Temper line

S

Stickypoint

Joined
Apr 2, 2000
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I've been a stainless steel stock removal guy for about 26 years now. I read Ed Fowlers Knife talk books (both of them) and it aroused my curiousity about differential hardening. Sooo..... I ground a small blade out of O 1 (Polished it) and took it to 1460 F in my furnace and edge quenched it. I used a block in the quenching oil so that approx 1/3 to 1/2 of the blade was submerged and let it cool that way. Then I tempered it twice for more than 2 hours (400 F) each time. This gave me a hardness of RC 59 on the ricasso near the edge and about RC 56-57 at the top of the blade. OK, I'm getting to my question here..... I then placed it in ferric chloride for approx. 10 minutes. Took it out and buffed it, I'm NOT seeing a temper line. But, I do have a slight difference in the etching. At the edge I have faint horizontal lines and at the top half of the blade the etching is in the form of small dots. The temper line is not clear.
Do I need to etch this longer to clearly bring out the temper line ? How long should it take. AND is a 2 to 3 point hardness difference between the top of the blade and the edge enough to bring out the good qualities of this method ?

Sorry for the length of this but I do tend to ramble on at times.
 
Stickypoint,
If you take the whole blade up to the critical temperature, and then edge quench, generally you will have differentially heat treated it. O1 has a long quench time (about 10 seconds) so that even if you edge quenched it, if the top of the blade got black before the 10 seconds were up, it would also air harden so that there would be very little difference in the Rc. If you had just heated the bottom half of the blade to above critical and then quenched it, then only the bottom half would have gotten hard. On the faster steels, 2 seconds or less, you can get the temper line to show by doing it the way you did the O1. If you can draw the back of the blade back softer by heating the blade while it is in water up to the depth of the temper line, it will also show a greater difference. For those that like a springy back, I think that is the way they do it and that is called differential tempering.
 
Ray,
Thanks for the reply...
If I wanted to heat the whole blade to critical temp and then just quench the edge, what would be a better steel to try ?? How much of a hardness difference would be normal between the top and edge? I am assuming that you don't want the back to be annealed ?? or do I ?
 
I achieve very distinct "temper lines" with 5160 and 52100. One thing you mentioned in your post is that you buffed the blade........not something you want to do if your seeking a "temper line". Hand finish the blade to 400 or 600 grit, clean it well with acetone, and then etch. As you know, a "sludge" will build up on the blade during the etching process. Just keep checking on the blade and when you feel it's etched long enough, take it out, soak it for a few minutes in TSP solution to neutralize the etch, and then scrb the blade in soap and water (I use the catch bucket under one of my grinders) with either #000 or #0000 steel wool. YOUR DONE! (Make sure you dry the blade well and oil/wax it) The blade may be polished with flitz or a similar porduct to produce a satin sheen.
 
Steels:
The usual ones are the 1070, 1080, 1084, 1095, and 52100. The 10xx series are usually around a second and the 52100 is just about 2 seconds. 5160 is about 5 seconds and it depends on the batch as some may quench a little faster. If you really need to do the O1 in the heat treat oven, put about a 3/16" layer of clay on the back part and quench the whole blade just as soon as it reaches slightly above critical. That way, the back hasn't gotten there yet so it won't harden, if everything goes right. :) The time of one second and two seconds is the time it takes to go from above critical to about 1000 degrees in order to miss the transformation zone known as the nose, where pearlite is formed (it doesn't harden if it passed through that area). After it missed the front part of the nose, you ususally have almost a minute to get to the martinsite transformation zone which usually will start at around 450 to 500 degrees. Once it drops to about 175 degrees, it has competed all that it is going to and you can remove it if you want. I heat my transmission fluid to about 300 degrees and quench in that and when it cools to the 175, I remove it, unless it is late at night.
I use 52100 and only heat the part of the blade I want to harden. I temper back to about 57 Rc and the back should be in the upper 40's, maybe, I really don't know for sure on the back. I have tested the edge on several occasions and it always comes out 57 Rc, + or - very little. I do know that you can do file work on the back and the blade will bend and straighten back out without cracking the edge. I have passed both the JS performance test with the 52100 and the MS performance test with a mix of ladder pattern damascus of 52100 and 15N20. Both blades did not crack at the edge and the JS knife was sent to Cliff to do the originial concrete block chop, which he hasn't lived down yet. :)
As far as having the back dead soft of springy hard, it just depends on the maker as to what he wants to do. I opt for soft back but that is me and I have learned to have repeatable results by doing it this way.
 
