Influence of carbon, manganese and nickel on microstructure and properties of strong steel weld metals Part 2 – Impact toughness gain resulting from manganese reductions E. Keehan* 1 , L. Karlsson 2 , H.-O. Andre ´n 3 and H. K. D. H. Bhadeshia 4 Two experimental high strength steel weld metals were produced with 7 wt-% nickel and either 2 or 0 . 5 wt-% manganese. Neural network predictions that it is advantageous to reduce the manganese concentration in high nickel alloys have been confirmed, with impact energy increasing from 32 to 113 J at –40uC. High resolution microstructural investigations showed that both weld metals contained mainly martensite at interdendritic regions and predominantly bainite at dendrite core regions, as a consequence of manganese and nickel segregation. In the high manganese weld metal significant amounts of coarse grained coalesced bainite formed whereas mainly upper bainite was seen with 0 . 5 wt-% manganese. Reducing manganese content increased the transformation temperature, promoting fine upper bainite at the expense of coarse coalesced bainite. Increased toughness was attributed to the finer grain size of bainite constituents and a more effectively tempered microstructure. Keywords: High strength steel weld metal, Nickel content, Manganese content, Impact energy, Microstructure, Martensite, Bainite, Segregation, Coalesced bainite, Toughness Background High strength steel is increasingly employed in greater amounts owing to the many advantages it offers, such as size and weight reduction, in many applications. However the joining of high strength steel must be carried out in a controlled manner, with particular attention placed on welding, if both strength and toughness requirements are to be met. 1–3 Since the 1960s, many investigators have carried out research by varying elemental composition and welding parameters, with the hope of achieving good strength above the region of 690 MPa (100 ksi) and good toughness using shielded metal arc welding. It was demonstrated in Part 1 4 of this series of papers that the common belief that the toughness of high strength steels and weld metals can be improved by adding nickel is not justified. It was found that whereas nickel increased the strength, it did not lead to an improvement in the impact toughness in alloys contain- ing some 2 wt-% manganese. These experimental obser- vations are consistent with predictions using neural network models. As pointed out in Part 1, 4 the physical basis of this behaviour is that the alloy transforms during cooling into coarse regions of coalesced bainite, which is expected to offer little resistance to cleavage crack propagation. The neural networks predict that this scenario should be different when the nickel concentra- tion is increased but the manganese concentration is decreased; the toughness should then improve at higher nickel contents. The most promising results to date have been achieved through the variation of manganese and nickel contents. 5–7 Zhang and Farrar 5 investigated a number of compositions with manganese content less than 1 . 6 wt-% and nickel less than 5 . 6 wt-%. With a combination of 0 . 36 wt-%Mn and 5 . 58 wt-%Ni, an impact toughness of y55 J at 260uC was recorded and a tensile strength of 904 MPa was predicted from hardness measurements. Increasing manganese con- tent to 0 . 7 wt-% and reducing nickel to 3 . 5 wt-% was found to increase impact toughness to y75 J at 260uC and a reduction in tensile strength to 745 MPa was predicted from hardness measurements. Mainly acicular ferrite with some Widmansta ¨ tten sideplates and grain boundary ferrite were reported, whereas increasing nickel content was found to promote martensite. 5 Lord 6 1 ESAB AB, PO Box 8004, SE–402 77 Gothenburg, Sweden. Work carried out in the Department of Applied Physics, Chalmers University of Technology, Kemiga ˚ rden 1, Fysikgra ¨nd 3, SE–412 96 Gothenburg, Sweden 2 ESAB AB, PO Box 8004, SE–402 77 Gothenburg, Sweden 3 Department of Applied Physics, Chalmers University of Technology, Kemiga ˚ rden 1, Fysikgra ¨ nd 3, SE–412 96 Gothenburg, Sweden 4 University of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK *Corresponding author, email enda.keehan@esab.se ß 2006 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 7 April 2005; accepted 22 June 2005 DOI 10.1179/174329306X77849 Science and Technology of Welding and Joining 2006 VOL 11 NO 1 9