Knife tempering

[Bruce]

     I am reworking an old knife (high carbon) and in the process of removing the crud and rust accumulate over time by draw filing I ran into a large area of the blade that is harder than the file so the detritus remains in that area.
     My thinking is to remove that hardness, finish draw filing and then reharden and temper the blade.  So my question is: what is the easiest way to perform these tasks?

                                   
                                                           Kinard Formplaster, a dupe for the mundane

[heinz]

Not in my experience.  Switch to a stone or emory paper for the hard areas. If you suspect the blade has been in a fire would be the only time I would reharden and retemper.

[Canute Rex]

My question is, what part of the blade is harder? If it is harder near the cutting edge and softer near the back, then I'm with Heinz, just stone the thing. ("Everybody must get stoned..." B. Dylan)

If the back is harder or the hard area is randomly distributed, there is a tempering problem and you should think about redoing it.

In that case, anneal, polish, harden, temper.

Heat uniformly to a dull cherry red (in dim light) and immediately stick it in a bucket of wood ashes for an hour or so, until cool. Test for hardness, repeat as necessary. A forge is best for this, but a rosebud tip an an oxy-acetylene torch will work. Your average propane torch will be a bit small for the task, unless the blade is small. You can gang a few propane torches in a line with the nozzles pointing about an inch apart.

Polish.

Wet the blade and coat with chalk-line chalk. Let dry. This (fragile) coating protects the finish during reheating.

Heat uniformly to a dull cherry red (in dim light). Quench completely, edge or point first, in.....hmmm. Here we have a conundrum. I'd recommend room temperature vegetable oil as the safest option. Some steels want water, some brine, some oil, but oil won't crack water hardening steel.

Carefully re-polish till bright.

Heat SLOWLY from the back edge till the bronze color reaches the cutting edge and the back edge is blue. This is tricky. A sponge soaked in water can slow down areas that are running past their proper tempering point. One technique is to heat up a couple of big bars of steel to a dull red and clamp the back of the knife in between them. Otherwise use a propane torch. SLOWLY. When the colors are right, quench.

But @!*%, if you can stone or emery the thing, save yourself the effort.

[ottawa]

as mentioned stoneing or emery will work and in less time the rehardening and then tempering a pic mite help

[LRB]

  Might as well add another recipe. Bring it to a bright red, bury in common vermiculite. After cooling, clean it up as far as you wish. In dim light, bring it to a red-orange, or two shades of red beyond non-magnetic. Quench point down, just the blade portion and about 1/2" of the tang, in canola oil that is around 130°. Canola oil is about as close to commercial heat treat oil as one can get, without buying the commercial oil. Use a candy thermometer in heating the oil. Warm oil quenches faster than cool up to around 150°. Once in the oil, move fore and aft. Never sideways, and remove after maybe one full minute. If warped, you have about 5 minutes to straighten it by hand pressure. Wear gloves. Give it two tempers at 425°, to 450° max., depending on how hard you want it, two hours each temper, in your kitchen oven. Use an oven thermometer. Never trust the oven dial. Remove the blade from the oven, and let cool to room temp in between tempers. This cool down in between allows unreformed steel solution to form to what it should be. No need for chalk, or any other protection. It will not decarb unless you soak it for a good while at the red-orange quench temp.

[Bruce]

      Thanks LRB, that's the information I was after.

[Canute Rex]

LRB's method will work fine. Step tempering from the back, as opposed to oven tempering, will give you a more durable knife, but it is a lot trickier. I've been doing it for a while and I still have to go back and do it over sometimes.

The chalk coating isn't about major decarburizing. It just helps to minimize the surface oxidation and resulting polishing after the hardening heat. It's especially useful with a coal forge.

The main problem is not knowing about the particular steel used in the knife. Hence a conservative room temperature oil quench.

Good luck!

[LRB]

  The reason to heat the quench oil is to thin it. All quench oils work best when heated as long as it's not over heated. When a red hot blade goes into oil a barrier of vapor is immediately formed around it, and the steel has no acual contact with the oil, or it's cooling effects until this vapor jacket is gone. The thinner the oil, the faster this vapor barrier dissapates, and the faster the steel cools. Moving the blade fore and aft in the oil aids in the vapor dissapation also and helps to speed the cooling. It is good to know what steel you are working with, but other than an air cooling type, which is very unlikely in your case, no steel I can think of would give a problem with a fast quenching oil. Especially canola, mineral, or peanut oil. If your knife is a kitchen or butcher type, chances are it is one of the 10xx steels, which will respond best with a very fast quench oil. If it is an old factory made hunting type, again, it is likely a 10xx type steel, or a close facsimile thereof, with few to no alloys in it.  Oil at 120° to 135° is the best heat range for quenching. Over 140° the quenching effects will start to slow, and over 150° will not get the job done in a good manner. Much under 120°, the vapor jacket will increase in size and duration, and you will not reach full hard in the steel, which is the desired goal of the quench process.

