Heat Treating with Dave Smucker
Improving Your Heat-Treatingby Dave Smucker
Rediscovering the Break-Test
What is a Break-Test ?
The "Break Test" has been around a long, long time. The first reference I find is in discussions of the manufacture of "Blister Steel" in the 15th century.
Blister steel was made by taking the best grades of wrought iron, in the form of bars, and packing them in finely ground charcoal in a large clay box. This was then heated for 6 to 8 days at about 2000 F so that the wrought iron absorbed the carbon from the charcoal and became "steel". (Real wrought iron had almost no carbon, being almost pure iron with several percent of silicon oxide slag.) The clay boxes were about the size of a coffin and had a small door or opening through which a sample could be removed for testing. This is kind of like testing to see if the cake is done.
The sample which, was well above the critical temperature was removed and quenched in water. It was then broken – hence the name ”break test”. Pure wrought iron would not break in a similar test, but would instead bend.
I have always assumed that this was just a simple test, with no tempering of the sample, just a quench followed by breaking the sample with a hammer. I also assumed that experience and judgment were then used to guess at the carbon content by looking the fracture. Carbon content could also be judged by testing the sample with a spark test on a grinding wheel. (While they didn't have modern testing methods in those days, they did have natural stone grinding wheels.)
Today I think I was wrong in assuming the test was quite this simple. Instead, I think a progressive temper was made to the sample and it was then tested by repeated breaking of the sample from least temper to most temper. This would add very little time to the testing process but give a much more meaningful test.
Mark Hopper Teaches the
Break-Test at Madison
At the Southern Blacksmith Association conference this past May, Mark Hopper was one of the featured demonstrators. Mark grew up in the England and actually started blacksmith when he was 13. He learned blacksmith in a formal British training program and then worked with a number of experienced smiths. He spent four years in Kenya, in East Africa where he taught tool making.
For me one of the outstanding things that Mark demonstrated is how he was taught to perform and used the Break-Test so that you could use the proper heat-treating procedure on almost "any" steel useful for tools. This test is very very useful for a steel that is a "NTS" (Not too Sure what it is steel) but is also very useful to just learn about the best way to heat treat and temper a know steel. It is specially useful to gain experience and confidence in heat-treating and to solve problems you might run into.
Complete article can be downloaded. Three PDF files are attached.
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Heat-Treating/break test
