Fracture mechanics and a fractographic study on thermal fatigue of cast irons

The problem of thermal fatigue is important in view of securing safety in heat engines, chemical plants, nuclear reactors and others. However, the studies in terms of the occurrence and propagation of cracks in thermal fatigue were scarcely carried out so far. In the present study, thermal fatigue test was carried out, and the behavior of crack development in thermal fatigue was clarified. Thus the possibility of estimating thermal fatigue life was shown, and further, the microscopic mechanism of thermal fatigue fracture was examined by observing fractographically fractured surfaces. When the thermal fatigue life in the range of small number of cycles till fracture is considered, the process of crack development controls the thermal fatigue life, and the equation for calculating thermal fatigue life was proposed by the authors. Using the flaky graphite cast iron for steel-making molds and that for machinery, the mechanical properties at room temperature and elevated temperature and the metallographic structures were examined, then the thermal fatigue tests were carried out with the thin-walled cylindrical test pieces. The speed of crack development can be represented with the equation in which the range of strain intensity factor was used. The thermal fatigue life of the cast iron can be estimated with the equation proposed by the authors. Striation was observed on the surfaces of the cast iron fractured by thermal fatigue same as the case of usual fatigue. (Kako, I.)