Carbon Steel Heat Treating: 1018, 1045, 4140

Karl Gillihan

🏗️ CARBON STEEL

Carbon Steel Heat Treating: Carburizing, Quench and Temper for 1018, 1045, and 4140

Carbon content is destiny when it comes to hardening steel, and the four grades buyers reach for most, 1018, 1045, 4140, and A36, span the full range from case-hardening-only to deep through-hardening. Get the grade-to-process match wrong and you either fail to harden a part at all or you crack it in the quench tank.

ISO 9001AS9100IATF 16949

Carbon Content Sets the Ceiling: Why 1018 and A36 Behave Differently Than 1045 and 4140

Steel hardens because rapid cooling traps carbon in a distorted body-centered-tetragonal lattice called martensite, and the more carbon dissolved in the austenite before the quench, the harder that martensite gets. 1018 has only 0.18 percent carbon, far too little to through-harden, the most you can do is carburize it to add carbon to the surface. A36 is a structural grade with loosely controlled chemistry (carbon around 0.25 to 0.29 percent and variable), so while it can be hardened somewhat, it is never specified for heat treatment because the properties aren't guaranteed lot to lot. 1045 with 0.45 percent carbon is a genuine through-hardening medium-carbon steel, you can quench and temper it to 55 HRC at the surface, though its shallow hardenability means thick sections stay soft in the core. 4140 adds chromium and molybdenum to 0.40 percent carbon, and those alloying elements slow the transformation enough that 4140 hardens deeply and uniformly even in thick sections, reaching 28 to 32 HRC in common quench-and-temper conditions and up to the high 50s if quenched hard. The buyer rule: if you need a hard wear surface on a low-carbon part, carburize 1018 or 8620. If you need a strong, tough part through-and-through, you specify 1045 for thin sections or 4140 for anything substantial. A36 is a weldable structural steel, not a heat-treat material.

01

Case Hardening 1018: Carburizing, Carbonitriding, and Effective Case Depth

1018 is the classic case-hardening steel. You pack the surface with carbon by holding the part in a carbon-rich atmosphere at 1650 to 1750F (gas carburizing) for hours, then quench, producing a hard martensitic case over a soft, tough, low-carbon core. A typical case depth runs 0.020 to 0.040 inch with a surface hardness of 58 to 62 HRC, ideal for gears, pins, and bushings that need wear resistance with shock tolerance. Case depth is a function of time at temperature, roughly proportional to the square root of carburizing time, so doubling the case depth quadruples the cycle time. That makes deep cases expensive. Carbonitriding adds ammonia to the atmosphere at lower temperatures (1450 to 1600F), producing a shallower but harder, more wear-resistant case with better temper resistance, and it distorts less, which is why high-volume automotive small parts often use carbonitriding instead. Buyers should specify effective case depth (the depth to 50 HRC), not total case depth, and call out the core hardness requirement. The soft core is a feature, it gives the part the toughness to survive impact loads that would shatter a through-hardened part.

02

Through-Hardening and Tempering 1045 and 4140

Through-hardening is austenitize, quench, temper. For 1045 you austenitize around 1550F and water or brine quench (oil isn't aggressive enough for plain carbon steel's poor hardenability), reaching as-quenched hardness near 55 to 58 HRC at the surface. Then you temper, reheating to 400 to 1100F to trade some hardness for toughness, because as-quenched martensite is brittle and full of residual stress. 4140 is more forgiving and far more common in industry. Its chromium-moly content lets it oil quench (gentler, less distortion and cracking) and still harden deeply. The popular 4140 'Q&T' or pre-hard condition is supplied at 28 to 32 HRC, oil-quenched and tempered around 1000 to 1100F, giving an excellent balance of strength (around 150 ksi tensile) and toughness used for shafts, gears, and oilfield tooling. For higher hardness, lower temper temperatures push 4140 into the mid-40s HRC at the cost of toughness. The tempering decision is the real engineering choice: every grade has a tempering curve and a tempered-martensite embrittlement zone around 500 to 700F that you avoid for impact-loaded parts. Buyers should specify the target hardness and the application loading so the heat treater picks a temper that won't leave the part brittle.

03

Quench Cracking, Decarburization, and Distortion Control

The most expensive failure in carbon steel heat treating is the quench crack. Sharp internal corners, abrupt section changes, and tight threads concentrate the thermal and transformation stresses of the quench, and plain high-hardenability cuts crack right in the tank. 1045 quenched in brine for full hardness is especially crack-prone, which is why parts are designed with generous radii and why 4140's oil-quench capability is a major manufacturing advantage. Decarburization is the other quiet defect, prolonged high-temperature soaking in an oxidizing furnace burns carbon out of the surface, leaving a soft skin on a part that tested hard internally. Controlled or endothermic atmospheres and vacuum furnaces prevent it, and on critical wear surfaces shops leave grind stock to remove any decarburized layer after heat treat. Distortion is managed by stress-relieving rough-machined parts before final hardening, using press quenching or fixture quenching for thin and gear-shaped parts, and leaving grind allowance. The order of operations rule mirrors aluminum: rough machine, stress relieve, harden and temper, then finish grind to remove distortion and any decarb.

