IFMASS10

3. The rapid cooling rate of the fillet weld, the dissolution of NbC precipitates, and some

grain growth in the HAZ of the bracing plate, all (together with the increased stress

concentrations) helped produce just the conditions likely to give rise to cold cracking.

4. Given the presence of cold cracks in the fillet welds, the increase in stress concentration

due to weakening of the flange plate, the poor weld profile, and the cyclic stresses experi-

enced at sea then all the necessary conditions for fatigue crack growth appear to be present.

As with most case studies, there are some unanswered questions. For example, why

did significant cracking occur in bracing D-6, but not in bracings A-5 and B-5, which also

contained sonar flange plates? A possible variable might be proper drying of the

electrodes for the fillet weld, particularly because this weld was considered of secondary

importance at the manufacturing stage. It is also not clear that the loading conditions are

the same for all three bracings under normal service at sea, and this may be another

important variable.

3. FRACTURE OF CARRIAGE MONOBLOCK WHEELS

The monoblock wheel was fractured radially in the zone corresponding to chuck

marks, Fig. 9. The data from report /4/ revealed that the wheel had been installed on 26

January, 1998 the tread diameter was reduced from 920 to 908 mm on 16

March, and

machined further to 890 mm on 15

May. After that the wheel had been broken with

tread diameter about 890 mm. This information points to possible braking problems.

It should be emphasised that a 5 mm gap was present in the fracture area (Fig. 9). The

gap indicates the presence of residual compressive stresses, in one wheel area at least.

The facts regarding breaking of this wheel, i.e. the influence of the brake shoe insert

(Fig. 9) that imparted traces of over-heating on the wheel and the insert itself are of an

equal importance. It should be mentioned that even more visible traces of braking are

found on N°6 wheel from the same bogie, only that this wheel was without chuck marks.

Figure 9: Fractured monoblock wheel

Figure 10: The appearance of fractured wheel surface