S136 vs 420 – Composition, Heat Treatment, Properties, and Applications
Share
Table Of Content
Table Of Content
Introduction
Selecting between S136 and 420 is a recurring decision for engineers, procurement managers, and manufacturing planners working on mold components, precision parts, or corrosion-exposed hardware. The choice often balances corrosion resistance and surface finish against cost and ease of fabrication — and it is typically driven by part function, expected service environment, and required life-cycle cost.
At a high level, both S136 and 420 are martensitic stainless steels used where a combination of hardness and some corrosion resistance is needed, but they are engineered with different priorities. The most consequential practical difference is that S136 is a stainless mold steel optimized for enhanced resistance to surface corrosion and superior polishability, while 420 is a general-purpose martensitic stainless steel with broader availability and lower cost. These differences influence alloying strategy, heat treatment response, surface finishing, and selection criteria in industry.
1. Standards and Designations
- S136: Commercially supplied as a stainless mold steel (commonly referenced by mold-steel suppliers and OEMs). It is typically specified for tooling and mold inserts where corrosion resistance and polishability are required. It is a martensitic stainless tool steel in the family of stainless mold steels rather than a conventional carbon tool steel.
- 420: Designated by AISI/SAE as AISI 420 / UNS S42000; it is a martensitic stainless steel in ASTM and many international standards. It is used broadly as a stainless tool and engineering steel.
Typical classification by type: - S136 — stainless martensitic mold/tool steel (tool steel family with stainless behavior). - 420 — martensitic stainless steel (general-purpose stainless alloy; often used in engineering and cutlery).
(Exact standard numbers and commercial designations can vary by supplier; always cross-check supplier material certificates for specific orders.)
2. Chemical Composition and Alloying Strategy
The following table presents the common alloying elements of interest and qualitative presence in each grade. Exact concentrations vary by supplier and product form; consult mill certificates for procurement.
| Element | S136 (typical, qualitative) | 420 (typical, qualitative) |
|---|---|---|
| C (Carbon) | Medium — tailored to achieve hardenability and surface hardness while enabling polishability | Medium — wide commercial range; controls final hardness and strength |
| Mn (Manganese) | Low–moderate (kept controlled to limit retained austenite and impurities) | Low–moderate (deoxidizer, affects hardenability) |
| Si (Silicon) | Low (deoxidation) | Low (deoxidation) |
| P (Phosphorus) | Trace / low (kept low for toughness and corrosion resistance) | Trace / low |
| S (Sulfur) | Very low (minimized to improve polishability and corrosion resistance) | Often higher than S136 in older grades (improves machinability, reduces corrosion resistance) |
| Cr (Chromium) | Relatively high (to provide stainless behavior and corrosion resistance) | High (12–14% classically; provides stainless behavior) |
| Ni (Nickel) | Low (may be present in trace amounts) | Low (usually low to trace) |
| Mo (Molybdenum) | Very low or absent (some mold grades deliberately control Mo to balance corrosion and toughness) | Often low or absent (unless specified) |
| V (Vanadium) | Low–moderate (if present, for wear resistance and grain refinement) | Low (may be present in some variants) |
| Nb, Ti, B | Typically controlled or absent (stabilizers and microalloying controlled to improve polishability and properties) | Usually absent or in trace amounts |
| N (Nitrogen) | Low (avoided in many variants because of effects on corrosion and toughness) | Low (typically) |
How alloying affects performance (high level) - Chromium: primary element for stainless behavior. Higher and well-distributed Cr with low sulfur promotes resistance to surface corrosion and pitting. - Carbon: raises achievable hardness and wear resistance after quenching & tempering but increases martensite hardenability and susceptibility to hard-zone cracking and reduced corrosion resistance if combined with high impurity levels. - Sulfur and manganese: higher S improves machinability but degrades polishability and corrosion resistance; S136 keeps S very low for mirror finishes. - Microalloying elements (V, Nb, Ti): added in small quantities to refine carbides and improve wear/toughness balance; their presence is tightly controlled in mold steels to preserve surface finish.
