Grade and Standard Basis
Q355GNH is a weathering structural steel defined in GB/T 4172, the standard for atmospheric corrosion resistant steel for welded structures. In the designation, Q marks structural steel, 355 is the minimum yield strength in MPa, G indicates the higher level of corrosion resistance within the family and NH confirms that the grade is weathering and intended for welded construction. The base material for the pipe is also covered by GB/T 1591, the general standard for high strength low alloy structural steels, so the pipe combines a controlled steel composition with a pipe manufacturing standard.
For export projects the grade is commonly cross-referenced to EN 10210 for hot finished structural hollow sections and to ASTM A847 for cold-formed welded and seamless high-strength low-alloy structural tubing with improved atmospheric corrosion resistance.
Specification Range of Q355GNH Circular Pipe
| Parameter | Range or description |
|---|---|
| Material grade | Q355GNH to GB/T 4172 with base steel to GB/T 1591 |
| Manufacturing route | Seamless, ERW, LSAW and SSAW welded pipe |
| Outside diameter | 21.3 mm to 2020 mm |
| Wall thickness | 1.5 mm to 25 mm |
| Length | 6 m, 9 m, 12 m or cut to order, up to 18 m |
| Tolerances | Outside diameter plus or minus 0.5 mm, wall thickness plus or minus 10 percent, length plus or minus 0.5 percent |
| End finish | Plain end, bevelled end or threaded with couplings |
| Surface | Pre-weathered, blast cleaned or oiled |
Mechanical Properties and Corrosion Performance
The grade delivers a minimum yield strength of 355 MPa, a tensile strength of 470 to 630 MPa and a minimum elongation of 20 percent, with impact energy of at least 34 J at room temperature for the standard supply class. Those values are high enough for the pipe to carry structural loads in exposed conditions while remaining ductile enough for erection tolerances, field welding and cold bending.
Corrosion behaviour is the reason the grade is specified. After the surface has gone through repeated wet and dry cycles, the alloying elements in Q355GNH concentrate in the oxide layer and produce a compact patina that restricts further attack. Comparative exposure trials and service records for Q355GNH report a corrosion rate several times lower than structural carbon steel, with a corresponding extension of outdoor service life. That advantage depends on detailing: the pipe must drain, dry and avoid permanent moisture traps.
Manufacturing Routes and Their Selection
Seamless pipe is produced by piercing and rolling a solid billet and is preferred for smaller diameters, higher pressures and where no longitudinal weld is acceptable.
ERW pipe is formed from coil and welded by high-frequency induction, giving good dimensional accuracy and wall uniformity at economic cost for general structural duty.
LSAW pipe is formed from plate into a cylindrical shape and welded with a longitudinal submerged arc seam, suited to larger diameters and heavier walls.
SSAW pipe uses a helical submerged arc seam and is used for the largest diameters in piling and pipeline applications.
The route is selected from diameter, wall thickness, pressure class and the inspection regime the project requires, and it is stated on the mill certificate because the weld type affects both the test programme and the acceptance criteria.
Inspection, Testing and Delivery
Every length is checked dimensionally for outside diameter, wall thickness, ovality, straightness and length before dispatch. Mechanical testing verifies yield strength, tensile strength, elongation and impact energy on samples taken from the pipe body. Welded pipe receives weld integrity examination, and where an ultrasonic examination class is specified, full body or weld scanning is carried out to the applicable standard. Surface condition is agreed in advance, because pre-weathered pipe is supplied deliberately oxidised, and the buyer should know in advance whether the pipe arrives in a bright, oiled or partially rusted state so that handling and painting plans can be prepared. Pipe ends are protected, and bundles are secured and marked with heat number, size and grade for traceability at site.
Frequently Asked Questions
Q: What is Q355GNH pipe used for?
A: Exposed structural work such as bridge railings, canopies, transmission structures, architectural frameworks, sign gantries and outdoor pipe supports where corrosion resistance and structural strength are both required.
Q: What does the G in Q355GNH mean?
A: It indicates the higher corrosion resistance class within the weathering steel family, distinguished from the standard weathering grades of the same strength level.
Q: Can Q355GNH pipe be welded on site?
A: Yes. Conventional arc welding processes are used with low-hydrogen, weathering-matched consumables, and the weld area is cleaned afterwards so the patina forms evenly.
Q: How much better is the corrosion resistance than carbon steel?
A: Comparative exposure data for weathering grades of this class show a corrosion rate several times lower than structural carbon steel under alternating wet and dry atmospheric conditions.
Q: Is the pipe supplied bare or pre-rusted?
A: Both are available. Pre-weathered or blast cleaned surfaces are supplied on request, and the chosen condition is stated in the order and on the delivery documents.
Q: What tests are carried out before delivery?
A: Dimensional inspection, chemical analysis, tensile and impact testing, and weld examination including ultrasonic testing where specified, with results recorded on the inspection certificate.
