Weldability is a sentence on the page, and it hands the job back to you

Almost every carbon steel datasheet carries a weldability claim that is warmer than the specification behind it. The Q235 plate page credits the grade with excellent weldability and resistance to cracking; the Q345 plate page says good weldability; the A36 plate page claims excellent weldability, adding that the material can be welded, bolted or riveted. None of that is wrong, and none of it is a welding procedure. Two pipe pages say out loud what the plate pages imply. The A53 building-structure pipe is described as suitable for welding, subject to certain qualifications. On A106 pressure pipe the wording is sharper: where the steel is to be welded, it is presupposed that a welding procedure suitable to the grade of steel and intended use or service will be utilized. The datasheet states the material is weldable. The procedure is still the buyer's problem.

Carbon and manganese set the risk, and the specification only caps them

Two elements decide most of the difficulty, and both are on the certificate. A53 caps carbon at 0.25 per cent for Grade A and 0.30 for Grade B, with manganese to 0.95 and 1.20 per cent. A106 runs carbon at 0.25, 0.30 and 0.35 per cent for Grades A, B and C, with manganese 0.27 to 0.93 per cent in Grade A and 0.29 to 1.06 per cent in Grades B and C. Both raise hardness in the heat-affected zone. Ordering A106 Grade C instead of Grade A moves the carbon ceiling ten points and the manganese ceiling thirteen, and moves the welding risk with it, without changing anything else on the order.

The residual line is not decoration

Below those two lines sits a row of caps most buyers never read. A53 lists copper 0.40, nickel 0.40, chromium 0.40, molybdenum 0.15 and vanadium 0.08 per cent as maxima, and the A106 table carries the same set. These are scrap-derived residuals, and all of them enter the carbon equivalent used to choose a preheat. With carbon and manganese alone, A53 Grade B reaches about 0.50 and A106 Grade C about 0.53. Add the permitted residuals at their caps and A53 Grade B lands near 0.68 and A106 Grade C near 0.71, because chromium, molybdenum and vanadium count at a fifth of their weight. Roughly 0.18 of carbon equivalent sits inside that residual row. These are worst-case stacks of published maxima and a real heat sits well below them, which is why the ladle analysis is worth reading before the procedure is written.

Thickness and workshop temperature move the preheat, not the grade

Preheat slows the cooling rate, so the section does as much work as the chemistry: A106 covers cold-drawn pipe from 15 to 100 mm outside diameter at 2 to 10 mm wall and hot-rolled pipe to 700 mm at 3 to 50 mm; A53 runs 3 to 12 mm wall from half-inch to 36-inch bore. Plate and coil run from 0.3 to 200 mm on the 16Mn, 45# and Q345 coil programmes. A 6 mm fillet in a workshop at 20 °C and the same fillet on a 40 mm flange at 2 °C are not the same weld. The Q215 and ST37 grades sit at the low-carbon end and tolerate the least care with draughts.

Alloy pipe changes the rules twice

Above the carbon-manganese grades the arithmetic changes. A335 P22 carries chromium at 1.90 to 2.60 per cent and molybdenum at 0.87 to 1.13 per cent, and its page states it as suitable for fusion welding. A335 P91 carries chromium 8.00 to 9.50, molybdenum 0.85 to 1.05, vanadium 0.18 to 0.25 and niobium 0.06 to 0.10 per cent. Once chromium is an intentional alloying element, the carbon equivalent expression stops meaning what it means on a plate: P91's own chemistry sits far outside the range it was fitted for. What replaces it is on the same page. P91 is delivered at 585 MPa minimum tensile, 415 MPa minimum yield and a hardness ceiling of 250 HB, and its proof stress falls from 410 MPa at 100 °C to 215 MPa at 600 °C.

Post-weld heat treatment belongs in the route, not in the repair column

Heat treatment after welding is a planned step, and the datasheets list it as one. A106 states that hot-finished pipe need not be heat treated, but where it is, the treatment must be at 1200 °F (about 649 °C) or higher, and that cold-drawn pipe must be treated after the final cold-draw pass. P22 writes its condition as letters: A for annealed, N+T for normalised and tempered, Q+T for quenched and tempered, all against one nominal chemistry. The hot-rolled H-shaped steel page describes controlled rolling temperature, speed and cooling followed by normalising and tempering before inspection. So the enquiry should say who heat treats, at what temperature and against which certificate. That last point catches people: the heat-affected zone produced by plate and pipe welding is not covered by any parent-material mill certificate.

The coating has to come off before the arc and go back on afterwards

Galvanized steel is welded every day, but the joint is not the same. Electro-galvanized sheet is supplied with a 40, 60 or 80 g coating, chromated and oiled or chromated and non-oiled, and lists welding among its processing services; the SECCN2 programme covers 0.13 to 5.0 mm at 600 to 1500 mm widths on the same basis. Zinc at the joint vaporises, and the porosity and fume it produces are avoidable by stripping the coating clear of the weld zone. The surface treatment page lists the whole cycle as line items: rust removal by sandblasting, shot blasting or pickling, then coating, galvanizing, polishing and heat treatment. The welding service page lists its own hazards — burns, electric shock, inhalation of toxic gases and ultraviolet exposure — which is why the coating belongs on the drawing rather than discovered at the bench. Where the part is Z275 galvanized coil, coating weight and weld preparation belong on the same line.

What to write into the welding clause

Six lines remove most of the argument: the grade and form; the carbon, manganese and residual chemistry, not the grade name alone; the section thickness and joint detail, because both set the cooling rate; the procedure qualification standard and who holds it; the post-weld treatment temperature and the responsible party; and the coating condition at the joint. Structural work in the ordinary grades is welded to a fabrication drawing, not a mill document: steel structure frames in Q235B and Q355B welded H sections, ASTM A29M H-shaped steel, A36 universal beams, Q235 beams to the IPE 450 pattern, and hot-dip galvanized H-beams. Where welding follows a cut, the sequence is usually cutting, bending and shearing, all offered as custom processing. When carbon steel stops being the answer the question changes from procedure to alloy, and moves to the stainless pipe range and the 304 and 304L coil or 316 and 316L coil programmes, where filler and heat input matter more than preheat.