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- Acicular ferrite (1)
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Within the framework of an AiF (The German Federation of Industrial Research Associations) project, investigations were conducted into the solidification behaviour of various high-alloy austenitic steels in laser and laser-GMA hybrid welding. For comparison purposes, reference was made to pure gas metalarc welding tests. It was possible to raise the welding speeds in hybrid welding compared with the individual gas metal-arc (GMA) and laser welding processes. The central subjects of the investigations were the type of solidification and the connected hot cracking behaviour during welding. In this respect, particular attention was paid not only to the kinetic effects during solidification which may be caused by the high solidification rates in laser and hybrid welding but also to their consequences.
Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.