Welding Procedure Development: Selecting the right welding process

Selecting the best welding process is a critical step in developing a welding procedure that attains the desired quality and optimizes productivity.  Many times we look for the welding process that can give us the highest deposition rate to optimize productivity. However, every welding process has limitations. For example, submerged arc welding (SAW) has extremely high deposition rates, but it is limited to the flat and horizontal positions, it is not feasible for short welds and it requires the use of flux which adds complexity to the process.  So it may not always be the best option.

Most times the welding processes selected for new welding procedures is that for which there is welding equipment available to the fabricator..  For example, when a shop receives a job that can be done with submerged arc (SAW) due to it having long straight welds in thick sections, many times the work will be done with GMAW or FCAW because there is no SAW equipment available. 

Below are the main considerations for welding engineers or anyone developing a new welding procedures in selecting the right welding process. 

Available Equipment

In the vast majority of cases the determinant factor in selecting a welding process is the equipment that the fabricator has available.  This makes perfect sense, but you must always consider other processes.  There are times when purchasing new welding equipment can lower the total cost of the project by increasing productivity and quality (less rework).  

Joint Type and Welding Position

There are 5 types of joints: Tee, butt, corner, edge and lap.  Certain joint types lend themselves better to certain processes.  For example, SAW can be used in butt and tee joints, but not really feasible for most edge, lap and corner joints. 

Joint types

The welding position (flat, horizontal, overhead or vertical) further narrows down the processes which may be used.  If you have a butt joint that is in the vertical position it would render SAW useless.  All other processes are still options, but then you can look at deposition rates for out of position welding. Typically, processes that produce slag (FCAW and SMAW) are good options for out-of-position welding.  GMAW and GTAW may be used but they would most likely result in lower deposition rates.

Welding positions by weld type

Indoor/Outdoor

Whether welding is being carried out indoors or outdoors may also dictate which welding process we use.  Any process may be used outdoors provided proper shielding from the elements is provided; however, this isn’t always practical.

The best processes for welding outdoors are stick (SMAW) and self-shielded flux-cored (FCAW-S). These processes can handle winds of up to 35 mph [56 kph] without experiencing negative effects.  If you need to use GMAW, FCAW-G or GTAW outdoors you would need to have barriers that block the wind as shown above in Figure 2.3.  Even a 5mph [8kph] draft is enough to cause porosity by blowing the shielding gas away when using gas-shielded welding processes. 

Welder Skill

The skill level of the welders is another important factor.  Certain processes such as GTAW and SMAW require a higher level of welder skill as compared to the GMAW and FCAW. SAW is done automatically in which case the welder skill may not be overly important as this person is a machine operator rather than a welder.  Furthermore, all processes can be automated by the use of robots or hard automation.  In this case, welder skill is not as critical of a factor. 

Productivity

Productivity is one of the most important factors which must be considered when developing a WPS.  The deposition rate of the different arc welding process should be taken into consideration. But keep in mind that welding speed alone is not all you need to take into account.  Processes that produce slag require clean up and also have a lower electrode efficiency as is the case with FCAW and SMAW.  

Below are typical deposition rates of arc welding processes.  These ranges are what’s considered practical.  We could certainly deposit 20 lb/hr with the GMAW process but it would not be practical.

GTAW  → 0.5 to 4 lb/hr

SMAW → 1.0 to 7.0 lb/hr

FCAW-SS → 4.0 to 12 lb/hr

FCAW-GS → 4.0 to 15 lb/hr

GMAW → 2.5 to 15 lb/hr

MCAW → 4.0 to 15 lb/hr

SAW    → 12 to 35 lb/hr

Return on Investment (ROI)

Once we understand the productivity factors of each process we can run a return on investment calculation. This can include the purchase of new equipment. If we determine that SAW is the most productive process, we may consider buying the equipment if we don’t already have it.  A SAW system can run between $20,000 and $80,000.  You will need to determine if the investment has a rate of return that is acceptable to your company.

These are all major considerations when selecting the welding process. Our next article will discuss another key component of developing a welding procedures: selecting the right filler metals. There may be more to it than you think.

Reference: Welding Procedure Development for Non-Welding Engineers

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Categories: Processes, Qualification, Welding Equipment
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