AISI S2 Tool Steel Round & Hex Bar Supplier
ASC LLP supplies AISI S2 tool steel bars – round, hexagon and flat – to ASTM A681 (UNS T41902). AISI S2 bars are the standard raw material for screwdriver bits, hex keys, chisels, punches and other hand and impact tools that must take repeated shock without chipping or breaking. All bars are supplied in the annealed, machinable condition with mill test certificates.
What is AISI S2 Tool Steel?
AISI S2 is a silicon–molybdenum shock-resisting tool steel (the “S” group in the AISI tool steel system). Its medium carbon (0.40–0.55%), high silicon (about 1%) and molybdenum give a combination of high hardness and very high impact toughness. S2 is a water-hardening grade: when quenched it forms a hard, wear-resistant surface over a tough core, which is why it is the go-to material for tools that are struck or twisted under load. If a drawing or bit packaging says “S2 material”, it refers to this grade.
AISI S2 tool steel is covered by ASTM A681, FED QQ-T-570, SAE J437 and SAE J438, and its UNS number is T41902.
What Does “S2” Mean on Tools?
When you see “S2” stamped on a screwdriver bit, hex key or socket bit, it means the tool is made from S2 shock-resisting tool steel. S2 bits are typically hardened to around 58–62 HRC, so they hold their tip shape and resist cam-out and snapping far better than bits made from ordinary chrome-vanadium (Cr-V) steel – especially in impact drivers.
Key Properties of AISI S2 Bars
- High impact toughness: resists chipping and cracking under repeated shock loading.
- High working hardness: typically 56–60 HRC after hardening and tempering.
- Hard case, tough core: in medium and large sections, water quenching gives a hard surface over a tough core.
- Good machinability when annealed: bars are supplied soft for turning, milling and forming before heat treatment.
- Economical: lower alloy content than air-hardening tool steels such as S7.
Applications of AISI S2 Tool Steel Bars
- Hand tools: screwdriver bits, power and impact bits, hex (Allen) keys, nut setters.
- Striking tools: chisels, punches, drifts, nail sets, hammers.
- Pneumatic tools: rivet sets, chipping and scaling chisels, pneumatic hammer tools.
- Industrial parts: shear blades, stamps, collets, pins and other parts that take shock loads.
- Mining & construction: impact-resistant tool tips and wear parts.
S2 vs S7 vs Cr-V Steel
| Property | AISI S2 | AISI S7 | Cr-V (e.g. 6150 / 50CrV4) |
| Type | Shock-resisting tool steel, water hardening | Shock-resisting tool steel, air/oil hardening | Spring / alloy steel, oil hardening |
| Key alloying | Si ~1%, Mo 0.3–0.6% | Cr 3.0–3.5%, Mo 1.3–1.8% | Cr ~1%, V ~0.15% |
| Typical hardness | 56–60 HRC | 54–58 HRC | 45–55 HRC |
| Toughness | Very high | Very high | Good |
| Distortion in hardening | Higher (water quench) | Low (air hardening) | Moderate |
| Typical use | Bits, hex keys, chisels, punches | Larger dies, punches, moulds | Wrenches, sockets, pliers |
Summary: choose S2 for bits and small striking tools where maximum hardness with toughness matters; choose S7 for larger or more complex tools where low distortion is needed; Cr-V is suited to wrenches and sockets rather than bits.
AISI S2 Bar Specifications & Size Chart
| Form | Size range | Length | Weight (approx.) |
| Round bar | 10 – 50 mm dia. | Up to 6 m | 0.62 – 15.4 kg/m |
| Round bar | 51 – 100 mm dia. | Up to 6 m | 16.0 – 61.7 kg/m |
| Round bar | 101 – 200 mm dia. | Up to 6 m | 62.9 – 247 kg/m |
| Hexagon bar | 6 – 50 mm A/F | Up to 6 m | 0.24 – 17.0 kg/m |
| Flat bar | On request | Up to 6 m | By size |
| Above 200 mm | Made to order | On request | By size |
Round bar weight (kg/m) = 0.00617 × d² (d in mm); hexagon bar weight ≈ 0.0068 × A/F². Bars are supplied annealed, black or peeled/ground, and cut to length on request.
Chemical Composition of AISI S2 Tool Steel (ASTM A681, UNS T41902)
| Element | Composition (%) |
| Carbon (C) | 0.40 – 0.55 |
| Manganese (Mn) | 0.30 – 0.50 |
| Silicon (Si) | 0.90 – 1.20 |
| Chromium (Cr) | 0.30 max |
| Molybdenum (Mo) | 0.30 – 0.60 |
| Vanadium (V) | 0.50 max |
| Phosphorus (P) | 0.030 max |
| Sulphur (S) | 0.030 max |
| Iron (Fe) | Balance |
The high silicon content is what distinguishes S2 – it raises strength and hardenability while keeping toughness.
