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Hardness for Sear Surfaces

Discussion in 'Gunsmithing and Repairs' started by Johnm1, Jul 12, 2020.

  1. Johnm1

    Johnm1 Member

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    Now that I am searching for a replacement sear/sear bar for the 25 I'm working on I thought I'd do some research on hand machining the part myself. Not at all likely to do it, but might as well learn something about the process.

    What hardness does a sear surface need to be to not wear out prematurely?

    I figure most sears are made from tool steel but there is a fairly large range of hardness for tool steel. General purpose tool steels are O-1 Rockwell C 65, A-2 C 60-62, and D-2 C 62-64. I read that better wear characteristics are found in the A-2 and D-2 grades but the hardness numbers don't seem to prove that out.

    I'm looking at https://www.hudsontoolsteel.com/technical-data/steelO1 for my basic information.

    It is my understanding that all of the tool steels purchased at Hudson would arrive in the annealed state and would be suitable for shaping with conventional tools before hardening. Does that mean what I think it means? As in it will be 'softer' as it arrives and needs to be hardened after it is shaped.

    I realize this option would require heat treating and that is another bit of research. I can reach the required temperatures but I have no idea how I'd maintain the required temperatures for the durations required. Keep in mind, this is a one off proposition. So I will have to make do with what I can get at an affordable cost. Meaning, I won't end up with a full size furnace with computer controlled temperatures. But heat treating is the second part of the equation that can be addressed later if this thought even has a chance of working. Heck, I may find someone locally that could heat treat my finished project.

    So for now, lets stick to the basic question, what material should a sear be made of and what hardness. I think the general purpose tool steels are a good place to start but maybe there are other possibilities.
     
  2. BBBBill

    BBBBill Member

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    Good sears available today run around 50-55 on the RC scale. 4340, S7, and A-2 are pretty common.
     
  3. Johnm1

    Johnm1 Member

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    Thanks Bill,

    As it turns out my wife has a jewelers kiln large enough for a piece this size with a maximum temperature of 1900 degrees. Just enough for A2 steel that has an Austenitizing temperature of 1725-1750. It would be done manually as this kiln has manual controls. So I'll have to monitor the temperatures and adjust by hand. I have a thermometer/thermocouple on the way. It's small and slow, but its a one off piece and will be fun to try. I'll get some tool steel locally and practice on some simple shapes to see if I can shape and harden the material.

    It will be interesting to see what tools I need to shape the material. Alost everything I do is by hand tool. The only machine I have is a drill press.
     
    BBBBill likes this.
  4. beag_nut

    beag_nut Member

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    The heat-treating oven we used at work (making surgical instruments) was operated under vacuum. The only time we did not draw a vacuum was when we were hardening air-hardening tool steel. (Starrett makes/made some of the best.) Just a warning ahead of time. You don't want to look at regular tool steel after hardening in normal atmosphere (in an oven).
     
  5. Jackrabbit1957

    Jackrabbit1957 Member

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    Johnm1 a small vise and a set of files will do you well on this project. I went to Colorado School of Trades a hundred years ago and one of the projects involved making a firing pin for a Remington slide action .22. using strictly files and a small ruler. This was a somewhat complex piece compared to yours so it can be done with files and time and patience. As to hardening the sear surface, Kasenit will get you there without a lot of fuss. Using tool steel is a good idea and heat treating is really the best option, though there are other methods available.
     
    SGW Gunsmith likes this.
  6. Jim Watson

    Jim Watson Member

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    What I was thinking. Tool steel and an industrial hardening service sounds like overkill for a .25. Mild steel you can work with and a dash of Kasenit on the sear surface should serve.
     
  7. Johnm1

    Johnm1 Member

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    Is the finished product just uglier than when hardening under vacuum?

    I'd like to see if I can harden something. It may not work. If all else fails I can always come back to the Kasenit.

    As I understand it, the tool steel will arrive annealed and at least somewhat workable. I doubt it will be as workable as mild steel. It isn't an expensive experiment to buy the tool steel. If I can't work it, I'll come up with an alternative.

