Submerged arc welding (SAW) is widely used for boiler tubes, pressure vessels, pipes and heavy fabrication. The quality of the weld depends heavily on the condition of the granular flux. Moist flux is one of the main causes of hydrogen-related weld defects. That is why many automated welding lines heat the flux in the hopper itself.
Why flux must be kept dry
Welding flux absorbs moisture from the air. When damp flux enters the arc, the moisture breaks down and hydrogen enters the weld metal. This can lead to:
- porosity and blow holes
- hydrogen-induced cracking
- rejected welds and costly rework
Flux manufacturers specify baking or re-drying temperatures for their products, and holding temperatures for storage. Keeping flux hot right up to the point of use prevents it from picking up moisture again.
Why heat the hopper and not just the oven?
Flux is often re-dried in an oven, but once it sits in an unheated hopper on the machine it starts cooling and absorbing moisture again. A heated hopper keeps the flux at the required temperature during the whole welding shift, which is especially important on long automated runs such as boiler pipe welding.
Design points for a flux hopper heater
Heating a granular material is different from heating a liquid or a metal block. Flux does not flow or conduct heat well, so the heater design must account for:
- Uniform heating – heaters placed around and sometimes inside the hopper so that no cold zones remain
- Target temperature – often in the range of a few hundred °C, for example around 300 °C, depending on the flux maker’s recommendation
- Watt density – kept moderate so that flux in contact with the heater is not overheated
- Temperature control – a thermocouple placed in the flux and a controller to hold the set point
- Space and mounting – the heater must fit the existing hopper geometry on the welding machine
- Robust construction – to handle vibration, abrasion and the workshop environment
Common heater types for this job include strip heaters or band heaters on the hopper wall, and tubular or cartridge heaters placed in the flux.
Custom flux hopper heating from Thermosys
Thermosys Technologies designs and manufactures custom heaters for welding flux hoppers on automated welding machines, including projects heating flux to around 300 °C. Send us a sketch or photo of your hopper with its dimensions and the required temperature, and we will propose a heater layout with control.