Creative Woodworks & crafts 2004-03, страница 73

Myths and Facts about Saw Blade Steel

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What does carbon do for saw blades?

Carbon steel, by definition, is steel that has only carbon added as the alloying element. The addition of carbon, followed by heating and quenching, changes the steel from a softer material to a harder one. Carbon steel has various amounts of carbon in it, which is classified by a numbering system. Depending on the use of the steel, a specific number related to the amount of carbon needed is used when purchasing steel, but I will not go into the specifics of the numbering system at this time.

The amount of carbon in the steel defines the hardness and strength. So, the more carbon in steel, the harder it is. The tensile (stretching) strength and the resistance to wear increase. In other words, the cutting edge of a saw blade will last longer. Other benefits of carbon are less warping or cracking and a decrease in ductility and malleability.

There are other classes of steel that contain many additional elements and can also be used for saw blades. But like everything else, the more features added, the more expensive it becomes. So, for saw blades, plain carbon steel is a fine choicc.

Finally, there is the testing of hardness. The tests are conducted by making a small dent in the steel with an instrument, and then measuring how deep the dent is. The tests cannot be conducted, however, without special equipment, because the depth of the dent is minuscule.

The most common tests used are Brinell, Rockwell, and Micro Hardness. Brinell and Rockwell are similar to each other in operation. Both systems use a penctrator under pressure to make an indentation in the steel from a calibrated machine. Then the impression depth is measured using a special gauge installed on the machine. The gauge reads the depth and a conversion number is assigned. The numbers obtained relate to a standard for hardness.

The third test, Micro Hardness, also uses pressure and penetration, but the difference is that micro hardness can actually determine the hardness of a single crystal. The machine is very expensive and resembles a large microscope. With this test, the indentation is in the shape of a diamond. The width of the indentation is measured under the microscope and then a cross-reference table is used to determine the hardness. If pinpoint accuracy is needed, then the micro hardness tester is the instrument to use.

There is a point where carbon steel reaches its maximum hardness and can

not get any harder. I mention this because I have seen hardness numbers that are not achievable, which sound good to the consumer.

So, as you can see, ordinary people will not be able to measure hardness at home. However, there is a simple method that can be taken to determine the relative hardness of a blade. Take a blade and hold it at each end. Then make a loop about three-quarters to one inch in diameter. Let go of one end. The blade should snap back close to the straight position but not entirely straight. If the blade breaks before the loop is made, il is too brittle; and, conversely, if the blade remains twisted, it is too soft. This is by no means a hard, fast rule, but if you feci there is a problem with

a blade, it will at least give you an indication about the heat-treating.

Well, J hope this has cleaved up some trouble spots some of you might have had about steel, and I also hope that it gave you some insight into what it takes to make the steel work for saw blades.

Ralph Costa is a design engineer and has worked in the saw blade industry for the past 15 years. He has designed equipment for fabricating saw blades, developed new processes, and consulted for companies worldwide. Ralph is currently Worldwide Sales and Product Development Manager for Scies Miniature .S'A and Grobet USA, the manufacturers of'Pegas saw blades, ca

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14 • Creative Woodworks S. Crafts March 2004