ON THE RESULTS OF THERMAL FATIGUE TESTING OF CAST IRON AND THE EVALUATION OF ITS DURABILITY

Abstract

The article notes that at present, despite numerous studies of thermal fatigue, the thermal fatigue strength of cast irons due to stresses in parts made of this material requires additional study. Issues of thermal fatigue strength are especially relevant for such heat-stressed parts of internal combustion engines of autotractor diesel engines as the cylinder head. Of particular importance for these parts in their design and restoration is the ability to predict thermal fatigue strength, which is the main criterion that determines their durability. The article describes an approach to solving the problem of predicting the thermal fatigue strength of cast iron parts based on the data on their stress state and the results of thermal fatigue testing of samples on a specially designed and manufactured installation. The setup consists of two massive plates mounted on racks, between which a sample with a hole was rigidly installed. During the tests, the sample was heated by a special annular furnace to a temperature of 550°C for two minutes, and then cooled with running water through the central hole. A sample bounded on both sides during heating and thermal expansion accumulated residual tensile stresses as a result of relaxation processes and, upon reaching critical values and the appearance of fatigue defects in the form of cracks, ruptured. The proposed mode quite accurately repeats the operating conditions of the heat-receiving surface of the cylinder heads in the area of the intervalve jumpers when changing the operating mode at rated power and subsequent engine shutdown. As a result of the tests, the curves of the number of cycles to failure depending on the achieved stresses were obtained and their approximation was carried out. On the basis of approximating expressions and dependences of "limited durability" under low-cycle loading of parts, the thermal fatigue strength of a part made of cast iron can be predicted when its stress state changes as a result of design changes or after restoration.