Heat Treatment of Stainless Steels
- redesiht
- Sep 4
- 3 min read
Stainless steels are the workhorses of industries as diverse as food processing, medical, chemical, energy, and automotive. Three reasons explain why: corrosion resistance, durability in aggressive environments, and the ability to maintain mechanical performance over the long term.
What is a Stainless Steel?
At the core, a stainless steel is an iron–carbon alloy, like any other steel, but with the addition of chromium and, in many cases, nickel. These two elements make the difference.
Nickel brings chemical stability and structural balance.
Chromium is the true hero: when exposed to oxygen, it spontaneously forms a thin layer of chromium oxide (Cr₂O₃). This “good oxide” acts like a shield, sealing the surface and protecting it against air, water, or salt.
👉 To form this passive film, stainless steels must contain at least 12% chromium by weight.
Types of Stainless Steels
Depending on alloying content, stainless steels fall into five main families:
Martensitic
High strength and hardness, lower ductility and toughness. The only stainless steels that can be hardened by martensite. Used in cutting tools, valves, turbines, shafts.
Ferritic
Low carbon, no nickel. Not hardenable, used in annealed or cold-worked condition. Medium strength with moderate corrosion resistance. Typical in appliances, exhaust systems, heat exchangers.
Austenitic
High chromium and nickel, low carbon (especially in “L” grades). Not hardenable by quenching, but strength improves by cold working. Excellent corrosion resistance plus high ductility and toughness. Used in food industry, medical devices, marine environments.
Duplex
Mixed ferritic–austenitic structure, with chromium, nickel, molybdenum, and nitrogen. Not hardenable, but outstanding against localized corrosion (pitting, stress corrosion cracking). Used in chemical plants, desalination units, subsea pipelines.
Precipitation-Hardening (PH)
Chromium and nickel with copper, aluminum, or niobium. Supplied annealed and hardened by aging. Ideal for aerospace, injection molds, precision structural components.
Mechanical Properties Compared
Hardness, ductility, dimensional stability, and corrosion resistance all vary with stainless steel type.
Heat Treatment of Stainless Steels: Conventional Routes
Each stainless steel family responds differently to heat treatment stainless steels, which modifies mechanical properties:
TREATMENT | OBJECTIVE | TYPICAL APPLICATION |
|---|---|---|
Quenching + tempering | Increase hardness, adjust toughness | Only martensitic stainless steels |
Aging (precipitation hardening) | Form stable hard phases | PH stainless steels |
Annealing | Soften, relieve internal stresses | Ferritic, austenitic, duplex |
Cold working | Raise strength by plastic deformation | Especially austenitic grades |
The Headache: High Temperatures
Here lies the trap: stainless steels suffer when exposed to excessive heat.
Decarburization
Carbon escapes from the surface, lowering hardness.
Grain growth & distortion
Reduced toughness, internal stresses, geometric changes.
Chromium carbide precipitation (sensitization)
Chromium ties up with carbon at grain boundaries, depleting the passive film and killing corrosion resistance.
Nitriding: The Alternative
When the goal is surface hardness without losing corrosion resistance, conventional heat treatment of stainless steels isn’t enough. The real answer lies in nitriding.
Unlike quenching-based methods, nitriding enriches the surface with nitrogen atoms, creating a hardened layer that boosts wear resistance while leaving the bulk untouched.
Plasma Nitriding: What Makes It Different
Plasma nitriding stands apart because it solves the long-standing dilemma: hardness vs. corrosion resistance.
Temperature control
The process works in the low-temperature ferritic range (350–450 °C / 662–842 °F), below the degradation threshold of the chromium oxide passive film. This avoids unwanted phase transformations, keeps chromium in solid solution, and preserves dimensional stability.
Optimized sputtering with argon
Through sputtering, a mix of hydrogen, nitrogen, and argon gases cleans the surface and opens it for nitrogen diffusion. The result is not a coating, but a nitrogen-enriched layer known as expanded austenite, or S-phase. This phase delivers high hardness, low friction, and intact corrosion resistance.
Oxygen-free process
Plasma nitriding also operates in a vacuum oxygen-free environment, avoiding unwanted oxides and ensuring consistent, reproducible results.
Double-Duty Layer
In practice, the treated steel becomes a “stainless stack”: a tough, undistorted core; a hardened layer by S-phase; and a regenerated passive film of chromium oxide.
Conclusion
Heat treatment of stainless steels has always been a challenge: conventional routes can improve hardness but risk losing corrosion resistance. Plasma nitriding changes the equation—delivering hardness, wear resistance, and dimensional stability while keeping the passive shield intact.
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