Most buyers think Stainless Steel is just "Rust-Proof Metal."
But for engineers, Stainless Steel falls into two worlds: Soft (Austenitic) and Hard (Martensitic).
STS 410 and 420 belong to the latter. They trade some corrosion resistance for massive mechanical strength.
Today, we explore the science of Martensitic Transformation and the hidden danger of Hydrogen Embrittlement.
1. Martensitic Transformation: The Hardening Core
Unlike STS 304, which stays soft no matter how much you heat it, STS 410/420 are designed to respond to Heat Treatment (Quenching & Tempering).
- The Phase Change: When heated to ~1000°C and rapidly cooled, the internal structure shifts from Face-Centered Cubic (FCC) to a stressed Body-Centered Tetragonal (BCT) structure called Martensite.
- Magnetism: This structure is strongly Ferrimagnetic. (Yes, magnets stick to it!).
- Hardness Potential: Can reach 50+ HRC depending on the Carbon content.
2. STS 410 vs. STS 420: Selecting the Right Grade
Choosing between 410 and 420 comes down to one question: "Do you need Toughness or Sharpness?"
| Grade | Carbon (C) | Application & Characteristic |
| STS 410 | Low (~0.15%) | General Purpose. High toughness, moderate hardness. Used for fasteners, valve parts, and pump shafts. |
| STS 420 | High (0.15-0.40%) | Cutlery Grade. Extreme hardness for edge retention. Used for surgical instruments, knives, and plastic molds. |
3. The Trade-off: Surface Integrity vs. Corrosion
There is no free lunch in metallurgy.
As Carbon increases to boost hardness, it reacts with Chromium to form "Chromium Carbides." This reduces the free Chromium available to fight rust.
- The Risk: STS 420 has lower corrosion resistance than 304.
- The Fix: Proper Surface Passivation is vital to restore the protective oxide film after heat treatment. Regular maintenance is required.
4. Critical Danger: Hydrogen Embrittlement
This is the "Silent Killer" of high-strength steel.
During plating or pickling (acid cleaning), tiny Hydrogen atoms can penetrate the steel lattice.
⚠️ The Failure Mode:
The steel looks fine visually. But under stress, it snaps instantly without warning (Brittle Failure).
The Solution (Baking): We verify that all plated 410/420 parts undergo a "De-embrittlement Baking" process (usually 200°C for 4+ hours) immediately after plating to drive the hydrogen out.
5. Engineering Best Practices
To get the best out of Martensitic steel, follow these rules:
- Avoid Temper Embrittlement: Do not temper in the 400°C–600°C range, or impact toughness will drop drastically.
- Pre-heating for Welding: Essential to avoid cold cracking in the Heat-Affected Zone (HAZ).
- Sourcing Logic: Success relies on the Carbon-Chromium balance. We analyze specific carbon drift in each batch to predict precise HRC outcomes for our clients.
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