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Annealing AISI 4140 and 4340: Parameters, Cycles, and Hardness Outcomes

AISI 4140 and 4340 are the two most widely specified alloy steels for industrial heat treatment work — 4140 (chromium-molybdenum) for general-purpose shafts, axles, and machine components in sections up to about 4 inches, and 4340 (nickel-chromium-molybdenum) for heavier sections, higher-strength applications, and parts where core toughness matters. 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. Annealing these grades differs from annealing plain carbon steel in three consequential ways: the alloy content lowers Ac3 modestly but substantially retards austenite decomposition on cooling, which makes a strictly controlled slow cool essential; the hardenability is high enough that a mistimed cool can produce bainite or martensite rather than pearlite, ruining the annealed condition; and the alloy elements provide modest surface protection against decarburization compared to plain carbon steels. This article covers grade-specific annealing parameters for 4140 and 4340, the reasons slow cooling cannot be compromised, when spheroidize annealing is the correct choice over full annealing, and typical hardness outcomes and verification practice.

Why is annealing 4140 and 4340 different from annealing plain carbon steel?​

Alloy steels differ from plain carbon steels in annealing behavior because their alloying elements (chromium, molybdenum, nickel, and manganese in 4140 and 4340) shift the transformation kinetics during cooling. On the time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) diagrams for these grades, the pearlite "nose" is displaced to longer times compared to carbon steel — meaning that at any given cooling rate, alloy steel produces a different microstructure than plain carbon steel would. For full annealing, this has three practical consequences. First, the cooling rate through the transformation range must be slow enough to pass to the right of the pearlite nose on the CCT diagram — typically 25–50 °F per hour for 4140, and slower still (15–40 °F per hour) for 4340 because of its higher alloy content and broader bainite field. Second, if the cool rate is too fast, the part will form bainite or martensite instead of pearlite — producing hardness of 30–50 HRC rather than the annealed target of roughly 12–15 HRC (163–197 HB for 4140). Third, the austenitizing temperature is modestly lower than for carbon steels at equivalent carbon content — Ac3 for 4140 is approximately 1,525 °F (830 °C) versus 1,500 °F for 1045 at the same 0.40% carbon level, because the Ni/Cr/Mo additions slightly lower the critical temperature. The net result: alloy steel annealing cycles run at roughly the same austenitizing temperatures as carbon steels, but with substantially more stringent cooling-rate control, and with greater consequence for getting the cooling rate wrong (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide: Practices and Procedures for Irons and Steels, ASM International, 2nd ed., 1995; SAE J406; SAE J1268).

What are the full annealing parameters for AISI 4140?​

AISI 4140 (0.38–0.43% C, 0.75–1.00% Mn, 0.80–1.10% Cr, 0.15–0.25% Mo per ASTM A29/A322) full annealing cycle: ramp to 1,500–1,600 °F (815–870 °C) at a rate not exceeding 400 °F per hour above 600 °F; soak one hour per inch of section thickness, minimum one hour, with additional soak (up to 1.5 hr/inch) advisable for heavy sections to ensure full carbide dissolution; furnace cool at 25–50 °F per hour through the transformation range (approximately 1,400 °F down through 1,150 °F — the critical window for pearlite formation); continue slow cooling below 1,000 °F, and do not remove from the furnace until the part has cooled below approximately 900 °F. Removing the part to still air at too high a temperature — even below Ac1 — risks a fast cool through the balance of the transformation range and unpredictable hardness. The resulting microstructure is coarse pearlite plus pro-eutectoid ferrite; typical hardness 163–197 HB (approximately 10–15 HRC on conversion). If the measured hardness comes in significantly above this range (230 HB or higher, for example), the cooling rate was almost certainly too fast — the part has formed bainite rather than pearlite and the anneal has not achieved its intended outcome. Total furnace occupancy for a 4140 full anneal on typical bar-stock or billet geometry is 18–36 hours depending on section size. For cycles where the part must be in the softest possible condition for heavy cold forming or severe machining, the lower end of the austenitizing range (1,500 °F) combined with the slowest controlled cool (25 °F/hr) produces the best result (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).

