Surface Treatment Carburetor Nit: How Engineers Choose a Case

• 1
  What Actually Happens Inside the Furnace
• 2
  Temperature, Time, and What They Do to the Part
• 3
  Which Steel Goes With Which Process
• 4
  Carburizing vs Nitriding vs Carbonitriding
• 5
  Choosing by Part Requirement

All three processes push carbon or nitrogen into the surface of steel while the core stays soft. The difference sits in which element diffuses, at what temperature, and for how long. Carbon diffuses fast at 850–950 °C because the iron lattice is austenitic and open. Nitrogen diffuses at 500–580 °C, where the lattice is ferritic and tight. That single fact explains most of the distortion gap between these processes.

Carburizing raises the surface carbon to roughly 0.7–1.0% C, then a quench converts that enriched zone into martensite. The core remains low carbon, so it stays tough. Case depth is governed by time and temperature: a typical gas carburize at 925 °C reaches about 0.8 mm in three hours, 1.5 mm in six hours. More time means a deeper case, not a harder one.

Nitriding does not need a quench. Nitrogen reacts with aluminum, chromium, or molybdenum in the steel to form hard nitride precipitates. Because the part never passes through the austenite-to-martensite transformation, it never sees the volume change that comes with quenching. That is why nitrided gears often need no grinding after treatment. The trade-off is time: 20–60 hours is normal for a 0.3–0.5 mm case.

Carbonitriding sits between the two. It runs at 800–880 °C and introduces both carbon and nitrogen from a gas atmosphere, then quenches. Nitrogen slows the carbon diffusion slightly and improves hardenability, so the same case depth is reached in less time than straight carburizing. When a drawing calls out a surface treatment carburetor nit process, this is usually the family being described.

Temperature sets the diffusion rate, not just the speed. Carburizing at 925 °C is standard, but shops push to 950 °C when they need depth fast. Above that, grain growth starts to hurt core toughness. At the other end, carbonitriding at 800–850 °C keeps grain size fine and distortion lower, which is why small, thin-walled parts often go that route.

Time follows a square-root law. Doubling case depth takes roughly four times the cycle. A 0.5 mm case might take two hours; a 1.5 mm case takes closer to six to eight. Engineers who need a deep case should plan the schedule early, because furnace time is the bottleneck and it shows up in lead time.

The quench is where distortion is born. Oil quench from 850 °C gives the fastest cooling and the most movement. Marquenching or high-pressure gas quenching reduces it but costs more and limits section thickness. For a part with a bore tolerance of ±0.005 mm, a post-treatment grind is usually required after carburizing. Nitriding avoids this entirely, which is its main selling point for finished parts.

Atmosphere control matters as much as the schedule. Carbon potential must be held within a narrow band, or the surface gets retained austenite or intergranular oxidation. Both show up later as pitting or flaking. A well-run furnace logs carbon potential every few minutes; a poorly run one produces a case that looks right on a hardness tester and fails in service.

• 1
  Carburizing850–950 °C, carbon only, quench required, deep cases to 3 mm
• 2
  Nitriding500–580 °C, nitrogen only, no quench, cases to 0.6 mm
• 3
  Carbonitriding800–880 °C, carbon plus nitrogen, quench required, cases to 0.8 mm

Carburizing needs low-carbon steel. Below about 0.25% C, the core stays ferritic and tough while the surface takes carbon. Grades like 1018, 4130, and 8620 are common. You cannot carburize 4140 and get a soft core, because it already has enough carbon to harden through. That is a frequent drawing error.

Nitriding needs alloy steel with nitride formers. Aluminum, chromium, and molybdenum do the work. That means 4140, 4340, and nitriding grades like 41CrAlMo7. Stainless 17-4PH responds well too. Plain carbon steel like 1018 will nitridе but the case is thin and weak, because there is nothing for the nitrogen to combine with. If a part is already made from 1018, nitriding is the wrong call.

Carbonitriding is more forgiving on material. It works on low-carbon and medium-carbon steels, and it tolerates leaner alloys than nitriding. Small parts in high volume benefit most: fasteners, bushings, thin plates. The nitrogen addition improves hardenability, so a mild steel can reach a hardness it could not reach with carbon alone.

The practical rule is simple. Match the steel to the process before you match the process to the part. If the print says 1018, you are choosing between carburizing and carbonitriding. If the print says 4140 and the part is finished, nitriding is likely the only option that keeps dimensions.

• 1
  Low-carbon steelCarburizing or carbonitriding; core stays tough
• 2
  Alloy steel with Cr or AlNitriding; hard case, minimal movement
• 3
  Already-hardened steelDo not carburize; use nitriding only