S136 steel is a high-quality stainless tool steel that is often used in high-precision plastic molding. It is known for its great resistance to corrosion, stability of dimensions, and ability to be polished to a mirror-like finish. The Electro-Slag Remelting (ESR) process improves its performance, making it perfect for molds that make clear plastics, corrosive materials, and medical or food-grade parts.
This guide looks at S136’s material properties, how it reacts to corrosion, how to process it, and some advanced uses. It gives designers, engineers, and procurement professionals useful tips on how to improve mold performance, make it last longer, and find the right balance between durability, quality, and cost-effectiveness.
S136 Steel Overview and Material Standards
What Is S136 Steel?
S136 steel is a high-purity, high-corrosion-resistance stainless mold steel belonging to the 420 modified family. Compared with standard 420, S136 contains optimized chromium content (≈13%) and is refined through Electro-Slag Remelting (ESR), resulting in extremely low inclusions and exceptional steel cleanliness. These characteristics give S136 three core functional advantages:
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High corrosion resistance
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Excellent mirror polishability suitable for optical-grade surfaces
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Superior cleanliness and uniformity for high-precision molding
For designers, this means smoother surfaces and tighter dimensional tolerances. For engineers, it ensures predictable processing behavior. For procurement managers, S136 offers a higher lifetime value in demanding molding environments.
Why S136 Is Critical in Modern Manufacturing
S136 stainless tool steel plays a vital role in modern manufacturing due to its unique combination of high corrosion resistance, exceptional polishability, and dimensional stability. These properties make it the material of choice for precision molds used in industries where surface quality and tight tolerances are essential, such as medical devices, food-grade components, and high-end consumer plastics. Its ability to withstand corrosive plastics like PVC or PC+ABS without compromising finish ensures consistent product quality and reduces mold downtime.
Additionally, the Electro-Slag Remelting (ESR) refinement of S136 further enhances its purity and uniformity, allowing manufacturers to achieve superior mirror finishes and extend mold life. For engineers, designers, and procurement professionals, S136 provides a reliable solution that balances durability, performance, and cost-effectiveness, making it indispensable in high-precision, high-volume production environments.
S136 Steel Equivalent Standards
Common equivalent or comparable materials include:
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AISI 420 Modified – similar chemical composition and corrosion-resistant properties
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DIN 1.2083 – European standard with close equivalence
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SUS420J2 – composition partially similar, but typically used for different applications
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ASSAB S136 vs Uddeholm Stavax ESR – both are premium ESR variants; chemical compositions are nearly identical, but performance may vary slightly based on melting and production processes
Differences Between S136 and S136H (Pre-hardened Grade)
S136H (pre-hardened):
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Delivered hardness: 33–38 HRC
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No additional heat treatment required
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Suitable for medium-demand plastic moulds
S136 (ESR, heat-treatable):
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After heat treatment: 48–52 HRC
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Superior wear resistance, polishability, and corrosion resistance
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Ideal for high-end transparent moulds, optical components, and long-life mould applications
Usage differences:
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S136H → general-purpose moulds, medium-wear plastics
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S136 → precision, high-transparency, or long-service-life moulds
Chemical Composition and Material Science Fundamentals
Chemical Composition Table
Typical S136 steel composition includes carbon, chromium, silicon, manganese, phosphorus, and sulfur. Each element contributes distinct functional properties:
| Element | Typical Content | Function |
| C (Carbon) | ~0.38–0.42% | Determines hardness, wear resistance, and strength after heat treatment |
| Cr (Chromium) | ≥13% | Provides stainless properties, corrosion resistance, and carbide formation capability |
| Si (Silicon) | ~0.8% | Enhances strength and improves hardenability |
| Mn (Manganese) | ~0.5% | Improves toughness and aids deoxidation during steelmaking |
| P (Phosphorus) | ≤0.03% | Controlled to maintain toughness and reduce brittleness |
| S (Sulfur) | ≤0.03% | Kept low to ensure high purity and excellent polishability |
High chromium content (≥13%) is the primary reason why S136 stainless steel exhibits strong corrosion resistance, even when processing corrosive plastics like PVC or acetate.
