Heat Treating AISI 4340: High-Hardenability Alloy Steel Parameters
AISI 4340 is the standard high-hardenability alloy steel for large-section industrial components — a nickel-chromium-molybdenum steel that through-hardens in oil in sections up to 6 inches or more, achieving higher hardness at any given tempering temperature than 4140, with substantially better impact toughness at the same hardness. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. It is specified over 4140 wherever section size, required hardness, or toughness demands exceed what 4140 can reliably deliver — heavy shafting, large gears, high-strength bolting, aerospace structural components, and large crane wheel hubs. This article covers 4340's composition, hardenability, all applicable heat treatment cycles, and the grade comparison logic for choosing between 4340 and 4140.
What are the composition and key characteristics of 4340 compared to 4140?
AISI 4340 per ASTM A29: 0.38–0.43% carbon, 0.60–0.80% manganese, 1.65–2.00% nickel, 0.70–0.90% chromium, 0.20–0.30% molybdenum, 0.15–0.35% silicon. Compared to 4140, the defining difference is the 1.65–2.00% nickel addition. Nickel has two effects: it substantially increases hardenability (nickel slows the pearlite and bainite transformation kinetics, allowing martensite to form at slower cooling rates in larger sections), and it improves the toughness and ductility of the tempered martensite at any given hardness level — the Charpy impact energy of tempered 4340 at 32 HRC is typically 60–100 ft·lb, versus 40–70 ft·lb for tempered 4140 at the same hardness. The Cr-Mo system in 4340 provides the same carbide stability and temper resistance as in 4140; the Ni adds the hardenability and toughness that makes 4340 the standard for heavy sections. Jominy end-quench hardenability for 4340 (H grade) shows minimum hardness of 25 HRC at J16 (versus 20 HRC for 4140), with substantially higher hardness at larger Jominy distances — confirming oil-quench martensite formation in sections up to 5–6 inches under production conditions. The price of 4340 bar is typically 20–40% higher than equivalent 4140, making grade selection a cost-versus-capability decision (ASM Handbook, Vol. 1, ASM International, 1990; SAE J1397; ASTM A29).
What are the annealing parameters for 4340?
Full annealing of 4340: austenitize at 1,500–1,560 °F (815–845 °C) — slightly lower than 4140 because the Ni-Cr-Mo alloy complex reduces Ac3 by approximately 30–40 °F; soak one hour per inch of section thickness; furnace cool at 25–50 °F/hr through the transformation range to below 900 °F. Resulting hardness: 187–229 HB — slightly harder than annealed 4140 (163–197 HB) because the higher alloy content of 4340 retards carbide coarsening even in the fully annealed condition. For heavy billet (6 inch and above), achieving the lower end of this range (187–200 HB) requires strict control of the furnace cooling rate in the 1,350–1,200 °F range — higher rates produce finer pearlite and higher hardness. Spheroidize annealing of 4340 — sub-critical hold at 1,350–1,400 °F for 12–20 hours — produces 179–207 HB with a spheroidal carbide microstructure that machines better than the full-annealed pearlitic condition. Spheroidize annealing is more commonly specified for 4340 than for 4140 because 4340's higher alloy content makes the pearlitic annealed condition somewhat more resistant to cutting (harder, more abrasive carbides) and the spheroidized structure provides a more meaningful machinability improvement. For shops machining large 4340 billets before quench-and-temper, the spheroidize anneal can meaningfully reduce cycle time and tooling cost (Heat Treater's Guide, ASM International, 1995; ASM Handbook, Vol. 4A, ASM International, 2013).
What are the quench-and-temper parameters for 4340?
