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The Case of the Careening Chip
Kenneth Russell, Professor Emeritus, MIT, Cambridge, MA -- Design News, September 27, 2004
Plain carbon steel saw blades—hacksaw, band saw, or circle saw—are just not all that great. They blunt easily and just won't cut really hard materials. Blade teeth much harder than the rest of the blade are needed. Carbide-tipped saw blades cut better and stay sharp a lot longer than do the simple carbon steel blades.
A near-new 7¼-inch circle saw blade was being used to cut 2-inch-thick pine, an easy task. One tooth tip came flying off and entered the eyeball of an unfortunate bystander, blinding him. The injured's attorney supplied me with the subject tooth, which had been recovered from the plaintiff's eyeball, and the remainder of the subject blade. My job was to find out why the tooth had detached.
The tooth tips turned out to be of a cemented carbide, made up of hard particles of tungsten carbide dispersed in a cobalt matrix. These tips had been silver soldered into right-angle notches at the tips of teeth in a plain steel blade. The foregoing is common and acceptable practice. There was none of the chipping of the teeth that would indicate such abuse as cutting into nails.
The outboard part of the joint fracture was through the silver solder, whereas that inboard was at the solder-carbide interface. Its combination of magnification and depth of field make the scanning electron microscope the tool for examining fracture surfaces such as these.
The figures on this page are scanning electron micrographs of the fracture surface. The top picture was taken near the outboard tip of the tooth. This fracture is of the ductile, "split English muffin" type. This micrograph is at a magnification of about 2,000×. This part of the joint failed as a result of half the silver solder being pulled away from the other half, as is intended. The lower picture is of the silver solder joint inboard of the ductile region. The surface resembles a moonscape. The circles are pores in the solder, which in some cases expose machining marks on the steel below. The solder had never wet the carbide, resulting in little if any adhesion. This magnification is at 100×, so as to show more of the pores. Higher magnification revealed nothing new.
Pelloux's Law had struck again: The fracture was in the joint. The carbide tip had never been properly bonded to the steel blade and had simply pulled away during what appeared to be normal use. The non-bonded region was under tension, whereas the bonded region was, at least initially, under compression. This is the worst possible arrangement. The good joint is needed most to resist tensile forces. The strength of the joint in the compressive region is not nearly so important.
So why was part of the joint so poor? My first boss when I started as an engineer posed three questions to be answered in case of a solder or weld joint failure. "Is it clean enough?" "Is it fluxed enough?" "Is it hot enough?" The purpose of the flux is to clean the surface to be joined. If no flux is to be used, cleanliness is even more important. Temperature has a dual role, both to increase fluidity of the solder and to accelerate any cleansing reaction between the flux and the surfaces to be joined.
Any one or a combination of these three factors could have been responsible for the defective joint. This case never reached the stage that the manufacturer has to reveal processing specifications so I never learned the details of the joining operation. I suspect that the manufacturer settled as soon as my report appeared, since the micrographs on this page are the sort of evidence that is likely to be convincing to a judge and jury.
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