4140 Steel
Also: AISI 4140, SAE 4140, UNS G41400, 42CrMo4, 1.7225, chromoly, chrome-moly steel, 4140 HT, 4140 pre-hard, 4140 Q&T, 4140/4142, SCM440 (JIS), EN19 / 709M40 (BS 970, near)
4140 is the go-to Cr-Mo alloy steel: stocked annealed or pre-hard at 28-32 HRC, it through-hardens and nitrides well. Values annealed and Q&T.
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At a glance
| Density | 7.85 g/cm³ (0.284 lb/in³) |
|---|---|
| Tensile strength | 655 MPa (95 ksi) annealed; about 1,000 MPa (145 ksi) Q&T pre-hard 28-32 HRC; over 1,700 MPa (250 ksi) when tempered low |
| Yield strength | 415 MPa (60 ksi) annealed; about 760-900 MPa (110-130 ksi) Q&T pre-hard 28-32 HRC; over 1,500 MPa (220 ksi) when tempered low |
| Hardness | About 197 HB (low 90s HRB) annealed; 28-32 HRC (about 270-300 HB) pre-hard; 50-55 HRC tempered low or induction hardened |
| Elongation | About 25% annealed; 15-20% pre-hard 28-32 HRC; about 10% when tempered low |
| Modulus of elasticity | 205 GPa (29.7 Msi) |
| Thermal | Melts at about 1,415-1,500 °C (2,580-2,730 °F); thermal conductivity ~42 W/m·K |
| Composition | Fe, 0.38-0.43% C, 0.75-1.00% Mn, 0.80-1.10% Cr, 0.15-0.25% Mo, 0.15-0.35% Si, up to 0.035% P, up to 0.040% S |
Typical values, Annealed and quenched-and-tempered pre-hard (28-32 HRC) given side by side; low-temper figures are for oil-quenched sections under about 25 mm (1 in).. Confirm against the mill certificate and the purchase spec.
- MachinabilityGood
- About 65% of the 1212 benchmark annealed. Pre-hard 28-32 HRC machines well with carbide at lower speeds and often gives the better finish; above about 40 HRC plan on hard milling, hard turning or grinding.
- WeldabilityFair
- Weldable with a procedure: preheat 175-315 °C (350-600 °F) by section, hold interpass, low-hydrogen filler, slow cool, then a post-weld temper or stress relief. On pre-hard bar keep the post-weld temper below the original tempering temperature or the part softens.
- FormabilityFair
- Annealed bar and plate bend only with generous radii; pre-hard does not cold form. Hot forging is routine and 4140 is one of the most common forging grades.
- Corrosion resistancePoor
- About 1% chromium is nowhere near stainless; it rusts like carbon steel. Black oxide with oil, electroless nickel, hard chrome or paint depending on the service.
4140 is a chromium-molybdenum alloy steel at about 0.40% carbon and the most widely stocked alloy steel in U.S. service centers. It is bought two ways. Annealed bar, at about 197 HB, machines easily and is then quenched and tempered to whatever the part needs. Pre-hard bar, quenched and tempered at the mill to 28-32 HRC and sold as "4140 HT" or "4140 pre-hard", is what most shops actually keep on the rack: it is machined straight to a finished part with no heat treat afterward.
The chromium and molybdenum give it far deeper hardenability than 1045. Oil quenched, it hardens through sections up to about 50 mm (2 in), and the tempering temperature then sets the strength anywhere from about 900 MPa (130 ksi) to over 1,700 MPa (250 ksi). Pre-hard bar runs about 1,000 MPa (145 ksi) tensile with good toughness and fatigue strength, and it nitrides and induction hardens well, so one bar can carry a hard wear surface over a strong core. That is why it turns up in shafts, spindles, fixtures, tool holders, gears, B7 studs and oilfield parts.
Three things catch buyers out. It welds only with a procedure: preheat, low-hydrogen filler and a post-weld temper, or the heat-affected zone cracks. Parts hardened after machining move and pick up a decarburized skin, so leave grind stock or buy pre-hard. And beyond about 50-75 mm (2-3 in) the core of a quenched section runs noticeably softer than the surface; that is the job for 4340.
