INDUSTRIAL RADIOGRAPHY

Transcription

1 1 BASICS INDUSTRIAL RADIOGRAPHY -Dr.O.Prabhakar, OP-TECH INTRODUCTION From the time Roentgen discovered X-rays and used it to radiograph his rifle, X-rays are being used in the industry to reveal internal flaws in manufactured components. In many industries like thermal and nuclear power generation, aircraft, chemical industries etc., this method plays a key role. THE METHOD The industrial radiographic method is based on the principle of Differential Absorption of Electromagnetic radiation in matter. In addition gamma radiography is extremely portable and does not need electrical power for its operation. LIMITATIONS Compared to other methods the initial costs are high. Large spaces are needed particularly if one is employing powerful isotopes like Cobalt 60. Inspection times are high. It is difficult to reveal cracks particularly tight cracks. Radiation safety and protection are major issues. ELECTROMAGNETIC RADIATION (EM) All the radiant energies like radio waves, heat radiation, visible or light rays, X-rays, gamma rays and cosmic Fig. 1 : BASIC ARRANGEMENT FOR RADIOGRAPHIC INSPECTION Fig. 2 : ELECTROMAGNETIC (EM) SPECTRUM X- or gamma rays are passed through a component and the transmitted rays are recorded on a photographic film, fluoroscopic screen or a detector. The basic arrangement to take a radiograph is shown in figure 1. The most common recording medium employed is the photographic film. The exposed film is further developed, fixed and washed just like an ordinary photo film in a dark room. The developed film called Radiograph is viewed under proper lighting arrangement for interpretation and evaluation. ADVANTAGES A direct view of the internal discontinuities is obtained making it relatively easier to interpret the radiographs. Discontinuities that are volume based like porosity, shrinkage etc. are easily detected. The method is readily accepted by various manufacturers because of the easy interpretation and extensive codes and standards available. rays belong to the same spectrum. The EM spectrum is shown in figure 2. They all can travel in vacuum and possess the same velocity in vacuum. They differ in their wavelength or energy values. One single ray is also known as Photon and does not possess any electric charge or magnetic moment. The straight line propagation of these waves is utilized in Industrial radiography (RT). These rays can also diffract but this is not of interest in RT but used in metallurgy. IONIZING ABILITY X- or gamma rays are not seen by the human eye directly. Fortunately these rays ionize matter, that is it splits them into positive ions and negative charge. There are four major ionization types of interest to RT and they are: 1. Photographic effect 2. Fluorescence effect

