Technical guide · 2026-08-21
Case-Hardening Steel
Case-hardening steels pair a low-carbon core chemistry with carburizing, carbonitriding or a related surface-enrichment route to create a hard wear surface over a tougher core. The finished case profile is a process result, not a property guaranteed by the grade name.
01
Definition and boundary
Keep carburized case/core design separate from induction hardening of medium-carbon steel and from through-hardening bearing steel.
Reference information only: final acceptance is controlled by the ordered product specification, condition, dimensions and certified test results.
02
How the material system works
Carbon potential and time-temperature exposure establish the carbon gradient and effective case depth.
Low-carbon alloy core chemistry supplies hardenability and impact support beneath the case.
Quench and temper convert the enriched surface and core into different hardness/toughness zones.
Geometry, stock allowance and carbon distribution govern retained austenite, distortion and grinding stock.
03
Representative grades and systems
| Grade or system | Selection focus | Qualification |
|---|---|---|
| 8620 | Ni-Cr-Mo carburizing chemistry used where a tougher core and controlled hardenability are needed. | Specify case method, effective case-depth definition, hardness traverse, core property, distortion allowance and final grinding. |
| 20MnCr5/16MnCr5 | EN Mn-Cr families whose delivery and hardenability requirements remain standard-specific. | Specify case method, effective case-depth definition, hardness traverse, core property, distortion allowance and final grinding. |
| Simpler low-carbon options for less demanding case/core and section requirements. | Specify case method, effective case-depth definition, hardness traverse, core property, distortion allowance and final grinding. |
04
Properties and trade-offs
Surface contact and wear resistance depend on case hardness and microstructure.
Core toughness depends on alloy hardenability, section and heat treatment.
Case-depth uniformity is sensitive to carbon transfer and geometry.
Distortion, retained austenite and grinding damage can control final dimensional capability.
05
Product forms and supply conditions
06
Applications and selection
Applications and selection
- Transmission gears and pinions
- Sprockets, cams and wear sleeves
- Carburized bearing races and large rings
- Pins and shafts needing a hard surface with a supported core
Properties and trade-offs
- Define required contact load, wear mode and effective case depth from the component design.
- Choose core chemistry and hardenability for the section beneath the case.
- Coordinate carbon potential, machining allowance and distortion strategy before material release.
- Select inspection positions that represent tooth root, flank or other critical geometry.
07
Fabrication, durability and inspection
Machine with stock for carbon removal, distortion correction and final grinding.
Control furnace carbon potential, load spacing and atmosphere circulation.
Set quench transfer and agitation to the part family rather than a coupon.
Verify hardness traverse, retained austenite, microstructure and grinding integrity after finishing.
08
Procurement checklist
- State base grade, product standard, bar/tube form and annealed or normalized condition.
- Order H-band, grain size, cleanliness and surface quality where core or fatigue risk requires them.
- Define effective case depth, surface/core hardness, microstructure and test locations on the process drawing.
- Agree distortion datum, grinding allowance, crack inspection and lot traceability.
09
Standards, sources and review
Standards and designation context
- SAE J404_200901 for applicable SAE alloy chemistry
- ASTM A304-20 or SAE J406_202402 where bar hardenability is ordered
- ASTM A534-17(2022) for applicable carburizing bearing-quality steel
- Component drawing and qualified case-hardening procedure control the finished case
Authoritative research sources
SAE International · J404_200901, current verified revision
ASTM International · A304-20
SAE International · J406_202402
ASTM International · A534-17(2022)