[Model19]

I'll throw in my two bits too, as another amateur knife maker...
   Unless you are going for a guild test grade show piece, you can use regular motor oil at room temp as well.  Most everybody has some and it's cheap.  Actually any of the previously mentioned oils will work at room temp as well.    For a complete newb lacking a knife making specific shop area, heating any oil in quantity can be risky.  How many garages have gone up in flames oil cooking turkeys, for example.
The edge tempering trick is tricky as other guys have said.. the color will turn from the bronze you want to the lovely purple you do not want in a heartbeat.  And it wil not heat evenly across the length of the blade by any means.  If you mess it up back then you can reheat to non-magnetic again and requench and do it again.  The oven temper is easy to do but also as mentioned, it does not yield the best results as far as blade flexibility vs. edge holding that you get with a real edge temper. 
If you are just reconditioning an old blade for fun and will never really use it to max potential in the wilds, then you can't really go wrong with any of the methods advised here.  Easy is best to begin with though, and be safe.  You are talking very metal and fire and possible burning oils....   but it's fun!

[LRB]

   As you yourself said. "Amatuer knifemaker". You need to do a bit more research before giving a lot of advise.  Motor oil is about the absolute worst quench one could use, and no quench works well at room temp. Non-magnetic heat is not enough heat for any steel to go into solution. Non-magnetic is merely an indicator that tells you to go 50° to 75° higher in heat, which is roughly two shades of red beyond.  If you want an edge temper, you get more strength softening the spine after a full quench in proper, or at least decent oil, and a controlled and timed temper heat. That does not include motor oil. If you can do no better, use canola oil, but warm it!

[Model19]

Yikes..  sorry if I stepped on any toes with my advice.   I understand that many more experienced people know more than I do about this subject, and my advice is not the end all be all.  I thought my last three sentences sort of framed where I was coming from and who my input was aimed at.  The beginner can often be overwhelmed by "the right way" to approach this art, and I had hoped to lessen some of the worry that comes up early in the learning curve. 
I'll sight Goddard's "$50 knife shop", page 73 :
 "Quenchants for hardening: There is nothing fancy or expensive required here.  Most any oil or combinations of oils will work. In order to obtain full hardness simple steels need to be cooled fom the hardening temprature to around 400 degrees F in somewhere between two and eight seconds.  The cooling rate neccesary for hardening changes with the number and percentages of alloy elements.  The method of cooling does not matter as long as the time/temprature rate is acheived."
 Or see David Boye's "Step by setp knifemaking", pg 115 , where he too advises that room temp or up to 140deg F is adequate for quenching, and that he likes olive oil.
  So there are many ways to skin this cat, and many ways to get an acceptable end result. Not all agree on the process.
And you are right that 50 deg above non-mag is best, but the beginner will need to learn to gauge that with practice.

[LRB]

  I did not intend to seem as harsh as my post reads, but after re-reading it I can see how it would be taken that way. Both of those books have been out a long time, and in my opinion, have caused more problems  for beginners than have helped. In the area of heat treat anyway. You are right to a point on time and temp, but just any oil will not do that. With the simple steels, the goal is to drop from the hardening temp to under 900° in from .8 of one second, to a couple of seconds, depending on which simple steel. Not to below 400°. You can remove the steel from the quench when it's under 600° if you wish. Motor oil will only work on the lower carbon simple steels, and even then must be heated in order to get any result. ATF would be a better choice than motor oil, and vege oils better than that. You also need to be aware that the only simple steels are those of the 10xx types, and even then 1095 is a problamatic steel in heat treat, and not for beginners. An 8 second cool down will not fully harden even simple steels. When you know what steel you are working with, you can look up the recommended heat treatment for it, but you need to realize that when the chart reads oil quench, they mean in the proper oil. If it reads water quench, they mean a brine solution. Not plain tap water. Yes, many ways to skin a cat, but only a few ways to do it and get a good hide as a result.

