Powder Coating Tool Steel: Cure Temperature vs. Tempering Is the Whole Story

It can, and this is the most important thing to control. A hardened tool steel holds its hardness because it was tempered at a specific temperature, and reheating it above that temperature softens it. The danger is with low-tempered grades: O1 oil-hardening steel is frequently tempered at 350 to 500 F to keep maximum hardness, which puts a standard 360 to 400 F powder cure right in its tempering range, so curing it can draw down a point or two of Rockwell hardness, exactly what you do not want. High-temperature grades are safe: H13 is tempered at 1000 to 1100 F, and A2, D2, and S7 are commonly tempered from 400 up to 1000 to 1150 F, all at or above cure temperature. The rule is to know the part's actual tempering temperature and keep the cure comfortably below it. For low-tempered parts, use a low-temperature-cure powder (some cure at 250 to 300 F) and have the coater log actual part temperature with an oven thermocouple, or do not powder coat the hardened part at all and protect it another way.

Generally no. Tool steel dies, punches, and molds exist to be hard, precise working surfaces, and powder coating is the wrong protective finish for them on several counts: the film is soft and thick and would be destroyed instantly in die service, the working surfaces cannot be coated at all, and the cure can soften low-tempered grades. The industry-standard protective and performance finishes for tool steel are thin, hard treatments that add hardness and lubricity without meaningful thickness: black oxide for light corrosion protection and break-in, nitriding for a hard diffused case, and PVD coatings such as TiN, TiAlN, and DLC for wear and friction reduction on cutting and forming surfaces. These survive die service and do not soften the part. If you simply need rust protection on stored or shipped tooling, a rust-preventive oil or black oxide is the right answer. Powder coating is appropriate only on the non-functional areas of tooling assemblies, frames, holders, and bodies, not on the hardened working surfaces of a die or punch.

Through careful masking and selective preparation, because the powder film and the prep would both destroy precision surfaces. A single powder coat adds 2 to 4 mils (0.05 to 0.10 mm) per surface, which would wipe out the few-tenths-of-a-thousandth tolerances held on ground and lapped tool-steel surfaces. So every ground locating face, bore, pin hole, and mating surface must be masked with silicone plugs, caps, and high-temperature tape, and those keep-out zones must be dimensioned on the drawing so the coater knows exactly what stays bare. Preparation is constrained too: you cannot abrasive-blast a ground or polished functional surface without ruining it, so blasting is confined to the non-functional areas being coated while the precision surfaces are masked through prep. Because hardened tool steel flash-rusts like any carbon steel, the precision surfaces also need rust protection during the wet prep stages, and the coated areas still need proper cleaning and a conversion coat. Provide a fully dimensioned drawing of every surface to mask, and expect per-feature masking charges.

Tool steel coating is priced as careful, handling-intensive work rather than bulk coating, because of the masking, selective prep, and cure control involved. For the non-functional areas of tooling in a standard color, the base coating rate is similar to carbon steel, roughly $1.50 to $4 per square foot of coated area, but masking of precision surfaces adds significant per-feature labor, often $2 to $8 per masked feature, and low-temperature-cure powder for low-tempered grades adds material cost. Batch minimums run $100 to $250. Lead times for in-stock colors run about 5 to 10 business days, longer when extensive masking or data-logged cure verification is required. The biggest variable is whether the part is hardened and at what tempering temperature, since that dictates the powder and cure process. For accurate quoting, give the coater the grade, the heat-treat condition and tempering temperature, quantity, and a fully dimensioned drawing marking every precision surface to be masked, and confirm whether a low-temperature cure is required to protect the hardness.