SUJ2 vs SUJ3 – Composition, Heat Treatment, Properties, and Applications

Introduction

SUJ2 and SUJ3 are JIS-designated high-carbon chromium bearing steels commonly considered for rolling-contact components, shafts, and other wear-critical parts. Engineers and procurement professionals often weigh trade-offs between maximum hardness and wear resistance versus toughness, machinability, and weldability when selecting between these two grades. Typical decision contexts include choosing materials for high-speed bearings, shafts subject to cyclic loading, or components where post-processing and joining operations influence cost and lead time.

The primary practical distinction between the two is a modest difference in chemical makeup that influences achievable hardness and hardenability after heat treatment. That small compositional shift translates into different heat-treatment responses and, therefore, different balances of strength, wear resistance, and toughness—hence the frequent direct comparison in design and manufacturing.

1. Standards and Designations

• JIS: SUJ2 and SUJ3 (JIS G4805 series for bearing steels)
• Common international equivalents or near-equivalents:
• SUJ2: often compared to AISI/SAE 52100 / EN 100Cr6 / DIN 1.3505
• SUJ3: generally categorized alongside high-carbon chromium bearing steels but with slightly different nominal chemistry than SUJ2
• Classification: Both are high-carbon, chromium-containing bearing steels (carbon-chromium tool/ bearing steels), not stainless, not HSLA.

2. Chemical Composition and Alloying Strategy

The following table summarizes the typical presence and relative levels of common alloying elements for SUJ2 and SUJ3. Values are described qualitatively (High / Medium / Low / Trace / None) to avoid implying exact numerical ranges where specific lot-to-lot or standard limits may vary.

How alloying affects performance: - Carbon and chromium are the dominant drivers of the bearing-steel behavior: carbon provides the martensitic hardness and wear resistance; chromium increases hardenability and improves wear and tempering resistance. - Low levels of Mn/Si act mainly as deoxidizers and have modest influence on hardenability. - Impurity levels (P, S) are controlled to preserve fatigue life; S is sometimes controlled to balance machinability vs fatigue.

3. Microstructure and Heat Treatment Response

Typical microstructures: - In the annealed condition both grades are supplied with spheroidized carbides in a ferritic matrix to facilitate machining. - After quenching and tempering, the target microstructure for both is tempered martensite with dispersed chromium carbides. The fraction and distribution of carbides and retained austenite depend on carbon content and quench severity.

Heat-treatment effects and differences: - Normalizing/refining: Raises toughness and refines prior-austenite grain size; both grades respond by forming a fine austenitic grain prior to quenching. - Quench & temper: SUJ2’s slightly higher combined carbon-plus-chromium tendency enables a higher as-quenched hardness and wear resistance for the same quench, but also increases the risk of quench-induced cracking and retained austenite if not properly tempered. SUJ3, with marginally lower effective hardenability in many specifications, typically achieves lower maximum hardness but somewhat improved toughness and reduced quench crack sensitivity under similar cycles. - Tempering: Both respond predictably—higher tempering temperature lowers hardness and increases toughness. Tempering parameters should be chosen based on the required balance between hardness and impact resistance. - Thermo-mechanical treatments: Not commonly applied to these bearing steels in the way they are to HSLA steels, but careful controlled-cooling following hot working can tune hardenability and residual stress.

4. Mechanical Properties

Below is a qualitative comparison table of typical mechanical behavior after representative bearing heat treatments. Absolute values depend on specific heat-treatment targets (e.g., 60 HRC vs 58 HRC vs tempered conditions).

Explanation: - When designers push to the highest hardness and wear resistance, SUJ2’s chemistry supports that target more readily. When service calls for higher fracture toughness or improved resistance to contact fatigue under repeated shocks, SUJ3’s slightly milder response can be advantageous.

5. Weldability

Weldability of high-carbon, chromium-containing bearing steels is generally poor relative to low-carbon steels; the risk of hydrogen-assisted cracking and hard, brittle heat-affected zones increases with carbon and hardenability.

Useful predictive formulas: - Carbon equivalent (IIW) often used to assess preheat and procedure needs: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$ - A more detailed parameter for modern steels: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Interpretation: - Higher $CE_{IIW}$ or $P_{cm}$ values indicate reduced weldability and greater need for preheat, controlled interpass temperatures, and post-weld heat treatment (PWHT). - Because SUJ2 tends to have a higher effective carbon and chromium contribution, it generally yields a higher carbon equivalent than SUJ3 and therefore poorer weldability. In practice, welding of both grades is avoided unless necessary; if welding is required, use low-hydrogen consumables, controlled preheat, and post-weld tempering to reduce cracking risk.

6. Corrosion and Surface Protection

• Neither SUJ2 nor SUJ3 is stainless; chromium levels are not high enough to produce a corrosion-resistant passive film.
• Common protections: oiling, phosphate coating, electroplating, painting, or hot-dip galvanizing (subject to dimensional and metallurgical considerations). For rolling elements, thin protective films (e.g., black oxide or specialized lubricants) are commonly used to protect stock before assembly.
• PREN (pitting resistance equivalent number) is not applicable to these non-stainless grades; for reference: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Use corrosion protection appropriate to the environment: closed bearings with lubricants require different protection than exposed shafts.

7. Fabrication, Machinability, and Formability

• Machinability: In the annealed (spheroidized) condition both grades are relatively easy to machine. If parts are machined in the hardened condition, SUJ2’s higher hardness will reduce tool life and increase cycle times relative to SUJ3.
• Grinding and finishing: SUJ2’s higher hardness often requires more aggressive grinding operations or finer abrasives; surface integrity and heat generation must be controlled to avoid tempering or inducing residual stress.
• Forming/bending: Cold forming is limited for both grades due to high carbon; shaping is typically done in softer, annealed condition. Deep drawing or severe forming is generally avoided.
• Heat-treatment distortion control: Both grades can distort during quench & temper; SUJ2 may be more sensitive to quench severity due to higher hardenability.

8. Typical Applications

Selection rationale: - Choose SUJ2 when maximum wear resistance and high contact-stress capacity are prioritized and when the manufacturing/assembly route can accommodate stringent heat-treatment and handling controls. - Choose SUJ3 when a marginal reduction in hardness improves toughness, reduces risk of quench cracking, or when machining/welding constraints favor a slightly milder hardenability.

9. Cost and Availability

• SUJ2 is widely specified and available in bearing steel stock forms (bars, rings, sheets, finished balls/rollers) from many suppliers; economies of scale tend to keep unit costs competitive.
• SUJ3 is also available but may be less commonly stocked depending on region and supplier focus; lead times can vary by product form.
• Relative cost: comparable for bulk commodity bar stock; cost differences are often driven more by product form, required heat treatment, and finishing than by mill-grade premium.

10. Summary and Recommendation

Summary table (qualitative):

Recommendations: - Choose SUJ2 if you need maximum rolling-contact wear resistance and high achievable hardness for bearings, rollers, or rings, and you can control heat-treatment, handling, and joining procedures to mitigate cracking and residual-stress risks. - Choose SUJ3 if your application benefits from a modestly improved toughness–machinability tradeoff, lower sensitivity to quench cracking, or if downstream fabrication/welding considerations weigh against the highest possible hardness.

Concluding note: Final selection should be driven by the specific performance targets (hardness, fatigue life, toughness), manufacturing flow (machining, joining, heat treatment capabilities), and service environment (lubrication, corrosion exposure). Validate material choice with sample heat-treatment trials and, where possible, fatigue or contact-wear testing representative of the intended service.