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  1. #1
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    Default Hardening of sawplates

    All,

    I've written some of this information elsewhere but decided to reproduce it here so that the thread will be complete, please forgive the repetition.

    Having recently acquired a 36 inch heat treating oven and a Chinese Rockwell tester at a good price I have decided to do some testing on saw plate materials.
    Why? Because it interests me and I am curious by nature. I like learning new things, I like building saws (among other things) and I want to build the best saws possible. This thread is in the same spirit as my threads on vacuum infusion of handles with resin and the use of bronze bushings/bearings for strengthening saw screw holes in handles. Why? Why not.

    I have read elsewhere on the web that Disston hardened its saw plates to Rc 52.

    As IsaacS and I have described in another thread on this site, the use of the C scale of Rockwell hardness, denoted "Rc", testing with the 120 degree Brale diamond indenter is limited by a combination of material hardness and thickness. Authorities differ somewhat on guidance but they state that the thickness of the material being tested should be at least three times the diameter of the indentation made by the indenter, other authorities call for 10X thickness. Thus, for a saw plate with a hardness of Rc 52 the minimum recommended thickness is 0.032". Rc testing of thinner or softer materials is not recommended as the scale becomes non-linear due to indenter induced stretching of the test article around the test point. However, I maintain that for materials of equal thickness that are harder than Rc 20 or so that the Rc test probably gives a good reading of relative hardness despite the invalidity of the Rc scale. I have decided to denote this scale Rc* indicating the application of Rc testing to materials that are regarded too thin to test by this method.

    Thus, after setting up my Rockwell tester and calibrating it, delta = 0.5 U over the range 27.2 to 62.2, dead on at 46.1, I set about measuring the hardness of my personal stock of saws.

    My first test was of an old (1917-1928 era I think) Disston D-8 I acquired on Ebay. The plate thickness measured at exactly 0.032" at the tooth line. Rc testing, at five different points, yielded five measurements of Rc 52.

    My second set of tests was performed on the five commercially available dovetail saws in my collection. These are from two manufacturers. I have one from manufacturer 1 and four from manufacturer 2. I also have a strip of 0.015" 1095 spring steel that I use for my straight handle gents saws. I used this material as a standard. Like many 1095 strips, this one is reported to have a nominal Rc hardness between 48 and 52. I meausured Rc* 42-43 with my tester. The saw from manufacturer 1 gave back readings identical to those taken on the standard 1095 strip. I did multi-point measurements of manufacturer #2 saws and found that they ranged from Rc* 35-36+. All of the plates at the points tested were 0.015" to the best of my ability to read the micrometer. I conclude therefore that manufacturer 1 used 1095 straight from the roll or something of the same hardness even if the chemistry was not the same. Manufacturer #2, given the consistently lower Rc* readings, used steel that was significantly softer.

    I then moved on to another saw, this on from manufacturer #3, that had a 0.030" thick plate at the test points. This plate gave the astonishingly low reading of Rc* 11. I have not been too pleased with the performance of this saw and visual inspection of the indentations produced by testing are consistent with the Rc* readings - it is soft.

    Not wanting to spend too much time at this point examining the work of others I then began heat treating experiments. I haven't taken pictures because the process is visually boring so I will just report the results.

    I have found that 0.015" 1095 steel can be hardened to Rc* 59+ using a range of soak times at temperatures from 749 to 807 Celsius. Soak times range from 45 minutes to 3 minutes for this material over this range. Given that I could produce adequate hardness I investigated annealing.

    Googled values for 1095 and a quick Excel calculation (R squared of the fit line = 0.9957) revealed that 1095 could be annealed to Rc 52 with soaking at a temperature of 342 Celsius for two hours - longer than necessary but I am using residual heat for economy. This worked out according to the book, no problems at all. The hardening and annealing produced nice uniform hardness throughout the material and I was easily able to hit my mark of around Rc* 45-46.

    Next comes the quenching. I started out using room temperature water. It was easy to get hard material but the temperature gradients that arose when the metal was transiting the surface of the quench bath caused severe distortion. I then proceeded to try hot water, cool brine, hot brine cool oil and hot oil. The best results so far have come with hot oil but I am still not satisfied.

    Unfortunately I am stopped as I am waiting on the components needed to make a molten salt bath. I'll post more results when I have them.

    Cheers,
    Rob

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  3. #2
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    Default

    Good work Rob. About 5 years ago I did some hardness measurements of a range of materials (mainly scraper blades) at local hardware shops to determine which could be used for saw blades. I bough a whole lot of stuff tested it and then returned it to the hardware stores for a full refund because the testing only left a pinprick indent in the material. The is how I discovered the Blue scraper material that quite a few Aussies are using as small saw blades.

    Just a couple of points.
    If you could set out your results on a table that would make it much easier to compare what is going on.

    RC hardnesses measurements are very difficult to measure accurately below ~30 RC as the RC scale was not designed for harnesses below this. To measure hardness in this range other scales should be used. However as you say you can still use the results for comparative purposes.

    Pictures are not at all boring especially if you can photograph the process, how you heat, transfer to quench bath, Orientation of material while quenching etc.

