Can 1018 steel be hardened?
Only at the surface. 1018 has too little carbon to through-harden; carburizing or carbonitriding gives a 58-62 HRC case over a soft, tough core.
Hardness in quenched steel comes from carbon, and 1018 has only 0.15-0.20% of it, with no alloying to help it harden deeply. Even ideal martensite at that carbon level is only about 40-45 HRC, and the hardenability of 1018 is so low that it gets near that only at the surface of thin sections quenched in water or brine. An oil quench does almost nothing, and a 25 mm (1") bar quenched in water ends up hard in a thin skin and soft through the middle. Flame and induction hardening fail for the same reason. So a drawing that says "1018, harden to 45 HRC" cannot be met, and in practice "hardened 1018" means case hardened: carbon is added to the surface, and only that layer becomes hard.
Carburizing is the standard route. The parts are held at 900-930 °C (1650-1700 °F) in a carbon-rich gas atmosphere (or in a vacuum furnace, salt bath or pack) until the surface reaches about 0.8-1.0% carbon, then quenched, usually in water or brine for a plain carbon steel like 1018 or in oil for thin parts, and tempered at 150-200 °C (300-400 °F). The surface reaches about 58-62 HRC. Effective case depth, measured to 50 HRC, runs from about 0.25 mm (0.010") to 1.5 mm (0.060"); roughly 4-6 hours at 925 °C (1700 °F) gives about 1 mm (0.040"). Carbonitriding at 790-870 °C (1450-1600 °F) adds nitrogen as well as carbon, allows an oil quench with less distortion, and suits thin cases of 0.08-0.5 mm (0.003-0.020") on small parts. Ferritic nitrocarburizing (gas FNC, or salt-bath processes sold as Tufftride, Melonite or QPQ) is a lower-temperature option that gives a thin, hard, corrosion-resistant compound layer with no quench, but no deep case.
The core is what limits 1018. It stays close to its original softness, typically below about 30 HRC except in small water-quenched sections, so the hard case sits on a support that yields under heavy point loads. That is fine for pins, bushings, cam followers, wear pads and light gears, where the case carries the wear and the tough core resists shock. Under heavy contact stress the case can crush into the soft core (the eggshell effect), and that is where 8620 takes over: its alloy content gives a core of roughly 25-45 HRC under the same case. Plan the sequence around the case: machine everything first, mask threads and surfaces that must stay soft with copper plate or stop-off paint (or leave stock and machine the case off before the quench), and leave grinding stock of 0.1-0.25 mm (0.004-0.010") on precision surfaces, with the case depth specified after grinding. Water-quenched parts distort, so long, thin and asymmetric shapes need straightening or grinding. For a through-hardened part in a simple carbon steel, start from 1045 or 1095, not 1018.
Carburizing for 1018 parts: where to send them
What to specify
- Process and result: "carburize and harden, surface 58-62 HRC, effective case depth 0.5-0.8 mm (0.020-0.030") to 50 HRC", with the depth adjusted to the load
- Case depth stated on the finished (ground) surface, measured per SAE J423 by microhardness traverse (ASTM E384) on a coupon from the same bar and load
- Areas to remain soft (threads, holes drilled later, weld areas): mask with copper plate or stop-off, or leave stock to machine the case off before the quench
- Temper after quench: 150-200 °C (300-400 °F), one hour minimum
- Grinding stock of 0.1-0.25 mm (0.004-0.010") per side on precision diameters and faces, and the straightness or flatness allowed after hardening
- Core hardness as "for information" or a wide range, since 1018 core response depends heavily on section size; AMS 2759/7 for aerospace-quality carburizing
Pitfalls
- Calling out a through-hardness on 1018: 45 HRC through a 25 mm (1") bar is not possible; specify a case, or change to 1045 or 4140
- Case depth specified before grinding: the grinder removes much of a thin case; set the depth on the finished surface
- Induction or flame hardening 1018: there is not enough carbon, and the surface barely changes
- Heavy point or line loads on a thin case over the soft core: the case crushes and flakes; use a deeper case or 8620
- Carburized threads: they come out brittle and chip in assembly; mask them
- Water-quench distortion on long pins and thin plates: plan for straightening and grinding
Frequently asked questions
- Can 1018 steel be through-hardened?
- Not usefully. With 0.15-0.20% carbon and no alloying, a water quench hardens only a thin skin of small sections to roughly 30-40 HRC and leaves the core soft. For through-hardness use 1045, 4140 or a tool steel.
- How hard can case-hardened 1018 get?
- About 58-62 HRC at the surface after carburizing, quenching and a 150-200 °C (300-400 °F) temper, with an effective case of 0.25-1.5 mm (0.010-0.060"). The core stays much softer, typically below 30 HRC.
- Should I use 1018 or 8620 for carburized parts?
- Use 1018 for lightly loaded pins, bushings and wear parts where cost matters. Use 8620 when the part carries high contact or bending stress: its nickel-chromium-molybdenum content hardens the core to roughly 25-45 HRC and supports the case.
- Can 1018 be nitrided?
- Ferritic nitrocarburizing works well and gives a thin, hard, corrosion-resistant compound layer. Conventional gas nitriding does little for 1018 because it has no chromium or molybdenum to form hard nitrides; that process is for 4140, Nitralloy and tool steels.
