Copyright Goodheart-Willcox Co., Inc. Chapter 18 Weld Inspection and Repair 271 of the ability of short wavelength radiations, such as X-rays or gamma rays, to penetrate material that is opaque to ordinary light. X-rays are a form of electromagnetic radiation that penetrates most materials. An X-ray test is similar to a photograph. A machine in a fi xed location trans- mits X-rays through the material being tested. A fi lm or sensor on the other side of the material is exposed by the X-rays that pass through the test material. Any defects or inconsistencies in the metal change the amount of X-rays that are able to pass through. Because more or fewer X-rays pass through those locations, they look different in the developed fi lm, or display. The area surrounding the X-ray machine may be lead-shielded to prevent the escape of radioac- tivity. An X-ray testing station usually includes all of the support equipment, such as fi lm-developing machines. The end result is a radiograph made in a minimum amount of time. Recently, more portable X-ray equipment has been developed and is becoming very common in the pipeline industry. This equip- ment consists of a small machine that is sent down the center of the pipe to X-ray each weld. Gamma rays are electromagnetic waves that are similar to X-rays, but with a shorter wavelength. Gamma rays are produced from radioactive mate- rials such as cobalt, cesium, iridium, and radium. These radioactive materials must be contained in a lead-shielded box and transported to the job site for in-place radiographs. The fi lm that is exposed by these rays is called a radiograph. Film is placed on one side of the weld, and the radiation source is placed on the other side of the weld. The radiation passes through the test material and exposes the fi lm, revealing any inconsistencies in the weld. Different types of radiation sources are more or less powerful. The thickness of the material usually determines the type of radiation source used for the test. A radiograph inspection of a fusion weld is shown in Figure 18-10. The fi lm is developed for viewing on a special viewer. The radiograph must be compared by a skilled technician to a specifi cation that defi nes discontinuities. The fi lm must have sharp contrast for proper defi nition of the weld and identifi cation of any defects. (Contrast is the degree of blackness of the darker areas compared with the degree of lightness of the brighter areas.) To ensure sharp images on the fi lm, image quality indicators (IQI), also called penetrameters, are used to indicate the quality of the radiograph. See Figure 18-11. A hole-type penetrameter consists of a thin shim of the base metal, usually with a thick- ness equal to 2% of the weld thickness. One, two, or more holes with various diameters are drilled into the metal shim. The shim is laid next to the weld before it is X-rayed. The ability of the radiograph to show defi nite-sized holes in the penetrameter establishes the radiograph quality. The resolution of the X-ray is indicated by the smallest hole that is visible. Another type of IQI is a wire type. A wire IQI is a series of wires embedded in plastic. The wires have decreasing diameters. The quality of the radiograph is determined by the thinnest diameter wire that can be seen on the image. Radiographs are expensive however, they provide a permanent record of the weld quality. It is often useful to compare a radiograph created when a weld was fi rst made with later radiographs made after the Gamma Ray Film Film Intensifying screen Lead X-ray Figure 18-10. Radiographic test operation. IQI Discontinuities Figure 18-11. This radiograph used an IQI to monitor clarity in the development process. Note the weld discontinuities below the IQI. (shinobi/Shutterstock.com)