When specifying alloy steel forgings for critical applications, two grades dominate the engineering conversation: AISI 4330 (and its vanadium-microalloyed variant, 4330V / 4330V MOD) and AISI 4340. Both belong to the chromium-nickel-molybdenum (Cr-Ni-Mo) family of low-alloy high-strength steels. Both are available as open-die forgings and seamless rolled rings. Both respond well to quench-and-temper heat treatment.
Yet choosing the wrong grade — even within this closely matched pair — can mean under-designed fatigue life in a drill collar body, an unnecessary cost premium on a gearbox shaft, or a failed NACE hardness qualification at the mill. This guide provides a complete, property-by-property comparison to help metallurgists, design engineers, and procurement professionals make the right specification decision.
AISI 4330 vs 4340: Chemical Composition Compared
AISI 4330 and AISI 4340 are both Cr-Ni-Mo low-alloy steels. The critical differences are: carbon content (4330: 0.28–0.33%; 4340: 0.38–0.43%), molybdenum content (4330: 0.30–0.40%; 4340: 0.20–0.30%), and vanadium in 4330V (0.05–0.10%), which is absent in 4340. These three differences directly drive all real-world performance distinctions between the two grades.
| Element | AISI 4330 / 4330V | AISI 4340 | Engineering Significance |
|---|---|---|---|
| Carbon (C) | 0.28–0.33% | 0.38–0.43% | Lower C in 4330 → better toughness; lower hardness ceiling; reduced H₂S cracking risk |
| Manganese (Mn) | 0.60–0.80% | 0.60–0.80% | Identical — equivalent hardenability contribution |
| Chromium (Cr) | 0.70–0.90% | 0.70–0.90% | Same — surface hardenability and oxidation resistance |
| Nickel (Ni) | 1.65–2.00% | 1.65–2.00% | Same — matrix toughness; low-temperature ductility |
| Molybdenum (Mo) | 0.30–0.40% | 0.20–0.30% | Higher Mo in 4330 → superior deep hardenability in heavy sections above 250 mm |
| Vanadium (V) | 0.05–0.10% (4330V only) | — | Unique to 4330V: austenite grain refinement → improved notch toughness and fatigue life |
| Silicon (Si) | 0.15–0.35% | 0.15–0.35% | Deoxidation — equivalent in both grades |
| Phosphorus (P) | ≤ 0.025% | ≤ 0.025% | Maximum limit — same for both grades |
| Sulfur (S) | ≤ 0.025% | ≤ 0.025% | Maximum limit — same for both grades |
The lower carbon content of AISI 4330 is not a weakness — it is a deliberate engineering trade-off. By reducing carbon from ~0.40% to ~0.30%, the alloy sacrifices approximately 50–80 MPa of peak tensile strength in exchange for measurably better notch toughness, Charpy impact energy, ductility, and resistance to hydrogen embrittlement in H₂S-containing sour environments.
AISI 4330V vs 4340: Mechanical Properties Side-by-Side
In quenched-and-tempered condition at 30–34 HRC: AISI 4330V achieves approximately 1,050 MPa tensile strength and over 68 J Charpy impact at −20 °C. AISI 4340 achieves approximately 1,170 MPa tensile strength but only ~45 J Charpy at the same temperature. Values depend on section size, chemistry, and heat treatment parameters.
Conclusion: AISI 4340 wins on peak tensile and yield strength. AISI 4330V wins on toughness, impact energy, ductility, and sour-service resistance. The right choice depends on which failure mode your application is most susceptible to.
Why AISI 4330 Outperforms 4340 in Heavy-Section Forgings
For large-diameter forgings — drill collar bodies, heavy ring forgings, and shafts with cross-sections above 250–300 mm — through-hardenability is often the single most critical selection criterion. A forging achieving 36 HRC at the surface but only 26 HRC at mid-radius will underperform in service and fail ultrasonic testing.
The higher molybdenum content in AISI 4330 (0.30–0.40% vs. 0.20–0.30% in 4340) suppresses the bainite nose in the TTT diagram, slowing transformation and allowing the quench more time to form a fully martensitic structure at depth. This is why 4330V MOD has become the global standard material for large-bore downhole tool body forgings. Jiangsu Liangyi manufactures AISI 4330V open-die forgings and seamless rolled rings from 30 kg to 30,000 kg with full hardenability documentation per heat.
For round forgings above 250 mm diameter or ring wall thickness above 150 mm: request Jominy hardenability band data from your supplier and verify the required hardness at ½-radius is achievable. AISI 4330V MOD consistently delivers uniform hardness through heavy sections; 4340 may require tighter chemistry control to match the same result at large cross-sections.
Heat Treatment: AISI 4330 vs 4340 Temperature Windows
Both grades follow a quench-and-temper (Q&T) cycle. The process steps are broadly similar, but critical temperature differences and constraints matter significantly for production outcomes on large forgings.
