SOLID-STATE WELDING OF A 027 CARBON, CHROMIUM MOLYBDENUM STEEL

Solid state welding is often carried out in a vacuum chamber under rather low pressure (2 to 50 MPa) and temperatures. It offers considerable advantages, especially metallurgical ones. Indeed, with good process conditions, it generates no important microstructural degradation such as those occurring with classical techniques of fusion or friction welding. It is a soft joining process that generates only a small deformation of the samples. However, classical diffusion welding requires welding durations ranging from some dozens of minutes to several hours. Because of the cost incurred, the process can be employed only in industries such as the electronic, nuclear, aerospace and military industries. A new process, dynamic diffusion welding (DDW), was improved in our laboratory (Ref. 4). It offers the advantages of both classical diffusion welding and rapidity. With DDW, new perspectives are opened for using this welding process in mass production, since it is consistent with the constraints of productivity. For this reason, dynamic diffusion welding of 27CD4 steel has been studied. The application targeted has been the welding of transmission shafts, which are presently made by friction welding. It was proven possible to carry out high-strength joints in a very short time: 30 s for heating to the welding temperature and a few seconds at the isothermal anneal. With the optimal conditions defined in this study, namely a welding time of several seconds, in a nitrogen with 5% hydrogen gas, with a proper surface preparation (Rmax = 0.4 mum) and under a modulated pressure during the whole welding process: 1) The tensile behavior of the joints is reproducible. 2) Failure occurs out of the interface with a tensile strength of 950 MPa and with an elongation of 22% (bainitic and martensitic structure). 3) Deformation of the welded specimens is limited to about 2% of the heated length. 4) The failure energy of joints measured with impact tests is lower than that of the base metal. This process is also efficient for making other ferrous alloy welds. An additional mechanical characterization is now going on in our laboratory, our purpose being, among others, to assess the fatigue endurance.