Before I buy Leaded Steel

I kinda asked this question once before, and got a bunch of answers that all seemed off the point. So I'm going to try again, and (hopefully)
make my question specific enough this time.
Everybody tells me "use 12L14, it's really easy to machine". I'm willing to accept that -- I'm using steel that I get surplus, and there's a wide range of machinabilities that I'm experiencing; I can certainly see how using the _right_ steel might make a significant difference. But:
If I take some annealed or normalized 1065 steel (or other 10"something big" steel), get it to the shape I want, and properly heat treat it, it's going to be a _lot_ harder than when I started. So assuming that I can machine it in the first place, then get it to hold its shape (or correct its shape after the fact) and not crack in the heat treat process, I'm way ahead.
On the other hand, if I take 12L14, get it to the shape I want, and do the _very best heat treat in the world_, then -- because of the low carbon content -- it's not going to be much harder. Right?
What about 1144 or 41L40? Do you get much by heat treating these? How easy are they to machine compared to 12L14? Why isn't there a 12L50?
I've heard tell that 1144 isn't as strong as other steels of similar carbon content -- true? What about the mechanical properties of 41L40 after heat treat?
I'm basically looking for a steel that'll be suitable for model airplane crank shafts. The advice that I get from the hobby builders of model airplane engines is "use 12L14 and don't crash". I'm not going to discount this -- but it'd be nice to be able to do something on par with what I buy at the store. So, basically, something that's easy to machine, but that'll harden up to something between a grade 5 bolt and a grade 8 bolt.
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These are really good questions, and I wish I had a specific answer, but I haven't machined some of those grades and I'm a little confused about the properties you want.
If "everybody" is getting away with 12L14, at least in normal flight, they must not need a lot of strength or hardness. They'll get decent toughness with that grade, at the expense of a low yield point. In other words, it may not break in a crash, but it will bend, right?
It sounds like you want more strength and toughness. If so, 4140 is a good choice. Unlike 12L14, adding the "L" in 41L40 doesn't result in a radically easier-to-machine steel. It gives about 25% better machineability than 4140, which is mostly of interest in production screw-machine operations.
4340 is better for toughness and strength. These 4000-series alloys get more hardenability than the 40 points of carbon would suggest, because the chromium content aids hardening. And those two are tough steels, reasonable to machine in the normalized condition (which is a way they're commonly sold), and slow-quenching for safe hardening. 4340 is basically oil-hardening -- almost air-hardening in small thicknesses.
If it were me, I'd go for 4340. If 12L14 is hard and strong enough in flight, then 4340 normalized (around 90 ksi yield, IIRC) is a lot stronger than you need for normal flight, and it's very tough. I'd think it would be a good choice to tolerate modest crashes. If you leave it normalized, you wouldn't even have to heat-treat. If you have a fairly rigid lathe, it's a little slow but not difficult to machine.
This is a semi-educated guess, but I think the ultimate material for small-engine cranks might be 8620. It's a low-carbon alloy steel that's really tough. It's made to be case-hardened. If you aren't going to case-harden, then I'd revert to 4340.
If you can get a piece of 4340 normalized to try, and if you find that you can machine it Ok, then you might be set. It's a common grade.
Another thing I know little about is the surface finish you can get, machining small pieces of these various alloys. 12L14 can produce a nice finish even for hackers. With 4340, it could be more difficult, especially if you use the annealed grade. I'd consider finishing the crank and bearing journal(s) by lapping, anyway.
Good luck.
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Ed Huntress



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Have you ever turned a grade 8 bolt? I think they machine pretty well with HSS.
jsw
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wrote:

Tim, I can't remember what you got for machines. I use plain old structural cold rolled for most stuff. Its cheap and i got a lot of it. Machines easily on my equipment.
If i need any perfomrance at all, I move up to 4140 (I assume that's what you mean by 41L40) It does machine a bit harder but not bad. On my equipment again.
IMHO 12L14 is an expensive low performance material. I'd only use it if my machines were marginal as it machines like butter.
karl
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On Mon, 29 Aug 2011 17:57:03 -0500, Karl Townsend wrote:

41L40 is in the McMaster catalog as an easier-to-machine version of 4140 -- I was wondering if (a) it's really easier to machine, and (b) if the lead interferes with the strength.
I've got a Smithy, and I rarely have trouble machining the "Mystery metal" that I get for cheap at the local surplus place. Sometimes I have to take really light cuts, and getting a really dynamite finish is still a bit of a mystery for me, but I never have _problems_.
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On Mon, 29 Aug 2011 16:20:09 -0500, Tim Wescott
<snip>

