SUJ2 (100Cr6 / AISI 52100) — bearing steel hardening, retained austenite and stability
How SUJ2 is hardened and low-tempered for pins and rolling elements, why retained austenite is a dimensional problem rather than a hardness problem, and what finish suits it.
Grade data
| JIS designation | SUJ2 |
|---|---|
| Closest equivalents | AISI 52100 / DIN 100Cr6 |
| Carbon (%) | 0.95–1.10 |
| Steel family | Bearing steel |
| Heat-treatment route | Quench and temper |
| Typical surface hardness (HRC) | 60–64 HRC |
| Typical hardness specified in service (HRC) | 58–64 HRC |
| Hardenable by heat treatment | Yes |
Hardness values are typical industry ranges for the small sections we process, and depend on the part, the section size, the quench and the governing specification. They are not a guarantee and do not replace your drawing or standard.
Reviewed:
What it is
SUJ2 is a high-carbon chromium bearing steel. It is bought for hardness, wear resistance and dimensional stability under contact load rather than for tensile strength, and it appears in our work as pins, rollers and small precision parts. Machining is normally done in the spheroidize-annealed condition, because the hardened material is not a machining proposition.
How it is heat treated
Harden from a narrow austenitizing window, quench, then temper low to keep the hardness the part is there for. The narrow window is the point: too low and the carbides do not dissolve, too high and you dissolve too much carbon and retain austenite. Where dimensional stability over time is specified, a sub-zero treatment before tempering is the usual answer to retained austenite.
Plating and hydrogen embrittlement
These parts are usually protected with oil rather than plated, and there is a functional reason as well as a metallurgical one: a coating on a running or fitted surface changes the fit. If a finish is specified, the part is above the ISO 4042 trigger and prompt de-embrittlement baking applies.
Indicated after electroplating
ISO 4042 calls for embrittlement-relief baking on electroplated fasteners at property class 10.9 and above, or at roughly 390 HV (≈ 40 HRC) and above. Many customer specifications set a lower trigger — the drawing decides, not this page.
Hydrogen de-embrittlement bake plannerTypical ISO 898-1 property classes
When to choose it
Choose it for pins, rollers and small precision parts where surface hardness, wear resistance and dimensional stability under contact stress decide whether the part works.
When not to choose it
Do not choose it for threaded fasteners, for parts that take impact or bending, or as a corrosion-resistant grade — it has no corrosion resistance to speak of, and it will not tolerate a notch.
What tends to go wrong
Frequently asked questions
What is retained austenite doing to our SUJ2 parts?+
It is soft, it is unstable, and over time or under load it transforms and the part changes size. On a fitted pin that shows up as a dimensional complaint months later, not as a failed hardness test. If stability is specified, it has to be addressed in the cycle.
Can SUJ2 be plated?+
It can, but ask why first — a coating changes the fit on a running surface, and at this hardness the part is above the ISO 4042 trigger, so baking after electroplating applies. Oil or a phosphate-and-oil system is often the more sensible protection.