23 FCAW Defects and Discontinuities

A discontinuity is an interruption of the typical structure of a material, such as a lack of its mechanical, metallurgical, or physical characteristics; any change to the normal pattern of work. A defect is a flaw in a part or product that is unable to meet minimum acceptance standards. The part or product can be rejected. Defects need to be either repaired or thrown away. All defects are discontinuities, but not all discontinuities are defects.

To ensure high-quality FCAW welding, follow proper welding procedures, maintain good welding technique, and implement effective quality control measures. To reduce all chances of discontinuities and defects, ensure regular inspections, proper material preparation, appropriate welding parameters, and skilled welder training.

The most common defects and discontinuities in FCAW include:

Porosity

Porosity consists of cavity-type discontinuities formed by gas that gets stuck in the weld as it cools. Porosity is usually the least harmful type of weld discontinuity. Many fabrication standards and codes provide comparison charts that show the amount of acceptable porosity. If there is more porosity than allowed, it must be ground out and repaired. There are three types of porosity: surface porosity, subsurface porosity, and linear porosity.

Surface porosity consists of discrete round or elongated holes on the surface of the weld. Surface porosity is formed if dissolved gases cannot fully escape before the weld metal cools. Causes of surface porosity include not enough shielding gas coverage and too high gas flow rates that expose the molten weld to oxygen in the air.

Subsurface porosity consists of discrete round or elongated holes within the weld. The holes can be many different sizes, from microscopic to 1/8” diameter. There are three types of subsurface porosity: uniformly scattered, cluster, and linear. Cluster porosity voids occur in the form of clusters separated by considerable lengths of pore-free weld metal. Cluster porosity is associated with changes in welding conditions, such as stopping or starting of the arc. Linear porosity usually happens in a relatively straight line. Causes of linear porosity include insufficient shielding gas coverage and dirt, rust, or moisture on the base metal or welding consumables. Linear porosity is most likely to occur in the root pass

Prevention methods include:

• Improve welding housekeeping conditions that can cause the porosity
• Use clean materials and well-maintained equipment and properly align fans and drafts
• Avoid the use of excessive current and arc lengths
• Avoid high currents and excessive arc lengths
  • This may cause the de-oxidizing elements in the electrode to break down, so there won’t be enough left to combine with the metal during cooling
• Change welding conditions such as gas flow rate and gas purity. This makes up for improper arc length, welding current, or electrode manipulation
• Reduce travel speed
• Turn on the gas before welding

Wormholes

Wormholes are elongated or tubular cavities caused by excessive entrapped gas. Wormholes have the appearance of sharply defined dark shadows of that are rounded or elongated. Wormhole porosity is the most severe type of porosity because it provides a leak path through the weld. This is especially serious when the vessel or pipe is intended to contain liquid.

To prevent wormholes:

• Use the same methods as those to prevent porosity
• Eliminate the gas and cavities

Methods of correcting wormholes include:

• Grinding
• Air gouging
• Rewelding

Slag inclusion

Slag inclusions occur when bits of slag get trapped in a weld. Slag is a material made when flux and impurities in metal react together. It’s not a metal and it forms during a chemical process when they mix. Slag inclusions aren’t as heavy as the surrounding metal, so they usually rise to the surface of molten metal. Slag inclusions are detected by radiographic testing where they appear as dark lines parallel to the edges of the weld. Slag inclusions are usually elongated, rounded, and run in the direction of the of the weld. Slag inclusions can be continuous, intermittent, or randomly spaced.

Multiple-pass welds are more prone to slag inclusions than single-pass welds. Slag from the preceding pass, if not completely removed when cleaning, will become entrapped in the subsequent pass.

Slag inclusions are prevented by using proper welding preparation:

• Thoroughly remove slag from the weld
• Clean the weld between each pass of a multiple-pass weld
  • If you don’t thoroughly remove slag between each pass, it increases the chances of slag entrapment and the production of a defective weld.

Incomplete fusion

Incomplete fusion is a lack of union (fusion) between adjacent weld passes or base metal. Incomplete fusion is usually elongated in the direction of welding, with either sharp or rounded edges. Causes of incomplete fusion include incorrect electrode manipulation by the welder and failure to heat the base metal or previously deposited weld metal to the melting temperature.

Incomplete fusion can be prevented or corrected by:

• Using proper settings depending on size of base metal
• Following weld sizes listed on the blueprint
• Paying attention to joint fit-up
• Grinding or air gouging of weld and rewelding

Spatter

Spatter is a discontinuity consisting of metal particles expelled during fusion welding that do not form part of the weld. Spatter is not usually considered a serious discontinuity unless it impacts future operations. Only spatter which sticks to the base metal is a concern. Large globules of spatter can create a heat-affected zone (HAZ) on the base metal surface, which has a similar effect as an arc strike. Spatter on the base metal surface also creates stress, which could cause problems during service of the finished product. Spatter detection is achieved by visual examination (VT).

Spatter can be prevented or corrected by:

• Adding argon to the shielding gas mixture can drastically reduce spatter
• Using anti-spatter spray
  • Be careful: too much anti-spatter spray can cause other discontinuities like porosity
• Reducing the welding current and arc length

Undercut

Undercut occurs when a groove melts in the base metal next to the toe of the weld but is not filled by the weld metal. It causes a weaker area at the toe of the weld and often leads to cracking. Low-thickness areas are often the first to crack under loading. Undercut can also decrease a joint’s strength by trapping water and dirt. This makes corrosion happen faster.

Undercut can occur for several reasons:

• Excessive heat from high current and voltage settings
• Too fast of a travel speed and the electrode moves away from the weld pool prematurely
• Edges are not prepared properly
• Dirt or grit are present on the welding surfaces

It’s important to use the proper welding current and voltage. Voltage, like current, is an indicator of the total heat input into the weld. When voltage goes up, more heat enters the weld zone, and more base metal melts Excess heat creates a larger than required cavity which is not always totally filled. This leads to undercuts on the sides of the weld due to a lack of filler material.

Undercut can be prevented or corrected by:

• Reduce amperage
• Reduce arc length
• Use correct electrode angle
• Adjust travel speed
• Grind out undercut and reweld