Posted by Paul Cook in Kempner, Texas on Sunday, July 09, 2006 at 11:52AM :
In Reply to: Any Metallurgists out there? posted by Dale on Saturday, July 08, 2006 at 7:00PM :
I'm not a metallurgist. But I'm older than any VC "pre-Power Wagon" and I have observed that there is a general metal fatigue in most steel parts resulting from age.
The alloys of the day were often primitive compared to those of the present. Old metal tends to "crystallize" which results in a weaker molecular structure.
When I decided to get serious about "old iron" I went to our local community college and earned a degree in Auto Body Repair and Body Shop Management. Fortunately, this was far enough back in time that we learned to repair metal instead of the current remove and replace technique.
One technique we learned was to heat and strike the metal to realign the molecular structure to make the metal more ductile so it could be worked without failure. This works very well on the heavier gauge sheet metal on older vehicles. A Power Wagon Flat Fender is the extreme example of heavy gauge sheet metal. Bent areas often break when straightened. The heat and strike method really does make the metal more ductile than when using a hammer and dolly on cold metal.
The current practice when welding is to avoid heating the metal and prevent warping. This is appropriate when welding modern metal where the alloys are different and the metal may expand more and warp.
On "old iron" I have come to prefer using a torch with a fine welding tip to get more heat into the weld. These welds seem to stay without additional cracking that sometimes occurs after arc welding.
This "crystallization" also occurs on drive line components. The other day I got to look at the end of an axle that broke off inside the differential. When the broken pieces were retrieved and cleaned up, the effects of "crystallization" were clearly visible like when you break a sugar cube in two. "Crystallized" metal breaks usually have multiple pieces. "Good" metal shows tearing or twisting but usually results in a single fracture with only two broken pieces.
This also means that a 60 year old "NOS" part is not as strong as a comparable part of current manufacture. The folks who are building up old trucks for extreme use usually replace the original axle shafts with "new" ones. The differential gears are strong because they were built heavy in the first place. The axle shafts are being subjected to much higher loads that they are were ever engineered to handle.
My experience and training says the answer to your question is, "All old steel is subject to metallurgical failure based solely on age. The extent to which this may happen is a factor of the stress or previous repairs to body parts and the loads on suspension and drive line parts."