Three Plate Families in One Supply Programme
Industrial fabrication usually needs three different plate families at the same time, and confusing them is the most common source of rejected material. Structural plate carries the frame and is ordered to a constructional steel standard. Pressure vessel plate contains or separates fluids and is ordered to a pressure equipment standard with a normalised delivery condition. Wear plate protects surfaces exposed to sliding abrasion and is ordered against a hardness specification rather than a yield strength specification.
The structural group is represented by Q355B to GB/T 1591-2018 and by P265GH to EN 10028-2 in lower stress service. The pressure group is represented by P355GH to EN 10028-2. The wear group is represented by NM400 and NM500 to GB/T 24186-2009. Each family has its own acceptance tests, so the material certificate must always be read against the standard named on the order.
Structural and Pressure Vessel Grades
| Grade | Standard | ReH min (MPa) | Tensile strength (MPa) | Delivery condition |
|---|---|---|---|---|
| Q355B | GB/T 1591-2018 | 355 up to 16 mm | 470 - 630 | As rolled or TMCP |
| P355GH | EN 10028-2 | 355 up to 16 mm | 510 - 650 | Normalised |
| P265GH | EN 10028-2 | 265 up to 16 mm | 410 - 530 | Normalised |
Q355B is the workhorse structural plate for welded frames, supports and general fabrication, and the B quality designation carries a room temperature impact requirement. The C, D and E quality designations in the same standard add progressively lower impact test temperatures for cold climate and dynamic service.
P355GH and P265GH are normalized pressure vessel steels intended for elevated temperature service, which is why their tensile ranges differ from those of structural grades of similar yield strength. Normalising refines the grain structure and produces consistent properties through the thickness, and the plate is supplied with test results taken from the product as required by the pressure equipment specification.
Abrasion Resistant Grades NM400 and NM500
Abrasion resistant plate is specified by Brinell hardness, since wear life in chutes, hoppers and truck bodies correlates with surface hardness rather than with tensile strength.
| Grade | Standard | Hardness range | Typical service |
|---|---|---|---|
| NM400 | GB/T 24186-2009 | 370 - 430 HBW | Chute liners, hopper walls, buckets |
| NM500 | GB/T 24186-2009 | 470 - 530 HBW | High wear points, crusher feed areas |
These plates are supplied quenched and tempered, and the hardness is verified on the plate surface or at a defined depth below it according to the purchase agreement. Higher hardness improves wear life but reduces formability and weldability, so NM500 is normally used only at the points of maximum abrasion while the surrounding structure is built from NM400 or from structural plate.
Common thickness for wear plate is 6 mm to 60 mm. Bending is carried out with generous radii, and cutting is done by plasma, laser or waterjet; thermal cutting leaves a hardened edge that must be removed by grinding before welding if the joint is highly restrained.
Welding and Fabrication Rules
Structural and pressure vessel grades are welded under procedures qualified to ISO 15614-1, with consumables matched to the parent metal strength. On P355GH and P265GH, welding is followed by any heat treatment required by the pressure equipment design, and the procedure includes the post weld heat treatment condition if applicable.
Wear plate requires a low hydrogen approach because the quenched and tempered microstructure is sensitive to hydrogen cracking. Typical controls are preheating of the joint area, use of low hydrogen consumables, and limiting heat input to avoid softening of the heat affected zone. Where the wear plate is welded to a structural grade, the procedure is qualified for the dissimilar combination, and the heat input is set by the more sensitive material.
Applications by Grade Group
Q355B plate is used for welded frames, equipment bases, bridge components and general industrial structures. P355GH and P265GH plate are used for pressure vessel shells, heads, storage tanks and heat exchanger components working at elevated temperature. NM400 and NM500 plate are used for mining chutes, transfer points, dump truck bodies, loader buckets, crusher liners and material handling ducts.
Corrosion protection is selected separately from the plate grade. Structural plate is usually blast cleaned and primed or painted, pressure vessel plate is normally supplied bare with a protective primer that is compatible with the subsequent welding and heat treatment, and wear plate is frequently supplied bare because a coating would be removed by the abrasive service.
Frequently Asked Questions
Q: Can Q355B replace P355GH in a pressure vessel?
A: No. The two grades have similar yield strength but P355GH is a normalised pressure equipment steel tested and certified to EN 10028-2, with a different tensile range and elevated temperature property data. Pressure equipment must be built from steel approved under the pressure equipment specification.
Q: What does the hardness number in NM400 mean?
A: The number denotes the nominal Brinell hardness level of the grade, with NM400 specified in a band of 370 to 430 HBW under GB/T 24186-2009. It is not a tensile strength value and should not be converted into a design stress for structural calculations.
Q: Is NM500 harder to weld than NM400?
A: Yes. The higher hardness of NM500 comes from a more highly tempered martensitic structure with lower ductility, so it needs more careful preheat control, strict low hydrogen consumables and a lower heat input limit. NM400 is generally the practical choice for joints that must be welded in the field.
Q: What thickness tolerance applies to hot rolled plate?
A: Thickness tolerance follows the order standard, and for structural plate it is usually stated as a class in the material specification or in the dimensional standard quoted on the order. Plates for pressure service are checked against the tolerance class agreed with the fabricator before rolling starts.
Q: How should wear plate be cut and formed?
A: Plasma or waterjet cutting is preferred for edge quality, and forming should use generous bend radii and be done at a controlled temperature. Any hardened cut edge should be ground back before welding when the joint is restrained, because a hard untempered edge increases the risk of cracking.
Q: Which documents accompany a plate shipment?
A: Each plate is supplied with a certificate showing the standard, grade, heat number, chemical composition, mechanical results and heat treatment condition. For pressure equipment steel the certificate is normally issued to EN 10204 3.1 so that the notified body can verify traceability.