What P355NL2 Is and Which Standard Covers It
P355NL2 is a weldable fine grain steel plate for pressure purposes specified in EN 10028-3, the part of the European flat product standard that covers normalized steels. The designation is built from four elements: P for pressure purposes, 355 for the minimum yield strength in MPa at the thinnest end of the thickness range, N for normalized delivery, and L2 for the low-temperature impact class, which is tested at -50 °C. The same standard defines the sister grades P355N for room temperature, P355NH for elevated temperature and P355NL1 for the -40 °C class, so ordering must always state the complete designation rather than the base grade alone.
Chemical Composition and Mechanical Properties
Fine grain practice is achieved through aluminium or micro-alloying additions, and the phosphorus and sulphur limits are tighter than for non-alloy structural plate because the steel must retain toughness at low temperature.
| Element | Requirement |
|---|---|
| C | 0.18 % max |
| Si | 0.50 % max |
| Mn | 0.90-1.65 % |
| P | 0.025 % max |
| S | 0.015 % max |
| Al (total) | 0.020 % min |
| Nb + Ti + V | 0.15 % max in total |
| Yield strength (min, up to 16 mm) | 355 MPa |
| Yield strength (min, over 60 to 100 mm) | 325 MPa |
| Tensile strength (up to 60 mm) | 490-630 MPa |
| Impact energy at -50 °C | 27 J min, transverse |
Minimum yield strength steps down with thickness: 355 MPa up to 16 mm, 345 MPa over 16 to 40 mm, 335 MPa over 40 to 60 mm, 325 MPa over 60 to 100 mm and 315 MPa over 100 to 150 mm. Tensile strength follows the same pattern, falling from the 490-630 MPa band to about 460-600 MPa above 100 mm.
Delivery Condition and Supplementary Options
P355NL2 is delivered in the normalized condition, and the normalized state is what the mechanical and impact values on the certificate refer to. Where a design requires improved through-thickness behaviour for welded connections subject to lamellar tearing, the plate can be ordered with a Z-direction quality designation under EN 10164, such as Z25 or Z35, which is verified by a through-thickness tensile test. Plates are commonly supplied from 6 mm to 150 mm, and thicker normalized plate is produced to controlled rolling plus normalizing schedules to hold the fine grain structure.
Applications
The grade is selected for pressure equipment in cold service: ethylene and propylene storage vessels, LPG and ammonia tanks, CO2 and nitrogen storage, cold boxes and cryogenic separation columns, low-temperature heat exchangers and their channels, compressor casings, and process vessels in LNG and petrochemical plants where the design metal temperature falls in the -40 °C to -50 °C band. It is also used for equipment that must meet European pressure equipment requirements and is therefore specified in EN designation rather than in national grade terms.
Welding and Fabrication Notes
The steel is designed for welding, but the fine grain structure must be protected. Heat input and interpass temperature are limited so that the heat-affected zone does not coarsen, and welding procedures are qualified with impact testing at -50 °C for both the weld metal and the heat-affected zone. Low-hydrogen consumables matched to the strength class are used, and preheat of 100-150 °C is applied for heavier sections. Post-weld heat treatment above about 600 °C softens the normalized structure and lowers the yield strength, so where the design code demands stress relief, the temperature and holding time must be agreed with the mill and with the design authority before the order is placed. Cold forming is straightforward for normal shell radii; hot forming above the tempering range destroys the normalized condition and requires re-normalizing, which is rarely practical for large fabricated components. Forming strains should be recorded so that the residual ductility allowance is confirmed against the design code.
Quality Control and Ordering Points
Ordering data should state the full grade designation, thickness and tolerance class, delivery condition, impact test temperature and energy, any Z-direction quality requirement, the ultrasonic examination class to EN 10160, and the surface condition acceptance level. Certification is issued to EN 10204 type 3.1 with heat number and plate identification traceable to the marking; type 3.2 with third-party witnessing is arranged for projects where the notified body requires it. Plates can be supplied cut to size, bevelled or rolled, with dimensional and flatness records issued with the shipment.
Frequently Asked Questions
Q: What does L2 mean in P355NL2?
A: It is the low-temperature impact class. The plate is impact tested at -50 °C with a minimum absorbed energy of 27 J in the transverse direction.
Q: Is P355NL2 the same as P355N?
A: No. P355N is tested for room temperature service, P355NH for elevated temperature service, and P355NL2 for low temperature. The mechanical property basis and the impact test temperature differ.
Q: Why is aluminium controlled in the specification?
A: Aluminium is the grain refining element used for fine grain practice. A minimum total aluminium content ensures the fine grain structure that gives the steel its low-temperature toughness.
Q: What thickness range is available?
A: Production is normally offered from 6 mm to 150 mm in the normalized condition, with the minimum yield strength stepping down as thickness increases.
Q: Can the plate be ordered with through-thickness properties?
A: Yes. A Z-direction designation under EN 10164, such as Z25 or Z35, is added at the order stage and verified by a through-thickness tensile test.
Q: When should post-weld heat treatment be avoided?
A: When the design code does not require it, because heating above roughly 600 °C softens the normalized structure. If stress relief is mandatory, the cycle must be agreed with the mill in advance.