What are the full annealing parameters for AISI 4340?​

AISI 4340 (0.38–0.43% C, 0.60–0.80% Mn, 0.70–0.90% Cr, 0.20–0.30% Mo, 1.65–2.00% Ni per ASTM A29/A322) differs from 4140 primarily in its higher nickel content, which increases hardenability substantially and further retards austenite decomposition on cooling. Full annealing cycle for 4340: ramp to 1,475–1,525 °F (800–830 °C) at a controlled rate; soak one hour per inch of section thickness, with heavier sections benefiting from 1.25–1.5 hr/inch to ensure full equilibration of the carbide and austenite phases; furnace cool at 15–40 °F per hour through the transformation range (1,400 °F down through 1,150 °F) — noticeably slower than 4140 because the pearlite nose on the CCT diagram is shifted to even longer times; continue slow cooling below 900 °F before removal. The resulting microstructure is coarse pearlite plus pro-eutectoid ferrite; typical hardness 197–241 HB — somewhat harder than fully annealed 4140 because even at the slowest practical cooling rates, 4340's higher alloy content produces slightly finer pearlite. If the measured hardness exceeds 260 HB after a nominal anneal cycle, the cool rate was insufficient and bainite has formed; re-annealing at a slower cool rate or shifting to a spheroidize anneal may be necessary to reach the required hardness. Total furnace occupancy for a 4340 full anneal is 24–48 hours — longer than 4140 for comparable geometry because of the slower required cooling. The 4340 grade-specific heat treatment article covers the full heat-treatment picture including austenitize-quench-temper and induction hardening parameters (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995; SAE J1397).

When should spheroidize annealing be specified for 4140 and 4340 instead of full annealing?​

Spheroidize annealing is specified for 4140 and 4340 when machinability is the dominant cost driver and a 10–20 HB hardness reduction plus improved chip control justifies the longer cycle. The mechanism is the same as for any steel: a subcritical hold at 1,350–1,400 °F (just below A1) for 8–24 hours converts the lamellar cementite of a pearlitic starting structure into rounded carbide particles dispersed in a ferritic matrix, producing the softest and most machinable condition the grade can reach. Spheroidize-annealed 4140 typically reaches 163–183 HB, compared to 170–197 HB for full-annealed 4140; the more important difference is chip morphology — the spheroidized structure produces short, well-broken chips under carbide tooling where full-annealed 4140 tends to form long continuous chips. For 4340, spheroidize annealing produces hardness 180–210 HB versus 200–241 HB full-annealed. Specify spheroidize annealing when: the part requires heavy rough machining and reduced cycle time or improved tool life justifies the longer anneal cycle; the part will receive substantial cold deformation (cold forging, cold heading, heavy cold forming) where the soft ferritic matrix and rounded carbides are necessary to avoid cracking; or the part requires very smooth machined surfaces (better than Ra 63 μin) where chip-control problems from lamellar pearlite would be limiting. For standard production machining of 4140 and 4340 on modern CNC equipment with appropriate carbide tooling and cooling, full annealing is typically adequate and the shorter cycle is preferred. The decision between full and spheroidize annealing is covered in detail in the companion article in this section (ASM Handbook, Vol. 4A, ASM International, 2013; ASM Handbook, Vol. 16: Machining, ASM International, 1989; Heat Treater's Guide, ASM International, 1995).

Why does alloy steel annealing require slower cooling than carbon steel?​

The slow-cool requirement for alloy steel annealing traces directly to the displacement of the pearlite nose on the continuous-cooling-transformation (CCT) diagram. For plain carbon steel (1045, for example), the pearlite nose sits at approximately 1–3 seconds at 1,050 °F — meaning that any cooling rate slower than roughly 100 °F per second will produce pearlite rather than martensite. Full annealing cooling rates of 25–50 °F per hour are well to the right of the nose and easily produce coarse pearlite. For 4140, the pearlite nose is shifted to approximately 5–15 seconds at 1,050 °F, and the bainite nose that sits below the pearlite nose extends out to 30–60 seconds — meaning that a cooling rate of only 10–30 °F per minute (much slower than carbon steel) is required to avoid bainite formation. For 4340, the effect is more pronounced: the pearlite nose is at approximately 30–90 seconds at 1,050 °F, and the bainite nose extends to several minutes. A cool rate of 10–20 °F per minute through the transformation range still risks bainite in 4340. This is why full annealing of these alloy grades requires a controlled cool of 15–50 °F per hour through the transformation range — two to three orders of magnitude slower than carbon steel — and why the furnace cannot be opened or the part removed until it has cooled well below the bainite field. UTEC Industrial's car-bottom furnace uses programmable ramp-and-soak control that holds the cooling profile precisely through the critical transformation window, producing reproducible annealed hardness on 4140 and 4340 bar stock, billets, and weldments — the alternative of attempting a controlled alloy-steel anneal in a furnace without programmed cooling control is why some heat treaters refuse to take in alloy-steel anneal work (ASM Handbook, Vol. 4A, ASM International, 2013; Totten, Steel Heat Treatment Handbook, 2nd ed., CRC Press, 2006; SAE J406).

