Laser welding of steels

• Main Author: Harvey, M. T.
• Published: University of Cambridge 1997
• Subjects: 671
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.603824

One of the aims of the study has been the evaluation of suitable wires for welding API-X80 and En8 grade steels, on the basis of experimental results. The experimental results (from Charpy, CTOD and tensile testing together with general microscopy) were obtained using commercially available flux and metal cored wires. Weld metal tensile properties similar to those of the parent metal were obtained, although the fracture properties were generally inferior. An exception to this was shown by the En8 steel welded with a high nickel content wire which exhibited toughness values greater than those recorded for the parent metal. However, the high nickel content makes the use of such a weld in certain corrosive environments problematic. The best toughness properties amongst the X80 welds were shown by the higher oxygen containing welds. For these X80 welds, mechanical test results correlated strongly with microstructure, which varied from coarse to fine ferrite. In order to improve the toughness properties, experimental wires were produced using a combination of computer modelling (weld prediction program) and statistical methods (Taguchi methods). These provided successful in that significant proportions of acicular ferrite were obtained. However, the toughness values did not show a corresponding improvement. This was believed to be due to the presence of inclusions, essential in order to nucleate acicular ferrite, but which have a direct effect on the fracture resistance of the weld metal. The presence of grain boundary ferrite morphologies, microphases and nitrogen may also have had an embrittling effect. It was suggested that reducing the oxygen levels and controlling the presence of other elements would have a beneficial effect with regard to toughness properties. Finally the problem of Fracture Path Deviation (FPD), where the crack path deviates out of weld metal during Charpy impact testing, was examined using autogenous and wire feed welds produced in BS 4360 50B steel. To examine the factors controlling FPD, both experimental work and finite element modelling of the Charpy specimen were undertaken. The results indicated the importance of weld width and orientation on the FPD phenomena. It was suggested that the toughness testing of narrow fusion zone laser welds (approximately 2 to 3 mm) should be undertaken using CTOD rather than Charpy testing.