A familiar legacy grade and a modern workhorse can share an application without sharing an approval
Compare 18CrNi8 / 1.5920 and 18CrNiMo7-6 / 1.6587 for large gears by standard identity, case and core targets, hardenability, distortion, inspection and substitution evidence.
Large carburized gears need a hard wear-resistant case supported by a tough core and controlled geometry. 18CrNi8 / 1.5920 and 18CrNiMo7-6 / 1.6587 must therefore be compared through their actual standards, section response and qualified carburizing route—not reputation alone.
This guide is intended for designers, buyers, heat-treatment teams and workshops supplying parts across Iran. It translates a familiar market enquiry into requirements that can be checked before payment, cutting, machining or furnace work.
Quick decision map
| Question | Why it matters | Required evidence |
|---|---|---|
| Standard and legacy basis | Old drawings may use chemistry or property rules no longer assumed | Original specification, revision, approved equivalents and heat analysis |
| Case-core system | Tooth flank wear and root fatigue depend on different layers | Effective case depth, surface hardness, core properties and test locations |
| Large-section hardenability | Rim thickness and blank geometry change the core response | Controlling section, furnace cycle, quench route and representative coupon |
| Distortion and inspection | Gear accuracy can be lost during carburizing, quenching and finishing | Datum plan, stock, straightening, grinding, NDT and final profile checks |
Identity, standard and the real engineering question
Treat 18CrNi8 / 1.5920 as a legacy or project-specific identity where applicable and 18CrNiMo7-6 / 1.6587 as its own documented grade. Historical equivalence claims require confirmation against the drawing, ordered edition and certificate.
1.6587 is widely specified for demanding carburized gears, but that does not automatically retire a validated 1.5920 route. Conversely, a legacy name cannot force supply if its property basis and current procurement evidence are unclear.
Standard and legacy basis
Old drawings may use chemistry or property rules no longer assumed. Record Original specification, revision, approved equivalents and heat analysis before commercial comparison begins. If this field remains verbal, purchasing, production and inspection can each interpret the same requirement differently.
Case-core system
Tooth flank wear and root fatigue depend on different layers. The order should therefore require Effective case depth, surface hardness, core properties and test locations. Catalogue data becomes useful only after product form, section and test condition are made comparable.
Section, process and delivered condition
Section size changes cooling rate, property distribution, sampling position and sometimes the applicable property range. Product form also matters: bar, plate and forging may share a grade name but follow different dimensional, testing and delivery requirements.
For replacement, map chemistry limits and hardenability evidence, then compare case-depth definition, core-property location, impact requirement, grain-size and cleanliness controls. Run process qualification where gear consequence or geometry justifies it.
Inspection evidence that closes the decision
Evidence should answer a defined question. An MTC records heat identity and declared results; a process record proves the route used; a supplementary test report proves a result at a stated location. None of them should be treated as a substitute for the other two.
Large-section hardenability
Rim thickness and blank geometry change the core response. Define Controlling section, furnace cycle, quench route and representative coupon and identify the point where production must stop if the evidence is missing or outside the agreed range.
Distortion and inspection
Gear accuracy can be lost during carburizing, quenching and finishing. Release should be based on Datum plan, stock, straightening, grinding, NDT and final profile checks, attached to the same heat and piece identity. A valid test report has limited conformity value after traceability has been lost.
A defensible purchase specification
Include exact grade and standard, forging or rolled form, blank dimensions, delivery condition, grain-size/cleanliness clauses, UT, effective case depth, surface and core hardness, mechanical properties, coupon, distortion stock, NDT and heat-number transfer.
Grade/material number: [complete]; standard and edition: [complete]; form and dimensions: [complete]; delivery condition: [complete]; quantity and cutting tolerance: [complete]; MTC type: [complete]; supplementary test and acceptance rule: [complete]. Topic-specific requirement: Include exact grade and standard, forging or rolled form, blank dimensions, delivery condition, grain-size/cleanliness clauses, UT, effective case depth, surface and core hardness, mechanical properties, coupon, distortion stock, NDT and heat-number transfer.
Ordering checklist for Tehran, Tabriz, Isfahan and Yazd
- Write the recognised market term, but place 1.5920 or 1.6587 and the documented designation on the same enquiry.
- Close Standard and legacy basis with Original specification, revision, approved equivalents and heat analysis.
- Close Case-core system with Effective case depth, surface hardness, core properties and test locations.
- Define Large-section hardenability before irreversible processing.
- Keep Distortion and inspection traceable through cutting, storage and loading.
- State destination, required dispatch date, packaging, piece count and who authorises release.
Related encyclopaedia pages and guides
- 18crni8 1 5920
- 18crnimo7 6 1 6587
- Previous Week-01 technical guide
- Next Week-01 technical guide
- Alloy-steel encyclopaedia
- Evidence and calculation methodology
- Contact Shaygan Steel
Frequently asked questions
Is 1.6587 the direct replacement for 1.5920?
Not universally. It is a candidate only after comparing the governing specifications, properties, section, heat treatment and design approval.
Why is core toughness important in a carburized gear?
The hard case carries contact and surface demands while the core supports bending, shock and crack resistance.
What should be qualified before serial production?
Material source, forging or rolling route, carburizing cycle, quench, distortion allowance, finishing and representative inspection.
Sources and method
This article uses the official references below as technical orientation and translates them into a procurement workflow for the Iranian market. A producer data sheet applies to the product and conditions it states; it does not replace the current purchase standard, drawing or MTC for the delivered heat.
- Ovako Steel Navigator — official technical reference
- Ovako Steel Navigator — official technical reference
- Steel-Iron-List — official technical reference
Conclusion and next step
Choose 1.5920 or 1.6587 from a verified case-core and manufacturing requirement. A successful large gear is the result of documented material, qualified heat treatment and final inspection working together.
For an enquiry, send material number, form, raw and final dimensions, quantity, delivery condition, inspection needs and destination in one message. Shaygan Steel can then review the evidence for current stock before proposing a supply route.
Comments
Comments are published after moderation.