Slag inclusion in Welding

What is slag inclusion?

Non-metallic particles of comparatively large size entrapped in the weld-metal are termed as Slag Inclusion in Welding.

Slag inclusions are detected by the normal non-destructive testing methods, while non-metallic inclusions are observed in the weld microstructure at high magnification.

Understanding what causes slag inclusion is key to reducing their occurrence and ensuring quality welds.

Slag inclusion in rt film or a radiography film are visible as dark spot due to their low density compared to the base metal as shown in the below picture.

Usually, slag inclusions have a three-dimensional elongated shape, but, in some, in cases, they can be both round and flat. Often slag inclusions are found along the boundaries between the base metal and the deposited metal.

When performing multilayer seams, the formation of slag inclusions often occurs along the boundaries between the individual Layers.

The cause of the formation of slag inclusions can be the presence of contaminants on welding edges (rust, slag, etc.), too little welding power current, and unnecessarily high welding speed.

Slag inclusions in welding or casting can be:

  1. Elongated slag,
  2. Scattered Slag,
  3. Cluster slug.

What causes slag inclusion in Welding?

Slag inclusions in welds are caused when non-metallic materials are trapped within the weld, preventing it from fully fusing with the base material as shown in the below sketch.

Why slag inclusion occur in welding is related to the good practices. Lack of proper interpass cleaning is the major cause of slag inclusion in welding.

Other important causes of slag inclusion in welding are:

  1. Slag inclusion usually occurs in multi-pass welds due to imperfect cleaning of the slag between the deposition of successive passes.
  2. It may also be caused by heavy mill scale, loose rust, dirt, grit and other substances present on the surface of the base metal or bare electrode.
  3. Slag trapped in undercuts or between uneven preceding runs may give rise to elongated lines of included slag when a subsequent weld pass is deposited.
  4. Slag inclusion is caused by several factors such as improper cleaning of metals before welding, incorrect joint preparation and inadequate coating on welding rods.

The melting characteristics of the welding consumable and particularly the viscosity of the resulting slag has an important bearing on inclusions. The molten slag should float freely to the surface of the weld pool and be easily removable on solidification.

How can slag inclusions in welding be avoided?

To avoid slag inclusions in welding, the following measures are necessary before & during the welding:

  1. Thorough cleaning of the surface of the welded edges and the surface of the inner layers of the weld seams during multilayer welding.
  2. Use weld weaving within limits. Excessive weld weaving will cause slag inclusion.
  3. Use proper welding consumables.
  4. Keep joint surfaces (especially gas cut surfaces) and bare filler wires perfectly clean.
  5. Avoid undercuts and gaps between deposited passes.
  6. Clean the slag thoroughly between weld passes.

The strength of a welded joint may be considerably reduced if large irregular inclusions, or elongated lines of inclusions at the weld junction are present. These sometimes give rise to radiating hair-line cracks.

The presence of small, isolated, globular inclusions may not however, seriously affect the static strength of a joint and these may normally be disregarded.

To rectify this defect, the portions of weld-metal which contain slag inclusions must be removed and then filled with sound weld-metal.

What is slag inclusion in casting?

Slag inclusion in casting is a defect that occurs during the metal casting process. It is caused when molten slag infiltrates the mold and gets trapped inside the cast part.

Slag inclusions weaken the casting overall soundness. The presence of slag in casting also causes an unsightly appearance, making it difficult to accept parts with slag defects.

To prevent slag inclusion, proper gating design and maintenance should be employed when pouring molten metal into the mold cavity. This ensures that any slag created during melting does not enter alongside the molten metal.

Additionally, appropriate fluxing materials need to be used so that any potential oxides created are removed before being poured into the mold cavity.

Finally, operators must constantly check for signs of clogging or debris in order to maintain a smooth flow of molten metal into the molds.

Porosity and slag inclusion in Welding

Porosity is an issue where gas bubbles form within the weld and create a porous structure, while slag inclusion is when material from a flux or electrode is trapped in the weld bead.

When porosity occurs, it appears as small holes throughout the material being welded. These pores are usually the result of air or other gases that weren’t able to escape before the filler material cooled and solidified.

On the other hand, slag inclusions manifest as thin strands of flux or other debris that has been melted into and surrounded by molten metal during welding.

Porosity and slag inclusion in Casting

Porosity and slag inclusion defects are two common types of casting defects that can have a negative impact on the quality of cast parts.

Porosity is defined as an open void or interconnected space in the material caused by gas entrapment during solidification, while slag inclusion occurs when foreign material such as sand or oxides contaminate the casting surface.

Read more: Welding Porosity, its types, causes and remedies.

Both porosity and slag inclusions can lead to significant reduction in strength and fatigue life, corrosion resistance, and overall part performance.

Slag inclusion acceptance criteria

Slag inclusion acceptance criteria is a set of guidelines as defined in the acceptance code and standard for final weld visual inspection.

"Slag inclusion as per AWS D1.1 is not acceptable and requires repair."

In ISO 5817, slag inclusion (301) is acceptable within certain limits. For Class D, maximum inclusion size is 4 mm with a maximum length of 75 mm.

For Class C, maximum slag inclusion permitted is 3 mm with a length of 50 mm. For Class B, maximum slag inclusion permitted is 2 mm with a length of 25 mm maximum.

In API 1104, Clause 9.3.8 covers the acceptance criteria for slag inclusion and based on:

  1. Large pipe diameter (>2.375 inches): Any Elongated slag inclusions (ESIs) more than 2 inch shall be considered as a defect.
  2. Small pipe diameter (OD less than 2.375 in. (60.3 mm): Any ESI of length more than 3 times of pipe thickness shall be considered as a defect.

Slag inclusion in tig welding

TIG welding produces high quality welds. Generally, Slag inclusions are not found in TIG Welding. Slag inclusion is not an inherent problem to TIG welding, but instead is caused by uncleaned base metal or mill scale on the base metal. However, Slag inclusion can appear when there is uncleaned base metal or mill scale on the base metal.

In order to prevent this issue during TIG welding, it is important that all surfaces be thoroughly cleaned prior to beginning the process. This will help reduce any potential contamination from occurring within the weld itself.

Slag inclusion in saw welding

Slag inclusion in saw welding is a common problem, and it can have a significant impact on the quality of the welding job.

As SAW process relies in external flux to shield the weld pool, it brings the risk of slag inclusion too. Improper cleaning of slag during interpass welding, can cause entrapped slag between weld beads.

To prevent these issues from occurring, welders must be diligent about keeping their weld area clean, make sure to clean every bit of slag- especially at weld toes and keeping welds free from contamination.

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