Welding, grinding or cutting work on containers, that had contained flammable or combustible materials in the past, are dangerous when appropriate precautions are not taken. They may explode when exposed to heat. Risks must be assessed beforehand to determine that the task is safe to perform.
Taking a chance can be very tempting! Think twice before experimenting with safety, it is your life (as well as that of around you) at stake. If you fail to control the risk, accident is almost guaranteed!