SUJ2 is a bearing-steel grade identifier
SUJ2 is a high-carbon chromium bearing steel used in bearing and precision-component manufacturing. Reading the designation helps identify its material family, but its technical requirements come from the applicable product standard. This article checks JIS G 4805:2008.
Classification and naming are different from acceptance
Japanese steel designations distinguish material families and individual grades. Familiar examples include structural steels such as SCM435 and SNCM220, spring steel SUP10, tool-steel names such as SK5, bearing steel SUJ2, stainless steel SUS304 and free-cutting steel SUM24L. These are classification examples; legacy or commercial names still require a matching product standard and edition.
A common teaching model separates the material symbol, steel-family symbol and distinguishing number. It is not a universal formula that replaces checking the complete identifier in its product standard.
What does the 2 mean?
JIS G 4805:2008 lists four type symbols: SUJ2, SUJ3, SUJ4 and SUJ5. The suffix 2 identifies SUJ2 within that series. It does not mean second-generation steel, the second entry in a table, 2% chromium or 0.2% carbon.
The standard does not provide a letter-by-letter expansion of SUJ as “S = Steel, U = Use, J = Bearing”. An informal mnemonic should not be presented as a definition from the standard.
For comparison, Table 1 of the reviewed edition gives SUJ2 carbon as 0.95–1.10% and chromium as 1.30–1.60%, by mass. These selected values illustrate why the suffix is not a literal composition; the full standard and its notes still govern the material specification.
Which standard defines the product?
JIS G 4805, High carbon chromium bearing steels, specifies composition, manufacturing and heat-treatment provisions, dimensions, surface requirements, inspection and other technical requirements. Grade naming and product conformity are separate matters. Neither the family name nor a nominal composition establishes that a delivered batch meets every requirement.
Where is SUJ2 used?
With suitable processing, heat treatment and finishing, this steel family is used in applications requiring hardness, wear resistance, rolling-contact fatigue performance and dimensional stability. Examples include ball and roller bearings, ball screws, precision guide components and other wear-resistant parts. Required performance must be verified for the component and service conditions.
How does it relate to other bearing steels?
| Grade or reference name | Standard family in the comparison |
|---|---|
| SUJ2 | JIS G 4805 |
| GCr15 | GB/T 18254 |
| 100Cr6 | EN ISO 683-17 |
| AISI 52100 | SAE/ASTM context; specify the actual product standard |
These are comparative references, not statements of automatic equivalence. Applicable composition tolerances, inclusion requirements, heat treatment, inspection and product form can differ. Confirm the actual standard and edition rather than treating the names as interchangeable.
From designation to a verified material choice
- Identify the steel family and exact product standard.
- Confirm composition limits, product form and delivery condition.
- Evaluate the required heat-treatment route and inspection evidence.
- Review potential alternatives against the customer drawing and technical agreement.
- Approve substitution only after the required engineering and production validation.
Creation Group studies JIS, ASTM, SAE, EN, ISO and GB/T systems to support cold-heading wire, shaped wire, bearing steels, structural steels and fastener-material projects. The aim is to help customers understand materials, select a suitable route, evaluate alternatives and assess total manufacturing cost. A database or a cross-reference name is a starting point for review, not a supply, performance or substitution approval.
SUJ identifies the bearing-steel family and 2 distinguishes the grade. The useful next step is to read the requirements and evidence behind that identifier, not to infer a composition or approval from the name alone.