Mechanical Properties & Hardness of AISI S2
| Property | Value (typical) |
| Hardness, annealed (as supplied) | approx. 192 – 217 HB |
| Hardness, hardened & tempered | 50 – 60 HRC (depends on tempering temperature) |
| Typical working hardness for bits and chisels | 56 – 60 HRC |
| Modulus of elasticity | 190 – 210 GPa |
| Poisson’s ratio | 0.27 – 0.30 |
| Impact toughness | Very high – the main reason S2 is chosen |
Tensile and yield strength of S2 depend on the final hardness and are not specified by ASTM A681; tools are specified by hardness.
Heat Treatment of AISI S2 Tool Steel
| Process | Temperature | Cooling / method | Result |
| Annealing | approx. 790 °C (1450 °F) | Furnace cool slowly (max. about 4 °C per hour) | Soft, machinable structure |
| Preheating | approx. 650 °C (1200 °F) | Heat slowly and uniformly | Reduces distortion and cracking risk |
| Hardening | approx. 845 – 900 °C (typically 870 °C / 1600 °F), soak 10–20 min | Quench in brine or water | Full hardness, hard case with tough core |
| Tempering | 175 – 425 °C (350 – 800 °F) | Air cool; temper immediately after quenching | About 60 HRC at the low end to 50 HRC at the high end |
S2 is sensitive to decarburisation because of its high silicon content, so hardening should be done in a protective atmosphere, salt bath or vacuum, or enough stock left for grinding.
AISI S2 Standards & Equivalent Grades
| Standard / system | Designation |
| AISI / SAE | S2 (SAE J437, J438) |
| ASTM | A681 – Tool Steels, Alloy |
| UNS | T41902 |
| US Federal | FED QQ-T-570 |
| DIN / Werkstoff Nr. (nearest) | 1.2103 – 58SiMo8 |
The DIN grade is a close, not identical, equivalent – check the full chemistry against your drawing before substituting.
Physical Properties of UNS T41902 (AISI S2)
| Property | Value (approx.) |
| Density | 7.79 g/cm³ (0.281 lb/in³) |
| Melting point | approx. 1421 °C (2590 °F) |
| Thermal expansion (20–100 °C) | 10.9 × 10⁻⁶ /°C |
| Modulus of elasticity | 190 – 210 GPa |
| Magnetic | Yes (ferromagnetic) |
Why Buy AISI S2 Tool Steel Bars from ASC LLP?
- Certified material: AISI S2 / ASTM A681 bars with EN 10204 3.1 mill test certificates.
- Round, hex and flat: a wide size range, cut to length for bit and tool makers.
- Testing on request: chemical analysis, hardness and ultrasonic testing; third-party inspection available.
- Export-ready: seaworthy packing and delivery to customers worldwide.
Looking for AISI S2 round bar suppliers? Send us your size, form and quantity for a quote.
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Frequently Asked Questions – AISI S2 Tool Steel Bars
What is AISI S2 tool steel?
AISI S2 is a silicon–molybdenum shock-resisting tool steel (UNS T41902, ASTM A681). It combines high hardness with very high impact toughness and is used for screwdriver bits, hex keys, chisels and punches.
What does S2 mean on a screwdriver bit?
It means the bit is made from S2 shock-resisting tool steel. S2 bits are harder and tougher than standard chrome-vanadium bits, so they last longer, especially in impact drivers.
What is the hardness of S2 tool steel?
S2 is supplied annealed at roughly 192–217 HB. After hardening and tempering it reaches about 50–60 HRC; bits and chisels are usually used at 56–60 HRC.
Is S2 steel better than Cr-V?
For bits and small striking tools, yes – S2 reaches higher hardness with better shock resistance. Cr-V steel is tougher in large sections and is preferred for wrenches and sockets.
What is the difference between S2 and S7 tool steel?
S2 is a water-hardening silicon–molybdenum steel used for smaller tools; S7 contains about 3.25% chromium and 1.4% molybdenum, is air or oil hardening, distorts less and is used for larger dies and punches.
What is the equivalent of AISI S2?
AISI S2 is UNS T41902 under ASTM A681 and FED QQ-T-570. The nearest European grade is DIN 1.2103 (58SiMo8).
Global Reach
ASC LLP serves customers around the world with prompt delivery and dependable support. Tool makers and engineering companies across the USA, Europe, the Middle East and Asia rely on our AISI S2 tool steel bars for consistent chemistry, hardness response and documentation.
Countries We Export To
UNITED STATES, EUROPE, UK, UAE, SAUDI ARABIA, OMAN, KUWAIT, JORDAN, QATAR, INDONESIA, MALAYSIA, BRAZIL, SPAIN, ITALY, FRANCE, GERMANY, BAHRAIN, SRI LANKA, AZERBAIJAN, AUSTRALIA, AUSTRIA, PHILIPPINES, TURKEY, RUSSIA, IRAN, IRAQ, CANADA, KOREA, JAPAN, YEMEN, FINLAND, ZIMBABWE, HUNGARY, PORTUGAL, VIETNAM, MOROCCO, THAILAND, SOUTH AFRICA, MEXICO, SINGAPORE, GREECE, KUWAIT, POLAND, NEW ZEALAND, SWEDEN, SWITZERLAND, BELGIUM, BHUTAN, BANGLADESH, TAIWAN, ROMANIA, CAMBODIA, KAZAKHSTAN, AFGHANISTAN.
