T8 is a carbon tool steel specified under the Chinese national standard GB/T 1298. With a nominal carbon content of approximately 0.80%, it occupies a balanced position in the T-series carbon tool steel family — offering higher toughness than T10/T12 while maintaining sufficient hardness for a wide range of cold-work tooling applications. This article provides a detailed technical reference covering composition, heat treatment, mechanical properties, machinability, and practical selection guidance.
T8 Carbon Tool Steel: Grade Designation
The Chinese carbon tool steel designation system uses the prefix “T” (from the pinyin tàn, meaning carbon) followed by a number representing the nominal carbon content in hundredths of a percent. T8 thus signifies an average carbon content of approximately 0.80%. The standard also specifies quality grades: T8 (standard), T8A (high-quality with stricter impurity limits), and T8E (extra-fine quality).
Chemical Composition per GB/T 1298
| Element | Content (wt%) | Role in T8 |
|---|---|---|
| Carbon (C) | 0.75–0.84 | Primary hardenability driver; forms martensite and fine carbides upon quenching |
| Silicon (Si) | ≤0.35 | Residual deoxidizer; contributes modestly to hardenability |
| Manganese (Mn) | ≤0.40 | Residual from steelmaking; improves hardenability and counters sulfur embrittlement |
| Chromium (Cr) | ≤0.25 | Residual; no intentional addition |
| Nickel (Ni) | ≤0.20 | Residual; low levels typical for plain carbon tool steel |
| Copper (Cu) | ≤0.30 | Residual from scrap; controlled to avoid hot shortness |
Note: T8 is essentially a plain carbon steel. Unlike alloy tool steels such as Cr12MoV or H13, it contains no intentional alloying additions. Its properties are governed almost entirely by carbon content and heat treatment.
Mechanical Properties
| Property | Value | Condition |
|---|---|---|
| Annealed hardness | ≤187 HB | Fully annealed (spheroidized) |
| Quenched hardness (water) | ≥62 HRC | Austenitized 760–780°C, water quenched |
| Quenched hardness (oil) | ≥60 HRC | Austenitized 800–820°C, oil quenched |
| Tempered hardness | 60–62 HRC | Quenched and tempered at 150–200°C |
| Tensile strength (quenched & tempered) | Approx. 1800–2100 MPa | Estimated; depends on tempering temperature |
Heat Treatment Practice
T8 is a hypereutectoid steel (above ~0.77% C), so its heat treatment requires careful temperature control to avoid grain growth and retained austenite issues:
- Spheroidizing annealing: Heat to 740–760°C, slow cool at ≤30°C/h to ~550°C, then air cool. Target microstructure: spheroidized cementite in ferrite matrix.
- Stress-relief annealing: 600–650°C for 1–2 hours, slow cool. Used after rough machining before finish grinding.
- Hardening: Preheat to ~500°C, austenitize at 760–780°C (water quench) or 800–820°C (oil quench). Hold time: ~1 min/mm of section thickness.
- Tempering: 150–200°C for 1–2 hours. Higher temperatures (200–300°C) reduce hardness to ~55–58 HRC but improve toughness.
- Critical temperatures: Ac1 ≈ 730°C, Acm ≈ 770°C. Overheating above Acm promotes grain growth and plate martensite formation.
Typical Applications
T8 is used where moderate wear resistance combined with acceptable toughness is required:
- Woodworking tools: Plane blades, chisels, carving knives, axe heads
- Cold-work tooling: Blanking punches (light to medium gauge), shearing blades, simple drawing dies
- Hand tools: Hammers, screwdrivers, pry bars, pliers
- Measuring tools: Simple calipers, go/no-go gauges, templates
- Springs: Flat springs, clip springs where fatigue is not critical
CNC Machining Guidelines
| Parameter | Annealed Condition | Hardened Condition |
|---|---|---|
| Machinability rating | ~65% (relative to AISI 1212) | N/A — grinding only |
| Cutting speed (carbide) | 60–100 m/min | N/A |
| Feed rate (roughing) | 0.1–0.3 mm/rev | N/A |
| Depth of cut (roughing) | 1–3 mm | N/A |
| Recommended insert grade | YT15, YT14 (ISO P20–P30) | CBN or ceramic for hard turning |
| Coolant | Water-soluble emulsion | Dry or MQL |
After quenching, T8 reaches 60+ HRC and conventional cutting is impractical. Finish machining on hardened T8 components should be performed by cylindrical grinding, surface grinding, or wire EDM. Allow 0.2–0.5 mm grinding stock on quenched surfaces.
T8 vs. T10 Comparison
| Parameter | T8 | T10 |
|---|---|---|
| Carbon range | 0.75–0.84% | 0.95–1.04% |
| Maximum hardness | 62–64 HRC | 62–65 HRC |
| Toughness | Good — suitable for impact tools | Lower — avoid impact loading |
| Wear resistance | Moderate | High |
| Typical use | Tools needing toughness + hardness | High-wear cutting edges |
| Dimensional stability | Good | Better (more carbides) |
Choose T8 when toughness cannot be compromised (e.g., hammers, chisels, punches). Choose T10 when wear resistance and edge retention are the priority (e.g., drills, reamers, dies).
Grinding and Finishing After Hardening
After quenching and tempering, T8 components typically require finish grinding to achieve final dimensions and surface finish:
- Surface grinding: Use aluminum oxide wheels (A46–A80, K–L hardness). Keep depth of cut ≤ 0.02 mm per pass on finish passes to avoid grinding burns.
- Cylindrical grinding: Wheel speed 25–35 m/s, workpiece speed 10–20 m/min. Flood cooling is essential to prevent thermal damage to the hardened surface.
- Wire EDM: Suitable for intricate profiles. White layer thickness should be removed by subsequent polishing or low-temperature tempering.
- Polishing: Achievable surface finish Ra 0.1–0.4 μm with sequential diamond paste polishing after grinding.
Post-grinding stress relief at 120–150°C for 1–2 hours is recommended to reduce the risk of delayed cracking in hardened T8 components.
Procurement and Specifications
Common supply forms for T8:
- Hot-rolled plate: thickness 3–60 mm
- Hot-rolled round bar: diameter Φ10–Φ200 mm
- Cold-drawn bar: diameter Φ2–Φ30 mm (tighter tolerance, better surface)
- Forged bar: larger diameters available on request
Standard delivery condition is annealed (spheroidized). Verify material certification against GB/T 1298 and confirm hardness ≤187 HB upon receipt. Magnetic particle or ultrasonic inspection is recommended for critical tooling applications.
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