[Dphariss]

  The reason to heat the quench oil is to thin it. All quench oils work best when heated as long as it's not over heated. When a red hot blade goes into oil a barrier of vapor is immediately formed around it, and the steel has no acual contact with the oil, or it's cooling effects until this vapor jacket is gone. The thinner the oil, the faster this vapor barrier dissapates, and the faster the steel cools. Moving the blade fore and aft in the oil aids in the vapor dissapation also and helps to speed the cooling. It is good to know what steel you are working with, but other than an air cooling type, which is very unlikely in your case, no steel I can think of would give a problem with a fast quenching oil. Especially canola, mineral, or peanut oil. If your knife is a kitchen or butcher type, chances are it is one of the 10xx steels, which will respond best with a very fast quench oil. If it is an old factory made hunting type, again, it is likely a 10xx type steel, or a close facsimile thereof, with few to no alloys in it.  Oil at 120° to 135° is the best heat range for quenching. Over 140° the quenching effects will start to slow, and over 150° will not get the job done in a good manner. Much under 120°, the vapor jacket will increase in size and duration, and you will not reach full hard in the steel, which is the desired goal of the quench process.

Thanks.
Knife/mould maker Steve Brooks used to live a block away and he always heated a big bar of steel in the forge and put it in the oil before quenching. Now I know the reason.
He used oil and diesel mixed as I recall.
Dan

[Model19]

Agreed, and no hard feelings.  I've been using used motor oil since I have plenty around and I'm in the early stages of the learning curve, 6 or 7 knives under my belt so far, all have been fairly simple designs using old files for steel and one forged in a very very primitive style and manner from an old (1900) steel spike. It got a few votes as the ugliest over on bladeforums this winter  
This is a fascinating science, and one can take it as far they like refining techniques and designs.  It can truly be a life long endeavor.  
 When I get too worried and stressed after reading more on the latest theories in books and forums and berating myself for not trying hard enough to implement them, I remind myself that our ancestors made serviceable blades and tools 300 years ago on these shores with only a tiny fraction of the knowledge we have in our time.
But I will be going to warmed ATF and/or Canola this summer as the next step in my education.  The snow is gone and the forge can come out of the garage!

[LRB]

  I used motor oil and diesel mixed myself for a few years. It does seem to work with files. Nicholson's and Simmonds anyway, but I ran into problems with 1095 using that. 1095 is a sneaky and devious steel. You can quench it as Goddard said, in most any oil, and it will pass a file test for hardness, but it really isn't. If it does not cool below 900° in a second or less it forms a mix of pearlite and martensite, in the form of tiny clumps, instead of a full homogenous martensite condition. A file skips off the martensite clumps leaving no marks behind, fooling one into believing the steel is full hard,  but the pearlite is there, and the blade will not hold a good edge. I carried such a blade for a few years wondering why it did not hold an edge that held up to 1095's reputation. This also can happen with 1080/84, but to a lesser degree. 1095 is very low in manganese which slows pearlite formation. 1080/84 has much more manganese, and is much more forgiving in quench speed time. You acually can quench 1080/84 in motor oil. It just will not be at it's potential, but will make a usable blade.

[Model19]

Yeah, I bought a piece of 1095 from Jantz early on and stored it for future use, only to find that as you said, it's not for beginners.   But I'll get there.
First project this year will be a colonial style flint striker to work on my shaping and forging skills.
Sorry we hijacked your thread BHB. 

[RobertS]

THanks, guys, it's been an interesting and informative discussion!

[Dphariss]

The difference between petroleum ATF and light petroleum motor oil is the additive package. Motor oil has metals added for wear protection, zinc and phosphorus usually, not sure what all is included in ATF but there is no need for such protection in transmissions but it has its own set of additives for its intended use. Both have corrosion inhibitors, anti-foam additives. Any oil used where foaming can occur will have something to reduce foaming.
How the additives in a light weight motor oil vs an ATF of the same approximate viscosity would effect the quench and how a multi-vis oil would effect it I do not know either. But multi-vis oils have viscosity improvers that keep the oil from thinning excessively when heated.
Low viscosity hydraulic oil, basically ATF without all the additives, friction modifiers etc. Might be a good choice for petroleum oil.

Dan

[J.D.]

Just a thought, though I came to this thread rather late.

If this old knife is an antique, "cleaning" it will destroy any value it might have had.

IMHO, from experience, there is a definite learning curve to hardening and tempering, even with good instructions. IMHO, I wouldn't do it unless I was beginning from scratch.

IMHO, your best bet is to use the stones and forget about annealing and heat treating.

God bless