35# Carbon Steel: Medium-Carbon Structural Steel for General Engineering Applications
35# carbon steel (GB/T 699-2015 standard) is a medium-carbon structural steel grade with 0.32-0.40% carbon content, offering an optimal balance between strength, machinability, and weldability. This grade is widely utilized in manufacturing mechanical components, fasteners, and structural parts that require moderate tensile strength (540-685 MPa) without excessive hardness. This article explores its chemical composition, mechanical properties, heat treatment processes, application scenarios, and performance advantages compared to similar carbon steel grades.
1. Chemical Composition (GB/T 699-2015 Standard)
| Element | Content Range | Function |
|---|---|---|
| Carbon (C) | 0.32 – 0.40% | Primary hardening element; determines strength and hardenability (0.35% average for balanced properties) |
| Silicon (Si) | 0.17 – 0.37% | Deoxidizer; improves strength and elastic limit without reducing ductility |
| Manganese (Mn) | 0.50 – 0.80% | Enhances hardenability and tensile strength; counteracts sulfur’s negative effects |
| Phosphorus (P) | ≤ 0.035% | Impurity; controlled to prevent cold brittleness and reduce weldability issues |
| Sulfur (S) | ≤ 0.035% | Impurity; minimized to improve machinability and prevent hot cracking |
| Chromium (Cr) | ≤ 0.25% | Residual element; slight improvement in hardenability and wear resistance |
| Nickel (Ni) | ≤ 0.30% | Residual element; minor toughening effect at low concentrations |
| Copper (Cu) | ≤ 0.25% | Residual element; can improve atmospheric corrosion resistance if present |
2. Mechanical Properties (As-Rolled Condition)
- Tensile Strength (σb): 540 – 685 MPa (varies with heat treatment and section size)
- Yield Strength (σs): ≥ 315 MPa (minimum guaranteed for structural integrity)
- Elongation (δ5): ≥ 20% (indicates good ductility for forming operations)
- Reduction of Area (ψ): ≥ 45% (high value reflects toughness and resistance to brittle fracture)
- Brinell Hardness (HB): 143 – 187 (as-rolled); can reach 200-250 HB after normalization
- Impact Toughness (Akv): ≥ 55 J at room temperature (suitable for dynamic loading applications)
3. Heat Treatment Processes
- Normalizing (850-870°C): Heating followed by air cooling to refine grain structure, improve machinability, and achieve uniform mechanical properties. Typical hardness after normalizing: 160-180 HB.
- Annealing (820-850°C): Slow cooling in furnace to reduce hardness (≤156 HB) for improved cold workability and stress relief. Produces a ferrite-pearlite microstructure.
- Quenching (840-860°C): Rapid cooling in water or oil to achieve martensitic transformation. Surface hardness can reach 45-50 HRC, but requires tempering to reduce brittleness.
- Tempering (550-650°C): Post-quenching treatment to adjust hardness and toughness. Tempering at 550°C yields optimal balance (25-30 HRC) for most engineering applications.
- Surface Hardening: Flame or induction hardening can achieve 50-55 HRC surface hardness while maintaining a tough core, ideal for shafts and gears.
4. Typical Applications by Industry
Automotive Components
Steering knuckles, axle shafts, connecting rods, and transmission gears — normalized or quenched-and-tempered for fatigue resistance under cyclic loads.
Machinery Manufacturing
Spindles, couplings, bolts, and hydraulic cylinder rods — often surface-hardened for wear resistance while maintaining core toughness.
Construction & Infrastructure
High-strength bolts (Grade 5.6/8.8), anchor rods, and structural fasteners — used in bridges, buildings, and heavy equipment assemblies.
Tooling & Fixtures
Drill jigs, fixture bases, and forming dies — typically annealed for machinability during production, then hardened for service.
Railway & Transportation
Axle boxes, brake levers, and coupling components — quenched-and-tempered to meet UIC/EN standards for dynamic stress resistance.
Energy Sector
Pump shafts, valve stems, and turbine components for non-corrosive environments — selected for cost-effective strength in power generation equipment.
5. Comparison with Similar Carbon Steel Grades
| Grade | Carbon Content | Key Properties | Typical Applications |
|---|---|---|---|
| 35# | 0.32-0.40% | Balanced strength/ductility; good machinability; weldable with preheat | General engineering components, fasteners, shafts |
| 45# | 0.42-0.50% | Higher strength (σb ≥ 600 MPa); lower ductility; requires stress relief after welding | Gears, cranks, high-stress mechanical parts |
| 20# | 0.17-0.24% | Lower strength (σb ≥ 410 MPa); excellent ductility and weldability | Low-stress components, cold-headed fasteners, pipes |
| Q235 | ≤ 0.22% | Structural steel; lower carbon for superior weldability; not heat-treatable | Construction profiles, sheets, non-critical structural parts |
| 1045 (AISI) | 0.43-0.50% | Equivalent to 45#; higher hardenability; often oil-quenched for uniformity | Automotive axles, machine tool components |
6. Machining & Fabrication Guidelines
- Machining: Best performed in normalized or annealed condition (hardness ≤180 HB). Use high-speed steel (HSS) or carbide tools with cutting speeds of 20-30 m/min for turning. Coolant recommended to extend tool life.
- Welding: Preheat to 150-200°C for sections >20mm to prevent cold cracking. Use E7018 (AWS) or J507 (GB) electrodes. Post-weld stress relief at 600-650°C recommended for critical applications.
- Forging: Hot forging range: 1200-850°C. Avoid forging below 850°C to prevent cracking. Air cool after forging to minimize residual stresses.
- Surface Treatment: Suitable for carburizing (case depth 0.5-1.5mm) to achieve 55-60 HRC surface hardness while retaining core toughness. Nitriding can also be applied for wear resistance.
- Quality Control: Verify mechanical properties via tensile testing (GB/T 228.1) and hardness testing (GB/T 231.1). Ultrasonic testing recommended for critical components to detect internal defects.
7. Request a Carbon Steel Quote
For customized 35# carbon steel products — including hot-rolled bars, cold-drawn rods, forged blanks, or machined components — contact our team with your specifications. We provide mill-certified material with full traceability, competitive pricing, and technical support for heat treatment or fabrication requirements.