Heat Treatment, Metallurgy and Material Selection
Heat treatment questions test whether you can choose a treatment for a given tool steel, control distortion, and specify the hardness a die or punch actually needs. The exam covers austenitizing, quenching, tempering, surface treatments, and the properties behind material selection.
Hardening and quenching
Steel is austenitized above its critical temperature, held to dissolve carbides, then quenched fast enough to avoid pearlite. The cooling rate required depends on hardenability, described by the Jominy end-quench test. Water and brine give the fastest cooling and the highest cracking risk; oil is intermediate; air and vacuum gas quenching are slow and dimensionally gentle. Sections with sharp corners or abrupt mass changes crack in water - choose an air-hardening grade or preheat.
Tempering and stress relief
As-quenched martensite is hard and brittle. Tempering relieves stress and trades hardness for toughness. H13 and other secondary-hardening steels need a double temper, at least two hours per inch of section, to eliminate retained austenite and stabilize dimensions. Temper immediately after quenching; delayed tempering risks cracking. Stress relief before finish machining reduces movement after hardening.
Surface treatments
Carburizing adds carbon to a low-carbon surface; nitriding diffuses nitrogen without a quench and produces a hard, wear-resistant case with little distortion. Coatings - TiN, TiAlN, hard chrome - add wear resistance without changing core toughness.
Selection
O1 for gauges and small punches, A2 for complex long-run dies where distortion matters, D2 for high-wear blanking, M2 for cutting tools, H13 for hot work and die casting, P20 for molds. Hardness follows function: D2 blanking dies commonly 58–62 HRC; H13 hot-work dies typically 44–52 HRC to resist thermal cracking.
Worked example
A D2 punch is austenitized at 1020 °C, quenched in oil, then tempered twice at 200 °C, giving about 60–62 HRC. If the same punch were tempered at 500 °C, hardness would fall to roughly 54–56 HRC and wear resistance would drop, though toughness would improve. Reasoning: tempering temperature is the main control of final hardness; the specified hardness range drives the tempering chart, not the reverse.
Traps
Single-tempering H13; forgetting to preheat thin, complex sections; over-tempering to chase a dimension; and neglecting decarburization, which leaves a soft skin that fails inspection.
Maintenance, Troubleshooting and Repair of Tooling
Maintenance, troubleshooting and repair tests whether you can diagnose why a tool is producing bad parts and restore it without introducing new problems. Expect wear analysis, sharpening and reconditioning, welding and EDM repair, root-cause methods, and the documentation that supports planned maintenance.
Planned maintenance and wear inspection
Tools wear in predictable places: punch and die cutting edges, draw radii, bending inserts, ejector pins, and guide pins and bushings. A maintenance schedule sets inspection intervals by stroke count. Record wear land, burr height, and part dimensions so the decision to sharpen is based on data rather than feel. Lubrication, cleanliness, and correct die height setting extend life more than any repair.
Sharpening and reconditioning
Regrind cutting edges square and maintain the designed clearance. Grind the minimum amount, cool generously, and avoid burning - a tempered blue edge is a soft edge. After grinding a punch shorter, shut height changes: grind the die to match clearance or compensate with shims and adjust the ram. Polish radii rather than sharpening them to a knife edge.
Repair methods
Laser and micro-TIG welding restore edges, but tool steels need preheat and a post-weld temper to avoid a brittle heat-affected zone. EDM can remove broken taps and restore detail. Hard chrome builds up worn diameters; confirm the coating thickness is accounted for in the fit.
Root cause
Use 5-Why or a fishbone diagram. A burr increase could come from a worn punch, excessive clearance, insufficient stripper force, or material variation. Isolate variables one at a time - changing three things at once makes the cause unknowable. Confirm with measurement before and after.
Worked example
A progressive die produces burrs that grow over a run. Measurement shows a 0.15 mm wear land on the punch edge and a 0.08 mm burr against a 0.05 mm specification. Reasoning: regrind punch and die to restore the 0.12 mm per-side clearance, then check shut height, because removing 0.15 mm from the punch face changes the punch-to-die relationship. Record the stock removal so the next sharpening starts from a known height.
Traps
Sharpening without restoring clearance; welding tool steel without preheat; replacing springs with unmatched ones; and changing several variables at once during troubleshooting.