Material Science Interpretation
From a materials-science perspective, the performance of S136 steel is driven by its carbide behavior and steelmaking process:
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Precipitation of Cr23C6 carbides This carbide improves wear resistance and contributes to hardness. Proper heat treatment prevents excessive carbide precipitation, which can otherwise compromise corrosion resistance.
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ESR (Electro-Slag Remelting) enhances purity The ESR process significantly reduces inclusions (oxides, sulfides), resulting in:
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superior mirror polishability
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uniform microstructure
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reduced risk of pitting or polishing defects
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better performance in optical-grade moulds
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Effect of carbon content
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Higher C → higher hardness & wear resistance
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Excessive C → reduced corrosion resistance and polishability S136 balances carbon content to ensure a stable combination of hardness and high surface quality.
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Microstructure Characteristics
After proper quenching and tempering, S136 steel typically exhibits:
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Tempered martensite – provides base hardness and dimensional stability
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Uniform carbide distribution (primarily Cr carbides) – improves wear resistance while maintaining polishability
Microstructural purity is one of the major reasons S136 is preferred for high-end moulds:
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ESR reduces A-type and B-type non-metallic inclusions
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Fewer inclusions → fewer polishing marks, better clarity for optical/transparent mould components
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Greater consistency → better dimensional stability during long production runs
This microstructure makes S136 steel exceptionally suitable for mirror-finish moulds, transparent parts, and corrosion-sensitive applications.
Mechanical and Physical Properties of S136 Steel
Mechanical Properties
S136 steel delivers robust mechanical performance after heat treatment:
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Tensile strength: High, suitable for demanding moulding environments
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Yield strength: Stable under repeated thermal cycling
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Hardness:
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48–52 HRC after heat treatment (typical ESR-grade performance)
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Wear resistance:
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Comparable to or better than DIN 1.2083
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Superior to standard AISI 420 due to improved carbide uniformity and ESR purity
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Physical Properties
Key physical characteristics include:
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Thermal expansion coefficient: Stable expansion behavior supports dimensional stability during moulding
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Thermal conductivity: Moderate, enabling controlled cooling rates for plastic moulds
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Elastic modulus: Ensures structural rigidity for complex mould inserts and cores
These properties allow S136 to maintain precision in high-volume injection moulding cycles.
Performance Comparison: 420 vs. 1.2083 vs. S136 ESR
S136 ESR (premium grade):
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Highest purity
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Best mirror-polish capability
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Excellent corrosion resistance
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Most suitable for optical or transparent plastic moulds
1.2083 (economical alternative):
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Good corrosion resistance
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Lower purity compared to ESR S136
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Adequate for general plastic moulds
AISI 420 (standard grade):
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Lower hardness and corrosion resistance
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Not ideal for high-gloss or transparent applications
Overall, S136 ESR offers superior mechanical stability, polishability, and service life, making it the preferred choice for high-end mould engineering.
Corrosion Resistance and Mechanisms
Mechanisms of Corrosion Resistance in S136
S136 stainless steel, a high-grade ESR-remelted 420 modified tool steel, exhibits outstanding corrosion resistance due to its high chromium content (≥13%), which forms a stable, self-healing passive oxide layer on the surface. This passive film effectively protects against general corrosion, pitting, and crevice corrosion.
In harsh environments, such as those containing chlorides from PVC or PC+ABS materials, S136 demonstrates superior performance compared to conventional 420 or 440C steels. Its low inclusion content and uniform tempered martensitic microstructure reduce localized breakdown of the passive layer, mitigating initiation points for stress corrosion cracking (SCC) and pitting.
Key corrosion types relevant to molding applications include:
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Pitting corrosion: localized attack often at micro-inclusions or surface defects.
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Crevice corrosion: occurs in narrow gaps or cooling channels with stagnant fluids.
