The microstructure of weld metals in low alloy steels

• Main Author: Barritte, G. S.
• Published: University of Cambridge 1983
• Subjects: 669
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596423

The microstructure of High Strength Low-Alloy steel weld metals has been investigated using a variety of techniques including transmission electron microscopy, incorporating energy dispersive X-ray analysis, and high-speed dilatometry. The nature of the constitutent commonly termed acicular ferrite, which is known to confer good toughness and strength in these materials, has been shown to comprise intragranularly nucleated Widmanstatten ferrite. Weld metal inclusions have been demonstrated to be the primary sites for the intragranular nucleation events and it is believed that the inclusions act to reduce the energy barrier to nucleation by providing a high energy interface which is destroyed during nucleation. The fine interlocking nature of the microstructure is attributed to sympathetic nucleation of ferrite on inclusion nucleated laths. The proportion of the acicular ferrite constituent in the final microstructure is found to be a complex function of cooling rate, alloy composition, inclusion content and the austenite grain size of the weld deposit. These latter three parameters appear to be interrelated in many instances due to the nature of the alloying elements considered. The dilatometric investigation of the decomposition of austenite in H.S.L.A. weld metals, under both isothermal and continuous cooling treatments, seems to be consistent with recent fundamental theories of phase transformations in steels. The general concept that two 'C' curves exist (ie. describing diffusional and shear transformations respectively) and can accurately describe transformations in steels is supported. The application of a thermodynamic analysis to transformations occurring on continuous cooling has introduced the concept of an 'effective' bainite start temperature to describe the condition of the austenite remaining untransformed during heat treatment. At the cooling rate and alloy composition considered in these weld materials, transformation has been shown to proceed at temperatures consistently higher than this calculated bainite start temperature at all stages of transformation. The morphology of the upper bainite microstructure has been investigated and shown to be consistent with a mechanism involving displacive sub-units of bainitic ferrite.