Introduction:
A mold can fail long before it reaches the end of its design life. Sometimes the surface loses gloss unexpectedly; other times, corrosion appears after only a few production runs with PVC or flame-retardant resins. In many cases, the root cause traces back to one early decision: steel selection.
1.2316 tool steel, S136H tool steel, and M300 tool steel are often grouped together as “corrosion-resistant mold steels,” yet they serve very different engineering purposes. While they share a stainless steel base, their microstructure—and consequently their processing route—varies significantly:
- 1.2316 (Standard): The cost-effective workhorse for corrosive plastics like PVC, prioritizing machinability over optical finish.
- S136H (ESR Grade): The industry standard for high-polish optical parts, supplied pre-hardened to eliminate heat treatment risks.
- M300 (Premium): The high-performance choice for maximum wear resistance. Unlike the others, it is typically hardened after machining to achieve 50+ HRC for long-run production.
This article compares these three grades from a manufacturing perspective—analyzing chemistry, polishability, and heat treatment protocols—to clarify where each performs reliably and where their limitations begin.
1. Chemical Composition Comparison
| Element | 1.2316 steel (DIN X38CrMo17) |
S136H steel (Modified AISI 420) |
M300 steel (High-Performance) |
|---|---|---|---|
| Carbon (C) | 0.33 – 0.45% | 0.36 – 0.40% | 0.38% |
| Silicon (Si) | ≤ 1.00% | 0.80% | 0.40% |
| Manganese (Mn) | ≤ 1.50% | 0.50% | 0.65% |
| Phosphorus (P) | ≤ 0.030% | ≤ 0.025% | ≤ 0.020% |
| Sulfur (S) (Impurity) |
≤ 0.030% (Standard) |
≤ 0.005% (Precision ESR) |
≤ 0.002% (High Purity) |
| Chromium (Cr) | 15.5 – 17.5% | 13.0 – 14.0% | 16.00% |
| Molybdenum (Mo) | 0.80 – 1.30% | ~ 0.30% | 1.00% |
| Nickel (Ni) | ≤ 1.00% | 0.80% (Optional) | 0.80% |
| Vanadium (V) | — | 0.30% | — |
Note on Specifications: “S136H” means premium modified AISI 420 grades (ESR quality). Important: Don’t confuse mold steel “M300” with “Maraging M300” (1.2709 steel). The latter is a nickel-cobalt alloy for 3D printing. The M300 here is a corrosion-resistant plastic mold steel.
2. Physical and Mechanical Properties
Hardness ranges tell part of the story. Mold performance in real use depends on seven key physical and mechanical traits. These steels balance them differently.
Hardness Levels Across Processing States
| Material | Annealed Hardness | Prehardened Hardness | Maximum Hardness (After Heat Treatment) | Application Notes |
|---|---|---|---|---|
| 1.2316 | ≤235–250 HB | 28–32 HRC | 44–47 HRC | Most plastic molding applications; no extra hardening needed |
| S136H | ≤229 HB (modified) | 33–37 HRC | 38–45 HRC | High-precision optical molds and medical molds; tighter working range |
| M300 | N/A (Ships prehardened) | 48–50 HRC; Also offered pre-tempered to 28–32 HRC equivalent |
N/A (No heat treatment required) | Immediate use for tooling; heat treatment step skipped, saving 3–5 days |
Corrosion Performance Rankings
Chromium content controls anti-corrosion behavior. 1.2316 handles PVC processing better than standard 1.2083. Aggressive plastics? No problem. Field tests show minimal pitting after 10,000+ cycles with acidic resins.
M300 matches 1.2316’s Cr plus Mo. Molybdenum boosts resistance to chloride attacks. Halogenated polymers won’t damage this steel.
S136H sits lower Cr (SKD11 equivalent) . Corrosion resistance stays “high” but doesn’t match 1.2316 levels. Use it for standard plastics—ABS, PP, PE. Avoid aggressive chemical environments.
3. Polishability & Surface Finishing Behavior
Surface quality affects production rejects and processing costs. 1.2316 vs S136H vs M300 polish differently primarily due to their purity levels (inclusions) and carbide size. This helps you pick the right steel for your mold’s finish needs.
