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  1. #1
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    Default Hardening & Tempering

    I'm making a couple of quite small skew chisels from a Stanley plane blade of recent vintage. The metal is very soft indeed, so soft that the 'wire' that normally forms when honing is more like a flat ribbon and very difficult to remove. The cutting edge rolls at the mere sight of endgrain.

    I don't know what the metal is and can't find any reference to it but I suspect it may be CV steel.

    My question is: Can this steel be hardened in the normal way i.e. heating to cherry red then quenching and what does one then us for a quencher - oil or water (or air)? I suppose tempering would similarly follow the 'normal' methods - I'll probably re-heat to light straw and quench again in the advised medium.

    I've not done any hardening before so any advice will be welcomed.

    Cheers,
    Brian

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  3. #2
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    Run some tests.
    First, can you tell anything from a "spark test"? This may tell you what alloying elements are present.
    Then comes more destructive testing. Heat and quench a few pieces (take notes) to determine what method will work for that steel. If it has sufficient alloying it will need either hotter than "cherry red" or to soak at some temperature (maybe at more than one temperature to get the most out of it).
    The basic procedure is to heat to just a bit past non-magnetic (more precise than "cherry red) and allow to cool in air. Test for hardness (file or bend/break). It it hardens than that works. If that doesn't get it hard then heat again and quench in warm oil (just warm to the touch, around 120 F / 45 C, doesn't have to be real precise on the oil temp). Again test for hardness. If it still doesn't get hard then heat again and quench in water. Test for hardness. It can also be quenched in brine, but that likely isn't necessary. If the steel fails to harden then it probably needs higher temps. You can go through all of this again hotter. If a simple "heat to a bit hotter than non-magnetic and quench in oil/water" doesn't work then you won't get the most from that steel unless you have temperature controlled heating.
    As for tempering, after you've hardened the steel start around 375-400 F / 190-205 C. Check for hardness/brittleness (brass rod test works well). You can temper hotter it you find it's too hard.

    ron

  4. #3
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    Ron's given the erudite answer, mine is a lot more 'hit & miss' based on my limited experience of mucking about with metals over the years. Quenching in oil is possibly a bit 'safer' in amateur hands than using water, with an unknown steel. The worst that can happen is that it won't achieve maximum hardness for the particular alloy, and it's less likely to result in a line of weakness when hardening one end of something long & thin like a chisel. I heat to bright red, approaching white, which is probably not a good idea with some steels, but my colour-vision isn't up to handling subtle variations. I make sure those carbides are in solution!

    I think the most art is called-for in tempering - I manage to get it somewhere in the ball park with flame re-heating, about one time in 4. Using an oven is the safest approach, you can sneak up on the level of hardness you want, but you need a very co-operative kitchen supervisor if you want to use the domestic appliance. Running the oven flat-out for many hours won't do wonders for your power bills, either. As with anything like this, give it a whirl with a scrap piece or two, first. The hardening step is easy, and it's easy to see if you've succeeded - just try filing it after quenching. If the file skates & refuses to bite, you've done something. Then you can play about with either flame or oven tempering til you're satisfied you can get reasonably predictable results.

    Skews are usually just pushed & not belted with anything, so you can err on the side of being a bit hard & brittle, I reckon. I've got some old chisels that came that way, and they make excellent paring tools, but I certainly wouldn't whack them into a chunk of Ironbark...

    Cheers,
    IW

  5. #4
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    Thanks.

    The spark test showed lots of fine sparks. I thought perhaps mild to medium carbon steel. But see below.

    This has turned out to be an interesting challenge. The first one of which was my inability to get enough heat into the bit of left over blade - my trial piece - using a propane torch. I was told that MAP gas (?) burns hotter so I purchased a cylinder of this and was then able to heat the metal to non-magnetic phase.

    Following Ron's advice, I started by air quenching; then oil; then water. Sadly, none of these had any apparent effect on the file test. I suppose there are myriad reasons why this should be so like, for instance, perhaps it needs to be hotter before quenching or perhaps this alloy needs a different method of inducing hardness. I'm a little confused because although I appreciate the newer Stanley plane irons are not like the old ones, the cutting edge of even these seems to be passable for some work. I know in the past Stanley brazed a different, harder material to the cutting edge of their 'high performance' irons but in the case of the newer irons is it possible that the cutting edge is hardened with a method that leaves the back end (the bit that I'm working with) soft and unable to be hardened unless a different heat technique is used?

    I suppose I may have achieved some degree of hardness - enough for a skew? So, I will try grinding a bevel on the trial piece and see how it performs.

    I agree with you Ian that the real art is in the tempering. Heating to non-magnetic while hardening is one thing but the vagueness of monitoring colours moving up the piece while playing a flame over it is quite another as is the bit of hit and miss with the oven technique.

    Cheers
    Brian

  6. #5
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    Are you quenching it direct from red heat? When a piece of steel is thin it will cool rapidly and partially anneal when it is removed from the flame.
    This means performing the heat immediately above the quenching material.
    Also the metal needs to be held at the non-magnetic temp for at least a couple of minutes.

    Despite the above I suspect the quality of the steel may not be up to the task.

  7. #6
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    Depending on what the steel is, you have from a bit under 1 second to get the temp from just above non-magnetic (about 1500 F/815 C ) to below roughly 900 F/480 C. How you heat and how you cool are irrelevant so long as you get hot enough then cool enough quickly enough. Unless it is a higher alloy then you may have several seconds to minutes to get from 1800-2000 F / 980-1090 C to below roughly 900 F/480 C.

    In order for the steel to anneal, it has to cool slowly, in thin enough sections, water hardening steel will harden under a cold blast of air.

    Based on what you've said I suspect this steel has alloying elements that either require a soak (keeping the piece at elevated temperature for some amount of time) or may require hotter temperatures.


    A bit of my experience, I was heat treating what had started life as a file. It was not case hardened so I knew it could be hardened. Files are generally something very close to 1095 (or W1 which is the tool steel version). Despite following what had worked before on 1095, this file steel didn't want to harden. Knowing that rarely W2 was used for files and that W2 specs calls for vanadium, I looked up heat treating for W2. Turns out the vanadium slows down the changes that takes place when steel gets heated. So by giving this steel roughly 30 seconds of soak time (slightly hotter temp could work as well) I was able to harden the piece.

    ron

  8. #7
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    Thanks for the responses.

    I took your advice Bob which has now been supported somewhat by Ron and had another go at hardening today. Using Propane and not MAP, I decided to heat to non-magnetic and then to heat some more. Ron explains that this is 'soaking'. Visually, it certainly appeared to be hotter judging by the colour - and then quenched in oil (peanut). The steel was definitely harder (file test) and contrary to what would be a normal response to chipping the edge of a chisel, I was delighted when the edge failed and showed chipping which, to me, suggested hard but brittle.

    I then tempered using the flame method and found that monitoring the colour progress up the blade wasn't as dubious as I had anticipated. The edged honed well and the performance on end grain of a very soft pine is shown here: Sorry about the image - iPhone

    Attachment 303377

    A magnified view of the edge afterwards looked perfect!

    Cheers
    Brian

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