I am making a woodworker's marking knife from an old chisel. I began grinding to shape the point, then when it started to turn neat shades of blue and green, I realized that I was probably destroying the temper of the steel. I think I am close to the shape that I want, so I tested it to see if it retained the original temper. It still reacts strongly to magnetism. So my question is does my knife still have good edge holding capability? Is the magnetism test a reliable one? Another question is how much grinding does it take to destroy the temper of steel?
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Grinding
- Thread starter BamaNoob
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when the metal turns blue (after heat treat) the temper is gone from the steel from over-heating. Magnetism has nothing to do with hardness. A magnet is used to test when a steel comes up to heat (non-magnetic) before quenching in oil/water.
For a marking knife that you will be using in the shop, I think you will be fine.
For a marking knife that you will be using in the shop, I think you will be fine.
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Welcome BamaNoob. Fill out your profile so we know where you live and a bit about you.
There might be a smith in your town who would help you out.
Non-magnetic is a state above1390°F in steel. It returns to magnet as it cools back down. This is a way to check the temperature of the steel when doing HT in a forge.
Tempering of a knife is done between 350°F and 450°F. The surface will change color as it heats up. Blue is around 500°F and shows that the temper has been affected at te edge. As you continue to grind, you may grind away the blue color, but the change to the temper is permanent (unless re-Heat Treated).Carefully grinding the edge back a little farther while keeping it cool may get past the damaged temper area if he edge only had a little blue area and it wasn't all turned blue.
Avoiding ruining the temper and making a soft spot on the edge is why sharpening of a woodworking tool or knife is always best done by hand on a stone.
When grinding, the thin edge can become several hundred degrees in a fraction of a second. To prevent "burning the temper" of an already hardened steel tool or knife, you have to dip it in water pretty much every pass to cool it down. Also, use a gentle hand and take material off very slowly.
Your marking knife will probably function OK, but will need re-sharpening more often. If it was a whittling knife it would be useless.
There might be a smith in your town who would help you out.
Non-magnetic is a state above1390°F in steel. It returns to magnet as it cools back down. This is a way to check the temperature of the steel when doing HT in a forge.
Tempering of a knife is done between 350°F and 450°F. The surface will change color as it heats up. Blue is around 500°F and shows that the temper has been affected at te edge. As you continue to grind, you may grind away the blue color, but the change to the temper is permanent (unless re-Heat Treated).Carefully grinding the edge back a little farther while keeping it cool may get past the damaged temper area if he edge only had a little blue area and it wasn't all turned blue.
Avoiding ruining the temper and making a soft spot on the edge is why sharpening of a woodworking tool or knife is always best done by hand on a stone.
When grinding, the thin edge can become several hundred degrees in a fraction of a second. To prevent "burning the temper" of an already hardened steel tool or knife, you have to dip it in water pretty much every pass to cool it down. Also, use a gentle hand and take material off very slowly.
Your marking knife will probably function OK, but will need re-sharpening more often. If it was a whittling knife it would be useless.
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