Combustion of finned steel tubing during stress relief heat treatment

An investigation has been made of two fires which occurred when finned plain carbon steel boiler tubes were being stress relieved at a nominal temperature of 1150° F (621° C). The furnace atmosphere consisted of the combustion products of natural gas with about 330% excess air. In both cases the furnace was being operated with one of the twenty-four burners out. This resulted in increased gas flow to the other three burners on the same manifold, and thus to local temperatures above 1150° F (620° C) near them. Both fires started at one of these regions of elevated temperature. Metallographic evidence indicates that repeated spolling of the scale occurred at the tips of the fins. The heat generated by the rapid oxidation of the bare metal exposed by this process led to a rise in temperature. This accelerated the spoiling, which in turn led to a further increase in temperature. Thus, the spoiling and subsequent oxidation resulted in an uncontrollable rise in temperature. When the temperature reached 2500° F (1371° C), iron and iron oxide (wüstite) reacted to form an eutectic liquid. As this ran off the steel surface under gravity, bare metal was continuously exposed. This led to an even higher oxidation rate. Eventually,the ignition temperature for the steel was reached.