D2 Punch Chips at a Square Corner After 1,200 Strokes
An 80 mm long progressive-die punch, 12 x 12 mm section, is made from D2 bar and vacuum hardened. The heat treater's certificate shows austenitize at 1,020 °C, 20 min soak, 2 bar gas quench, double temper 2 x 2 h at 510 °C. Hardness checked in the toolroom is 60.5 HRC on the shank and 59.8 HRC near the tip. After 1,200 strokes punching 1.5 mm mild steel, the punch chips at a square corner. Punch-to-die clearance is 0.08 mm per side and the die set was aligned with a dial indicator before running. The punch was wire-EDM cut to final size; the tip was not ground after EDM. A micrograph shows a 0.04 mm decarburized layer at the tip, a white recast layer on the EDM surfaces, and coarse carbide bands. The customer wants a corrected punch in ten days.
1. Rank the likely causes of the corner chipping and justify the ranking.
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Ranked causes:
- Decarburization, 0.04 mm deep - the surface at the tip is lower in carbon and softer, so it cannot support the cutting edge. A vacuum furnace leak-up or insufficient partial pressure during the 1,020 °C soak would do this; the certificate alone does not prove atmosphere integrity.
- EDM recast layer left on the cutting edge - the white layer is brittle and microcracked, and a square corner concentrates stress. This is a direct contributor at the chip location.
- Coarse carbide bands - segregation or insufficient forging reduction in the bar; coarse carbides act as crack initiation sites and cut toughness.
- Hardness too high for the job - 60-62 HRC D2 on 1.5 mm mild steel is on the brittle side; 58-60 HRC would be tougher.
- Square corner geometry - a sharp corner is a stress riser; a 0.3-0.5 mm radius is normal practice.
Clearance of 0.08 mm per side is about 5.3% of thickness, reasonable for mild steel, so clearance is not the primary cause - though alignment and side loading should still be confirmed.
2. List the verifications you would request from the heat treater and the toolroom before building the replacement.
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From the heat treater:
- Furnace leak-up rate and partial pressure chart for the run - the chart, not just the certificate.
- Quench pressure and transfer time record, since a slow gas quench can leave an unstable structure.
- Temper furnace record confirming 510 °C and the soak time.
From the toolroom and incoming inspection:
- Microhardness traverse from the surface inward to establish true decarb depth; if hardness rises after removing 0.10 mm, the layer is deeper than reported.
- Microstructure: carbide size and distribution, retained austenite, and any banding.
- Recast depth on an EDM coupon, plus the number of trim passes and wire condition used.
- Bar certification and forging reduction ratio for the D2.
- Punch-to-die alignment, stripper clearance and any side load from the strip.
This separates material, heat treat and EDM contributions before another punch is cut.
3. Recommend the corrective actions for the replacement punch and the acceptance checks after it is installed.
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Corrective actions:
- Remove all EDM recast at the cutting edge: EDM with multiple trim passes, then grind 0.05-0.10 mm off the tip and flanks. No white layer should remain at the edge.
- Vacuum harden with a verified leak-up rate and partial pressure; specify no decarburization and prove it by microhardness.
- Temper to 58-60 HRC rather than 60-62 to gain toughness, since the failure is chipping, not wear.
- Add a 0.3 mm corner radius or chamfer at the chipping corner.
- Consider a tougher grade such as DC53 or A2, or a PVD coating if adhesive wear is also present.
Acceptance after installation:
- First-article inspection of hole size and burr height at 50 and 200 strokes.
- Visual and magnification check of the punch edge at 200 and 500 strokes for microchipping.
- Confirm die clearance and alignment after the first run.
- Record the heat-treat lot and EDM parameters for traceability if the failure recurs.