Slag inclusions

The characteristic features and principal causes of slag imperfections are described

Identification

Slag is normally seen as elongated lines either continuous or discontinuous along the length of the weld. This is readily identified in a radiograph, Fig 1. Slag inclusions are usually associated with the flux processes, ie MMA, FCA and submerged arc, but they can also occur in MIG welding.

Fig. 1. Radiograph of a butt weld showing two slag lines in the weld root

Causes

As slag is the residue of the flux coating, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs, Fig 2.
As they both have an effect on the ease of slag removal, the risk of slag imperfections is influenced by

• Type of flux
• Welder technique

The type and configuration of the joint, welding position and access restrictions all have an influence on the risk of slag imperfections.

Type of flux

One of the main functions of the flux coating in welding is to produce a slag which will flow freely over the surface of the weld pool to protect it from oxidation. As the slag affects the handling characteristics of the MMA electrode, its surface tension and freezing rate can be equally important properties. For welding in the flat and horizontal/vertical positions, a relatively viscous slag is preferred as it will produce a smooth weld bead profile, is less likely to be trapped and, on solidifying, is normally more easily removed. For vertical welding, the slag must be more fluid to flow out to the weld pool surface but have a higher surface tension to provide support to the weld pool and be fast freezing. 

The composition of the flux coating also plays an important role in the risk of slag inclusions through its effect on the weld bead shape and the ease with which the slag can be removed. A weld pool with low oxygen content will have a high surface tension producing a convex weld bead with poor parent metal wetting. Thus, an oxidising flux, containing for example iron oxide, produces a low surface tension weld pool with a more concave weld bead profile, and promotes wetting into the parent metal. High silicate flux produces a glass-like slag, often self detaching. Fluxes with a lime content produce an adherent slag which is difficult to remove.

Prevention of slag inclusions by grinding between runs

The ease of slag removal for the principal flux types are:

• Rutile or acid fluxes - large amounts of titanium oxide (rutile) with some silicates. The oxygen level of the weld pool is high enough to give flat or slightly convex weld bead. The fluidity of the slag is determined by the calcium fluoride content. Fluoride-free coatings designed for welding in the flat position produce smooth bead profiles and an easily removed slag. The more fluid fluoride slag designed for positional welding is less easily removed. 

• Basic fluxes - the high proportion of calcium carbonate (limestone) and calcium fluoride (fluospar) in the flux reduces the oxygen content of the weld pool and therefore its surface tension. The slag is more fluid than that produced with the rutile coating. Fast freezing also assists welding in the vertical and overhead positions but the slag coating is more difficult to remove. 

Consequently, the risk of slag inclusions is significantly greater with basic fluxes due to the inherent convex weld bead profile and the difficulty in removing the slag from the weld toes especially in multi-pass welds.

Welder technique

Welding technique has an important role to play in preventing slag inclusions. Electrode manipulation should ensure adequate shape and degree of overlap of the weld beads to avoid forming pockets which can trap the slag. Thus, the correct size of electrode for the joint preparation, the correct angle to the work-piece for good penetration and a smooth weld bead profile are all essential to prevent slag entrainment.

Fig. 2. The influence of welder technique on the risk of slag inclusions when welding with a basic MMA (7018) electrode

a) Poor (convex) weld bead profile resulted in pockets of
slag being trapped between the weld runs 

b) Smooth weld bead profile allows the
slag to be readily removed between runs 

In multi-pass vertical welding, especially with basic electrodes, care must be taken to fuse out any remaining minor slag pockets and minimise undercut. When using a weave, a slight dwell at the extreme edges of the weave will assist side-wall fusion and produce a flatter weld bead profile.

Too high a current together with a high welding speed will also cause side-wall undercutting which makes slag removal difficult.

It is crucial to remove all slag before depositing the next run. This can be done between runs by grinding, light chipping or wire brushing. Cleaning tools must be identified for different materials e.g. steels or stainless steels, and segregated.

When welding with difficult electrodes, in narrow "V" butt joints or when the slag is trapped through undercutting, it may be necessary to grind the surface of the weld between layers to ensure complete slag removal.

Best practice

The following techniques can be used to prevent slag inclusions: 

• Use welding techniques to produce smooth weld beads and adequate inter-run fusion to avoid forming pockets to trap the slag 

• Use the correct current and travel speed to avoid undercutting the side-wall which will make the slag difficult to remove 

• Remove slag between runs paying particular attention to removing any slag trapped in crevices 

• Use grinding when welding difficult butt joints otherwise wire brushing or light chipping may be sufficient to remove the slag. 

Acceptance standards

Slag and flux inclusions are linear defects but because they do not have sharp edges compared with cracks, they may be permitted by specific standards and codes. The limits in steel are specified in BE EN ISO 5817: 2003 for the three quality levels. Long slag imperfections are not permitted in both butt and fillet welds for Quality Level B (stringent) and C (moderate). For Quality Level D, butt welds can have imperfections providing their size is less than half the nominal weld thickness. Short slag related imperfections are permitted in all three quality levels with limits placed on their size relative to the butt weld thickness or nominal fillet weld throat thickness.