Abstract
The thermal fatigue strength of cast steel for turbine casings was examined. The thin-walled cylindrical type specimens with a through-hole were used for thermal fatigue tests. We observed the propagating crack from the hole by means of a travelling microscope, and took the photographs of the ruptured surface by a scanning electron microscope.
Basing on those results, we studied the factors affecting the rule of crack propagation rate. We also examined the propagation of a part-through crack, which is important in practical use. Both results obtained above were compared with each other and discussed. The conclusions obtained are as follows;
(1) When striations were almost dominant on the ruptured surface, the rate of thermal fatigue crack propagation was expressed by
dl/dn∝(Δεin)αl
where Δεin, l and α are the inelastic strain range, half crack length and material constant, respectively.
(2) The ruptured surfaces consisted of three types of rupture mode, that is, intergranular fracture surface, striation pattern and dimple pattern, and the striation spacing agreed with the macro-rate of thermal fatigue crack propagation. As the dimple pattern began to get dominant on the ruptured surface, under the condition of out-of-phase, many surface cracks began to grow on the specimen's surface, and the main crack propagated coalesing with them rapidly.
(3) In thermal fatigue fracture, the rate of part-through crack propagation was about half of that of through crack.
