Unless it has a high carbon content the fire might not have done to much damage to the steel and most likely is was a cold cool down. Only higher carbon steels can be heat treated such as M-1, A-2, 1075 etc. Before you say it yes low carbon can be heat treated but it takes some special circumstances and a really good heat treater. Standard soft cold rold steel has very little carbon in it to affect anything and if the steel got red hot maybe but it looks like the annealing process was a slow cool and not a fast cool down. Maybe if a fire hose blasted it directly to cool it down but still most of that tractor is low carbon steel. The rims might have taken some warpage with the heat because tires burn pretty hot. Engine block probably didn't get hot enough to affect anything, head seals on the outside might have taken some damage but there is also radiator coolant and oil in the engine, even if the hoses burnt through. Same with the trans, there is oil in that sealed metal and most likely absorbed some heat but kept it from getting to hot. Now aluminum may have melted some. I don't know the circumstances how hot it got, if the fire department blasted it with water and so on. I do know if you weld on metal it can weaken that area but again you are technically getting that area red hot.
