What is 20Cr (AISI 5120 Case Hardening Steel)?
AISI 5120 Case Hardening Steel is a high-performance case hardening steel designed for demanding applications requiring high strength, wear resistance, and good hardenability. This comprehensive guide covers chemical composition, mechanical properties, heat treatment, and machining characteristics per ASTM/GB standards.
Chemical Composition (ASTM A29/GB/T 3077)
| Element | Content (%) | Specification |
|---|---|---|
| Carbon (C) | 0.38-0.45% | GB/T 3077 |
| Chromium (Cr) | 1.35-1.65% | GB/T 3077 |
| Molybdenum (Mo) | 0.15-0.25% | GB/T 3077 |
| Aluminum (Al) | 0.70-1.10% | GB/T 3077 |
| Manganese (Mn) | 0.45-0.70% | GB/T 3077 |
| Silicon (Si) | 0.20-0.45% | GB/T 3077 |
Mechanical Properties (Quenched & Tempered)
| Property | Value | Standard |
|---|---|---|
| Tensile Strength | ≥980 MPa | GB/T 3077 |
| Yield Strength | ≥835 MPa | GB/T 3077 |
| Elongation | ≥14% | GB/T 3077 |
| Impact Strength (KV) | ≥63 J | GB/T 229 |
| Hardness | 28-34 HRC | ASTM E18 |
Key Features
- Excellent nitriding properties: Forms hard nitride layer for superior surface hardness
- High strength: Excellent tensile and yield strength after heat treatment
- Good toughness: Balanced mechanical properties for demanding applications
- Wear resistant: Superior wear resistance after nitriding treatment
- Good hardenability: Uniform hardness throughout cross-section
- Fatigue resistant: Excellent fatigue strength for cyclic loading applications
Common Applications
- Injection molding: Mold cores, cavities, inserts for high-volume production
- Extrusion: Extrusion screws, barrels, dies for plastic processing
- Automotive: Crankshafts, camshafts, gears, cylinder liners
- Machinery: High-strength bolts, studs, shafts, spindles
- Power generation: Turbine blades, valve components, boiler parts
- Petrochemical: Reactor components, heat exchanger parts, valve bodies
Heat Treatment
Quenching & Tempering
- Austenitizing: 930-950°C, oil quench
- Tempering: 580-620°C for desired hardness
- Hardness after Q&T: 28-34 HRC
Nitriding Treatment
- Gas nitriding: 500-530°C, 20-50 hours
- Plasma nitriding: 480-520°C, 10-30 hours
- Surface hardness: 900-1200 HV (65-70 HRC equivalent)
- Case depth: 0.3-0.6mm typical
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: 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 | 80-120 | 0.2-0.4 | 2-5 |
| Finish Turning | 120-180 | 0.05-0.15 | 0.2-0.5 |
| Milling | 100-150 | 0.1-0.3 | 1-4 |
| Drilling | 50-80 | 0.1-0.2 | 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
- Nitriding: Surface hardening for wear resistance
- Chrome plating: Decorative and corrosion resistant
- Nickel plating: Corrosion resistant coating
