P355GH is a fine-grained non-alloy pressure vessel steel standardized under EN 10028-2, specifically engineered for high-temperature service in demanding pressure equipment. The number "355" denotes a minimum yield strength of 355 MPa at room temperature, which is significantly higher than P265GH, enabling thinner walls and lighter weight constructions for the same design pressure.
Microstructure and Mechanical Properties
P355GH is supplied in the normalized condition, with a heat treatment temperature typically between 880°C and 960°C. This produces a homogeneous ferrite-pearlite microstructure with a fine grain size (ASTM grain size number 7 or finer). The fine-grained structure enhances both strength and toughness. Typical chemical composition limits are: carbon ≤0.20%, manganese 1.10–1.70%, silicon ≤0.60%, phosphorus ≤0.025%, sulfur ≤0.010%. Additional trace elements such as aluminum and nitrogen are controlled to promote grain refinement. At room temperature, the tensile strength ranges from 460 to 610 MPa, with minimum impact energy values of 27 J at 0°C (for thicknesses up to 16 mm). The elevated temperature yield strength values are specified up to 450°C. For example, at 350°C the minimum yield strength is 195 MPa, and at 400°C it is 185 MPa. This superior high-temperature strength allows P355GH to be used in thicker-walled boilers and high-pressure vessels operating at steam temperatures up to 420°C.
Fabrication and Welding
Despite its higher strength, P355GH retains good weldability provided that appropriate procedures are followed. Preheating is recommended for plates thicker than 20 mm or when ambient temperatures fall below 5°C, typically in the range of 50–150°C depending on thickness and restraint level. Low-hydrogen welding consumables are mandatory to prevent hydrogen-induced cracking. PWHT is usually required for wall thicknesses exceeding 30 mm to relieve residual stresses. The PWHT temperature range is 580–620°C with a holding time of 1 hour per 25 mm of thickness. When welded correctly, P355GH joints can achieve full strength matching and retain adequate toughness in the heat-affected zone. Applications include boiler drums, steam accumulators, heat exchanger shells, and high-pressure gas storage tanks operating at elevated temperatures. For service conditions involving both high pressure and temperature, P355GH represents a cost-effective alternative to Cr-Mo alloy steels, offering ease of procurement and fabrication.
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