Masonry drills (not)

[John Rumm]

In which case I stand corrected... ;-)

[Dave]

At that temerarure, the tips would fall off due to the brazed joints melting.

Dave

[Arthur]

From my days as an App' more than 25 years ago. Mild steel is a low carbon steel that can only be case hardened. Case hardening is a process of heating to erm red hot and immersing the steel in a carbon powder for some time. High carbon steels can be hardened by heating to red hot and then rapid cooling by quenching in water. This treatment would not make the steel suitable for driiling for example because the steel would be too brittle. To achieve the optimum hardness for a particular function eg, steel cutting, drilling, scraping, spring, etc. the steel must be Annealed/Temprered by heating and quenching This is process of reheating to a particular temperature measured by monitoring the colour of the steel as it is heated, dark straw, straw, yellow, , , blue, dark blue ..then immediately quenching again at the required colour.

Normalising is another process of heating and then cooling naturally. This is done to usually mild steel if it has undergone various streeses. eg, bending. Bending a piece of steel or copper will cause weakness to the bend area. Normalising will restore the area to its original state and minimise weak spots.

I hope my memory has served me well and proves to be helpful to some.

Arthur

[Geo]

I have my doubts about the red-painted tips being anything special at all... "After" photo (150k) at

Geo

[Chris Bacon]

Could be TCT bits, this can happen with bits that have a thin section of TC at the tip, and too much feed has been applied, or where the bits have been cooled too rapidly (even during manufacture). Pieces of TC flake off in the hole & cause extremely fast self-destruction of the remaining piece. A "before" picture would be interesting...