Heat treatment
Anneal at about 845 °C (1,550 °F) with a slow furnace cool for the softest machining condition. Normalize at 870-900 °C (1,600-1,650 °F) to refine forgings; normalized 4140 air cools to about 300 HB, so it is usually tempered or annealed before machining. Harden from 845-870 °C (1,550-1,600 °F) with an oil or polymer quench, which carries full hardness through about 50 mm (2 in) and gives roughly 55 HRC as quenched. Temper to set the hardness: about 205 °C (400 °F) for the low 50s HRC, about 540 °C (1,000 °F) for the mid to high 30s, and 600-650 °C (1,100-1,200 °F) for the 28-32 HRC pre-hard range. Avoid tempering in the 260-370 °C (500-700 °F) band, where impact toughness drops. Nitride at about 500-530 °C (930-990 °F) over a pre-hard core for a 50-55 HRC case, or induction harden journals and teeth to 50-55 HRC. Stress relieve after heavy roughing at about 30-50 °C below the last tempering temperature.
Tempers and conditions
- Annealed (as stocked)
- Q&T pre-hard 28-32 HRC (4140 HT)
- Normalized
- Normalized and tempered
- Q&T to a specified range (for example 34-38 HRC)
- Cold drawn, annealed or stress relieved
- Turned, ground and polished (TGP)
- Induction hardened and chrome plated rod
Stock forms
- Round, square, hex and flat bar
- Plate (annealed and pre-hard)
- Seamless mechanical tube
- Precision-ground shafting
- Forgings and rolled rings
- Induction-hardened chrome-plated rod
Relative cost
Tier 2 of 5. Roughly 1.5-2x the price of 1018 bar by weight and about 1.5x 1045. Pre-hard costs a little more than annealed but usually saves a heat-treat trip.
Typical applications
- Shafts, spindles, arbors and axles
- Fixtures, jigs, tool holders and die shoes
- Gears, sprockets, couplings and splined hubs
- ASTM A193 B7 studs, bolts and high-strength fasteners
- Oil and gas parts: drill collars, subs, valve stems and pump shafts
- Hydraulic cylinder rods and piston rods
- Firearm barrels and receivers
- Mold bases, holder blocks and press tooling
Process compatibility
How 4140 takes each process, and what to specify. Each process links to its own guide and to the shops that do it.
| Process | Rating | Notes |
|---|---|---|
| CNC Machining | GoodGood | Pre-hard 28-32 HRC is machined straight to finish with carbide; annealed bar cuts faster but must be heat treated after. Can hardened 4140 be machined? |
| CNC Turning | GoodGood | The standard alloy shaft bar; pre-hard turns to a clean finish and holds size with no heat treat to follow. |
| CNC Grinding | ExcellentExcellent | Ground after hardening or nitriding to hold bearing fits, flatness and runout to a few microns. |
| Heat Treating | ExcellentExcellent | Oil quenches through about 50 mm (2 in) and tempers to any level from 28 to 50+ HRC; specify a hardness range. How is 4140 steel heat treated? |
| Nitriding | ExcellentExcellent | Chromium and molybdenum form hard nitrides: a 50-55 HRC case over a pre-hard core with very little distortion. Can 4140 be nitrided? |
| Induction Hardening | GoodGood | 50-55 HRC on journals, cam lobes and gear teeth over a 28-32 HRC core; temper lightly after. |
| Welding | FairFair | Preheat, low-hydrogen filler, controlled interpass and a post-weld temper; skip them and the heat-affected zone cracks. Can 4140 steel be welded? |
| Forging | ExcellentExcellent | One of the most forged alloy grades; normalize and temper or anneal the forging before machining. |
| Black Oxide | GoodGood | Common on pre-hard fixtures, tooling and firearm parts; mild protection only, and only when oiled. Can 4140 be black oxide coated? |
| Hard Chrome Plating | GoodGood | Hydraulic rods and wear surfaces over a ground, often induction-hardened, base; bake hardened parts after plating. |
| Electroless Nickel Plating | GoodGood | Uniform 10-25 um (0.0004-0.001") coat for corrosion and wear on fixtures and mold parts; bake hardened parts. |
| Zinc Plating | FairFair | Fine on annealed or pre-hard parts; hardened parts need a hydrogen-embrittlement relief bake right after plating. |
| Carburizing | Not recommendedNot recommended | Already at 0.40% carbon; carburizing adds a brittle high-carbon case, not a useful one. Use 8620 for carburized parts. |
| Anodizing | Not recommendedNot recommended | Anodizing is an aluminum and titanium process and does not work on steel; use black oxide, electroless nickel or zinc. |
Compare 4140
Design and sourcing tips
- Name the condition: "4140 pre-hard, 28-32 HRC" when the part is machined to finish with no heat treat, "4140 annealed, Q&T to 40-44 HRC" when it is hardened after machining.
