Most plating defects on heat-treated screws are decided before the parts reach the plating line. They come from what was left on the surface after quenching and tempering. Six process faults account for most of what we see.
Residue after silicate cleaning
Screws are cleaned with silicate after quenching and then rinsed. If the rinse is too short, a silicate film stays on the thread and the plating sits on top of that film rather than on steel.
Stacking damage during heating
A screw that changes colour after heating, and leaves an oily residue after an ether soak, was not clean when it went into the furnace. The usual cause is stacking: parts packed tightly enough that the oil between them oxidises during the quench instead of being flushed away.
Phosphide white residue
High-strength screws that skip the acid cleaner, combined with a rinsing tank nobody inspects, come out with a white residue. Inspection identifies it as phosphide. Both halves of the cause have to be fixed — running the acid cleaner without checking the rinse tank only moves the defect further down the line.
Alkali burn
Alkaline residue carried into the quenching oil burns onto the surface under the heat of the quench. The part turns black, with an even, flat, oil-black appearance that looks like a finish and is not. Tempering makes it worse. Clean the alkaline residue off before heat treatment, not after.
Rust during rinsing
Large screws quenched in a polymer solution and then rinsed in an alkaline cleaner can rust before they leave the line; we have caught it in internal testing. The rinse water has to be changed on a schedule, not when someone notices.
Black stripes from aged quenching oil
Quenching oil that has been in service too long leaves black stripes on dried high-strength screws. That is burnt residue from the gas phase of the quench, not a plating fault, and no amount of pre-treatment removes it. Add new oil.
All six are process faults rather than material faults. They are cheap to catch at the rinse tank and expensive to catch after plating.