• X-RAY

    A form of radiant energy derived from the bombardment of a material by electrons in a vacuum at a high voltage. The wave length of these rays is between 10-8 and 10-11 cm. 

  • X-RAY TESTING OF WELDS

    A nondestructive radiographic testing procedure which uses X-rays or gamma rays to penetrate the weldment or brazement to detect and indicate discontinuities. An image is rendered on photographic film, sensitized paper, a fluorescent screen, or an electronic radiation detector.

    Photographic film is normally used to retain a permanent record of the test. The print from the developed film is known as a radiograph, and the science of making and interpreting such photos is called radiography. A radiograph produced by X-rays is called an exograph.

    X-rays most suitable for welding inspection are produced by high-voltage X-ray machines. The wave-lengths of the X-radiation are determined by the voltage applied between elements in the X-ray tube. Higher voltages produce X-rays of shorter wave lengths and increased intensities, resulting in greater penetrating capability. Typical applications of X-ray machines for various thickness of steel are shown in Table X-1. The penetrating ability of the machines may be greater or lesser with other metals, depending on the X-ray absorptionproperties of the particular metal. X-ray absorption properties are generally related to metal density.

    The use of X-ray machines for examination of welds has been largely supplanted by various isotopes that provide a radiation source. Among them are Cobalt-60, Cesium-137, and Iridium-192. The approximate thickness limitations in steel for these radioisotopes are shown in Table X-2.

    The advantages and limitations of the sources of radiation are shown in Table X-3.

    Historical Background

    In 1895 Professor Konrad Roentgen of the University of Wurtzburg, Bavaria. first observed the effects of X-radiation while passing an electric current through a vacuum tube. The Roentgen rays, as they were officially named after the discoverer, quickly became known as X-rays because of their enigmatic origin and qualities.

    The importance of X-rays in the medical field is well known. Industrial X-ray applications lagged considerably behind medical, but by the 1930s, radiography had begun to grow into a powerful metalworking inspection tool. 

    In 1918, steel of 25 mm (1 in.) thickness represented the absolute limit of X-ray penetration. As equipment manufacturers improved the process by raising the voltage across the tube elements, however, increased thickness of metal could be radiographically examined.

    H. H. Lester, a physicist at the Watertown Arsenal, Watertown, Massachusetts, was one of the pioneers in the radiography of metal sections. In 1924, Lester conducted radiographic examinations of castings which were to be installed in the United States’ first 8.3 MPa (1200 psi) steam pressure power plant for the Boston Edison Company, Radiographic inspection of the welded joints of pressure vessels soon followed. In 1930, the United States Navy specified that X-ray tests must be made of the main longitudinal and circumferential joints of welded boilerdrums. Subsequently, the 1931 ASME Boiler Code made X-ray examination of welded seams mandatory for power boiler drums and other pressure vessels designed for severe service conditions. Other code requirements for X-ray testing followed.

    Applications

    X-ray weld testing is particularly well suited to butt joints, where weld and parent metals lie in the same plane. The rays penetrate the metal without damaging it, and the entire weld may be readily inspected.

    Fundamentals

    X-rays are produced in an evacuated tube through the impact of a high-velocity electron stream on a metal plate, or target, at the anode (positive electrode) of the tube. The electrons are “boiled” from the cathode (negative electrode) by means of a heated filament and are accelerated by impressing an extremely high potential (on the order of hundreds of kilovolts) across the tube. X-ray voltages may reach as high as one million volts. The currents however, are extremely low, usually on the order of 6 to 25 milliamperes.

    Since they are much shorter in wave length than visible light, X-rays can penetrate solid objects. They do not, however, penetrate all objects with equal facility, but are absorbed to a degree depending on the thickness and density of the material. Since density is a function of atomic weight, the heavier metals offer the greatest resistance to the passage of X-rays. Lead, a substance with the high atomic weight of 207.20, has a very high degree of X-ray absorption and so is used as shielding against X-rays.

    Like visible light, X-rays will travel in straight lines unless deflected. As a result, the projected image of an object will be accurate in size and shape. When the image is recorded on film, it becomes a “shadow picture” dependent on the thickness and density of each partthrough which the rays travel.

    X-rays darken a photographic film in much the same way as visible light. The less dense regions of a weld offer the least resistance to the passage of X-rays. These portions, consequently, will show darkest when the weld is radiographed. Denser regions, offering greater X-ray resistance, will permit fewer rays to reach the film and will show as areas of comparative whiteness. The process based on this principle permits the quick detection of weld faults. Such welding defects as porosity, slag inclusion, cracks, lack of fusion, gas pockets and blowholes all show up in radiographs as dark areas.

    The most important factor of any nondestructive weld test method is the ability of the inspector to correctly interpret the indications of discovered defects. Only through careful study of many radiographs exhibiting known defects can such ability be gained.

    The common welding faults revealed by radiographs are (in order of frequency): porosity, entrapped slag, cracks and lack of fusion.

    When there are defects and the weld must be chipped out, finding the exact location and depth of the defect will facilitate the task of the welder or gouger. This can be done with double exposure radiation. In this method, exposures are made from two different angles on the same film or on separate films. The distances are measured between the two positions of the radiation source and between each position of the identification markers on the surfaces of the plate. Images of both the marker and the defect are projected on the film. By comparing the known distances and solving similar triangles, the exact location of the fault is readily found. This enables the welder to begin work on the side closest to the defect and remove, then replace, a minimum of weld metal.

    X-ray Diffraction

    X-ray equipment can also be used to investigate the properties of weld metals by creating and examining diffraction patterns. These are produced by localizing a narrow beam of X-rays through a tube, passing the X-rays through pinholes, then through a small, thin sample of the material to be investigated. A film held behind the sample will show a dark central spot surrounded by a collection of rays, rings, and spots. This is called the difraction pattern, and its analysis makes it possible to peer into the molecular structure of matter and visualize the arrangement of the molecules themselves. Diffraction analysis is very important in the steel and alloying industries, where stresses and strains are a vital factor.

    X-ray diffraction patterns can indicate the ductility of the weld metal or parent metal, and also the presence of strained areas. In practice, it is customary to make a number of patterns to determine the condition of various areas of the metal: in the center of the weld, at the edge of the weld near the line of fusion, the edge of the parent metal near the line of fusion, two or more points in the parent metal which have undergone considerable changes in temperature during welding, and finally, a point in the parent metal far enough removed from the weld so that it can safely be assumed to be unaffected by the heat. It should be noted that although only very small specimens are needed for investigation by means of diffraction patterns, considerable care must be exercised in preparing specimens to be sure that the patterns will not show conditions introduced by the method of preparation itself, which were not originally present in the specimens. See RADIOGRAPHIC EXAMINATION. See also RADIOGRAPHY. 

  • XENON

    (Chemical symbol: Xe). A rare, heavy, colorless, inert gas. Xenon is present in the atmosphere to the extent of one part in twenty million by volume. It is used in electronic control components. Atomic number, 54; atomic weight, 131.30.