The code letters are a product definition, not a processing note

Most order confirmations carry a grade, a size and a condition code, and only two are checked. The condition code is the one that changes what the steel does. An A335 P22 alloy pipe order is a clean example, because the specification table writes the options as letters rather than words: A, standing for annealed; N+T, normalised and tempered; and Q+T, quenched and tempered. Those three descriptions apply to one nominal chemistry and do not deliver the same yield strength, hardness or behaviour in service. In the ferritic alloy pipe family the letter set is the difference between a grade that will hold a hot wall for years and one that will not, and the same logic runs through plate, coil and sheet.

Hot rolling is already a thermal process, and the plate pages say so

Before any separate heat treatment, the rolling itself has done work. The hot-rolled pages describe rolling above the recrystallisation temperature, which breaks up the coarse grains left by casting, heals cracks, reduces or eliminates casting defects and converts an as-cast structure into a deformed one. That is why the 16Mn plate programme runs 0.3 to 200 mm in 3 to 18 tonne coils at regular 1000, 1250 and 1500 mm widths against 6000 tonnes of monthly inventory without a separate annealing step. The same text appears on the Q345 plate, Q235 plate, Q345 coil and ST37 coil pages, because it describes how the material is made rather than a selling point.

Annealing: a softening step with a temperature window and a grain caveat

Annealing is used when the next operation is forming, not when the next operation is load-bearing. The 410S sheet page gives a worked example: the grade is annealed, or softened, to make it less brittle by heating to between 1600 and 1650 °F, which is 871 to 899 °C, then air cooling it slowly to room temperature to relieve cold-working stresses. It then adds a detail worth reading twice: if excessively large grains appear after annealing mildly cold-worked material, the annealing temperature should be decreased. Annealing is therefore not simply hotter is softer: above the right band the grain coarsens, and a coarser grain is weaker and less tough than the one that went into the furnace. The same family shows what happens without it: 410 sheet is normally supplied in the hardened but still machineable condition, and the 410 and 410S coil range offers soft, deep draw, extra deep draw, quarter hard, half hard and full hard tempers as separate line items.

Normalising, and normalising plus tempering

Normalising takes the steel above the critical range and cools it in air, refining the grain and evening out the rolled structure; tempering afterwards trades a little strength for toughness and relieves internal stress. The hot-rolled H-shaped steel page describes exactly this sequence, stating that rolling temperature, speed and cooling mode must be precisely controlled and that a series of heat treatment processes such as normalising and tempering is carried out to improve the comprehensive properties. It then lists the checks: chemical composition analysis, mechanical properties testing and metallographic inspection. The universal beam programme and the steel structure frames built from Q235B and Q355B welded H sections sit on the same principle. Pipe makers write the same rule differently: on A106 seamless pressure pipe, hot-finished pipe need not be heat treated at all, but where it is, the treatment must be at 1200 °F or higher, and cold-drawn pipe must receive that treatment after the final cold draw pass. Cold work, in other words, has to be paid back.

Quenching and tempering is where the hardness ladder comes from

Quenching hardens by cooling fast enough to trap a hard structure; tempering restores enough ductility to make the material usable. The martensitic grades are the clearest teachers. 410 is described as displaying maximum corrosion resistance when it has been hardened, tempered and polished — three operations, in that order — from a grade with a minimum of 11.5 per cent chromium and carbon between 0.08 and 0.15 per cent. 422 plate is a hardenable martensitic grade usable to 1200 °F, capable of being heat treated to various strength levels, and quoted at 897 MPa minimum tensile, 207 MPa minimum yield, 15 per cent elongation and hardness capped at 290 HB or 95 HRB. The 416HT and 420 sheets belong to the same family, and 422 lists its value-added services explicitly: cladding, heat treatment, annealing, pickling, polishing, rolling, cutting, bending and forging, with a mill test certificate to EN 10204 3.1. On the alloy pipe side the same ladder appears as a hardness ceiling: P91 is delivered at 585 MPa minimum tensile, 415 MPa minimum yield and 250 HB maximum.

Where the hardening must not happen

Some grades are chosen because they will not harden, and the wording is blunt. 410S is a low-carbon, non-hardening modification of 410 in which the low carbon and a small alloy addition minimise austenite formation at high temperature, restricting the alloy's ability to harden. It remains soft and ductile even when rapidly cooled from above the critical temperature, and that characteristic is credited with preventing cracking when the alloy meets high temperatures or welding. It is completely ferritic in the annealed condition. The same logic explains the stabilised ferritics: 430 coil strip is titanium stabilised, keeps good mechanical properties at the weld and has better thermal conductivity and lower expansion than austenite, and 409 sheet is stabilised with niobium, titanium or both for the same reason. Where a grade must harden, that is handled separately, which is the subject of the 17-4PH, annealed 17-4PH and 17-7PH sheets.

Six lines that decide the answer before the furnace is lit

The condition code is worth little alone, so the enquiry should carry the operating limits. State the condition in full letters, not a single symbol; state the mechanical or hardness target the code is meant to deliver; state whether the mill or the fabricator will heat treat, because a mill furnace and a site furnace differ; state the certificate level; state whether the finished assembly needs a post-weld treatment, since the heat-affected zone produced by plate and pipe welding is not covered by any parent-material certificate; and state the surface the part must leave with, which the surface treatment page lists as rust removal, coating, galvanizing, polishing, electroplating and heat treatment. After that the route is mechanical. Plate and coil in the ordinary structural grades — A36 plate, Q195, Q215, 45#, SPHC, A36 coil, 16Mn coil, 45# coil and Q345 coil — is supplied as rolled for most fabrication. High-temperature and hydrogen duty moves to the alloy pipes, and severe corrosion moves to 304/304L coil, 316/316L coil, 321/321H plate, 347H pipe, 904L, Alloy 718 and C-276. Where the part must be cut to a profile first, keep steel cutting, plate bending, plate shear and plate punching inside the condition conversation and have the mill process to order against a finished drawing rather than a raw plate size.