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S136 Stainless Mold Steel
S136 (also known as AISI 420 modified, DIN 1.2083, or ASSAB 840 Supreme) is a premium corrosion-resistant martensitic stainless mold steel that occupies the top tier of injection mold steel selection. What distinguishes S136 from standard martensitic stainless like 420 is its carefully balanced chromium content (13.6%) combined with vacuum arc remelting (VAR) or electro-slag remelting (ESR) refining, which produces exceptional cleanliness — minimal non-metallic inclusions that are critical for achieving flawless mirror-quality surface finishes. S136 delivers the unique trifecta of **corrosion resistance** (inherent in the 13% Cr content), **mirror polishability** (Ra < 0.1 μm achievable), and **high hardness** (HRC 48–54) — a combination that no other mold steel matches. It is the definitive material for optical-quality lenses, medical device molds, pharmaceutical containers, food-contact packaging, and clear/transparent polymer components. Its corrosion resistance eliminates the risk of rust spots and pitting that would ruin a polished mold cavity, making it indispensable for long-term production of high-value parts.
Mechanical Properties
- Density: 7.85 g/cm³
- Tensile Strength: 1250 MPa
- Yield Strength: 1100 MPa
- Elongation at Break: 10%
- Reduction of Area: 30%
- Hardness: HRC 48–54 (heat-treated, working condition)
- Modulus of Elasticity: 200 GPa
Advantages
✔ Superior corrosion resistance — 13.6% chromium content provides inherent rust resistance in humid storage, with cooling water exposure, and against corrosive polymers (PVC, acidic resins); eliminates pitting and rust staining that destroy polished cavities
✔ Exceptional mirror polishability — vacuum/ESR refinement produces ultra-clean microstructure; Ra 0.05–0.1 μm finishes consistently achievable; essential for optical, medical, and clear-product molds
✔ High working hardness (HRC 48–54) — excellent wear resistance for long-run production of 500,000–2,000,000+ shots; resists abrasion from glass-filled and mineral-filled polymers
✔ Good thermal conductivity (25 W/m·K) — superior heat dissipation compared to conventional tool steels; reduces cycle time in injection molding
✔ Good dimensional stability — low thermal expansion coefficient (10.8 × 10⁻⁶/°C) combined with through-hardening provides consistent dimensions in production
✔ No special storage requirements — unlike standard mold steels, S136 does not require oil coating or desiccant storage; surface rusting is minimal even in humid environments
Machining & Manufacturing
| Parameter | Specification | Notes |
| Cutting Speed | Annealed (HB ~230): 80–150 SFM (24–46 m/min)Hardened (HRC 48–54): 50–90 SFM (15–27 m/min) | Hardened material requires lower cutting speed |
| Feed Rate | Annealed: 0.004–0.010 ipr (0.10–0.25 mm/rev)Hardened: 0.002–0.005 ipr | — |
| Tooling | Hardened material: Carbide insertsAnnealed material: TiN/TiAlN coated carbidePre-hardened material: Tools with sharp polished edges | Polished edges help obtain better surface finish |
| Coolant | Water-soluble coolant with biocide | Chlorine-based coolant is forbidden to avoid stress corrosion cracking; keep coolant clean |
| Machining Difficulty | Moderate (annealed) / Difficult (hardened) | Use rigid fixtures and high-grade coated tools for hardened stock. Complex cavities are recommended for EDM/wire-cut EDM; remove EDM recast layer prior to polishing |
Common Applications
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