INVESTIGATIONS ON THE USE OF NITROGEN SHIELDING GAS IN WELDING AND ITS INFLUENCE ON THE HOT CRACK BEHAVIOUR OF HIGH-TEMPERATURE RESISTANT FULLY AUSTENITIC Nl- AND FE-BASE ALLOYS

摘要

Due to their excellent high-temperature behaviour and corrosion-resistance properties, as well as to their equally good high-temperature strength and long time strength, high-temperature resistant fully austenitic Ni-base alloys are widely employed in industrial branches for especially demanding and exacting use. The outspoken tendency of the weld metal to hot cracking, however, considerably restricts the outstanding working-life properties of this group of materials. In our research project is investigated systematically the metallurgical effect of nitrogen for avoidance of hot cracks in fully austenitic weld metal. As objects of the investigation, were selected hot-crack sensitive fully austenitic high-temperature Ni- and Fe-base alloys (alloy 602 CA, alloy 601 H, alloy 617, alloy 690, alloy 800), which were processed with GTAW and pulsed GMAW procedures under the use of suitable weld filler materials as applied in practical use. The element nitrogen was added as gaseous component with the weld shielding gas. The hot-crack behaviour of the base metals and of the welds was evaluated by the Programmed Deformation Rate-Test. As results of the hot cracking tests, the positive effect of nitrogen was shown at first and foremost only with the primary carbide containing Ni-base alloy 602 CA and slightly with alloy 601 H. The reasons for this effect are metallurgical. Starting from the presented results and the experience meanwhile gained in practice in matched arc welding the high-temperature resistant Ni-base alloy 602 CA, nitrogen additions between 1 and 3 % for the GTAW process and between 5 and 12 % for the pulsed GMAW process are recommended.