What are typical applications for annealing 4140 and 4340?​

Annealing of 4140 and 4340 appears in several distinct manufacturing situations. Pre-machining softening of as-forged or hot-rolled billet stock is the most common — forged 4140 billet often arrives at 255–302 HB (hot worked plus air cool), which is hard enough to significantly reduce tool life during rough machining; full annealing drops this to 163–197 HB and allows efficient material removal with standard carbide tooling. Similarly, 4340 arrives from hot-rolling at 241–302 HB and is annealed to 197–241 HB for easier machining. Inter-operation softening of cold-worked parts: alloy steel that has been cold-drawn, deep-drawn, or heavily cold-formed and work-hardened to 300+ HB may require full annealing between forming operations to restore ductility; subcritical annealing (covered in a companion article) is sometimes adequate but supercritical annealing is the safer choice when substantial ductility recovery is needed. Post-weld annealing of thick-section 4140 weldments is occasionally specified when both stress reduction and microstructure normalization are required — typically this is full annealing followed by controlled cooling, and is a more extensive cycle than stress-relief alone. Re-softening of quench-and-tempered parts that must be reworked — for example, a Q&T 4140 shaft that must be modified or repaired; a full anneal to 170–190 HB restores machinability, after which the part can be reworked and re-hardened if necessary. Restoration of proper microstructure after an improperly executed heat treatment — if a previous cycle produced mixed microstructure or excessive grain growth, a careful full anneal can reset the microstructure for a subsequent austenitize-quench-temper cycle. In each of these applications, the key specification question is whether the target hardness and microstructure uniformity justifies the long furnace time of a full anneal, or whether a shorter cycle (normalizing, stress relief, spheroidize anneal) would serve equally well (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).

What verification and documentation applies to alloy steel annealing?​

Verification of a successful alloy steel anneal rests on three indicators: hardness measurement, furnace chart review, and (for critical applications) microstructure examination. Hardness measurement: after cooldown, Brinell hardness readings (per ASTM E10) are taken at representative locations on the annealed part — typically at one end and in the middle for bar stock, at multiple locations on the surface for weldments or irregular parts. Typical acceptance for 4140 full anneal is 163–197 HB; for 4340 full anneal, 197–241 HB. A hardness reading above the specified range indicates that cooling was too fast and some bainite or fine pearlite formed — the part may be usable for its intended application if the hardness is only slightly elevated, but a substantially out-of-range reading (for example, 260 HB on 4140) requires either re-annealing at a slower cool rate or acceptance by the customer with documented deviation. Furnace chart review: the programmable ramp-and-soak record shows the complete cycle — ramp rate, soak temperature and duration, cooling profile through the transformation range, and removal temperature. This chart documents the heat treatment and accompanies the part as the quality record. Microstructure examination (per ASTM E3 and E407 for preparation and etching) is specified for critical applications — a representative section is polished, etched with 2% nital, and examined at 100–500× magnification to confirm coarse pearlite plus pro-eutectoid ferrite (indicating a successful anneal) rather than bainite, fine pearlite, or partial spheroidization. For most industrial 4140/4340 annealing work, hardness plus furnace chart is sufficient; microstructure examination is reserved for aerospace, pressure vessel, or other high-consequence applications. For comprehensive documentation requirements and what a complete heat-treatment record contains, see the companion documentation article (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM E10; ASTM E3; ASTM E407; Heat Treater's Guide, ASM International, 1995).

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References​

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  • ASTM E3: Standard Guide for Preparation of Metallographic Specimens. ASTM International.
  • ASTM E407: Standard Practice for Microetching Metals and Alloys. ASTM International.

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