Correct Tempering Of Your Picks

[Engineer]

There's so much written about tempering of picks, that I thought a guide might be helpful for pick makers?

Metals, especially carbon steels like hacksaw blades can be soft (and so bendable), or hard (and so brittle). Tempering is about the fine balance between these two opposite conditions. Think of bending a bit of metal, it bends easily to begin with, but then starts to "work-harden" and become brittle, until it breaks. Even hammers will harden and an engineer will replace them every 20-30 years or so (depending on how much they are used) as the face will become too brittle and chips will break off and fly off at dangerously high speeds.

So all picks are likely to break eventually, but unless you are using the same pick for several hours a day, every day, then a good pick is likely to last decades.

Generally you work the metal into the shape you want the finished pick to be and then you temper it, to give it that trade-off of flexibility against hardness. Even a cigarette lighter can do this, but you will not get good results. You need to get the end of the pick, from the tip to the start of the handle, as even a cherry red color as you can. At home, I use a plumber's gas torch. I went to a plumber's supply and bought a gas torch end (£9) and gas canister (£4). They do very posh ones with their own built-in ignitior if you want, but mine was just a basic model.

If you want a very hard workpiece, you cool the metal very rapidly, such as by rapidly quenching it in water. This is likely to make the pick too brittle and so break. I've read about heating them up and leaving them to cool down on their own in the air. This will tend to cool them too slowly and so lead to a soft pick that will bend and wear down very quickly. The best way is to plunge the cherry-red hot end of the pick into oil instead. Car engine oil will work, even cooking oil at a pinch. The idea being that the heat from the metal is drawn out by boiling the oil. Water boils at 100 degrees, so cools it very quickly, but oil boils at about 300-350 degrees Centigrade, so cools it more slowly.

There are "partial-tempers", where you just heat the metal up a bit and let it cool down. This works surprisingly well quite often, but isn't as repeatable or accurate, so you might find one pick is fine, one is too soft and one breaks. Full-tempering usually gives the best results.

Tempering is not always necessary, needed or even possible with alloys like stainless steel though. Here certain other metals are added, to modify the characteristics enormously. If you know your exact steel, you can look it up. Most of us though will know only that it's stainless steel from a knife or something like that. Here you will have to experiment to get the best results, as the exact composition of the steel will be unknown.

Spring steel tends to just refer to steel with about 1% carbon and tempered at a fairly narrow range of temperatures.

Commercially made picks often are made of a stainless steel that doesn't need tempering, as this cuts down manufacturing costs.

Windscreen wiper blades are made from stainless steels like 301:

http://www.azom.com/details.asp?ArticleID=960

[ToolyMcgee]

You are being super helpful this month. It would be a royal pain to outline everything involved in metal tempering by oneself. Great thread to start.

It might be important to mention that bringing certain kinds of steel to very hot temperatures very quickly can have negative effects and also is not a very good way to evenly heat any piece of steel. Cherry red is not as hot as some people would think, it is actually barely visible in good light. Orange hot is commonly mistaken to be red hot. Red, orange, white. In that order. Soapy water, or soapy ice water will leave a piece cool enough to touch almost right out of the quench, but oil does NOT evaporate like water. The oil on the end of your piece can still be quite hot, so let it cool a bit before you try to wipe it off. 15 seconds is more than adequate. Do not let your attention switch from the open flame to the quenching bucket. You don't want to tip over a still lit torch, or lean into the flame and burn yourself or set your clothes on fire. It sounds stupid, but most accidents are mindless. Always be mindful of the dangers in the shop.

