Fatigue strength of 30ХГСА–40ХМФА welded joints produced by rotary friction welding

Rotary friction welding (RFW) is used in the production of drill pipes for solid mineral prospecting. The need for the creation of the lightened drill strings for high-speed diamond drilling of ultradeep wells dictates the necessity of a greater focus on the study of a weld zone and setting the RFW technological parameters. This paper presents the results of experimental studies of a welded joint of a drill pipe of the H standard size according to ISO 10097, made of the 30ХГСА (pipe body) and 40ХМФА (tool joint) steels under the cyclic loads. The authors evaluated the influence of the force applied to the workpieces in the process of friction of the contacting surfaces (force during heating), and postweld tempering at a temperature of 550 °С on the cyclic life of welded joints, under the conditions of alternate tension-compression at the cycle amplitude stress of ±420 MPa. The study determined that with an increase in the force during heating, the microstructure changes occur in the zone of thermomechanical influence, contributing to an increase in the fatigue strength of welded joints. The authors identified the negative effect of postweld tempering on the fatigue strength of welded joints, which is expressed in the decrease in the number of cycles before failure by 15–40 %, depending on the magnitude of the force during heating. The optimal RFW mode of the specified combination of steels is determined, which provides the largest number of cycles before failure: the force during heating (at friction) Fh=120 kN, forging force Ffor=160 kN, rotational frequency during heating n=800 Rpm, and upset during heating l=8 mm. A series of fatigue tests have been carried out at various values of the cycle amplitude stress of the welded joint produced at the optimal mode and the 30ХГСА steel base metal; limited endurance curves have been plotted. It is shown that the differences in the limited endurance curves of the pipe body material (30ХГСА steel) and the welded joint are insignificant. The obtained results are supplemented by the microhardness measurement data and fractographs of fractured samples, revealing the mechanism of crack propagation under the cyclic loads.