Steel Changes with Age
Date: Spring 2012
I took the back saw from my Millers Falls miter box to have the teeth sharpened and reset. The saw was made by Disston and is probably 60 or 70 years old. I was told they couldn't sharpen or set the teeth because the saw is old and the metal has become brittle. 1 or 2 teeth were broken in their attempt to sharpen it. The saw has always been stored in the house and only has some rust stains on it. My question is ,does steel become brittle with age
High carbon, heat treated steel, such as your saw was undoubtedly
made from, does not become brittle with age. Hardened steel is simply
iron with about 1% added carbon dissolved in the iron. Hardness is
imparted by heating the steel to a red heat which causes the carbon to
react with some of the iron to form an ultra-hard iron-carbon
compound called Martinsite. Rapid cooling (by plunging the red hot
steel into water or oil) results in the martinsite being distributed
throughout the steel, imparting harness to the entire piece.
If the red hot steel were slowly cooled, this allows the Martinsite time to
revert back to iron and carbon. This process is called annealing and
results in a very soft steel.
But importantly, there is no known process that, over long periods, can
cause additional Martinsite to form at normal temperatures, further
increasing the hardness and brittleness of hardened steel. My
suggestion is that your saw sharpener needs a little more practice!
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Update: June 2012