Well not sure of that ... I have seen that thing being done on my old Maruti 800... and nothing ever gave way ... Metallurgically I agree what you are saying will hold true if the metal is allowed to cool slowly (similar to annealing that is used to soften metal), instead if you cool the metal rapidly it hardens (similar to Tempering that hardens the metals).
So what I have seen these guys do is cool the hot metal by dipping in water or spraying it with water. Regards Anupam Kansal [Non-text portions of this message have been removed]
