S136 vs 420 – Composition, Heat Treatment, Properties, and Applications

Introduction

S136 and 420 are both martensitic stainless steels frequently considered for components where hardness, polishability, and some corrosion resistance are required. Engineers, procurement managers, and manufacturing planners commonly face the decision between a specialized mold-grade stainless (S136) and a general-purpose martensitic stainless (420) when balancing corrosion resistance, surface finish, cost, and manufacturability.

The primary practical difference between the two centers on their alloying and processing intent: S136 is formulated and processed for high surface finish, low inclusion content, and improved corrosion resistance for mold applications, while 420 is a broader-family martensitic stainless used where straightforward hardenable stainless properties are acceptable at lower cost. These similarities in family (martensitic stainless) but differences in composition control and processing underpin the common comparison.

1. Standards and Designations

• S136
• Not a single international ASTM/ISO canonical number; it is a commercial mold steel designation used by European and international tool steel suppliers (often listed in mold steel catalogs).
• Classified as a corrosion-resistant stainless tool/mold steel (martensitic stainless, tool steel family).
• Often supplied as pre-hardened plate, blocks, or polished mold components and referenced in tool-steel supplier standards (proprietary designations).
• 420
• Standardized in many specifications as AISI/SAE 420 and in ASTM product standards (for example, ASTM A276 for stainless bar/rod).
• Also appears under EN and JIS as martensitic stainless grades with similar chemistry.
• Classified as a martensitic stainless steel (hardenable stainless).

Classification summary: - S136: Stainless tool steel (martensitic), tool/mold grade, commercial designation. - 420: Stainless steel, martensitic, standardized AISI/ASTM grade.

2. Chemical Composition and Alloying Strategy

Notes: - S136 is produced and specified with tighter control of impurities (P, S) and nonmetallic inclusions; its alloying strategy emphasizes a fine, uniform distribution of carbides and low segregation to optimize polishability and consistent corrosion resistance across the surface. - 420 uses a simpler alloy balance to provide economical hardenability and corrosion resistance; wide range of carbon contents in the 420 family yields different subgrades (e.g., 420A, 420B, 420C variants historically differ by C).

3. Microstructure and Heat Treatment Response

• Typical microstructures:
• Both grades are martensitic stainless steels after appropriate quench. The microstructure consists primarily of martensite with varying amounts and morphologies of chromium carbides.
• S136: produced and heat-treated to yield a fine, homogeneous martensite with a low level of large primary carbides and controlled secondary carbide distribution. Powder metallurgy or refined melting practices may be used by suppliers to reduce segregation, yielding superior surface finish after polishing.

• 420: wrought or conventionally melted; martensite matrix with chromium carbides of coarser distribution relative to highly controlled mold steels. Carbide population depends strongly on carbon content and thermal history.

• Heat treatment response:

• Both grades respond to austenitizing, quenching, and tempering. Hardness, strength, and toughness are strongly a function of carbon content, austenitizing temperature, quench media, and tempering regimen.
• S136 is commonly supplied in a pre-hardened and surface-finished condition; it accepts sub-zero or cryogenic treatments and tempering cycles optimized to retain corrosion resistance and minimize distortion for tooling.

• 420 is heat-treated to a broad range of hardnesses for different applications: lower-carbon variants for machining and moderate hardness; higher-carbon variants for higher hardness after quench and tempering.

• Normalizing / thermo-mechanical processing:

• S136 benefits from controlled normalization and potentially sub-zero treatment to reduce retained austenite and refine carbides.
• 420 may be normalized/annealed to improve machinability prior to final hardening.

4. Mechanical Properties

Explanation: - S136 typically achieves high surface hardness and surface integrity with carefully controlled carbides, which improves wear resistance and polishability but can reduce bulk toughness if pushed to extreme hardness. - 420’s properties vary more with carbon grade. Lower-carbon 420 is more ductile and tougher at modest hardness; higher-carbon 420 approaches S136 in hardness but may show larger carbides and less predictable surface finish.

