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As a welding inspector, in order for you to successfully conduct your profession, it is important to understand weld discontinuities. To fully understand weld discontinuities, we must first examine some welding terminology. The term discontinuity is defined as an interruption of the typical structure of a material, such as a lack of homogeneity in its mechanical, metallurgical, or physical characteristics. A discontinuity is not necessarily a defect. A defect on the other hand is defined as a discontinuity, or discontinuities, that by nature or accumulated effect (for example, total crack length) render a part or product unable to meet minimum applicable acceptance standards or specifications. The term “defect” designates rejectability. Because in this article we are examining these phenomena outside the requirements of any specific welding code or standard, and we will not be discussing their limitations in terms of these documents, we will use the term discontinuities.
Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Porosity is caused by gases that are present in the molten weld. These gases may be trapped and form bobbles or gas pockets as the weld solidifies. The main reason for the presence of gases that cause porosity are dirty base material, moisture on joint surface or electrode, insufficient or improper shielding during the welding process, or incorrect welding conditions or techniques. Base material that is contaminated with hydrocarbons such as oil, grease or paint, will be susceptible to porosity during the welding process. Moisture in the form of water or hydrated oxides on base material and/or welding electrodes, or water leaks from poorly maintained equipment cooling systems, can introduce hydrogen into the welding process and cause major porosity problems during welding. The use of shielding gas, which has an inadequate flow rate, is contaminated from its source or within its delivery system, or is prevented from adequately protecting the molten weld metal through its removal by wind or draft, can seriously affect porosity levels.
Porosity is often classified into types based on its shape and distribution within the weld. Such descriptions as uniformly or randomly scattered porosity, cluster porosity, and linear porosity, or similar terms, are used to describe its distribution. Each of these porosity distributions may provide for different levels of acceptance within a welding code or standard. The presence of linear porosity, for instance, will usually have greater restrictions than those applied to scattered porosity, as linear porosity is often associated with fusion problems within the weld. The most practical methods for controlling or eliminating porosity is to use clean base materials, suitably stored and non-contaminated welding consumables, adequately maintained welding equipment, acceptable environmental conditions, and proven welding procedures.
Incomplete Fusion and Incomplete Joint Penetration
Since these terms are sometimes misused, it is important to understand the difference between these two weld discontinuities.
Incomplete fusion is a weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads. This absence of fusion may occur at any location within the weld joint and may be present in fillet welds and/or groove welds. Incomplete fusion may be caused by the inability, during the welding process, to elevate the base material or previously deposited weld metal to its melting temperature. It is often found on one leg of a fillet weld and is caused by incorrect welding angle that allows for an imbalance of heat between both sides of the joint. It may also be caused by failure to remove oxides or other foreign material from the surface of the base material to which the deposited weld metal must fuse.
Incomplete joint penetration is described as a joint root condition in a groove weld in which weld metal does not extend through the joint thickness. It is the failure of filler metal or base metal to completely fill the root of the weld. Some common causes of incomplete joint penetration are related to groove weld design or set up not suitable for the welding conditions. These problems develop in situations where the root face dimensions are too large, the root opening is too small, or the included angle of a v-groove weld is too narrow. All of these joint design characteristics restrict the ability of the weld to penetrate through the joint thickness. To help prevent this discontinuity, care should be taken to ensure the use of correct joint design and joint fit-up in accordance with welding procedure requirements.
The full understanding of these weld discontinuities will assist the welding inspector to identify them and, more importantly, help to prevent them from occurring in production.It is often possible to use the welding inspection function as a preventative tool within the quality system. This offers far more efficiency to the overall quality system than purely using welding inspection as an appraisal technique to sort bad from good.