Last Updated on September 2021
The fundamental reason why welds are subjected to quality testing is to establish whether their quality correctly matches the function for which it is intended.
For proper evaluation of the quality of a weld, it is important to have the necessary measurement standards against which you can compare and contrast its features. For accuracy purposes, this process should be handled by a qualified individual who is well acquainted with the welding field and procedures involved therein.
It does not always mean that a weld is terrible just because it does not look good at face value. To test a weld with certainty, you will need to employ various tests broadly classified as destructive and nondestructive tests. We shall evaluate these in more detail below.
The Importance of Weld Testing
Testing the welding work done is vital in providing assurance that products are fit for their intended purpose.
When you ensure that the weld measures up to the required set standard, you facilitate the smooth running of assets with minimal to no errors, saving time and extra costs.
Without proper testing, you can experience welding errors that cause significant damage to weld metals leading to weak welds and structural failures of unimaginable loss.
Below are some of the basic requirements that define a good quality weld product:
- When there are no holes or cracks found in the bead.
- When the appearance of the welding work aligns with the required exterior form.
- When the product finishing is accurately aligned with the required design dimensions with no distortion.
- When the product aligns with the required strength and functionality.
- When the bead has uniform waves, width, and height.
- When the welding joint is tight as loose joints are a sign of a weld problem.
Weld Testing Methods
Weld Testing methods are broadly classified into two; destructive and nondestructive testing (NDT) testing.
This can also be referred to as Mechanical testing. It involves the physical destruction of a completed weld to reveal and evaluate the material properties under static or dynamic force. It aims to ensure that materials are appropriate for their intended applications,
Destructive weld testing includes bend tests, tensile tests, hardness tests, impact tests, and material analysis. These testing methods are most fruitfully used when testing products that are not going to be mass-produced, as it goes against economic interests to destroy a large number of specimens.
These tests are used on applications such as sampling inspection of production welds, failure analysis work, welding procedure qualification, and welder performance qualification testing and research inspection. Destructive welding tests evaluate weld performance and integrity by deconstructing the welded element and assessing various physical and mechanical properties. It is vital to measure the physical properties and components of base metals for a better understanding of a material’s performance under certain conditions.
Destructive testing methods are generally easier to perform. Their result is easily interpreted and offers more evaluation information compared to nondestructive testing.
Nondestructive weld testing is the opposite of destructive as it involves evaluating the structural quality of components without causing any damage to them. It is the process of inspecting, testing components/material for discontinuities or property differences without destroying the usefulness of the part. For this reason (among others), most engineers utilize nondestructive testing methods, such as electrical, mechanical, forensic, and petroleum engineering.
One of the main advantages of nondestructive testing compared to its destructive counterpart is that it saves a lot of time and money dedicated to product evaluation and research.
Guided Weld Bend Test
Guided bend tests determine the quality of the weld metal at the face and root of the welded joint, together with the degree of penetration and fusion to the base metal. It also shows the efficiency of the weld.
This type of weld testing is made in a jig where test specimens are machined from welded plates, whose thickness must align with the capacity of the bending jig. The test specimen is positioned across the lower portion of the jig, also known as the supports of the die. Use a hydraulic jack to operate the plunger from above, therefore, forcing the specimen into the shape of the dice.
The top and bottom surfaces of a welded plate are labeled as the face and root surfaces, respectively. These tests requirements mandate that the specimens must bend 180 degrees and have no cracks greater than 1/8 in. (3.2 mm) in any dimension be seen on the surface. The face bend tests are made by placing the face of the weld on the outside of the bend or in tension. The root bend tests are made by placing the root of the weld in on outside/convex of the bend or in tension.
In the case of bend testing thick plates, cut transverse slices from the welded joint and bend so that one of the cut sides surfaces becomes the side bends on the bend specimen’s convex side. The transverse bend tests effectively evaluate and qualify welders because of their precision in revealing defects that are otherwise missed in tension tests. The defects are primarily inadequate or lack penetration or root fusion. Other bend tests are useful in evaluating the flexibility and safety of welded joints.
Weld Hardness Test
Hardness testing of welds is usually performed on the polished, ground, or polished and etched cross-section of the joint space.
