Almost every rolling bearing in the world is made from one steel under three different names. 100Cr6 in the German and European system, GCr15 in the Chinese standard and SUJ2 in the Japanese standard are not three materials - they are one through-hardening, high-carbon chromium steel, historically known as 52100. Knowing that they are the same family changes how you read a quotation, because a supplier quoting SUJ2 and a supplier quoting GCr15 are quoting the same grade, and the difference in the part lies in cleanliness, heat treatment and verification rather than in the name.
This guide covers the chemistry, the hardening and tempering that give 58-62 HRC, the heat-stabilisation classes and their temperature limits, retained austenite, case-hardening and stainless alternatives, and how a buyer verifies a grade with a material certificate.
- 100Cr6, GCr15 and SUJ2 are one grade. About 1% carbon and 1.5% chromium, through-hardening, equivalent to the American 52100 grade. The name reflects the standard, not a different material.
- Standard bearings run at 58-62 HRC. Hardening followed by tempering sets the balance between hardness and toughness, and the tempering temperature sets the usable temperature.
- Heat-stabilisation classes define the ceiling. S0 to S3 trade dimensional stability at temperature for other properties; a standard S0 bearing is not suitable for continuous service above about 120 degC.
- Cleanliness is the real differentiator. Oxygen content and non-metallic inclusions decide fatigue life at least as much as the grade name, and both are verified with a material certificate and a heat number.
One steel, three names
The naming systems are parallel, not competing. 100Cr6 is the DIN and EN designation (material number 1.3505), GCr15 is the Chinese GB/T designation, SUJ2 is the JIS designation, and all three map onto the American 52100 chemistry. A quotation for a SUJ2 bearing and a quotation for a GCr15 bearing are quotes for the same steel family.
Why the name matters in a quote
Because it tells you which standard the supplier is working to and which certificate you should expect. A supplier who quotes SUJ2 should be able to produce a JIS-conformant certificate; one who quotes GCr15 should produce a GB/T certificate. When the two quotes carry the same grade under different names, the decision should turn on chemistry verification and cleanliness rather than on the label.
Specify the grade against one of the recognised standards so that the requirement is unambiguous: ISO 683-17 internationally, ASTM A295 in the United States, GB/T 18254 in China and JIS G4805 in Japan are the four that cover this family, while the German and European route uses the grade name EN 100Cr6. Each of these fixes a composition range and a set of cleanliness requirements, and any of them can be quoted against once you know which one the supplier works to.
The chemistry in round numbers
The composition is simple and it is the reason the steel behaves the way it does.
| Element | Approx. content | Role |
|---|---|---|
| Carbon (C) | About 1.0% | Provides the hardness after hardening by forming martensite and carbides |
| Chromium (Cr) | About 1.5% | Improves hardenability, wear resistance and corrosion resistance slightly |
| Manganese (Mn) | About 0.3% | Aids hardenability and deoxidation |
| Silicon (Si) | About 0.25% | Deoxidation and some tempering resistance |
| Sulfur and phosphorus | Low, controlled | Impurities kept low for cleanliness |
Roughly 1% carbon is high for a steel and is what allows the surface to reach 60 HRC. About 1.5% chromium adds hardenability and a modest amount of corrosion resistance, though not enough to make the steel stainless. The remainder of the composition is about keeping the steel clean: low sulfur, phosphorus and oxygen, because inclusions are where fatigue cracks start.
Hardening, tempering and the heat-stabilisation classes
Through-hardening means the whole section is heated to austenitising temperature and quenched, so the hardness extends through the ring rather than only the surface. Tempering afterwards relieves stress and sets the final hardness, normally 58-62 HRC for a bearing. The tempering temperature also fixes the maximum service temperature, which is expressed as a heat-stabilisation class.
Heat-stabilisation classes
The classes are identified by a suffix and each corresponds to a different maximum continuous operating temperature.
| Class | Approx. max temperature | Note |
|---|---|---|
| S0 (standard) | About 120 degC | Default; not suitable for sustained higher temperatures |
| S1 | About 200 degC | Additional tempering for elevated temperature |
| S2 | About 250 degC | Special heat treatment |
| S3 | About 300 degC | Highest class, used for extreme environments |
Exceeding the class limit is not a gradual degradation, it is dimensional instability. A standard S0 bearing run continuously above about 120 degC will grow in size as retained austenite transforms, and that growth changes the fit and the clearance. This is why the temperature and clearance interaction has to be considered together with the material class.
Retained austenite and dimensional stability
After quenching, some austenite remains untransformed at room temperature - retained austenite. Over time, particularly at temperature or under stress, it gradually transforms to martensite, which is slightly larger, so the ring grows. A small amount of retained austenite is desirable for toughness, but too much makes the bearing dimensionally unstable. The heat-stabilisation tempering is what reduces the retained austenite to a level matched to the intended service temperature, which is exactly why a higher S class is needed for a hotter position.
When case-hardening grades are the right answer
Through-hardening has a limit: it needs enough carbon throughout the section, and a large section may not harden uniformly, while a part that must absorb shock benefits from a tough core. In those cases a low-carbon, case-hardening grade such as 20CrMnTi or 8620 is used, carburised to produce a hard surface over a soft, tough core. The case depth is specified and measured, and the result tolerates shock loads and section sizes that a through-hardened part would not. A carburised case is typically 1-3 mm deep at about 60 HRC, over a core of 30-45 HRC.
| Grade | Hardening | Max temperature | Typical use |
|---|---|---|---|
| 100Cr6 / GCr15 / SUJ2 | Through-hardened, 58-62 HRC | About 120 degC (S0) | Standard ball and roller bearings |
| 100Cr6, class S1-S3 | Through-hardened, heat-stabilised | 200-300 degC | Furnace, kiln and hot-equipment bearings |
| 20CrMnTi / 8620 | Case-carburised over a tough core | About 200 degC | Large-section and shock-loaded parts |
| 440C (X105CrMo17) | Martensitic stainless, about 58 HRC | About 250 degC | Food, beverage and marine positions |
| Silicon nitride (ceramic) | Not applicable | Above 800 degC | Hybrid and full-ceramic, high speed |
Stainless, ceramic and hybrid alternatives
Where 100Cr6 is not enough, several alternatives are available, each with a trade-off.
