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Certifications: EN 10204 3.1 / 3.2 material test certificates, Jominy hardenability test reports where applicable, and complete export documentation packages.
Alloy Steel
A world-class technical reference for mechanical, structural, and machinery engineers navigating the complete AISI/SAE alloy steel system beyond 4140 and 4340 — covering the numbering system itself, the fundamental distinction between through- hardening grades and case-hardening carburizing grades (8620, 9310), specialty bearing steel (52100), the Jominy end-quench hardenability test as the standard engineering tool, and the QC and documentation discipline required for critical alloy steel component supply.
AISI/SAE Alloy Steel
Numbering System
RR Hydraulic’s dedicated Alloy 4140 and 4340 references discuss those specific grades in detail — this reference explains the underlying numbering logic that makes the entire alloy steel family, including grades not individually covered elsewhere, immediately interpretable.
1.1 — How the 4-Digit Designation Works
43xx — Nickel-chromium-molybdenum series (4340)
86xx — Nickel-chromium-molybdenum, lower-alloy carburizing series (8620)
93xx — Nickel-chromium-molybdenum, higher-alloy carburizing series (9310)
52xxx — High-carbon chromium bearing steel series (52100)
1.2 — Complete Grade Reference Across the Family
| Grade | Series | Heat Treatment Approach | Primary Application | RR Hydraulic Reference |
|---|---|---|---|---|
| 4130 | 41xx (Cr-Mo) | Through-hardening (lower carbon than 4140) | Welded chromoly tubing, aircraft structures | This reference, Section 1.1 |
| 4140 | 41xx (Cr-Mo) | Through-hardening (quench & temper) | General high-strength shafts, A193 B7 studs | Alloy 4140 reference |
| 4340 | 43xx (Ni-Cr-Mo) | Through-hardening, deep hardenability | Landing gear, high-stress aerospace/defence components | Alloy 4340 reference |
| 8620 | 86xx (Ni-Cr-Mo, carburizing) | Case-hardening (carburizing) | Gears, camshafts — tough core + hard wear surface | This reference, Part 2 |
| 9310 | 93xx (Ni-Cr-Mo, carburizing) | Case-hardening (carburizing) | Aerospace gears — higher core toughness than 8620 | This reference, Part 2 |
| 52100 | 52xxx (high-C Cr) | Through-hardening (very high hardness) | Bearings — rolling contact fatigue resistance | This reference, Section 2.3 |
Case-Hardening (Carburizing) —
8620 and 9310
The 8620 and 9310 carburizing grades represent a fundamentally different heat treatment philosophy from the through-hardening approach discussed throughout RR Hydraulic’s 4140 and 4340 references — worth understanding as a genuinely distinct design strategy, not simply “another alloy steel grade.”
2.1 — Through-Hardening: The 4140/4340 Approach
Through-hardening — the heat treatment philosophy discussed in detail throughout RR Hydraulic’s Alloy 4140 and 4340 references — quenches and tempers the component to achieve a relatively uniform hardness and strength throughout its entire cross-section, governed by the alloy’s hardenability (its ability to form martensite at depth, discussed via Jominy testing in Part 3). This approach suits applications requiring uniform strength throughout the component — shafts, high-strength fasteners, and structural components where the entire cross-section must resist the applied load.
2.2 — Case-Hardening (Carburizing): The 8620/9310 Approach
2.3 — 8620 vs. 9310: Selecting Between the Carburizing Grades
8620 — General Purpose Carburizing Grade
The standard, most widely used and cost-effective carburizing grade for general gear, shaft, and camshaft applications across general machinery and automotive applications — providing a good, well-proven combination of case hardness and core toughness for the large majority of carburized component applications.
9310 — Aerospace-Grade Carburizing Steel
A higher-nickel-content carburizing grade providing superior core toughness and fatigue strength compared to 8620, at correspondingly higher cost — the standard specification for aerospace and defence gear applications (per RR Hydraulic’s Defence & Aerospace reference) where the higher consequence of gear failure justifies 9310’s improved core properties over the more general-purpose 8620.
2.4 — Bearing Steel: 52100
AISI 52100 is a high-carbon (approximately 1.0%), chromium-alloyed (approximately 1.5% Cr) through-hardening steel specifically engineered and near-universally specified for rolling element bearings (ball and roller bearings) — its very high achievable hardness (typically 60–65 HRC after through-hardening heat treatment) and specifically controlled, very low non-metallic inclusion content (critical for rolling contact fatigue resistance, since inclusions act as fatigue crack initiation sites under the repeated rolling contact stress bearings experience) make it a distinctly specialised grade compared to the general-purpose structural and shaft alloy steels discussed elsewhere in this reference.
