Grinding Burn and Cracks in Hardened A2 Tool Steel
An A2 tool steel block 150 × 100 × 20 mm, hardened and tempered to 60-62 HRC, must be surface ground to 20.00 ±0.01 mm, flatness 0.005 mm and Ra 0.2 µm. The wheel is WA46K8V, 200 mm diameter, run at 32 m/s. Parameters are 0.05 mm downfeed per pass, 3 mm crossfeed and 15 m/min table speed. The wheel is dressed with a single-point diamond at 0.02 mm depth and a fast traverse. Flood coolant is available, but the nozzle is misdirected and the operator grinds dry to see the sparks. After 0.15 mm total stock removal, the surface shows blue temper colours, a patchy finish of Ra 0.35 µm and fine transverse cracks under a magnifier. The operator proposes to continue with the same wheel and 0.01 mm passes until size, then polish.
1. Identify the mechanism producing the blue colour and cracks. Explain why polishing after cracking is not acceptable.
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Blue temper colours mean the grinding zone exceeded the tempering temperature of the hardened steel. The patchy finish and fine transverse cracks indicate a rehardened, brittle white layer over a softer tempered zone. The wheel is too hard (K grade) for 60-62 HRC steel, so the abrasive dulls and glazes instead of self-sharpening; rubbing generates heat. Grinding dry with misdirected coolant removes the heat sink, and the 0.05 mm downfeed is high for this hardness.
Polishing after cracking is unacceptable because it hides the cracks and does not remove the damaged layer. The cracked layer can spall in service. The part must be inspected by magnetic particle or dye penetrant, and any damaged layer, typically 0.05-0.10 mm deep, must be ground away only if enough stock remains.
2. Recommend a wheel grade and dressing change, plus revised grinding parameters. Justify each change in terms of heat generation.
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Use a softer, more open wheel, such as 46H or 46I vitrified white alumina for this hardness. A softer grade lets dull grains release instead of glazing and rubbing. Dress with a sharp single-point diamond at 0.02-0.03 mm depth with a faster lead or traverse so the wheel stays open and free-cutting; a dull diamond polishes the wheel and increases heat.
Reduce downfeed to 0.010-0.015 mm per pass, increase table speed to 20-25 m/min, and reduce crossfeed to 2-2.5 mm. Aim flood coolant directly at the contact zone and keep it on continuously. Spark out for two or three passes at the end. These changes reduce the heat per unit area by taking a thinner chip and removing it faster, while the softer wheel keeps sharp abrasive exposed.
3. How would you verify crack depth and decide whether the part can be salvaged?
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First clean the surface and inspect by magnetic particle or dye penetrant. If cracks are present, the part is normally scrap because grinding out the full crack depth may take it below the 20.00 ±0.01 mm size. If only temper colour is present, remove 0.05-0.10 mm per side, re-inspect by magnetic particle and etch, and confirm hardness remains 60-62 HRC and flatness is within 0.005 mm.
Do not continue with 0.01 mm passes on a glazed wheel; that increases rubbing. Prove the new wheel, dressing and downfeed on a test coupon of the same hardened steel before committing the part.