Through-hardening of 4340: austenitize at 1,475–1,525 °F (800–830 °C); soak one hour per inch of section; quench in agitated oil. The lower austenitizing temperature compared to 4140 reflects 4340's lower Ac3 and finer carbide distribution — the carbides dissolve more readily. As-quenched hardness: 55–60 HRC for fully martensitic sections, depending on carbon content variation. Temper immediately. Representative tempering response for 4340 (oil-quenched, surface measurement):
| Tempering Temperature | Approx. Hardness |
|---|---|
| 400 °F (205 °C) | 54–57 HRC |
| 600 °F (315 °C) | 51–54 HRC |
| 800 °F (425 °C) | 47–50 HRC |
| 900 °F (480 °C) | 43–47 HRC |
| 1,000 °F (540 °C) | 38–43 HRC |
| 1,100 °F (595 °C) | 33–38 HRC |
| 1,200 °F (650 °C) | 30–36 HRC |
At every tempering temperature, 4340 runs 2–5 HRC higher than equivalent 4140 — the benefit of the higher alloy content for applications where maximum hardness at a given toughness level is needed. The temper embrittlement range for 4340 (450–570 °F) is more severe than for 4140 due to the higher nickel content — nickel accelerates the phosphorus grain-boundary segregation mechanism. For impact-loaded applications, avoid this range rigorously; temper at 400 °F or below, or at 600 °F or above. Some high-performance applications for 4340 (aerospace landing gear, ordnance components) specify a rapid cool from tempering temperature to suppress embrittlement kinetics during cooling — a practice not routine for industrial production but worth noting for specifications that reference AMS 6414 or equivalent aerospace material standards (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995; AMS 6414).
How does 4340 hardenability compare to 4140 in heavy sections?
The hardenability difference between 4340 and 4140 is most consequential in sections above 2 inches. In a 1-inch bar, both grades reach full martensite in oil — the core hardness difference between 4140 and 4340 at the same tempering temperature is minimal. At 3-inch section, 4140 begins to show core softening from non-martensitic transformation products (bainite at the core); the core hardness after oil quench and 1,050 °F temper may be 4–6 HRC below the surface. At 3-inch section, 4340 retains full or near-full martensite through-section — core hardness matches surface hardness within 2–3 HRC. At 5-inch section, 4140 in oil quench has substantial bainite at the core and may reach only 26–28 HRC at the center after 1,050 °F temper, even if the surface is at 32 HRC. 4340 in oil at 5-inch section still reaches 32–35 HRC at the core. This hardenability gap drives the specification decision for large industrial shafts, heavy crane wheel hubs, large gear blanks, and similar parts where core properties are functionally important: if through-section hardness is required at diameters above 2–3 inches, 4340 (or an equivalent high-hardenability grade like 300M, 4340 Mod, or 8740) is the specification, not 4140. For sections below 2 inches where both grades perform equivalently, 4140 is the cost-effective choice (ASM Handbook, Vol. 4A, ASM International, 2013; ASM Handbook, Vol. 1, ASM International, 1990).
What normalizing and stress relief parameters apply to 4340?
Normalizing 4340: austenitize at 1,600–1,650 °F (870–900 °C), soak one hour per inch of section, air cool. Resulting hardness: 269–321 HB — significantly harder than normalized 4140 (197–241 HB), because 4340's higher hardenability means the air-cooled microstructure contains more bainite and less coarse pearlite, especially in heavy sections. For machining purposes, normalized 4340 above 285 HB is difficult to machine economically with standard carbide tooling — annealing or spheroidize annealing is preferred as the pre-machining condition for 4340, rather than normalizing. Normalizing of 4340 is most useful as a pre-quench conditioning step to refine grain structure before the final Q&T, not as a pre-machining treatment. Stress relief of previously Q&T'd 4340: the same rule as 4140 applies — stress relief temperature must be at least 50 °F below the original tempering temperature. Because 4340 is often tempered at higher hardness than 4140 (the grade is selected for maximum hardness in heavy sections), the window between tempering temperature and an effective stress relief temperature is sometimes tight. For 4340 parts tempered at 800–900 °F for service hardness, stress relief can be performed at 750–850 °F — effective for stress reduction (60–70% typical) with modest hardness drop (2–4 HRC, within specification if the range was set appropriately) (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).
