Radiograph

Radiograph Interpretation - Welds

In addition to producing high quality radiographs, the radiographer must also be skilled in radiographic interpretation. Interpretation of radiographs takes place in three basic steps:

 (1) detection,

 (2) interpretation, and (3) evaluation. 

All of these steps make use of the radiographer's visual acuity. Visual acuity is the ability to resolve a spatial pattern in an image. The ability of an individual to detect discontinuities in radiography is also affected by the lighting condition in the place of viewing, and the experience level for recognizing various features in the image. The following material was developed to help students develop an understanding of the types of defects found in weldments and how they appear in a radiograph.

Discontinuities

Discontinuities are interruptions in the typical structure of a material. These interruptions may occur in the base metal, weld material or "heat affected" zones. Discontinuities, which do not meet the requirements of the codes or specifications used to invoke and control an inspection, are referred to as defects.

General Welding Discontinuities

The following discontinuities are typical of all types of welding.

Cold lap is a condition where the weld filler metal does not properly fuse with the base metal or the previous weld pass material (interpass cold lap). The arc does not melt the base metal sufficiently and causes the slightly molten puddle to flow into the base material without bonding.

Porosity is the result of gas entrapment in the solidifying metal. Porosity can take many shapes on a radiograph but often appears as dark round or irregular spots or specks appearing singularly, in clusters, or in rows. Sometimes, porosity is elongated and may appear to have a tail. This is the result of gas attempting to escape while the metal is still in a liquid state and is called wormhole porosity. All porosity is a void in the material and it will have a higher radiographic density than the surrounding area. 

Cluster porosity is caused when flux coated electrodes are contaminated with moisture. The moisture turns into a gas when heated and becomes trapped in the weld during the welding process. Cluster porosity appear just like regular porosity in the radiograph but the indications will be grouped close together.

Slag inclusions are nonmetallic solid material entrapped in weld metal or between weld and base metal. In a radiograph, dark, jagged asymmetrical shapes within the weld or along the weld joint areas are indicative of slag inclusions. 

Incomplete penetration (IP) or lack of penetration (LOP) occurs when the weld metal fails to penetrate the joint. It is one of the most objectionable weld discontinuities. Lack of penetration allows a natural stress riser from which a crack may propagate. The appearance on a radiograph is a dark area with well-defined, straight edges that follows the land or root face down the center of the weldment.

What is Computed Radiography

Computed Radiography (CR)

Sometimes called “film replacement technology”, Computed Radiography (CR) uses a flexible phosphor Imaging Plate (IP) to capture digital images instead of conventional photographic film.

CR has several significant adva

ntages:

–Phosphor Imaging Plates are reusable
– Chemical processing and a darkroom are not required
– Plate exposure times are typically less than film
– ScanX View Software offers the tools to optimize image quality
– Digital information is easily stored and shared

CR follows a simple 3-step process:

When the IP is exposed to X-ray or gamma radiation, the phosphor layer in the plate stores the image.
As the IP is read by the scanner, a focused laser releases the stored image in the form of visible light photons.
These photons are collected and amplified by the scanner, converted to a digital signal and then transferred to a computer for processing and display.

What are the advantages?

Film radiography offers little opportunity to vary the technique, other than picking a different film type. CR also offers different imaging plates, but also there are many scanner control parameters that can be adjusted to the required inspection task, optimizing image quality and maximizing productivity.

High definition Computed Radiography

When used with correspondingly high resolution phosphor storage plates, the ScanX Discover HR meets all the stringent requirements of the EN14784, EN17636 and ASTM E2446 standards.

This combination of high resolution imaging plates and the ScanX Discover HR scanner achieves a Basic Spatial Resolution (BSR) of 30 µm across all system classes.

          Nondestructive Testing Equipment for Nuclear Safety

Within the nuclear industry, non-destructive testing (NDT) plays an important role in maintaining the safety and constant inspection of the most critical parts of nuclear power plants at a cost-effective rate. 

Nuclear power plants accounted for approximately 11% of the world’s electricity production; therefore, maintenance of such an important industry can often be associated with high costs to ensure the safe and reliable production of power from these sources.

NDT serves as a basic technique in detection flaws of all sizes, as well as the constant monitoring of material performance and degradation. The following parts of a typical nuclear power plant are monitored by NDT:

• Valves
• Concrete
• Nozzles
• Vessel head welds
• Vessel welds
• Piping components¹

 

The principal NDT techniques used to ensure the reliability and safety of nuclear power plant operations include ultrasonic measurements, eddy current measurements, radiographic testing, acoustic techniques and several others.

Ultrasonic Testing

Of the NDT methods used in nuclear power plants, ultrasonic testing accounts for the most commonly applied as it is primarily used to detect leaking fuel rods and end plug weld control. By sensing the presence of coolant between the cladding and the fuel, ultrasonic methods can accurately measure and determine the characteristics of the cladding in order to provide information to the technician on the complete failure of the system in this area of the plant².

When ultrasonic waves are applied to the material within the nuclear power plant, the cladding will typically decay as a result of the energy radiation present within the environment. Any reduction of the wave propagation indicates the presence of water leaking from beneath the cladding.

For example, through a number of applied pulse voltages, phased array ultrasonic testing is a type of inspection technique that focuses its applied ultrasonic beam to create an image of the material that accurately reflects the size and performance of the material.

