Submerged Arc Welding Fluxes:
The criticality of particle size
by Naddir M. Patel
(Calgary, Alberta, Canada)
These data were then stratified as per the type of weld defect and charted. Analyzing the data collected for a week, it stood out that porosity (Pinholes, pock marks on the weld surface and surface discoloration) was the main culprit that needed to be focused on, followed by burn-through and slag inclusions.
Whereas burn-through is strictly a welding heat input based problem, the data analysis clearly indicated, that the flux designed to weld water pipes needed a drastic makeover for this application of welding thin gauge gas cylinders.
This was of-course not a 2 step process, but a repetitive process of continuous improvement identified as PDCA (Plan, Do, Check, Act) and incorporating Design of Experiments (DOE) protocols. Design of Experiment (DOE) is a structured methodology of effectively and efficiently exploring the cause and effect relationship between numerous process variables (Xs) (in this case material inputs and their proportions), and the output or process performance variable (Y) (in this case, welding characteristics, mainly unacceptable flaws).
It was determined that uniformity of the welding flux components, flux fluidity, flux reducing capability and flux surface tension after welding were the Critical to Quality components.
- If a uniform mixture was to be ensured, the mineral components would have to be melted. Physical mixing would never result in a 100% mixed material. This validated the customers’ contention that agglomerated/bonded fluxes had already been tried and were rejected due to persistent operational inadequacies.
The solution was to offer a Fused SAW flux.
- High welding speeds and high currents resulted in molten flux run-off. Flux fluidity therefore required reduction through adjustment of input proportions.
- Lack of joint cleanliness required a superior de-oxidation capability to address both the oxide inclusions and porosity due to ambient moisture. Components that increased Si and Mn deposition in the weld metal were then added or increased.
- At roughly 45seconds/revolution, Slag not detaching immediately after the first run would cause defects in the form of inclusions in the next run. Components that influenced the surface tension at the metal-slag interface were then added or increased.
A fused Mno-SiO2
-CaO system based SAW flux, duly tweaked, as explained above was offered and accepted.
Whereas a customized variation of this fused welding flux (Fluxomelt BRD-1, discontinued since 2003) became the standard flux used for this process in India and Bangladesh, from the "lean" point of view (meaning elimination of all sources of waste like process variation, including re-work) though, there were still random weld defects
that could not be accounted for even after welding parameters were all fine tuned.
As users of SAW flux are aware, fluxes are sold in mesh size fractions
such as 8X48, 12X100 or 20X150. These fractions indicate particle size distribution
within the upper and lower control limits of the mesh sizes indicated. Thus 8X48 implies 100% of the material passed a sieve of mesh size 8 and 0% passed the sieve of mesh size 48. Mesh sizes are measured in various standards worldwide such as Tyler, ASTM (US), BSS (UK) etc.
A general rule of thumb
requires decreasing the flux particle size as both the welding speed and the current increase.
For this application, unfortunately, finer sizes created porosity based weld defects
, probably due to the flux not being heated prior to use. As the chemical/mineral components of the flux had already been fine tuned, attention was therefore focused on the mesh size (particle size) of the flux granules.
By collecting the experimental data in Tables and Bar Charts it was made evident that a particular mesh size fraction, namely that described as 8x48 was responsible for the least number
of weld defects.
This mesh fraction was then re-sieved into narrower fractions of material passing a series of sieve meshes. Welding runs
were then made with each individual fraction and a bar chart generated against defects.
The end of the development program was reached when the mesh fraction 18x24 was singled out as that minimizing the number of defects
The mineral/chemical composition and particle (granular) size of a Submerged Arc welding flux are two elements, very critical
to quality parameters for ensuring defect free welds.
Welding of CS (carbon Steel), SS (Stainless steel) or ultra low C steels require fluxes of different chemical components
to facilitate the optimal weld microstructure.
Whereas similarly acceptable welding results
are possible from two or more fluxes having totally different
components, the user should explore, possibly with external help, which set of flux components are most optimal for their application, based on the chemistry and microstructure
of the parent metal and that desired of the weld metal.
Similarly the particle/granular size affects the rate of melting and wetting
of the parent metal (within that limited time window of arcing) and is unique to the user and needs to be optimized.
It is therefore important for SAW shops to take maximum advantage
of this versatile and highly efficient process by implementing statistically valid (Six sigma based) data collection and analysis drive to arrive at both the optimal base composition of the flux required, and the optimal granule size fraction, required for their specific process parameters.
The goal is to ensure a robust, defect and breakdown free
production process, insulated from human error. Not only are these processes simple and economical to implement, but in this economical situation, squeezing out processing waste would go a long way in addressing operational profitability
and international competitiveness.