[quote ="engineer"]There are "partial-tempers", where you just heat the metal up a bit and let it cool down. This works surprisingly well quite often, but isn't as repeatable or accurate, so you might find one pick is fine, one is too soft and one breaks. Full-tempering usually gives the best results.
[/quote]

So true, especially by flame. As for the stainless wiper inserts, they can be hardened a notable bit from their original temper. I stuck my foot in my mouth many times before when I said it wasn't worth a try. Especially for tension wrenches where you can change the temper of a twist wrench to make it more rigid. You will almost always loose some of the characteristics of the metals memory however, and I think it is unavoidable. I don't know very much about it though other than what I have attempted to do by torch. I do know theory, but that depends entirely on the consistency and type of steel, not unknown scrap steel. I would deter anyone from trying to temper a stainless pick using a fuel torch only because once the metal is this thin it becomes very difficult to heat slowly or control the even heat effectively. It would be a shame to shape a pick and then scorch the middle of it leaving further tempering useless. This seems to require an oven, and I doubt the oven in your home gets hot enough to turn steel red hot at between 800 and 1200 degrees. Depending on the type of steel. You can try to temper a stainless piece beforehand and then work with the tempered piece, but honestly it doesn't yield very much hardness to a thinner pick and makes it more difficult to work with.

I have been reading alot about metal working, but knowing temperatures doesn't seem to mean jack at the end of a torch flame. If I had a brick oven that would be a different. Color is most important to gauge temperature without a termostat. Someone correct me if I am wrong about any of the above as I am not sitting in front of open reference material I might be wrong about temperature. Not about method or what I consider color.

Thank you Engineer for the topic post and for the invaluable info on the stainless wiper inserts possible steel makeup. It gives me a starting point to work from and is certainly greatly appreciated by anyone who is curious enough to search it out. Another great homebrew topic.

-Tooly

[LearningTheArt]

Wow nice run down on tempering, I have never thought of using motor oil, I wonder what liquids would help in the tempering process? Thanks for the info.

[TMIB]

See my earlier posts on the subject (as well as your earlier responses. ):

viewtopic.php?p=313859&highlight=#313859

Nothing you have said is really incorrect, but I'd just like to clarify a couple of points:

Technically speaking, heating then quenching in oil is not tempering- it's actually a hardening process. Doing it as you describe is kind of the opposite approach to tempering; you're softening by heating then hardening by quenching, hopefully quenching it slowly enough that the hardening stops at the point you want. That will work-- you're just approaching it from the opposite end than tempering does; you may be arriving at the same level of hardness when you're done as a tempering process would, and it has the advantage to being very simple.

My only problem with this method is that it is difficult to ensure repeated consistency. Even with the same type of metal used between jobs, the method you describe makes it hard to get the same level of hardness each time, and leaves little flexibility to adjust if you find the result is too hard or too soft.

As an alternative, you can heat the metal up to a specific temperature (color is one way to get in the right range, or you can go all the way to the Curie point- which is easily tested.) then quench it. (most likely in oil, so it doesn't crack)

At this point you can temper the steel by drawing the temper colors. This will allow you to soften the metal to the exact degree that you wish and stop the softening process right at that point. One simple method is to heat up a metal bar and lie the back end of the polished pick or wrench on it. You'll see the color change spread down the pick towards the tip. This is the "temper line" as the temperature of the steel changes. Once the tip has reached the proper color (depending on how hard you want it) you can stop the process instantly by picking it up off the heat source (The metal bar in this case) and quenching it again.

This allows you to "fine tune" the hardness wherever you want it.


Whatever method you use, my suggestion is the same: try it out and see what kind of results you get. Adjust accordingly. Results will vary by method and type of steel used. Once you arrive at a combination for a particular type of steel that gets you the right combination of hardness and flexibility, stick with it for that batch of steel.

Thanks for the interesting discussion.

[ToolyMcgee]

Ok, my mistake. I'm only just weeks into working with steel and the literature I read seems to be technical jargon just for the sake of it, so it's difficult to sort out and absorb. I say tempering because to me it has the same effect. The process I am referring to is more of a heat treating than a true tempering. No more reading, time to let the steel tell me where I'm messing up, then back to the books.

