4140 Steel: Properties, Hardness and Heat Treatment

What Is 4140 Steel?

4140 is a medium-carbon chromium-molybdenum alloy steel (AISI/SAE 4140) and one of the most widely used engineering steels. Roughly 0.40% carbon, 1% chromium and 0.2% molybdenum give it a strong balance of strength, toughness and hardenability, so a single grade covers a wide range of strength levels through heat treatment.

Internationally the same steel is sold as 42CrMo4 (1.7225) under EN/DIN, SCM440 under JIS, 42CrMo under GB, and G41400 under UNS. The chemistry limits are close but not identical, and the differences are listed in the equivalent grades table below.

One point matters for material selection: 4140 is an alloy structural steel, not a tool steel. Its home is shafts, gears, axles, bolts, couplings and machinery parts that carry load and resist fatigue. It is not designed for cutting, blanking or forming tools, where wear resistance and high working hardness dominate. Knowing where that line falls is the difference between a part that lasts and one that fails early.

Download the 4140 card, 414 KB

Chemical Composition

Element	AISI 4140 (wt %)	42CrMo4 / EN (wt %)	GB 42CrMo (wt %)
Carbon (C)	0.38–0.43	0.38–0.45	0.38–0.45
Manganese (Mn)	0.75–1.00	0.60–0.90	0.50–0.80
Silicon (Si)	0.15–0.35	0.40 max	0.17–0.37
Chromium (Cr)	0.80–1.10	0.90–1.20	0.90–1.20
Molybdenum (Mo)	0.15–0.25	0.15–0.30	0.15–0.25
Phosphorus (P)	0.035 max	0.035 max	0.035 max
Sulfur (S)	0.040 max	0.035 max	0.035 max

Chromium and molybdenum are what separate 4140 from a plain carbon steel such as 1045. Chromium raises hardenability and wear resistance; molybdenum improves hardenability further, holds strength at moderate temperature and limits reversible temper embrittlement. Together they let thicker sections harden more evenly than carbon steel does.

Tempering Response and Mechanical Properties

4140 is rarely used in a single fixed condition. The same bar can be a tough shaft near 30 HRC or a hard wear surface above 50 HRC, and the tempering temperature alone decides which. The table below is the standard tempering response for a 25 mm bar oil quenched from 845–870 °C.

Tempering	Tensile strength	Yield strength	Hardness HB (~HRC)
205 °C	1965 MPa	1740 MPa	578 (55)
260 °C	1860 MPa	1650 MPa	534 (53)
315 °C	1720 MPa	1570 MPa	495 (50)
370 °C	1590 MPa	1460 MPa	461 (47)
425 °C	1450 MPa	1340 MPa	429 (45)
480 °C	1300 MPa	1210 MPa	388 (41)
540 °C	1150 MPa	1050 MPa	341 (37)
595 °C	1020 MPa	910 MPa	311 (34)
650 °C	900 MPa	790 MPa	277 (30)
705 °C	810 MPa	690 MPa	235 (22)

Values are typical, not guaranteed minimums. Hardness is printed in HB in the source material; the HRC figure in brackets is a conversion. Certified properties must always be tied to a defined section size and a stated test location.

Hardness by Section Size

Surface hardness after an oil quench falls as the bar gets bigger, because 4140 has only moderate hardenability. In thin sections the quench produces close to full martensite; past roughly 75–100 mm the centre never gets there.

Bar diameter	HRC	HB (approx.)	Condition
13 mm (0.5 in)	57	596	As quenched
25 mm (1 in)	55	560	As quenched
50 mm (2 in)	49	469	As quenched
25 mm (1 in)	—	335	Tempered 540 °C
50 mm (2 in)	—	202	Tempered 540 °C

The last two rows are the point. At the same tempering temperature a 25 mm bar comes out at 335 HB and a 50 mm bar at 202 HB. Where a part must be hard through the section, this is the number that decides the grade or the process — a longer temper does not close the gap.

Hardness by condition:

Annealed: 197 HB. Soft, machinable, and the condition bar stock is normally supplied in.
Normalized: 241–302 HB.
Oil quenched and tempered: 229–341 HB, set by the tempering temperature and the section.
Pre-hardened supply: at the top of the tempered band, roughly 300 HB (about 30 HRC).
Induction, flame or nitrided surface: the case is much harder than the core; the core still sets the load capacity.

Equivalent Grades

The same steel travels under different names, and the chemistry limits are not identical. Manganese is the range that moves most: 0.75–1.00% in AISI 4140, 0.60–0.85% in SCM440, 0.50–0.80% in GB 42CrMo.

Standard	Grade	Chemistry (wt %)
ASTM A29/A29M-03	4140 (G41400)	C 0.38–0.43 · Mn 0.75–1.00 · Si 0.15–0.35 · Cr 0.80–1.10 · Mo 0.15–0.25 · P 0.035 · S 0.040
SAE J404 JUN00	4140 (G41400)	C 0.38–0.43 · Mn 0.75–1.00 · Si 0.15–0.35 · Cr 0.80–1.10 · Mo 0.15–0.25 · P 0.030 · S 0.040
JIS G4053:2003	SCM 440	C 0.38–0.43 · Mn 0.60–0.85 · Si 0.15–0.35 · Cr 0.90–1.20 · Mo 0.15–0.30 · P 0.030 · S 0.030
EN 10083-1:1991	42CrMo4 (1.7225)	C 0.38–0.45 · Mn 0.60–0.90 · Si 0.40 max · Cr 0.90–1.20 · Mo 0.15–0.30 · P 0.035 · S 0.035
EN 10083-1:1991	42CrMoS4 (1.7227)	C 0.38–0.45 · Mn 0.60–0.90 · Si 0.40 max · Cr 0.90–1.20 · Mo 0.15–0.30 · S 0.020–0.040
GB (China)	42CrMo	C 0.38–0.45 · Mn 0.50–0.80 · Si 0.17–0.37 · Cr 0.90–1.20 · Mo 0.15–0.25

These are nearest grades, not interchangeable parts. The manganese and sulphur ranges differ, and so does the hardenability band each standard allows. Before substituting one for another, check the chemistry tolerances in the purchasing standard and the hardness the part must reach at its section size — a grade that meets the chemistry can still miss the hardness.

