For previous reading, please refer to A Literary Look at Plastic Mold Steel: NAK80.

 

Both S136 and S136H are corrosion-resistant plastic mold steels developed by ASSAB of Sweden. Featured for their "mirror-like polish" and "rust resistance," they are core materials for high-end precision plastic molds. Essentially, they are from same family of steels, with highly consistent core compositions: approximately 0.38% carbon and 13.5% chromium (key to their excellent corrosion resistance), supplemented by 0.4% manganese, 0.3% silicon, 1.0% molybdenum for enhanced hardness and toughness. The key difference between two lies in their factory-delivered conditions: S136 is annealed, with a factory hardness of less than 235 HB, requiring subsequent quenching and tempering to reach a service hardness of HRC 48-52. S136H, on the other hand, is pre-hardened. Through factory aging, its hardness stabilizes at HRC 30-36. It eliminates need for through-hardening and can be used directly in manufacture of small and medium-sized molds. This difference makes S136 more suitable for large, complex molds (which require through-hardening to ensure strength), while S136H is more suitable for small, precision molds requiring quick delivery.

Superior Corrosion Resistance: A "Shield" Against Harsh Environments
High chromium content of S136/S136H (13.5%) forms a dense chromium oxide film on the surface, providing protection against corrosion from weak acids, weak bases, and corrosive plastics (such as PVC and fluorinated plastics). Even under long-term use in hot and humid environments, or when molds require frequent cleaning with acidic cleaners, S136 maintains a smooth surface, preventing product defects caused by rust. Its corrosion resistance is 3-5 times higher than that of ordinary mold steels, making it preferred choice for bathroom plastic parts and medical plastic molds.
Super Mirror Polish: "Creator" of Ultimate Appearance
Refined through electroslag remelting (ESR) process, steel boasts exceptional purity, minimal non-metallic inclusions, a uniform and fine microstructure. S136 can easily be polished to 18,000 grit after quenching, with a surface roughness of Ra ≤ 0.01μm, reproducing every detail of mold surface. It is suitable for products requiring an extremely precise "mirror effect," such as optical lenses, mobile phone covers, and high-end appliance panels. Although S136H has a slightly lower pre-hardened polishing limit (12,000 grit), it meets requirements of most precision appearance parts and eliminates heat treatment, shortening production cycles.
Excellent Hot and Cold Stability: A Reassurance for Complex Molding
After quenching and tempering, S136 maintains a hardness above HRC45 at operating temperatures of 200-250℃. Its coefficient of thermal expansion is only 11.5×10^-6/℃, far lower than that of ordinary steel, making it less susceptible to deformation due to temperature fluctuations during mold forming. Even when used for engineering plastics requiring high-temperature injection molding (such as PC and PA66), dimensional accuracy is maintained. S136H's pre-hardened microstructure is more stable, eliminating risk of heat treatment deformation after processing, making it suitable for precision molds with tolerances of ±0.005mm.
Excellent Machinability and Repairability: A Booster for Efficient Production
S136 exhibits low cutting resistance in its annealed state, making it easily machined with high-speed steel or carbide tools. While its hardness increases after quenching, it can still be efficiently milled with right tooling (such as cubic boron nitride tools). Both offer excellent welding performance. After local mold damage, argon arc welding repair and tempering can restore hardness to over 90% of parent material, significantly extending mold's service life.

Applications of S136 and S136H are primarily focused on high-demand plastic molds, but require differentiation based on specific needs:
S136's core applications include large, complex molds (such as automotive headlight molds and washing machine drum molds), molds for corrosive plastics (such as UPVC pipe fitting molds), and ultra-mirror molds (such as optical lens molds), which require quenching to achieve high hardness and wear resistance.
S136H's core applications include small and medium-sized precision molds (such as mobile phone charger housing molds and medical syringe molds), and quick-turnaround molds (custom molds with short order cycles). Pre-hardened state allows for direct processing, reducing production steps. Compared to similar steels, S136/S136H and NAK80 complement each other: NAK80 offers a high cost-effectiveness in its pre-hardened state, but lacks corrosion resistance, making it suitable for molds with medium- to high-quality mirror finishes for non-corrosive plastics. S136/S136H offers superior corrosion resistance and mirror finish, but at a higher cost (imported S136 is approximately 120 yuan/kg, while S136H is approximately 150 yuan/kg), making it suitable for demanding applications. Domestically produced S136 in Dalian or Fushun costs approximately 20 to 45 yuan/kg ex-factory. If mold will come into contact with corrosive materials or be used in a humid and hot environment for extended periods, S136/S136H is the only choice.

Processing Taboos: S136 has a high hardness after quenching, so avoid cutting with ordinary high-speed steel tools. Carbide or cubic boron nitride tools are recommended. When machining S136H, control cutting speed; excessive speed can easily lead to tool wear.
Heat Treatment Key Points: Quenching temperature for S136 steel must be strictly controlled between 1020-1050℃, typically between 1020-1030℃. After holding, cool slowly to avoid cracking. Tempering temperature should be selected based on requirements (hardness HRC 52 at 180℃, HRC 45 at 500℃), and multiple tempering cycles are required to eliminate internal stresses.
Maintenance Tips: After use, mold surface should be cleaned with a neutral detergent to avoid residual corrosive plastic. When stored for extended periods, surface should be coated with anti-rust oil and stored in a dry, ventilated environment to prevent oxidation and rust.
S136 - Mold Steel Welding: Excellent welding results can be achieved with preventative measures such as proper welding procedures, preheating, weld preparation, and electrode selection. If polishing or photoetching of mold steel is required after repair welding, electrodes with same chemical composition as mold steel should be used.
S136 Mold Steel Photoetching: S136's low impurity content makes it well-suited for photoetching. Due to its excellent corrosion resistance, a specialized photoetching process is required. This property is widely recognized by photoetching companies.

S136 Tool Steel Polishing: Quenched and tempered S136 offers excellent polishability. Follow "rough grinding → fine grinding → polishing" process, using diamond abrasive paste to avoid surface scratches. Clean promptly after processing to prevent abrasive residue that can cause corrosion.