Pressure Vessel Plate Selection: Carbon, P355GH and Cr-Mo Grades

Vessel plate choice follows service temperature and environment: carbon steel for the ordinary range, C-Mn for elevated temperature, Cr-Mo where creep and hydrogen rule.

Start with the service window

Pressure vessel plate selection is driven by design temperature range, pressure, and fluid chemistry, under codes such as ASME Section VIII or EN 13445. Three plate families cover most duty.

Carbon steel: A516 Grade 70

The workhorse. A516 Gr.70 (tensile 485–620 MPa) is fine-grained, normally supplied normalised for thicker or lower-temperature work, and covers ambient and moderate temperatures. Low-temperature toughness is not automatic: impact testing is a supplementary requirement that must be ordered — vessels for cold service are commonly specified with Charpy tests well below 0 °C (often around −46 °C in LPG-type practice). HIC-tested variants are specified for wet H₂S (sour) service.

Elevated temperature: P355GH

EN 10028-2 grade P355GH is a normalised carbon-manganese steel with specified elevated-temperature proof strength, intended for boilers and vessels running hotter than plain carbon practice allows. It fills the band between ordinary A516-type duty and alloy territory, roughly the few-hundred-°C range where creep data begin to govern design.

High temperature and hydrogen: A387 Cr-Mo

Chromium-molybdenum plates — A387 Grade 11 (1.25Cr-0.5Mo) and Grade 22 (2.25Cr-1Mo) — serve where carbon steel loses strength or is attacked:

Creep service in refinery and power equipment at temperatures beyond C-Mn capability.
Hydrogen service, where chromium ties up carbon and resists high-temperature hydrogen attack (selection follows the Nelson-curve/API RP 941 logic).

Cr-Mo plate is supplied normalised and tempered, and welded fabrications almost always receive post-weld heat treatment (PWHT) — also common for thick carbon-steel vessels to relieve stress and restore toughness.

Cross-cutting requirements

Whatever the family: confirm impact-test temperature against the minimum design metal temperature, check whether PWHT is in the material plan (it can shift properties), and treat the plate specification, delivery condition and test extras as one package — the grade name alone buys none of them.

Toughness, thickness and MDMT

The governing low-temperature figure is the minimum design metal temperature (MDMT), and codes link allowable materials to it through toughness curves and thickness: thicker plate needs better demonstrated toughness at the same temperature, because thick sections stress in triaxial constraint and cool welds more slowly. This is why two vessels in "A516 Gr.70" can legitimately carry different impact-test temperatures on their data sheets — the test temperature is an engineered choice, documented on the mill certificate, not a property baked into the grade name.

A practical ordering checklist

Grade and delivery condition (as-rolled, normalised, or normalised-and-tempered).
Impact test temperature and average/single-value criteria, if the service needs them.
Simulated-PWHT test coupons where the vessel will be post-weld heat treated, so the certified properties reflect the finished vessel, not the as-shipped plate.
Ultrasonic examination class for thick or critical plates, and HIC/SSC testing for sour service.
The construction code edition the certificates must reference — ASME and EN paperwork are not interchangeable, even for near-equivalent grades.