After 4 or 5 cycles the exhaust fan will come on as normal for about 1 minute and the burners will go out. The exhaust fsan will continue to run for 2 or 3 minutes then the inside circulation fan comes on with heated air and a very strong hot metal smell. What is the problem?

What causes the heat exchanger to go out and the exhaust fan continues to run for 2 to 3 minutes, then the circulating inside fan comes on heated with a very strong hot metal smell.