Carbon Steel Carburizing Process: Case Depth, Temperature and Quality Control

Carburizing is the go-to case hardening process for carbon and low-alloy steels that must combine a tough core with a hard, wear-resistant surface: gears, shafts, cam followers, chain pins, and bearings. Because the process runs at 850–980°C for hours and adds carbon to the surface layer, getting it wrong is expensive — a scrapped carburized gear set can cost more than the raw forgings. This guide explains how the process works, how to specify case depth and hardness, what furnace data you should demand from your heat treater, and the classic defects buyers encounter during incoming inspection.

How Carburizing Works

Carburizing diffuses carbon into the steel surface at austenitizing temperature, followed by quenching to transform the carbon-rich case into martensite while the low-carbon core remains tougher (bainitic or ferritic-pearlitic). Only steels with roughly 0.10–0.25% initial carbon respond usefully — the surface carbon rises to 0.75–1.00% at the end of the carburizing cycle. Typical production variants:

• Gas carburizing (dominant, >90% of tonnage): parts cycle in endothermic atmosphere (CO/CO₂/H₂) at 900–925°C, with carbon potential controlled by dew point, oxygen probe, or infrared CO₂ analysis.
• Vacuum (low-pressure) carburizing: acetylene or propane pulses at 900–980°C in a vacuum furnace. Faster diffusion at higher temperature, no intergranular oxidation, cleaner parts — preferred for aerospace and EV gearing.
• Pack (solid) carburizing: parts buried in charcoal compound in boxes. Slow, labor-intensive, but still used for single-piece repairs and very large sections.
• Carbonitriding: carburizing with 2–5% ammonia addition, used on thin sections (<3 mm case) for improved hardenability at lower quench severity.

Case Depth: How to Specify It Correctly

Case depth is the single most disputed specification on carburized parts. Drawings must state which definition applies, because three different numbers circulate:

Definition	Meaning	Typical use
TCH (Total Case Depth)	Depth where hardness equals core + 50 HVC (ISO 2639) or drops to 50 HRC	Gearing, wear surfaces
ECD (Effective Case Depth)	Depth to 50 HRC (550 HV1)	Standard for most drawings
CHD	Effective case depth measured by microhardness traverse (ISO 6507)	ISO drawing notation

As a sizing rule for through-hardened gearing, ECD ≈ 0.15–0.20 × module; for a module 4 gear that is 0.6–0.8 mm. Diffusion time follows Fick’s law roughly as depth ∝ √time: reaching 1.0 mm effective case at 925°C takes about 6–8 hours, while 1.5 mm takes 14–18 hours. Specifying excessive case depth is a real cost driver — every extra 0.2 mm can add 2–4 hours of furnace time and increase distortion.

Process Control Numbers Worth Checking

When auditing a heat treater or reviewing certificates, these are the numbers that separate controlled shops from hopeful ones:

• Carbon potential accuracy ±0.05%C: verified weekly by shim stock or foil test; a drifting oxygen probe silently produces soot or decarburized cases.
• Temperature uniformity ±7°C (AMS 2750 Class 3 typical): surveyed quarterly with certified thermocouples.
• Quench oil temperature 50–70°C: warm oil reduces distortion on thin webs; agitation level (propeller or spray) must match the steel’s hardenability.
• Temper within 1–2 hours of quench at 150–200°C: skipping or delaying temper is the top cause of grinding cracks found days later at final machining.
• Intergranular oxidation (IGO) depth <13 µm: gas carburizing always oxidizes grain boundaries slightly; deeper IGO pits act as fatigue initiators on gear roots and must be removed by grinding stock.

Vacuum carburizing eliminates IGO entirely, which is why high-reliability gearbox manufacturers pay the 30–60% heat treatment premium for it on premium product lines.

Typical Defects and How They Show Up

Defect	Cause	Detection
Soft spots	Uneven quench, part contact in basket, soot on surface	File test, hardness map, Nital etch
Grinding cracks	Retained austenite + overheat during grinding, no temper	Magnetic particle, nital etch
Excess retained austenite	Surface carbon >0.9%, low temper	XRD, metallography (X-ray >25% reject)
Carbide network at surface	Carbon potential too high, slow cool from carburizing	Metallography — brittle network causes spalling
Distortion beyond drawing	Poor racking, asymmetric sections, quench severity mismatch	First-piece CMM after HT
Decarburized case	Carbon potential collapse mid-cycle	Hardness traverse flat-topped instead of peaked

Grade Selection for Carburizing

Grade	Core carbon	Best for	Note
1018 / 20#	0.15–0.20%	Light-duty pins, small sprockets	Water quench, distortion risk
1117 / 1215*	0.15%*	Small fasteners, moderate wear	*1215 marginal — high S welds badly
5120 / 5115	0.17–0.22%	Gears module ≤ 4	Chromium adds case hardenability
8620 / 20CrMnTi	0.18–0.23%	Automotive gears, shafts	Industry default for power transmission
4320	0.17–0.22%	Heavy-duty, large cross-sections	Nickel-moly, tough core
9310	0.08–0.13%	Aerospace gearing	3.5% Ni, premium cost

What to Ask For on the Purchase Order

A complete carburizing PO should lock down: ECD with measurement method (microhardness traverse per ISO 6507 or file test per drawing note), surface hardness range (typically 58–63 HRC), core hardness range, tempering temperature window, retained austenite limit if gearing (≤ 25–30%), IGO depth limit, straightness/flatness after HT with grinding stock stated separately, and process records retention (chart printouts per load). Reputable Chinese heat treaters supplying export gears routinely attach load charts and hardness traverse plots per batch; if a supplier refuses to share furnace charts, treat that as a red flag equal to missing MTCs on bar stock.

Frequently Asked Questions

How long does it take to reach 1.0 mm case depth?

About 6–8 hours at 925°C in gas carburizing, including a boost-and-diffuse cycle. Vacuum carburizing at 950–980°C shortens this to roughly 4–5 hours of equivalent processing.

Can 1045 steel be carburized?

It is not recommended. With 0.45% core carbon, the case and core hardness difference becomes meaningless, and quench cracking risk rises. 1045 is normally induction hardened instead of carburized.

Why do carburized parts need grinding stock on gear flanks?

To remove the intergranular oxide layer (IGO) and any surface carbide network — both sit within the first 10–25 µm and would otherwise become fatigue initiation sites under root bending stress.

How do I verify case depth on incoming parts?

Request the heat treater’s hardness traverse on a coupon processed with the load, and spot-check by sectioning a sacrificial part and running a microhardness profile per ISO 6507 on the metallographic cross-section.