Page 348 - Failure Analysis Case Studies II

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primary crack

secondary crack

Fig. 2. Samples of a cracked radiant tube for failure analysis.

through the other end of the tube. The tubes are hung from the ceiling of the furnace and laid at
the bottom of the furnace. Steel plates moving between the tube rows are heat-treated by radiated
heat from the tubes. As shown in the figure, the U-type tube is supported by three hangers.
Cracking problems have been frequently reported near the locations where supporting hanger A,
which is close to the burner, is in contact with the tube. Cracks initiated at the inside of the tube
and propagated to the outside inducing internal combustion gas to leak and cause problems to the
heat-treated rolled plates. The cracking direction was random and not related to the principal
stress direction. From this observation it can be argued that stresses generated in the tube due to
internal pressure or due to thermal gradients are not the major cause of cracking. Hence, for the
analysis of the cause of failure of this case, a metallographic investigation of the failed component
seems more appropriate than stress analysis.
Cracked specimen D was sampled from the cracked area around the supporting guide A as
shown in Fig. 2 for metallographic analysis. Specimen D contained a big primary crack and several
secondary cracks which were smaller than the primary crack. Considerable wall thinning was
observed at the inside of the cracked tube. Since it is known that a failure analysis with a secondary
crack (which usually shows the initial stage of cracking) is preferred to that with a primary crack
(which shows almost the final stage of the failure), a failure analysis on the secondary crack which
is 8-9 mm long was conducted. The failed radiant tube was fabricated locally by centrifugal casting
with 25 Cr-20 Ni steel. The chemical composition of the tested sample is shown in Table 1 with
the standard composition.

Table 1
Chemical composition of the 25 Cr-20 Ni radiant tube steel (in wt%)
~~ ~~~ ~~~
Elements C Si Mn P S Ni Cr Mo Fe
Test sample 0.43 1.83 - 0.025 0.008 22.28 22.65 0.068 bal.
Manufacturing standard 0.20-0.60 <2.00 ~2.00 <0.04 <0.04 18.00-22.00 24.00-28.00 <OS0 bal.