Low-cycle fatigue of steel 15NiCuMoNb5 (WB36) and its welded joint

The results of studying the low-cycle fatigue characteristics of steel 15NiCuMoNb5 (WB36) and its welded joints are presented. Brief information is given on the specific features of WB36 steel proceeding from the nalysis of literature data. An emphasis is made on the absence of available information on the studies of WB36 steel and its welded joints produced in Russia. The study was carried out on an experimental welded metal billet produced by standard technology and simulating a full-scale fragment of a high-pressure drum. The methodological aspects of the conducted research are described. Tests for low-cycle fatigue of WB36 steel and its welded joints were performed in the mode of hard loading in a symmetrical cycle at room (20 – 25°C) and operating (350°C) temperatures. Corset-type tensile and compression samples were used; the onset of a macro-fracture on the surface of the corset part was taken as the criterion for the destruction of the sample. It is shown that with an increase in the test temperature, the resistance to low-cycle fatigue of steel and weld metal decreases. At the same time, the base metal and the weld metal demonstrate identical characteristics of low-cycle fatigue at each of the test temperatures. A certain part of the study touched the effect of temperature ageing on the low-cycle fatigue of steel. It is shown that the values of the low-cycle strength of the material under study slightly exceeds the results of similar tests of WB36 steel presented in foreign technical literature. A comparative analysis of experimental low-cycle fatigue curves and corresponding standard calculation curves obtained using analytical expressions for the actual mechanical properties of steel has shown that experimental data on the low-cycle fatigue of steel demonstrate a higher fatigue resistance at room temperature and approximately a similar level at operating temperature compared with calculated fatigue curves obtained using the mechanical properties of the metal. An example of the low-cycle fatigue calculation for a high-pressure drum of a steam-gas recovery boiler is given. It is confirmed that for the given initial parameters and resource indicators, conditions of the drum strength met the criteria of low-cycle fatigue.