It is all about this irreversible fast tire decomposition called pyrolysis.
You do not know you have started it and then you cannot stop it. This degradation process can also be started in ways other than welding such as:
overheated brakes
equipment contact with high voltage power lines
tire or equipment fires
lightning strike
driving long distances with underinflated tires
In most North American jurisdictions it is illegal to weld a rim with the tire on it because people have been killed.
https://www.youtube.com/watch?v=HiLeji8bLOk
Dave
Pyrolysis is a thermochemical decomposition of organic material which can occur at elevated temperatures in the absence of oxygen. This decomposition process only requires the presence of rubber and heat. When heat is applied to the tire or to tire components, hydrocarbon vapors are released inside the tire. As the vapors are released, the air pressure and temperature within the tire increase dramatically. These vapors are highly flammable. If the flammable vapor inside the tire makes contact with oxygen at a temperature in excess of 400 degrees Celsius, auto-ignition can occur, resulting in an explosion.
Once the decomposition process begins, it is irreversible. There is no visual indication that the decomposition process is taking place. Depending on the particular circumstances, an explosion may occur immediately, within minutes or several hours after the exposure to the heat source. In many instances, an explosion can cause the tire and/or wheel components to be thrown great distances, resulting in severe worker injuries and/or fatalities as well as significant property damage.
