Author: Site Editor Publish Time: 2026-10-07 Origin: Site
AISI 4140 is the workhorse medium-carbon Cr-Mo alloy steel. With 0.40% carbon, 1.0% chromium and 0.20% molybdenum, it sits at the sweet spot of strength, toughness, hardenability and cost that makes it the default choice for oil & gas tool joints, automotive shafts, machine tool spindles, gears and high-strength fasteners. This article lays out the full technical file: chemistry per ASTM A29 / SAE J404, mechanical properties in the annealed, normalized and quench-and-tempered conditions, the heat-treatment temperatures that produce each strength level, and the application map that drives selection of 4140 alloy steel bar.
| Item | Value |
|---|---|
| Designation | AISI 4140 / SAE 4140 |
| Standards | ASTM A29, SAE J404, ASTM A322 (bearing quality), ASTM A331 (cold-finished alloy bar), ASTM A646 (premium QA) |
| Equivalent grades | 42CrMo4 (DIN EN 10083-3 / 1.7225), SCM440 (JIS G4105), 42CrMo (GB/T 3077), 42CrMo4 (ISO 683-1) |
| Category | Medium-carbon, low-alloy Cr-Mo steel |
| Carbon, % | 0.38–0.43 |
| Chromium, % | 0.80–1.10 |
| Molybdenum, % | 0.15–0.25 |
| Manganese, % | 0.75–1.00 |
| Silicon, % | 0.15–0.35 |
| Annealed hardness | ≤ 197 HB |
| Normalized hardness | 187–229 HB |
| Q&T hardness range | 20–55 HRC (selected by temper) |
| Typical Q&T tensile strength | 690–1100 MPa |
| Machinability rating (annealed) | ~65% of B1112 |
| Weldability | Limited — preheat 200–300 °C, low-hydrogen consumables, PWHT 550–650 °C |
| Through-hardening section (oil) | ~50 mm full hardness; ~100 mm with aggressive quench |
| Element (%) | Spec range | Typical, major mill |
|---|---|---|
| Carbon (C) | 0.38–0.43 | 0.40–0.42 |
| Manganese (Mn) | 0.75–1.00 | 0.85–0.95 |
| Silicon (Si) | 0.15–0.35 | 0.20–0.30 |
| Chromium (Cr) | 0.80–1.10 | 0.95–1.05 |
| Molybdenum (Mo) | 0.15–0.25 | 0.20–0.23 |
| Phosphorus (P), max | 0.035 | 0.015–0.020 |
| Sulfur (S), max | 0.040 | 0.015–0.025 |
| Nickel (Ni) | — (residual) | 0.10 max |
| Copper (Cu) | — (residual) | 0.20 max |
Three alloying decisions define 4140. The 0.40% carbon gives as-quenched hardness of about 55 HRC and the strength baseline. The 1% chromium deepens hardenability — 4140 through-hardens in oil to about 50 mm section, where 1045 (no Cr) tops out at 12 mm. The 0.20% molybdenum is the secret: molybdenum suppresses temper embrittlement, which means 4140 can be tempered at 540–650 °C for high toughness without the impact values dropping — a property that plain carbon steel and Cr-only alloy steel (5140 / 41Cr4) do not share. For applications where fatigue and impact both matter, 4140 earns its cost premium over 1045.
4140 is supplied in one of three conditions: annealed (soft, for machining), normalized (medium hardness, for stock), or quenched and tempered (Q&T, for service). Each route has a specific temperature window.