- Specify hardness as a range at least 4 HRC wide, not a single number or a bare minimum, and say where it is tested if the section is large.
- Parts hardened after machining move and decarburize: leave 0.25-0.5 mm (0.010-0.020") per side for grinding, or ask for vacuum or protective-atmosphere hardening.
- Tell the heat treater the hardness you need rather than a tempering temperature; a good shop will keep clear of the 260-370 °C (500-700 °F) band where 4140 loses impact toughness.
- For nitrided parts, finish machine first (the case grows only a few microns), buy pre-hard so the core was tempered above the nitriding temperature, and call out case depth and surface hardness.
- Put the welding procedure (preheat, filler, post-weld temper) on the drawing; many shops treat "steel" as mild steel and skip it.
Frequently asked questions
- Can 4140 be welded?
- Yes, with a procedure. Preheat to about 175-315 °C (350-600 °F) depending on thickness, hold that temperature between passes, use low-hydrogen filler, let the part cool slowly and give it a post-weld temper or stress relief. Welding it cold, the way you would weld 1018, gives a hard, brittle heat-affected zone that cracks, often hours later. Weld in the annealed or pre-hard condition, not after hardening to 40+ HRC.
- What is the hardness of 4140 pre-hard?
- Pre-hard 4140, also sold as 4140 HT or 4140 Q&T, is quenched and tempered at the mill to 28-32 HRC, about 270-300 HB, with a tensile strength around 1,000 MPa (145 ksi). It is hard enough for most fixtures, shafts and tooling yet still machines well with carbide. Some mills also stock higher ranges; confirm the hardness on the cert.
- Is 4140 the same as chromoly?
- Chromoly is a loose name for the 41xx chromium-molybdenum steels. Machinists usually mean 4140 bar; fabricators and racers usually mean 4130 tube. 4130 has less carbon, so it welds and forms more easily, while 4140 hardens deeper and reaches higher strength. Put the grade number on the drawing, not "chromoly".
- Can 4140 be hardened after machining?
- Yes. Machine it annealed, then oil quench from about 845 °C (1,550 °F) and temper to the range you need, up to the low 50s HRC. Expect distortion and a thin decarburized skin, so leave stock for grinding critical features or use vacuum hardening. If 28-32 HRC is enough, buy pre-hard and skip the heat treat.
- Does 4140 steel rust?
- Yes. Its chromium content is far too low to make it stainless, and bare 4140 rusts about as fast as carbon steel. Protect it with black oxide and oil, electroless nickel, hard chrome, zinc or paint, or choose 17-4 PH stainless when you need comparable strength in a corrosive environment.
Processes that commonly use it
Related materials
Where to verify these values
- ASTM A29/A29M, ASTM A322 and ASTM A434 (bar)
- SAE J404 (chemistry); AMS 6349 and AMS 6382 (aerospace bar and forgings)
- ASM Handbook Vol. 1 (properties) and Vol. 4 (heat treating)
- Producer data sheets for annealed and pre-hard 4140 bar