Steel is the metal bar stock you are talking about heating to lay a finished pick onto to draw temper colors I assume. By drawing temper colors you are talking about letting the steel of the pick reach the desired color of the desired hardness? Didn't we already do that once? When you are talking about reaching the desired temperature possibly up to the Currie point then quenching how is this any different the second time around? More control? I thought you only wanted to reach the Currie point for annealing by cooling at room temperature. If I have posted a question previously discussed than just say so, I'm going to read the thread anyway. I'm just unsure if you are talking about starting with already tempered steel, annealing, then returning an even temper, or something that I'm ignorant of altogether.

[wolfy_9005]

Oil is less dense aswell, so it helps reduce the amount of heat "leached" from the metal at any given time.

And, on a side note, do it outside. The heat from the bit will burn a small portion of the oil off, giving a white plume of smoke. Ive never had it catch on fire, which is always a good thing.

If it does catch on fire, dont put water on it. Just get a plate/fire blanket and stick on top of the bucket(preferably metal....use an old cooking oil tin with the lid removed)

[TMIB]

... By drawing temper colors you are talking about letting the steel of the pick reach the desired color of the desired hardness?

We are letting the pcik reach the desired color of the desired *softness*. Remember that tempering is by its nature a softening process. Ferrous metals get harder when heated and quenched. (The amount of carbon in the metal determines how hard it gets when you do this.) Then tempering is used to soften it back from that hard brittle state to something more flexible. If you go too far and make it too soft, you've "lost your temper" and need to re-heat/quench to harden it again and then start over on drawing the temper colors.

Didn't we already do that once? When you are talking about reaching the desired temperature possibly up to the Currie point then quenching how is this any different the second time around? More control?

This is essentially the same as what was described as the method in the first post in this thread, only we're concentrating on getting the metal much hotter, so that we get a good hardening when we quench it. The original post described heating it to a low red temperature then quenching it- this will still harden the pick, but it is an attempt to approximate the right hardness in one step. What I am describing uses this first step to make the pick extremely hard, then goes on to temper it in a controlled manner.

I thought you only wanted to reach the Currie point for annealing by cooling at room temperature. If I have posted a question previously discussed than just say so, I'm going to read the thread anyway.

No worries. (Cooling at room temperature is normalizing, by the way, not annealing. Annealing is done much slower.) We are using the Curie point in my example to ensure we have a very high heat in the metal before quenching it in oil. This ensures we get a good amount of hardness all the way through the metal.

I'm just unsure if you are talking about starting with already tempered steel, annealing, then returning an even temper, or something that I'm ignorant of altogether.

We are starting with metal in an unknown state (the pick that we've worked & cut & ground, etc.) then getting it to a very hard state (heating/quenching) then tempering it back to the balance of softness/flexibility and hardness/brittleness that we want.

My example of laying the pick on a steel bar that had come out of the forge was just a method of applying a heat to one end of the pick to draw the temper colors down it. You could do it with a torch at one end of the pick as well, but that's going to go a lot faster, and is harder to watch.

For an exercise, do the following:

Get a piece of carbon steel rod (spring steel for example)
Heat the rod very hot (Curie point if you can)
Quench it in oil, then when cool, clean it up with a rag.
Run a file on it, and see how the file almost skates off and doesn't bite in.
Use some sandpaper to clear off any scale so you can see the shiny metal.
put one end of the rod in/on a heat source.
Watch the colors change down the rod as the temperature line (or "temper line") moves down the metal.
Quench it when you have a good spectrum running down the rod.
take the file again and test the hardness at varying colors on the rod.

[ToolyMcgee]

Thank you very much for the attention to detail this time around. It gives me a much better idea of what you are talking about, and a great comparison to a similar process. I still probably have it wrong, but we heat to around the Currie point depending on the steel and quench to get the hardest possible temper. Then the second process is to fine tune the temperline for the desired softness of the steel along the pick. For instance if I wanted the tip to be very hard, but the shaft to be progressively softer I would place the tip on the heated steel bar and let the color advance down the pick shaft until it was to my liking for the desired effect and quench. Obviously brighter on the tip and more red down the shaft. I suspect the more evenly tempered I wanted the pick to be the more of it would have to be in contact with the heated bar? Man, I'm a rookie. See this is the part I was mixed up at with edge tempering, but comparing the two I think I have a better understanding of both. This would be a royal pain with a torch. I've gotta get a good bed of coals to tackle this one. Definately not just for one pick. Steel tranfering heat to steel makes alot of sense. Once again I used a term incorrectly simply because it produced the desired effect. Normalizing does make the steel much softer. I remember reading now the different stages of exposures to lower heat for extended periods required to fully anneal steal.

