1. Introduction
Material selection in demanding industrial environments, including the aerospace and energy sectors, leaves no room for improvisation. The reliability of components directly depends on the resistance to corrosion, thermal and mechanical fatigue phenomena. This technical guide examines the selection criteria for austenitic stainless steels (304, 316) and duplex stainless steels for industrial maintenance equipment.
2. Fundamentals
Stainless steel is defined by a chromium content greater than 10.5%. Corrosion resistance comes from the passive layer of chromium oxide which reforms spontaneously in the presence of oxygen. The addition of nickel stabilizes the austenitic structure, improving ductility and weldability. The addition of molybdenum, particularly in grade 316, significantly increases resistance to pitting corrosion. Duplex steels combine an austenitic and ferritic structure (approximately 50/50), providing a higher yield strength and excellent resistance to stress corrosion cracking.
3. Technical specifications and standards
The classification of shades must comply with international standards to guarantee traceability and performance:
- NF EN 10088: Stainless steels.
- ASTM A240 / A276: Specifications for stainless steel sheets and bars.
- ISO 3651: Determination of resistance to intergranular corrosion.
The pitting resistance index (PREN - Pitting Resistance Equivalent Number) is a key technical criterion: PREN = %Cr + 3.3 x %Mo + 16 x %N.
4. Selection and Sizing Guide
The choice of grade is based on analysis of the corrosive environment, mechanical stresses and service temperatures.
| Shade | PREN (approx) | Yield limit (MPa) | Preferential use |
|---|---|---|---|
| 304L | 18-20 | 200-230 | Mildly corrosive environments, moderate temperatures |
| 316L | 23-26 | 200-240 | Chlorinated environments, chemical industry |
| Duplex 2205 | 34-36 | 450-500 | High pressure, SCC resistance, marine environment |
5. Installation and commissioning
Soldering is a critical step. For grades 304L/316L (low carbon), the risk of precipitation of chromium carbides at grain boundaries (sensitization) is minimized. For duplex, controlling the heat input is essential to maintain the ferrite/austenite phase balance. After welding, chemical stripping or glass bead blasting is necessary to restore the passive layer.
6. Failure Modes and Cause Analysis
Common failures include:
- Corrosion by pitting: Localized, linked to the presence of chlorides.
- Stress corrosion (SCC): Brittle cracking in a hot chlorinated environment. Grade 316 is sensitive above 60°C; duplex 2205 is superior in these conditions.
- Galvanic corrosion: Contact between metals with different electrochemical potentials.
7. Predictive maintenance and monitoring
Maintenance is based on:
- Control of electrochemical potential.
- Ultrasonic inspections to detect internal cracks.
- Chemical analyzes of process fluids to detect an increase in chloride ions.
8. Comparison Matrix
| Feature | 304L | 316L | Duplex 2205 |
|---|---|---|---|
| Corrosion resistance | Standard | High | Very high |
| Elastic limit | Standard | Standard | Double of 316L |
| SCC resistance | Low | Low | Excellent |
| Weldability | Excellent | Excellent | Requires control |
9. Conclusion
The choice of stainless steel grade is a critical engineering decision directly impacting the MTBF (Mean Time Between Failures) of equipment. The use of Duplex 2205 is recommended for aerospace and energy applications subject to high corrosion and mechanical stresses. UNITEC-D GmbH supports its partners in the rigorous selection of components that meet the most demanding standards.
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10. References
- AFNOR, NF EN 10088-1:2014, Stainless steels - List of stainless steels.
- ASTM International, ASTM A240/A240M-20, Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate.
- International Nickel Company, Corrosion Resistance of Stainless Steels in Industrial Environments.
