| In the present work, two methods have been used to improve the toughness of weldjoint of high strength low alloy steels. Firstly, fast cooling was carried out to investigatethe effect of cooling modes on microstructure and toughness of the E690offshore steel.Secondly, the addition of copper in weld metal was utilized to investigate the effect ofcopper on microstructure and toughness of weld metal of high strength low alloy steels.(1) E690is a newly developed high-strength steel for offshore structures. Theeffect of different cooling modes on the microstructure and toughness of theheat-affected zone in E690weldment was investigated in this work. The outcome ofmicrostructural examinations and mechanical tests showed that fast coolingimmediately after submerged welding can reduce the width of heat-affected andcoarse-grained zones, as well as improving the low-temperature impact toughness(-40oC). It was also shown that reduction in the grain size of the parent austenite andhaving lower amounts of martensite austenite constituents are the main causes of theobserved improvement.(2) The base metal was welded by three-wire double-pass submerged arc welding,three different level of copper was added in the weld metal, i.e.0.35,0.82,0.49wt%respectively. According to the optical microscopy analysis, scanning electronmicroscopy analysis, Leco M-400-H1hardness tester, Charpy V-notch impact test, theeffect of cooper on microstructure and toughness in weld metal was studied. Resultsshowed that the proportion of acicular ferrite increased while side-plate ferrite decreases,the toughness of the sample containinig0.82wt%Cu had the highest value with thelowest standard deviation. The reason is that Cu is an austenite stabilizer, whileprecipitation of copper will impair the toughness of weld metal. |
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