Busting the Myths of Welding of High Strength S690 and S960 Steel

Despite both the S690 and the S960 steel are produced in an industrial scale in many countries in the world since the 1990s, their applications in construction are rather limited owing to their high material costs, difficulties in welding and associated quality assurance, and a lack of efficient structural design. However, many design and construction engineers believe that a wide application of these steel will introduce a fundamental change to how multi-storey buildings and long span bridges are built. Nevertheless, to some structural engineers, there are technical concerns on applying these steel in large scale construction projects. Moreover, there are occasionally conflicting reports in the literature on the mechanical properties of the welded sections of these steel, and also the structural behaviour of their welded members and joints over the past 30 years. 

In many modern steel mills, these high strength steel are manufactured with a highly controlled heat treatment process known as “Quenching and Tempering” to produce a homogeneous microstructure of tempered martensite. This is a fine-grained crystalline form of steel with high strengths and toughness. However, a transient heating / cooling cycle induced during welding of these steel may initiate phase transformation, re-crystallisation and grain growth, i.e. to transform into a heterogeneous microstructure, within the heat affected zones of the welded joints, as shown in Fig. A. 
Fig. A   Microstructural changes in Q690-QT steels after welding

It is important to control the heating up as well as the cooling down processes so that any microstructural change in the heat affected zones of the welded joints is kept to be a minimum. This is generally achieved through a careful selection of various welding parameters according to geometrical details of the weld joints so that the heat input energy during welding is kept to be a practical minimum. Then, the welded joints are cool down quickly, giving little time for a significant microstructural change to take place within the heat affected zones. In general, the time for the steel to cool down from 800 ^oC to 500 ^oC, i.e. t_8/5 , is often adopted for this purpose. In practice, the value of t_8/5 should not exceed 10 to 15 seconds in those steel plates of 10 to 50 mm thick.

Hence, welding in these steel will potentially cause a significant reduction in their mechanical properties if both the maximum temperatures during welding and the cooling rates after welding are not properly controlled, in particular, in those steel plates with practical thicknesses in construction, i.e. 6 to 80 mm thick.  It is highly desirable to assess or predict reductions in various mechanical properties of these welded sections, and more importantly, how to eliminate any such reduction in these welded sections.