How Hardenability Is
Actually Measured and Specified
RR Hydraulic’s dedicated 4140 and 4340 references discuss hardenability as a key concept differentiating alloy steel grades — the Jominy end-quench test is the specific, standard engineering test method used to measure and specify it.
3.1 — How the Jominy Test Works
The Jominy end-quench test (per ASTM A255/SAE J406) heats a standardised cylindrical test specimen to its austenitizing temperature, then quenches only one end of the specimen with a controlled water spray while the rest of the specimen air-cools — creating a controlled, progressively slower cooling rate along the specimen’s length moving away from the quenched end. Hardness is then measured at standard intervals along this length, producing a hardenability curve (hardness vs. distance from the quenched end) that directly reflects the specific heat’s actual hardenability — how deep a martensitic, high-hardness structure can be achieved before the cooling rate becomes too slow to prevent softer transformation products from forming.
3.2 — Why Jominy Testing Matters for Alloy Steel Specification
Confirms Actual Hardenability for the Specific Heat
Since hardenability varies somewhat between different heats of the same nominal grade (due to normal, specification-compliant variation in exact alloy content and grain size), Jominy testing on the actual production heat provides direct confirmation of hardenability for that specific material lot, rather than relying solely on typical/nominal hardenability data for the grade.
Directly Relevant to Large-Diameter Component Qualification
For large-diameter through-hardened components (particularly relevant to the diameter-dependent property step-down discussed in RR Hydraulic’s Alloy 4340 and A193 B7 references), Jominy hardenability data helps confirm whether the specific heat can achieve adequate hardness/strength at the component’s core, not just at the surface, for large cross-sections.
Standard Specification Requirement for Critical Applications
Many critical component specifications (aerospace, defence, and high-consequence mechanical applications) explicitly require Jominy hardenability test reports as part of the material certification package, particularly for 4340 and other deep-hardenability-critical grades, ensuring the specific heat meets the application’s minimum hardenability requirement rather than relying on nominal grade chemistry alone.
Industry Applications
& Documentation
RR Hydraulic maintains full traceability across the complete alloy steel family, from certified heat through finished, tested, and packed component shipment.
4.1 — Inspection & QC Protocol
4.2 — EN 10204 / Documentation Requirements
| Certificate | Content | EPC Requirement | When Mandatory |
|---|---|---|---|
| 2.1 / 2.2 | Declaration / non-specific | Acceptable for non-critical general applications | Low-consequence general machinery components |
| 3.1 (EN 10204) | Heat-traceable chemical + mechanical test report | Mandatory — EPC/critical supply | Critical shafts, gears, fasteners, and high-stress components |
| Jominy hardenability report | End-quench test data per ASTM A255/SAE J406 | Mandatory — critical through-hardening applications | Large-diameter or safety-critical 4140/4340 components |
| 3.2 (EN 10204) | 3.1 + TPI countersign | Critical / owner-specified critical items | Aerospace, defence, and other high-consequence supply |
4.3 — Applications by Industry
Gears, Camshafts, and Wear Components
8620 and 9310 carburizing steel for gears, camshafts, and similar components requiring the combined hard wear surface and tough core discussed in detail in Part 2 — a fundamentally different design approach from the through-hardened shafts and fasteners discussed throughout RR Hydraulic’s 4140/4340 references.
Bearings and Precision Rolling Contact Components
52100 bearing steel for ball and roller bearing manufacturing, leveraging its very high achievable hardness and controlled cleanliness discussed in Section 2.4 — a distinctly specialised grade within the broader alloy steel family.
Welded Tubular Structures
4130 chromoly for welded tubular structures (aircraft, roll cages, general welded tubing) where lower carbon content than 4140 provides the weldability discussed in Section 1.1 while retaining good strength and toughness.
4.4 — Export Packaging Specification
- Alloy steel bar and components packed by grade and heat treatment condition with clear labelling, given the meaningful performance differences across the family discussed throughout this reference
- Heat/lot number marked or tagged on each item, cross-referenced to the accompanying material test certificate and Jominy hardenability report where applicable
- Rust-preventive oil treatment for uncoated bar stock and components per standard practice discussed throughout RR Hydraulic’s surface treatment references
- Documentation in a waterproof pocket: EN 10204 3.1/3.2 (or 2.1/2.2 where acceptable) MTC, chemical composition report, mechanical properties report, Jominy hardenability report (where applicable), case depth report (carburizing grades), and packing list with grade/condition/size breakdown per item
- ISPM-15 timber or export cartons for international shipment, with country of origin and HS tariff code documentation matched to the alloy steel product category
Submit your grade, heat treatment condition, and quantity to RR Hydraulic for a complete, certified commercial offer.