lawn mower blade turned into a knife

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lawn mower blade turned into a knife

Postby coon4492 » Sun Aug 15, 2010 3:41 pm

I have 4 old lawn mower blades hanging up in my barn and I want to make them into knives. My question is do I have to heat treat them in any way? Or could I just cut out the shape of the knife and grind an edge on it as it is?
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Re: lawn mower blade turned into a knife

Postby LDS » Sun Aug 15, 2010 5:08 pm

If you are making something like a machette or Kruki I would not worry with heat treat. That is prety tough steel.

If you are going to cut it in half and make two or three smaller bushcraft blades I would do a simple HT.

LM blades are basically "found steel" with no knowledge of the content, but they do have to be tough, so they are not simply mild steel.

The standard for simple heat treat has always been heat to red, then quench in oil, then draw the brittleness with 3 hours in a 500 degree oven.

It will not be perfect, but it will be tougher than nothing.

As you grind keep you fingers near the grind section. When the metal gets too hot for comfort dip in water and cool off. With heavy grinding you will grind for a second and dip, grind and dip. If you do not keep the metal cool you will burn the carbon out of the steel. You do not want to see that metal turn brown, black or blue while you are grinding.

A kruki with the hole for mounting the LM blade worked into the curve would be different.
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Re: lawn mower blade turned into a knife

Postby dixieangler » Sun Aug 15, 2010 9:25 pm

LDS wrote:The standard for simple heat treat has always been heat to red, then quench in oil, then draw the brittleness with 3 hours in a 500 degree oven.


I heard or read somewhere that after heating red hot and quench, that reheat the steel to yellow color and requench will temper the steel to toughen the steel and keep it from being brittle. The steel treated in this way will keep it safe from impacts and breaking. The hardened and tempered steel should also spark well on flint. So maybe no need for the oven. :)

I may have to give those masonry nails another try on the little propane torch but in a darker area so I can see the color changes of the steel better. It might be that I was missing color changes in areas where there was too much light. If not then the little propane torch might not be able to heat the steel up to red hot.
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Re: lawn mower blade turned into a knife

Postby LDS » Mon Aug 16, 2010 10:53 am

Your color method will work, but requires a pretty even heat source to accomplish, which is why I recommended the oven. The kitchen stove oven will hot a consistant 500 and hold it with no problem. You can also control things better. After a couple of hours you can pull the blade out using an oven mit and hit it with a file. the file will probably skip. You can pullit out and check it evey 30 minutes until you feel the file just barely brab the metal. It is perfect at that point. You can never meintain that kind of control in the fire.

Tempering and quenching temp is really not as critical on these all around steels. Get it good and red or hot enough that a magnet will not stick and plop it in a good oil bath. That will make it brittle and allign the structure. The trip through the oven will draw the brittleness out. Time in the oven can gauge the brittleness but that takes trial and error on "found steels". Not enough time in the oven and it will be hard, and spark, but will probably break. Too much time in the oven and it will be back to its anneled state.

It is the same with the forge drawing process you discribe, only very quick with no room for error. A second too long or past the color and the temper is disrupted and the metal goes soft. Not hot or long enough and you have a blade resemvling a piece of glass. I have several knives I have tempered and drawn at the historic sites where the process you discribe is necessary. I like to place a big slab of metal in the fire and get it white hot, lay the spine of the blade on the white hot slab and watche the heat creep up the width of the blade. When the spine heats and the "Blue Line" starts creeping up the blade I remove the knife from the fire as the color line reaches the middle of the blade, whatching and hoping it disipears before it hits the edge. This gives you a soft spine and hard edge, if all goes perfect, the fire was perfect, and the color was actually what you thought you saw. Occasionally it actually works.

If I am making blades for others they usually go in the oven, just so I can say they are "right", will sharpen and will not break.

For fire steels you skip the annealing process. You want them hard, brittle and sparky. The concrete nails are very inconsistant.
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Re: lawn mower blade turned into a knife

Postby cue606 » Tue Aug 21, 2012 9:38 am

I've found this an excellent guide for blades:

http://www.anvilmag.com/smith/flintstr.htm
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Re: lawn mower blade turned into a knife

Postby dixieangler » Tue Aug 21, 2012 10:37 am

Thanks, cue
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Re: lawn mower blade turned into a knife

Postby cue606 » Tue Sep 11, 2012 11:35 am

No problem dixie.
“The opposite of fear is love - love of the challenge, love of the work, the pure joyous passion to take a shot at our dream and see if we can pull it off.” ― Steven Pressfield, Do the Work
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