What Is ASTM A240 TP405 Plate?
Type 405, UNS S40500, is a ferritic stainless steel supplied as plate, sheet and strip under ASTM A240 / ASME SA240. Its defining feature is a deliberate aluminium addition, and that addition is what keeps the grade soft and ductile after welding and cooling instead of hardening like type 410. The difference sounds small on paper but it decides whether a welded vessel or heat exchanger can be fabricated without a post-weld heat treatment.
The grade contains no nickel and stays ferritic at all temperatures, so it is magnetic, it cannot be hardened by heat treatment, and it does not suffer the chloride stress corrosion cracking that attacks austenitic grades such as 304 and 316 when they are stressed in warm chloride-bearing environments.
Chemical Composition of 405 (UNS S40500)
| Element | Content, % |
|---|---|
| Chromium (Cr) | 11.5–14.5 (13 typical) |
| Aluminium (Al) | 0.10–0.30 |
| Carbon (C) | 0.080 max |
| Manganese (Mn) | 1.00 max |
| Silicon (Si) | 1.00 max |
| Phosphorus (P) | 0.040 max |
| Sulfur (S) | 0.030 max |
| Iron (Fe) | Balance (about 85) |
The chromium level gives useful resistance to oxidation and to mildly corrosive media, while the aluminium addition improves scaling resistance at elevated temperature and, just as importantly, suppresses the hardening that would otherwise follow welding.
Physical and Mechanical Properties
| Property | Metric | Imperial |
|---|---|---|
| Tensile strength | 448 MPa | 65 000 psi |
| Yield strength, 0.2 % offset | 276 MPa | 40 000 psi |
| Elongation in 50 mm | 25 % | 25 % |
| Elastic modulus | 200 GPa | 29 000 ksi |
| Poisson ratio | 0.27–0.30 | 0.27–0.30 |
| Izod impact at 20 °C | 41 J | 30.2 ft·lb |
| Hardness, Brinell | 131 | 131 |
| Hardness, Rockwell B | 75 | 75 |
| Hardness, Vickers | 140 | 140 |
| Density | 7.80 g/cm³ | 0.282 lb/in³ |
These are the values normally quoted for annealed plate. Because the grade is ferritic and not hardenable, the same figures apply after welding and after most elevated-temperature service exposures, which is one of the reasons designers accept its lower strength compared with the austenitic grades.
Why Type 405 Is Chosen for Welded and Hot Service
Freedom from weld hardening: the aluminium addition prevents the martensite formation that makes type 410 crack after cooling, so welded construction is straightforward.
Stress corrosion cracking resistance: a nickel-free ferritic structure is not susceptible to the chloride stress corrosion cracking that limits austenitic grades.
Elevated-temperature scaling resistance: the grade resists oxidation and scaling in hot gas service, which suits heat exchanger and ducting components.
Thermal stability: no phase change and no hardening on cooling means properties stay predictable through repeated thermal cycles.
The trade-off is corrosion resistance: 405 performs well in mild atmospheres, fresh water and mildly corrosive media, but it is not a substitute for a 316 or duplex grade in aggressive acid or high-chloride service.
Fabrication, Heat Treatment and Availability
Welding: 405 was developed to be welded. In most cases preheating and post-weld heat treatment are not required, which keeps fabrication simple.
Forming: good ductility in the annealed condition; ferritic grades work harden less than austenitic grades, so forming loads are moderate.
Machining: use sharp tooling and adequate lubrication, since ferritic stainless steels tend to gall.
Heat treatment: the grade cannot be strengthened by heat treatment; plate is supplied annealed and pickled or with a mill finish.
Ranges: thickness 0.25–200 mm, width 1000–3500 mm, length up to 12000 mm, in finishes 2B, 2D, BA, No. 1, No. 4, No. 8, mirror, checkered, embossed, hairline, sand blast and brush.
Documentation: mill test certificate to EN 10204 3.1, with heat number traceability.
Frequently Asked Questions
Q: What is the difference between type 405 and type 410?
Type 410 hardens when it cools after welding and can crack in the heat-affected zone, so it usually needs preheat and post-weld heat treatment. Type 405 contains an aluminium addition that prevents this hardening, so it can be welded and formed without those extra steps.
Q: Is 405 stainless steel magnetic?
Yes. The grade is ferritic and nickel-free, so it is magnetic in the annealed condition and remains magnetic after welding or service exposure.
Q: Can 405 plate be hardened by heat treatment?
No. Type 405 is a ferritic grade and does not respond to hardening or tempering. Its mechanical properties come from the annealed structure and from cold forming where higher strength is needed.
Q: When should 405 be chosen instead of 304 or 316?
When a welded component or an elevated-temperature part needs freedom from chloride stress corrosion cracking and does not face aggressive acids or heavy chlorides. Where broad corrosion resistance is the priority, an austenitic grade is the better choice.
Q: What equivalent designations does 405 have?
It is known as UNS S40500 in the unified numbering system and as DIN 1.4002 in European designation, and it is ordered to ASTM A240 / ASME SA240 for plate, sheet and strip.
