Aço 4140: Propriedades, Dureza e Tratamento Térmico

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, allowing a single grade to cover a wide range of strength levels through heat treatment.

Internationally, it is known as 42CrMo4 (1.7225) under EN/DIN, SCM440 under JIS, 42CrMo under GB, and G41400 under UNS.

One point matters for material selection: 4140 is an alloy structural steel, not a tool steel. Its home is in shafts, gears, axles, bolts, couplings, and machinery components that must carry loads 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.

Composição Química

Chromium and molybdenum are what set 4140 apart from a plain carbon steel such as 1045. Chromium raises hardenability and wear resistance; molybdenum further improves hardenability, holds strength at moderate temperatures, and reduces susceptibility to reversible temper embrittlement. Together, they allow thicker sections to harden more uniformly than carbon steel does.

Propriedades mecânicas

4140 is rarely used in a single fixed condition. Its value lies in how predictably it tempers trades hardness for toughness, so the same bar can be set from a tough shaft near 28 HRC to a hard wear surface above 50 HRC.

Actual values depend on section size, exact heat-treatment parameters, and the testing standard, so these are typical ranges rather than guaranteed minimums. Certified properties to a standard such as EN 10083-3 must be tied to a defined section size and test location.

Hardness and Hardenability

As-quenched in thin sections with a full martensitic structure, 4140 reaches roughly 54–58 HRC. After tempering, the practical working hardness is usually 50–55 HRC, and most service applications run lower to retain useful toughness.

Hardness by route:

• Annealed: ~197 HB, soft and machinable
• Through-hardened (oil quench): up to ~55 HRC in thin sections (under 25 mm); ~45–52 HRC at 25–75 mm; ~35–45 HRC above 75 mm
• Induction- or flame-hardened: 50–58 HRC case over a tough core.
• Nitrided: ~500–650 HV surface (roughly 49–58 HRC equivalent)

The real limit is hardenability, not peak hardness. 4140 has only moderate hardenability, so in heavy sections the core cools too slowly to form full martensite, and core hardness drops. Above roughly 100 mm, oil quenching no longer hardens the center to the same level as the surface. For large parts that must be hard throughout, this is the deciding factor, and a deeper-hardening grade or a different process is needed.

Tratamento térmico

Recozimento: heat to 830–870°C, hold, then furnace-cool slowly to a soft, machinable structure near 197 HB.

Normalizando: heat to 870–900°C and air-cool to refine grain and even out the structure before hardening or machining.

Endurecimento: austenitize at 830–870°C and quench in oil. Oil is standard; water or brine increases the risk of cracking in complex shapes and is generally avoided.

Têmpera: temper between about 205°C and 650°C to set the final properties.

• Low (~205°C): highest hardness, ~50–55 HRC, lowest toughness
• Medium (~425–500°C): balanced strength and toughness, ~35–40 HRC
• High (~540–650°C): maximum toughness for dynamic and fatigue loading, ~28–35 HRC

Two embrittlement effects are worth respecting:

• Tempered martensite embrittlement occurs during tempering in the 260–370°C range, reducing impact toughness. Where toughness matters, this band is normally skipped.
• Reversible temper embrittlement can develop in thick sections that cool slowly through the 375–575°C range after high-temperature tempering, and is linked to phosphorus and similar impurities. The molybdenum in 4140 reduces this tendency, and faster cooling from the tempering temperature further limits it.

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 based solely on price. The right choice follows from what the part actually 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 (with preheat and post-weld heat treatment), 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 can, and it loses strength quickly above about 400°C. Three lines are worth drawing clearly:

• Cutting and blanking tools require the hardness (60+ HRC) and carbide volume that 4140 lacks. D2 or A2 lasts far longer in a wear die.
• Hot-work tooling running above ~400°C needs the hot hardness of H13, which 4140 lacks entirely.
• Any part that must hold 58+ HRC in a real section, per section size rules, 4140 out, since it cannot reliably reach that hardness.

Standards and References

4140 and its equivalents are covered by SAE J404 (chemical composition), ASTM A29/A29M (bar general requirements), and EN 10083-3 (42CrMo4, quenched and tempered alloy steels). Certified mechanical properties should always be tied to a stated standard and section size.

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