Family overview
Bearing steels and spring steels both get their performance from quenching and tempering, but the loads their parts must withstand are different. A bearing steel has to resist wear and fatigue under rolling contact, where high pressure acts repeatedly on a small contact area. A spring steel has to keep returning to its original shape after being deformed under load, cycle after cycle. The sections below describe SUJ2, SUP9 and SUP12 using general engineering knowledge.
SUJ2 is a high-carbon chromium steel. After quenching it has a hard martensite matrix with fine carbides dispersed in it, and this structure gives the hardness, wear resistance and resistance to rolling contact fatigue that rolling bearings need. Rolling contact fatigue often starts around non-metallic inclusions beneath the surface, so bearing steel is judged on cleanliness (how few non-metallic inclusions it contains) as well as on composition.
SUP9 and SUP12 are spring steels used mainly for hot-formed springs. SUP9 is a Mn-Cr steel that gets its hardenability from manganese and chromium, and SUP12 is a Si-Cr steel with higher silicon and added chromium. Silicon raises the elastic limit and the resistance to sag (the permanent deformation left after loading), so Si-Cr steel is often chosen for coil springs under heavy cyclic loads. Parts with large sections, such as leaf springs and torsion bars, depend on hardenability, and Mn-Cr steel is often used for them. Springs coiled cold from fine wire usually use wire of separate standards, such as piano wire or oil-tempered wire.
Role of the main elements
- Carbon (C): the basic element that sets hardness and strength after quenching. In SUJ2 the high carbon leaves carbides alongside hard martensite, and spring steels use medium to high carbon to secure the elastic limit and fatigue strength. Too much carbon lowers toughness and raises the risk of quench cracking.
- Chromium (Cr): raises hardenability so that the section hardens evenly to the core. In SUJ2, fine chromium-bearing carbides add to wear resistance. In SUP9 and SUP12, chromium slows softening during tempering and helps give a uniform structure and strength in larger sections.
- Silicon (Si): dissolves in the matrix, raises the elastic limit and the resistance to sag, and slows softening during tempering. Its drawback is that the surface decarburizes more easily during heating, so the decarburized layer needs particular attention in SUP12, which has higher silicon. In SUJ2 and SUP9 it is present mainly in small amounts for deoxidation.
- Manganese (Mn): raises hardenability and ties up sulfur as manganese sulfide, which reduces cracking during hot working. In SUP9 it is, together with chromium, the main element that provides hardenability. Too much can make the steel more prone to temper embrittlement.
- Phosphorus (P) and sulfur (S): treated as impurities and kept low. Phosphorus weakens the grain boundaries and lowers toughness, and sulfur forms sulfide inclusions that can reduce fatigue performance.
- Oxygen and non-metallic inclusions (cleanliness): hard oxide inclusions can start fatigue cracks under repeated loading. For bearing steel, where rolling contact fatigue life matters, and for spring steel, where fatigue life under cyclic loading matters, keeping oxygen and inclusions low is important, and order specifications sometimes include items such as an inclusion rating test or ultrasonic testing.
Heat treatment and delivery condition
- SUJ2 material condition: it is often specified in the spheroidize-annealed condition, a softening heat treatment that rounds the carbides. In the as-rolled state, high-carbon steel contains a lot of lamellar pearlite (plate-like carbides), which is hard and difficult to machine. A spheroidized structure makes cutting and cold working easier and helps the structure transform evenly during quenching.
- SUJ2 part heat treatment: unlike steels that are case hardened by carburizing, the usual method is through hardening (hardening to the core): the part is heated to the quenching temperature, quenched in oil and tempered at a low temperature. Because it is a hypereutectoid steel, with more carbon than the eutectoid composition, some carbides are deliberately left undissolved during heating, and these remaining carbides contribute to wear resistance. The depth that hardens is limited by section size, so thick rings use separate bearing steels with manganese and silicon or molybdenum added to raise hardenability.
- Dimensional stability: retained austenite left after quenching can transform over time or at higher temperatures and change part dimensions. Precision parts and parts used at elevated temperatures are sometimes given sub-zero treatment or a higher-temperature stabilizing temper to reduce dimensional change.
- Material structure and surface requirements: high-carbon steel tends to decarburize at the surface during rolling and annealing, and segregation of carbon and chromium can gather carbides into bands, while carbides can also precipitate along grain boundaries as a network during cooling after rolling. These can lead to uneven hardness and shorter fatigue life after heat treatment, so bearing steel specifications often state decarburized depth, carbide structure, degree of spheroidization and non-metallic inclusions together.
- SUP9 and SUP12 heat treatment: a hot-formed spring is made by shaping heated material into a coil or leaf, quenching it in oil and tempering it at a medium temperature to obtain a tempered martensite structure. A higher tempering temperature gives more toughness and less strength, so it is set to balance the required elastic limit and toughness.
- Shot peening and presetting: after heat treatment, shot peening puts compressive residual stress in the surface and extends fatigue life. Presetting, which applies a large load in advance to remove initial sag, is often carried out as well.
- SUP9 and SUP12 material condition: hot-rolled round and flat bar is commonly used, and heat forming and heat treatment take place at the spring-making stage. Decarburized layers and surface flaws directly reduce fatigue strength, so a decarburization depth limit and the surface condition are specified together, and peeled or ground bar is sometimes specified.
Standards cross-reference
| Group | Grade | SAE/AISI | EN |
|---|---|---|---|
| High-carbon chromium bearing steel | SUJ2 | 52100 | 100Cr6 |
| Spring steel | SUP9 | — | — |
| SUP12 | — | — |
Composition range
Confirm the composition range against the applicable standard.
Typical applications
- Rolling bearings: SUJ2 is used for the inner and outer rings and the rolling elements (balls and rollers) of ball and roller bearings.
- Linear shafts: SUJ2 is often used for shafts that travel in a straight line inside a linear bushing, where surface hardness, wear resistance and straightness are all required.
- Small precision parts: SUJ2 is also used in some parts that need high hardness and wear resistance, such as gauges, pins, rollers and bushings.
- Automotive coil springs and stabilizer bars: SUP9 and SUP12 are used. SUP12, with its higher silicon, is often chosen for coil springs under heavy cyclic loads.
- Leaf springs and torsion bars: these have large sections, so hardenability matters, and SUP9, which gets its hardenability from manganese and chromium, is often used.
Products covering these grades
Grades not listed on a product page (SUP9, SUP12): send us the standard, size and quantity, and we will confirm availability. Quote & technical inquiry