Burned (Blued) Edge on the Grinder
Symptom
A flash of straw, bronze or blue blooms at the edge mid-pass — usually on a thin section, near the tip, or during a moment's dwell — on a blade that already cost a heat treat.
Likely Causes
Heat concentrating faster than steel can shed it: thin cross-sections (edges, tips) have nowhere to send the heat. Dull, glazed or loaded belts rubbing instead of cutting (the top cause — worn abrasive converts pressure to pure heat). Too much speed for hardened work, too much pressure chasing a worn belt, dwelling at plunges and tips, or dry-grinding a near-finished bevel that needed water every pass. Premium hard steels raise the stakes, not the temperature — a 66 HRC ApexUltra edge blues exactly like 1084 and costs five times more to re-treat.
Quick Checks
Read the color like a temper chart, because it IS one: pale straw = roughly 400°F touched, bronze/purple = 500s, blue = 550-600°F — compare against your tempering temperature. The killer detail: color shows the SURFACE’s peak; a thin edge likely saw it through the full cross-section. Check how localized it is — a tip kiss versus three inches of blue edge are different verdicts.
Fixes
Be honest about the math. Color BELOW or at your temper temperature (light straw on a 400°F-tempered blade): cosmetic — sand it out and continue. Color meaningfully above (purple/blue anywhere near the working edge): that zone is over-tempered soft, and grinding the color off does not restore the hardness — the choice is (a) grind PAST the affected depth if geometry allows (a kissed tip often grinds away entirely), (b) live with a soft spot on a user blade and tell the truth about it, or (c) re-heat-treat: re-normalize, re-quench, re-temper — the only real restoration, detailed in the heat treat guide. For a customer blade, (c) is the answer.
Prevention
Fresh sharp belts and the discipline to bin worn ones — belts are cheaper than heat treats. Slow the machine for hardened work (variable speed is exactly this insurance), lighten pressure, keep the blade moving, dunk on cadence, and grind post-HT edges with bare fingers on the spine — flesh is the best over-temperature alarm ever fitted to a grinder. Leave edges 0.030 thick through heat treat so finish grinding has thermal mass to work with.
When to Stop and Get Help
Three blues on the same blade is the machine talking: stop grinding and fix the setup — belt, speed, technique — before feeding it more steel. And blue INSIDE a finished sharpened edge means stop selling: test hardness honestly before that blade leaves the shop.