What is S136 (AISI 420 Stainless Mold Steel)?
AISI 420 Stainless Mold Steel is a high-performance plastic mold 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