Non-destructive testing (NDT) of welds

Non-destructive testing of weld (NDT) is the process of testing, inspecting or evaluating materials, components or assemblies for discontinuities without causing damage. An error in welding process may cause damage of weld metals significantly resulting in loss of strength, durability and failure of structure. To ensure satisfactory performance of a welded structure, the quality of weld must be determined by adequate testing procedures. If a weld is required to withstand severe loading conditions, it is critical to meet its minimum quality standards and hence proof testing becomes essential under conditions that are same or more severe than the field.

NDT of welds is commonly performed to examine weld integrity. Nondestructive testing of welds are essential as destructive testing necessitates replacing it with new but untested weld. Nondestructive tests are performed in the cases when test piece that is tested is too precious to be destroyed or to be re used after testing or test piece is in service. Nondestructive testing of welds detects internal flaws that could cause weld failure.

Types of Non-destructive testing of welds

1. Visual inspection
2. Magnetic particle inspection
3. Dye penetrant inspection
4. Radiography testing
5. Ultrasonic testing

1. Visual inspection:

Visual inspection is one of the most important methods of inspection yet the most underrated and often misused method of inspection. Because of its absence of sophisticated equipment, potential of this method is often underestimated but if conducted correctly, this type of inspection can be extremely effective in maintaining acceptable weld quality and preventing welding problems. Visual inspection is mainly performed to determine many important factors in the weld such as shape, size, structural stress concentration and defects like undercut. All welds that are visually inspected and shall be acceptable if following criteria are satisfied:

1. Crack prohibition: Any crack is unacceptable regardless of its size and location
2. Weld/base metal fusion: Thorough fusion shall exist between adjacent layers of weld metal and between weld metal and base metal.
3. Crater cross section: Crater is the unfilled end of a weld. The full cross section of the weld is not filled and considered as a weld defect. All craters shall be filled to provide the specified weld size, except for the ends of intermittent fillet welds outside of their effective weld length.

• Weld profile: All welds shall be free from cracks, overlaps and unacceptable profile discontinuities.
• Time of inspection: As soon as the completed weld reaches ambient temperature, the visual inspection of weld in all steels should begin. The visual inspection shall be performed not less than 48 hours after the completion of the weld for ASTM A 514, A 517, and A 709 grade 100 W steel.
• Undersized weld:  The size of fillet weld shall not be less than specified nominal size without correction by the following amounts.

In all cases, undersized portion shall not exceed 10% of weld length.

• Undercut: undercutting in welding is when weld reduces the cross-sectional thickness of base metal. The reason for this defect is excessive current, which causes edges of the joint to melt and drain into the weld which leaves drain like impression along the length of weld. This type of defect can also be due to poor technique that does not deposit enough filler material along the edges of weld.

• For the material less than 25 mm thick, undercut shall not exceed 1mm except that a maximum 2 mm is permitted for an accumulated length of 50 mm in any 300 mm. For material equal to or greater than 1 inch thick, undercut shall not exceed 2 mm for any length of weld.
• In primary members, undercut shall be no more than 0.25 mm deep when weld is transverse to tensile stress under any design loading condition. Undercut shall be no more than 1 mm deep for all other cases.
• Porosity: Porosity in a welding happens when a contaminant or gas is puddled into weld puddle. These impure welds are commonly referred to as cavities.
• Complete joint penetration grove welds in a butt joint transverse to the direction of computed tensile stress shall have no visible piping porosity.

For all other grove welds and for fillet welds, the sum of visible piping porosity of diameter 1/32‘’ (1 mm) or greater shall not exceed 3/8 in. (10mm) in any linear inch of weld and shall not exceed ¾ in. (20 mm) in any 12 in. (300 mm) length of weld.

• The frequency of piping porosity in fillet welds shall not exceed one in each 4 in. (100mm) of weld length and maximum diameter shall not exceed 3/32 in. (2.5 mm). Exceptional case is where in fillet welds connecting to stiffeners to web, the sum of diameters of piping porosity shall not exceed 3/8 inches (10 mm) in any linear inch of weld and shall not exceed ¾ inch (20 mm) in any 12 inch. ( 100 mm) of length of weld.
• Complete joint penetration groove welds in butt joints transverse to the direction of computed tensile stress shall have no piping porosity. For all other groove welds, the frequency of piping porosity shall not exceed one in 4 in. of length and the maximum diameter shall not exceed 3/32 in. (2.5 mm).

Advantages of visual inspection:

• Economical
• Low-cost equipment
• No power requirement
• Easy to apply and Quick

Disadvantages of visual inspection:

• This method is only applicable for surface identification of flaws.
• Only large flaws can be detected.
• Possible misinterpretation of flaws.

