On 2011-08-29, Tim Wescott wrote:
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?
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.
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?
Probably because the leading and sulfur which aid machinability
fight the carbon in the supposed 12L50.
I've heard tell that 1144 isn't as strong as other steels of similar
carbon content -- true?
Look it up on the web. There are sites which a Google search
will discover.
What about the mechanical properties of 41L40
after heat treat?
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'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.
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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