• RADIATION

    A combination of the processes of emitting, transmitting and absorbing waves or particles. 

  • RADIO INTERFERENCE

    The high-frequency radiation used to stabilize a-c gas tungsten arc welding may cause telephone, radio and television interference. This problem can be alleviated by using an earth ground to ground the work-piece and the welding power supply case. It is also helpful to keep cables as short as possible and to shield the primary wiring. 

  • RADIOACTIVE

    The property of some elements to emit charged or uncharged particles as alpha or beta rays, and sometimes gamma rays, caused by the disintegration of the nuclei of atoms. See RADIUM and RADIOGRAPHIC EXAMINATION. 

  • RADIOGRAPH

    A “shadow picture” or image produced by passing radiation, such as X-rays, gamma rays, or high-energy neutrons, through an object and recording the variations in the intensity of the emerging radiation on a sensitized film or screen.

    A radiograph shows the gross structure of a metal or weld, such as the presence of blowholes, slag, high- or low-density inclusions, porous spots, cracks, or other defects or abnormalities which could not otherwise be found except by cutting through the material. 

  • RADIOGRAPHIC EXAMINATION

    The use of radiant energy in the form of X-rays, gamma rays, or high-energy neutrons for the nondestructive examination of visually opaque objects which yield a record of their soundness on a sensitized film or screen.

    Radiography is a nondestructive test method based on the principle of preferential radiation transmission, or absorption. Areas of reduced thickness or lower density transmit more, and therefore absorb less radiation. The radiation which passes through a test object will form a contrasting image on a film receiving the radiation.

    Areas of high radiation transmission, or low absorption, appear as dark areas on the developed film. Areas of low radiation transmission, or high absorption, appear as light areas on the developed film. Figure R-1 shows the effect of thickness on film darkness. The thinnest area of the test object produces the darkest area on the film because more radiation is transmitted to the film. The thickest area of the test object produces the lightest area on the film because more radiation is absorbed and thus, less is transmitted. Figure R-2 shows the effect of the material density on film darkness. 

    Of the metals shown in Figure R-2, lead has the highest density: 11.34 g/cm3 (0.409 lb/in3), followed in order by copper: 8.96 g/cm3 (0.323 lb/ in3); steel:  
    7.87 g/cm3 (0.284 lb/in3), and aluminum: 2.70 g/cm3 (0.097flb in3). With the highest density (weight per unit volume), lead absorbs the most radiation, transmits the least radiation, and thus produces the lightest film.

    Lower energy, non-particulate radiation is in the form of either gamma radiation or X-rays. Gamma rays are the result of the decay of radioactive materials; common radioactive sources include Iridium 192, Cesium 137, and Cobalt 60. These sources are constantly emitting radiation and must be kept in a shielded storage container, referred to as a “gamma camera” when not in use. These containers are often shielded with lead or steel. 

    X-rays are man-made; they are produced when electrons, traveling at high speed, collide with matter. The conversion of electrical energy to X-radiation is achieved in an evacuated (vacuum) tube. A low current is passed through an incandescent filament to produce electrons. Application of a high potential (voltage) between the filament and a metal target accelerates electrons across this voltage differential. The action of an electron stream striking the target produces X-rays. Radiation is produced only while voltage is applied to the X-ray tube. Whether using gamma or X-ray sources, the test object is not radioactive following the test.

    The following are essential elements of radiographic testing:

    (1) A source of penetrating radiation, such as an X-ray machine or a radioactive isotope

    (2) The object to be radiographed, such as a weldment

    (3) A recording or viewing device, usually photographic (X-ray) film enclosed in a light-proof holder

    (4)A qualified radiographer, trained to produce a satisfactory exposure

    (5) A means to process exposed film or operate other recording media

    (6) A person skilled in the interpretation of radiographs

    When a test object or welded joint is exposed to penetrating radiation, some of the radiation will be absorbed, some scattered, and some transmitted through the metal to a recording medium. The variations in amount of radiation transmitted through the weld depend on the following:

    (1) The relative densities of the metal and any inclusions

    (2) The relative thickness of materials in the radiation path

    (3) The penetrating power of the radiation source. Nonmetallic inclusions, pores, aligned cracks, and other discontinuities result in more or less radiation reaching the recording or viewing medium. The variations in transmitted radiation produce optically contrasting areas on the recording medium. The most important factor of any nondestructive weld test method is the ability of the inspector to correctly interpret the meanings of the 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.

