I've
noted that there is a deafening silence from
qualified metallurgists, so as a chartered
mechanical engineer, I'll advise what little I
know on this subject. ?NO specialist is going to
proffer free advice or any advice that allows you
to proceed safely without a.) knowledge of what
the steel consists of, b.) the application,
including stresses, safety implications etc. and
c.) professional indemnity insurance, so what
follows will deliberately be vague while
explaining the difficulties against which you are
up.
?
The
standard advice to welders who want to weld high
strength steels is just don't. ?If steel is heat
treated, it implies that there was some objective
in heat treating it, such as hardening, and
therefore it is at least medium if not high
strength.
?
That
instruction can be qualified to some extent by
someone with a higher level of qualification.
?
Steel
that is nominally not hardenable by heat treatment
is close to pure iron and is referred to as mild
steel. ?This is used for structures, boilers, etc.
and the whole point is that the steel and its weld
are ductile.
?
To
make it hardenable, various alloying elements are
added, the most basic of which is carbon. ?This
allows the formation of iron carbide, which is
very hard. ?To achieve maximum hardness, this
requires a very quick quench, so you cannot
through harden thick sections. ?To slow down the
quench requirements and allow thicker sections to
be through hardened various other alloying
elements can be added, such as Chromium, Vanadium,
Cobalt, Nickel, Manganese etc. ?(Manganese is in
virtually all steel anyway, just as a means of
mopping up impurities like Phosphorous and
Sulphur. ?In higher quantities than required to
balance the impurities, it leads to extreme work
hardening.)
?
In
the periodic table of elements, Hydrogen is
grouped with the metals. ?It is soluble in liquid
iron. ?For reasons I don't understand, it doesn't
cause so much of a problem in thin mild steel, but
in hardenable steel, or even thick mild steel, it
comes out of solution during the solidification
process and congregates at the grain boundaries,
so the grains are not stuck together as well,
thereby causing gross lack of strength. ?There is
a limit to the % carbon in the steel beyond which
you just cannot successfully weld. ?Each alloying
element is given a carbon equivalent, usually less
than 1. ?You add all those up in proportion to
their % in the composition and add them to the
carbon %. ?At a lower figure than the limit,
welding can be successful but only if a suitable
procedure is devised and followed. ?This typically
includes the exclusion of hydrogen (moisture,
which is why rods are baked for extensive
periods), preheating the job for a period to a
specified temperature, maintaining a certain
minimum temperature during the welding,
maintaining a specified temperature for a
significant period after the welding to allow the
hydrogen to diffuse out of the job, which it does
only slowly, and finally cooling very slowly.
?This is fine if you don't mind losing all the
high strength properties as a result of the slow
cooling. ?Never gone further than this myself, but
it may then be possible to reharden the whole job
in the normal way.
?
Very
sorry, but I feel you're going to have to pay a
pro for a procedure if the job is critical, or try
some of the above and test it if a few failures
are of little consequence. ?There should be info
on t'interweb thingy about carbon equivalents and
limits.
?
Eddie
?
?
------ Original Message ------
From: ask_derrick@...
To: [email protected]
Sent: Wednesday, May 15th 2024, 19:25
Subject: [SouthBendLathe] Anyone in the Group a
Degreed Metallurgist?
I am working on a project and need some advice on
welding heat treated materials.
Ask_derrick@...
