The role of titanium in the non-metallic inclusions which nucleate acicular ferrite in the submerged arc weld (SAW) fusion zones of Navy HY-100 steel

The article of record as published may be found at http://dx.doi.org/101016/0956-716X(95)00075-7The origin of acicular ferrite in the weld metal of submerged arc weldments on high strength steels is very complex and depends upon the chemical composition for the steel base plate and filler wire, the composition of the flux used during welding and the cooling rate of the weld metal during the transformation of the undercooled metastable austenite. The strength and toughness of weld metal improves as the amount of acicular ferrite increases due its fine basket weave microstructure and so it is important to understand the mechanism of its formation so that the volume fraction of acicular ferrite can be maximized in steel weld metal. The chemical composition of the filler wire mostly determines the final composition of the weld metal although the composition of the base plate is important because of dilution effects. In high strength steels the alloying elements such as carbon, nickel, chromium, copper nd niobium are present to achieve the required strength levels and a fortuitous outcome of this is a continuous cooling transformation (CCT) diagram with features that mean that bainite is the major transformation product during the arc welding of these steels provided a suitable weld power and preheat/interpass temperature is chosen during multi-run welding. Once a suitable weld-metal hardenability and cooling rate has been established the amount of acicular ferrite nucleated will depend on the size, number, distribution and chemical composition of the non-metallic inclusions. Suitable inclusions appear to be in the size range 0.2--2.0 micrometers with a mean size of 0.5 micrometers being about an optimum value. These inclusions usually contain manganese, silicon, aluminum and titanium as their major constituents and do not appear to be exactly spherical but have a faceted or slightly angular appearance