How is 4140 steel heat treated?
Austenitize at about 845 °C (1550 °F), oil quench to roughly 55-57 HRC, then temper to the hardness you need: 205 °C (400 °F) leaves about 54 HRC, 540 °C (1000 °F) about 38 HRC and 650 °C (1200 °F) about 28 HRC.
4140 is the standard oil-hardening chromium-molybdenum steel, and its heat treatment is the textbook quench-and-temper cycle. Austenitize at 830-870 °C (1525-1600 °F), typically 845 °C (1550 °F), soaking about one hour per 25 mm (1") of section; quench in agitated oil (water cracks all but the simplest shapes; polymer quenchants are used for heavy sections); then temper immediately, before the part reaches room temperature and while it is still warm, to stop quench cracks. As-quenched hardness is about 55-57 HRC at the surface, and the alloy through-hardens in oil to about 50-65 mm (2-2.5") round; heavier sections come out softer in the core, which is where 4340 takes over. Normalizing at about 870 °C (1600 °F) with an air cool gives a uniform 200-230 HB starting structure, and a full anneal at 815 °C (1500 °F) with a furnace cool gives about 197 HB for machining.
Tempering sets the properties, and the numbers below are typical for a 25 mm (1") oil-quenched section; the mill cert and a test coupon rule. 205 °C (400 °F): 53-55 HRC, for wear parts. 315 °C (600 °F): 49-51 HRC. 425 °C (800 °F): 44-46 HRC, about 1450 MPa (210 ksi) tensile. 540 °C (1000 °F): 37-39 HRC, about 1200 MPa (175 ksi) tensile with 15% elongation, the usual high-strength shaft condition. 595 °C (1100 °F): 32-34 HRC, roughly 1030 MPa (150 ksi). 650 °C (1200 °F): 27-30 HRC, about 900-950 MPa (130-138 ksi), the pre-hard 28-32 HRC condition sold as 4140 HT. Two hours at temperature is normal, and double tempering is cheap insurance on parts that will be nitrided or that see impact. Avoid tempering in the 260-370 °C (500-700 °F) range: 4140 shows tempered-martensite embrittlement there, with a toughness dip that does not show up in hardness.
Two cautions and two options. Decarburization: 4140 loses surface carbon in an unprotected furnace, leaving a soft skin of 0.1-0.5 mm (0.005-0.020"); heat treat in a controlled atmosphere, a vacuum furnace or with stock to grind off. Distortion: expect 0.05-0.2% size change and warp on long or asymmetric parts; rough machine, harden, then finish grind, and fixture-quench flat parts. The options: 4140 is a good candidate for nitriding (harden and temper at 565-620 °C first, then nitride at 500-565 °C), and for induction hardening of shaft journals and gear teeth (surface 50-55 HRC over a pre-hard core). AMS 2759 and its slash sheets govern aerospace heat treatment of low-alloy steel; commercial work is usually done to a hardness range and a named cycle.
Heat Treating for 4140 parts: where to send them
What to specify
- Final hardness range, for example "quench and temper to 28-32 HRC" or "38-42 HRC", and where on the part it is measured (surface after grinding, or a test coupon)
- Sequence: rough machine, heat treat, finish; state grinding stock (0.2-0.4 mm) and which features are finished after hardening
- Heat-treat spec when it matters: AMS 2759/1 for aerospace low-alloy steel, or "quench and temper per SAE / ASM practice" with the cycle written out
- Atmosphere or decarburization limit: "no decarburization permitted on finished surfaces" or the allowed depth, with stock to remove it
- Quench medium and fixturing for distortion-prone shapes, and the straightness or flatness tolerance after heat treatment
- Tempering constraint if the part will be welded, nitrided or plated later: temper at least 30 °C (50 °F) above any later process temperature
- Mechanical property verification: hardness test (ASTM E18), and a tensile or Charpy coupon (ASTM A370) if the drawing calls for strength or toughness
Pitfalls
- Water quenching 4140: it cracks at corners, keyways and section changes; oil or polymer only
- Letting the quenched part sit overnight before tempering: quench cracks form in fully hardened steel at room temperature
- Tempering in the 260-370 °C (500-700 °F) embrittlement range: hardness looks right, impact toughness is a fraction of expected
- Decarburized skin on a part hardened in open air: the surface reads 10-20 HRC soft, and a grinder reveals the real hardness underneath
- Hardening a finished part and expecting +/-0.025 mm: it moves; harden rough and finish afterwards
- Heavy sections over 65 mm (2.5"): the core does not harden fully in oil; check the core or move to 4340
Frequently asked questions
- What temperature do you harden 4140 at?
- Austenitize at about 845 °C (1550 °F), soak about one hour per 25 mm (1") of section, and quench in agitated oil. That gives roughly 55-57 HRC as quenched, which is then tempered down to the working hardness.
- What is the tempering temperature for 4140 to get 30 HRC?
- About 650 °C (1200 °F) for two hours, giving 27-30 HRC and roughly 900-950 MPa (130-138 ksi) tensile. For 40 HRC, temper near 510-540 °C (950-1000 °F); for 50 HRC, near 315 °C (600 °F). Confirm with a coupon from the same heat.
- Can 4140 be case hardened?
- Yes, by nitriding (surface 50-55 HRC, 0.2-0.6 mm case, on a pre-hardened core) or by induction hardening of specific surfaces to 50-55 HRC. It is not carburized: its 0.40% carbon already through-hardens, and 8620 is the carburizing grade.
- How much does 4140 distort during heat treatment?
- Typically 0.05-0.2% dimensional change plus warp on long, thin or asymmetric parts. Rough machine, harden, then grind 0.2-0.4 mm (0.008-0.015") of stock off the finished faces. Long shafts are often straightened after tempering.