<snip>
It is possible to get 12L14 glass/file hard by case hardening. You can do this at home with a propane torch. You may get better results/less distortion from a commercial heat treater if they can/will piggyback your parts with a commercial load.
about http://en.wikipedia.org/wiki/Case_hardening
buy supplies from http://www.dixiegunworks.com/product_info.php?products_idd65 http://www.midwayusa.com/viewProduct/?productNumber 9479 there are other brands available
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Take a look at Stressproof. It machines fairly well and has pretty good mechanical properties without requiring heat treatment. It is a premium 1144.
http://www.niagaralasalle.com/product-stressproof.html
Dan
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    Right -- as far as you went. Do you want a surface hardness (e.g. to serve as a bearing) or as a through hardness? Note that the through hardness is going to make it more likely to break under stress.
    And you can get the surface hardness with a case hardening compound, without the stresses which through hardening can produce.
    But the leaded steels don't weld well.
    41L42 has a machinability rated at 70% of 1212 (135 SFM)     12L14 has a machinability rated at 197% of 1212 (325 SFM)     1214 is also called "Leaded Grade A"     Leaded Grade B 236% of 1212 (390 SFM)     Leaded Grades AX, AY, AZ 303% of 1212 (500 SFM)
    This is all from the Joregensen Steel book No. 71.

    Probably because the leading and sulfur which aid machinability fight the carbon in the supposed 12L50.

    Look it up on the web. There are sites which a Google search will discover.

    After what cycle of heat treat? You heat it to between 1525 F and 1625 F, and then quench in oil. This gives the maximum hardness (and brittle, too). Then you heat it so somewhere between 400 F and 1300 F depending on how much of that hardness you want to get rid of.
    And no, the pages which I was looking at don't tell me how hard it will be after quenching, so I would have to do a Google search to look that up too -- *that* is your job, not mine.

    I don't think that you will find both "easy to machine" and Grade 5 to Grade 8 bolt hardness in the same metal.
    Since this is for a model, so a life is not hanging in the balance, I would go for the 12L14 to make it easier to make the crankshaft, perhaps use case hardening for bearing surfaces (and grind to a finish) and try to follow the "and don't crash" advice. It is probably easier to make four of the 12L14 or better alloy crankshafts at one time, than to do the same machining on a 14L40 for one. (Also, the case hardening should produce less distortion during the heat treating, so you won't have to leave as much extra metal for finish grinding.)
    I know that I try to keep 12L14 on hand and use it for any project which does not need the extra strength and hardenability of the other alloys. It is a true pleasure to machine and produces a very nice finish. Some of these days, I need to get some of the "Leaded Grades AX, AY, or AZ" steels and try them.
    Good Luck,         DoN.
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wrote:

"Stressproof" (a modified 1144) has a yield around 100ksi, about halfway between grade 5 and grade 8 screws. It isn't as easy to machine as 12L14, but it's not bad considering its strength. This table rates it about the same as 1018, which seems about right. http://www.carbidedepot.com/formulas-machinability.htm
McMaster carries generic Stressproof on p. 3633.
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I made my replacement vertical mill drawbar out of 1144 stressproof. It was easy to machine on the Clausing 5914, although the machining forces were about double those of 1018.
Joe Gwinn
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For me, the free machining leaded steel 12L14 is very easy to cut, but bad for lapping together parts, because is galls so quickly.
For me, the tool steel, annealed 4140 is easy to drill, until the high speed drill gets dull, and then the 4140 gets locally hard. I have to order a carbide drill of that size to cut through the hard spot.
For me, 1018 steel is the go to steel.
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On Tue, 30 Aug 2011 11:20:26 -0700 (PDT), " snipped-for-privacy@gmail.com"

I've most certainly workhardened a spot in 4140. But its not that bad. Just sharpen the drill and go real slow with oil till you get a couple chips. Then sharpen the drill again. After learning this the hard way, I watch tool condition pretty close. Far easier to not get in trouble Karl
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It isn't going to get much harder, you would have to case harden it (carbon packing), and it doesn't weld well. Sure is nice to turn though. I have a bunch of barends from when we ran a Index G200.
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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