fxdlrider said:
my welding buddy is a welder in an aircraft repair shop. He is used to working with exotic metals and stuff, so I am going to assume he knows what he is talking about. Maybe I'm wrong too, but wouldn't less heat over a wider area spread that stress out more?
Before you draw any conclusions, I'd suggest you look up <Heat Affected Zone> and research the heat-treatment, tempering, hardening, annealing, and normalizing processes. You'll be able to find it explained online rather better than I'm about to do here (because it would take up
far too much space for me to do properly...)
However, what you end up with in the HAZ is an area where the heat-treatment and/or basic "hardness" of the iron or steel has been altered - usually increased - by the fusion heat and relatively rapid cooling (there are some "air-hardening" tool steels out there - I've worked with a couple.) Yes, letting iron/steel sections cool in open fresh air counts as "rapid" cooling from a metallurgical point of view - so you wind up with a hard "band" in between two zones of normal metal.
"Hard" - in a metallic sense, means "strong." "Hard" also means "brittle" - like glass. That's why I sometimes advice
against using harder screws in some applications - the softer grades don't fail as dramatically (read: "explode under stress") and give you more warning...)
Post-weld heat-treatment is a variation of annealing which is done to reduce the relative hardness in the HAZ (are your eyes glazing over yet?) This is especially important in cast sections, due to the microscopic grain structure of the metal.
In "wrought" stock (like the tubes,) the metal is hot- and/or cold-worked into shape. This results in a definite microscopic grain structure to the metal - which can be maximised in a structural sense, and you take advantage of it when you need strength in a particular direction (in beams, tubes, bars, and the like - it's invariably a "long grain." Sheets can be cut either way.) This is
just like the grain in wood - but much smaller.
In "cast" stock (like the centre section or the knuckles,) the metal is melted and poured into a mould. It cools in no particular manner, leading to what is called an "amourphous" grain structure. Think "particle board" here, and you're on the right track.
Cast items can be heated and formed (called "forging") or heated to a critical temperature (the "transformation point" of the alloy) and the microscopic grains can be realigned. "Forging" is similar to what is done to "wrought" stock - it can even be done "hot" or "cold" (above or below transformation temperature points.) "Hot working" is better from a structural point of view, but results in scale formation on the metal (which must be removed before fusion processes can be used.) "Cold working" doesn't give you the scale, but doesn't allow for as much refinement of the grain structure, either.)
The transformation point of the alloy is
always below the melting point. The welding ("fusion") temperature is
always at the melting point
or slightly higher. So, welding can instantly wreck heat treatment and hardening - and you'll have to do it over again. Also, the weld zone itself is more like "cast" material - even if welding two pieces of "wrought" stock together. The only way to get away from that - "forge welding" (what blacksmiths used to do on the anvil.)
I've probably given you a headache by now, so I'll keep shut. However, searches for <iron metallurgy> or just pillaging around aisi.org should prove rewarding, if you want further information.