Two more newbie questions if I can. I also remember reading similar processes of gradually heating steel to the desired temperature for tempering after it had been brought to it's hardest temper of course. What is the benefit of the super gradual heating? Also for subzero quenching. Obviously it isn't cryo quenching, but I have been freezing my oil to around -55 in the deep freeze. Is there any benefit to that, or am I just being rediculous. It seems to give a better result on the one low heat treatment stainless quench. Is this just placebo effect?

Thanks for the excellent feedback TMIB.

-Tooly

[TMIB]

I still probably have it wrong, but we heat to around the Currie point depending on the steel and quench to get the hardest possible temper.

Yes. Water would quench harder than oil as it cools the metal even faster, but that's usually too fast, and the metal gets so hard so fast that a carbon steel will crack or shatter that way- so we use oil instead, as it cools the metal slightly slower than water does.

Then the second process is to fine tune the temperline for the desired softness of the steel along the pick.

Yep!

For instance if I wanted the tip to be very hard, but the shaft to be progressively softer I would place the tip on the heated steel bar and let the color advance down the pick shaft until it was to my liking for the desired effect and quench.

Whoops, no, you have it backwards. You'd place the back of the pick (the handle) on the heat source and let the heat travel up to the tip. The areas that get the hottest will be softer and the areas that get no heat will stay as hard as they were when they came out of the initial quench.

Obviously brighter on the tip and more red down the shaft. I suspect the more evenly tempered I wanted the pick to be the more of it would have to be in contact with the heated bar?

Wrong colors. If you've got the pick turning red at all, you're already too hot for tempering. The color range I'm talking about looks almost like the rainbow effect you see when you have oil floating on water. It's a range from yellow through straw/bronze to blue, then purple.

Here's a temper color chart. Note the temperatures indicated are in Celsius, so the hottest it gets is around 626 Farenheit. In comparison, steel at a dark cherry red temperature is around 1250-1300 degres- way too hot for tempering.

The colors here are NOT the glow of the metal coming out of the fire; these colors appear just on the surface layer of the metal.

http://www.giantimpex.com/imgs/temperchrt.gif

Again, this is a bit hard to describe, but very easy to see when you do it. The surface of the metal must be clean to see the temper colors change. I'd recommend giving it a try with a larger bar, say 1/4"x1" flat bar. That'll give you an idea of what to look for.

Man, I'm a rookie. See this is the part I was mixed up at with edge tempering, but comparing the two I think I have a better understanding of both. This would be a royal pain with a torch.

It's not too bad, the only hard part is that on thin steels it's hard to get the metal heated evenly for the intial heat before quenching, and the whole thing heats up pretty fast when you go to temper it since it is so thin.

I've gotta get a good bed of coals to tackle this one. Definately not just for one pick.

You can get by much simpler with a MAPP gas torch and a few firebricks. Since you're not heating large pieces, that's usually enough to get a good even heat on small stock.

Steel tranfering heat to steel makes alot of sense. Once again I used a term incorrectly simply because it produced the desired effect. Normalizing does make the steel much softer. I remember reading now the different stages of exposures to lower heat for extended periods required to fully anneal steal.

When I'm teaching metalwork to Boy Scouts, we start out by annealing carbon steel for punches. These are heated up to the curie point, then let cool very very slowly. I do this either by dropping them into a bucket full of ashes (which acts as an insulator) or just keeping them in the hot forge after turning it off for the night.