| Treatment | Temperature | Cooling | Resulting hardness / structure |
|---|---|---|---|
| Anneal | 845–900 °C (typ. 870 °C), hold 1 h per inch of section, slow furnace cool at ≤ 22 °C/h to 540 °C, then air cool | Furnace | ≤ 197 HB; spheroidized carbide in ferrite matrix, free-machining condition |
| Normalize | 870–925 °C (typ. 900 °C), hold 1 h per inch, air cool | Still air | 187–229 HB; fine pearlite + ferrite, uniform structure for subsequent heat treat |
| Stress relief (after heavy machining or welding) | 540–650 °C × 2 h, slow cool | Still air | No structure change; relieves residual stress |
| Austenitize (for quench) | 815–870 °C (typ. 845 °C), hold 30–45 min per inch | — | Fully austenitic at quench temperature |
| Quench | Oil at 40–60 °C, agitated (preferred). Water for sections under 8 mm only — high cracking risk | Oil | As-quenched hardness ~55 HRC at surface, martensite |
| Temper (high strength, ~1100 MPa UTS) | 425–480 °C × 2 h, air cool | Air | ~330–360 HB / 36–40 HRC |
| Temper (medium-high strength, ~930 MPa UTS) | 510–560 °C × 2 h, air cool | Air | ~290–320 HB / 31–35 HRC |
| Temper (medium strength, ~800 MPa UTS, max toughness) | 600–650 °C × 2 h, air cool | Air | ~240–270 HB / 23–28 HRC |
| Surface harden — induction | 850–900 °C surface, water spray quench, temper 150–200 °C | Water | 55–60 HRC case, 25–30 HRC core |
| Surface harden — nitride (gas or plasma) | 500–540 °C × 24–60 h, in NH3 atmosphere | Slow cool | 950–1150 HV surface (~68–72 HRC equivalent), 0.2–0.4 mm case |
The Q&T response curve is what controls 4140 selection. The same steel can be heat-treated to a tensile strength anywhere from 700 MPa (tempered at 650 °C) to 1100 MPa (tempered at 450 °C) — and the engineer chooses the temper based on the strength-toughness balance the application demands.
| Temper temperature, °C | Hardness, HRC | Tensile strength, MPa | Yield (0.2%), MPa | Elongation in 2", % | Charpy V-notch, J at 20 °C |
|---|---|---|---|---|---|
| 200 (low temper, very hard) | ~52 | 1500–1700 | 1300–1500 | 9–11 | 12–18 |
| 315 | ~48 | 1400–1500 | 1200–1300 | 11–13 | 18–24 |
| 425 | ~40 | 1100–1200 | 950–1050 | 14–16 | 30–42 |
| 480 | ~36 | 1000–1100 | 850–950 | 16–18 | 42–55 |
| 540 | ~32 | 900–1000 | 750–850 | 18–20 | 55–70 |
| 600 | ~26 | 780–860 | 650–730 | 20–23 | 70–90 |
| 650 (high temper, tough) | ~22 | 690–780 | 580–680 | 22–25 | 85–110 |
Three Q&T principles drive selection. First, tempering below 425 °C produces high strength but brittle steel — Charpy values drop below 30 J, and hydrogen cracking risk in service rises. Most structural 4140 is tempered between 540 °C and 650 °C. Second, tempering between 425 °C and 540 °C is the "magic zone" for high-strength structural applications (oil tools, gears, shafts) — tensile strength 1000–1200 MPa with 30–55 J impact toughness. Third, avoid the temper embrittlement zone (425–540 °C) for slow-cooled parts — 4140 is molybdenum-bearing precisely to suppress this, but slow cooling through that range after temper can still cause impact loss. After tempering, cool in water or fast air.