    Porosity- Porosity usually (but not always) appears as small, black circular spots. A certain amount of porosity is allowable in a weld; how much is allowable is determined by comparing the radiograph with standard radiographs of acceptable welds.

    Inclusions- Entrapped slag is readily distinguished from porosity because of its large and irregularly shaped shadows. Slag often extends parallel to a side-wall of the joint, and is easily and quickly identified. Only a very limited amount of entrapped slag is permissible in acceptable welded structures.

    Slag inclusions show as dark areas in ferrous materials, but may appear as comparatively light streaks in lighter weight metals. The dark areas are created because the slag is less dense than the ferrous alloy, but may be of greater density than the lighter weight metal.

    Tungsten inclusions in aluminum welds, produced by improper GTAW techniques, appear as very light areas on the film; the density of tungsten is 19.3g/cm(0.697 lb/in3).

    Cracks- Cracks appear as dark lines in the weld. Shrinkage and stress cracks may be readily distinguished by their appearance. Shrinkage cracks are generally irregular, while stress cracks are regular and well defined

    Lack of Fusion- Lack of fusion is usually easy to recognize, since it has the appearance of a thin line of slag, or a crack, close to the joint wall.

    Equipment

    The equipment required to perform radiographic testing begins with a source of radiation; this source can be either an X-ray machine, which requires electrical input, or a radioactive isotope which produces gamma radiation. The isotopes usually offer increased portability. Either radiation type requires film and a light-tight film holder, and an alphabet of lead letters which are used to identify the test object. Because of the high density of lead and the local increased thick- ness, these letters form light areas on the developed film. Image Quality Indicators (IQI), or penetrameters (“pennys”), are used to verify the resolution sensitivity of the test. These IQIs are usually one of two types: shim or wire. They are both specified as to material type. The shim type will have a specified thickness and included hole sizes, and the wire type will have specified diameters. Sensitivity is verified by the ability to detect a given difference in density due to the penetrameter thickness and hole diameter, or wire diameter.

    Shim penetrameters vary in thickness and hole diameters, depending on the metal thickness being radiographed. Figure R-3 shows the essential features of various penetrameter designs. When the penetrameter thickness is 0.025 in., it will have the designation of #25, for the shim thickness in mils (a #10 is 0.010 in. thick; a #50 is 0.050 in. thick). The hole diameters and positions are specified, and are noted in terms of multipliers of the individual shim thickness. The largest hole in a #25 penny is 0.100 in., and is called the “4T”hole, indicating that it is equal to four times the shim thickness. A “2T” hole (0.050 in.) is equal to two times the shim thickness. The smallest hole between the 4T and 2T hole is referred to as the “IT” hole and is exactly equal to the shim thickness, 0.025 in. These holes are used to verify resolution sensitivity, which is usually specified to be 2% of the weld thickness. However, a l% sensitivity can also be specified, but is more difficult to attain.

    Film processing equipment is required to develop the exposed film and a special film viewer with intense lighting is best for interpretation of the film. Because of the potential dangers of radiation exposure to humans, radiation monitoring equipment is always required.

    The major advantage of this test method is that it can detect subsurface discontinuities in all common engineering materials. A further advantage is that the developed film serves as an excellent permanent record of the test if properly stored away from excessive heat and light.

    Along with these advantages are several disadvantages. One of those is the hazard posed to humans by excessive radiation exposure. Many hours of training in radiation safety are required to assure the safety of both the radiographic test personnel and other personnel in the testing vicinity. For that reason, the testing may be performed only after the test area has been evacuated, which may present scheduling problems. Radiographic testing equipment can also be very expensive, and the training periods required to produce competent operators and interpreters are somewhat lengthy. Interpretation of film should always be done by those currently certified to a minimum Level I1 per the AWS NDE Certification or ASNT’s SNT TC-1A. Another limitation of this test method is the need for access to both sides of the test object (one side for the source and the opposite for the film).