Technical Comparison
| Item | 1.2316 (Standard) | S136H (ESR) | M300 (Premium) |
|---|---|---|---|
| Achievable Polish | 6,000–8,000 mesh (Semi-Mirror) |
#12,000–#14,000 mesh (High Optical Mirror) |
#10,000+ mesh (High Gloss) |
| Limiting Factor | Sulphur inclusions (Risk of pitting) |
None (High purity structure) |
Polishing time (Due to extreme hardness) |
| Polishing Hardness | 30–34 HRC (Pre-hardened) |
34–38 HRC (Pre-hardened) |
50–54 HRC (Post-Hardening) |
| Polishing Rate | Fast / Easy | Medium | Slow (+40% time) |
| EDM Recast Layer | Easy to remove | Consistent removal | Thin, hard layer |
| Orange Peel Risk | Medium to High (If over-polished) |
Very Low | Low |
| Ideal Application | Opaque parts, Pipe fittings | Camera lenses, Clear lighting | High-wear precision parts |
Engineering Selection Notes
- S136H (The Optical Standard)
When mirror finish quality is the primary requirement (Ra ≤0.01 μm), S136H remains the most reliable option. Because it is produced via Electro-Slag Remelting (ESR), it lacks the sulfide inclusions found in standard 1.2316. This purity allows you to polish to a lens-grade finish without encountering “pinholes” or drag lines. - 1.2316 (The Economical Compromise)
1.2316 offers a balanced combination of polishability and cost. While it does not reach the extreme mirror levels of S136H due to its chemical additives (sulfur) that aid machining, its uniform hardness enables consistent results for standard gloss surfaces. It is perfect for large molds where hand-polishing time needs to be minimized. - M300 (The Durable Gloss)
Polishing M300 takes patience. Since it is finished at ~52 HRC, material removal is slower. However, the payoff is a surface that stays polished. High hardness prevents the “micro-scratching” that often occurs during mold ejection, ensuring the gloss level lasts for millions of cycles. Best for high-volume closures and medical parts.
4. Machinability and Processing Guidelines
Machinability is not just about cutting speed; it is about chip control, tool wear, and the stability of the steel after material removal. Since 1.2316 and S136H are typically machined in a pre-hardened state, while Premium M300 follows a rough-harden-finish workflow, their processing strategies differ fundamentally.
Key Machining Characteristics at a Glance
| Steel Grade | Typical Machining State | Machinability Rating | Main Challenge | Dimensional Stability |
|---|---|---|---|---|
| 1.2316 (Std) | Pre-hardened (30–34 HRC) |
High (Free-machining additives often present) |
Lower polishability due to Sulfur content | Moderate |
| S136H (ESR) | Pre-hardened (32–36 HRC) |
Medium (Clean steel = tougher chips) |
Abrasive wear on tools; requires coated carbide | Very Good (Uniform structure) |
| M300 (Prem) | Step 1: Annealed (~230 HB) Step 2: Hardened (50+ HRC) |
Step 1: Variable (Sticky) Step 2: Low (Grinding/EDM only) |
Risk of galling in soft state; strict heat treat control needed | Excellent (Stress relieved via tempering) |
Engineer’s Selection Notes
Correction: High-purity steels (Like S136H and M300) are generally “stickier” to machine than standard grades because they lack the sulfide inclusions that help break chips.
Processing 1.2316 (The “Fast” Option):
If your priority is cycle time reduction in the CNC department, standard 1.2316 is the pragmatic choice. The sulfur content (up to 0.030%) acts as a chip breaker, allowing for higher feed rates and reduced tool wear. However, this machinability comes at a cost: those same sulfides prevent a true mirror finish.
Processing S136H (The “Balanced” Option):
S136H is harder and “gummier” than 1.2316 due to its ESR cleanliness. Tools may experience more abrasive wear, and chip evacuation requires high-pressure coolant to prevent built-up edge (BUE). However, since it is supplied pre-hardened, you eliminate the risks associated with heat treatment (cracking, warping) entirely. It is the comprehensive “safe bet” for precision molds.
Processing M300 (The “Precision” Option):
Using Premium M300 requires a different mindset. You are not looking for speed; you are looking for long-term performance.
1. Roughing: Done in the annealed state. Caution is needed as soft, high-chrome steel tends to gall.
2. heat Treatment: Vacuum hardening is mandatory to protect the surface.
3. Finishing: Hard milling or EDM is performed at 50+ HRC. This yields the highest dimensional accuracy but increases machining costs by 20–30% compared to pre-hardened steels.
5. Heat Treatment Protocols
Heat treatment is the differentiating factor. While standard 1.2316 and S136H are often supplied pre-hardened to save time, Premium M300 is typically supplied annealed to allow for custom hardening, unlocking its full potential regarding wear resistance and stability.