    In reality this is just an experiment to see if I can do it. I have 2 1/2 years before I retire. I wish I were there now.
     
    unclenunzie likes this.
  8. stringbeany

    stringbeany Member

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    For heat treating a2, it is done in a vacuum furnace or wrapped in stainless tool wrap. The tool wrap is like a heavy tin foil. When heating a2 above 1000 degress or so, the presence of air will cause Decarburization. This will look heavily scaled and will remove carbon from the steel.
    This is how I used to heat treat a2;
    Wrap steel with stainless tool wrap. I would make a pouch and fold each of the sides 3 times. I would start by folding a 18 by 36 inch piece in half to get a 18 inch square. Then you fold in each of your sides 1 inch and crease it, then fold again 2 more times creasing each time.This makes it air tight and curl up a corner so you can grab it with long tongs.
    Place your pouch in the oven, and set it for 1250f
    Once its at that temp, let it sit for a hour minimum and/or hour per inch of thickness.
    This normalizes and preheats everything.
    Then turn up oven to 1775f
    Once at that temp, let sit for another hour per inch, hour minimum.
    Once your time is up, you need to pull it out and set it on a cooling rack while still in the pouch.
    Have a fan ready and blowing on the pouch. I would have a fan blowing into oven also to cool it down.
    Keep your pouch sealed until it is black cherry or 1000f.
    Then rip open and cool completely.
    Your part should be about 62 rockwell C.
    This is too brittle.
    You then can put it back into a cool oven, without any tool wrap and set it for 400f.
    Once its at temperature, leave it in for 4 hours.
    Take it out and let cool without fans.
    Part should be about 59 rockwell C.
    Once heat treated, it will be next to impossible to shape or drill. You want your part useable before this. You can sand or hone while hard of course. You could mill on it if you had a ridgid setup in a mill. Even soft, a2 can be pretty tough on any high speed drill bits, carbide is the best.
    Be careful moving around that hot of steel, and be aware that sharp part edges of your part could puncture the pouch, especially if its a heavy part. Don't even think of touching that with anything other than metal tools or tongs. I would suggest something like 2 feet long.
    Good luck and keep us posted !
     
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  9. beag_nut

    beag_nut Member

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    "Is the finished product just uglier than when hardening under vacuum?"

    Scale, sometimes in the form of thin layers, will tend to form on the surface. That then needs to be removed before tempering or use. stringbeany's method also works well, but requires something like the wrap, to protect the surface. Thus, someone invented the vacuum oven.
    Air-hardening steel is somewhat more forgiving and easier to use. But you'll have to order it, unless you have a true industrial supplier nearby.
    I also wish you good luck. No matter what, you will learn something new.
     
  10. Johnm1

    Johnm1 Member

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    Very interesting, and thanks for your detailed steps. They coincide with what I have read on Hudson's website. The more I read the more of this sinks in. I have read of the foil wraps before but was unclear why they were used.

    I live just outside of Phoenix Arizona and we do have a couple of industrial suppliers including Arizona Tool Steel in Tempe, and Apache Steel in Phoenix. I haven't contacted them for availability yet.

    So, I know what I just learned from the internet these last 36 or so hours (that ought to scare anyone) and I generally know that general purpose tool steel comes in the three basic flavors, O1, A2, and D2. I figure there are dozens of other possibly suitable steels. I focused on the general purpose steels only because I thought I could learn more about a general product than one of the more special materials. From what I have learned, again in the last 36 hours, the selection between O1, A2, and D2 is pretty safe and I'm leaning towards the A2. Is that a good choice? The part is small, 1.5" long, 0.27" wide, by 0.60" thick with the thickest portion at 0.34" thick. H R 25 Sear with Spring.jpg

    Thanks again. Hopefully this experiment won't end like the last one that destroyed the above part starting me on my quest to machine a new piece and harden it.

    In the end, I may well take Jackrabbit's and Jim Watson's advice and use a lesser steel and Kasenit to harden the surfaces that need it.


     
  11. David Hoback

    David Hoback Member

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    Hello Johnm1,
    As it happens, besides a gunsmith/builder I’m also a custom Knifemaker. And I’ve spent the last 8 years in the study of metallurgy. Lot of good info being given...but much of it is mixed up. You are correct, the medium carbon Tool steels are the best candidate. Although simple Carbon steels like 1080 also work well. A2 works fine...S7 is a favorite of many. There are other steels that I’m sure very few here have ever heard of which are also good picks. Hudsen is good, although they don’t sell in small quantity. You can purchase smaller pieces here. https://www.alphaknifesupply.com/shop/product-category/blade-materials

    You could also PM me if you want to try something else for free. I have various scraps of CPM3V Tool steel, which would also work. It has much of the characteristics as S7..I.e., toughness, HRC range. I would gladly drop a piece in an envelope and send you to try. Let me know. Or if you have any other steel questions... feel free to ask.
     
