The questions that come up most often when someone is specifying stock metal for the first time. Answers are orientation, not specification — the governing documents are always the relevant ASTM, SAE or ASME standard.
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Is a higher gauge number thicker or thinner?
Thinner. The gauge system counted drawing operations rather than measuring thickness, so more operations meant a thinner result and a higher number. Worse, the series was standardised separately for different metals, so 16 gauge steel, 16 gauge stainless and 16 gauge aluminium are three different thicknesses. Order by decimal thickness and treat a gauge number as shorthand to be confirmed — see sheet and plate and the background under sheet metal.
Where does sheet become plate?
By convention, at about 0.1875 inch (3/16) for steel and stainless, and at about 0.250 inch for aluminium. The distinction is practical, not arbitrary: sheet is cold rolled and can be sheared and brake formed, while plate is hot rolled and is normally sawn or burned.
What is the difference between tube and pipe?
Tube is specified by its actual outside dimension and its wall thickness. Pipe is specified by a nominal size — which is not a measurement — and a schedule number that sets the wall. A 1-1/4 inch schedule 40 pipe has a 1.660 inch outside diameter; a 1-1/4 inch round tube has a 1.250 inch outside diameter. They are not interchangeable. See stainless steel tubing.
Hot rolled or cold finished?
If the part is welded into an assembly and painted, buy hot rolled: the finish is irrelevant and the price is lower. If the part is machined, ground, plated or left bright, buy cold finished for the tighter tolerance, sharper corners and brighter surface — and expect it to move when you machine it asymmetrically.
Why is my stainless rusting?
Almost always because carbon steel particles are embedded in its surface — from a shared grinding wheel, a steel wire brush, or contact with a steel bench. Those particles rust and the stain looks like the stainless has failed. Keep stainless abrasives dedicated, and passivate after fabrication where it matters. Genuine failure of the alloy itself is usually chloride pitting, which points at using 316 rather than 304.
304 or 316?
316 wherever chloride is present — coastal exposure, pool atmospheres, road salt, chemical process. 304 for everything else. If the part is welded and then used wet, specify the low-carbon variant, 304L or 316L. See stainless steel grades.
Does a magnet tell me if something is stainless?
No. Annealed 304 and 316 are essentially non-magnetic but become magnetic after cold work, so a bent or machined part can attract a magnet while the plate it came from does not. Ferritic 430 is magnetic from the start. A magnet distinguishes alloy families, poorly, and grades not at all.
What does a mill test report actually certify?
The chemistry and mechanical test results for the specific heat the material came from. It travels with the material and it is the only real evidence of what a piece is. Once a bar has been taken off a rack unmarked and mixed with others, its heat number is gone and no subsequent test restores the traceability. If certification is required, it must be stated at the point of order.
Is galvanised steel maintenance free?
No, but it is long-lived. A zinc coating protects both as a barrier and sacrificially, so a scratch does not immediately start rusting, and service life scales with coating weight and with how aggressive the atmosphere is. Cut edges and welds break the coating and need a zinc-rich repair coating. Welding galvanised material releases zinc fumes and requires extraction — see galvanisation.
Why does 1/4 inch plate cost more than twice what 1/8 inch sheet costs?
Because it weighs twice as much and is processed differently. Sheet is cold rolled from coil and can be sheared; plate is hot rolled in discrete pieces and normally has to be sawn or burned, which is slower. Crossing the sheet/plate boundary changes the process route as well as the weight.
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