5. Weldability

Weldability depends on carbon content, hardenability (Cr and alloying), and impurity/microalloying content. Two common empirical indices are helpful for qualitative interpretation:

• International Institute of Welding carbon equivalent: $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$

• Dearden and O’Neill-type parameter ($P_{cm}$): $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$

Qualitative interpretation: - Higher carbon and higher chromium increase hardenability and risk of cracking in the heat-affected zone (HAZ). S136, because it is optimized for high hardness and often higher effective carbon and fine carbide populations, will generally be more challenging to weld than low‑carbon 420 variants. - 420 weldability varies across subgrades: lower-carbon 420 is relatively more weldable; higher-carbon 420 and S136 require preheat, controlled interpass temperature, and post-weld heat treatment to avoid HAZ cracking and excessive martensite formation. - Additionally, S136’s stringent cleanliness and low impurity strategy means that welding can locally alter surface condition, potentially degrading corrosion resistance near welds unless welded and post-treated carefully.

6. Corrosion and Surface Protection

• For stainless martensitics, general corrosion resistance is primarily from chromium content and surface condition. Neither S136 nor 420 offer the pitting resistance of austenitic or duplex stainless steels when exposed to chloride-rich environments.
• PREN (Pitting Resistance Equivalent Number) is commonly used for assessing pitting resistance in stainless grades: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
• Application of PREN:
• For both S136 and 420, PREN values are modest because Mo and N contents are low; S136’s cleaner surface and controlled chemistry often yield better local corrosion performance than generic 420 after polishing and in less aggressive media.
• Non-stainless alternatives / protection:
• When increased corrosion protection is required for either grade, consider surface treatments: electroless nickel, hard chrome plating (noting environmental/regulatory constraints), nitriding (for wear, limited corrosion benefit), passivation, or barrier coatings (paints, polymer coatings).
• For environmental resistance beyond what these martensitics provide, consider austenitic stainlesss, duplex, or Ni-base alloys.

7. Fabrication, Machinability, and Formability

• Machinability:
• Lower-carbon 420 variants are easier to machine in annealed condition. S136, due to higher carbon and hardenable nature and potential high hardness as supplied, can be more difficult to machine; many S136 plates are supplied pre-hardened and may require EDM or grinding rather than conventional machining.
• Formability:
• Both alloys are limited in cold formability when hardened. In annealed condition, 420 can be formed reasonably well; S136’s formability depends on supplier temper but is primarily targeted at mold fabrication rather than sheet forming.
• Surface finishing:
• S136 is optimized for polishing and high-gloss surface finishes, with attention to low inclusion content and fine carbide dispersion.
• 420 can be polished but often produces a less consistent mirror finish compared to S136 unless specially processed.

8. Typical Applications

Selection rationale: - Choose S136 where surface finish, polish retention, and consistent corrosion resistance in mold cavities are paramount. - Choose 420 for cost-effective hardened stainless parts where superb polishability is less critical.

9. Cost and Availability

• 420: Widely available worldwide in bars, rod, plate, and sheet; generally lower cost due to standardized metallurgy and broad supply base.
• S136: Specialized mold steel available from tool-steel suppliers and distributors; typically higher cost per kg and sometimes longer lead times for large dimensions or specific surface finishes. S136 is often offered in pre-hardened and pre-polished formats which add value for tooling applications.

10. Summary and Recommendation

Conclusions and recommendations: - Choose S136 if: - Your application requires mirror-quality polish, consistent surface integrity, and improved localized corrosion resistance for tooling or molds. - You accept higher material cost and potentially more restrictive fabrication (EDM, grinding, specialized welding/post-weld treatment). - Choose 420 if: - You need a cost-effective, hardenable stainless steel for general components where excellent polishability and top-tier mold corrosion resistance are not required. - You need broad availability in many product forms and easier conventional machining (for lower-carbon variants).

Final note: Both grades must be selected with attention to the specific subgrade, supplier processing (e.g., pre-hardened vs. annealed), and the intended heat treatment schedule. For critical tooling or high-surface-finish components consult the steel supplier for heat-treatment recipes, delivery condition, and weld/repair recommendations tailored to the chosen alloy variant.