Grinding Burn and Concavity on a 60 HRC D2 Forming Block
A D2 die block, 150 x 200 x 40 mm, hardened and tempered to 60 HRC, is surface ground on a reciprocating grinder using a 250 x 25 mm WA46JV wheel at 30 m/s. The operator takes 0.05 mm depth per pass at 15 m/min table speed with flood coolant: 5% soluble oil at 20 L/min. The wheel was dressed 40 minutes earlier with a single-point diamond at 0.02 mm/rev and has not been re-dressed since. After grinding, a blue-brown tint covers a 30 x 60 mm patch near the centre of the block. A 0.01 mm indicator on a surface plate shows 0.03 mm concavity along the 200 mm length. The block is a forming die face that will run against 60 HRC tool steel; final thickness is already at nominal, and the next operation is polishing.
1. Diagnose the tint and the concavity, and explain how the two symptoms are related.
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The blue-brown tint is a temper colour: the surface has exceeded the tempering range of D2, roughly 200 °C and upward, which is the signature of grinding burn. The concavity of 0.03 mm along 200 mm is consistent with wheel wear and with thermal expansion of the block during grinding.
Contributing causes in the data:
- 0.05 mm depth per pass on 60 HRC D2 is heavy; heat is generated faster than a 20 L/min flood can remove it.
- The wheel has not been dressed for 40 minutes. A glazed WA46JV - J is on the hard side - rubs rather than cuts, raising both heat and normal force.
- 15 m/min table speed with that depth gives high specific energy at the contact zone.
The two symptoms share one cause: a dull, relatively hard wheel combined with an aggressive depth. The dull wheel wears and deflects, producing taper, and rubs, producing burn.
2. Specify the corrected grinding parameters and wheel conditioning for the rework.
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Corrective plan:
- Dress the wheel properly with a sharp single-point diamond at 0.02-0.03 mm per pass with coolant on, and dress frequently - every two to three passes at this hardness, not every 40 minutes.
- Reduce depth of cut to 0.005-0.010 mm per pass for finishing; keep roughing to 0.02-0.03 mm with a softer grade.
- Direct coolant at the contact zone, check nozzle aim and concentration, and increase flow.
- Reduce table speed slightly and add two or three spark-out passes to relieve deflection and improve flatness.
- Consider a softer grade (H or I) or a more open wheel structure if loading continues.
These changes lower specific energy and keep the wheel sharp, which addresses burn and taper together rather than one at a time.
3. Decide whether the block can be accepted, and define the verification steps for the reworked part.
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Reject the block as ground. The burned patch is tempered back, likely to the low 50s HRC, with tensile residual stress. Running it against 60 HRC tool steel will cause pickup, spalling or cracking, and the damage will print onto every part as a witness mark.
Because the block is already at nominal thickness, removing 0.05-0.10 mm is not free: check whether the die can be shimmed or whether the block must be scrapped. If rework is feasible, grind off at least 0.10 mm with corrected parameters, then verify:
- No temper tint after the final pass.
- Hardness 60 ± 1 HRC at the surface.
- Flatness within 0.01 mm on the surface plate.
- A nital etch burn check on a representative area, documented.
Acceptance requires all four. Polishing alone will not remove a burned layer of this depth.