    Another disadvantage of radiographic testing is that it may not detect those flaws which are considered to be more critical (e.g., cracks and incomplete fusion) unless the radiation source is preferentially oriented with respect to the flaw direction. Further, certain test object configurations (e.g., branch or fillet welds) can make both the performance of the testing and interpretation of results more difficult. However, experienced test personnel can obtain radiographs of these more difficult geometries and interpret them with a high degree of accuracy. See also X-RAY TESTING OF WELDS. 

     

     

  • RADIUM (Ra)

    A rare, brilliant white, radioactive metallic element used in luminous materials. Atomic number 88;  atomic weight, 226.05. Melting point, 700°C (1292°F). 

  • RADON

    (Chemical symbol: Rn). A heavy, gaseous element which is given off as the initial product during radium disintegration. Radon is a gas which has a half-life period of 3.85 days. Either radium salts or radon may be used in industrial radiography, however, when radon is used, complicated corrections in exposure time estimates are necessary because of its short half- life. See RADIOGRAPHIC EXAMINATION. 

  • RAIL JOINT WELDING

    Rails are joined in the field by either flash butt welding (a resistance welding process) or by thermite welding. In the shop, flash butt welding is used to weld the standard (1 1.9 m [39 ft]) lengths of rail into 300-ft sections. For additional information, refer to ANSUAWS D15.2, Recommended Practices for the Welding of Rails and Related Rail Components for Use by Rail Vehicles.

    Rail joints are welded for the following reasons: smoother riding qualities, reduced track maintenance, and to eliminate the need for shimming and building up worn rail ends. Welded joints increase the life of ties and reduce the effects of vibration on cars and locomotives. Most American railroad systems are operating on trackage that has continuous welded rail. See FLASH WELDING and THERMITE WELDING.

  • RAIL-END HARDENING

    In railroad tracks, the wearing down, or battering, of rail ends is caused mainly by the cold flow of metal. When trains pass over a rail, the concentrated load applied under the wheels produces at times a stress greater than the elastic limit of the steel in the rail.

    This stress is further increased by the hammer-like blows resulting from any unevenness in the height of the abutting rails, or poor joint or surface maintenance. In addition, the metal at the ends of the rail can flow in two directions, laterally and longitudinally. This causes a much more rapid lowering of the surface of the rail at the very ends.

    In the past, rails were heat treated to raise the elastic limit of the steel in the tread portion of the rail sufficiently to overcome the cold flow effect. The oxy-acetylene process was used. Heat treating was accomplished by heating the end portions of the tread surface of the rails until they were well above the transformation point, then quenching the rail ends. If necessary, a second heat treatment was applied to obtain the required degree of hardness, about 400 on the Brinell hardness scale.

    The advent of rail joint welding almost completely did away with the need for rail-end hardening. 

  • RAILROAD CAR REPAIR

    Information on the repair of railroad cars is contained in ANSUAWS D15.1, latest edition, Railroad Welding Specifcation-Cars and Locomotives. This publication contains material on processes, consumables, base metals, operator and procedure qualification, and design of welded joints. Reference: American Welding Society, 550 N.W. LeJeune Road, Miami, Florida 33126. 

  • RAILWAY EQUIPMENT, Welding

    The primary source of welding information relating to the construction of new railway equipment is the Manual of Standards and Recommended Practices prepared by the Mechanical Division, Association of American Railroads (AAR). This manual includes specifications, standards, and recommended practices adopted by the Mechanical Division. Several sections of the manual relate to welding, and the requirements are similar to those of ANSVAWS D1.l, Structural Welding Code-Steel. This code is frequently referenced for weld procedure and performance qualification. In 1986, the American Welding Society published AWS D15.1, Railroad Welding Specification, which has been endorsed by AAR. 

  • RANDOM INTERMITTENT WELDS

    Intermittent welds on one or both sides of a joint in which the weld increments are made without regard to spacing. 

  • RANDOM SEQUENCE

    A longitudinal sequence in which the weld bead increments are made at random. 

  • RANDOM WOUND

    Spooled or coiled filler metal that has not been wound in distinct layers. See LEVEL WOUND. 

  • REACTANCE

    The property of a device to impede the flow of an alternating current while allowing direct current to flow without opposition. 

  • REACTANCE COIL

    A choke coil. It is used to oppose the flow of high- frequency currents in a circuit. See REACTOR. 

