Engineering has always recognized the existence of imperfections and working with tolerances. Tolerance is the term that defines the degree of acceptance before imperfection. Thus, any tolerance should only be defined for a certain application, process and material used.
Imperfection is understood to be any difference regarding the project of the equipment or structure. The difference between the nominal values (values prescribed in project) and the actual values (measured in the welded part) describe the impossibility of reaching expectations in a given scenario. This is the measure of imperfection. Imperfections are inevitable in engineering, but not all of them should be treated as unacceptable.
Imperfections will be considered defects, that is, unacceptable, only if certain limits are exceeded, fixed by the project of the equipment with criteria of acceptance according to standarts.
As the welding is a process involving extreme temperatures, imperfections are generated and can be grouped into three categories:
Dimensional Imperfections;
In the weld;
In the workpiece;
Discontinuities.
Imperfections in material properties.
This article will address the imperfections treated as discontinuities, while the others will be dealt with later in the next articles.
Discontinuity is an interruption in the typical physical structure of a material, which abruptly changes its properties. Thus, the simple variation of the properties does not characterize a discontinuity.
Among the discontinuities related to the welding process, one can mention:
Lack of Penetration:
Occurs when the root of the weld bead does not reach the root of the joint to weld the opposite surface in the workpiece. To correct this discontinuity, you can increase the current, decrease the welding speed, or change the joint geometry.
Lack of Fusion:
It occurs with localized lack of fusion, either at the joint edge or at the face of the previously deposited strand. To correct this discontinuity, you can increase the current, decrease the welding speed, change the joint geometry or use some artifice to avoid magnetic blow.
Undercut:
This type of defect reduces the strength of the weld and workpieces. One reason for this defect is excessive current, causing the edges of the joint to melt and drain into the weld; this leaves a drain-like impression along the length of the weld. Another reason is if a poor technique is used that does not deposit enough filler metal along the edges of the weld. A third reason is using an incorrect filler metal, because it will create greater temperature gradients between the center of the weld and the edges. Other causes include too small of an electrode angle, a dampened electrode, excessive arc length, and slow speed. It occurs with a depression, as a notch, at the edge of the weld bead. To correct this discontinuity, you can reduce the current or reduce the welding speed.
Spatter:
It occurs with the projection of molten particles projected out of the weld bead. To correct this discontinuity, one can reduce the current and control the instability in the metal transfer.
Check our article about the causes and consequences of the Spatter in Welding.
Slag Inclusions:
It occurs with the retention of solid materials, metallic or not, within the weld metal.
Among the discontinuities of metallurgical origin, we can mention:
Cracks:
Cracks may appear in the zone affected by the weld (Fused Zone or Heat Affected Zone) due to several factors, such as the contraction of the solidifying metal and the growth of grains, and can be classified as cold cracks, solidification cracks, cracks and reheating cracks.
Porosity:
It occurs with the formation of gas bubbles retained within the melt zone. It can occur internally and also surfacing on the surface. To correct this discontinuity, it is possible to correct the flow of the protection gas and to use gases of better quality (with greater purity in its composition).
References:
Terminologia Usual de Soldagem e Símbolos de Soldagem, Prof. Paulo J. Modenesi, Universidade Federal de Minas Gerais, Departamento de Engenharia Metalúrgica e de Materiais, Belo Horizonte. 2008.
Terminologia da Soldagem: Imperfeições dimensionais, de propriedades e descontinuidades, Prof. Dr. Almir Quites. 2011
Metalurgia da Soldagem – Introdução, Prof. Carlos Enrique Niño, Dr. Eng., Universidade Federal de Santa Catarina
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