TWI Industrial Member Report Summary 162/1981
By D J Abson and P H M Hart
In a previous study, the influence of niobium and vanadium on weld microstructure and toughness was investigated in submerged-arc welds deposited under an agglomerated basic flux. In the present phase of the work, two fused fluxes, one a calcium silicate (CS) flux and the other a calcium silicate, low silica (CSLS) flux, were used in order to produce a wider difference in initial as-deposited microstructure than had been achieved previously by the use of different wires and the basic flux. Relevant data from the previous study have been incorporated into the present report. As in the previous work, two-pass tandem submerged-arc welds were produced in 25mm thick microalloyed steel plate using a 1%Mn wire. The arc energy for the first side weld was 4.1kJ/mm and for the second side weld was 4.9kJ/mm. Weld niobium levels were approximately 0, 0.02, 0.03 and 0.078, and welds made in plate containing vanadium plus 0.02%Nb were also investigated. Weld vanadium levels were 0.06% (for the fused fluxes) and 0.11% (for the agglomerated flux).
For the as-deposited weld metal, the predominant microstructural constituent in the microalloy-free deposits was ferrite with aligned MAC* for the CS flux and acicular ferrite for the other two fluxes, as expected from the oxygen and titanium contents. Niobium increased the proportion of acicular ferrite and gave substantial increases in strength and hardness for all fluxes. For as-deposited weld metal, niobium had a marked deleterious effect on crack tip opening displacement (CTOD) (for surface-notched B x B specimens) and Charpy toughness with the CS flux, but little effect with the other fluxes. In respect of reheated weld metal, mechanical tests on 0.07%Nb deposits showed that the CTOD toughness (of through-thickness notched 2B x B specimens) was below that of the surface-notched (B x B) specimens for deposits produccd under all three fluxes, and that the Charpy toughness was worse than that of the as-deposited weld metal.
Vanadium increased the proportion of acicular ferrite in the CS flux deposits, but had only a small microstructural effect in the deposits produced under the other fluxes. Vanadium had minimal effect on the CTOD toughness (in surface notched B x B specimens) for the CS and basic flux deposits but had a marked deleterious effect for the CSLS deposit. The Charpy toughness of the second-pass weld metal was influenced adversely by the vanadium addition for the CSLS and basic flux deposits while for the CS deposit, vanadium had little effect. Overall, the work has shown that greatest tolerance to dilution of niobium and vanadium into two-pass welds is obtained by consumables giving a predominantly acicular ferrite microstructure, with minimum grain boundary ferrite.
*The microstructural terminology used in this report is that proposed by Abson and Dolby (3).