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273422 Matthew Groves <grovesthegrey@g...> 2021‑04‑09 Re: Froe made from vehicular leaf spring
From Gary
> Scott---
>   In practical terms, is there a difference between truck spring steel and
tool steel? Serious question!
>


From Scott
> OTS, old truck spring, would make a dandy froe.


>  But forged, hardened and tempered to spring temper it would make a 
> great froe!


There are a dozen other metallurgists and blacksmiths on the list that could
(and I hope do) say it better than I.

Here it goes.

1. There’s nothing wrong with making a froe from a truck spring, but it’s more
about convenient starting shape than it is about metal.

2. Not to disagree too heavily with Scott, but froes have been made out of soft
(ish) steel for quite a while and they work just great. No need for hardening or
tempering. No need for edge retention or anything else that would steer a person
toward some steel alloy capable of such. Mild steel, the kind that’s plentiful
is usually either 1018 or 1020 or A36. The 1018 and 1020 describe a steel that
has .18 or .20 percent of carbon in it’s formula. A36 doesn’t describe a steel
composition, but instead sets minimum performance standards for that steel.

3. Old truck springs usually get the label of 5160 steel, which again is a
composition label that among other things includes .60 percent carbon. It’s
often referred to as 60 “points” of carbon.

1018 has .18 percent carbon, so 18 “points”. It is not hardenable, yes even with
the galoot superquench.

Lots of automotive parts use medium carbon steels. These have enough carbon to
be hardenable, but that hardness depends on how much carbon there is.

Axles and tie rods are often 4140 steel (40 “points”, you now get it) and can
get hard. Spring is often 5160, and it gets harder than 4140 at it’s hardest.

“High carbon” steels are up in the 80 and 90 and 100 and 110 “point” range.

4. Here’s two things most non-metallurgists need to know about steel and
strength.

a) ALL steels have very similar modulus of elasticity, which means that if you
apply a force that makes them bend, they all bend about the same amount given
the same amount of force. This is called ELASTIC Deformation.

We’ve all seen steel get bent by a force, but spring back when that force is
removed. And we’ve all seen steel get bent to an extent that when the force is
removed, even though it springs back some, there is some permanent bend
introduce. This is called PLASTIC Deformation.

b) higher strength steels, even though they have the same amount of ELASTIC
deformation as low carbon steels, can withstand a LOT more force before they
PLASTICALLY deform. This is why spring applications (where you WANT it to return
to form) involve alloys of at least a medium carbon steel.

5. The medium and higher carbon steels do not need to be hardened in order to
have this higher elastic deformation limit. A froe made from truck spring (5160)
will be tougher just because it’s 5160, even when you don’t harden or temper it.
It’s automatically a superior froe, and there’s no need to proceed with those
steps.

6. A hardened steel is virtually always a more brittle steel. That’s the
tradeoff. So chisels aiming for rockwell 62 for that super keen edge are also
gonna be brittle. This is a generalization. Fancy alloys are always coming down
the pike. A froe needs to be tough way more than it needs to be hard. It’s
subject to many forces form the handle and the froe club. Also if it DOES fail,
you’d much rather it bend than snap.

7. The upper limit for carbon that contributes to hardenability is in the range
of 1.3%. Past that, and the carbon does things that reduce the hardenability.
Cast iron has 2 to 4% carbon, for instance. Not hardenable AND brittle!

8. There are tons of alloys out there and each is like a recipe with different
methods of achieving it’s max softness and hardness. Sometimes designated by the
speed of it’s quench. O1 and O2 are very forgiving alloys that harden fine in
Oil, hence the O. W1 and W2 are alloys that need the speed a Water quench
provides, hence the W. A1 and A2 are air quenching alloys, as are D2, L2, A33,
H13, and S7.

Obviously, I’m leaving a lot out, but these are basics.


Here’s the short version: You CAN make a froe out of truck spring, but mild
steel works fine and hammers much easier. Don’t bother hardening or tempering.


A metallurgist in another life,
Matthew Groves
Springfield, MO

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