Differential Tempering Process

[CrookedPath]

       First time posting here.  This is a great site and good resource.  I had some questions about differential tempering.  I see alot of people on this site talking about heating their steel in an oven at certain temps (dependant on what kind of steel it is) in order to temper them.  Twice at 2 hours at a time at whatever temp is appropriate for their steel, seems to be standard.  This should make the entire blade one temp. and thus not be differential tempered. 

       At home Ive been tempering with a butane torch, running the colors from the spine to the edge.  I'll do this twice, generally, and so far the results have been good.  My edge retention is passable, I've been able to chop through a 2x4 and still shave some hairs off my arm, but I haven't been able to bend a knife more than about 30 degrees in my post vice, without it snapping.  I try to get the edge to be medium/dark straw, with the spine as blue as I can.  All this is with 1095 i've been getting from Jantz.

     How can I do this better?  When other people use the oven temper method, is there another step I am missing that they will do in order to achieve a differential temper?  Is a differential temper really all that important or am I waisting my time?  Thanks in advance.

 

[ThomasPowers]

What's the grain like in your breaks?  What are you quenching it in?  What temperature are you quenching it at?  All stages of heat treat affect the toughness of a blade.

[Steve Sells]

Use an oven for your entire blade at the low temperature for the cutting edge, then use the torch for the spine only. Another issues could be 1095 is higher carbon than is needed unless you want a razor, and if you do, dont bend it.

 

A lower carbon content steel will be tougher, such as 1065, 9260, 5160

[Rich Hale]

i temper springs made from 1095 quite often. i use a torch and watch the colors. It is great you are shop testing your knives. I think you are close. Remember that it is not only color (temperature)  that decides a temper it is also time at temperature. Think of when you see a chart or post that suggests a certain temp at one hour..and often..repeat that twice...That is quite different than running colors with a torch for less than a minute,

[Geoff Keyes]

There is a difference between a differential heat treat and tempering.  If you want a blade with a hard edge and soft spine, you have to find a way to heat or quench only the parts of the blade you want hard.  The Japanese sword smiths do this by putting a clay mask on the blade before the HT, thick on the spine as an insulator, and thin on the edge for a radiator.  I use an edge quench for my differential blades.  I quench a third to a half of the blade width and let the spine air cool to black.  You still need to oven temper the whole blade, to keep the edge from stress cracking, but you can eliminate the torch draw.

 

A friend of mine made an ABS test blade in 5160.  Because 5160 is s deep hardening steel, a differential quench often gives you hardness in the spine, he then took the blade and packed the edge in wet sand and, using an O/A torch and a big rosebud, drew the spine to blue/white, almost to the point where you can see red.  He did this 10 times (which might have been overkill).  After the cut and chop tests we took turns bending the blade 90 degrees, back to straight, and 90 degrees the other way.  After an hour of this, we lost count of the number cycles we'd done and went for beers.  We were unable to break that blade, but it still had a good edge, no cracks, no chips, no roll overs.

 

Just my .02

 

Geoff

[ThomasPowers]

A differential temper will give you a blade with a harder edge than spine.  A differential hardening will give you an edge harder than a spine and the spine may be softer than you get drawing

 

BUT THEY BOTH HAVE AN EDGE HARDER THAN THE SPINE---A SIMPLE HARDNESS TEST ACROSS THE BLADE SHOWS THIS QUITE WELL!

[CrookedPath]

@Thomas      I quench in Preheated Canola.  I try to get the edge to a non magentic state, while keeping the spine from getting too hot.  I always test the edge with a file to make sure it's hardened.  Grain at the break seems pretty fine.  I'll try to post a picture of the grain if I can firgure out how.  I dont have any thermometers for measuring exact temps.  I try to do without such things if I can, but I am not at all above being convinced I need to start using one.

 

Sounds like I need to start using an oven for the edge temp hardness, and then switch to the torch to further soften up the spine.  Will bringing the spine up to Blue/Purple 3 times be enough? 

I am not ready to change blade steels yet, I've been working with the 1095 for about a year now, and have made maybe 20 blades.  Will 1065 really hold enough of an edge to go through the 2x4 and still shave with?  I admit I dont know much about the qualities of other steels.

[CrookedPath]

There we go, figured out the picture thing.

[ThomasPowers]

I think I can see the grain which is not very fine; but that could be just not seeing the break just the top of it.  Can you post a picture of the break not the side of the knife?  Did you normalize?

[Geoff Keyes]

I have made test blades, that is, blades that are designed to pass the ABS JS test, in 5160, 1095, 1080/84, 1060, and 52100.  My experience is that a torch draw is hard to get really consistent results with.  An edge quench and an oven temper get me the best and most consistent results.  Within broad limits, the steel is not the most important factor.  Good thermal cycling, proper temperature control (both in the quench and the temper cycles) and good geometry are the most important factors.

 

1060/65, BTW, is used by many makers as a sword steel, properly heat treated.

 

Geoff

[ThomasPowers]

 "Good thermal cycling, proper temperature control (both in the quench and the temper cycles) and good geometry are the most important factors."

 

Worth repeating!

[Steve Sells]

1095 is a good steel, I also use it.   I never meant to imply other,  Just giving you options