    I'm not sure, but you seem to be confusing annealing with tempering?
    Even if you are not your post might sound confusing to a newbie.,
    The sequence is usual annealing to as soft as practicable, working the material, hardening, and then tempering.

    For most tool steels, only hardening usually requires a quench.
    Annealing and tempering are done by heating and usually letting the steel cooling off in air.
    For SS its the reverse.

    Quenching a big piece of flat steel is indeed very tricky.
    It should be dunked as quickly as possible into the quench bath and moved around within the bath to cool the material off asap.
    A sandwich between thicker material may help but will need longer heating times.
    Ideally the bath should be consderably deeper than the dimension of the material being dunked so the material can be moved up and down to assist the quench.
    Also the material does not need to be brought down to room temperature to be quenched so pre-heating the oil (~100C) may reduce distortion.

    Be really careful with a molten salt bath, especially with any water as it explodes on contact with molten salt.

  4. #3
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    Rob
    I won't say much.
    But the reason is you have said it all and so much more in the last six months or so here.
    Tho I may never but never say never go to the lengths you are.
    They do make for some fascination reading.
    Your enthusiasm for your art form and yes I mean art form is very encouraging.
    Please keep posting
    And will be trying some more ideas my self once I'm back in my creative zone(I hate the man cave thing lol)
    Matt

  5. #4
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    Default

    Quote Originally Posted by BobL View Post

    Just a couple of points.
    If you could set out your results on a table that would make it much easier to compare what is going on.

    Pictures are not at all boring especially if you can photograph the process, how you heat, transfer to quench bath, Orientation of material while quenching etc.

    I'm not sure, but you seem to be confusing annealing with tempering?
    Even if you are not your post might sound confusing to a newbie.,
    The sequence is usual annealing to as soft as practicable, working the material, hardening, and then tempering.


    Quenching a big piece of flat steel is indeed very tricky.
    It should be dunked as quickly as possible into the quench bath and moved around within the bath to cool the material off asap.


    Be really careful with a molten salt bath, especially with any water as it explodes on contact with molten salt.
    Hi Bob,

    Yes, I plan data tables - I was at the end of the day at the time I posted the above and was too tired to make them up, I also want them to reflect all of the data I acquire.

    Yes, technically the adjustment of hardness is tempering/ drawing, fatigue dulls the mind.

    I am learning that heat distortion of thin sections can be dramatic. Cold water produced the worst, followed by warm water and so on up to hot oil which wasn't nearly as bad as cold water but still was unacceptable for a saw plate. I'm pretty close with hot oil so I think a bath that is just a little hotter will do the trick.

    Yes, I forgot the safety admonition. Hot troughs of molten oxidizers can get exciting in a hurry. Water, sawdust, windblown leaves, fingers and other organic materials can combine with hot oxidizers in important, surprising and extraordinarily unpleasant ways. The usual firefighting measures will often make the problem worse as well. I keep a fire blanket, sand bucket and a dry chemical extinguisher ready in my work area. Amazingly however the reference I am reading on salt bath tempering states that water can be added to them. Sounds a little too exciting to me so I think I'll check the results without water first.

    So no one who is not thoroughly familiar with safety techniques and the proper handling of dangerous processes such as I describe should try reproducing my projects. I have 30+ years of laboratory safety training and I still occasionally get my fingers nipped.

    I'll post more later, this is turning into a busy week.

    Cheers,
    Rob
    Attached Files Attached Files

  6. #5
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    Default Work hardening

    Please take this in the spirit it is intended - only to progress the discussion, and not in any way to detract from your good work. It's great to see actual measurement in action (as opposed to the usually more subjective posts that seem to proliferate)

    Have you considered the effects of work hardening? (post heat treatment - pretensioning or hammering as applied to circ saw baldes, and roll straightening of sawplate or setting of the teeth on handsaws?)

    I can imagine (...subjectively...) that setting (and perhaps reverse setting) teeth would have a significant work hardening effect at the tooth line. It is afterall only the teeth that are cutting.

    Regards,
    John
    Last edited by jcge; 21st February 2014 at 09:08 AM. Reason: spelling

  7. #6
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    Quote Originally Posted by jcge View Post
    Please take this in the spirit it is intended - only to progress the discussion, and not in any way to detract from your good work. It's great to see actual measurement in action (as opposed to the usually more subjective posts that seem to proliferate)

    Have you considered the effects of work hardening? (post heat treatment - pretensioning or hammering as applied to circ saw baldes, and roll straightening of sawplate or setting of the teeth on handsaws?)

    I can imagine (...subjectively...) that setting (and perhaps reverse setting) teeth would have a significant work hardening effect at the tooth line. It is afterall only the teeth that are cutting.

    Regards,
    John
    Hi John,

    I agree completely. The cold working will certainly have an effect on the hardness of the saw plate.