| Property | Annealed (≤197 HB) | Normalized (≈ 200 HB) | Q&T 425 °C (~40 HRC) | Q&T 540 °C (~32 HRC) | Q&T 650 °C (~24 HRC) |
|---|---|---|---|---|---|
| Tensile strength, MPa | 655 | 870 | 1100 | 950 | 760 |
| Yield strength (0.2%), MPa | 415 | 635 | 950 | 800 | 660 |
| Elongation in 2", % | 25.7 | 17.5 | 15 | 19 | 22 |
| Reduction of area, % | 56.9 | 45 | 45 | 55 | 60 |
| Charpy V-notch impact, J at 20 °C | 27 (annealed values are low despite soft structure — carbide morphology) | 14 | 38 | 60 | 95 |
| Hardness, HB | 197 | 302 | 360 | 293 | 248 |
Note: the Charpy values in the annealed and normalized conditions are lower than the Q&T conditions at similar hardness. This is the fundamental argument for Q&T: the tempered martensite microstructure gives better impact toughness than pearlite at the same hardness. The Q&T 540 °C condition (32 HRC, 950 MPa UTS, 60 J Charpy) is the most commonly specified 4140 service condition.
| Designation | Standard | Country / Region | Notes |
|---|---|---|---|
| 4140 | AISI / SAE J404 / ASTM A29 | USA | The base designation |
| 42CrMo4 | DIN EN 10083-3 | Europe | Werkstoff 1.7225; chemistry is identical; the European standard has tighter P (0.025 max) and S (0.025 max) for quality grade |
| 42CrMo4 | DIN EN 10083-3, Werkstoff 1.7225 | Europe | The "+A" (annealed), "+N" (normalized), "+QT" (quench and tempered) suffixes indicate delivery condition |
| SCM440 | JIS G4105 | Japan | Identical chemistry; the JIS tolerance on S is 0.030 max, slightly tighter than ASTM |
| 42CrMo | GB/T 3077 | China | Identical chemistry; the GB standard also specifies SCM440H (harder, narrower hardenability band) for high-spec applications |
| 42CrMo4 | ISO 683-1 / ISO 683-3 | International | Global reference |
| 38CrMoAl (modified) | GB/T 3077 | China | Not directly equivalent — 0.38% C plus Al for nitriding; often used where 4140 + nitriding is specified |
For procurement, 4140 / 42CrMo4 / SCM440 / 42CrMo are interchangeable for 95% of applications. The European 42CrMo4 with the "+QT" suffix (e.g., 42CrMo4 +QT) indicates the bar has been quenched and tempered to a specified mechanical property level — a delivery condition the U.S. ASTM A29 spec addresses separately (e.g., ASTM A322 covers quenched and tempered alloy steel bar).
4140 is specified wherever medium-to-high strength, good toughness and through-hardening in moderate sections are required. The dominant application categories:
| Application category | Specific parts | Why 4140 | Q&T strength level |
|---|---|---|---|
| Oil & gas | Drill collars, tool joints, subs, kelly bars, stabilizers, mud motor shafts | Through-harden in oil to 100 mm; high fatigue; sour-service variants (4140 modified, low S) resist H2S cracking | 950–1100 MPa UTS |
| Automotive | Axle shafts, CV joints, steering knuckles, transmission shafts, gears, connecting rods | Good fatigue, moderate impact; induction-hardenable gear teeth | 800–1000 MPa UTS |
| Fasteners | ASTM A193 B7 bolts and studs, ASTM A320 L7 (low-temperature), nuts and washers | 4140 is the base grade for B7 — oil-quenched and tempered to 125 ksi YS min | ~880 MPa YS min |
| Machine tools | Spindles, lead screws, gear teeth, housing bolts | Wear + fatigue resistance; nitriding spindles for 950–1150 HV surface | 900–1100 MPa UTS, often nitrided |
| Mining and construction | Gear teeth, pinions, track pins, bushings, shafts | Impact + wear; case or induction-hardened surfaces | 1000–1200 MPa UTS |
| Agriculture | Tractor axles, PTO shafts, plow shares, gear teeth | Cost-toughness balance better than 1045 in larger sections | 850–1000 MPa UTS |
| Power transmission | Couplings, sprockets, gears, chain links | Through-hardened, tempered for fatigue | 900–1000 MPa UTS |
| Aerospace | Landing gear cylinders, actuator shafts (often VAR grade) | High strength, fatigue critical; VAR route for inclusions | 1100–1300 MPa UTS, premium quality |
| Firearms | Receiver bodies, bolt carriers, barrel extensions | Medium strength + impact; Q&T to ~32 HRC | ~1000 MPa UTS |
| Hydraulic | Cylinder rods, piston rods, valve bodies | Induction-hardened wear surface over tough core | 900 MPa UTS core, 55 HRC surface |
The pattern: 4140 is the answer when 1045 lacks hardenability (large section) and 4340 is overkill (4340's 1.8% Ni is needed only for impact-critical very-high-strength service above 1200 MPa). For applications between 800 and 1200 MPa UTS in sections 20–100 mm, 4140 is the standard.
