Image credit @BOC
INTRODUCTION TO DEFECTS IN WELDING PROCESSES
The capsizing of the Alexander L. Kielland oil rig is one of history’s most significant engineering disasters. The cause of this unfortunate event in March 1980 was a fatigue crack defect in the welding joint, which resulted in the demise of 123 workers. The seemingly minor weld defect in the hydrophone tube caused one of the five supporting columns to collapse, as the hydrophone tube was held within a brace that connected and supported the columns that were arranged in the form of a pentagon.
Welding processes are used in major industries, namely automotive, petrochemical, power generation, manufacturing, and large-scale food production industries. These welds are used to hold together vital parts of various mechanisms, structures, and components. Failure and deformities can occur at multiple stages of the welding process due to reasons like human error, environmental factors, unskilled labour, etc. These issues are sometimes unavoidable.
Hence organizations require a thorough understanding of the nature of welds and are required to analyze, monitor, and test the quality of welds during the welding process, during operation, and after failure of the structure to better understand their requirements and to ensure that industry standards are maintained since conformity to pre-set standards ensure safety and optimal function.
Welding structures can be subject to multiple defects and deformities, which can appear on the exterior or interior of a structure or test subject and are hence classified as exterior defects and interior defects.
Poor welding technique and structural obstructions may cause these dangerous deformities which may lead to the weld displaying poor penetration and a lack of fusion in the welds between the filler material used during the welding process and the edge preparation. The types of welding defects are of an extensive variety, and include the following defects:
Lack of fusion in welds
Blowholes or porosity in the weld caused due to contamination of metal used in welding.
Undercutting or formation of grooves in the toe of the weld.
Cracking in the weld region is considered unacceptable by every code and standard because of the dangers involved.
Slag inclusions are caused due to melting of the flux within or on the surface of the weld zone.
Incomplete penetration is caused when the welding grooves are not thoroughly filled with the weld.
Spattering is caused due to the expulsion and deposition of specks of metal around the weld zones.
Distortion in the size of the weld caused due to uneven weld deposition.
Hot tears are caused due to cracks formed while the weld solidifies.
Mechanical damage is caused due to improper operation of welding equipment.
Misalignment of welds
Excess reinforcement caused to overuse of filler material in the weld zone.
Overlap of the weld in which excess material covers to base material without adhering to it.
Lamellar tearing is caused due to incorrect materials and the orientation of welds.
Whiskers formed due to protrusion of electrode wires.
Blowing through of weld through the material due to excess temperature of the weld.
Also Read, non-destructive testing vs destructive testing
LACK OF FUSION IN WELDS
The scarcity of adherence of the weld to the base metal leads to the formation of a gap inside the weld joint. This defect is called a lack of fusion in the weld and can also be formed between the weld beads which causes the genesis of an adhesive joint in the weld zone.
Lack of fusion is a very dangerous defect, and it can result in the summation of geometrical irregularities in the surface of the material that lowers its fatigue strength by increasing the stress concentration, hence creating deformities.
A lack of fusion in welds can be categorized according to the position of its formation, which include:
Lack of fusion of weld in the side wall
Lack of fusion of weld among runs
Lack of fusion at the weld root
Types of Lack of fusion in welds a) side walls b)inter-run c)root of the weld (Image credits: Jovanović, M., Rihar, G. and Grum, J., 2006. Analysis of ultrasonic indications in lack of fusion occurring in welds.)
Most research and scientific focus have been on crack formation in welds, although lack of fusion in welds has proven to be equally catastrophic. The IIW (International Institute of welding) and ASTM E-390 Vol. 2 have however focused on and set strict criteria for the lack of fusion in the weld, hence setting industry safety and testing standards in its regard.
Lack of fusion can also be classified as per the oxide inclusions. They are classified as:
Lack of fusion due to melted oxide inclusions (Pure lack of fusion): This type of lack of fusion requires Visual Inspection or microscopic methods help study this defect that appears as a fault line on the weld. In case this type of fusion is present in the runs, its detection procedure proves to be even more complex.
Open lack of fusion: Internal stresses are present while the weld is cooling and solidifying, resulting in the separation of stuck faces and the creation of the open style of lack of fusion in the weld.
Lack of fusion due to unmelted oxide inclusions: Here, oxides that are difficult to melt and inclusions that are non-metallic in nature are confined within the weld. This leads to a linear arrangement of oxides along the length of the zone of lack of fusion. In the event of the oxides melting, globular non-metallic inclusions are formed within the zone.
The lack of fusion defect in welds is a structural defect that is planar in nature. This makes the detection process of lack of fusion deformities complex. Ultrasonic testing and Radiographic methods of non-destructive testing and evaluation are the most productive methods for the detection of such defects.
General quality testing and evaluation processes do not manage to gauge the presence of a lack of fusion defects, hence in case the speculation arises of the presence of such defects, Radiographic and Ultrasonic Testing methodologies must be adopted.
As per ASME (American Society of Mechanical Engineers) code B31.3, lack of fusion in welds is applicable as an acceptance criterion for the following:
In normal fluid service: Girth and branch connection welds, Longitudinal grove welds, and fillet welds.
