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m16a2soldier

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Jul 12, 2008
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been lurking here a while now so I thought Id pipe up now that it looks like I will be able to make a few attempts at making blades, but had a question for the wisemen here.

Been doing some studying on making knives and am having a bit of a hard time finding info on the tempering process. i know this varies a bit from steel to steel. but say I was using 5160 from a car/truck spring and had heat treated it with 3 thermal cycles. how would I go about the temper to get the hard edge and soft spine thereby creating a blade that could handle a little punishment like chopping through hardwood?
 
I am by no means an expert or a wiseman in the world of knife making, but if I remember correctly, many will suggest that the blade is heat treated properly according to steel type. Then do the tempering. When that is done, use an A/O torch set to heat up the spine. While doing this, the edge is placed in water to keep it cooler than the rest of the blade. Not completely sure, but I think this is called a differential HT.

If I'm wrong about this, hopefully someone will pipe up and correct me before you do any work on your blade.
 
For tempering 5160, I temper them in a regular oven at 400f, twice, for 2 hours each time, letting them cool to the touch in between. Some people temper 5160 at slightly lower temps, but 400f works well for me and my knives.

The hard edge/soft back tempering process is to immerse the edge into water, and heat the back/spine of the blade with a torch until it turns a pretty blue/gray color... this is usually referred to as a 'soft-back draw'. the most important thing to remember when doing this is not to allow the edge to get hot along with the spine, which can be difficult on a narrow blade.
 
The HT process for carbon steels usually goes like this:
1. normalize (for grain refinement/stress relief)
2. austenize (heat to hardening temp. and hold at temp if called for)
3. quench
4. temper
Your question involves an additional selective temper cycle, where you would ideally heat just the spine area of the blade to a higher tempering heat to increase the toughness, without further affecting the cutting edge. This is most commonly done with a torch, while the edge of the blade is immersed in water, to keep it from being heated/tempered beyond the original tempering temperature used in step 4.
 
thanks for the info. appreciate it. just out of curiosity, is there a guide out there that describes the recommended process for the common blade steels?
 
also to make sure I understand this right, normalizing would go something like heating the steel to non-magnetic temp and letting it air cool, and from there use 1 or 2 progressively lower temps, air cooling between cycles? or would you simply heat the steel once and hold it a temp for a certain time period and then let it cool?
 
m16a2soldier said:
also to make sure I understand this right, normalizing would go something like heating the steel to non-magnetic temp and letting it air cool, and from there use 1 or 2 progressively lower temps, air cooling between cycles? or would you simply heat the steel once and hold it a temp for a certain time period and then let it cool?
Click to expand...

Your first sentence describes a triple normalizing process with reducing heats. "Non-magnetic temp" is incorrect, it should read 1600F. for most carbon steels.
The second sentence describes a single cycle (hold time is usually minimal, just long enough to be sure the part is evenly heated). I often use a single cycle on new bar stock if I need to anneal it, but I do the triple cycle before hardening.
 
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There are lots of "guides" to heat treating online. The ones that are easy to read usually offer only the most shallow understanding of how it actually works, and leave you with just a recipe. I suggest starting with the "Rosetta Stone of Metallurgy" sticky at the top of the Shop Talk forum. If you use this as a base and do searches as needed to brush up on terminology and get focused explanations, you should come away with a few answers, and a bunch more questions. Welcome to heat treating, it isn't a subject one can pick up in an afternoon and then hit the ground running. Our ancestors struggled with it for centuries and the struggle is not over.
 
M16,

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