3. Microstructure and Heat Treatment Response
Typical microstructures - Both S136 and 420 are intended to form martensite after appropriate quenching. In the annealed condition they contain ferrite/pearlite or annealed martensitic structures depending on processing. The as-quenched microstructure is martensite plus retained austenite and carbides; tempering reduces hardness and stabilizes the microstructure.
Heat-treatment behaviors and considerations - S136: supplied frequently in vacuum-annealed or pre-hardened condition with a focus on cleanliness and controlled carbide distribution. It accepts standard quenching and tempering cycles to reach target hardness while preserving corrosion resistance. Because S136 is specified for high-quality surface finish, vacuum or controlled atmosphere heat treatment is common to limit decarburization and surface oxides. - 420: responds predictably to conventional hardening (austenitize → quench → temper). 420’s heat treatment is flexible and can be optimized for higher toughness or higher hardness depending on tempering temperature. Atmospheric heat treatments are commonly used in general engineering shops.
Normalizing, quenching & tempering, and thermo-mechanical processing - Normalizing refines grain size in both steels and is useful before machining operations. - Quenching & tempering sets the hardness–toughness balance. S136 often uses more conservative austenitizing and tempering cycles combined with vacuum treatment to preserve corrosion behavior. - Thermo-mechanical processing (rolling and controlled cooling) is more relevant to bar/plate production; both steels’ final properties are determined largely by subsequent heat treatment.
4. Mechanical Properties
Because both grades are heat-treatment sensitive, absolute numbers depend on supplier, heat-treatment schedule, and product form. The table below gives a qualitative comparative view of typical mechanical tendencies rather than fixed numerical values.
| Property | S136 | 420 |
|---|---|---|
| Tensile strength | High when fully hardened (designed for high surface hardness) | High when fully hardened (similar achievable ranges depending on C) |
| Yield strength | High after tempering to typical mold hardnesses | Comparable; varies with tempering |
| Elongation (ductility) | Moderate — tends toward lower elongation at higher hardness | Moderate — can be tailored; some 420 variants give better ductility at lower hardness |
| Impact toughness | Moderate to low at very high hardness (mold steels often sacrifice some toughness for hardness and surface quality) | Moderate — can be slightly higher than S136 at equivalent hardness depending on carbon and processing |
| Hardness (hardened) | High achievable hardness and sustained surface hardness after polishing | High achievable hardness; wide commercial range depending on carbon |
Interpretation - Both steels can be hardened to similar hardness levels usable for tooling and wear-resistant parts; final mechanical balance depends on carbon content and tempering. S136’s manufacturing and processing emphasis is on producing a clean microstructure and surface finish, which supports wear resistance and corrosion resistance but may be specified with slightly different carbon and tempering choices than generic 420 grades.
5. Weldability
Weldability of martensitic stainless steels is challenging relative to austenitic grades because of carbon and alloy content that promote hard, brittle martensite in heat-affected zones (HAZ). Two widely used weldability indices:
-
Carbon equivalent (IIW): $$CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}$$
-
Equivalent Pcm (older BE) formula: $$P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000}$$
Qualitative interpretation - Higher $CE_{IIW}$ or $P_{cm}$ values imply greater risk of HAZ cracking and need for preheat/post-weld heat-treatment. Both S136 and 420 typically require careful welding procedures: preheating, low heat input, and post-weld tempering or stress-relief are common practices. - S136 may be somewhat less forgiving if the grade has higher carbon or more tightly controlled cleanliness (to avoid sensitization and preserve surface finish). Conversely, some 420 variants formulated for general engineering may include sulfide inclusions and higher Mn that facilitate shop welding but compromise corrosion resistance. - For critical assemblies, welding should be qualified with procedure specifications (PQR/WPS) and trials; brazing or mechanical fastening may be preferable for high-integrity mold surfaces.
6. Corrosion and Surface Protection
- For martensitic stainless steels with chromium in the 12–14% range, corrosion resistance is moderate — sufficient for many indoor, non-aggressive environments and for injection molding of many polymers. S136 is engineered for higher surface corrosion resistance and mirror polishability by controlling sulfur, non-metallic inclusions, and surface decarburization.