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  12. Johnm1

    Johnm1 Member

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    Wow David, that is very generous of you. And, I'm likely to take you up on your offer. I'm still in the gathering information stage at this point, so give me a little bit. I'll do the internet thing on S7 but in annealed state, how difficult will S7 be to work/shape? Remember the most sophisticated machines I have to work with are a drill press, bench grinder, and a Dremel. The rest is all hand files. Though I must admit that I have gathered quite a few files for many different applications.

    The exercise of hardening tool steel is very intriguing to me and even if I can't shape the piece, I'd like to follow up on that part.

    Well, I did the unthinkable. I purchased another working firearm so I would have a part that I could duplicate. My old piece is in two pieces and even though I could place it together for a decent sample I couldn't be sure it was exactly right. In the end, I'll only have one copy of this firearm though. I'll continue that story on the thread about an amateur mistake.

    I will continue on this task of making the part and attempting to harden it. It will start with a simple piece with a sear type surface that I try to harden just to see if I can with the equipment I have. That being a jeweler's oven and a thermocouple/thermostat. Not sure how well that is going to work as I'll have to monitor and adjust the temperature for the entire duration.
     
  13. David Hoback

    David Hoback Member

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    Not a problem my friend. I’ll check a couple of my steel suppliers. See if they have some S7 in smaller pieces. That’s the problem with places like Hudsen. Smallest you can get is like a 1”x36” in the thickness you want. I’m thinking 1/8” thickness will work well.
    But I’ll see if I can find one of the suppliers that you can order from.

    S7 isn’t very difficult to work. Your bench grinder will do the bulk shaping guide nicely. Then finish with your Dremel, files & sanding. This is exactly how I make my knives. It’s called “Stock Removal”. I start with a piece of bar-stock and use my belt grinder for the bulk of work.

    You said you have an electric kiln you can use? If so, you’ll be fine to heat treat it yourself. I can even even give you a good heat treat “recipe“ for S7. I’m quite familiar with it.
     
  14. Johnm1

    Johnm1 Member

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    I do have a small kiln. The next test will be to see if I can manually control the temperature for a couple of hours. It is a dumb kiln without a controller. I have a thermocouple/thermometer on the way. So we'll be doing some testing this weekend. If I were to be doing this a lot I'd invest in something a bit more robust.
     
  15. David Hoback

    David Hoback Member

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    Your kiln needs to be able to reach 1725F degree. Soak the part for at least 30mins.

    Check your kiln with the thermocouple. If you can’t reach 1725F, don’t even try it. Won’t harden correctly without. Let me know your outcome. If you have a problem, I’ll give you info to my heat treater. Peters Heat Treat. Best in the country!
     
  16. Johnm1

    Johnm1 Member

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    The kiln is older but is supposed to reach 1900 degrees. I'm going to test it when my thermocouple/thermostat arrives. Should be today or tomorrow.

    It does have a rheostat with markings that appear to function. It does get hotter as it is turned up. If I want to monitor temperature continuously I'll have to drill a hole through the top to mount it inside.
     
  17. WestKentucky

    WestKentucky Member

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    I have been around a lot of heat treating in an industrial capacity, though not normally tool steels although one of the in-house die shops did harden trims for stamping presses. Scale or decarburization typically is indication of a chemistry change, and usually not a good one. If I remember correctly, decarburized spring steels would break easily, and had a very grainy look when they did break.

    For the at-home hardening, I would be very skeptical about doing anything more than surface hardening because metals quite often twist and contort as they cool because it is VERY difficult to get them to cool uniformly. You could literally spend days to get a perfect part only to have it twist in cool down and be out of spec and no longer functional. There are good temperature indicative paints available and a lot of knife makers use them to know what temperature their blades are at to control the hardness characteristics. Those paints would be a very wise investment, even for someone who has a decent handle on temperature control.
     
  18. Johnm1

    Johnm1 Member

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    Considering the size and shape of the piece I am working on, distortion is a pretty large concern of mine. That is why I want to test on a sample first.

    As far as temperature control goes, I'm still not sure exactly how I'm going to manage it. The kiln is tiny and opening the top will drop the temperature dramatically and very quickly. Inserting a thermocouple into the kiln will actually take up a measurable amount of volume of the kiln. I'd prefer to rely on the setting of the rheostat to maintain the temperature but I'm not sure I can do that. Only testing will tell.