The weld hardness test is useful in providing information about the metallurgical changes caused by welding. For instance, in construction steels, rapid cooling from high HAZ temperature may be the reason why martensite of much higher hardness than the base metal is formed.
The process is usually sensitive to welding conditions such as the heat input, interpass temperature, electrode compositions, methodology used, interpass temp, and plate thickness. The type of hardness test employed is dependent mainly on the size of the welded joints, the strength of the material, and the desired information output.
Indentations for these tests are made in accordance with the area of interest. These could be the base metal, the face or root regions of the deposit, the weld centerline, or even the HAZ.
- The Rockwell test is best suited for hardness traverses and produces more minor indentations.
- The Brinell test is best suited for large welds. The large indentations it produces range 2 to 5.6 mm in diameter.
- The Vickers and Knoop tests are best suited for hardness measurements of the different sections of the HAZ and produce relatively small indentations.
Tensile Strength Test
The tensile strength weld testing is typically used to measure the strength of a welded joint. Before testing, you have to take the test specimen, measure its width thickness, and calculate the area in square inches.
The tensile physical weld testing sample is then mounted in a machine to yank it with enough force that it can break the specimen. You may use either stationary or portable testing. The mobile testing machine has the double capability of pulling as well as bending test specimens when it operates on the hydraulic principle.
The primary areas of focus when it comes to tension tests are strength, elongation, and reduction. The tension testing of welds is relatively more demanding compared to base metal because of the weld test section, which is heterogeneous, and composed of the deposited weld metal, the HAZ, and the unaffected base metal.
Depending on how the welded joint is loaded, the tensile test specimen can be either transverse or longitudinal. The common methods include
- Testing one-dimensional tensile stretching of welds prepared from the welding zone.
- Create an accurate model simulating the welded compound and then apply the actual load.
- As we continue to advance, different simulation models have been used with CAD / CAE systems to solve such tasks then compared with real experiments. Sometimes, the input data is set by the experiment to be an adequate simulation test.
Typically, the strength of weld joints is equal to that of the base materials, except for when using special base metals. The overall welding process and strength of the material used determine the strength quality of a weld joint and its joint efficiency.
This is the most familiar nondestructive testing technique used. It is the cheapest yet remain extremely effective inspection method that can be employed throughout the production process of a welding product. It is a very effective quality control tool because it ensures that errors are highlighted and corrected early enough, therefore, conforming to procedures and overall quality output.
Visual inspection techniques are classified into three categories below:
- Before welding visual inspection: prepping tools such as dimensions, edge preparation, drawings, material specifications, cleanliness of the welding joint, working area, and conditions.
- During welding visual inspection: operating conditions, preheat needs, welding process, electrode selection, welder performance, weld sequence, and size.
- After welding visual inspection: on the finished product checking on defects (such as cracks, porosity, poor penetration, undercut and underfill), warpage, spatter base metal defects, and weld size by use of a weld gauge.
This weld testing method uses radiography to reveal the presence and nature of internal defects in a weld. It can detect problems such as porosity or cracks, slag, blowholes, sections with poor fusion. Also, sections where thickness has been altered by porosity or cracks, will show a darker appearance with dark outlines on the film. These discontinuities are spotted by observing the shape and variations in the density of the processed film.
First, you place an X-ray tube on one side of the welded plate and then an X-ray film (with a special sensitive emulsion) on the other side. When developed, the defects in the metal manifest as dark spots and bands. An added advantage is that the film record of defects produced is permanent and easy to interpret.
Its disadvantage is that it is a slow and expensive technique of nondestructive examination.
Dye Penetrant Testing
The liquid-penetrant examination is a highly sensitive, nondestructive technique that is applicable to many material types, including ferrous and nonferrous metals, glass, and plastics. It is an ideal method for detecting even the tiniest defects such as cracks, pores, and porosity, which are open on the surface of the welded product being examined.
First, clean the surface. Then apply a liquid penetrant on the surface and let it seep into any opening. Clean the surfaces after sufficient penetration has been achieved and remove any excess penetrant.
The indication is shown clearly and effectively because of the red penetrant against a white background.
To ensure welded structures perform as desired consistently, there is a need to invest in a proper weld quality evaluation and testing system.