    I do projects like this because I want to know. I have a background in the sciences and I learn things through systematic and controlled studies. I reason that if Henry Disston did, or commissioned, studies that lead him to conclude that Rc 52 was the optimum hardness for saw blades he must have had very good reason to do so. However, over the intervening years his standard for hand saws has not evolved, at least publicly. I know that the blades of today's power driven saws using modern materials are true marvels, even those lacking tungsten carbide or other, now commonplace, exotica. But what of handsaws? What innovations have happened in the hand saw world in the last fifty years?

    There seems to be a consensus among present day saw makers and wood workers that 1095 spring steel is the best available material for making saw blades and I do not dispute that some makers are turning out really fine products. However given my inquisitive nature, my strong technical bent and my interest in tool making I want to know if it can be done better. These studies may be dead ends, but if they are at least we know that.


    Cheers,
    Rob

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    Interesting read. I should send you a couple of my saw plates and see what you find.

  9. #8
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    Quote Originally Posted by rob streeper View Post
    These studies may be dead ends, but if they are at least we know that.
    Cheers,
    Rob
    I really like your perspective Rob. There is clearly a defacto "standard" or benchmark but from where was it bourne? Was it established empirically, or by limitation or reason of economy, availability or applied study? And as to innovation beyond the benchmark ? Perhaps its all silently held close to the modern (boutique?) makers chests (who's work I also do admire).

    I'd not for a moment believe you're headed toward a dead end, and as you rightly say, at least we will know. I applaud your efforts and particularly your willingness to share your results.

    Regards,
    John

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    Quote Originally Posted by Ron Bontz View Post
    Interesting read. I should send you a couple of my saw plates and see what you find.
    Hi Ron,

    Sure, if you would like to send samples I am happy to help. I don't need a whole saw plate, just a couple of pieces a few square inches in size will do. The diamond indenter point makes a mark <1 mm to ~1.5/2mm in radius so I can test the area around the teeth easily.

    Cheers,
    Rob

  11. #10
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    Quote Originally Posted by jcge View Post
    I really like your perspective Rob. There is clearly a defacto "standard" or benchmark but from where was it bourne? Was it established empirically, or by limitation or reason of economy, availability or applied study? And as to innovation beyond the benchmark ? Perhaps its all silently held close to the modern (boutique?) makers chests (who's work I also do admire).

    I'd not for a moment believe you're headed toward a dead end, and as you rightly say, at least we will know. I applaud your efforts and particularly your willingness to share your results.

    Regards,
    John
    Thanks John,
    The apparent disparity between the levels of development of handsaws vs. power saw blades is really what prompted me to begin here. I recently took delivery of some Lenox bandsaw blades, their Diemaster II, that are made from M42. The blades are amazing stuff - sharp, durable and amazing performers. Why not use such steel for handsaws? Is such a step needed? Is it worth the expense? Can 1095 be better than it comes from the mill?

    Cheers,
    Rob

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    Here is what NIST has to say about Rockwell testing.

    NIST_Rockwell_Hardness-2.pdf

  13. #12
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    When I was bricklaying we use to go into a shop to the trowel section and hit the trowels against each other edge to edge. The soft ones would be cut about 5mm. We only bought the hard ones that did the cutting.
    I am learning, slowley.

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    Quote Originally Posted by rob streeper View Post
    ... But what of handsaws? What innovations have happened in the hand saw world in the last fifty years?

    ...
    Cheers,
    Rob
    I can answer this one
    Totes have become ugly and uncomfortable (and plastic)

    sorry you were talking about steel etc, carry on
    regards
    Nick
    veni, vidi,
    tornavi
    Without wood it's just ...

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    Quote Originally Posted by rob streeper View Post
    ... I am learning that heat distortion of thin sections can be dramatic. Cold water produced the worst, followed by warm water and so on up to hot oil which wasn't nearly as bad as cold water but still was unacceptable for a saw plate. I'm pretty close with hot oil so I think a bath that is just a little hotter will do the trick.
    William Clemson wanted me to point this out ...
    http://www.wkfinetools.com/hus-saws/...merican-01.asp
    Great job here.
    Cheers,
    Paul

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    Clemson might have something to contribute here ... but he's lost in on the razor thing I think ...

    http://www.wkfinetools.com/hus-saws/...gesInSteel.asp

    "SCIENTIFIC AMERICAN of December 8th, 1866

    Messrs. Editors "The singular quality of steel" alluded to in the SCIENTIFIC AMERICAN of Nov. 3d, is familiar to all saw makers.


    When a hand saw is hardened and tempered it is crooked but elastic. Hammering it straight takes from it its elasticity, but not its hardness; for it yet grinds and files hard. Grinding and, polishing will destroy the spring nearly as effectively as the hammering, either of which affects soft steel in a manner exactly the reverse.


    To restore the elasticity of saws, they are exposed to a degree of heat that will produce a straw color, and if not done too quickly this will be effectual. The color is removed with an acid; the presence or absence of it makes no difference in the elasticity of the saw; that is doubtless due to a certain definite arrangement of the grain of the steel."



    This link reminded me of the japanese swordsmiths coating the backs (edge?) of the blades in clay so the edge became hard and the back remained softer. Any application to backsaws? would a difference in hardness make a difference in straightness while cooling?
    http://www.wkfinetools.com/hus-saws/...ntAmerican.asp

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