R&D

WELDING

Wherever components cannot be produced economically and functionally of form- and force-locking connections (screws, rivets, shrink connection ...), or primary shaping and forming (casting, forging ...) welds are used. The biggest advantages to rivet and screw constructions are that no overlaps and seals are necessary, that smooth walls formed which can be easily protected against corrosion and that there are no additional weakenings caused by rivet and bolt holes. The greatest advantage over cast designs is that much smaller wall thicknesses are possible, that no expensive casting model is required and that an infinitely large component can be produced theoretically. The biggest drawbacks of welds are that they are indissoluble connections, that shrinkage can lead to large stresses, distortion and warping, and a highly qualified staff is necessary in order to avoid welding defects. In addition to the connection-welding also build-up welding for the production of wear-resistant armouring has a wide application.

Challenges

The biggest challenge is to optimize the welding process in a way (for a seam geometry and a material) that no welding errors may occur. For example in MIG, TIG or MAG welding processes the preheating temperature, the welding speed, the bead geometry and the cooling rate must be adjusted that no hot and cold cracking can occur. An excessive nitrogen content, too long arc, a too flat working angle, too little protective gas, poor ground contact, and impurities can for example cause pores in the weld. Too low current and wire feed or incorrect welding technique can lead to lack of fusion. Another challenge is to adjust the microstructure in the weld seam and in the heat-affected zone in a way that no poor material properties result in terms of strength, toughness and corrosion resistance. Such negative factors are for example grain coarsening or strongly different ratios of ferrite and austenite in the heat-affected zone of duplex steels.

The control of welding processes for the error-free production of joint- and overlay-welds is a core competence at BHDT. Thereby welds between pipes or pipes and flanges are produced, but also valve stems with wear-resistant hard-faced layers. To specifically influence particular weld properties and avoid possible sources of errors, top-trained staff is necessary together with the latest developments in welding technology.

BHDT GmbH is increasingly focusing on automated welding processes such as orbital welding to increase the weld quality and reproducibility. In addition, automated welding of complex high alloy materials leads especially for larger quantities to an increase in productivity compared with manual welding.

WELDING

In the left picture is a micrograph of a weld seam and the heat affected-zone of a sample prepared by orbital welding can be seen. Here, the height of the heat input and the composition of the shielding gas supplied was varied to investigate their effect on microstructure and corrosion resistance. The area of the weld seam shows a very fine-grained microstructure in which the ratio between ferrite and austenite has nearly the desired 50 %. The right figure shows the significant increase in pitting- and repassivation-potential when using orbital welding processes as opposed to manual welding in a current density-potential test.