What is 40Cr (AISI 4140 Alloy Steel)?
AISI 4140 Alloy Steel is a high-performance alloy steel widely used in automotive, machinery, tooling, and industrial applications. This comprehensive guide covers its chemical composition, mechanical properties, heat treatment, machining characteristics, and practical applications per ASTM/GB standards.
Chemical Composition (ASTM A29/A681)
| Element | Content (%) | Specification |
|---|---|---|
| Carbon (C) | 0.43-0.50% | ASTM A29 |
| Manganese (Mn) | 0.60-0.90% | ASTM A29 |
| Silicon (Si) | 0.10-0.35% | ASTM A29 |
| Chromium (Cr) | 0.80-1.10% | ASTM A29 |
| Molybdenum (Mo) | 0.15-0.25% | ASTM A29 |
| Phosphorus (P) | ≤0.035% | ASTM A29 |
| Sulfur (S) | ≤0.040% | ASTM A29 |
Mechanical Properties (Quenched & Tempered)
| Property | Value | Standard |
|---|---|---|
| Tensile Strength | ≥625 MPa | ASTM A29 |
| Yield Strength | ≥415 MPa | ASTM A29 |
| Elongation | ≥16% (in 50mm) | ASTM A29 |
| Reduction of Area | ≥40% | ASTM A29 |
| Hardness | 170-220 HB (Annealed) | ASTM E10 |
| Hardness (Q&T) | HRC 28-32 | ASTM E18 |
Key Features
- High strength: Excellent tensile and yield strength after heat treatment
- Good toughness: Balanced mechanical properties for demanding applications
- Wear resistant: Good abrasion resistance in hardened condition
- Machinable: Good machinability in annealed condition (60% of free-cutting steel)
- Hardenable: Can be induction hardened or flame hardened for surface wear resistance
- Weldable: Can be welded with proper preheat and post-weld treatment
Common Applications
- Automotive: Axles, crankshafts, connecting rods, gears, bolts
- Machinery: Shafts, spindles, pins, bushings, couplings
- Tooling: Jigs, fixtures, dies, molds, machine components
- Construction: High-strength bolts, structural components
- Agricultural: Plow shares, cultivator tines, harrow discs
- Railway: Rail fasteners, wheel axles, coupling components
Heat Treatment
Annealing
- Full annealing: Heat to 820-850°C, furnace cool
- Hardness after annealing: ≤200 HB
Quenching & Tempering
- Austenitizing: 830-860°C, oil quench
- Tempering: 500-600°C for desired hardness
- Hardness after Q&T: HRC 28-32 (depends on tempering temperature)
Surface Hardening
- Induction hardening: 850-900°C, water or polymer quench
- Flame hardening: Oxy-acetylene flame, water quench
- Case depth: 2-5mm typical
- Surface hardness: HRC 50-55
CNC Machining Guidelines
Tool Selection
- Tool material: Carbide (YG/YW class) or coated carbide
- Tool geometry: Positive rake angle (5-10°), relief angle (8-12°)
- Nose radius: 0.4-0.8mm for finishing, 0.8-1.2mm for roughing
- Coating: TiN, TiAlN, or AlTiN for improved tool life
Recommended Cutting Parameters (Annealed Condition)
| Operation | Cutting Speed (m/min) | Feed Rate (mm/rev) | Depth of Cut (mm) |
|---|---|---|---|
| Rough Turning | 100-150 | 0.2-0.4 | 2-5 |
| Finish Turning | 150-220 | 0.05-0.15 | 0.2-0.5 |
| Milling | 120-180 | 0.1-0.3 | 1-4 |
| Drilling | 60-100 | 0.1-0.25 | N/A |
Coolant & Lubrication
- Coolant type: Emulsion or semi-synthetic coolant with EP additives
- Coolant pressure: ≥5 MPa, directed at cutting zone
- Through-tool coolant: Recommended for deep hole drilling
Surface Finish Options
- As-machined: Ra 1.6-3.2 μm
- Fine turning: Ra 0.8-1.6 μm
- Grinding: Ra 0.4-0.8 μm
- Polishing: Ra 0.2-0.4 μm
- Black oxide: Decorative and mild corrosion resistance
- Phosphate coating: Improved wear resistance and paint adhesion
- Nitriding: Surface hardening for wear resistance