When should 4340 be specified instead of 4140 — the decision logic?
The specification decision between 4340 and 4140 is driven by five factors, in rough priority order: (1) Section size — above 3 inches in any dimension requiring through-hardness, 4340 is the standard upgrade path. (2) Required hardness at core — if the drawing specifies minimum core hardness above approximately 30 HRC in sections above 2 inches, verify with Jominy data that 4140 can achieve it; if not, specify 4340. (3) Impact toughness — for components in dynamic loading (impact, fatigue, cyclic bending), 4340 provides 30–50% higher Charpy impact energy than 4140 at the same hardness; this advantage is material for crane and hoist components, driveline shafts, and any part in cyclic loading below 32 HRC. (4) Cost — 4340 is 20–40% higher raw material cost than equivalent 4140 bar; for thin sections or low-hardness applications where both grades perform identically, 4140 is the default. (5) Availability — 4140 is stocked in a broader range of sizes and forms than 4340 in most North American steel service centers; 4340 may require longer lead time for non-standard sizes. For UTEC's crane wheel production, the decision is typically: axles and hubs in sections above 3 inches specify 4340; smaller shafts and pins in sections below 2 inches specify 4140. For custom fabrication customers, the choice is made case-by-case based on drawing requirements and the hardenability-versus-cost trade-off (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).
What are the common specification and processing errors with 4340?
4340 heat treating errors follow similar patterns to 4140 but with grade-specific wrinkles. Most common: (1) Austenitizing at the 4140 temperature (1,550–1,600 °F) instead of the correct 4340 range (1,475–1,525 °F) — 4340 responds to slightly lower austenitizing temperature; the higher temperature causes grain coarsening that reduces toughness without improving hardness. (2) Tempering in the embrittlement range (450–570 °F) — more damaging for 4340 than 4140 due to the higher Ni content amplifying phosphorus grain boundary segregation; the resulting impact toughness loss can be dramatic (from 80+ ft·lb to under 20 ft·lb). (3) Specifying 4340 for sections under 1 inch where 4140 is identical in performance — an unnecessary cost premium. (4) Expecting 4340 to through-harden in water when the geometry or section would benefit from oil — 4340 in most sections oil-quenches successfully; water quench adds cracking risk without hardenability benefit. (5) Failing to specify tempering temperature minimum on the drawing — a hardness callout of 36–40 HRC on a 4340 drawing could legally be satisfied by tempering at 530 °F (in the embrittlement range). The fix: specify the grade, Q&T process, hardness range, hardness location and method, and a minimum tempering temperature of 600 °F (or maximum of 400 °F for very high hardness applications) (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).
- Heat Treating AISI 4140: Austenitize, Quench, and Temper Parameters — the more common, lower-cost alternative for lighter sections
- Through-Hardening and Quench-and-Temper: Process Fundamentals — the general Q&T process framework
- Tempering Temperature and Hardness: Relationship by Steel Grade — tempering curves and embrittlement range details
- Quench Media: Water, Oil, Polymer, and Air — Selection and Effects — why oil quench is standard for 4340
References
- ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
- ASM International. (1990). ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys. ASM International.
- ASM International. (1995). Heat Treater's Guide: Practices and Procedures for Irons and Steels (2nd ed.). ASM International.
- Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.
- Machinery's Handbook (31st ed.). (2020). Industrial Press.
- SAE J1397: Estimated Mechanical Properties and Machinability of Steel Bars. SAE International.
- ASTM A29: Standard Specification for General Requirements for Steel Bars, Carbon and Alloy, Hot-Wrought. ASTM International.
- AMS 6414: Steel Bars, Forgings, and Tubing, 0.80Cr - 1.8Ni - 0.25Mo (0.38 - 0.43C). SAE Aerospace.
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