The Casting ProcessCasting is the process of causing liquid metal to fill a cavity and solidify into a useful shape. It is a basic method of producing shapes. With the exception of a very small volume of a few metals produced by electrolytic or pure chemical methods, all material used in metal manufacturing is cast at some stage in its processing. Castings of all kinds of metals, in sizes from a fraction of an ounce up to many tons, we used directly with or without further shape processing for many items of manufacture. Even those materials considered to be wrought start out as cast ingots before deformation work in the solid state puts them in their final condition. A vast majority of castings, from a tonnage standpoint, are made from cast iron. A relatively small number of these are subjected to NDT. In most cases they are designed for non-critical applications with principally compressive loading and oversize dimensions to eliminate the problem effect of the innumerable discontinuities inherent in the material. However, some of these castings and many others made of different material may be used in such a way that careful inspection is essential for satisfactory service. Penetrant testing may be in order for surface examination. Radiography or ultrasonic testing may be needed to detect internal defects regardless of the material or type of casting. Ultrasonic methods are difficult to use with some castings because of noise created by grain structure. The rough surfaces of many castings also can produce problems in transducer coupling, but ultrasonic testing is used extensively in the examination of critical coolant passages in turbine engine blades to measure thickness. Eddy current and penetrant methods are also used to detect leading and trailing edge cracks before and during service of turbine blades.
80 Materials and Processes for NDT Technology
THE PROCESS The Process Starts with a Pattern. The casting, or founding, process consists of a series of sequential steps performed in a definite order, as shown in Figure 8-1. First, a pattern to represent the finished product must be chosen or constructed. Patterns can be of a number of different sytles, but are always the shape of the finished part and roughly the same size as the finished part with slightly oversized dimensions to allow for shrinkage and additional allowances on surfaces that are to be machined. In some casting processes, mainly those performed with metal molds, the actual pattern may be only a design consideration with the mold fulfilling the function of a negative of the pattern as all molds do. Examples would be molds for ingots, die casting, and permanent mold castings. Most plastic parts are made in molds of this type, but with plastics, the process is often called molding rather than casting. A Mold Is Constructed from the Pattern. In some casting processes, the second step is to build a mold of material that can be made to flow into close contact with the pattern and that has sufficient strength t o maintain that position. The mold is designed in such a way that it can be opened for removal of the pattern. The pattern may have attachments that make grooves in the mold t o serve as channels for flow of material into the cavity. If not, these channels, or runners, must be cut in the mold material. In either case, an opening to the outside of the mold, called a sprue, must be cut or formed.
PATTERN IN S A N D M O L D
Mold Cavity Filled with Molten Material. Liquic metal is poured through the channels to fill the cavit: completely. After time has been allowed for solidifj cation t o occur, the mold is opened. The product i then ready for removing the excess metal that ha solidified in the runners, cleaning for removal of an: remaining mold material, and inspecting to determin if defects have been permitted by the process. Th casting thus produced is a finished product of th foundry. This product occasionally may be used i~ this form, but more often than not needs furthe processing, such as machining, to improve surfac qualities and dimensions and, therefore, becomes ra.i material for another processing area. Casting Is a Large Industry. The tonnage o u t p ~ of foundries throughout the United States is ver large, consisting of close to 20 million tons (18 mi lion tonnes) per year. Foundries are scattered all ovc the United States but are concentrated primarily i the eastern part of the nation with a secondary cor centration on the west coast in the two areas wher the main manufacturing work is carried on. Foundries Tend to Specialize. Because of diffe: ences in the problems and equipment connected wit casting different materials, most foundries specializ in producing either ferrous or nonferrous casting: Relatively few cast both kinds of materials in appn ciable quantities in the same foundry. A few foundries are large in size, employing sever: thousand men, but the majority are small with fror one t o one hundred employees. Most large foundaric are captive foundries, owned by parent manufactu~ ing companies that use all, or nearly all, of th foundry's output. More of the small foundaries ar independently owned and contract with a number c different manufacturers for the sale of their casting: Some foundries, more often the larger ones, ma produce a product in sufficient demand that thej entire facility will be devoted to the making of tha product with a continuous production-type operl tion. Most, however, operate as job shops tha produce a number of different things at one time an1 a r e continually changing from one product t' another, although the duplication for some parts ma, run into the thousands.
SOLIDIFICATION O F METALSThe casting process involves a change of state o material from liquid to solid with control of shap being established during the change of state. Th problems associated with the process, then, ar primarily those connected with changes of physice state and changes of properties as they may be i n f l ~ enced by temperature variation. The solution t c many casting problems can only be attained with a 1 understanding of the solidification process and th effects of temperature on materials.
COMPLETE CASTING WlTH ATTACHED G A T I N G SYSTEM
M O L D CAVITY WlTH G A T I N G SYSTEM
Figure 8-1 Casting steps for a pulley blank
The Casting Process 81
Energy in the form of heat added to a metal changes ,he force system that ties the atoms together. Eventu~lly, heat is added, the ties that bind the atoms are as proken, and the atoms are free to move about as a lipid. Solidification is a reverse procedure, a s shown in 2igure 8-2, and heat given up by the molten material nust be dissipated. If consideration is being given mly to a pure metal, the freezing point occurs a t a iingle temperature for the entire liquid. As the temperiture goes down, the atoms become less and less nobile and finally assume their position with other itoms in the space lattice of the unit cell, which grows nto a crystal. Ciystal Growth Starts a t the Surface. In the case l a casting, the heat is being given up t o the mold f laterial in contact with the outside of the molten lass. The first portion of the material t o cool to the reezing temperature will be the outside of the liquid, nd a large number of these unit cells may form imultaneously around the interface surface. Each nit cell becomes a point of nucleation for the rowth of a metal crystal, and, as the other atoms 001, they will assume their proper position in the pace lattice and add t o the unit cell. As the crystals Drm, the heat of fusion is released and thereby inreases the amount of heat that must be dissipated efore further freezing can occur. Temperature radients are reduced and the freezing process re~ r d e d .The size of crystal growth will be limited by lterference with other crystals because of the large umber of unit cell nuclei produced at one time with ~ n d o morientation. The first grains to form in the kin of a solidifying casting are likely t o be of a fine quiaxed type with random orientation and shapes.HEAT ADDED AT CONSTANT KATE HEAT REMOVED-2
Second Phase Slower. After formation of the solid slrin, grain growth is likely t o be more orderly, providing the section thickness and mass are large enough t o cause a significant difference in freezing time between the outside shell and the interior metal. Points of nucleation will continue t o form around the outside of the liquid as the temperature is decreased. The rate of decrease, however, continues t o get lower for a number of reasons. The heat of fusion is added. The heat must flow through the already formed solid metal. The mold mass has been heated and has less temperature differential with the metal. The mold may have become dried out t o the point that it acts as an insulating blanket around the casting. Second Phase Also Directional. Crystal growth will have the least interference from other growing crystals in a direction toward the hot zone. The crystals, therefore, grow in a columnar shape toward the center of the heavy sections of the casting. With the temperature gradient being small, growth may occur on the sides of these columns, producing structures known as dendrites (Figure 8-3).This pine-treeshaped first solidification seals off small poclrets of liquid t o freeze later. Evidence of this kind of crystal growth is often difficult t o find when dealing with pure metals but, as will be discussed later, can readily be detected with most alloy metals. Third Phase. As the wall thickness of frozen metal increases, the cooling rate of the remaining liquid decreases even further, and the temperature of the remaining material tends t o equalize. Relatively uniform temperature distribution and slow cooling will permit random nucleation at fewer points than occurs with rapid cooling, and the grains grow t o