Influence of microalloy design on heat-affected zone toughness of S690QL steels
Three high-strength Nb-, Ti- and Ti + V-bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on heat-affected zone (HAZ) toughness. Charpy V notch impact tests from three microalloyed welds under different cooling rates have been performed. Fractographic examination shows that several factors, including large-sized grain, upper bainite or hard second phase, interact to determine brittle fracture and impaired toughness in Nb-bearing weld with high heat input. In contrast to this reduced toughness, Ti-bearing welds exhibits satisfied toughness regardless of at fast or slow cooling. This is attributed to its limited austenite grain and refined favourable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the fracture process. The crystallographic results also confirm that high-angle boundaries between fine ferrites plates provide effective barriers for crack propagation and contribute to improved toughness.
KeywordsHigh-strength steels Microalloyed steels Toughness Cooling rate Microstructure
The authors thank R. S. Neumann, M. Buchheim, S. Brunow, D. Schroepfer and E. Steppan from Federal Institute for Materials Research and Testing (Germany) for their kind support. One of the authors (Lei Zhang) also appreciates the funding support from China Scholarship Council.
- 1.Gladman T (1997) The physical metallurgy of microalloyed steels. Institute of Materials, LondonGoogle Scholar
- 2.Mitchell P, Hart P, Morrison W (1995) The effect of microalloying on HAZ toughness. In: Microalloying ‘95 Conf Proc pp 149–162Google Scholar
- 7.Lazor RB, Glover AG, McDonald RD (1984) Properties of welds in thick section Nb-containing steels. In: Welding in energy-related projects. Pergamon pp 1–9. doi: https://doi.org/10.1016/B978-0-08-025412-8.50005-X
- 9.DIN EN ISO 9016:2013-02. Destructive tests on welds in metallic materials—impact tests—test specimen location, notch orientation and examination (ISO 9016:2012); German version EN ISO 9016:2012 (2013). vol DIN EN ISO 9016:2013–02Google Scholar
- 10.DIN EN ISO 148-1:2011. Metallic materials—Charpy pendulum impact test—part 1: test method (ISO 148-1:2009); German version EN ISO 148–1:2010 (2011)Google Scholar
- 14.Wang G, North T, Leewis K (1990) Microalloying additions and HAZ fracture toughness in HSLA steels. Weld J 69(1):14–22Google Scholar