Page 354 - Failure Analysis Case Studies II

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Fig. 8. Ni distribution map of crack tip region (same location as shown in Fig. 6(b))

oxidation product in the oxide scale. Other minor elements appeared in the scales such as 0.4-
0.6% of S and 0.2-0.3% of V apparently produced by contact with combustion gas. A high content
of S may generate a corrosion problem; vanadium usually forms V205, which, together with high
velocity combustion gas, may cause erosion problems. In this tube failure, however, S and V
seemed not to contribute to the failure.
Figure 8 is a composition map obtained by EDAX that shows the Ni distribution at the crack
tip region of Fig. 6(b). This picture shows that the oxidation layer is not correlated with Ni. Figure
9 shows the Cr distribution at the same location. Focusing on the Cr concentration of the oxide
scale in Fig. 9, it is evident that the oxide layer at the crack tip is of Cr oxide product.

3. Discussion
Abnormal oxidation problems of high temperature steels are generally caused by improper
operating temperature exceeding the recommended temperature range. The manufacturing process
of the radiant tube of this study is similar to a well-known centrifugally-cast HK steel, but the
chemical composition of the tube is different from that of the HK steel since Si was added to the
tube metal. Ifwe consider the composition only, which is Cr = 23-26%, Ni = 19-22%, C = 0.25%,
Si = 1.5-3.0%, and Fe = balance, it is a typical composition of 314 stainless steel (UNS31400
steel). This material is known to have excellent high temperature oxidation resistance by forming
Cr203 protective oxide film. However, if the service temperature exceeds 1 OOO'C, the stabilized
Cr203 film becomes unstable and transforms into volatile Cr03 losing its protective effect. There-
fore, the current radiant tube material should be used in a temperature range which does
not exceed 1000°C, to prevent abnormal oxidation. Also, it was reported that for high Cr-Ni