Cracking Causes of Radiant Tube in Heat Treatment Gas Heating Furnace

The radiant tube of heat treatment gas heating furnace in a factory cracked after serving for less than 2 years. By means of macroscopy observation, chemical composition analysis, metallographic examination, scanning electron microscope and energy spectrum analysis, the reasons for cracking of radiant tube were analyzed. The results showed that the carbon potential of the protective gas outside radiant tube was not properly controlled, radiant tube was carburized from the outer wall to the inner wall, and a large amount of coarse Cr₂₃C₆ was precipitated in the grain boundary and in the grain, which increased the brittleness of radiant tube. Under the action of thermal stress, the radiant tube cracked from the outer wall and expanded to the inner wall. Due to the large amount of carbide precipitation, the content of chromium element dissolved in the matrix of radiant tube decreased, which reduced the oxidation resistance of radiant tube. In the high temperature oxidizing atmosphere in the tube, the radiant tube was oxidized and corroded, and the internal casting defects of radiant tube accelerated the crack propagation, which eventually led to the cracking of radiant tube.