Effect of Niobium and Vanadium on the Microstructure and Mechanical Properties of Submerged-Arc Welds Produced Under a Basic Flux in Carbon-Manganese Steels

Two-pass tandem submerged-arc welds have been deposited in 25-mm-thick, silicon-killed, aluminum-treated steel plate containing different levels of niobium and vanadium, all of the same base composition, 0.14C-0.3Si-1.4Mn-0.04Al. Two vanadium-containing steels with a high nitrogen content were also included. The range of plate compositions included those which would be used for controlled rolled and quenched and tempered steels. The effects on weld metal Charpy toughness of the microalloying additions were compared with those from previous work, and crack-tip opening displacement (CTOD) data were generated for submerged-arc welds in such steels. For this, the first phase of a larger program, a basic agglomerated flux was used, and the arc energy (for the second pass) was 4.9 kJ/mm. Welding was carried out using S1 (0.5Mn), S2 (1Mn), and S3Mo (1.5Mn-0.5Mo) wire so that the influence of the microalloying additions on different initial microstructures could be studied. Three weld deposit levels of niobium (∼0.2, ∼0.03, and ∼0.07 percent) and two of vanadium (∼0.06 and ∼0.11 percent) were studied.

For the majority of the welds, the Charpy and CTOD properties of the as-desposited weld metal were good (the 27-J temperature was generally < −50 °C and the 0.1-mm CTOD temperature generally < −60 °C). The microalloy additions had comparatively little effect on the microstructure except when low levels of niobium (<0.02 percent) were added to lean alloy (1.2Mn) deposits, when a marked increase in acicular ferrite content occurred. An increase in 27-J temperature and in 0.1-mm CTOD temperature was commonly observed but was generally <15 °C for the highest levels of microalloy additions. Hardness increases in reheated weld metal generally became greater with increases in microalloy content, and this was reflected in a rise in the 0.1-mm CTOD temperature, particularly for the S3Mo deposits. In contrast, the Charpy tests revealed little influence of microalloying in the S3Mo deposits, while in the S2 deposits the effect depended strongly on the level of niobium.