• Magnetic particle inspection (MPI):

MPI is one of the most widely used method of nondestructive testing which identifies both surface and subsurface defects such as cracking, pores, cold lap etc. The testing involves introducing magnetic field with high flux density into the object and Iron fillings are dusted on the surface to be tested. If the object is defect free, then the magnetic flux lines pass through the object without any leakage shown in figure 4. The flux lines spread out when they pass through non ferro magnetic material such as air. This spreading action may force some of the flux lines out of the material called flux leakage. If the flux leakage is strong enough, the fillings will cluster along edges of a flaw to reveal cracks or other defects (figure 5). The amount of flux leakage at discontinuities depends on flux density of material, size, orientation and proximity to the surface of a discontinuity.

However, It should be noted that magnetic flux will only leak out of the material if the discontinuity is at perpendicular to the flow of field lines and if there is any discontinuity parallel to the lines of magnetic flux, there will be no leakage and therefore no indication is observed. To resolve this, each area is to be examined twice. Second examination needs to be done perpendicular to the first, so that discontinuity in any direction is detected. The examiner must maintain enough overlap of areas of magnetic flux throughout the process so that discontinuities are not missed.

Advantages of magnetic particle method:

• Both surface and subsurface discontinuities are detected.
• This method does not require surface preparation as surface contaminants will not hinder detection of a discontinuity
• Quick test and low cost
• This method is a portable NDT method which is used with battery powered yoke equipment.
• This method is a safe technique as materials are not combustible or hazardous.
• Indications show the size and shape of the discontinuity
• This method of testing is simple to use and requires minimal training.

Disadvantages of magnetic particle method:

• Power requirements are necessary for the examination of large parts.
• Post demagnetization is often necessary.
• Alignment between magnetic flux and indication is necessary.
• Only small section or small parts can be examined at one time.

3.   Dye penetrant inspection: 

Dye penetrant inspection also called liquid penetrant inspection or penetrant testing which is a oldest and simplest method of NDT of welds. It was earlier in use back at 19^th century where test used to be conducted using kerosine and oil mixture. This type of NDT testing involves detecting only surface flaws such as cracks, seams, laps, cold shuts, laminations, through leaks, or lack of fusion.

Dye penetrant method is based on capillary action where low surface tension fluid is applied on to a clean and dry surface under examination and is permitted to remain there for sufficient time (Dwell time – generally 5 to 30 min.) allowing the liquid to penetrate into any defects open at surface. Smaller flaws require longer penetration time. Liquid penetrant used may be a visible or a florescent material. The penetrant is usually a brilliant-colored mobile fluid with high wetting capability. Application of penetrant may be by dipping, spraying or brushing. After adequate penetration time is allowed, excess penetrant which remains on surface is removed. Then a developer (a light-colored powder absorbent) is applied on to the surface. This developer draws out penetrant which had previously seeped into the flaws to form a visible indication, commonly known as bleed-out.

Visible penetrant examination uses a penetrant that can be seen in visible light. The penetrant in this method is usually red so that the indications produce definite contrast with the white background of the developer. Visible penetrant process does not require the use of black light.

Florescent penetrant examination utilizes penetrants that fluoresce when excited by black light. Florescent indications are many times brighter than surroundings when viewed under black light illumination. 

Advantages of dye penetrant testing method:

• Quick test and relatively inexpensive.
• Highly sensitive as very small discontinuities can be detected.
• Rapid inspection of large areas and volumes and suitable for parts with complex shapes.
• Flaws can be visibly seen indicated directly on the surface.
• This method reveals relative size, shape and depth of flaw.
• This method is easy and relatively requires minimal training

Disadvantages of dye penetrant testing method:

• Only surface detection of flaws is possible
• This method is suitable for only nonporous materials
• Precleaning of surface is required (rust, dirt. Paint. Oil and grease must be removed)
• Metal smearing from power wire brushing, shot blasting, or grit blasting must be removed prior to liquid penetrant examination
• Direct access to the surface to be examined is required

4.   Radiographic testing of welds:

In this method, short wavelength electromagnetic radiation such as X-rays produced by X-ray tube or gamma ray produced by radioactive isotope is used as a source for testing. This type of method can detect hidden flaws or discontinuities present in weld such as cracks, porosity, blow holes, slag, flux or oxide inclusions, lack of fusion, incomplete penetration, mismatch, tungsten inclusion etc. The first step in the methods is surface preparation. It is necessary to remove weld ripples or weld surface irregularities so that the image of irregularities should not be confused with discontinuities.

