the term heat-treating is misleaing...many different types of treating with heat produce drastically different results
Hardening....heating up a material red hot and queching or cooling it quickliy
making metal harder
Case Hardening....same as above but heating and then adding carbon...hardens just the top layers of the metal but not throuought like hardening
Annealing.....heat red hot and cool VERY slowly....makes hard metals more pliable or "softer"
shor blasts of high heat follwed by quick cooling, hardens steel...on the other hand,long periods of applied heat follwed by retarded cooling (slower then by air) softens steel
you don`t want to harden a frame....I worked for a custom V-twin MC aftermarket
parts manufacturer and we also built tens of thousands of custom frames for V-twin builds for big-name bike builders as seen on TV
soft grade metals like cold-rolled 1018 steel bend very easily ....it has a low carbon content....carbon molecules bond to nearly every element easily and in steel they act like a binding agent...more binder means a tighter hold and the steel can retain its rolled,forged,or cast shape
too much hardness from heattreating makes the metal very brittle...since the bond molecularly is so tight, the steel just snaps at the weakest spot....less hardened steel can stretch and bend quite a bit before it breaks...like the difference between a dead tree limb and a green one
on another note...welding two pcs. of metal together has it`s own dynamic...the weld itself is in a constant state of draw in every direction....or "stressed" as they call it....when a area that was welded cools by air or other means, it hardens a bit...add stress from the weld itself and you have a pre-stressed and hardened joint...technically it`s more brittle after you weld the joint and under stress looking for a weak link
some frame companies on the higher end had their stuff "stress-relieved"...I hear that process involves cooking the thing in hot oil for an extended period of time to relieve the stress on the welds
you can also "anneal" the metal....annealing is the process of softening a hardened metal using heat, and cooling naturally....the process involves continued heating of the metal to "burn-off" carbon molecules, thus softening the metal by lowering carbon content....we had to anneal lot`s of tool steel dies at work just so an endmill would even cut the stuff
steel like 4130 has a high carbon content and so it is harder stuff....not anywhere near as hard as a heat-treated part but strong enough not to bend or tear like 1018 tends to
most folks think frames are tubes welded together to hold the motor and wonder why they cost so much...chromoly tubing is expensive...there is probably close to $200 in chromoly material in a trike frame minimum...It took us roughly two weeks for an engineer to design each one of our various frame tables (jigs) and build them frome scratch...equated to about $6000 in labor for the least expensive jig and maybe $700 in material for the jig...we had 7 of them...takes a lot$$ to build a safe, quality product and unless you sold several thousand like we did, you have trouble justifying the cost up front for the venture
each Tig welder spent about 8 hours per frame cutting,bending,handfitting,and welding...they also weld in a sequnce to reduce stresses which cause warpage...just because you bolted the thing in 48 places to the jig doesn`t mean it won`t pop into a banana shape as soon as you unbolt the finished product
frames are NOT easy or cheap to produce and $1000 bux for a race frame is actually a VERY economical investment
moral is....don`t heat treat a stock frame....it will break faster than it did before....replace that thin-wall frame with a quality frame if you want reliability and strength
hardening
hope all that crap answered yer question