Importance of Root Fusion

Depth of Penetration Affects Weld Strength

When welds are sized from a design standpoint, the theoretical throat is used to calculate weld strength.  This means that the desired strength of a weld is achieved by attaining fusion at the root and side walls.

Before going on we’ll define fusion and penetration as these terms are not necessarily interchangeable.  AWS defines fusion as “the melting together of filler metal and base material which results in coalescence.”  This basically means that the filler metal and base material become one homogenous piece of metal.  Penetration, which is also called depth of fusion, is defined by AWS as “the distance that fusion extends into the base metal or previous pass from the surface melted during welding.”

So does depth of penetration affect weld strength? See the image below.

root throat leg combined

Figure 1 – Theoretical Throat and Effective Throat

As you can see in the above figure, you can have two welds with equal leg size (fillet size) but differ in amount of penetration.  If a weld size of 5/16” is required to achieve a specific weld strength there needs to be fusion at the root.  If there is penetration beyond the root then the weld strenght does not necessarily increase, but a smaller fillet can be use to achieve the same strength. However, before you go downsizing your fillets you have to make sure that you are consistently achieving the needed penetration.

On the flip side, if we do not achieve fusion at the root then the weld drops in strength dramatically, thus giving rise to potential failures.  See the figures below.

Figure 2 - Complete root fusion

Figure 2 – Complete root fusion

The above figure shows a weld with adequate root fusion. This is what we want to see when we do an etching of the cross section of the fillet weld. If we do not have fusion at the root we are prone to weld failure because the weld does not have the strength it was designed to achieve.

Figure 3 - Lack of fusion at the root

Figure 3 – Lack of fusion at the root

The red, dotted line denotes the weld which achieves fusion into the side walls but does not achieve fusion at the root.  The blue line denotes the new theoretical throat.  This weld will be much weaker than the one in figure 2. Because the weld does not have the proper strength there is a high probability of failure.

It is important to make sure that you always get fusion at the root.  If you are not achieving this you need to adjust your parameters. Having welding procedures that achieve proper penetration is absolutely necessary. Procedures should be qualified by testing, or in some cases can be prequalified.  If using prequalified welding procedures you must still be careful as there are some dangers to doing this.  We recommend you read “Dangers of Using Prequalified Welding Procedures” if you will be going this route.

Source: Fabricators’ and Erectors’ Guide to Welded Steel Construction

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4 thoughts on “Importance of Root Fusion

  1. Thanks for this page, i learn a lot about my job, i am a welder too, keep up the good work, to share to other about welding process and technics.