Back scattered radiation (radiation reflected from surfaces behind the film may reduce radiographic contrast and may reduce radiographic quality. Therefore, a 1/8  inch lead sheet is placed behind the film generally furnishes adequate protection against back scattered radiation. Penetrant radiation (X rays or gamma rays) is passed through the solid object and this radiation is made to fall on the photographic film kept behind the solid object (figure 7). This results in deposition of object’s internal structure on the film.

By studying the radiographic data, defects can be easily recognized. Thicker and denser material absorb more radiation and allow less radiation to pass through the specimen. Thinner portion will absorb less radiation and allow more radiation to pass through the specimen. Entire rays will evenly pass through the material if the material is sound without any defect.

Defects in metal reduces its density and hence they transmit radiation much better than sound metal. Therefore, radiographic film appears to be darker at the defected area (figure 8). Inclusion of low density such as slag will appear as dark spots on the film. Inclusions of high density such as tungsten will appear as light areas (figure 9).

Advantages of radiographic testing:

• Radiographic testing is a positive approach for finding defects such as porosity, cracks, inclusions and voids inside the welds.
• Minimum surface preparation is required.
• Both surface and subsurface flaws are detected.
• Can be used for wide range of materials.
• Flaw size and shape can be directly seen and evaluated.
• Permanent record of inspection results.

Disadvantages of radiographic testing:

• Radiographic interpretation requires trained personal since false interpretation of radiographs can be expensive and interfere with productivity.
• This method is usually suited having access to both side of joint.
• Slow and expensive method.
• As X-ray and Gamma ray are invisible to naked eye, they can have serious health and safety implications.

5.   Ultrasonic testing:

Ultrasonic test is one of the common methods of NDT of materials. The biggest advantage of this method is it does not harm human operator.  This method involves using mechanical vibrations in the form of ultrasonic waves for internal flaw detection.  The first step in ultrasonic testing involves removal of all the paint and rust from welding seams. Then, the piezoelectric transducer is connected to a flaw detector. Now the transducer is placed over object under test. For more accurate results, weld seam and the metal surface around weld seam should be treated with “couplant”. This couplant efficiently transmits sound energy from transducer into the object. Now a beam of ultrasonic energy is directed into the object to be tested. This beam travels through object with insignificant loss till it is intercepted and reflected by a discontinuity (figure 10).

Ultrasonic test can be performed using any of the following methods:

1. Pulse echo transmission: In this method, transducer alone emits and receives the ultrasonic energy. This method uses echo signal at an interface (usually at an imperfection) to reflect the waves back to the transducer. The results are shown graphically, with amplitude representing intensity of reflection on Y-axis and distance or time on X-axis.
2. Through transmission: In this method, separate emitter to emit ultrasonic waves and receiver to receive reflected waves is used. Imperfection in the material causes lesser amount of sound waves at the receiver so that the location of flaw is detected.

Ultrasonic testing can also be split again into two main types:

1. Contact ultrasonic testing: This type of ultrasonic testing is typically useful for onsite inspections. This test is only applicable when one side of test specimen is reachable and the parts to be tested are large, irregular in shape or difficult to transport.
2. Immersion ultrasonic testing: This type of ultrasonic testing is lab-based test and mainly applicable for curved components, complex geometries. In this method, the material under test is immersed in water which acts as a couplant. Immersion Ultrasonic testing utilises pulse-eco method and robotic probe trajectories can be used to inspect complex surfaces which are hard to cover with contact probes. Immersion UT is used for detecting wide range of wall thickness and material types.

Ultrasonic testing for welds makes use of contact pulse reflection technique. In this method, transducer used converts electrical energy to mechanical energy. The transducer is excited by high frequency voltage which causes crystal to vibrate mechanically. The crystal probe becomes source of ultrasonic vibration. These vibrations are transmitted through the test piece passing through couplant.

As the pulse of ultrasonic waves strikes discontinuity in the test piece, it reflects back to the transducer which now serves as receiver. The initial signal, returned echoes from the discontinuity and echo of rear surface of test piece are all traced on the screen (Figure 12). This method of NDT has ability to determine exact position of discontinuity in a weld.

Advantages of ultrasonic testing:

• High sensitivity which allows detection of very small flaws.
• This test is useful for thicker sections and when only one side of an object is accessible.
• This test can detect finer lines or plainer defects which cannot be detected in radiographic testing.
• Estimates size, shape and orientation and nature of defects.
• Non hazardous to the human operator.
• Portable operations are possible.
• Immediate results are obtained.

Disadvantages of ultrasonic testing:

• Requires trained and experienced technicians for data interpretations
• Some times this method gives false positive results, also known as spurious signals. This may result in tolerance anomalies.
• Surface preparations such as removal of loose paint is necessary. But clean properly bonded paint can be left in place.
• Couplants are necessary.