Page 356 - Failure Analysis Case Studies II

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Fig. 11. Microstructure of cracked region of the tested radiant tube showing void formation

Referring to the known reference microstructures of HK tubes degraded at high temperature [3],
the microstructure shown in Fig. 10 corresponds to the microstructure similar to one aged at 950-
1000°C for 60,000 h. Since the service period of the failed tube was only 15,000 h which is much
less than the 60,000 h of the corresponding microstructure, it can be predicted that the service
temperature of the tube was above 1000°C. Figure 11 shows a microstructure near the cracked
area which shows internal void formation. Voids of this kind were reported to be formed when
the service temperature reaches 1090-1230°C in the case of Ni-Cr steel [4]. Hence, it can be argued
that the local metal temperature during the service must go up to this high temperature. This
overheating can be induced by touching of the flame to the tubes near the supporting guide A in
Fig. 1. Therefore, to prevent radiant tube failures methods should be sought to lower the tube
metal temperature below lOOO"C, particularly in the vicinity of supporting guide A. Modification
of burner tips or improving combustion systems can be considered.

4. Conclusions

By conducting a failure analysis on the cracked radiant heater tubes used in a high temperature
furnace, the following conclusions are derived.
The radiant heater tube which was centrifugally-cast with the same chemical composition as a
typical HK steel except additional Si could be used-without problems if it is operated at a
temperature less than 1000°C so a protective Cr,03 oxidation film forms. However, as the operating
temperature exceeded 1000-1 lOO"C, the stabilized Cr203 transformed into volatile Cr03 and
abnormal oxidation or rapid oxidation occurred. The failed tube of the current study must have
been used at or above the recommended temperature range and as a result, locally thinned areas
were formed by excessive oxidation. Some of the oxidation pits were filled with oxide scales formed