3 Fluxes can be further classified as active or neutral: ACTIVE FLUXES are those fluxes that will cause a substantial change in the weld metal chemistry when welding voltage (and consequently, the amount of flux fused) is changed. Active fluxes generally add large amounts of manganese and silicon to the weld metal and cause the weld strength to increase. If an active flux is used for multipass welding, excessive manganese and silicon build-up can occur, causing the weld to be brittle and crack prone. Active fluxes should be limited in the number of passes, especially over rust and mill scale. n When an active flux is used for multipass welding, the voltage must be carefully controlled to avoid excessive build-up of manganese and silicon. Active fluxes should not be used to multipass weld plates more than 1" (25 mm) thick. NEUTRAL FLUXES are fluxes that will not cause a significant change in the weld metal chemistry even with variations of welding voltages. Neutral fluxes will not significantly affect the strength of the weld metal regardless of the welding voltage used or the number of passes. As a rule neutral fluxes should be specified for multipass welding.
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