- For severe environments, neither S136 nor 420 matches the pitting resistance or general corrosion resistance of austenitic (304/316) or duplex stainlesses. When assessing localized corrosion resistance, the Pitting Resistance Equivalent Number (PREN) is useful: $$\text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N}$$
- For typical S136 and 420 chemistries (low Mo and low N), PREN values are modest; thus S136 achieves better practical surface corrosion resistance largely through cleanliness and optimized Cr distribution rather than high PREN by Mo or N additions.
Surface protection options for non-ideal scenarios - If part service requires additional protection: electroless nickel plating, PVD coatings, nitriding (where compatible with corrosion and hardness goals), or polymer coatings can be used. For general corrosion protection of 420 (if chosen for cost reasons), standard coatings (galvanize is not typical for stainless parts) and paints or passivation treatments are options.
7. Fabrication, Machinability, and Formability
- Machinability: 420 in its annealed or free-machining variants is generally easier to machine than highly hardened S136. S136 is often supplied in a pre-hardened or vacuum-annealed state; machining should be done in softer condition and followed by final heat treatment and finish grinding/polishing.
- Grinding and polishing: S136 is optimized for mirror-polishability; its low sulfur and inclusion control produce superior surface finishes with fewer surface defects. 420 can be polished to high shine but may produce more surface features due to inclusions.
- Formability/bending: Both are limited in cold formability when hardened; forming should be performed in annealed condition.
- Surface finishing: S136’s processing encourages final electro-polishing or mechanical polishing for optical or medical molds; 420 can be finished but often requires more correction of surface defects.
8. Typical Applications
| S136 (typical uses) | 420 (typical uses) |
|---|---|
| High-gloss injection-molding dies and cores (plastics, optical parts) | Cutlery, blades, and general-purpose knives |
| Corrosion-resistant mold inserts for medical or food-contact parts | Shafting, valve components, and general tooling |
| Molds for medical devices and precision components that demand mirror finishes | Simple molds, fixtures, and hand tools where cost is a factor |
| Components where surface quality and resistance to mild corrosive media are key | Pump parts, bearings, and components requiring stainless behavior at lower cost |
Selection rationale - Choose S136 for molds and tooling where surface finish, resistance to corrosion from process fluids or cleaning agents, and long-term dimensional stability in polished condition are priorities. - Choose 420 when cost sensitivity, broad availability, and general-purpose stainless behavior are more important than optimized polishability and specialized corrosion resistance.
9. Cost and Availability
- 420: widely available, produced by many mills worldwide in bars, plate, sheet, and forgings. Generally lower unit cost than specialized mold steels because of large production volumes and multiple suppliers.
- S136: a specialty stainless mold steel typically available through tool-steel distributors and selected mills. Cost is higher per kilogram due to additional processing (e.g., vacuum melting, inclusion control) and more limited production runs. Availability in standard tool-stock sizes is good but may be less ubiquitous than 420 in commodity forms.
10. Summary and Recommendation
Summary table (qualitative)
| Attribute | S136 | 420 |
|---|---|---|
| Weldability | Moderate to challenging; requires qualified procedures | Moderate to challenging; dependent on C and variant |
| Strength–Toughness (balance) | High hardness with moderate toughness (optimized for surface integrity) | Comparable hardness achievable; can be tailored for more toughness |
| Cost | Higher (specialty mold steel) | Lower (commodity martensitic stainless) |
Choose S136 if... - You need a mold or tooling material that delivers superior surface finish, mirror polishability, and improved resistance to surface corrosion in aggressive cleaning or polymer-processing environments. S136 is the preferred choice for high-value injection molds, medical-device tooling, and applications where surface defects and corrosion pits are unacceptable.
Choose 420 if... - You need a broadly available, lower-cost martensitic stainless steel for general tooling, cutlery, shafts, or parts where extreme polishability and optimized corrosion resistance are not primary requirements. Use 420 when you require flexibility in heat treatment and wide supplier choice.
Final note - Both grades are heat-treatment sensitive and require specification of exact supplier composition, product form, and intended heat-treatment cycles at procurement. For critical parts, request mill certificates, specify surface condition (e.g., vacuum-annealed, pre-hardened), and qualify welding and finishing processes before production.