  • REACTION FLUX, Soldering

    A flux composition in which one or more of the ingredients reacts with a base metal upon heating to deposit one or more metals. 

  • REACTION SOLDERING

    A soldering process variation in which a reaction flux is used. 

  • REACTION STRESS

    A stress that cannot exist in a member if the member is isolated as a free body without connection to other parts of the structure. 

  • REACTOR

    A device used in arc welding circuits to minimize irregularities in the flow of the welding current. Reactors are choke coils used in an electrical circuit for protection or for changing the power factor.

    On an arc welding machine, a reactor is an inductive coil of copper wire or strap, surrounded by a laminated iron circuit provided with an air gap. The reactor slows the rate of change of the current, and stores electromagnetic energy. The first feature enables the operator to strike the metal electrode arc more easily, because the tendency of the electrode to freeze to the work is minimized. The second feature gives the arc additional stability, counteracting any influences, such as air drafts or gas formation caused by impurities in metal being welded, which tend to extinguish the shielded metal electrode arc. 

  • REACTOR CONTROL

    As used in a-c welding machines, reactor controls provide for remote adjustment of welding currents. The reactor control consists of a motor-driven gear device that may be applied to crank-adjusting units, or a rheostat at the work station for reactors that are adjusted electrically. Foot-operated remote control units are available which permit a gradual buildup or reduction of the welding current. This type of control device is useful in preventing weld craters. 

  • RECALESCENCE

    The liberation of heat when steel is cooled from a white heat to a dull red heat, at which point it suddenly brightens, then continues to cool to ambient temperature. See METALLURGY. 

  • RECOVERY

    The amount of alloying elements in a weld deposited from the filler metal. For example, the deposit from a bare rod containing 0.50% carbon generally will not contain over 0.05% carbon. In this case, the recovery is 10%.If the electrode is coated, the carbon recovery may rise to 50 or loo%, depending on the coating. Low recovery of an alloying element may be due to the low boiling point of the element, its tendency to join with the slag because of its affinity for oxygen, nitrogen or other gases, or may be due to incorrect welding procedure, such as overheating the base metal. 

  • RECTIFIER

    A device for changing alternating current into direct, or continuous, current. 

  • RECTIFIER WELDING MACHINE

    A rectifier welding machine is used for welding processes or electrodes that require direct current rather than alternating current. It is a machine in which a-c input power is changed to d-c welding power. Alternating current is supplied to the rectifier from the power line through a transformer. The welding current control may be incorporated in the transformer, or may be a separate reactor between the transformer and the rectifier.

    Rectifier welding machines may have either single-phase or three-phase input. While some machines may supply either a-c or d-c output, the most efficient are those designed for d-c welding only. The three-phase welding machine will show the lowest ripple percentage; that is, it will exhibit very smooth arc characteristics. Rectifier welding machines may be divided broadly into two general types, according to volt-amperage curves and application.

    Constant-Current Welding Machines- A constant-current welding machine has characteristically drooping volt amperage curves, producing relatively constant current within a limited change in load voltage. This type of welder is conventionally used with shielded metal arc welding, gas tungsten arc welding, plasma arc welding or air carbon arc cutting. Constant current welding units, when adjusted for full-rated output, should maintain the current within 5% of its rated value, with a variation of 1% above or below normal arc voltage.

    A constant current welding machine is best suited for most manual operations where variations in the arc length are most apt to occur because of the individual technique of the operator. It may also be used, however, in automatic and semi-automatic operations with a variable electrode feed mechanism, and in operations in which an effort is made to maintain a constant arc length by automatic changes in the wire feed speed.

    Constant Potential- Constant-potential power supplies are designed specifically to power the various automatic welding processes which use a continuous wire electrode that is fed at a constant speed. In this type of welding machine, the arc voltage curve approaches a horizontal line and maintains its voltage within 5% of the rated full-load setting, over the range from open circuit to full load.

    The methods of current control on rectifier type welders vary between different equipment manufacturers. Among commercial designs, the means of current control are movable coil transformers, movable core reactors, saturable reactors, magnetic linkage controls, and various solid-state devices.

    The advantages of mechanical current controls are stability and the capacity of duplicating current settings. The principal advantages of electrical controls are convenience of operation and adaptability to automatic welding process control.