Weldability of P355NH
P355NH is a normalized, weldable fine-grain structural steel specified in EN 10028-3, material number 1.0565, designed for pressure vessels, boilers, and piping operating at elevated temperatures up to about 400 degrees C. It is explicitly engineered to be weldable, and with correct procedures it produces sound joints that match the base material properties.
Chemical Composition and Weldability
Heat analysis limits are carbon 0.18 percent, silicon 0.50 percent, manganese 1.10 to 1.70 percent, phosphorus 0.025 percent, sulfur 0.010 percent, chromium 0.30 percent, nickel 0.50 percent, molybdenum 0.08 percent, vanadium 0.10 percent, titanium 0.03 percent, copper 0.30 percent, niobium 0.05 percent, nitrogen 0.012 percent, and aluminum 0.020 percent minimum. The maximum carbon equivalent is 0.43 percent and the maximum Pcm is 0.27 percent, which is why the steel welds without difficulty in thin and medium sections.
Mechanical Properties
The minimum yield strength is 355 MPa for thicknesses up to 16 mm, 345 MPa for 16 to 40 mm, 335 MPa for 40 to 63 mm, 325 MPa for 63 to 80 mm, 315 MPa for 80 to 100 mm, and 295 MPa for 100 to 150 mm. Tensile strength is 490 to 630 MPa up to 100 mm and 470 to 610 MPa above, with a minimum elongation of 22 percent and a guaranteed impact energy of 40 J at minus 20 degrees C longitudinal.
Welding Procedures
P355NH welds by SMAW, GMAW, GTAW, SAW, and FCAW. Preheating of 50 to 150 degrees C is normally applied depending on thickness, restraint, and ambient temperature, and interpass temperature is controlled to avoid overheating. Post-weld heat treatment is not mandatory for most applications but is applied where required by the design code, typically at 550 to 650 degrees C, to relieve residual stress.
Transformation Temperatures
Typical transformation temperatures are Ms 432 degrees C, Ac1 717 degrees C, and Ac3 847 degrees C. These values guide preheat and post-weld heat treatment selection and help avoid hardening of the heat-affected zone.
Applications
P355NH is used for reactors, heat exchangers, separators, spherical tanks, boiler drums, and pressure-bearing components in petrochemical, power generation, energy storage, marine, pharmaceutical, and food equipment. Its guaranteed toughness at minus 20 degrees C supports cold-region and low-temperature storage applications.
Frequently Asked Questions
Q: Is P355NH easy to weld?
A: Yes, with a carbon equivalent of 0.43 percent maximum and standard processes.
Q: What preheat is required for P355NH?
A: Typically 50 to 150 degrees C depending on thickness and restraint.
Q: Is post-weld heat treatment mandatory?
A: Not for most applications; it is applied where the design code requires stress relief.
Q: What is the yield strength of P355NH?
A: 355 MPa minimum for thicknesses up to 16 mm.
Q: What is the difference between P355NH and P355N?
A: P355NH requires impact testing at minus 20 degrees C, P355N at plus 20 degrees C.
Q: What is the impact energy of P355NH at minus 20 degrees C?
A: 40 J minimum longitudinal.
Q: What applications use P355NH?
A: Pressure vessels, boilers, heat exchangers, reactors, and storage tanks.
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