Pressure Vessel Stainless Steel Plates: Grades, Welding and Corrosion Control
Leave a message
Plate Grades Used for Pressure Vessel Construction
Pressure vessel plate is ordered against a plate standard, not a tube or bar standard, and the certificate must state the plate specification, heat number and heat treatment condition. The grades below are the workhorses of vessel fabrication; all mechanical values are the minimum requirements of ASTM A240 / ASME SA-240 for plate in the solution-annealed condition.
| Grade | Composition (wt%) | Tensile min. | Yield min. | Elongation min. | Typical vessel duty |
|---|---|---|---|---|---|
| 304 | 18.0–20.0 Cr, 8.0–10.5 Ni | 515 MPa | 205 MPa | 40% | Chemical containers, storage tanks, condensers |
| 304L | 18.0–20.0 Cr, 8.0–12.0 Ni, C ≤ 0.030 | 485 MPa | 170 MPa | 40% | Welded vessels without post-weld heat treatment |
| 316 | 16.0–18.0 Cr, 10.0–14.0 Ni, 2.00–3.00 Mo | 515 MPa | 205 MPa | 40% | Chemical reactors, mixing vessels, process columns |
| 316L | As 316 with C ≤ 0.030 | 485 MPa | 170 MPa | 40% | Vessels in chloride-bearing or acidic service |
| 2205 duplex | 22.0–23.0 Cr, 4.5–6.5 Ni, 3.0–3.5 Mo, 0.14–0.20 N | 620 MPa | 450 MPa | 25% | High pressure, high chloride, weight-sensitive shells |
Design values come from the allowable stress tables of the applicable code, usually ASME BPVC Section VIII Division 1 with Section II Part D, or the European route through EN 10028-7. The plate specification and the design code are separate documents, so a plate that meets ASTM A240 does not by itself prove that a vessel built from it satisfies the design code.
Why 304L and 316L Are Specified Instead of Standard Grades
The low-carbon versions exist for one reason: welding. When an austenitic plate containing more than about 0.030% carbon is held between roughly 450 °C and 850 °C, during welding or in service, chromium carbides precipitate on the grain boundaries and leave the adjacent zones chromium-depleted. Those boundaries are then attacked preferentially in a corrosive medium, and the resulting intergranular corrosion can penetrate a full plate thickness while the surface still looks clean.
Capping carbon at 0.030% in 304L and 316L keeps the carbides in solution, so heavy sections can be welded without a subsequent solution anneal. Low-carbon filler metals such as ER308L and ER316L should be used even when the base plate is a standard-carbon grade, because weld metal solidifies with a cast structure that is more sensitive to carbide precipitation than wrought plate. Where a vessel runs continuously above roughly 400 °C in a corrosive stream, a stabilised grade such as 321 with titanium or 347 with niobium is the alternative, because those carbides form preferentially and leave the chromium in solution.
Duplex 2205 Plate: Higher Strength and Stress Corrosion Behaviour
Grade 2205 is a duplex stainless steel with roughly equal parts austenite and ferrite, and a composition of about 22% chromium, 3% molybdenum, 5.5% nickel and 0.18% nitrogen. Its minimum yield strength of 450 MPa is more than double the 170 MPa of 304L, which allows a thinner shell for the same design pressure and can offset part of the higher price per kilogram. It also resists chloride stress corrosion cracking far better than the austenitic grades and performs well against pitting and crevice attack in chloride-bearing streams.
Duplex is not a universal substitute. Code design normally limits it to a service range from about −50 °C to 600 °F (316 °C); above that range the ferrite phase embrittles and sigma phase forms, and toughness collapses. Fabrication is stricter as well: heat input must be controlled, interpass temperature is usually capped, and the weld must be balanced to give roughly 30% to 60% ferrite in the weld metal.
Welding Hot Cracking: Cause and Prevention
Austenitic stainless steel has low thermal conductivity and a high coefficient of linear expansion, so the weld pool is deep and narrow, the surrounding plate restrains it strongly, and the weld metal stays hot for a relatively long time. Under that combination the last liquid films between growing dendrites can be torn apart by contraction stresses, producing solidification cracks along the weld centreline and liquation cracks in the partially melted zone. These defects are often invisible from the surface and are found only by radiographic or ultrasonic examination.
Prevention follows three rules. Keep out the impurities that form low-melting phases, chiefly sulfur and phosphorus, which is why filler metal is normally purchased to limits near S ≤ 0.010% and P ≤ 0.020%. Promote a small amount of delta ferrite in the weld metal, typically 4% to 12% by ferrite number, because ferrite dissolves those elements and breaks up the continuous liquid films. Then control the process through moderate heat input, correct filler selection, tight fit-up and proper crater treatment. Matching fillers such as ER308L and ER316L normally deliver the required ferrite balance.
Intergranular Corrosion, Residual Stress and Post-Weld Care
Corrosion control does not end at the weld. Intergranular corrosion is prevented by low-carbon or stabilised base and filler metals as described above, and by a final solution anneal at roughly 1040 °C to 1120 °C with rapid cooling where the fabrication sequence allows it. Stress corrosion cracking arises from the combination of tensile stress, chlorides and elevated temperature; welding residual stress alone can be sufficient and the risk rises sharply above about 60 °C. Countermeasures include reducing restraint through welding sequence and peening, keeping insulation free of leachable chlorides, controlling chloride concentration in the process fluid, and selecting duplex 2205 where the environment is aggressive.
The surface must also be restored after fabrication. Weld discolouration and heat tint are chromium-depleted layers and should be removed mechanically or by pickling, followed by passivation so that a uniform passive film reforms. Contamination from carbon steel tools, grinding wheels or lifting gear must be avoided, because embedded iron particles become corrosion initiation sites.
Frequently Asked Questions About Pressure Vessel Stainless Steel Plates
Q: Should 304L or 316L plate be used for a vessel that will be welded?
Yes, in most cases. The low-carbon grades resist sensitisation and allow welding without a post-weld solution anneal, which is why they are the default choice for welded shells and heads.
Q: Is duplex 2205 suitable for all pressure vessel service?
No. It offers high strength and excellent chloride resistance between about −50 °C and 316 °C but loses toughness above that range and needs tighter welding control than austenitic plate.
Q: When is a stabilised grade such as 321 or 347 needed?
When the vessel operates continuously at elevated temperature in a corrosive stream, typically above about 400 °C, where carbon precipitation cannot be controlled by low carbon content alone.
Q: Does stainless steel plate require post-weld heat treatment?
Usually not. Austenitic grades are not stress relieved by ordinary thermal treatment; where residual stress must be reduced, a full solution anneal is used instead, following the applicable code.
Q: How can hot cracking in a stainless weld be detected?
By radiographic or ultrasonic examination of a sample or production weld. Surface inspection is not sufficient because these cracks run along the weld centreline inside the metal.
