NAK80 vs S136 – Composition, Heat Treatment, Properties, and Applications

Introduction

NAK80 and S136 are two widely used mold steels in the injection-molding and tooling industries. Engineers, procurement managers, and manufacturing planners commonly face the decision between these grades when designing molds for corrosive environments, high-gloss finishes, or parts that demand long service life. The choice often balances corrosion resistance, polishability, hardenability, toughness, cost, and downstream fabrication needs.

The principal practical distinction between the two is their alloying and processing emphasis: one is optimized primarily for elevated resistance to corrosive attack and superior polishability in molded plastics, while the other offers a different balance of corrosion resistance with improved mechanical strength and toughness for heavier-duty mold applications. Both are discussed here in terms of standards, chemistry, microstructure and heat treatment response, mechanical behavior, weldability, corrosion characteristics, fabrication attributes, applications, and purchasing considerations.

1. Standards and Designations

• Common standards and trade designations encountered:
• S136: often referenced as a martensitic stainless mold steel used in Europe and worldwide (commonly sold by multiple producers under the S136 designation or equivalents).
• NAK80: a commercial stainless mold steel designation used primarily in Asia and internationally by specific manufacturers; sometimes listed in product literature rather than as a single international standard designation.
• Typical standard families to consult for equivalents and specifications include:
• ASTM / ASME (tool steel and stainless tool steel specifications)
• EN (European steel standard nomenclature and equivalents)
• JIS (Japanese Industrial Standards; some commercial grades originate in Japan)
• GB (Chinese national standards may list equivalent stainless mold steels)
• Classification:
• Both NAK80 and S136 are stainless tool/mold steels (martensitic stainless mold steels), i.e., they are tool steels with a corrosion-resistant chemistry rather than plain carbon or HSLA steels.

2. Chemical Composition and Alloying Strategy

Notes: - Exact quantitative compositions vary by producer and should be verified on the mill certificate for each heat. The table reports typical presence and alloying strategy rather than specific weight percentages. - Alloying strategy summary: both grades rely predominantly on chromium to deliver corrosion resistance. NAK80 variants from some producers emphasize nitrogen and carefully balanced alloying to improve resistance-to-corrosion while maintaining machinability and through-hardening. S136 is engineered for high corrosion resistance and polishability, often optimized for mold environments where staining or rusting must be minimized.

3. Microstructure and Heat Treatment Response

• Typical as-processed microstructures:
• Both grades are martensitic stainless steels after appropriate heat treatment. In the as-annealed condition they are generally soft, ferritic/partially annealed microstructures suitable for machining.
• Heat treatment routes and responses:
• Annealing: Performed to soften and stabilize composition for machining; produces relatively soft ferrite/pearlite or tempered martensite depending on grade and exact anneal cycle.
• Hardening (Solution treat / Austenitize and Quench): Both grades are austenitized at grade-specific temperatures to dissolve carbides and then quenched to form martensite. Carbide distribution and retained austenite are influenced by alloying (Cr, Mo, N).
• Tempering: Multiple tempering cycles are used to achieve target hardness and toughness; tempering reduces internal stresses and adjusts toughness at the expense of some hardness.
• Influence of alloying:
• Chromium and molybdenum stabilize carbides and affect tempering resistance; higher Cr content supports corrosion resistance but can influence the size and distribution of carbides affecting polishability.
• Nitrogen (when present) can increase hardness and corrosion resistance through solid solution strengthening and nitride precipitation; it can also reduce retained austenite if used carefully.
• Processing considerations:
• Control of austenitizing temperature, quench severity, and tempering schedule is critical to produce the required combination of hardness, uniformity, and polishability.
• Electro-slag remelting (ESR) or vacuum melting processes are often used for S136 and high-end NAK80 to ensure cleanliness and fine carbide distribution crucial for polish and corrosion properties.

4. Mechanical Properties

Interpretation: - Both grades can be heat treated to similar hardness ranges suitable for injection molds; however, subtle differences in alloying mean NAK80 variants can offer slightly higher toughness under some tempering conditions, while S136 is commonly used when corrosion resistance and surface finish are paramount. - The exact mechanical metrics depend on mill chemistry and the selected heat treatment schedule; consult supplier data sheets for specific tensile, yield, and impact values.

