ASTM A240 sets the 0.2% offset minimum yield strength of type 304 stainless plate at 205 MPa (30 ksi) and of duplex 2205 (UNS S32205) at 450 MPa (65 ksi), a 2.2x ratio that directly drives plate-thickness calcs in pressure-vessel and heat-exchanger work [S4][S3].
The standard covers chromium and chromium-nickel stainless plate, sheet and strip used in pressure vessels, food processing, architecture, transport and medical service, and 304 plus 2205 are the two grades engineers compare most often when chloride resistance and weight both matter [S3].
Where the Numbers Come From: ASTM A240 Mechanical Property Floors
ASTM A240 specifies minimum tensile strength of 515 MPa for grade 304 plate, with a 205 MPa (30 ksi) 0.2% offset yield floor and a 40% minimum elongation in 2 in. (50 mm) for the plate product form [S4][S3]. Duplex 2205 plate under the same specification sets 655 MPa minimum tensile strength, 450 MPa (65 ksi) minimum 0.2% offset yield, and 25% minimum elongation, with maximum hardness 293 HB [S4]. For comparison, 316L plate under A240 carries roughly 25 ksi (≈170 MPa) minimum yield strength, which is why duplex 2205 is described as having about twice the yield strength of standard austenitic stainless [S2][S3]. Independent industry data places the typical 0.2% proof range at 450–550 MPa for 2205 versus about 240 MPa for 304, a 1.9 to 2.3x envelope that designers use as a sanity check against mill certs [S5].
Why the Gap Exists: Microstructure and Alloying
Type 304 is a fully austenitic (fcc) grade, with the A240 composition envelope typically at 18.0–20.0% Cr, 8.0–10.5% Ni, 2.0% Mn max, 0.08% C max, and nitrogen not intentionally added beyond roughly 0.10% [S4]. Duplex 2205 is a balanced two-phase alloy (about 50% ferrite, 50% austenite) containing 22.0–23.0% Cr, 4.5–6.5% Ni, 3.0–3.5% Mo, and 0.14–0.20% N, with carbon capped at 0.030% [S1][S4]. The nitrogen plus the dual-phase structure is what delivers the yield-strength jump, while the high Cr and Mo content drives pitting and chloride-stress-corrosion-cracking resistance to a PREN (pitting resistance equivalent number) of about 35 minimum versus a 24–26 typical range for 316 [S2][S4].
Side-by-Side Spec Comparison
The decision matrix below is built directly from A240 mechanical floors and the associated composition limits, so the trade-off is visible at a glance: [S4]
Minimum yield strength (0.2% offset): 304 = 205 MPa (30 ksi); 2205 = 450 MPa (65 ksi), ratio 2.2 to 1 [S4]. Minimum tensile strength: 304 = 515 MPa; 2205 = 655 MPa [S4]. Minimum elongation: 304 = 40% in 2 in. (plate); 2205 = 25% in 2 in. (plate) [S4]. Maximum hardness: 304 = 201 HB; 2205 = 293 HB [S4]. PREN (typical): 304 ≈ 18; 2205 ≈ 33–35 [S2]. Usable service temperature ceiling: 304 up to about 800°F (427°C) in non-stressed, oxidising service; 2205 capped near 600°F (315°C), with embrittlement risk in the 475–885°F (246–474°C) range on prolonged exposure [S1][S2]. Welding and forming: 304 is the easier grade to form and weld with no preheat and a wide heat-input window; 2205 needs controlled heat input, interpass limits and matching duplex filler to preserve the austenite-ferrite balance [S4]. Material cost ratio per pound typically runs roughly 1.8 to 2.5x higher for 2205 than 304, partially offset by the thinner sections the higher yield allows [S1].
Selection Criteria: Pick 2205 When, Pick 304 When
Specify 2205 (UNS S32205) under ASTM A240 when any of the following applies: chloride-containing process fluid above roughly 60°C, seawater or brackish service, pressure-vessel or heat-exchanger shells where wall-thickness reduction pays back the alloy premium, offshore platform and topside structural plate, or chloride-laden chemical and pulp-and-paper digesters [S1][S2][S4]. Specify 304 (or 304L for welded assemblies, dual-certified 304/304L is common) when the environment is atmospheric or mildly corrosive, when deep drawing or heavy forming is on the routing, when field welding has to be forgiving of variable heat input, or when the service temperature climbs above 600°F in a non-chloride, oxidising setting where 2205 embrittlement is a real risk [S3][S4]. The plate thickness saving at equal design stress is roughly a factor of 450/205 ≈ 2.2, which for pressure-vessel and pressure transmitter connection-flange calculations often lets the designer drop one or two nominal pipe schedules. For piping systems that tie into flow meter bodies and industrial valve bodies, the 2205 advantage also reduces flange class requirements at a given pressure rating.
Limits, Failure Modes and Spec Pitfalls
2205 is not a drop-in for 304: the 25% minimum elongation versus 40% for 304 means cold-forming operations (deep draws, heavy cold rolling) need larger bend radii and intermediate anneals, otherwise the dual-phase structure can crack at the ferrite islands [S4]. The 475°C embrittlement window means 2205 is unsuitable for sustained service near 280–475°C, so high-temperature furnace components, fired-heater tubes, and boiler parts stay in 304/304H/316H territory [S1][S4]. Welding 2205 outside the procedure-spec window (typical heat input roughly 0.5–1.5 kJ/mm, interpass below 150°C with matching 2209 filler) produces sigma phase, chi phase or excessive ferrite, all of which tank toughness and SCC resistance; a 304 weld procedure is not transferable [S4]. On the 304 side, expect chloride SCC above roughly 60°C in concentrated chloride brines, and pitting in stagnant seawater at ambient temperatures, both of which is what pushes most offshore specs toward 2205 [S4]. Finally, A240 plate is not the only product form to check: 2205 pipe is covered by ASTM A790 with a 450 MPa yield floor and 620 MPa tensile floor, and forgings by ASTM A182 F51/F60, so a pressure sensor or PLC rack fastener spec written only against A240 will miss the relevant product form for machined parts [S4].
Sourcing, Standards and Procurement Notes
ASTM A240 is the governing plate, sheet and strip specification; for pipe, A790 (2205) and A312 (304/316) carry the corresponding mechanical floors, and ASME SA240 is the pressure-vessel-code dual of A240 [S3][S4]. European dual is EN 10028-7 with the 1.4462 / X2CrNiMoN 22-5-3 designation for 2205, and 1.4301 / X5CrNi18-10 for 304 [S1]. Chinese practice maps 2205 to 022Cr23Ni5Mo3N under GB/T 24511 for plate [S4]. Plate thickness availability for 2205 commonly runs 4.8–152.4 mm (3/16″ to 6″) from major service centres, with standard widths 60″/79″/96″/120″ and typical lengths 240–252″, with select grades available above 300″ [S1]. Per ASTM A240, the manufacturer must report the 0.2% offset yield on the mill test certificate, and the value must be at or above the 205 MPa (304) or 450 MPa (2205) floor, so always cross-check the MTC against the A240 revision cited on the purchase order.
Track these signals next: the 2026 nickel and molybdenum price spread, which sets the 2205-to-304 cost ratio and therefore the breakeven plate thickness, and any ASTM A240 revision affecting 2205 nitrogen or PREN limits, which would shift the chloride service ceiling. For a deeper read on stainless procurement economics, see 304 Stainless Sheet Price per Kg in 2026: Cost Drivers, Ranges, Procurement, and for the forming and machinability trade-off that usually shows up alongside yield strength on the same drawing, see 303 vs 304 Stainless Steel: Machinability, Corrosion, Spec Pick.