Full Description
Introduction - Chapter 9
It has been well known in the field that industrial radiographers are called upon to interpret radiographic images of components, subassemblies and structures either at the time of manufacture or during service. This may include a variety of specialized welding techniques. Consequently, radiographers become not only associated with welding techniques but also benefit by the association, as the knowledge gained will assist them in the interpretation of weld radiographs.
It can be appreciated that a wide variety of specialized welding processes is being used in industry. Techniques and metallurgical details are numerous; however, an attempt will be made to briefly summarize the essentials of some of the processes that are commonly encountered. Nonetheless, the industrial radiographer is encouraged to refer to some of the publications of the professional societies whose names appear in par. 9.9. It is hoped, however, that the information covered in this chapter will enable industrial radiographers to become aware of the problems associated with the welding process and its effect on interpretation of weld radiographs.
A "weld" is considered to be an intimate and homogeneous union between pieces of metal at interfaces rendered pasty or molten by heat or pressure or both. A filler metal whose chemical composition and melting temperature are similar to that of the parent metal may or may not be required depending upon the type of process used.
"Welding" is a metal-joining process involving several metallurgical principles. Welded joints are expected to be as strong as the parent metal. Modern welding processes sometimes produce joints which are stonger than the parent material.
The term "welding technique" is generally used to describe the preparation of metal joints and the control of factors such as heat, pressure, speed of deposition and other variables which directly influence the strength of the joint.
The term "weldment" describes a unit or a structure formed by welding together an assembly of pieces.
Introduction - Chapter 10
Several metal-shaping processes are in use in modern manufacturing industries. Industrial radiographers may be called upon to examine parts either during the manufacturing process, or at the assembly stage, as well as when the parts are used in service.
Some aspects of the "casting" and "forging" processes that are necessary for the routine functions of industrial radiographers are discussed here. Other metal-shaping processes to name a few more recent ones, are extrusion, drawing, rolling, forming (hot or cold), explosive forming, high-energy rate forming, and sintered metal (powder metallurgy) techniques. As can be appreciated, the details are far too many to include in this manual.
Radiographers who become associated with these processes during the course of their work will be able to find the process details in the Tool and Manufacturing Engineers Handbook published by the Society of Manufacturing Engineers.
A "Casting" is a metal object shaped by pouring molten metal into a die or a mould having the desired shape. The term "casting" is used not only to describe the end-product, but also very loosely al of the "processes" that contribute to achieve the end result.
A "Forging" is a metal object shaped by hammering or pressing the metal in a semi-soft condition, in a pair of dies having the desired shape. The term "forging" is also used to describe the "processes" of achieving the end product.
Generally speaking, the radiography of "castings" is carried out on a much wider scale than the radiography of "forgings". This is due to the fact that there are several different casting processes, and that there are innumerable applications where castings are employed compared to the number of forgings, as well the fact that the cast material does not have the machanical strength of fibrous flow of the forged part.
Radiography can be used for inspecting castings of any of the common metals including cast iron, steel, aluminum, magnesium, copper, zinc and their alloys. Practically all sizes and shapes of castings can be radiographed. However, there are limitations as to the section thickness that can be inspected with the sources of radiation that are available in a particular industrial radiographic facility. Additionally, the geometric construction of the part should be such as to make it accessible to the radiation source and that a film can be placed against it on the side remote from the source. The thickness of the most complex shaped light alloy casting generally does not restrict its inspection by radiography. Most aluminum and magnesium casting can be satisfactorily X-rayed with commonly available X-ray equipment. X-ray equipment in the 140kV (peak) range seems to be most widely applicable. For steel castings, 220, 400 and 1000kV (peak) units are most suitable and will permit inspection of steel thicknesses from about 50 to 125 mm (2 to 5 inches).
Certain radioactive isotopes or high-energy X-ray equipment, suck as linear accelerators and betatrons, permit inspection of heavier metal sections, in some instances a maximum of up to 450 mm (18.0 inches) of steel, or the equivalent thickness of other materials of different density. Thus, Cobalt 60 may be used for the inspection of steel up to approximately 200 mm (8.0 inches) in thickness.
There are two ways in which radiography serves the casting industry. First, it can be used in the development work at the initial stages of casting design. It can also be used to improve the foundry techniques. Prototype castings can be examined in order to determine the position of runners, risers, gates, chills, and to study and control shrinkage, cooling rates, segregation of impurities, and porosity. Secondly, radiography can be employed as an on-going inspection method to control the quality and to meet specifications in a production environment. By revealing subsurface discontinuities, it can detect unacceptable castings before costly machining operations are performed. Also, it can detect discontinuities that would not be uncovered by machining which could ultimately cause catastrophic service failure. The amount of radiographic inspection required depends upon the design and end use of the casting. Castings that are subjected to relatively low stresses in service may require radiographic inspection only during the development of suitable foundry technique. On the other hand, castings that will be highly stressed in service and whose design does not permit a large safety factor, may require 100% inspection. On large castings, only certain critical areas, determined by past experience with such castings, may need to be radiographed. The amount of inspection to be applied is usually specified in the contract or specifications.
The quality standard required will vary with the application and service conditions and different acceptance standards may apply to different parts of the same casting.
Introduction - Chapter 11
The radiography of aircraft structures requires that the radiographer have general knowledge of the structure of an aircraft, the principles of flight, the terms used to describe components and their function, and the radiographic procedures and techniques unique to aircraft construction and maintenance. This chapter will attempt to provide this general knowledge without going into detail on the myriad differences between present and past aircraft, civilian and military aircraft or problems associated with aerospace programs. The radiographer must be prepared to initiate new radiographic procedures or modify present ones in order to provide proper quality control for all types of aircraft with which he/she may be associate. Before reading further, it is recommended that you the reader become familiar with the terms in the glossary. See par. 11.12.
Radiography of critical aircraft structural members and component parts has proven to be an effective and economical inspection method when properly applied in an aircraft maintenance program. In the interest of public safety, routine radiography of critical aircraft structural members is carried out at most busy airports. The primary advantage of radiography is that it facilitates the inspection of inaccessible structures or closed assemblies without the need for time consuming, and therefore expensive, disassembly. As well, disassembly of structural members can be detrimental to the continued airworthiness through enlargement of fastener holes and scratches or other induced damage to critical areas. Radiography has the potential to inspect such critical, hard-to-reach areas without added risks.
The application of radiographic inspection to aircraft structures must be approached with care; the part to be inspected may not be readily accessible for ideal placement of the film. The radiographic beam may be obstructed by other structural members amd components in its vicinity. The best focal and part-to-film distances may be impossible to attain. The presence of sealants, trapped fuel, wiring harness and similar items will interfere with the radiographic work and hence quality of the radiograph. The effect of these and other obstacles must be carefully evaluated during the development of a radiographic procedure.
Since the defects are contained in the hard-to-reach structural members or components, it may become very difficult to place the penetrameter at the appropriate location, hence the best radiographic sensitivity may be hard to achieve. Often the best available guide to sensitivity is the structure itself. If, for example, it is possible to differentiate between components of different thicknesses, this may be used effectively as a measure of sensitivity. It is important to remember that radiographs of airframes, which normally possess high contrast are, for this reason, very deceptive when evaluating the sensitivity obtained. It is easy to be optimistic in considering what can be successfully accomplished with radiography. Fatigue cracks in large fittings, for example, should never be investigated by means of radiography as they are normally too small to be located by this method.
While determining a radiographic technique for the inspection of a definite aircraft-structure-failure area, a surface indication such as a crack, when confirmed visually, becomes the most efficient penetrameter. Knowledge thus gained may be used as future reference for similar work.
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