70# Carbon Steel: High-Strength Medium Carbon Structural Steel for Industrial Applications
70# carbon steel (equivalent to AISI 1070, UNS G10700, and DIN C70) is a high-strength medium carbon steel grade widely used in industrial applications requiring superior wear resistance, moderate toughness, and cost-effective machinability. With a nominal carbon content of 0.67-0.75%, this grade achieves a balanced combination of hardness (up to 250 HB after heat treatment) and ductility, making it ideal for springs, axles, hand tools, and high-stress components. This article provides a technical breakdown of its chemical composition, mechanical properties, heat treatment processes, and industry-specific applications.
1. Chemical Composition (GB/T 699-2015 & ASTM A29 Standards)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | 0.67 – 0.75% | Primary hardening element; increases tensile strength and wear resistance while reducing weldability |
| Manganese (Mn) | 0.50 – 0.80% | Enhances hardenability and deoxidation; counteracts sulfur’s negative effects on hot brittleness |
| Silicon (Si) | 0.17 – 0.37% | Improves strength and elastic limit; acts as a deoxidizer during steelmaking |
| Phosphorus (P) | ≤ 0.035% | Impurity element; controlled to prevent cold brittleness and reduce toughness |
| Sulfur (S) | ≤ 0.035% | Impurity element; minimized to avoid hot shortness and improve machinability |
| Chromium (Cr) | ≤ 0.25% | Residual element; may slightly improve hardenability in trace amounts |
| Nickel (Ni) | ≤ 0.25% | Residual element; negligible impact at low concentrations |
| Copper (Cu) | ≤ 0.25% | Residual element; may improve atmospheric corrosion resistance in trace amounts |
2. Mechanical Properties (As-Rolled & Heat-Treated Conditions)
| Condition | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HB) | Impact Toughness (J) |
|---|---|---|---|---|---|
| Hot Rolled (Annealed) | ≥ 630 | ≥ 375 | ≥ 10 | ≤ 207 | ≥ 27 |
| Normalized (830-860°C) | ≥ 700 | ≥ 420 | ≥ 9 | 170-217 | ≥ 20 |
| Quenched & Tempered (820°C oil quench, 400°C temper) | ≥ 900 | ≥ 700 | ≥ 8 | 200-250 | ≥ 15 |
| Cold Drawn (Hard) | ≥ 800 | ≥ 600 | ≥ 6 | 200-240 | ≥ 12 |
3. Heat Treatment Processes & Microstructural Transformation
- Annealing: Heat to 780-820°C, hold for 1-2 hours, then furnace cool at ≤30°C/h to produce a ferrite-pearlite structure (softest condition, HB ≤ 207) for improved machinability.
- Normalizing: Heat to 830-860°C, soak for 30-60 minutes, then air cool to refine grain structure and achieve uniform mechanical properties (HB 170-217).
- Quenching: Austenitize at 800-840°C, oil quench to form martensite (hardness up to 60 HRC). Critical cooling rate: ≥ 50°C/s to avoid bainite formation.
- Tempering: Reheat quenched parts to 300-600°C (typical: 400°C) to relieve stresses and adjust hardness/toughness balance. Tempering at 200-300°C risks temper brittleness.
- Surface Hardening: Flame or induction hardening (850-900°C) for localized wear resistance (e.g., gear teeth, shafts) while maintaining core toughness.
4. Key Industrial Applications
Automotive Components
Axle shafts, suspension springs, clutch discs, and steering linkages — heat-treated to 200-250 HB for fatigue resistance under cyclic loads.
Hand Tools & Hardware
Hammers, chisels, wrenches, and punches — quenched and tempered to 45-55 HRC for durability and edge retention. Cold-drawn wire for nails and rivets.
Machinery Parts
Gears, sprockets, camshafts, and roller bearings — surface-hardened to 50-60 HRC with case depths of 0.5-2.0mm for wear resistance.
Railway & Construction
Railroad spikes, track bolts, and concrete reinforcement bars — normalized or hot-rolled for high tensile strength (≥ 700 MPa) and vibration resistance.
Agricultural Equipment
Plowshares, harrow tines, and cultivator blades — flame-hardened to 50-58 HRC for abrasion resistance in soil contact applications.
Wire Rope & Cables
High-strength wire for bridges, elevators, and industrial lifting — cold-drawn to ≥ 1500 MPa tensile strength with zinc or aluminum coating for corrosion protection.
5. Comparison with Similar Carbon Steel Grades
| Grade | Carbon Content | Tensile Strength (MPa) | Key Characteristics | Typical Applications |
|---|---|---|---|---|
| 65# (AISI 1065) | 0.62-0.70% | ≥ 600 | Lower strength than 70#; better toughness; easier to cold-form | Light springs, low-stress shafts, wire forms |
| 70# (AISI 1070) | 0.67-0.75% | ≥ 700 | Balanced strength/toughness; optimal for heat treatment; cost-effective | Automotive parts, hand tools, machinery components |
| 80# (AISI 1080) | 0.75-0.88% | ≥ 800 | Higher hardness (up to 280 HB); reduced ductility; prone to quenching cracks | High-wear parts, blades, heavy-duty springs |
| 60Si2Mn (AISI 9260) | 0.56-0.64% | ≥ 1200 | Silicon-manganese alloy; superior elasticity; higher fatigue limit | Heavy-duty springs, suspension coils, valve springs |
6. Machining & Fabrication Guidelines
- Machinability: Rated at 55-60% of AISI 1212 (free-machining steel). Use carbide tools for hardened states (>200 HB); HSS tools for annealed/normalized conditions. Recommended cutting speed: 20-30 m/min (annealed) or 10-15 m/min (hardened).
- Welding: Poor weldability due to high carbon content. Preheat to 200-300°C and post-weld stress relieve at 550-650°C to prevent cracking. Low-hydrogen electrodes (E7018) recommended.
- Forging: Hot-forge at 900-1100°C; avoid working below 800°C to prevent cracking. Anneal after forging to relieve stresses.
- Surface Treatment: Phosphate coating or black oxide for corrosion resistance; zinc plating (ASTM B633) for outdoor applications. Avoid nitriding (risk of brittle white layer formation).
- Quality Control: Verify hardness via Rockwell (HRC) or Brinell (HB) testing; conduct ultrasonic testing for critical components to detect internal defects post-heat treatment.
7. Request a Carbon Steel Quote
For customized 70# carbon steel products—including hot-rolled bars, cold-drawn wire, forged blanks, or heat-treated components—contact Baoli Iron & Steel’s technical team. We provide mill-certified materials with full traceability, precise dimensional tolerances, and tailored heat treatment solutions to meet your specifications.