In severe cyclic conditions: Girth and branch connection welds, Longitudinal grove welds, and fillet welds.
In category D fluid service: Girth and branch connection welds, Longitudinal grove welds, and branch connection welds.
CAUSES OF LACK OF FUSION
Lack of fusion defects, like any weld defects, are caused due to a plethora of functional and operational errors which may include:
Contamination of the surface of the metal to be welded with oxides or scaling.
Low arc current applied or lower heat input, depending on the type of welding used.
Higher speed of travel of the welding apparatus than necessary.
Incorrect diameter of the weld probe for the region to be welded.
Large weld pool that travels further than the welding arc.
Incorrect positioning of beads.
Incorrect angle of the positioning of the electrode.
Incorrect alignment of the root.
PREVENTION OF LACK OF FUSION IN WELDS
The errors in performing the welding operation can be easily averted and the lack of fusion in welds can be avoided by following certain remedial or preventative measures, such as:
Voltage and current should be actively adjusted during the welding process to ensure an appropriate amount of heat induced in the metal.
The surface of the workpiece to be welded should be thoroughly cleaned to avoid contamination.
The edges should be aligned correctly, and the welding material should be meticulously placed at the edge.
The beads should be correctly placed at welding points to ensure thorough sealing when the beads melt and to avoid the formation of slots.
Wire feed output in Metal inert gas welding processes can be adjusted or increased to raise the temperature.
In Tungsten inert gas welding, the beads need to be safely placed individually by the operator to avoid a lack of fusion defects.
Welding work should be carried out neatly and thorough training must be conducted before the welding operation.
NON-DESTRUCTIVE TESTING METHODS FOR DETECTION OF LACK OF FUSION IN WELDS
As per the industry standards set by the ASME (American Society of Mechanical Engineers) code B31.3 – Acceptance criteria of welds, non-destructive testing methodologies like Visual Testing, Visual Testing, Radiographic Testing, and Ultrasonic Testing are useful in detecting and analyzing defects and deformities without affecting the operating conditions of the workpiece under study.
The radiographic method of non-destructive testing uses X-rays and gamma rays and is a volumetric method of detection of defects. This method of NDT proves useful for workpieces that display a lack of fusion in welds that are larger in nature and distinct. These lack of fusion defects that are detectable by radiographic testing may also contain large inclusions and voids within the weld zone.
The radiographic method of non-destructive testing lacks sensitivity to detect the lack of fusion defect and is considered to only be capable of detecting voids and inclusions associated with the defect. Research suggests that the radiographic method of non-destructive testing can be used to accurately detect the lack of fusion in welds if the incident angle is the same as the angle of the bevel of the weld on the workpiece.
To make the two angles equal, the angle of disorientation between the defect and the x-ray or gamma-ray beam from the radiographic test apparatus must be lowered.
Simulations can be conducted using radiographic testing software to adjust parameters to reach optimal testing conditions to detect such deformities in the weld.
Ultrasonic testing is the preferred method of non-destructive testing for the detection of a lack of fusion in welds. However, ultrasonic testing cannot be used to detect pure lack of fusion or lack of fusion due to melted oxide inclusions.
Larger-sized defects produce weak indications in ultrasonic testing, which are considered acceptable defects as per code. Ultrasonic testing may produce weak reflections in the following situations:
The presence of multiple smaller defects on the surface passes the defect over to a pure lack of fusion.
The angle of reflection of the ultrasonic waves produced a clash with the lack of fusion defect located on the edge of the V-weld.
While inspecting for lack of fusion in welds, the above factors should be taken into consideration as weak, repeating ultrasonic reflections may be otherwise overlooked or accepted. The incident waves in ultrasonic testing should be made perpendicular to the sticking faces.
In case the defect is accessible from all four directions by the ultrasonic testing apparatus, a direct testing approach is used. A single bounce technique of inspection is used to examine the upper region of the weld, as in this scenario the testing can be carried out only from the cover layer.
In ultrasonic testing, the defect has to be half the wavelength to be detected by the apparatus.
The non-destructive testing methods of penetrant testing can also be utilized to assess the defect in sealing that the lack of fusion may cause in welds.
The presence of a lack of fusion defect can be speculated if there are indications present at the edge of the run. Fillet welds are complex and need to be carefully analyzed for lack of fusion defects. On loading a workpiece that may contain a lack of fusion defect, the operator may risk the failure and splitting open of the weld.
Lack of fusion in the weld can be avoided by adequate knowledge and experience on the operator’s part. The organization also needs to have a thorough understanding of the welding apparatus, properties of filler material, and the workpiece to produce a defect-free weld that is eligible for loading and operation.
Lack of fusion defects of welds produces significant notch formations. The presence of such notches and deformities formed due to the presence of defects like lack of fusion in welds causes a drastic reduction in the operational safety and load-bearing capability of the structure under non-destructive analysis.
Welding processes are highly dependent on the capabilities of the operator, hence conformity to industry safety codes and quality standards is essential to prove that the workpiece or test subject is fit for use.
Accurate and high-quality test results and welds are possible only if the operators, workers, and supervisors are highly trained, skilled, and meticulous in their understanding of the welding process and the systems.