Annealed 4140 has a machinability rating of about 65% of B1112 free-machining steel — better than 4340 (~50%) but worse than 1045 (~57%) or free-machining 12L14 (~190%). In the Q&T condition at 32 HRC, machinability drops to about 45% of B1112 — still machinable but with carbide and ceramic tooling. Practical machining notes:
Annealed condition: turning speed 60–80 m/min with HSS, 180–250 m/min with coated carbide; drilling 25–30 m/min
Q&T at 32 HRC: turning speed 50–65 m/min with coated carbide; drilling 18–22 m/min; expect shorter tool life
Q&T above 40 HRC: requires ceramic or CBN tooling for production turning; drilling becomes slow
Thread milling and tapping: annealed 4140 cuts clean threads; Q&T 4140 needs sharp tools and adequate lubrication
For improved machinability in higher-volume parts, 4140 alloy steel is sometimes specified with resulfurization (0.035–0.05% S) or as 41L40 (leaded). Both trade some impact toughness for machinability — useful for moderate-strength components but not for fatigue-critical parts.
4140 has limited weldability. The carbon equivalent (CE) sits around 0.83 (using IIW formula on nominal chemistry), well above the 0.45 threshold where preheat becomes mandatory. Practical welding rules:
| Process | Recommendation |
|---|---|
| Preheat | 200–300 °C for sections 12–25 mm; 300 °C+ for sections above 25 mm |
| Interpass temperature | Maintain at preheat level, do not let it drop |
| Consumables (SMAW) | E10018-D2 (matched chemistry) or E9018-B3 (higher Cr-Mo) |
| Consumables (GMAW/FCAW) | ER100S-D2 (matched) or E110T1-K3 (high-strength) |
| Consumables (GTAW) | ER100S-D2 |
| Post-weld heat treatment | 550–650 °C for 1 h per 25 mm of section, mandatory for restrained joints |
| Hydrogen control | Mandatory — low-hydrogen consumables (baked properly), dry shielding gas, no moisture on joint |
For general structural fabrication, 4140 is rarely the right choice if welding is the dominant joining method — a lower-carbon grade like 4130 (0.30% C, similar Cr-Mo) welds much more easily. 4140 is specified when the as-heat-treated strength of the parent metal is the design driver, and welds are local attachments.
For wear surfaces on tough cores, 4140 supports three surface-hardening routes:
Induction hardening: 55–60 HRC surface, 1–3 mm case depth. Standard for gear teeth, spindle journals, hydraulic rod surfaces. The 0.40% carbon is enough to form 60 HRC martensite locally; the induction coil heats only the surface, the water spray quench forms the case, the core stays at the Q&T toughness.
Carburizing: not typically used — 4140 has too much carbon for carburizing to be effective; the case hardness drops as carbon diffuses. For carburized wear surfaces, use 8620 or 4320.
Gas nitriding / plasma nitriding: 950–1150 HV surface (68–72 HRC equivalent), 0.2–0.4 mm case. Standard for spindle surfaces, sliding guideways, and gear teeth where wear + dimensional stability at temperature matter. 4140 nitrides well because the chromium and molybdenum form hard nitride precipitates. Nitriding runs at 500–540 °C for 24–60 hours — temperature is below the temper temperature, so the core strength is not affected.