Raker explained it well. A better choice for what you want to do would be 1080. In my opinion the back should be pearlite which you will get with differentially hardened . The pearlite spine and martensite edge will show in etching.Edge hardness would be perhaps 45-50 Rc. Edge hardening would give you an annealed spine .
 
Check raymonds posting 'a very simple camp knife ' to see what it looks like .
 
Goes to show there are a number of ways.

If you have a big enough bath to put the whole blade in, then cover the area you want to be soft in refractory cement, and dump the whole thing in. You should also be able to get it harder, which O1 can take. That will stop it getting quite so hard on the back, and harder still with the fast quench, on the edge.

Your method might create a better knife. I don't necesarily want a blade I can bend, it just isn't as strong. So quite possibly the blade without enough difference in hardness that you can see a line, might still be better. On the other hand, if the blade a has loads of beef anyway, then a soft back is just fine
 
It sounds like the first thing to do is change steels. I used O1 because I had a piece laying around. I am stuck with heating the steel with a heat treat furnace but this gives me some options to try.
Thanks for the help.
 
The 52100 works very good using the heat treat oven and the clay asd well as the other steels. I have set the temp on the oven at 1550 and as soon as it reached that temperature, pulled it out and quenched it. It is very consistant doing it that way. No problem on over heating and causing the grain to grow.
 
A wise Mastersmith once told me that O1 is not the steel to use, if a very visible temper line is your goal. O1 is designed to get really hard, really easy and it makes for great difficulty as the whole blade wants to get hard, instead of just the edge. People can and do get visible temper lines with O1, but I'll bet they have to go to a lot more trouble than if they had used a simpler steel like W1, W2, 1084, 1095, etc...
 
Little Hen gets excellent hardening lines in O-1. He clay coats the spine.
 
Not sure if this was mentioned but alot of times what works best instead of a long etch is a number of short etches 30 seconds or less and rubbing with 00000 steel wool inbetween while it is still wet and just do your harder rubbing on the lower part. Terry Premos had told me that . The steel wool is something I like using.
 
O1 will work, but as stated it has a tendency to air harden enough that it won't show much difference.

Personally, I feel W1 and W2 are the way to go. I can get just as active (if not more so) than steels like the 10XX, and better cutting ability.

I always feel clay-coated is the way to go. It will give (IMHO) optimal results, and the most beautiful line. Just look at some pics of Japanese blade work. It adds a whole new element of artistic expression to your knives!

Nick
 
To get a temper line on O-1 without using clay, you will need to heat only the area to be hardened up to critical, then quench as usual (I edge-quench everything except daggers).

I believe that is what Mr. Fowler does with his 52100, using a torch.
 
I was able to get a temper line with the O1 but I had to heat the back of the blade with a torch to soften it.(the edge was in water) The temper line came through kind of faint. I've etched it 3 times for 3-4 minutes each time. I'm going to try it a couple more times to see if it will come through better. I came up with a hardness of RC 59 at the edge and RC 54 at the top of the blade. I also ordered some 52100 yesterday.
 
You will be glad you got the 52100. I think it is about as good as you can get when it comes to holding and edge and cutting. What ever you use, learn to get the best out of it and when you can, buy a large amount of the same batch. It does make a difference in your heat treat.
 
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