    Where does one procure the temperature sensitive paints?
     
  19. David Hoback

    David Hoback Member

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    LOL! Warp is a number 2 concern for us knifemakers. The part you want to make..., a sear is actually much less prone to warping than a knife blade. But there is a good counter to warping I can teach you. It’s actually part of the quenching technique.

    S7 is an air quenching steel. As is my favorite steel to make knives, CPM3V. A favorite method is to use two pieces of aluminum and CLAMP the part between immediately after removing from the kiln. The aluminum, acting as a heat sink, quickly draws heat away from the part. Allowing the proper hardening, while also keeping the part from warping.

    This is how ALL Knifemakers quench their blades made high allow, high speed steels which require air hardening. It is extremely efficient and quite safe for the part. It allows knifemakers to work with extremely thin steels. Some of my CPM3V knives are “through hardened“, 60-61HRC & .090” at the spine. And I’ve never encountered a single warp. As I said before...CPM3V is quite similar to S7.
     
  20. Johnm1

    Johnm1 Member

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    That is very interesting David,

    On another note that I'll have to deal with first, while shaping, or stock removal, how does one control the heat generated by either a bench grinder or other high speed tool? The grinder doesn't have the ability to flow oil across the surface to cool the piece. Do you just go slow and not allow the piece to get too hot?

    I'm going to follow up on the offer for a piece of the S7 in a PM. The thermocouple/thermometer hasn't shipped yet, so I might as well start shaping a piece and see how that goes.
     
  21. David Hoback

    David Hoback Member

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    I responded to your PM. I’m sorry, you misunderstood. I have scrap CPM3V. It’s a Tool steel with several characteristics very much like S7.

    As for heat while grinding. Keep a bucket of cool water next to you.Just grind until it’s too hot to hold, and dunk in a bucket of water. Exactly what I do.
     
  22. Johnm1

    Johnm1 Member

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    I can't understand my wife and we've been married for 30 years. Whatever you have is fine.

    I understand on the heat.
     
  23. Johnm1

    Johnm1 Member

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    I just looked over the specs for the CPM3V and I have just enough heat in that little kiln to harden it. And actually at that temperature it should be the most applicable to the service. At this point I'm more focused on shaping the material and what I'll have to do to accomplish that.
     
  24. SGW Gunsmith

    SGW Gunsmith Member

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    I remember reading a paragraph from a P.O. Ackley book on doing gun repair when I was a "cub" tool maker. In that book he mentioned how difficult it would be to actually make a replacement firing pin for the Remington Model 12 pump action .22 rifle. So, he gave me a challenge: ;)

    MBd05rKl.jpg

    Any tinkerer, with the will and the stubbornness to succeed, using basic tooling, along with the required patience, can accomplish most anything, if they want too, bad enough. I needed to make the tap ( used 0-1 oil-hardening steel ) for the nut that goes into the receiver for the take-down screw on the Remington Model 12 rifle, only because I couldn't find that size thread anywhere. I can only encourage you to stick with your tinkering, as it can be very therapeutic, and rewarding when you get the job done.
    Get yourself a Brownells catalog. They sell various paste on liquids that once heated will tell you approximately what the metal hardness has been reached.
     
  25. David Hoback

    David Hoback Member

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    I like your thinking. My thoughts exactly. Anyone with the desire can accomplish this. Precisely what prompted me to start making knives in the first place! I started out, and things were a bit crude.. yes. But after just a couple years and some practice, I was turning out pieces like this
    05-D48943-98-F9-401-A-997-F-91-E5-AD15532-D.jpg
    E6-F3-A6-D2-2260-46-F1-AE8-E-C8-AFC6-DAD95-F.jpg

    And getting into machining, I’ve made several small gun parts & even a full custom shoulder stock for my bench rifle. Like SGW said, all it takes is the drive.
    8-D3991-CF-9052-4559-822-C-8-A01-E75971-D8.jpg


    The liquid you’re thinking of is called Tempilaq. Unfortunately, it’s only for meant for basic carbon steels. Steels which are hardened at under 1500F degrees. The Tool steels we are talking about are High alloy/High speed steels. These are hardened at 1725-1950F degrees. No temp guessing liquid is made that withstand those temps.

    For accurate temp control, either a thermocouple or testing material is needed. (If the kiln has presets).
     
    Last edited: Jul 18, 2020
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