2 2 BASICS 3. Electrical conductivity 4. Biological effect Of these four, the first two are used to take radiograph and the third is used to detect and quantify radiation levels. In RT one has to take safety precautions against the biological effects. A doubt one may have is whether the component becomes radioactive after being exposed to X- or gamma rays. Under the conditions RT used in the industry this danger does not exist as most of the interactions between the X- and gamma rays and the matter involve shell electrons and not the nucleus. However care must be taken while exposing the modern digital detector panels to X- or gamma rays. Above a certain kilovoltage they may damage the detector panels. X-RAY GENERATION The variables that one needs to consider while taking a radiograph are: 1. Thickness of the sample. 2. Kilovoltage to be employed. 3. Tube current 4. Time of exposure These four variables are plotted in a graph called exposure charts that are used by radiographers to take an acceptable quality radiograph. These charts are dependent on the film to focus distance, film type used, material being radiographed and the darkness of the film desired. Equations are available for taking into account all these factors and arrive at the final exposure conditions. One should try to employ the lowest kilovoltage for any given job that gives adequate transmitted X-rays for a satisfactory radiograph. Increasing the kilovoltage would increase the penetrating power of the X-rays but the radiograph will be of poor contrast. GAMMA RAY PRODUCTION Fig. 3 : SCHEME OF X-RAY GENERATION WITH AN X-RAY TUBE If an electrical source is used to generate the electromagnetic waves of the required wavelength, then it is known as X-rays. A basic X-ray tube is shown in Fig. 3. The tube is essentially a vacuum tube (diode) with an anode and cathode. Electrons are emitted by a heated filament and they are further accelerated by employing a very high electrical voltage between the cathode and the anode. The accelerated electrons are made to strike a target like tungsten and suddenly decelerated. The kinetic energy of the electrons is converted into heat and X-rays. So the production of X-Rays consists of three steps: 1. Thermionic emission of electrons from a heated filament. 2. Accelerating the electrons by employing a high voltage of the order of 50 to 400 kev. 3. Suddenly decelerating these accelerated electrons by striking them against a target made of high melting point metal. The type of element in the periodic table is determined by the number of electrons or protons in the nucleus. If the number of neutrons in the nucleus is altered the element type is not altered and is known as isotope. Some of these isotopes decay giving out radiation and particles and such isotopes are known as Radioactive isotopes. These radioactive isotopes may occur naturally or may be produced artificially in a nuclear reactor. Some of these radioactive isotopes give out electromagnetic waves that can be used to take radiograph of components. Just to distinguish the source employed to produce the EM waves, the term Gamma Rays is used to denote the EM rays given out by radioactive isotopes. The most commonly used radioactive isotopes are Iridium 192 and Cobalt 60. How the unstable Co- 60 decays producing gamma rays is described in Fig. 4. Fig. 4 : DISINTEGRATION OF COBALT -60.

3 3 Fig. 5 : GAMMA RAY SOURCE CONTAINER The parameters that one needs to consider while selecting a source are: 1. Energy of the gamma rays 2. Half life of the isotope 3. Size of the isotope 4. Cost of the isotope Figure 5 shows a typical manually operated gamma ray source container. Exposure charts for gamma radiography are different from that of X-rays. In this case we need to consider the source strength instead of kilovoltage and tube current. The gamma rays from cobalt 60 have relatively good penetrating ability as the wavelength is smaller. Co 60 can be used to radiograph sections of steel 9 inches thick, or the equivalent. Radiations from iridium 192 have lower energy. Ir-192 emits radiations equivalent to the x-rays emitted by a conventional x-ray tube operating at about 600 kv. The intensity of gamma radiation depends on the strength of the particular source used specifically, on the number of radioactive atoms in the source that disintegrate in one second. This is measured as curies (1 Ci = 3.7 x 10 s -1 ). PHOTOGRAPHIC DENSITY Photographic density is the measure of blackness of the fully developed and fixed radiograph. This determines the viewing facility one needs to interpret and evaluate radiographs. FILM RADIOGRAPHY In film radiography one employs a film to record the information carried by the transmitted X- or gamma rays. Films used in RT consist of a flexible and transparent base coated with a radiation sensitive silver compound. The coating is applied on both sides of the base. Fig. 6 : CHARACTERISTICS OF INDUSTRIAL RADIOGRAPHIC FILM The characteristic curve, sometimes referred to as the H and D curve (after Hurter and Driffield), expresses the relationship between the exposure applied to a photographic material and the resulting photographic density. The characteristic curves of a fast and slow film are shown in Figure 6. The characteristic curve can be used to solve quantitative problems arising in radiography and in the preparation of technique charts. The simple logic we use is that Pairs of exposures having the same ratio will be separated by the same interval on the log relative exposure scale, no matter what their absolute value may be. (Ref: RT in Modern Industry- Kodak) RADIOGRAPHIC CONTRAST Radiographic contrast between two areas of a radiograph is the difference between the photographic densities of those areas. It depends on both subject contrast and film contrast. Subject contrast is the ratio of x-ray or gamma-ray intensities transmitted by two selected portions of a specimen. Subject contrast depends on the nature of the specimen, the energy of the radiation used, and the scattered radiation, but is independent of time, milliamperage or source strength, and distance, and of the film characteristics or film processing. Film contrast refers to the slope (steepness) of the characteristic curve of the film. It depends on the type of film, the film processing, and the density and is independent of subject contrast. GEOMETRIC UNSHARPNESS The area over which the electrons strike the anode determines the size of the focal spot. In order to obtain a good radiograph one would prefer to have as small a focus size as possible. However as the focal spot size decreases the intensity of the EM radiation obtained is also less. The focal spot should be as small as

