November 10, 2009
Suitable for welding various materials and plate thicknesses, submerged arc welding is considered an economical, efficient process. Yet, in the search for fine tuning this process, obvious solutions have been ignored in favor of exotic and expensive welding heads and power sources. Applying sound expertise and creative insight can lead to less costly solutions.
Editor's Note: Naddir M. Patel, Calgary, Alb., Canada, former director of operations at Silico Products, Mumbai, an Indian manufacturer of fused and bonded fluxes for submerged arc welding, has written two articles that relate his experiences solving two separate, unrelated manufacturing problems.1 A summary of those articles has been compiled by Elia E. Levi, with Patel's permission.
When working for Silico Products, Naddir Patel noticed that despite the SAW process being highly efficient in depositing weld metal, some of his customers were experiencing significant downtime between processes. They also were getting burn-through defects because welders were not proactively measuring the inter-pass temperatures.
Selecting a different high-deposition power source, such as twin wire, tandem wire, or power wave source was not a viable option simply from a capital cost standpoint. Retraining welders and requalifying weld procedures also would increase a project's capital cost. This high capital investment still would have limited versatility in a typical multitasking shop that makes circumferential welds on 6-in. to 42- in. diameters in various plate thicknesses.
A published survey2 indicated that increasing electrode extension would seem to be the ideal remedy for increasing production rates and decreasing the heat input.
Whereas earlier attempts to increase electrode stickout (extension) were not very successful or consistent, Patel found a company that produced ceramic nozzles3 that could be screwed onto the contact tip of the welding head to guide the hot wire without jamming it.
This simple ceramic nozzle allows electrode extensions of 3 in. or more without wire straying. It eliminated arc wandering, provided adequate weld penetration, and produced quality welds. As welding parameters were not changed (except for a very small voltage increase), there was no need to requalify weld procedures.
The electrode extension was increased in increments to yield deposition rate improvements of 45 percent without any radiographic testing (RT) defects occurring.
As the heat-affected zone (HAZ) was narrower, narrower grooves were designed for greater efficiencies and less distortion.
Tests found that a 3/32-in. electrode with a 2 ¼-in. extension could be used for normal welds on various material thicknesses. Heavy-walled welds were completed with a 5/32-in. dia. electrode.
Tests were conducted at various customer locations and on a variety of groove welding applications for both mild steel and stainless steel in a 350- to 650-amp range.
A different SAW application is high-speed welding (at 450 to 500amps; 28 to 32 volts; 55 to 59 inches per minute, two weld runs) of liquefied petroleum gas (LPG) cylinders in India.
Ten to 12 years ago, Patel's company was approached by several customers that manufactured LPG cylinders with a complaint that their current SAW operation resulted in a weld integrity reject rate of up to 20 percent—clearly an unsustainable condition.
Electrode specifications in terms of chemistry and wire diameter typically are very clear. However, because flux compositions are proprietary, no welding-related operational details are available for guiding the SAW flux user..
In this LPG application involving mild steel plate barely 0.12 in. thick, burn-through and oxide inclusions and porosity were serious time and cost multipliers.
As process parameters required two runs, the challenge was to design a uniform melting and fast self-peeling flux that could handle welding abuse, such as unclean surfaces, no preheating, and no flux preheating.
Consumable SAW fluxes are complex, multipurpose, ceramic compositions with mineral and alloy ingredients mixed together in proprietary combinations and processed (either by electric fusion or agglomeration) to yield a granular ceramic product.
The SAW flux takes on the triple role of shielding the weld from the atmosphere, refining the weld metal (through addition of alloying elements and removal of tramp oxides), and peeling off as a slag once the welding is complete.
The formulation must melt uniformly at specific temperatures (dictated by the parent metal chemistry); possess operational characteristics, such as a specific gravity and fluidity, to refine the weld metal; and have surface tension characteristics that ensure speedy slag peel-off.
To research the LPG SAW problem, Patel formulated an action plan to collect process data and operation details from the manufacturers—mainly, operational parameters(amperage, voltage, welding speed, parent material thickness and chemistry, and electrode chemistry) and defect identification (burn-through, porosity, slag inclusions, pockmarks, and surface discoloration).
Whereas the operational parameters were readily available, weld defect identification and tabulation documentation were not. Management was concerned about information leaking out, and the union was worried that the information could be used as justification for decreasing productivity-based wages.
A compromise was finally reached, with the data guaranteed to remain strictly confidential. Neither the welder nor the manufacturer would be identified.
Analyzing data collected over a week revealed that porosity (pinholes, pockmarks on the weld surface, surface discoloration) was the main problem that needed addressing, followed by burn-through and slag inclusions.
This was, of course, not a two-step process, but a repetitive process of continuous improvement. Uniformity of the welding flux components, flux fluidity, flux-reducing capability, and flux surface tension after welding were determined to be the critical to quality components.
A fused MnO-SiO2-CaO system-based SAW flux, duly tweaked, was finally offered and accepted. Whereas a customized variation of this fused welding flux became the standard flux used for this process in India, there still were random weld defects that could not be accounted for, even after welding parameters were all fine-tuned.
Fluxes are sold in mesh size fractions describing the particle size distribution within the upper and lower control limits of the mesh sizes indicated. For example, 8 by 48 implies that 100 percent of the material passed a size 8 sieve of mesh, and 0 percent passed a size 48 sieve. A rule of thumb requires decreasing the flux particle size as both the welding speed and the current increase.
As the chemical/mineral components of the flux already had been fine tuned, attention was therefore focused on the mesh size (particle size) of the flux granules.
It was made evident from the experimental data collected that a particular mesh size fraction was responsible for the least number of weld defects.
This mesh fraction was then resieved into narrower fractions of material passing a series of sieve meshes. Welding runs were then made with each individual fraction and a Pareto bar chart generated against defects.
The end of the development program was reached when a specific mesh fraction was singled out as that minimizing the number of defects.
These two unrelated examples of possible SAW problems led to the following conclusions:
1. With weld deposition rates increasing by 50 percent and minimal change in existing welding parameters, it was obvious that increasing electrode extension was a force multiplier, not only in terms of production throughput, but also in a drastic decrease in burn-throughs and subsequent repair, with no additional capital expense.
2. The mineral/chemical composition and particle (granular) size of a SAW flux are two parameters very critical to ensuring quality, defect-free welds.
It is important for SAW shops to take maximum advantage of this versatile and highly efficient process by implementing data collection and analysis to arrive at both the optimal base composition of the flux and the optimal granule size fraction required for their specific process parameters, along with process optimization.
Welding consultant, Elia Levi, and Naddir Patel, a process optimization (lean, six sigma) welding consultant, can be reached through their respective Web sites: www.welding-advisers.com and www.rayatechnicalservices.com.