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Stress corrosion cracking: combination of tensile stress and corrosive environment, rare in ESR S136 due to its purity and uniform structure.
Quantitative Corrosion Performance
Salt spray (fog) and immersion tests provide empirical data for engineers and procurement decisions. Typical ESR S136 results:
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Salt spray test (ASTM B117): 48–96 hours with no visible pitting or surface corrosion.
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Immersion in PVC-containing solutions: negligible material loss over repeated cycles.
Comparative performance:
| Steel Grade | Salt Spray (h) | Observations | Notes |
| S136 ESR | 48–96 | No pitting | High purity, passive film stability |
| 420 | 24–48 | Minor pitting | Conventional 420, higher inclusion |
| 440C | 12–24 | Early pitting | Hard but less corrosion-resistant |
| 1.2316 | 24–36 | Some localized corrosion | Good mechanical properties, moderate corrosion |
This demonstrates that S136 offers significantly longer corrosion-free service under demanding molding environments.
Engineering Case Studies: Mold Corrosion
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High-chloride plastics (PVC, PC+ABS): S136 molds maintain surface integrity for 20–30% longer cycles compared to standard 420 steel.
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Cooling channel corrosion: ESR S136’s uniform microstructure reduces micro-pitting in water channels, preventing early leakages and maintaining dimensional accuracy.
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Surface pitting to crack propagation link: Localized corrosion pits can act as initiation sites for stress-induced cracks. S136’s enhanced purity and passive layer stability minimize this risk, ensuring longer fatigue life for precision molds.
Summary: S136’s corrosion resistance arises from its high chromium content, ESR refinement, and tempered martensitic microstructure. For engineers, designers, and procurement managers, understanding these mechanisms is crucial for selecting the right steel for high-volume, corrosive, or optical-grade molding applications. Correct material choice directly extends mold life, reduces maintenance, and improves production reliability.
Heat Treatment of S136 Steel
Heat treatment is one of the most critical process steps that determine the final hardness, corrosion resistance, dimensional stability, and polishing behavior of S136 tool steel. Because S136 is an ESR-refined martensitic stainless steel, its heat-treating window is narrower than conventional 420 stainless steels, requiring precise temperature control to avoid carbide coarsening or distortion.
Heat Treatment Process Overview
Annealing (Softening Treatment)
Annealing is typically performed at 800–850°C, followed by slow furnace cooling.
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Purpose: restore uniform microstructure, relieve machining stress, and improve subsequent CNC machinability.
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Resulting hardness: ≤ 230 HB. Designers benefit from predictable machinability, while procurement teams often specify annealed blocks for complex cavity milling.
Pre-Hardened Variant: S136H
S136H is supplied in a pre-hardened 33–38 HRC condition.
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Suitable for medium-demand molds such as cosmetic housings.
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Eliminates the need for quenching, reducing cost and lead time. Engineers often select S136H when transparency requirements are moderate and mold polishing demand is lower.
Quenching (Hardening)
Standard hardening temperature: 1020–1080°C.
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Typical holding time: 20–40 minutes depending on section thickness.
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Cooling medium: high-pressure gas quench or oil quench with controlled cooling rate. This step transforms the microstructure to martensite while preserving the chromium content responsible for corrosion resistance.
Tempering
Tempering is performed at 180–250°C, often twice or even three times to stabilize hardness. A higher number of tempering cycles is recommended for large electrodes and optical-grade mold inserts.
Hardness Range and Process Control
Post-Quench Hardness
Properly hardened S136 stainless steel typically reaches 48–52 HRC, the optimal range for maintaining polishability and mold wear resistance.
Two vs. Three Temper Cycles
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Two temper cycles: sufficient for general injection mold applications.
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Three temper cycles: recommended for precision optical molds or large blocks where retained austenite must be minimized.
Stress Relief and Distortion Control
Stress-relief annealing (approx. 600–650°C) is performed prior to rough machining or after heavy milling.
