Heat Treat of blade: advice seek

Alarion

Alarion

Joined
Nov 26, 2001
Messages
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I heat treated two knives. Since they will end up in the hands of two guys I know well for being quite ham-fisted, I decided to temper the blades to purple-blue, instead of the usual heat treat in the kitchen oven, to trade some hardness for toughness. I'll also do a differential heat treat on the spine, just to stay on the safe side.
This will also make the blade easier to resharpen with normal tools (one of the knives will be used to cut salami and has to be resharpened easily with normal kitchen tools.
The other will be used to eat but also as an utility by a friend which is in historical reenactment, so better it be strong.

Do you think it's a wise idea?
I have this doubt that I should have kept the blade harder, on the yellow-brown range... but after all making custom knives is also adapting the blade to its user... what do you think?
is it ok this way or should I reanneal the blades and re-heat treat them?
 
Purple / blue is pretty high heat for a knife (but whats the steel used) and thats up to you and the user. I usually bring my hawks up to a brown/purple, that seems to make them good and tough and still hold a good sharpness. But I have started tempering my knives at higher heats, I like a temper to where the blades edge will just pass the brass rod test and still rebound. Everyone talks about low temper heats for hardness and edge holding ability, and thats fine, but then when that blade needs resharpening the user has a heck of a time, trying to resharpen.
I'd rather have a knife that will make several cuts and then be easy to resharpen, then one that makes a bunch of cuts and then almost impossible to resharpen, a dull knife, unsharpenable knife is dangerous.
But I have found too, that if I don't like something about a knife I've made. That I'll go back and redo it, and have always been glad I did, be it heat treat, blade grinds or handle shape. So let your conscience be your guide.

hope this helps a little,

Bill
 
I suppose it depends on the steel. I think since your going to do a differential temper you could get by with a decently hard edge (maybe shoot for RC 57-58) where an arkansas stone will easily cut it still, that way they could sharpen it with just about any common sharpening system, or stone.
Then maybe put a convex edge, or sharpen at a wider angle than usual.

Experiment a little before you finish them though, if the spring temper takes a good edge and seems like it will hold it then go for it.
 
Thank you guys.
The steel is C70, about the same as 1075, a low alloy steel with 0.70 carbon content. Since it went through edge bevels forging, double anneal, grind, anneal again, the carbon has to be somewhat lower. The file still skates nicely on the edge.
 
I don't know much about that stuff. I have some 1084 dowel (music wire) that is hardened to a spring temper. Its really tough, I make punches and things out of it. Points have to be reground fairly often if your using them on steel but they hold up very well on everything else. I think I would want it a little harder for a knife though.

You might try a search on ABS tests and see if you can find some details on what the target range is for the chopping and flex tests. I really think the differential temper would take care of most of your concerns on the knife being tough. An edge in the mid to high 50's would probably sharpen easily and resist impact without dulling too easily.
 
Excuse me, Could I please get some clarification here. I consider a 58 RC edge to be a happy medium between too soft and too hard. It is easily sharpened in the field, with just a simple pocket stone or pocket steel.
Ken Beatty
 
With the annealing I did the hardness should be somewhat between 52 and 55 Rockwel C degrees, which is pretty soft for a knife blade, but adds a lot of toughness on the other hand.
My doubt was of having been too worried about breaking, and having sacrificed too much blade hardness in favour of toughness. Since there's no "rule", but it's a matter of compromise and experience, I was just seeking out expert advice about the subject.
 
I'd like to add a few more thoughts.

There seems to be a general belief that softer equals tougher when it comes to blades. How are you defining "tough"? Do you mean you want a knife that can be bent back and forth 90 degrees without breaking a dozen times? Then softer may be better, or "tougher". In fact, a blade made from soft low carbon steel or iron would be "toughest" according to that definition. OR, do you mean you want an edge that will resist damage if it accidentally hits a rock or nail? Do you prefer a blade that will take a lot more force before bending, but may break if pushed past its limits? In that case, then harder is actually "tougher".

Each steel we commonly use has been thoroughly tested by the manufacturer, and you can easily get a data sheet showing just what hardness to leave you blade for maximum toughness. (you will have to choose which kind of toughness you want. Impact toughness or bending toughness) Granted, these data are only guidelines, and you'll have to experiment and test to find the best heat treat for your needs, but they can help you get a rough idea pretty quickly.

Also, do not assume softer blades are easier to sharpen. Softer steels tend to form a wire edge rather easily, which can be hard to remove or avoid. Each steel has a specific range of hardness where it will perform best.
 
Obviously I don't want a soft steel blade :)
Or a blad ethat can be bent 90° a dozen times, otherwise I wouldn't have tempered it, I would have annealed it :)
I just added a little more flex sacrificing a little hardness. I just wondered if this was good, given the premises I explained above.
 
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