Technical guide · 2026-08-03
Case-Hardening Steel
Case-hardening steels are low-carbon carbon/alloy grades designed to develop a hard wear-resistant surface by carburizing, carbonitriding or related treatment while retaining a tougher core. Case depth, surface carbon, hardness profile and distortion are process outcomes, not properties guaranteed by grade name alone.
01
Definition and boundary
Differentiate carburizing steels from induction-hardened medium-carbon steels: carbon source, hardening depth and core design are different.
Reference information only: final acceptance is controlled by the ordered product specification, condition, dimensions and certified test results.
02
How the material system works
Alloy chemistry changes hardenability, tempering response, toughness or elevated-temperature stability.
Heat treatment develops the usable microstructure and must be tied to section size.
Cleanliness, segregation and surface condition influence fatigue and reliability.
03
Representative grades and systems
| Grade or system | Selection focus | Qualification |
|---|---|---|
| 8620 | Ni-Cr-Mo carburizing grade for gears and wear components. | Verify the ordered product specification, condition and dimensions before substitution. |
| 20MnCr5/16MnCr5 | EN Mn-Cr case-hardening families requiring standard-specific verification. | Verify the ordered product specification, condition and dimensions before substitution. |
| Simple low-carbon options for shallower cases and less demanding cores. | Verify the ordered product specification, condition and dimensions before substitution. |
04
Properties and trade-offs
Properties are condition- and section-specific rather than chemistry-only.
Higher hardenability or strength can reduce welding and machining margin.
Fatigue performance requires cleanliness and surface-integrity control.
05
Product forms and supply conditions
06
Applications and selection
Applications and selection
- Transmission gears and sprockets
- Pins, cams and wear sleeves
- Components needing a hard case and machinable/tough core
Properties and trade-offs
- Define final component loads, life, temperature and failure mode.
- Select grade and hardenability for section and heat-treatment capability.
- Set inspection and process controls around the critical property.
07
Fabrication, durability and inspection
Match austenitizing, quench and temper or case-hardening procedure to section size and required microstructure.
Control decarburization, grain size, residual stress, straightness and distortion through the full route.
Qualify welding and repair separately; preheat, consumables and post-weld treatment depend on grade and restraint.
08
Procurement checklist
- Specify exact designation, product standard, melt practice, size and supply/heat-treatment condition.
- Define hardenability, cleanliness, grain size, decarburization, UT, hardness and mechanical tests as applicable.
- State machining allowance, surface class, straightness, traceability and final component heat treatment.
09
Standards, sources and review
Standards and designation context
- SAE designation/composition standards for SAE grades
- ASTM A29/A322 for applicable bar products
- EN/ISO/AMS or component standards for alternate systems and final acceptance