Frequently Asked Questions

A36 can be hardened to a limited degree, but it should not be specified for heat treatment, and most heat treaters will tell you the same. A36 is a structural steel defined by its mechanical properties (36 ksi minimum yield) and a loose chemistry range, with carbon typically between 0.25 and 0.29 percent and significant lot-to-lot variation. That carbon level is enough to form some martensite if you austenitize and quench, but the result is unpredictable, you cannot guarantee a target hardness across different heats of A36, and the inconsistent chemistry means cracking risk and uneven response. If your application needs a known, repeatable hardness, you switch to a controlled-chemistry grade: 1045 for medium-carbon through-hardening, 1018 for case hardening, or 4140 for deep, uniform hardening. A36 is excellent as a weldable, machinable structural and fabrication steel, and that is exactly what it is designed for. Treat any request to heat treat A36 as a sign the wrong material was specified for the part.

Case hardening creates a hard outer shell over a soft, tough core, while through hardening makes the part roughly uniform in hardness from surface to center. Case hardening is used on low-carbon steels like 1018 or 8620 that lack the carbon to harden on their own, you diffuse carbon (carburizing) or carbon and nitrogen (carbonitriding) into the surface at high temperature, then quench, producing a 58 to 62 HRC case typically 0.020 to 0.040 inch deep over a 20 to 30 HRC core. That combination resists surface wear while the ductile core absorbs impact, which is ideal for gears, cams, pins, and bearings. Through hardening is used on medium-carbon and alloy steels like 1045 and 4140 that already contain enough carbon, you austenitize the whole part, quench, and temper to a uniform hardness, giving consistent strength throughout the cross-section for shafts, fasteners, and structural components. The trade-off is that a fully through-hardened part is more brittle and crack-prone than a case-hardened one, which is why high-impact wear parts favor the soft-core case-hardened approach.

4140 generally costs a bit more in both material and processing, but it is usually the better choice for any substantial part. The material itself runs higher because it contains chromium and molybdenum, and those alloying elements are exactly what make 4140 worth it: they give the steel high hardenability, meaning it hardens deeply and uniformly even in thick sections, and they let it be oil quenched instead of the aggressive water or brine quench that 1045 requires. Oil quenching is gentler, so 4140 distorts less and cracks far less often, which lowers scrap and rework cost and frequently offsets the material premium. 1045, by contrast, has shallow hardenability, so a thick 1045 part hardens only near the surface and stays soft in the core, and the brine quench needed for full surface hardness raises the cracking risk on any part with sharp corners. Choose 1045 for thin, simple, cost-sensitive parts where surface hardness in the cross-section is enough. Choose 4140 for shafts, gears, oilfield tooling, and anything thick or geometrically complex where you need uniform strength and toughness and want to avoid quench cracking.

Through-hardening (quench and temper) of 1045 or 4140 typically runs $0.75 to $2.50 per pound at production volume, with lot minimums of $150 to $350, and turns in 3 to 7 business days depending on furnace batching. Case hardening (carburizing) 1018 is priced more by case-depth and cycle time than weight, since a 0.040 inch case can mean 8 or more hours at temperature, expect $1.50 to $4.00 per pound and 5 to 10 business days, with deeper cases costing disproportionately more because case depth grows with the square root of time. Carbonitriding is often cheaper and faster than deep carburizing for shallow cases. The real cost drivers across the board are secondary operations: stress relief before hardening, straightening distorted parts, and grinding to remove decarburization or restore tolerance after the quench. Automotive work under IATF 16949 with full traceability and per-lot hardness and case-depth verification adds 25 to 50 percent and a few days. Expedited service is widely available at a 25 to 50 percent premium for the simpler quench-and-temper cycles.

Quench cracking happens when the stresses generated during rapid cooling exceed the strength of the freshly formed, brittle martensite. Two stresses stack up: thermal stress from the surface cooling and contracting faster than the core, and transformation stress from the volume expansion that occurs as austenite turns into martensite. When those concentrate at a stress riser, they tear the part open, usually within minutes of the quench. The biggest culprits are sharp internal corners, abrupt section thickness changes, drilled holes near edges, sharp thread roots, and too-aggressive a quench for the grade. Prevention starts at design: add generous fillet radii, avoid abrupt section changes, and plug or chamfer holes that act as crack initiators. On the process side, use the gentlest quench that still achieves the required hardness, which is a major reason 4140's oil-quench capability is preferred over 1045's brine quench, and temper the part promptly after quenching, ideally before it cools fully to room temperature, to relieve the brittle as-quenched stress before it can propagate a crack. Press or fixture quenching and pre-hardening stress relief also help on thin or complex geometries.

Last updated: July 2026

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