Scope and Designation of P355QH
P355QH belongs to EN 10028-6, the part of the European pressure-purpose flat product series that deals with weldable fine-grain steels supplied in the quenched and tempered condition. The designation reads as follows: P for pressure purposes, 355 for the minimum yield strength in MPa, Q for the quenched and tempered delivery condition, and H for the impact quality tested at -20 °C. The related QL1 and QL2 qualities move the Charpy test temperature down to -40 °C and -60 °C for equipment operating in cold climates or in low-temperature process service.
Because the plate is quenched and tempered rather than normalized, the strength level is reached with a leaner chemistry and a finer, more uniform microstructure than a normalized grade of the same strength class, which improves the balance between strength and toughness and helps in heavier sections.
Chemical Composition
| Element, max % | C | Si | Mn | P | S | N | B | Mo | Cu | Nb | Ni | Ti | V | Zr |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| P355QH | 0.16 | 0.40 | 1.50 | 0.025 | 0.015 | 0.015 | 0.005 | 0.25 | 0.30 | 0.05 | 0.50 | 0.03 | 0.06 | 0.05 |
Carbon is capped at 0.16% to keep the quenched structure tough and to limit hardness in the heat-affected zone during welding. Manganese at up to 1.50% provides the base hardening response, while niobium, vanadium and titanium pin the austenite grain boundaries during rolling so that the quench produces a fine martensitic or bainitic structure. Boron at up to 0.005% improves hardenability at very low addition levels and is carefully controlled because small variations change the transformation behaviour. The carbon equivalent is calculated as CEV = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 and is reported for each heat.
Mechanical Properties and Impact Quality
| Thickness, mm | ReH min, MPa | Tensile, MPa | Elongation min, % | Impact quality |
|---|---|---|---|---|
| 6-50 | 355 | 490-630 | 22 | QH at -20 °C, 27 J min |
| >50-100 | 335 | 490-630 | 22 | QH at -20 °C, 27 J min |
| >100-150 | 315 | 450-590 | 22 | QH at -20 °C, 27 J min |
Yield strength falls gently with thickness, from 355 MPa in the 6-50 mm band to 315 MPa above 100 mm, because the cooling rate at the mid-thickness of a heavy plate is slower and the transformation product is coarser. Impact testing is carried out on Charpy V-notch specimens for every heat and every thickness band; the QH quality is tested at -20 °C with a minimum absorbed energy of 27 J, the QL1 quality at -40 °C and the QL2 quality at -60 °C. All three share the same strength requirement, so the choice between them is made on the minimum design metal temperature rather than on strength.
Applications
P355QH is used for boiler drums and shells, steam accumulators, heat exchangers, reactor vessels, storage tanks and high-pressure piping components in oil, gas, petrochemical and power generation plants. It is also selected for hydroelectric penstocks, gas storage spheres and pulp digesters. The combination of a 355 MPa yield with toughness tested well below freezing makes it suitable for equipment installed outdoors in cold regions and for vessels that must withstand a rapid depressurization event.
The grade works well where wall thickness has to be limited: compared with a normalized steel of the same strength, the quenched and tempered structure allows heavier sections to pass the same toughness requirement, so designers can avoid the cost of a thicker normalized wall or of a stress-relief treatment that would otherwise be needed.
Supply Range, Testing and Fabrication
Standard production covers thickness from 6 mm to 300 mm, width from 1500 mm to 4050 mm and length from 3000 mm to 15000 mm, with cut-to-length and profile-cut plates available. Supplementary requirements frequently ordered with this grade include ultrasonic examination to EN 10160, thickness-direction testing to EN 10164 in Z15, Z25 and Z35 classes, hydrogen-induced cracking testing and compliance with NACE MR0175 for sour service, and third-party witness testing.
Welding a quenched and tempered pressure steel requires a qualified procedure with low-hydrogen consumables, controlled heat input and a preheat based on the plate thickness and the certified carbon equivalent. Post-weld heat treatment, where the design code requires it, is performed at a temperature that keeps the tempered structure stable and the impact properties within specification, and the cooling rate after treatment is controlled for the same reason. Forming is carried out cold for normal radii; hot forming above the tempering temperature is not permitted because it destroys the quenched and tempered condition.
Every consignment is documented with an inspection certificate covering heat analysis, product analysis, tensile properties, impact energy at the ordered test temperature and dimensional inspection, together with non-destructive examination reports where ordered. Plate identification carries the grade, heat number, thickness and the impact quality letter so that material can be traced from goods-in to final assembly.
Frequently Asked Questions
Q: What does the H in P355QH mean?
A: H identifies the impact quality tested at -20 °C with a minimum absorbed energy of 27 J; the QL1 and QL2 qualities are tested at -40 °C and -60 °C.
Q: Is P355QH a normalized steel?
A: No. It is delivered quenched and tempered, which is why it reaches 355 MPa with a leaner chemistry and better toughness than a normalized grade of the same strength class.
Q: How thick can P355QH be supplied?
A: EN 10028-6 covers up to 300 mm; production commonly runs from 6 mm to 300 mm with the 6-50 mm band carrying the full 355 MPa minimum yield.
Q: Can P355QH be hot formed?
A: Only within limits that preserve the quenched and tempered condition. Forming above the tempering temperature requires re-quenching and tempering, which is normally done by the plate producer.
Q: What is the difference between P355QH and P355NH?
A: Both are EN 10028 pressure steels, but P355NH is normalized under EN 10028-3 while P355QH is quenched and tempered under EN 10028-6.
Q: Is impact testing mandatory for P355QH?
A: Yes. Charpy V-notch testing at the temperature associated with the ordered quality is a requirement of the specification.
