Bearing steel testing is the set of cleanliness, microstructure, mechanical, and chemical tests that verify a high-carbon-chromium bearing steel (GCr15 / 100Cr6 / SAE 52100) meets the strict purity, carbide-uniformity, hardness, and fatigue requirements that a rolling-element bearing's life depends on. The governing standards are GB/T 18254-2016 (High-carbon chromium bearing steel, national standard platform), ISO 683-17 (Ball and roller bearing steels), ASTM A295 (American bearing steel), and JIS G 4805 (Japanese high-carbon chromium bearing steel). Bearing steel testing is not a re-run of structural-steel testing — both owe chemistry and tensile, but only bearing steel owes the non-metallic inclusion rating after quench-and-temper (the cleanliness that decides rolling-contact-fatigue life), the carbide-uniformity rating (banded, network, ledeburitic carbides), and the oxygen content that measures the cleanliness that the inclusion rating partly captures. These are the tests a structural steel never owes, because a structural member does not fail by rolling-contact fatigue from a subsurface inclusion. It is the bearing-grade counterpart to our Stainless steel testing and beyond-the-surface metal-reliability testing — the inclusion and carbide methods come from our Metallographic analysis programme, and the rolling-contact-fatigue failure mode feeds the same life-prediction work covered by our fatigue stress testing.
What Makes Bearing Steel a Distinct Test Subject?
A rolling-element bearing fails by rolling-contact fatigue (RCF) (bearing steel background) — the repeated cyclic contact stress between ball/roller and raceway spawns a subsurface crack, typically at a non-metallic inclusion or a coarse carbide, that grows to a spall. This failure mode is what makes bearing steel testing fundamentally different from structural-steel testing:
- Cleanliness is the headline property, not strength. A structural steel is judged on yield and tensile; a bearing steel is judged on how few and how small its non-metallic inclusions are, because every inclusion is a potential RCF initiation site. The oxide, sulfide, and silicate inclusions are rated to a defined microscopic method, and the steel must meet the rated limits — a bearing steel that meets the tensile spec but fails the inclusion rating is scrap.
- Carbide uniformity is the second headline. High-carbon-chromium steel (≈1 % C, ≈1.5 % Cr) forms chromium carbides that must be fine and uniformly distributed; the failure modes are banded carbides (segregation from forging/rolling), network carbides (along grain boundaries from poor cooling), and ledeburitic carbide (large primary carbides from solidification). Coarse or non-uniform carbides lower wear resistance and fatigue life, and they are rated on a metallographic section to defined limit charts.
- Oxygen content is the cleanliness proxy. The total oxygen in the steel tracks the oxide-inclusion content (the most damaging inclusion type for RCF), so a low oxygen level is a bearing-steel requirement — modern vacuum-degassed bearing steels target ≤ 10–15 ppm oxygen, and the oxygen value is reported as a cleanliness indicator alongside the inclusion rating.
The fact the SERP obscures: a bearing-steel certificate that quotes only chemistry and hardness is unverifiable. The non-metallic inclusion rating, the carbide-uniformity rating, and the oxygen content are the properties that decide whether the steel will deliver its rolling-contact-fatigue life — and they are the tests a structural-steel certificate (chemistry + tensile + impact) does not produce.
What Are the Headline Cleanliness Tests?
The tests that define bearing steel, and that structural steel does not owe:
- Non-metallic inclusion rating (microscopic) — GB/T 10561 / ISO 4967 / ASTM E45 method A — a metallographic section of the steel, quenched and tempered to standardise the matrix, is examined under a microscope at defined magnification, and the inclusions are classified by type (A sulfides, B aluminates, C silicates, D globular oxides) and rated by thickness and length against standard charts or by the extreme-value method. The rating limits are set by GB/T 18254 / ISO 683-17 / ASTM A295, and a steel that exceeds the limit in any inclusion type fails. The quench-and-temper before rating is mandated because the inclusion appearance depends on the matrix condition.
- Macro-inclusion / cleanness test by magnetic particle (ASTM A295 / the step-down method) — a defined specimen is machined and magnetised, and the total length of macro-inclusions revealed by magnetic particles is measured. This catches the large, sparse inclusions that the microscopic rating can miss but that will single-handedly spawn a fatigue spall.
- Oxygen content (GB/T 11261 / inert-gas fusion) — the total oxygen in the steel, reported in ppm, as the proxy for oxide-inclusion content. Modern bearing steels target ≤ 10–15 ppm total oxygen; higher oxygen means more oxide inclusions and shorter RCF life.
What Are the Carbide-Uniformity and Microstructure Tests?
High-carbon-chromium bearing steel forms carbides whose distribution decides wear and fatigue, and the carbide microstructure is rated on a metallographic section to defined limit charts:
- Banded carbide rating — carbides segregated into bands by forging/rolling (chemical segregation stretched along the working direction). Banded carbides make the steel anisotropic and lower fatigue life across the band; the rating is against a defined chart limit.
- Network carbide rating — carbides precipitated along prior-austenite grain boundaries, typically from slow cooling through the carbide-precipitation range. Network carbides are brittle and crack under contact stress; the rating limit catches the heat-treatment/cooling defect.
- Liquid / ledeburitic carbide rating — large primary carbides retained from solidification, which do not break up in subsequent working and act as fatigue initiators; rated against a limit chart.
- Grain size (GB/T 6394 / ASTM E112) — the prior-austenite grain size, which affects hardenability and fatigue; bearing steels require a fine, uniform grain.
These carbide ratings are bearing-steel-specific — no structural steel owes a banded-carbide or network-carbide rating, because a structural member does not fail by the mechanism that these defects drive.
What Mechanical and Chemical Tests Complete the Profile?
