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

Solid state welding is often carried out in a vacuum chamber under rat her low pressure (2 to 50 MPa) and temperatures. It offers considerabl e advantages, especially metallurgical ones. Indeed, with good process conditions, it generates no important microstructural degradation suc h as those occurring with classical techniques of fusion or friction w elding. It is a soft joining process that generates only a small defor mation of the samples. However, classical diffusion welding requires w elding durations ranging from some dozens of minutes to several hours. Because of the cost incurred, the process can be employed only in ind ustries such as the electronic, nuclear, aerospace and military indust ries. A new process, dynamic diffusion welding (DDW), was improved in our laboratory (Ref. 4). It offers the advantages of both classical di ffusion welding and rapidity. With DDW, new perspectives are opened fo r using this welding process in mass production, since it is consisten t with the constraints of productivity. For this reason, dynamic diffu sion 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-strengt h joints in a very short time: 30 s for heating to the welding tempera ture and a few seconds at the isothermal anneal. With the optimal cond itions defined in this study, namely a welding time of several seconds , in a nitrogen with 5% hydrogen gas, with a proper surface preparatio n (Rmax = 0.4 mum) and under a modulated pressure during the whole wel ding process: 1) The tensile behavior of the joints is reproducible. 2 ) Failure occurs out of the interface with a tensile strength of 950 M Pa 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 te sts is lower than that of the base metal. This process is also efficie nt for making other ferrous alloy welds. An additional mechanical char acterization is now going on in our laboratory, our purpose being, amo ng others, to assess the fatigue endurance.