approver
- 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
- ISO 683-2:2016 Alloy steels for quenching and tempering
- SAE J403_202402 Chemical Compositions of SAE Carbon Steels
- SAE J404_200901 Chemical Compositions of SAE Alloy Steels
- ISO 3320:2013 Cylinder bores and piston rod diameters and area ratios
- Vorburger et al.: NIST roughness calibration and measurement uncertainty
Inquiry support
Comparing two material routes?
Send the drawing or RFQ with the grade, size, condition, processing, inspection, quantity, and destination. We can flag open points before quotation.