Sell your carbide saw blade scrap, worn teeth and all

A carbide-tipped tooth is three things: a cemented carbide tip, the steel plate behind it, and between them a thin layer of braze, a filler metal melted into the joint.

Brazing is one of several ways carbide is fixed to steel; ITIA lists brazing, laser welding, pressure welding and projection welding. A university machining lecture puts it plainly: carbide tool bits are either brazed to the shank with soldering or brazing materials, or clamped on mechanically. Clamped tools give you loose inserts. Brazed tools give you carbide that stays stuck to steel until somebody takes it off.

What the filler is varies by tool. In one 2019 study, steel and carbide were joined with a silver-copper filler (BAg-24) at a brazing temperature of 750 degrees C. The same study gives the two metals' expansion as 11.6 millionths per degree C for its steel and 5.7 for its carbide, so the steel grows about twice as much when heated. As the joint cools they shrink by different amounts, and a 2000 study of brazed carbide-steel tools says the stress left behind can crack the cutting edge early.

Brazed tool bits with a carbide tip on a steel shank belong in the same pile; NTP counts carbide-tipped saw blades among cemented carbide products in their own right.

Sources: ITIA, Cemented carbides (June 2010 newsletter excerpt); King Saud University lecture, fundamentals of metal cutting; Materials (Basel) 2019, steel-to-carbide brazing (PMC6947482); Materials Science Forum 2000, brazed carbide-steel tools (OSTI ETDE); NTP, Report on Carcinogens background document for hard metals (2010).

Send blades the way they came off the saw. Do not grind, cut or knock the tips off to make the lot smaller. NTP says dust containing both cobalt and tungsten carbide can come from finishing or maintaining hard-metal products, sharpening and grinding among them, and IARC says the maintenance and sharpening of hard-metal tools can put cobalt into the air at several hundred micrograms per cubic metre. Sawing with a finished blade is different: IARC puts exposure during use much lower than during manufacture, and ITIA expects little or no exposure from normal use.

Tips that already broke off, or a jar of them from a sharpening bench, are the concentrated part. Keep them in a small sealed, labelled bag or jar, apart from the blades.

Two blades that can sit in the same rack are not carbide at all, so bag them separately:

• Bimetal blades. ITIA notes that some bimetal saw blades have high-speed steel teeth, such as M42, not carbide. We buy high-speed steel as well.
• Stellite-tipped blades. These carry a cobalt alloy tip, and OSHA notes that sharpening them makes a fine dust containing cobalt. We buy cobalt alloys and Stellite too.

Swarf and sludge from the sharpening machine are soft scrap with a page of their own: grindings, sludge and powder.

Sources: NTP, Report on Carcinogens profile: cobalt-tungsten carbide; IARC Monographs Volume 86 (2006); ITIA, health and environment; ITIA, Tungsten in steel (December 2008 newsletter excerpt); OSHA sawmills eTool, cobalt dust.