Normalizing does make the steel softer, but not as soft as fully annealing it does. Normalizing is normally used as a stress-reducing method rather than just a softening method though.

Two more newbie questions if I can. I also remember reading similar processes of gradually heating steel to the desired temperature for tempering after it had been brought to it's hardest temper of course. What is the benefit of the super gradual heating?

This is often done to get an overall, even temper. It depends on what you are building. Take an ax or hatchet for example; you want the back to be tough, not brittle, but you want the blade to hold a good edge, yet you don't want it so hard that it will chip or snap when you hit a knot. So you temper it from the back to the front.

On thinner stuff where you want an even heat like some knife blades, or perhaps even our picks, you temper the whole piece at once. A slow even heat is applied (usually in an oven with a thermometer) to bring the overall temperature of the metal to the right amount gradually. This has the dual effect of giving a very even temper as well as reducing the stress in the metal.

This may actually be a better method for our picks than drawing temper colors. My instinct is that you want a harder tip and a more flexible shaft/handle, but an even temper may produce better results. Obviously some experimentation is in order.

Also for subzero quenching. Obviously it isn't cryo quenching, but I have been freezing my oil to around -55 in the deep freeze. Is there any benefit to that, or am I just being rediculous. It seems to give a better result on the one low heat treatment stainless quench. Is this just placebo effect?

I'm not familiar with subzero quenching. I'd be concerned that with some carbon steels you're going to have the same problem you do when quenching in water- that is too rapid cooling can cause the metal to crack or fracture. Since you're not heating the metal up to a super-high temperature though, it may be that the change is not as drastic and you're not getting cracks that way.

Again, experiment and see what kind of results you get. Test the hardness with a file and see how it performs. Sometimes you can get really good results by discovering a new method that works well, rather than just doing what folks have done before.

--TMIB


[/img]

[ToolyMcgee]

Awesome. 3 books now I have read in their entirety and as many online sources as I could hunt down and this is the first time I have actually seen the color scale laid out. To me that is unbelievable, and it is the only thing I have really been dying to know. When I said red hot again it was out of ignorance, what I was refering to was the cornflower blue to deep blue/violet color the steel turns before it goes red hot at the end of a torch. Very, very fast and difficult to control. The best way I have been able to do it so far is right before the steel reaches red hot it begins to sing. Like physically vibrate. Also the flame tip reaches out to the piece and takes it in, but you don't want that I find as it can very quickly turn the blue flame orange and the metal to red. I doubt it would do this vibrating with a larger piece, but it's why I started to mess around with it so much. Finally I had a literal bell that would ring when it was getting too hot. From there I began to notice the subtle blue to violet glow the metal was getting just before it started to vibrate. If it was on color alone I might have missed it altogether. I can only assume it is a product of the metal expanding on one side and less on the other even though dispite my best attempts to keep the temperature even it still does this especially with the stainless steel.

The reason I started to freeze the oil is because the stainless seemed to take on a darker color I like when quenched in the cold oil. Again it may not in fact be the oil, but a different temperature I am bringing each wrench too. I thought you might know something about the cryo quenching they claim to do with some knife blades. Ahem, Cold steel, but they aren't the only place that does it and they don't do it for all their knives. It gives the metal a really brilliant shine.

Thanks a ton for all the great advice. Full annealing in ash is something I am going to have to try. I assume that sand would work too? I don't know why I said tip of the pick to the heat source as I know from experience getting the tip super hot makes it too bendable. I like the idea of the temperlines on the pick. Even temper is great, but something about the slightly differing tempers of the pick from the tip down the shaft really appeals to me. I doubt it will make a positive difference, but I have to try it out. Thanks again for the great info.

-Tooly

[TMIB]

"The Blacksmith's Craft" by Charles McRaven is a book that has probably the best description and instructions for tempering stel that I've read yet. Might be worth checking your local library to see if they have a copy.

(they may also have it as "the Country Blacksmith" which is the original title of the same book by McRaven.)

I don't know how annealing in sand would work. Wood ash is common, as is vermiculite clay.