Two columns on the certificate, and only one of them is normally discussed

Every quotation for structural or pressure steel carries the same two mechanical columns: tensile strength and yield strength. The numbers are checked against the grade and filed away. They say nothing about what the same steel does at −20 °C, and that is the gap a cold-climate enquiry falls into. An ASTM A106 pressure pipe ordered as Grade A, B or C, for instance, is bought on 48,000, 60,000 or 70,000 psi minimum tensile with 30,000, 35,000 or 40,000 psi minimum yield, the three grades separated mainly by carbon, from 0.25 to 0.30 to 0.35 per cent. Nothing there tells a buyer how the pipe behaves in February, because nothing in it measures toughness.

Strength and toughness are different properties, tested differently

Toughness is the energy required to break metal that already contains a defect, measured by breaking a notched bar. In the Charpy V-notch test a standard bar with a machined notch is struck by a swinging pendulum and the absorbed energy is recorded in joules. The notch matters because it concentrates stress at a point; the temperature matters because most carbon and low-alloy steels lose absorbed energy as they cool, moving from a ductile fracture to a brittle one across a transition range rather than at a single threshold. A tensile test pulls a smooth bar slowly. A Charpy test hits a notched bar hard. Steel that passes the first can fail the second, which is why cold-service certificates quote an energy figure with the temperature it was measured at — one number is meaningless without the other.

Three datasheets, three ways of writing the same limit

The super duplex S32760 / F55 data sheet states it in two rows: a minimum of 80 J at ambient temperature, and 45 J average with 35 J minimum at −46 °C. Same alloy, same heat treatment, one figure per temperature. The 2205 duplex plate page takes the other route and quotes a band instead: the alloy is described as suitable for applications covering −50 °F to +600 °F, with the note that temperatures outside that band may be considered but need restrictions, particularly for welded structures. Minus 50 °F is about −46 °C, so the two duplex grades converge on the same practical ceiling expressed in different units. The Grade 2 titanium bar sheet goes further, stating that toughness is maintained at −196 °C without low-temperature brittleness.

Carbon structural steel is where the question has to be asked

Plate pages describe their products the way a mill does. Q345 plate is credited with high tensile strength, high yield strength and good toughness; Q235 plate with moderate tensile strength, excellent weldability and good toughness; A36 plate with weldability and formability good enough to be welded, bolted or riveted. All of it is true and all of it is silent on the figure a cold-weather buyer needs. The programme runs 0.3 to 200 mm in Q345 coil and other grades, at regular 1000, 1250 and 1500 mm widths with 3 to 18 tonne coils — material is not the constraint. What has to come from the enquiry is the impact test temperature and the minimum energy required at it. The same applies to 16Mn plate, ST37 coil and A36 hot rolled coil. It matters more for welded assemblies, which the steel structure page describes as Q235B and Q355B welded H sections with C and Z purlins, tied together by welds, bolts or rivets: the fabrication introduces a heat-affected zone that the plate certificate never covered.

Where the transition largely disappears

Austenitic stainless is the practical answer for genuinely cold duty, because its face-centred cubic structure does not show the same sharp ductile-to-brittle transition. A 304 or 304L seamless pipe is quoted at 85,000 to 105,000 psi tensile with 20 to 55 per cent elongation, and the L grade simply drops carbon from 0.08 to 0.035 per cent to protect the weld. The 316 and 316L grades keep that austenitic behaviour and add molybdenum, bought normally for corrosion resistance, which also makes the grade the default for coastal and chemical cold service; the same chemistry is available in 316/316L coil and 304 sheet. Duplex grades are a partial exception, because they are half ferritic and the transition returns. 2507 is capped at 316 °C on the hot side and carries a low-temperature restriction like 2205; the lean duplex LDX 2101, at 21.5 per cent chromium and 1.5 per cent nickel with corrosion resistance superior to 304L, is a strength-and-corrosion substitute for 304 rather than a cryogenic material.

Where the specimen is taken from decides what the number means

An impact figure is only meaningful with its location attached. On P22 alloy pipe the specification lists the impact test alongside tension, flattening, hardness and bend tests, and sets sampling explicitly: for material heat treated in a batch furnace, 5 per cent of the pipe per treated lot, and for continuous heat treatment a quantity constituting 5 per cent of the lot but never fewer than two lengths. The P91 grade is delivered at 585 MPa minimum tensile, 415 MPa minimum yield and 250 HB maximum, from 2 to 14 mm wall and lengths to 16,000 mm, with every length hydrostatically tested or given a nondestructive electric test. Base metal, weld and heat-affected zone are three different specimens and the weld is normally the weakest. The same principle shows in line pipe chemistry: API 5L PSL 2 keeps the X42 to X70 yield ladder of PSL 1 but tightens phosphorus from 0.030 to 0.025 per cent and sulphur from 0.030 to 0.015 per cent, with yield-to-tensile ratio capped at 0.93 — the discipline that makes a repeatable impact result possible.

Four lines to add to the enquiry

Four items turn a general enquiry into a buyable order: the grade with its full designation; the impact test temperature, stated in the units the mill works in; the minimum absorbed energy at that temperature, and whether it is a single value or an average with a separate minimum; and the coupon location, base metal or weld. For pressure work, name the acceptance standard the third-party inspector will work to, because that decides whether the certificate is issued as 3.1 or 3.2 — the S32760 sheet is supplied with 3.1 as standard and 3.2 at surcharge. After that it is a straightforward ladder. Where service is cold but not extreme, coated carbon steel with a stated impact value is the economical answer, with the coating chosen for the atmosphere rather than the thermometer. Where duty is genuinely sub-zero and welded, the answer moves to the austenitic grades, seamless line pipe for buried or above-ground runs, and A53 or square stainless tube for structural work. Sections built from Q235B and Q345B H sections and galvanized H structural steel should be qualified as finished assemblies, which puts plate welding and steel cutting inside the specification conversation instead of in the workshop. State the temperature and the mill will process to order; leave it out and the first cold snap writes the test report.