Q: What is the difference between 4140 and 42CrMo4?
Chemically, nothing — they are the same composition to the limit of the specifications. 42CrMo4 is the DIN EN 10083-3 designation (Werkstoff 1.7225); 4140 is the AISI/SAE designation. The European standard has slightly tighter phosphorus (0.025% max vs 0.035%) and sulfur (0.025% max vs 0.040%) limits on the quality grade, which means a 42CrMo4 mill cert from a European mill typically holds cleaner metallurgy. For 95% of applications, the two are interchangeable.
Q: Why is 4140 used for ASTM A193 B7 bolts?
B7 is the most common high-strength bolting spec for pressure vessels and pipework, and 4140 is the base chemistry. B7 requires oil quench and temper to a minimum yield of 105 ksi (725 MPa), max hardness of 35 HRC, and the Cr-Mo chemistry gives the elevated-temperature strength needed for pressure service (B7 is rated to 400 °C). The molybdenum is what makes 4140 (and B7) suitable for service at 350–400 °C where carbon-steel fasteners would soften.
Q: Can 4140 be flame-hardened?
Yes. Flame hardening uses an oxy-acetylene flame to heat the surface to 850–900 °C, followed by water spray. The case is 55–60 HRC, depth 1.5–3 mm. Flame hardening is cheaper than induction for large or irregular parts (gear teeth on big gears, sliding ways on long beds).
Q: What is the largest section that 4140 will through-harden?
In oil, 4140 through-hardens (to >90% martensite at center) to about 50 mm. With aggressive oil quench and slightly higher austenitizing temperature, sections to 75 mm reach 80% center martensite. For sections above 75 mm requiring through-hardened strength, the upgrade paths are 4140H (broader hardenability band, mill-controlled), 4145 or 4150 (higher carbon for deeper hardening), or 4340 (1.8% Ni for deeper hardening and higher toughness).
Q: Is 4140 corrosion resistant?
No. 4140 has 1% chromium, which is not enough to form a passive oxide layer like stainless steel. 4140 rusts in shop atmospheres and requires protective coating or oil for storage. The chromium is there for hardenability, not corrosion resistance. For corrosion-critical applications with similar strength, look at 17-4 PH stainless or precipitation-hardening stainless.
Q: Can I weld 4140 to mild steel (A36)?
Yes, with proper procedure. Use E7018 (SMAW) or ER70S-6 (GMAW) consumables that match the lower-strength side. Preheat is still needed (200 °C minimum for the 4140 side), and PWHT at 600 °C is recommended for restrained joints. The weld will be slightly under-strength relative to the 4140 parent metal, but the joint will be sound for structural service.
Q: Why choose 4140 over 4340?
4140 is cheaper (no nickel) and easier to machine. 4340 (with 1.8% Ni) gives higher impact toughness and deeper hardenability — useful for very large sections or very high strength (>1200 MPa). For applications between 800 and 1200 MPa UTS in sections under 75 mm, 4140 is the right call; above those thresholds, step up to 4340.
If you are specifying AISI 4140 Cr-Mo alloy steel and want a second opinion on the Q&T strength level, the hardenability for your section size, or the surface-hardening route for a wear surface — send the drawing and the load case. Qilu Metal supplies 4140 in hot-rolled, cold-drawn, peeled, Q&T and induction-hardened bar to ASTM A29, ASTM A322, DIN EN 10083-3 (42CrMo4 / 1.7225), JIS G4105 (SCM440) and GB/T 3077 (42CrMo), in rounds, flats and hexagons from 6 mm to 300 mm, with mill test reports and full traceability. For oil & gas and aerospace applications, we also supply 4140H (hardenability-guaranteed) and ESR / VAR premium routes. Email enquiry@qilumetal.com and our metallurgy team will quote the size, the quality level and the heat-treatment recommendation matched to your section size and load.