440C and 316 for food and marine
440C (X105CrMo17) is a martensitic stainless steel that hardens to a usable bearing hardness and resists corrosion in food, beverage and marine positions, at the cost of a reduced load rating compared with 100Cr6. 316 stainless does not harden to bearing hardness and is used for cages, washers and housings rather than for loaded raceways. The stainless bearing steels reference has the detail, and the corrosion context is in the corrosion protection guide.
Ceramic and hybrid options sit at the other end of the trade. A hybrid bearing uses ceramic balls in steel rings, which permits higher speed, reduces friction and electrically insulates the bearing; a full ceramic bearing takes that further at higher cost. The ceramic hybrid guide covers the high-speed case.
Steel quality, certificates and hardness verification
Two bearings can carry the same grade name and differ in useful life, because life depends on how clean the steel is.
Material certificates and heat numbers
Ask for a material certificate that states the grade, the heat number and the measured composition, including oxygen content in ppm. A heat number lets you trace the steel back to the melt, so a certificate without a heat number is a claim rather than a record. Oxygen content and non-metallic inclusion ratings are the numbers that correlate with fatigue life, and they are the ones to compare between suppliers of the same grade.
Hardness testing: surface versus core, and case depth
Hardness is verified on the finished bearing, typically as a surface reading converted to HRC. For a through-hardened ring the core hardness should be close to the surface value; for a case-hardened part the surface is hard and the core is softer, so the case depth must be measured and compared with the specification. A single surface reading does not prove that the section was hardened correctly, which is why depth-related checks matter for thick or case-hardened parts.
How to Select Bearing Steel: a Checklist
- Fix the operating temperature and choose the heat-stabilisation class that covers it with margin, not the default S0.
- Confirm the grade family and write the name in the specification together with its standard, so a SUJ2 and a GCr15 quote can be compared directly.
- Decide through-hardening or case-hardening from the section size and the shock load, not from habit.
- Specify cleanliness - oxygen content and inclusion rating - where life is critical.
- Require a certificate with a heat number and the measured composition.
- State the corrosion requirement if the position is wet, and move to 440C or a coated part if plain carbon steel cannot hold up.
- Specify the hardness range and, for case-hardened parts, the case depth and core hardness.
Common mistakes
- Specifying through-hardening for a very large section where uniform hardness is not achievable, when a case-hardening grade or a different design is needed.
- Ignoring the temperature class and running a standard S0 bearing above about 120 degC, where dimensional growth then changes the fit.
- Accepting a certificate without a heat number, which cannot be traced to the melt.
- Treating 100Cr6, GCr15 and SUJ2 as different grades and paying a premium for a name rather than for verified cleanliness.
- Assuming stainless means same rating. 440C resists corrosion but carries less load than 100Cr6 in the same envelope.
The grade choice interacts with tolerance and fit; see the tolerance classes guide and the material supply context in the bearing steel supply chain notes. Compare grades on any SKF listing, and see bearing accessories for the sleeves, nuts and housings that complete an assembly. Our deep groove ball bearing range records the material and heat class on published items, and the glossary defines each grade name. Standards to cite when specifying are ISO 683-17, ASTM A295, GB/T 18254 and JIS G4805, with the German and European grade named as EN 100Cr6.
Frequently asked questions
Are 100Cr6, GCr15 and SUJ2 the same steel?
Yes, they are the same through-hardening bearing steel family under different national standards. 100Cr6 is the German and European designation, GCr15 the Chinese designation and SUJ2 the Japanese designation, all equivalent to SAE and AISI 52100 with about 1% carbon and 1.5% chromium. The names reflect the standard, not a different material.
What hardness is a bearing made from 52100 steel?
Standard bearings are hardened and tempered to about 58 to 62 HRC. Hardening produces the martensitic structure and tempering sets the final hardness and the maximum service temperature. A higher tempering temperature lowers the hardness slightly but raises the usable temperature class.
What do the S0, S1, S2 and S3 suffixes mean?
They are heat-stabilisation classes that define the maximum continuous operating temperature. S0 is standard and suits service to about 120 degC, S1 to about 200 degC, S2 to about 250 degC and S3 to about 300 degC. Using a class below the real temperature causes dimensional growth as retained austenite transforms.
Why is steel cleanliness important for bearing life?
Fatigue cracks start at non-metallic inclusions, so cleaner steel with lower oxygen content and fewer inclusions gives longer life for the same loads. Cleanliness is measured as oxygen content in ppm and as an inclusion rating, and both should appear on the material certificate alongside the grade and heat number.
When should I use a case-hardening steel instead of 100Cr6?
Use a case-hardening grade such as 20CrMnTi or 8620 for very large sections where through-hardening cannot give uniform hardness, or for parts that must absorb shock loads. Carburising produces a hard surface over a tough core, and the case depth is specified and measured against the requirement.
Is stainless steel stronger than 52100 for bearings?
No. Martensitic stainless grades such as 440C harden to a usable bearing hardness but carry a lower load rating than 100Cr6 in the same envelope. They are chosen for corrosion resistance in food, beverage and marine positions. Grades like 316 do not harden to bearing hardness and are used for cages and housings rather than loaded raceways.