Normalize (Recommended for Sections Above 150 mm)
Both grades: 870–900 °C, air cool. Refines as-forged grain structure and reduces chemical banding. Adds lead time but strongly recommended for forgings over 150 mm and all premium quality applications.
Austenitize
AISI 4330V: 843–871 °C (1,550–1,600 °F). Tighter window — exceeding 880 °C risks coarsening vanadium carbide precipitates. AISI 4340: 800–845 °C — slightly lower due to higher carbon content lowering the Ac₃ temperature.
Quench
Oil quench is standard for sections above 50 mm for both grades, to control thermal gradient and distortion. AISI 4330V has slightly lower quench cracking risk than 4340 due to its lower carbon content — an advantage in large-section oil quench operations.
Temper
AISI 4330V: 540–650 °C — vanadium carbide precipitation provides secondary hardening and fatigue resistance. AISI 4340: 425–650 °C. Critical for both grades: Never temper in the 260–370 °C range (tempered martensite embrittlement / TME zone) — toughness collapses and cannot be recovered without full re-heat treatment.
Stress Relieve (Post-Machining)
Both grades: 425–480 °C, 2–4 hours, air cool. Mandatory before final NDT. 4340 is more sensitive to grinding burn at high hardness — grinding parameters must be carefully controlled.
| Step | AISI 4330V | AISI 4340 |
|---|---|---|
| Normalize | 870–900 °C, air cool | 870–900 °C, air cool |
| Austenitize | 843–871 °C | 800–845 °C |
| Quench media | Oil (standard) | Oil (standard) |
| Temper range | 540–650 °C | 425–650 °C |
| Avoid (TME zone) | 260–370 °C — BOTH grades — NEVER temper in this range | |
| Stress relieve | 425–480 °C, 2–4 h, AC | 425–480 °C, 2–4 h, AC |
AISI 4330 vs 4340: Which Grade for Which Application?
The selection reduces to one fundamental question: does your application's primary failure mode favour toughness or maximum tensile strength?
- Downhole MWD/LWD drill collar bodies and stabilizers
- Mud motor mandrels and bearing housings (NACE MR0175 hardness limit HRC ≤ 34)
- Mining rotary drill rods and high-impact hammer components
- Subsea wellhead bodies and BOP components (to API 6A material requirements)
- Sour gas / H₂S service meeting NACE MR0175 / ISO 15156 hardness limits
- Heavy sections above 250 mm with mandatory uniform through-hardness
- Charpy impact above 68 J at −20 °C or below
- Cyclic / dynamic loading where fatigue governs design life
- Aircraft landing gear struts and structural airframe forgings (AMS 6415)
- High-strength fasteners, bolts and studs requiring UTS above 1,100 MPa
- Crankshafts, connecting rods, and high-load rotating power-train shafts
- Gears requiring high surface hardness after carburizing or nitriding
- Tooling and die components requiring maximum achievable hardness
- Military and defence applications (AMS 6414 / MIL-S-5000D)
- Non-sour operating environments without H₂S exposure
- Applications where peak tensile strength governs — not toughness
API 6A specifies that sour service material classes (DD, EE, FF) mandate maximum hardness of HRC ≤ 34. This limit is consistently achieved with AISI 4330V MOD. AISI 4340's higher carbon (0.38–0.43%) significantly increases susceptibility to hydrogen stress cracking (HSC) at equivalent hardness, making it generally unsuitable for wellhead, BOP, and subsea forgings in environments where NACE MR0175 or ISO 15156 hardness limits apply.
AISI 4330V vs 4340: Commercial Considerations
AISI 4340 is typically slightly more commodity-priced than 4330V MOD, available from a broader range of global steelmakers. The vanadium addition in 4330V adds approximately 3–8% on ingot price, with tighter chemistry control for 4330V MOD adding a further 5–12% premium in some markets. However, total cost of ownership in critical service often favours 4330V — a single unexpected downhole tool failure can cost orders of magnitude more than the material premium across a full production run.
| Factor | AISI 4330V MOD | AISI 4340 |
|---|---|---|
| Global ingot availability | Select specialist mills | Widely available |
| Relative material cost | +5–15% vs 4340 | Baseline (lower cost) |
| Forging process complexity | Moderate (tighter reheat window for V) | Standard |
| Typical production lead time | 4–8 weeks (custom chemistry) | 3–6 weeks (standard grade) |
| Mill Test Certificate | EN 10204 3.1 / 3.2 | EN 10204 3.1 / 3.2 |
| Key standards compliance | ASTM A646 · NACE MR0175 · API 6A material req. | AMS 6415 · AMS 6414 · ASTM A646 |
| Total cost of ownership (critical service) | Lower (fewer in-service failures) | Standard |
Applicable Standards: AISI 4330 vs 4340 Forgings
Both grades are produced to ASTM A646 (premium quality alloy steel for critical applications), with mandatory ASTM A388 ultrasonic examination and ASTM A370 mechanical testing for aerospace and energy sector applications.
