Thermal Shock and Thermal Fatigue of Cast Iron

Abstract

In recent years, the problem of thermal shock of metallic materials has been the subject of many studies, and several researchers have made fundamental works in this line. Surveying the previous works, it is found most of the studies on thermal shock are limited within the experiments on non-metallic materials, and so-called “thermal shock parameter” is defined as a criterion for thermal shock resistance. Thermal shock parameter is analysed from the concept based on maximum principal stress theory and elastic thermal stress therory. In the past study on nonmetallic materials such as ceramic, the thermal shock parameter is adopted as the measure of thermal shock resistance. However, this is true within the limited case of non-metallic materials. For the cases of most metallic materials, there are several questions as to criterion with which thermal shock resistance is evaluated. Furthermore, there is an open question as to the study on transition phenomenon from thermal shock fracture to thermal fatigue fracture. In this connection, it is necessary to establish the relation between thermal shock strength chracteristics and thermal fatigue strength characteristics.The authors, as well, have been interested in the problem concerned with thermal shock, and have done some works on the relation between tensile strength characteristics and thermal fatigue strength characteristics by employing heat-resisting metallic materials. In the present paper, the results of thermal shock tests for different heat-transfer conditions are presented, and the results are compared with those of thermal fatigue tests. Two types of tests of thermal shock and thermal fatigue are performed by solid cylindrical specimen of cast iron (3.79% C). Two sorts of heat-transfer conditions are realized by using air cooling and air mist cooling. Both tests are made by employing a conventional thermal fatigue test apparatus, and both ends of specimen are constrained at upper temperature level in thermal cycling. Maximum temperature level is chosen as 600°C.The results of the present study are summarized as follows:(1) Cast iron tested behaves transition phenomenon from thermal shock fracture to thermal fatigue fracture. Under air cooling condition (heat-transfer condition β=0.035), the material fractures at the begining of cooling path in the first cycling of temperature, when the upper temperature level is 500°C. If the upper level of temperature cycling is lower than 500°C cycle-dependent fracture occurs. On the other hand, under air-mist cooling condition (β=0.15), thermal shock fracture occurs for the case of the upper temperature level of 400°C. The parameter β is a non-dimensional heat-transfer coefficient and is denoted as β=r0h/k where r0 is outer radius of the specimen, h heat-transfer coefficient and k heat conductivity. The influence of heat-transfer condition on the temperature level under which such a transitional phenomenon occurs is related with the effect of increase in axial thermal stress at the outer surface of specimen due to multiaxial thermal stresses. In this case, the influence of strain rate on breaking strength of the material is not predominant.(2) From the results of thermal fatigue tests of which were carried out by taking the upper temperature level, the thermal strain amplitude and the thermal stress amplitude as variables, it is found that number of cycles to fracture is shorter in the case of air mist cooling condition than the case of air cooling condition. This trend is interpreted by taking account of plastic deformation produced in the material during a cycle of thermal strain cycling.(3) For the transition phenomenon of cast iron tested from thermal shock fracture to thermal fatigue fracture, simple maximum principal stress theory is applicable.