Beyond the cleanliness and carbide tests, the bearing-steel profile includes:
- Chemistry (GB/T 4336 / ISO 14284, by OES or XRF) — the carbon, chromium, manganese, silicon, sulphur, and phosphorus content; GCr15 is ≈ 1.00 % C, ≈ 1.50 % Cr, with tight limits on the impurity elements (low S and P). The chemistry must match the grade and the impurity limits must be met, but chemistry alone does not certify a bearing steel.
- Hardness (after the standard quench-and-temper, GB/T 230 / Rockwell C) — bearing-steel components are through-hardened to high hardness (typically ≥ 60 HRC for the finished component; the steel stock is tested after a defined heat treatment to verify it can reach that hardness).
- Tensile and impact (as applicable) — secondary for bearing steel (hardness and microstructure dominate), but reported for completeness on the annealed stock.
- Macro-etch / sulphur print — the steel section is etched to reveal macro-segregation, porosity, and inclusion stringers; the sulphur print specifically shows the sulphide distribution.
- Dimensional and surface defect check (UT or magnetic particle on bar) — the bar stock is inspected for surface seams, internal stringers, and pipe, because a surface seam on a finished raceway is a stress-raiser that fails in service.
How Do the Standards Differ?
| Standard | Origin | Scope |
|---|---|---|
| GB/T 18254-2016 | China | High-carbon chromium bearing steel (GCr15 family) |
| ISO 683-17:2014 | International | Five groups of wrought ball/roller bearing steels, through-hardening and others |
| ASTM A295/A295M | USA | High-carbon anti-friction bearing steel (52100 family) |
| JIS G 4805 | Japan | High-carbon chromium bearing steel (SUJ2 family) |
These standards are broadly equivalent in scope (through-hardening high-carbon-chromium bearing steel) but differ in grade names (GCr15 / 100Cr6 / 52100 / SUJ2 are near-equivalent), in exact chemistry limits, and in the inclusion-rating method and limits. A steel certified to GB/T 18254 cannot be assumed equivalent to one certified to ASTM A295 without checking the chemistry, inclusion, and carbide limits side-by-side — so a bearing-steel specification must name the standard and the grade.
Frequently Asked Questions
What standard governs bearing steel testing?
GB/T 18254-2016 (High-carbon chromium bearing steel) in China; ISO 683-17 internationally; ASTM A295 in North America; JIS G 4805 in Japan. The methods: GB/T 10561 / ISO 4967 / ASTM E45 for inclusion rating, GB/T 11261 for oxygen, GB/T 18254 carbide-rating charts for carbide uniformity.
What is the difference between bearing steel testing and structural steel testing?
Both owe chemistry and mechanical tests, but only bearing steel owes the non-metallic inclusion rating (after quench-and-temper), the carbide-uniformity rating (banded, network, ledeburitic), and the oxygen content — because these are the properties that decide rolling-contact-fatigue life, the failure mode that a structural member does not owe.
Why is the inclusion rating done after quench-and-temper?
Because the appearance of non-metallic inclusions on the metallographic section depends on the matrix condition. GB/T 18254 (and the ASTM/ISO methods it aligns with) require the specimen to be quenched and tempered to a defined hardness before the section is examined, so the rating is comparable across steels and labs — a rating on an annealed or as-rolled section is non-comparable and non-compliant.
What is oxygen content and why does it matter?
The total oxygen in the steel (in ppm) tracks the oxide-inclusion content — the inclusion type most damaging to rolling-contact fatigue. Modern vacuum-degassed bearing steels target ≤ 10–15 ppm total oxygen; higher oxygen means more oxide inclusions and shorter fatigue life. Oxygen is the cleanliness proxy reported alongside the inclusion rating.
What are banded and network carbides, and why are they rated?
Banded carbides are chromium carbides segregated into bands by forging/rolling; network carbides precipitate along grain boundaries during slow cooling; both lower wear and fatigue resistance. They are rated on a metallographic section against limit charts, because a bearing made from banded or network-carbide steel fails earlier under contact stress.
Is GCr15 equivalent to 52100 or SUJ2?
Near-equivalent but not identical — they are the through-hardening high-carbon-chromium bearing grades of GB/T 18254, ASTM A295, and JIS G 4805 respectively, with near-matching chemistry (≈1 % C, ≈1.5 % Cr). But the exact chemistry, inclusion, and carbide limits differ slightly across standards, so equivalence must be checked case-by-case and the standard named on the certificate.
Our Testing Capabilities
Beijing ZKGX Research (ISO/IEC 17025 testing laboratory) provides bearing steel testing across cleanliness, microstructure, mechanical, and chemical properties:
- Non-metallic inclusion rating (microscopic) to GB/T 10561 / ISO 4967 / ASTM E45 method A — after the standard quench-and-temper, by inclusion type (A/B/C/D).
- Macro-cleanness — magnetic-particle / step-down method for total macro-inclusion length.
- Oxygen content to GB/T 11261 — inert-gas fusion, ppm-level.
- Carbide microstructure — banded, network, and ledeburitic carbide ratings to GB/T 18254 limit charts; grain size to GB/T 6394 / ASTM E112.
- Chemistry to GB/T 4336 / ISO 14284 — OES or XRF, full grade analysis with impurity (S, P) limits.
- Hardness (Rockwell C, after the defined heat treatment), macro-etch / sulphur print, and UT / magnetic-particle for surface and internal defects on bar stock.
If you have a bearing-steel heat to qualify to GB/T 18254 / ISO 683-17 / ASTM A295, an inclusion-rating or carbide-rating requirement to verify, an oxygen-content target to confirm, or a bar-stock cleanliness lot to accept, contact our testing team to scope the applicable tests, the heat-treatment condition for rating, and the acceptance limits.