Standards for AISI 4330 / 4330V / 4330V MOD Forgings
Standards for AISI 4340 Forgings
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 quality management certification. References to ASTM, API, NACE, AMS, and MIL specifications on this page describe the material and testing standards that forgings are produced to meet — they do not imply that Jiangsu Liangyi holds API Monogram licensing, NACE membership certification, or AMS/MIL qualification status. Customers requiring API Monogram or other third-party certification should confirm specific compliance requirements with our sales team prior to order placement.
Jiangsu Liangyi produces custom AISI 4330 forgings to these material and testing specifications, and supplies all shipments with EN 10204 Type 3.1 Mill Test Certificates as standard, covering chemical composition, mechanical test results, hardness, and NDT outcomes with full heat traceability. EN 10204 Type 3.2 certificates (witnessed by customer-nominated third-party inspectors — Bureau Veritas, SGS, Lloyd's Register) are available on request.
AISI 4330 vs 4340: The Bottom Line
Provide your forging supplier with: (1) operating temperature range, (2) H₂S / sour service exposure yes/no, (3) dominant loading type (static vs. cyclic vs. impact), (4) required hardness target and section size. From those four inputs, an experienced forging metallurgist can unambiguously recommend the correct grade and heat treatment target.
For full technical details on AISI 4330 forging capabilities — chemistry ranges, product forms (open die forgings, seamless rolled rings, hollow forgings), heat treatment documentation, and NDT requirements:
AISI 4330 vs 4340: Frequently Asked Questions
The main differences are carbon content (4330: 0.28–0.33%; 4340: 0.38–0.43%), molybdenum (4330: 0.30–0.40%; 4340: 0.20–0.30%), and vanadium in 4330V (0.05–0.10%) which is absent in 4340. Result: 4330V offers superior toughness (68+ J Charpy at −20 °C vs ~45 J for 4340) and sour-service resistance; 4340 offers higher tensile strength (~1,170 MPa vs ~1,050 MPa for 4330V).
AISI 4330V MOD is the industry standard for downhole tool forgings — drill collars, mud motor mandrels, MWD/LWD tool housings. Its lower carbon (0.28–0.33%) allows it to meet the hardness limits required by NACE MR0175 and API 6A sour service material classes (HRC ≤ 34). AISI 4340's higher carbon (0.38–0.43%) greatly increases susceptibility to hydrogen stress cracking in H₂S environments, making it unsuitable for most downhole applications.
At Q&T to 30–34 HRC: 4330V achieves ~1,050 MPa UTS and ~950 MPa yield strength. 4340 achieves ~1,170 MPa UTS and ~1,080 MPa yield strength. However, 4330V outperforms on Charpy impact (68+ J vs ~45 J at −20 °C), elongation (14% vs 11% min), and reduction of area (50% vs 38% min).
AISI 4330V MOD is an enhanced version of SAE 4330V with elevated nickel (typically 1.8–2.0%) and tightly controlled vanadium (0.05–0.10%). Nickel improves matrix toughness and low-temperature ductility; vanadium refines austenite grain size during hot working, improving notch toughness and fatigue resistance. It is the global standard for large-bore downhole oil and gas tool forgings where heavy sections, impact loading, and sour environments coincide.
AISI 4330V forgings are produced to: ASTM A646 (chemical composition and premium quality), ASTM A388 (ultrasonic examination), ASTM A370 (mechanical testing), NACE MR0175 / ISO 15156 (for sour service hardness limits), and API 6A material requirements (when specified by customer). Material test certificates are supplied to EN 10204 Type 3.1 standard, with Type 3.2 (third-party witnessed) available on request.
AISI 4340 is the standard aerospace forging grade, specified under AMS 6415 and AMS 6414 (formerly MIL-S-5000). It is the material of record for aircraft landing gear struts, structural airframe forgings, and high-strength fasteners requiring UTS above 1,100 MPa. AISI 4330V is not a standard aerospace grade but can be applicable in specific sub-applications where fatigue resistance under dynamic loading is the primary performance driver.
Key differences: Austenitize — 4330V at 843–871 °C; 4340 at 800–845 °C. Temper — 4330V at 540–650 °C (vanadium secondary hardening); 4340 at 425–650 °C. Both grades must avoid the 260–370 °C tempered martensite embrittlement (TME) zone. 4330V has lower quench cracking risk due to lower carbon content, an advantage in large-section oil quench operations.
AISI 4340 has severely limited suitability for NACE MR0175 sour service. Its higher carbon (0.38–0.43%) significantly increases susceptibility to hydrogen stress cracking (HSC) in H₂S environments. API 6A sour service material classes DD, EE, FF specify HRC ≤ 34, far more reliably achieved and maintained with AISI 4330V MOD. Engineers should default to 4330V MOD for all H₂S-containing environments.
Need AISI 4330 or 4340 Forgings? Get a Free Technical Quote
Send us your drawing, material specification, or application description. Our engineering team responds within 24 hours with full material compliance confirmation, heat treatment plan, and competitive pricing.