(Please note: I've crossposted this to both rec.crafts.metalworking and sci.engr.joining.welding since it's valid both places, and possibly of equal interest to readers of both, so set your follow-ups accordingly if you're concerned about it!)
So as my welding classes are coming to a close, I happened across some factoids in my welding text that have me wondering about the wonders of chromoly steel. The main one, though, is this whole thing about post-weld stress relieving using a torch. Just about every book I've read on fabrication suggests the same post-weld process, except for one; that book is "Performance Welding" by Richard Finch. Now previous discussions on the subject of Mr. Finch have led me to believe that he's not always playing with a full deck of cards, but my welding textbook for class said something that actually matches his opinion on the subject.
Finch writes that post-weld stress relieving of a weld in chromoly steel using a torch is completely worthless because proper stress relieving requires a six hour long process that simply can't be achieved with a torch. My textbook makes a vaguely similar assertion in that it says that proper stress relief of a welded joint can take anywhere from one hour to six hours for the heating segment of the process, with the point of diminishing returns on the increase in strength starting at around the six hour mark. However, its implication is that SOME amount of stress relieving will still have a benefit, but that the percentage of effectiveness is based on the total amount of time the part to be stress relieved is soaked in the heat.
Here, then, is my first question. Who is right on this subject? Is it worthless to even attempt post-weld stress relieving of a chromoly part, or can an appreciable amount of strength be regained through using a torch and allowing the part to cool in still air, or better yet, buried in sand? Does anyone know of a chart that might exist someplace that shows the relationship between gained strength and duration of applied heat?
That first question then leads me to my second question. According to Ron Fournier in his book "Metal Fabricator's Handbook" the best rule to follow with chromoly is to simply not use it unless you know EXACTLY why it is needed. And from my reading, I'm beginning to think that he's absolutely right on the money with that. So, when then would you actually need to use chromoly? I can only think of two times, that being when weight is a critical issue and when its strength makes it the only metal appropriate for the part while its deficiencies do not make for an undesirable failure mode (see my example in the next paragraph). Does that sound accurate?
As I tinker with cars a lot, I especially think of this in terms of car parts, and one part in particular where chromoly shows up a lot in the aftermarket is with suspension and chassis components. Mr. Fournier says to stay away from chromoly roll cages because they tend to break instead of bend, and that a broken up cage is infintely more likely to kill a driver than a bent up cage since bends absorb impact and breaks create sharp spears that turn a driver into hamburger. This sounds absolutely reasonable to me, after reading about chromoly's deficiencies. But now I also wonder, in anything but a track driven race vehicle, couldn't the decreased weight of a chromoly part have its value offset by the fact that it would break instead of bend? After all, if you were to, say, break a chromoly control arm on the track, there'd be a vehicle to tow you back to the pits. However, if you were offroading in the desert or being an idiot on the street, a broken control arm could leave you completely stranded whereas a bent up one might still allow you to limp home. It seems like chromoly's only place for street and offroad vehicles exists for parts like sway bars and other things where breakage is either statistically impossible or not particularly hazardous/lethal.
So those are the questions and my moment of pondering... I look forward to hear comments from the smart folks out there with more knowledge and experience on the subject than I. Thanks!