Moved from C & V Appraisals.
Look around the lugs to see if the brazing material melted.
The brazing material (likely brass) has a melting point lower than the steel (or steel ally) used in the tubes, and the steel tubes (at least the ends of them) were heated to above the melting point of the braze material when the frame was made. Plus carbon steel frames can be repaired by heating the lugs to melt the braze material, allowing a damaged tube to be removed, and a new one inserted. So if the braze material did not melt, the steel tubes were not heated by the fire to a higher temperature than they were when the frame was made.
Steel and steel alloys can lose strength if they are kept for a long time at high temperature that is lower than their melting point. For structural steel building I think the rule of thumb is 1 hour at 900 degree F will start to weaken the steel. Plain carbon steel will regain at least some of its strength when cooled. Some steel alloys may not regain strength when cooled, but I think these alloys have to be kept at a temperature close to what was used to temper them in the first place, and I’m pretty sure that is above the melting point of brass.
Last edited by 0.2HP; 08-08-13 at 02:55 PM.
The bluish coloration means that if the tubing was harden and tempered (which it may not have been then the temper is lost. I don't know if that means the bike was ruined. Though I would agree with the prior posters concerns about the brazing of the lugs; however, the heat source doesn't seem to have affected the whole bike and bluing can occur at much lower temperatures than required to melt brass or silver solders used in brazing.
Fire "next door", eh? A little far down the I-5 to evaluate in person, perhaps.
Looks cool in it's way...could be worthwhile.
Frames get exposed to heat in the middle of the tubes when a builder adds water-bottle braze-ons or anything else and I've never heard of anyone heat-treating a frame after doing that.
Personally, I wouldn't mess with it.
Do you know what the tubing is?
As an example, 753 is the same chemical composition as 531, but is heat treated to gain higher tensile strength and can therefore be drawn thinner than 531 so the tubes are lighter. Subjecting heat treated tubing like 753 to high temperatures weakens it. In fact, Reynolds certified 753 brazers to ensure that the work never exceeded the temperature that would lead to loss of strength, and insisted that 753 be silver brazed because of the lower temperatures. Reynolds wouldn't sell 753 to builders who weren't certified.
To me, it seems your problem is right out there in the middle of the grey area, between the "frame is trashed for sure", and, "heck, no problem". That means you won't be able to get a definite answer on the internet, but maybe we can point you down some avenues that will provide the information you need to determine whether the frame is good as-is, needs to be refurbished, or is now wall art. BTW, I think it looks pretty cool, and I hope you keep it that way regardless of the outcome.
Off the top of my head, perhaps a Brinnell or Rockwell hardness test would tell the true story. (probably Rockwell, because I think Brinnell will not work due to the thinness of the tubing - not even sure about Rockwell). Because the temperature exposure varied over the extent of the frame, this would have to be done in many distributed locations on all the tubes.
I also don't know if there is readily available test equipment that can be used on an arbitrary shape like a bike frame tube. I would suggest calling around to local machine shops. Most of them may not have have any hardness testing capability, but odds are they'll know someone to put you onto.
The test is not completely non-destructive, as there will be a small indentation left behind at each test location. But generally, you could place them in relatively discreet locations, i.e. on the bottom side of the top & down tubes, and the rear side of the seat tube.
A little more research will be required to determine what the desired minimum hardness is, for the 753 tubing, but that information is available, surely.
Another way to ascertain the condition of the tubes, it seems to me, would be to have a frame builder check it out on a straightening jig. First of all, to see if it's still straight, but secondly, to input a specific amount of displacement, to the expected limit of the elastic range. If the frame returns to its original shape, I say it's good. This should be done in as many iterations, in different axes, as necessary to verify each of the tubes.
I don't have any other ideas at the moment, but if I think of anything, I'll let you know via this thread.
"I think that I think, therefore I think that I am"