5. Weldability

• Factors affecting weldability: carbon equivalent, alloying elements that increase hardenability (Cr, Mo, V), and presence of nitrogen or carbide formers.
• Helpful formulas for assessing weldability (interpret qualitatively rather than numerically):
• $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
• $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
• Qualitative interpretation:
• Both NAK80 and S136 are stainless tool steels with alloying that increases hardenability relative to plain carbon steels; therefore preheat, interpass temperature control, and post-weld tempering are typically required to avoid cracking.
• Nitrogen in some NAK80 variants can complicate welding by altering solidification behavior and promoting porosity; S136 with high Cr and Mo can be prone to martensitic hardening in the heat-affected zone.
• For critical molds, welding is often minimized; when necessary, use matched filler metals designed for martensitic stainless tool steels and follow supplier-recommended preheat and post-weld heat treatment procedures.

6. Corrosion and Surface Protection

• For stainless mold steels, inherent corrosion resistance is derived from chromium (and sometimes Mo, N). Use of indices:
• Precipitation Resistance Equivalent Number (PREN) is useful for austenitic stainlesses but can be used to give directional insight: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Interpretation: PREN is most applicable to austenitic grades; for martensitic stainless mold steels the formula offers directional indication but is not a definitive predictor of service corrosion behavior.
• Non-stainless alternatives:
• If a non-stainless tool steel is selected, standard corrosion protections include galvanizing, painting, plating, or local coatings. For stainless mold steels like NAK80 and S136, surface passivity reduces the need for sacrificial protection in many molding environments.
• Practical notes:
• S136 is often chosen when long-term resistance to staining and mild corrosive environments is required, particularly for water-cooled molds or molds producing corrosive plastics or corrosive molded media.
• Surface finish and microstructural cleanliness (low non-metallic inclusions) are crucial for corrosion resistance and shine; ESR or vacuum-processed steels excel here.

7. Fabrication, Machinability, and Formability

• Machinability:
• Both grades in the annealed condition machine reasonably well; however, stainless martensitic tool steels can work-harden and demand appropriate tooling, feeds, and speeds.
• Carbide tooling and stable machine setups are recommended for high-precision machining.
• Formability:
• These grades are not typically used for extensive forming; bending is possible in annealed condition but requires attention to spring-back and edge cracking.
• Finishing:
• Polishing to mirror finish is a common requirement; S136 is often selected for its superior polishability when produced with clean microstructure and fine carbide size.
• Surface treatments (nitriding, chrome plating) are sometimes used depending on wear and corrosion needs; nitriding behavior depends on base chemistry (presence of N and diffusion behavior).

8. Typical Applications

Selection rationale: choose based on the dominant requirement—load and wear resistance versus maximum corrosion resistance and polishability.

9. Cost and Availability

• Cost:
• Both are specialty stainless tool steels and generally cost more than conventional mold steels (e.g., P20). Relative cost depends on market, mill, and processing (ESR/Vacuum melts increase cost).
• S136, especially when supplied as ESR or vacuum-refined material, is often at the higher end of the price spectrum relative to common stainless tool steels.
• NAK80 pricing depends on producer, nitrogen addition, and processing; it can be competitive with other stainless mold steels but exact cost must be checked with suppliers.
• Availability by product form:
• Both are available in block, plate, bar, and pre-hardened plates from major tool steel suppliers; lead times vary by mill and whether the product is ESR or specially treated.
• Global availability is generally good, but procuring large sections or specific processed conditions (pre-hardened & polished plates) may require longer lead times.

10. Summary and Recommendation

Recommendation: - Choose NAK80 if you need a stainless mold steel that provides a balance of corrosion resistance with improved mechanical strength and toughness for molds subject to higher loads, repeated mechanical stress, or where nitrogen-enhanced metallurgy offers benefits in performance. - Choose S136 if the priority is maximum resistance to staining, superior polishability, and a proven track record for high-gloss, corrosion-sensitive mold applications (for example, cosmetic, medical, or water-cooled molds where surface appearance and long-term corrosion resistance are critical).

Concluding note: NAK80 and S136 are both strong candidates for corrosion-resistant mold applications. The correct grade depends on the specific interplay of service environment (corrosive exposure, cooling media), required surface finish, load and wear conditions, and fabrication constraints. Always verify mill certificates, supplier heat-treatment recommendations, and conduct trial processing where critical performance or qualification is required.