Overview
4140 is the workhorse heat-treatable medium-carbon alloy steel. The chromium-molybdenum chemistry (~1% Cr, 0.2% Mo, 0.4% C) gives 4140 excellent hardenability — it through-hardens in oil quench up to several inches of section thickness, which lower-alloy steels (1045) cannot. Combined with response to tempering, 4140 covers a strength range from ~95 ksi UTS (annealed) to ~280 ksi UTS (low-temper Q&T) — all from the same alloy.
What 4140 does:
- Hardenable. Q&T to any strength level 95–280 ksi UTS depending on temper. Tempering temperature is the strength selector.
- Tough at moderate strength. Better Charpy impact than carbon steels at equivalent yield strength.
- Good fatigue life. Standard for fatigue-loaded shafts, axles, gears.
- Reasonable cost. ~$1.20/lb vs A36’s $0.55/lb — modest premium for major property gain.
What 4140 doesn’t do:
- Corrosion resistance. Same as A36 — rusts in moist conditions. Cr content is not enough for stainless behavior.
- Easy welding. Hardenable means HAZ cracking risk. Preheat and PWHT for welded structural applications.
- Cryogenic toughness. Becomes brittle at low temperatures, especially in low-temper Q&T conditions.
The “heat-treatable” superpower is the design driver. 4140 components get machined in annealed or pre-hardened condition, then heat treated to final strength. The temper temperature controls the strength-vs- toughness trade-off. Aerospace specifies temper temperature precisely because impact toughness matters at altitude and low temperature.
Heat treatment is the design space
For 4140, specifying the temper is specifying the part properties. The same alloy at different tempers behaves like different materials:
| Temper | UTS | Yield | Elong. | Hardness | Use |
|---|---|---|---|---|---|
| Annealed | 95 ksi | 60 ksi | 25% | 200 HB | Machining stock |
| Normalized | 148 ksi | 95 ksi | 18% | 290 HB | Intermediate |
| Q&T 1200°F (650°C) | 138 ksi | 116 ksi | 17% | 28 HRC | Pre-hardened 4140HT |
| Q&T 1100°F (600°C) | 152 ksi | 130 ksi | 16% | 32 HRC | High toughness |
| Q&T 1000°F (540°C) | 175 ksi | 150 ksi | 14% | 40 HRC | Balanced |
| Q&T 800°F (425°C) | 220 ksi | 195 ksi | 12% | 45 HRC | High strength |
| Q&T 400°F (205°C) | 270 ksi | 245 ksi | 8% | 52 HRC | Maximum strength |
Higher temper temperatures soften 4140 by allowing carbide precipitation and stress relief; lower temper temperatures retain more of the as-quenched martensite hardness. Designers select temper to balance strength against toughness for their specific application.
Pre-hardened 4140 (“4140HT”, “4140 PHT”) at 28–32 HRC is the standard supply condition for many machining applications. The bar arrives already heat treated to medium strength; machining proceeds to final dimensions without subsequent heat treatment. Most industrial shafts, gears, and machinery components use this condition.
Machining notes
Annealed 4140 machines reasonably — harder than A36 but predictable. 4140HT (28–32 HRC) machines slower but still cleanly. Above ~40 HRC machining gets specialized; most parts are machined soft, heat treated, then finish-ground.
Practical recipes:
Annealed 4140:
- Coated carbide, speed 100–200 SFM, feed 0.005–0.020 in/rev
Pre-hardened 4140HT (28–32 HRC):
- Coated carbide (TiAlN preferred), speed 70–150 SFM, feed 0.005–0.015 in/rev
Hardened above 40 HRC:
- CBN or ceramic tooling
- Speed 200–500 SFM at very light depths
- Most production work avoids machining at this hardness
Tool life on 4140 is ~50–70% of A36 due to alloy content abrasion. Cutting fluid (water-soluble emulsion) extends tool life significantly.
Welding considerations
4140 is weldable but requires care. The same alloy chemistry that gives 4140 its hardenability creates HAZ cracking risk during welding:
- Preheat to 200–300°C before welding to slow cooling and prevent martensite formation
- Low-hydrogen electrodes (E11018, E10018) to prevent hydrogen cold cracking
- Post-weld heat treat at 600–700°C for 1 hour per inch of section to temper any martensite formed in the HAZ
- Cool slowly in still air or under insulation after welding
For Q&T 4140 assemblies, post-weld heat treatment is essentially mandatory — otherwise the weld zone is over-quenched and brittle.
Most 4140 weldments are designed to be:
- Welded in annealed condition with standard preheat practice
- Heat treated as an assembly afterward
- Finish machined to final tolerances
This sequence avoids the welded-Q&T problems.
Failure modes worth designing around
Hydrogen embrittlement in high-strength temper conditions (>1200 MPa UTS). Sources: pickling acids, electroplating (especially cadmium and zinc), cathodic protection systems, hydrogen-containing service environments. Mitigation: bake-out at 200°C for 4 hours after plating to drive out absorbed hydrogen. Mandatory practice for aerospace and high-strength structural fasteners.
Stress corrosion cracking in chloride environments at high strength. Aerospace specifications restrict maximum hardness for chloride- exposed 4140 components.
Decarburization during heat treatment without protective atmosphere reduces surface hardness and fatigue strength. Critical for shafts and fatigue-loaded parts. Use protective atmosphere (endothermic) or salt bath; alternatively, leave grinding stock and remove decarb after heat treatment.
Fatigue at surface defects. 4140 in fatigue service is sensitive to surface finish — machined fillets, threads, tool marks. Polish fatigue-critical surfaces, shot peen for compressive surface stress.
Brittle fracture in low-temper Q&T at low temperatures. Ductile-brittle transition temperature varies with temper. For sub- zero service, use higher temper temperatures or alternative alloys (e.g., 4340 with better cryogenic toughness).
Galvanic corrosion when in contact with stainless or copper alloys in moist environments — 4140 corrodes preferentially.
Variant selection guidance
- Annealed — machining stock and starting material for heat treatment.
- 4140HT (Q&T 28–32 HRC) — pre-hardened supply for machined shafts, gears, machinery components. The default supplied condition.
- Q&T 1100°F (600°C) — balanced high-strength + toughness for structural service.
- Q&T 1000°F (540°C) — high-strength applications: aerospace brackets, drive shafts, gear teeth.
- Q&T 800°F (425°C) — maximum strength with acceptable toughness. Fasteners (Grade 8 bolts), high-stress brackets.
- Q&T low-temper (<\125°C) — wear-critical surfaces, springs, tooling. Hydrogen embrittlement risk peaks here.
Applications by industry
- Automotive — crankshafts, camshafts, connecting rods, axles, drive shafts. The default heat-treated structural alloy for engine and powertrain.
- Aerospace — landing gear, structural fittings, fasteners, high-strength brackets. AMS 6382 / AMS 6395 govern aerospace 4140.
- Oil and gas — drill collars, tool joints, pump shafts, downhole tooling. NACE MR0175 limits hardness for sour service.
- Industrial machinery — drive shafts, gears, sprockets, hydraulic cylinder rods. The workhorse machinery alloy.
- Power generation — turbine shafts, generator components, pump impellers. Heavy industrial mechanical service.
- Defense — historic gun barrels (still used in some applications), structural components, fasteners.
- High-strength fasteners — Grade 8 SAE bolts, AN/MS aerospace fasteners, ASTM A574 socket head cap screws — these are typically 4140-class steel.