4 4 BASICS PENETRAMETERS Fig. 7 : GEOMETRIC UNSHARPNESS conditions permit, in order to secure the sharpest possible definition in the radiographic image. The degree of sharpness of any shadow depends on the size of the source of X-rays and on the position of the object between the X-ray source and the film whether nearer to or farther from one or the other. When the source of X-rays is not a point but a small area, the shadows cast are not perfectly sharp (in Fig. 7) because each point in the source of X-rays casts its own shadow of the object, and each of these overlapping shadows is slightly displaced from the others, producing an ill-defined image. From simple geometry one can derive an expression for the geometric unsharpness as: Ug = F ( b/a) INHERENT UNSHARPNESS Even without any geometric unsharpness blurring of the sharp edge may occur due to movement of electrons in the film. This is termed as inherent unsharpness. This depends on the energy of the photon striking the film. Penetrameters are used while taking every radiograph to check whether the radiograph is satisfactory or not. The test piece is commonly referred to as a penetrameter in North America and an Image Quality Indicator (IQl) in Europe. Examples are shown in Figure 8. It contains some small features (holes, wires, etc.), the dimensions of which bear some numerical relation to the thickness of the part being tested. The image of the penetrameter on the radiograph is permanent evidence that the radiographic examination was conducted under proper conditions. A penetrameter is used to indicate the quality of the radiographic technique and not to measure the size of cavity that can be shown. (Ref: RT in Modern Industry- Kodak) DISCONTINUTIES In castings shrinkage, pipes, gas porosity, lack of fusion of the chills, hot tears and core shift are revealed. However thin cracks are difficult to be revealed. Weld defects like gas porosity, lack of penetration, slag inclusions and tungsten inclusions can be revealed. However, laminations in the base plate can not be revealed by RT. When dealing with castings, it may be better to use penetrameters based on finished rather on rough-wall thickness and this way penetrameter sensitivity is not compromised. Individual casting that are more prone to non-systematic flaws (random) require more radiography. Contrary to common misconception, there is no such thing as 100 % radiographic coverage for all castings. To make sure that no coverage problem occurs between the foundryman and the user, it is essential to follow proper and early planning of the radiography. Fig. 8 : PENETRAMETERS Fig. 9 : A TYPICAL ILLUMINATOR TO VIEW RADIOGRAPH

5 5 CODES & STANDARDS Fig. 10: A TYPICAL RADIOGRAPH (Schematic). RADIOGRAPHS Figure 9 shows a typical illuminator to view a radiograph. A typical radiograph of a weld showing Porosity is shown in Fig. 10. Weld bead is thicker than the base metal. So it appears white. Defects like porosity are of low density material and hence appear as dark spots in the radiograph as shown in Fig. 10. The American Society for Testing and Materials has committee on Non-Destructive Tests. This committee has prepared reference materials concerning recommended practices for radiographic testing, and radiographic references for various industrial processes and materials. For example, it has comparison radiographs for steel castings, aluminium and magnesium castings, steel welds and castings for aerospace applications. All the alloys are not represented. Hence a mutually acceptable document between the foundryman and the user may be adopted. As an example, titanium alloy castings can be judged by aluminium and steel reference radiographs. AWS D1.1/D1.1M:2004 code contains the requirements for fabricating and erecting welded steel structures. In this code, section 6 on inspection section contains criteria for the qualifications and responsibilities of inspectors, acceptance criteria for production welds, and standard procedures for performing visual inspection and NDT (Nondestructive testing) including RT.