Weld Fusion vs. Weld Penetration

When you have a welding procedure that produces a deeper weld penetration (and a resulting wider penetration profile), you increase the chances of still achieving complete fusion at the root, even with welders that have limited skills. A deeper and broader penetration profile covers a bigger area. Thus you are more likely to still hit the root (i.e., achieve fusion) even if the arc is not focused directly at it.

Benefit: Figure 6 shows examples of CJP groove welds in a butt joint with a root-face dimension (i.e., the square edge or non-beveled portion of the plate edges in a butt joint). These joints will be welded from the first side (with one or more passes, depending on plate thickness). Then typically the weldment is flipped over and welded from the second side (again, with one or more passes).

To achieve complete joint penetration, the plates must be beveled, as in the double V joint shown in the top picture. Or if it is a square-edge joint (shown in the bottom illustration in Figure 6), then after the first side is welded, the second side of joint must first be back gouged to sound weld metal. Then the second side is welded.

If welding procedures that produced a deeper weld penetration were used, then the depth of the joint bevels would not need to be as deep, making the root face longer. Or in the case of square edges, not as much base plate on the second side would need to be removed by back gouging before sound weld metal was reached.

In either case, the volume of weld metal required to fill the joint would be reduced. This reduces both the amount of filler metal required to fill the joint and the welding time. Less welding would also reduce potential plate warpage issues.

Benefit: For fillet welds with a flat face and even leg sizes, the distance from the weld face to the root is called the theoretical throat. If you achieve fusion beyond the root, then the actual or effective throat length increases (see Figure 3 for identification of the theoretical and actual throats).

Generally no design credit of extra weld strength is given for normal root penetration. However, if significant and consistent root penetration can be achieved, which significantly increases the effective throat depth, the fillet leg size then can be reduced without sacrificing weld strength (see example in Figure 7).

Deeper weld penetration does not produce a fillet weld with more weld strength. Rather, it allows a smaller fillet weld to be made with the same strength level as a larger fillet weld made with less weld penetration. Smaller fillet welds decrease the amount of weld metal needed, and may even allow for increased travel speeds. This benefit could be potentially realized by using the submerged arc welding (SAW) process, known for its deep-penetration capabilities.

Other arc welding processes can be capable of achieving deep penetration as well. However, the fabrication shop must be capable of producing the deeper penetration level on a consistent basis, so this concept may not always be applicable. Click here to link to a Welding Innovations article on the James F. Lincoln Foundation website, which discusses this topic in more detail.

There are also situations in which deeper weld penetration can be detrimental. Here are three examples.

Limitation: Deep penetration can be troublesome when burn-through is a concern. When welding on thin material, such as gauge thickness sheet metal, too much penetration can cause the weld to burn all the way through the joint and fall out the bottom. In other cases, a thin root pass is made in an open root joint (e.g., pipe joint). If the second pass has too much penetration, burning through the root pass can be an issue.

Limitation: If penetration is too deep, centerline cracking (a form of hot cracking) may become an issue. See Figure 8 for an example of a centerline crack in a fillet weld. A balance must be maintained between the depth of penetration and the width of the root pass. The depth to width ratio (W/D ratio) should not exceed 1 to 1.2. This keeps the shape of the weld fairly uniform. As the weld metal solidifies, the shrinkage stresses are thus fairly uniform in all directions.

However, if the weld is significantly deeper than it is wide, then the shrinkage stresses are unequal and the weld will crack in the center of the bead as a result.

Limitation: Too much admixture with the base plate may also be a problem with deep-penetration welds. As penetration increases, so does the volume of base plate that is melted and combined with the filler metal in the resulting weld puddle. This can possibly add additional elements into the weld puddle that makes the weld more crack-sensitive.

Examples of this would include welding on free machining grades of steel with higher levels of sulfur, phosphorus and/or lead. These softer elements have lower melting (and solidification) temperatures than steel. So in the liquid weld puddle, they tend to migrate to the center of the weld where they are the last elements to solidify. This high concentration of softer elements in the center of the weld bead often leads to centerline cracking from the solidification shrinkage stresses of the weld.

Additionally, in the case of hardfacing or overlay applications, deeper penetration may dilute the weld-deposit chemistry and potentially decrease its resulting wear resistance properties. Overlay welds are simply “bead-on-plate” welds. Figure 9 shows a weld overlay with minimum penetration and thus minimum admixture between the weld metal and base plate.

Figure 10 shows a bead-on-plate weld with deeper penetration and, thus, much more admixture between the weld metal and base plate.