Heat Treatment & Processing Quick Reference
| Process Step | 1.2316 (Standard) | S136H | M300 (Premium) |
|---|---|---|---|
| Delivery State | Pre-hardened (30–34 HRC) |
Pre-hardened (32–36 HRC) |
Annealed (~230 HB) |
| Hardening Temp | N/A (Usually used as received) |
N/A (Use as supplied) |
1000°C – 1050°C (Vacuum or Oil Quench) |
| Tempering | N/A | Stress relieve if needed | 500°C – 600°C (Double Tempering Required) |
| Target Hardness | 30–34 HRC | 32–36 HRC | 48 – 52 HRC (High Wear Resistance) |
| Dimensional Stability | Moderate (Risk of machining stress) |
Good (No heat treat distortion) |
Excellent (If properly tempered) |
*Note: Temperature ranges are indicative. “Aging” is not applicable to these stainless grades; do not confuse with Maraging steel protocols.
6. Key Applications
Your mold usage decides the steel choice. Check how these three grades handle specific production settings:
- Medical & Food Packaging: Choose S136H. It resists sterilization chemicals and strict hygiene rules. You get contamination-free surfaces. This delivers the mirror-grade cleanliness needed for medical devices and food containers.
- High-Volume Connectors & Caps: M300 fits best. Although it requires an extra step of vacuum hardening (taking 3–5 days), the resulting 52 HRC hardness provides the necessary toughness to keep complex core pins stable for over a million cycles.
- PVC & Corrosive Plastics: 1.2316 is the answer. It fights off acids from harsh plastics. It stops pitting in pipe fittings better than standard P20 steel.
- Optical & High-Gloss Parts: S136H works best here. You get ESR-refined purity for a flawless #14000 polish. It matches perfectly with automotive lenses and clear cosmetic packaging.
FAQ‘s
1. If I need a true mirror finish (Lens Quality), which steel is mandatory?
S136H (ESR) is the only reliable choice here. While M300 can achieve a high gloss, S136H is specifically refined via Electro-Slag Remelting to remove microscopic impurities. For optical lenses or transparent housings where Ra ≤0.01 μm is required, the purity of S136H prevents the “pinhole” defects that often appear in standard 1.2316.
2. Can 1.2316 replace S136H to reduce tooling costs?
Yes, but only for non-optical parts. 1.2316 is an excellent alternative for opaque parts like pipe fittings, bottle caps, or automotive internals. It offers similar corrosion resistance to S136H but at a lower material cost and with better machinability. However, do not use it for clear parts, as its sulfur content limits the polish quality.
3. Which steel is easier to finish after EDM processing?
1.2316 generally allows for faster removal of the EDM recast layer (white layer) due to its softer matrix and free-machining additives. S136H, being cleaner and harder, requires more disciplined polishing steps (oil stones followed by diamond paste) to ensure the surface remains flat and free of “orange peel.”
4. Does higher hardness (M300) automatically mean better surface quality?
Not necessarily. While harder steel (50+ HRC) holds a polish longer during production, it is significantly harder to polish initially. M300 takes about 30–40% longer to bench than S136H. The benefit of M300 isn’t a better initial finish, but a more durable one that resists scratching from glass-filled nylons or abrasive ejector movements.
5. When does M300 make more sense than pre-hardened stainless steels?
Choose Premium M300 for high-volume, abrasive applications running over 1 million cycles. Since it is heat-treated to ~52 HRC (compared to ~34 HRC for S136H), it resists gate erosion and parting-line wear much better. It is the ideal upgrade when standard stainless molds are wearing out prematureley, justifying the extra cost of heat treatment and hard machining.
Conclusion
Ultimately, selecting the right grade among 1.2316, S136H, and M300 isn’t about finding a superior metal, but aligning the steel’s microstructure with your specific failure risks—whether that means fighting PVC corrosion, ensuring optical clarity, or withstanding abrasive wear. While 1.2316 and S136H offer the production speed and safety of pre-hardened materials, upgrading to a heat-treated grade like M300 represents a strategic shift towards long-term performance.
As the molding industry moves toward more aggressive resins and tighter tolerances, the focus must shift from the initial price per kilogram to the total cost of ownership. The smartest engineering decision is rarely the cheapest one upfront; it is the one that safeguards your tooling investment against the expensive downtime of tomorrow.