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Reduces warpage during quenching
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Improves dimensional accuracy This is especially valuable for designers working with tight tolerances on multi-cavity molds.
Common Heat Treatment Issues and Solutions
Quench Cracking
Causes: overly fast cooling, sharp internal corners, or inconsistent part thickness.
Mitigation: smooth transitions, optimized cooling, and uniform section design.
Tempering Brittleness
Occurs when tempering temperature overlaps the embrittlement zone of martensitic stainless steels.
Solution: keep tempering below 250°C and apply multiple temper cycles.
Heat Treatment Distortion
Due to martensitic transformation and residual stress.
Compensation strategies include:
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Pre-machining with stock allowance
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Symmetrical machining sequences
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Post-quench corrective grinding
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Use of vacuum heat treatment for improved uniformity
Machinability, EDM Behavior, and Polishing Guidelines
S136 stainless steel, particularly in ESR (Electro-Slag Remelted) form, offers exceptional corrosion resistance and polishability, but its machinability and post-processing behavior require careful control to maintain dimensional stability and surface quality. This section provides detailed guidance for CNC machining, EDM processing, and mirror polishing of S136 tool steel, addressing key performance considerations for engineers, mold designers, and procurement specialists.
CNC Machining Guidelines
Machining S136 ESR steel requires attention to tool material, feed rates, and heat management. Recommended tooling includes coated carbide inserts (TiAlN or AlTiN) or high-speed steel end mills for smaller or pre-hardened sections.
Key Machining Parameters:
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Rough machining: Moderate cutting speeds (60–120 m/min), higher feed per tooth to reduce heat accumulation.
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Finish machining: Lower cutting speeds (30–60 m/min) with reduced depth of cut to minimize surface work hardening.
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Coolant: Flood coolant or mist cooling is advised to dissipate heat and reduce microstructural alteration.
Best Practices:
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Avoid prolonged dwell at one location to prevent localized heat buildup.
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Leave minimal machining allowance for post-heat-treatment operations to compensate for potential distortion.
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Use rigid fixturing to control vibration and maintain surface integrity.
EDM (Electrical Discharge Machining) Considerations
EDM is commonly used for intricate features and hard-to-machine areas of S136 molds. However, attention must be paid to the formation of the recast layer (white layer) and potential microcracks.
Guidelines for EDM:
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Minimize discharge energy to limit white layer thickness (ideally ≤20 μm).
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Post-EDM stress-relief or tempering reduces the risk of microcracks propagating during service.
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EDM → Polishing → Light Tempering sequence is recommended to maintain corrosion resistance and surface finish.
Tips:
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Use fine-pulse settings for precision features to reduce thermal damage.
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Regularly inspect critical mold surfaces after EDM for microstructural defects.
Mirror Polishing Guidelines
Achieving a high-quality optical finish on S136 molds relies heavily on ESR purity and systematic polishing steps.
Polishing Procedure:
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Sequential sanding from coarse to fine grit (240 → 2000 mesh).
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Diamond paste or oxide compounds for final mirror finish.
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Intermediate cleaning to remove residual abrasives that could scratch or pitting.
Best Practices:
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ESR S136 reduces non-metallic inclusions, enhancing polishability and minimizing “orange peel” texture.
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Maintain consistent polishing pressure and motion to avoid surface undulations.
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Avoid aggressive polishing on corners and edges to prevent rounding, which can affect mold performance.
Engineering Value:
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Designers: Ensures dimensional accuracy and surface quality for transparent or medical-grade molds.
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Engineers: Maintains mechanical properties and prevents residual stress-induced distortion.
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Procurement: Reduces scrap rates, minimizes post-processing labor, and improves mold lifespan.
Common Failure Modes and Prevention Strategies
S136 stainless steel excels in corrosion resistance and polishability, but high-performance molds can still experience failures if design, processing, or maintenance is suboptimal. Understanding typical failure modes and implementing preventive strategies is critical for prolonging mold life, ensuring product quality, and optimizing total cost of ownership.
