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Identifying Pockmarking Causes in Structural SAW
Q: Our company manufactures various structural components, typically made from A36 or A572 steel grades in plate, I-beam, or channel. Based on the application we'll use GMAW, FCAW, or SAW. On random occasions we experience pockmarking on the surface of the SAW joints. While the timing is usually unpredictable, we've found that it typically shows up on horizontal fillet (2F) T-joint welds or on the cover pass of exterior weld joints of rolled plate components. I understand that this issue is not considered a weld defect, but the appearance is not desirable. Is there a remedy for this?
A: There are several explanations for the pockmarking you are experiencing. For starters, if this problem began happening fairly recently, you may want to revisit any news process, consumable, or material changes. It's not uncommon for companies nowadays to make changes that might appear insignificant to the manufacturing processes in an effort to run lean. Sometimes, however, these changes end up having unforeseen effects.
Assuming you have approved weld procedures, make sure your welders are following them closely. Excessive electrical stick-out (ESO) has been known to cause a similar weld surface issue. Additionally, most flux-delivery nozzles are attached directly to the welding torch and if the ESO is excessive, the amount of flux burden may be too deep. This could lead to improper degassing of the molten weld puddle.
Pockmarking also may be
caused by an improper ratio of recycled-to-virgin flux or by lack of
flux-recovery system cleaning. Every time you recycle flux, the particles tend
to break down into smaller mesh sizes. This could allow too many flux fines to
accumulate in the system, which also prohibits proper degassing of the weld
puddle during the welding process.
A more common culprit of pockmarking is excessive moisture in the flux. Proper intermediate- and long-term storage of flux is essential. That information is supplied by your flux manufacturer, but typically keeping the flux temperature above the boiling point of water (212 degrees F or 100 degrees C) should prevent any moisture buildup.
Without knowing your complete history and assuming you have always had issues with pockmarking, the next thing to consider is welding process variables. How well are your ESO, voltage, current, and travel speed controlled? How is the flux delivered to the joint, and what is the flux burden depth? What is the ratio of recycled flux to virgin flux, and how often is the flux-recovery system cleaned? Is your flux stored in a heated holding tank, and if so, what is the temperature? Is the material surface clean? Is there any oil, rust, or mill scale in the welding joint area? If the flux delivery is through an air pressure hose, make sure the air is dry and clean (no oil).Each one of these items can play a significant role in weld performance. If more than one variable is out of the recommended range, they may combine to cause welding issues that also make determining the root cause more challenging.
This article originally appeared in The WELDER magazine.
It is reprinted here with permission of the Fabricators & Manufacturers Association, Intl.