Man oh man, is it ever hard to do a brief
article on heat treating! I'm usually pretty good at summarizing technical information,
and putting it into layman terms. However, with heat-treating there are a lot
of complicated definitions and explanations.
Fortunately, for me there
are some good resources on the web and I'll list them in my conclusion on pre
and post-heat treating.
It is good that more people are starting to
realize how important pre and post-heat treating can be depending on the steel
used. It is a real shame how many welders out there know nothing about the steel
they are welding on. To be a good hand, you HAVE to know about the steel you are
working on, by knowing what takes place when it is heated and cooled.
once stopped a guy who was going to use re-bar for rungs on a ladder for a 200-foot
tall tower. What he didn't realize was the re-bar has a high carbon content and
when heated and rapidly cooled becomes brittle. Therefore, when some 220-pound
Iron Worker with 35 pounds of tools hanging on his belt, stepped down on one of
those rungs it could have easily broke. NOT a good scenario 200 feet in the air!
steel there are microscopic crystals, some bigger than others. The smaller the
crystals, and the closer they are to each other, the more strength, and hardness.
However, with that comes the downside of the steel being brittle. The larger the
crystals, and the farther away from each other, the steel will have less strength,
but will be more ductile. (flexible)
Using dashes from my laptop keyboard
let's say the small, close ones are from the high strength steel on my pocketknife
- - - - - -- - --
- -- - - - - -
- - - - - - - -
- - - - - - - - -
Using larger dashes, further from each other, a section
from a mild steel column or beam on a high-rise office building would be
___ ___ ___
___ ___ ___ ___
___ ___ ___ ___ ___
___ ___ ___
___ ___ ___
That's why the old timers changed from cast iron to mild steel
when they started building upwards back in the day. Cast iron would crack and
shatter when loads shifted from wind or weight of the floors built. With mild
steel the building will flex, yet return to its original shape. Another advantage
is that mild steel is not changed much when heat treated, thus welding in the
field has little effect on the joints.
When welding, there is a HAZ (heat-affected
zone.) The steel on each side of the weld is heated by the welding process, and
then cooled when the weld cools.
Iron MUST be pre-heated and post-heated to slow the cooling process or it will
crack. When repairing a crack on cast iron it can be very frustrating when the
crack spreads just ahead of the welding pool. This can make even the most laid-back
welder want to punch a wall!
Drill a hole in front of and at the
end of the crack. Pre-heat, weld, then post heat and cool. The hole will keep
the crack from spreading, and after properly welding the crack, you can fill in
Learn more about out Batch Ovens.