Heat Treatment

Normalizing: heat to 845–925 °C and air cool. This refines grain and evens out the structure before hardening or machining.

Annealing: heat to 830–870 °C, hold, then furnace cool at about 15 °C/h to 480 °C and air cool. The result is a soft, machinable structure near 197 HB.

Hardening: austenitize at 830–870 °C and quench in oil. Oil is standard. Water quenching raises cracking risk in complex shapes and is rarely used; where it is, the austenitizing temperature drops to 815–845 °C. Hold 1 hour minimum, or 15–20 minutes per 25 mm of section.

Tempering: two windows are used. 175–230 °C for maximum strength, or 385–705 °C for everything else, down to a yield strength below 1380 MPa. The tempering temperature, not the quench, sets the final properties.

Two embrittlement effects are worth respecting:

Tempered martensite embrittlement — 4140 is normally not tempered between 230 and 370 °C, the blue-brittleness band, because impact toughness drops sharply inside it.
Reversible temper embrittlement — can develop in thick sections that cool slowly through 375–575 °C after high-temperature tempering, and is linked to phosphorus and similar impurities. Molybdenum in 4140 reduces the tendency; faster cooling from the tempering temperature limits it further.

Welding 4140

4140 is not a prequalified welding steel. A weld procedure has to be written and qualified for the joint, and the four controls below are not optional.

Preheat above the martensite-start temperature: 290 °C (550 °F). Above that level the heat-affected zone transforms to bainite instead of untempered martensite, which is what cracks.
Hold the preheat as the interpass temperature. If the part drops below it, stop and reheat.
A low-hydrogen process and low-hydrogen consumables are mandatory. Hydrogen is the other half of cold cracking.
Post-heat no higher than 28 °C (50 °F) below the tempering temperature the part was given. Cooling an untempered HAZ to room temperature is how a repair turns into a crack.

Machining 4140

In the annealed condition at 187–229 HB, 4140 machines at about 65% of the rate of AISI 1212 taken as 100%. That is a normal alloy-steel rating, not a difficult one — the hardness condition is what changes the number.

Annealed (187–229 HB): the fastest condition, and the one to machine in when the part is heat treated afterwards.
Quenched and tempered (229–341 HB): lower speeds, more rigid setup, more tool wear. Machining in the pre-hardened condition avoids distortion and scale from a later heat treatment.
Typical parts cut from 4140: connecting rods, crankshafts, steering knuckles, axles, gears, studs, high-strength bolts, tool holders and shanks.

4140 or a Tool Steel? Matching the Grade to the Job

4140 and tool steels overlap in some hardness ranges, so it is tempting to substitute one for the other on price alone. The right choice follows from what the part has to survive.

4140 is the better answer when the part carries load and fatigue rather than cutting or abrasion: shafts, gears, arbors, holders, bolsters and structural machinery parts. It is tougher and far cheaper than tool steel, welds more readily, and comes in a wider range of sizes and forms. When extreme wear life and 58+ HRC are not required, paying for tool steel buys nothing.

Tool steel is the right answer when the failure modes are wear, edge retention or heat. 4140 cannot hold a cutting edge or resist abrasive die wear as well as a high-carbide grade, and it loses strength quickly above about 400 °C. Three lines are worth drawing clearly:

Cutting and blanking tools need the hardness (60+ HRC) and carbide volume that 4140 lacks. D2 or A2 lasts far longer in a wear die.
Hot-work tooling above ~400 °C needs the hot hardness of H13, which 4140 does not have.
Any part that must hold 58+ HRC in a real section is out of reach for 4140, because it cannot reliably get there at the section size involved.

Property	4140	D2	H13	A2
Max hardness (practical)	~50–55 HRC	58–62 HRC	44–52 HRC	57–62 HRC
Wear resistance	Low	Excellent	Moderate	Good
Hot hardness	Low (under 400 °C)	Low	Excellent (to ~600 °C)	Low
Toughness	Good	Moderate	Good	Moderate
Relative cost	Low	High	High	Medium–high
Carbide volume	None	High (Cr carbides)	Low (alloy carbides)	Moderate (Cr carbides)

Standards and References

4140 and its equivalents are covered by ASTM A29/A29M and SAE J404 (composition of bar stock), ASTM A322 (billets and bars), EN 10083-1 (42CrMo4), JIS G4053 (SCM440) and GB/T 3077 (42CrMo). Certified mechanical properties should always be tied to a stated standard, section size and test location.

Data on this page:

Mechanical properties and heat treatment practice — ASM Handbook Vol. 4, Heat Treating, 4140 section.
Hardness by section size, annealed / normalized / tempered bands, machinability rating — Cutting Data for Turning of Steel, AISI 4140 chapter.
Equivalent grades and chemistry — Handbook of Comparative World Steel Standards (ASTM DS 67B), Cr-Mo steels table.
Welding practice — ASM Handbook Vol. 6, Welding, Brazing and Soldering.
AISI 4140 reference card, PDF, 2 pages, 414 KB. The same tables, printable.