Typical Failure Modes
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Corrosion Pitting Leading to Fatigue Cracks Even with high Cr content (>13%), exposure to aggressive chemicals or improper maintenance can initiate corrosion pits. These localized pits act as stress concentrators, accelerating fatigue crack formation under cyclic loading, particularly in thin-wall or high-stress mold features.
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Heat Treatment-Induced Cracks Improper quenching or tempering may cause microcracks in S136 molds. Rapid cooling after austenitizing can induce residual stresses, while uneven tempering may result in local brittleness.
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Edge Chipping and Fracture High-stress corners and thin sections are susceptible to edge chipping or small-scale fracture, especially during high-pressure molding cycles or when post-EDM surfaces are inadequately finished.
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EDM Heat-Affected Zone (HAZ) Cracking EDM can leave a recast layer with tensile residual stress. Without subsequent stress relief or tempering, this HAZ becomes a potential origin of brittle fracture or microcracks under service conditions.
Lifetime Enhancement Strategies
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Surface Hardening & Nitriding Low-temperature nitriding enhances surface hardness without compromising bulk corrosion resistance. This improves wear resistance and reduces crack initiation on contact surfaces.
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PVD / DLC Coatings Applying TiN, CrN, or DLC coatings can minimize adhesion, reduce friction, and protect against minor corrosion or mechanical wear, extending mold life in high-cavity or high-volume production.
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Optimized Hardness vs. Toughness Design Targeted heat treatment can balance hardness (48–52 HRC) with sufficient toughness to avoid brittle failure. In critical areas, slightly reduced hardness can prevent cracking without significantly affecting wear resistance.
Maintenance Recommendations
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Regular Cleaning: Remove chemical residues and prevent pitting corrosion.
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Cooling System Management: Maintain uniform mold temperature to reduce thermal gradients and stress accumulation.
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Chemical Control: Avoid incompatible plastic additives or aggressive cleaning agents that could compromise corrosion resistance.
Engineering Value:
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Designers: Consider mold geometry to reduce sharp corners and stress concentrators.
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Engineers: Implement controlled heat treatment and surface finishing sequences.
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Procurement/Operators: Adopt preventive maintenance schedules and coating selection for long-term mold durability.
Applications and Engineering Case Studies
S136 stainless steel is widely recognized for its high corrosion resistance, excellent polishability, and dimensional stability, making it the preferred material for advanced plastic mold applications. Understanding its practical applications and validated case studies helps engineers, designers, and procurement managers make informed material choices.
Typical Applications
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Transparent Plastic Molds (Optical Lenses & Covers) S136’s high ESR purity and low inclusion content allow for superior mirror polishing, achieving surface roughness as low as Ra 0.02 μm. This ensures optical clarity for lenses, display covers, and other transparent components.
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Medical and Food-Grade Molds With Cr content ≥13% and exceptional corrosion resistance, S136 is ideal for molds in contact with medical-grade plastics, food packaging, and pharmaceutical devices. Its resistance to pitting and staining ensures hygiene compliance and long-term performance.
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Corrosive Environment Injection Molds (PVC, PC+ABS) S136 resists chemical attack from PVC and ABS blends, preventing mold degradation and improving lifetime. The steel maintains dimensional stability under repeated thermal cycles and exposure to aggressive plasticizers.
Engineering Case Studies
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Transparent Housing Mold: A consumer electronics company achieved Ra 0.02 μm on critical optical surfaces using S136 ESR steel. Post-polishing and multiple tempering cycles resulted in minimal residual stress and high dimensional accuracy.
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PVC Injection Mold Longevity: Comparative studies showed that S136 molds outperformed standard 420 stainless steel by extending service life by 20–30% under identical processing conditions, particularly in corrosive PVC molding environments.
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ESR Purity Advantage: High-purity ESR S136 consistently reduced polishing time and improved defect-free surface ratio, demonstrating the direct link between steel quality and mold efficiency.