The software used to control the dimensions to be focused on in a given image is particularly unique, as it provides information on the direction of the applied wave. The inspected materials that can particularly benefit from this Hitachi tool include materials or welds composed of carbon steel, stainless steel, aluminum, carbon graphite, ceramics and even carbon fiber-reinforced plastic (CFRP).

 

Industrial and Nuclear Radiography

Industrial radiography is a portable way to identify and inspect welds within pipelines, pressurized vessels and important structural component. When prompted, a radiographic camera releases radiation from a radioactive source material that is enclosed within the heavily shielded device. A detector then picks up an image captured by the radiation, which can then be analyzed by trained personnel in order to determine the quality and safety of the weld.

Nuclear radiography, which is similar to that which is employed for industrial purposes, but instead utilizes a neutron radiation source, is capable of conducting a better interaction between the camera and the organic material of interest as compared to the gamma sources used in industrial radiography³.

This property allows for a better imaging capability to be permitted for more dense materials, such as metals. Therefore, the detection of corrosion and other moisture-based damage, such as that which occurs following the damage induced by a leak, typically utilizes neutron radiography as its choice method of detection within the nuclear power plant industry.
Radiography testing within the nuclear power plant industry can also provide important information on the position, depth, amount, dimensions and configuration of the needed reinforcements, damage present within steel parts and thickness measurements.

Nondestructive Testing Types

Nondestructive Testing and its Types:

Nondestructive testing (NDT) is the process of inspecting, testing, or evaluating materials, components or assemblies for discontinuities, or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed the part can still be used.

 In contrast to NDT, other tests are destructive in nature and are therefore done on a limited number of samples ("lot sampling"), rather than on the materials, components or assemblies actually being put into service.

 These destructive tests are often used to determine the physical properties of materials such as impact resistance, ductility, yield, and ultimate tensile strength, fracture toughness and fatigue strength, but discontinuities and differences in material characteristics are more effectively found by NDT.

 Today modern nondestructive tests are used in manufacturing, fabrication and in-service inspections to ensure product integrity and reliability, to control manufacturing processes, lower production costs and to maintain a uniform quality level. During construction, NDT is used to ensure the quality of materials and joining processes during the fabrication and erection phases, and in-service NDT inspections are used to ensure that the products in use continue to have the integrity necessary to ensure their usefulness and the safety of the public. Here a deeper look at some of the most common types of NDT/NDE:

Radiographic Testing 

(RT) is a nondestructive examination (NDE) technique that involves the use of either x-rays or gamma rays to view the internal structure of a component. In the petrochemical industry, RT is often used to inspect machinery, such as pressure vessels and valves, to detect for flaws. RT is also used to inspect weld repairs.

Compared to other NDE techniques, radiography has several advantages. It is highly reproducible, can be used on a variety of materials, and the data gathered can be stored for later analysis. Radiography is an effective tool that requires very little surface preparation. Moreover, many radiographic systems are portable, which allows for use in the field and at elevated positions.

Ultra Sonic Testing

It is a non-destructive testing (NDT) method in which beams of high frequency sound waves that are introduced into the material being tested are used to detect surface and sub-surface flaws. The sound waves travel through the materials with some attenuation of energy and are reflected at interfaces. The reflected beam is detected and analyzed to define the presence and location of flaws. Cracks, laminations, shrinkage, cavities, bursts, flakes, pores, bonding faults and other discontinuities can be easily detected. Inclusions and other in homogeneities in the metal being inspected can also detected by causing partial reflection or scattering of the ultrasonic waves, or by producing some other detectable effect on the ultrasonic waves. Ultra-high frequency sound is introduced into the part being inspected and if the sound hits a material with different acoustic impedance (density and acoustic velocity), some of the sound will reflect back to the sending unit and can be presented on a visual display.

 By knowing the speed of the sound through the part (the acoustic velocity) and the time required for the sound to return to the sending unit, the distance to the reflector (the indication with the different acoustic impedance) can be determined. The most common sound frequencies used in UT are between 1.0 and 10.0 MHz, which are too high to be heard and do not travel through air. The lower frequencies have greater penetrating power but less sensitivity (the ability to "see" small indications), while the higher frequencies don't penetrate as deeply but can detect smaller indications (the ability to "see" small indications), while the higher frequencies don't penetrate as deeply but can detect smaller indications.

Liquid Penetrant Inspection

The basic principle of Liquid Penetrant Inspection (LPI) is that when a liquid with low viscosity – called a penetrant, is applied to the surface of a part, it will penetrate into fissures and open to the surface.

Once the excess penetrant is removed, the penetrant trapped in those voids will creep back out through capillary action, creating an indication.

Penetrant testing can be performed on magnetic and non-magnetic materials, but it doesn’t work well on porous materials. Penetrants may be “visible”, meaning they can be seen in ambient light, or fluorescent, requiring the use of a “black” light.

After applying the penetrant, it sits on the surface for a specified period of time, coined as the “penetrant dwell time”, then the part is carefully cleaned to remove excess penetrant from the surface. A light coating of developer is then applied to the surface and given time to allow the penetrant from any voids or fissures to seep up into the developer, creating a visible indication.

Following the developer dwell time, the part is inspected visually, with the aid of a black light when using fluorescent penetrants. Most developers are fine-grained, white talcum-like powders that provide a color contrast to the penetrant being used.