Bearings, springs, gears and fasteners each fail by fatigue, and fatigue performance depends on parameters that a grade name does not capture: cleanliness, surface condition and the heat treatment actually delivered. This guide covers the four component steel families, the specifications that predict fatigue life, and what to verify before releasing material into production.
The four families
Bearing steel
GCr15, also designated 52100 and EN31, is the standard through hardening bearing steel, with roughly 1 percent carbon and 1.5 percent chromium, used for balls, rollers and rings. Its critical property is very high cleanliness, because oxide inclusions act as fatigue initiation sites. Higher performance bearings move to carburising grades for tough cores or to high nitrogen corrosion resistant grades for wet or contaminated environments.
Spring steel
60Si2MnA and 65Mn are the common spring steels, and spring steel strip is supplied for leaf springs, coil springs, retaining rings and flat spring components. Springs operate at high stress, so their specification emphasises cleanliness, uniform hardness after hardening and tempering, and a controlled surface, since surface decarburisation and defects are the dominant fatigue drivers.
Gear and carburising steel
20CrMnTi and its variants, together with 8620 where an international grade is preferred, are case carburising steels: a hard, wear resistant case over a tough core. 40CrNiMoA is a quenched and tempered grade used for larger gears and shafts where through hardening plus toughness is required. The specification that matters is case depth, core hardness, and the hardness gradient across the case.
Fastener steel
42CrMo and 35VB are used for high strength bolts, and cold heading grades such as SWRCH are used for forming fasteners without prior machining. Property classes such as 8.8, 10.9 and 12.9 define the strength requirement, and the material and heat treatment must be selected to achieve the class while remaining resistant to hydrogen embrittlement, which is a major risk for the highest strength classes.
Specification axes that predict fatigue performance
Cleanliness and inclusion rating
Specify the cleanliness method and acceptance level, because inclusion content is the single strongest predictor of rolling contact and bending fatigue life, and it is invisible in a chemistry certificate.
Surface condition and decarburisation
For springs and gears, specify the maximum allowable decarburisation and the permissible surface defect depth, and specify whether the surface must be free of grinding burns or other damage that initiates fatigue.
Case depth and hardness gradient
For carburised parts, specify case depth by a defined method, the surface hardness, the core hardness and the shape of the gradient, since a case that is too shallow or too steep will spall or fail prematurely.
Hardenability and core properties
For through hardening grades, confirm hardenability so the section will harden as designed, and confirm the core mechanical properties after the intended heat treatment rather than relying on the annealed condition values.
Verification steps before releasing material
- Confirm chemistry by independent analysis, including the elements that set hardenability.
- Check cleanliness against the specified method for bearing and spring applications.
- Verify hardness and, where relevant, case depth on a sectioned sample rather than at the surface alone.
- Inspect for decarburisation and surface defects relevant to the fatigue duty.
- Confirm dimensional accuracy and straightness, since distortion after heat treatment is costly to correct.
- For high strength fasteners, confirm hydrogen embrittlement control through the plating and baking process.
Common sourcing mistakes
- Selecting bearing steel on grade and price without specifying cleanliness, then seeing unpredictable fatigue scatter.
- Specifying spring steel without a decarburisation limit, then finding surface initiated failures.
- Specifying carburising steel without case depth and gradient, then encountering spalling or wear.
- Using annealed condition values to predict performance after hardening.
- Ignoring hydrogen embrittlement risk on class 12.9 fasteners.
Sourcing component steel from Hunan and China
China supplies the full range of bearing, spring, gear and fastener steels, with integrated special steel capacity and a large downstream industry in automotive, machinery and wind energy. Hunan hosts advanced steel and metal suppliers within its advanced materials base. LiiFoo maintains verified supplier profiles in the advanced steel materials cluster.
For grade matched quotations, mill certificates and sample material, see the cluster above or contact our sourcing team.
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