Procurement Guidelines
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S136 vs S136H vs 1.2083: Choose S136 ESR for maximum corrosion resistance and mirror polishing; S136H for pre-hardened, moderate-performance molds; 1.2083 for cost-sensitive applications where ultimate corrosion resistance is not critical.
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Quality Verification: Inspect for uniform hardness, ESR-certified purity, and low non-metallic inclusions. For high-precision optical or medical molds, prioritize ESR S136 with documented chemical composition and microstructure reports.
Engineering Value:
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Designers: Ensure mold geometry aligns with S136’s polishing and thermal properties.
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Engineers: Optimize heat treatment and finishing sequences based on application-specific wear and corrosion exposure.
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Procurement Managers: Make informed material selection decisions to balance cost, performance, and mold lifespan.
Summary
S136 steel is a type of stainless tool steel that is great for making precision plastic molds because it doesn’t rust easily, polishes well, and stays the same size. The ESR (Electro-Slag Remelting) process makes the material purer and fewer impurities, which leads to better mirror finishes and more reliable performance in applications like clear plastics, corrosive materials, and medical or food-grade parts.
To get the best performance out of S136, you need to be very careful when machining, EDM, polishing, and heat treating it. Proper maintenance, like cleaning, cooling, and preventing corrosion, also helps the mold last longer and lowers the chance of failure. These tips help designers, engineers, and procurement managers make molds that last a long time and work well while keeping costs and efficiency in mind.
FAQ
What is S136 steel and what makes it different from AISI 420?
S136 is a modified 420 stainless tool steel specifically optimized for high corrosion resistance, mirror-polishing, and dimensional stability. Unlike standard AISI 420, S136 is typically ESR-refined, which reduces non-metallic inclusions and enhances polishing performance, making it ideal for high-precision plastic molds.
Is S136 stainless steel suitable for optical-grade mold polishing?
Yes. ESR S136 achieves excellent surface finish due to its ultra-low inclusion content and fine tempered martensite structure. Mirror finishes with Ra ≤ 0.02 μm are routinely achievable for transparent plastic applications such as lenses and covers.
What are typical S136 steel properties after heat treatment?
After hardening and tempering, S136 steel typically reaches 48–52 HRC, exhibits excellent wear resistance, good toughness, and maintains dimensional stability. Thermal expansion, thermal conductivity, and elastic modulus are well-suited for precision molds.
What is the S136 steel equivalent in DIN / AISI standards?
S136 is equivalent to DIN 1.2083, and can be roughly compared to AISI 420 Modified. However, the ESR process gives S136 superior purity and polishability compared with conventional 420 grades.
Can S136 be EDM-machined and still achieve mirror finish?
Yes. EDM machining is compatible with S136, but the process must control white layer formation and avoid microcracks. The recommended workflow is EDM → tempering → mirror polishing, leveraging the ESR-refined steel for optimal surface quality.
How to avoid corrosion in S136 mold cooling channels?
Regular cleaning, controlling aggressive chemicals, and maintaining proper water treatment are essential. S136’s high Cr content ensures excellent corrosion resistance, but design features like sharp corners or stagnant zones must be minimized to prevent localized corrosion.
S136 vs S136H: which one should I choose for transparent parts?
S136H is pre-hardened at ~33–38 HRC and is suitable for medium-duty molds. For high-precision, optical, or long-lifetime molds, fully hardenable S136 ESR (~48–52 HRC) is preferred to maximize corrosion resistance, hardness, and polishability.
Does ESR-remelted S136 significantly improve mold life?
Yes. ESR refinement reduces inclusions and segregation, improving surface finish, wear resistance, and fatigue life. Field data show 20–30% longer mold life in high-corrosion or transparent plastic applications compared to non-ESR 420-grade steels.
About the Author: Gavin Xia
This article was written by engineers from the RAPID PROTOS team. Gavin Xia is a professional engineer and technical expert with 20 years of experience in rapid prototyping, metal parts, and plastic parts manufacturing.
