Comparing Castings and Forgings


Many cell types will grow when attached to a rigid surface but not in suspension, a phenomenon termed ?anchorage dependence”?. Anchorage dependence can be studied by incorporating solid particles of varying size into gels. It has been found that colonies will form on glass fibrils 500 μ in length, but not in the presence of silica fragments smaller than the cells. This shows that the suspending medium is not itself inhibitory, and confirms the requirement for a rigid surface of adequate size.


The state of inhibited cells in suspension culture was examined by dispersing them in a methyl cellulose gel, in vessels lined with agar. In this system aggregation is prevented and the cells may be recovered quantitatively. Normal, as well as transformed, cells increase in size, and a proportion synthetize DNA during the first 24 hours in suspension culture. Growth and DNA synthesis in normal cells then virtually cease, while transformed cells continue to grow into colonies. The stationary normal cells remain competent for further growth for at least a week in suspension. When such cells are allowed to attach to a rigid surface in the presence of colchicine, DNA synthesis occurs and is followed by mitosis. These results indicate that suspended cells are blocked between mitosis and the end of the S phase of the cycle.


Anchorage Classification


To anchor is to hold or resist the movement of an object; anchorage is the gaining of that hold. In orthodontics, terms such as “critical anchorage”, “noncritical anchorage”, or “burning anchorage” are often used to describe the degree of difficulty of space closure. Anchorage may be defined as the amount of movement of the posterior teeth (molars, premolars) to close the extraction space (Fig. 10-1A) in order to achieve selected treatment goals. Therefore, the barrier anchorage needs of an individual treatment plan could vary from absolutely no permitted mesial movement of the molars/premolars (or even distal movement of the molars required) to complete space closure by protraction of the posterior teeth.






When designing large structural components it’s critical to make an informed decision between castings and forgings. The following paper by Rexnord provides an in-depth examination.


Material selection is one of the most crucial decisions made in the design, manufacture, and application of large structural components. Material selection naturally influences the entire performance of the design, and thus it is critical that informed decisions are made during the design stage. Steel castings and steel forgings are two alternatives for large structural components. For many design engineers it is often assumed that a forging is a better product because it is formed or worked during the manufacturing process. It also assumed that castings are inferior because they may contain porosity. Nothing could be further from the truth. Each process has its advantages and disadvantages. It is just as possible to produce an inferior product whether it is a forging or a casting. This paper will present an honest evaluation of castings and mining forgings, so that those in the design community can make an informed choice.


Introduction


This paper will concern itself with the differences between forged and cast steels in heavy sections. Heavy sections will be interpreted to mean parts in excess of 10 tons and a minimum metal section of 200 mm (5”). All steel products, whether they are cast or wrought (forged), start from a batch of molten steel that is allowed to solidify in a mold. The difference is that a wrought product is mechanically worked by processes such as rolling or forging after solidification, while a casting is not.


Melt Shop Practice


The process of steel making is essentially the same for both wrought and cast steels. Liquid steel is principally an alloy of iron and carbon. Other metals such as chromium, nickel, manganese, and molybdenum are added as alloying agents to impart particular properties to the steel. The raw materials used to make steel also contain undesirable elements such as phosphorus and sulfur, which form inclusions in the steel that can never be completely removed from the steel. Thus the quality of both forgings and castings is dependent upon the quality of the molten steel that is poured into the mold.


Since most forge shops purchase their steel ingots, they are dependent upon the steel mill to control the quality of the raw material that is used in their product. This also limits forge shops to supplying the standard alloy grades that the steel mill offers. Conversely, steel foundries have to both make and pour their own steel to produce a casting, and thus have full control of the metal that is used to produce the casting. This also allows the foundry to supply virtually any alloy grade that the customer may want.


Liquid steel has a high affinity for oxygen, and it will form oxide inclusions that can also become trapped in the final product. Molten steel must be handled properly to minimize the formation of re-oxidation products. Once the steel is refined in the melting furnace it is tapped into a ladle, which is a refractory lined vessel made to handle molten steel. Good steel making practice dictates the use of a bottom pouring ladle. The reason for this is that a slag layer is developed on top of the molten steel by use of fluxes. This slag layer is less dense than steel, and thus floats on top while at the same time forming a protective barrier from the atmosphere. This protective barrier is maintained since the steel is poured from the bottom of the ladle. The bottom pouring technique is used for both steel castings and for steel ingots.






One important distinction between wrought and cast steels is the de-oxidation practice that is used. Wrought steels are typically “aluminum killed,” which means that a small amount of aluminum is added during the melting process for the purpose of removing oxygen from the steel. While very effective at removing oxygen, the aluminum forms microscopic aluminum oxide particles, which are abrasive during the CNC machining process. Some steel casting shops de-oxidize with calcium, which also removes the oxygen but produces a softer, more machinable inclusion.


Forging Process


Wrought or forged materials by definition are made from cast ingots, which are then mechanically worked after solidification. Ingot castings are the raw materials from which all wrought products such as forgings, plate, and barstock are produced, and they are nothing more than a casting that is produced by pouring the liquid steel into a reusable metal mold. The cast ingot structure consists of different zones that contain porosity and segregation.


After solidification the ingot is hot forged into the desired shape using a hammer, press, or ring-rolling machine. As the forging is hot worked into shape, the inclusions, porosity, and grains within the steel ingot are forced to flow in the direction the part is being worked. This imparts directionality to the finished part. According to the forging industry, this grain flow makes forgings superior to castings. However, the fact is that although the mechanical properties of a forging are higher in the longitudinal direction (direction of working), they are significantly lower in the transverse direction, or perpendicular to the grain flow. Thus, when using a forging the design engineer needs to evaluate the loading characteristics in both the transverse and longitudinal direction. 


Large forgings are hammered or pressed into rough shapes, which then require extensive machining parts or welding to other components to produce a more complex shape. This adds to the cost of the overall product. Large forgings are limited as to the amount of mechanical working that can be done.


The forging industry typically refers to the term “reduction ratio,” which is the ratio of cross-sectional area before and after forging and is used as a means to specify the quality of the forging. The typical standard for very large forgings is to require a minimum of three reductions. It is recognized by the forging industry that excess hot working can impart too much directionality into the part.


Forgings are subject to process variables and have the same potential for defects as any manufacturing process. For example, a large forging may actually burst or crack internally during forging if not heated properly


Casting Process


Most steel mining castings are produced in expendable sand molds. The mold is produced by forming sand around a pattern, which is a replica of the finished part. Molding sands are mixed with materials that will allow it to hold the desired shape after the pattern is removed. Holes or cavities are created by assembling sand cores in the mold. The pattern equipment also includes the gates and risers which are needed to produce a quality casting. The gating system is designed to allow the metal to flow into the mold in a controlled manner. Risers are reservoirs of molten metal which allow the casting to solidify without shrinkage porosity.


Post solidification processing includes sand removal or shakeout, removal of gates and risers, inspection, weld upgrading, and heat treatment. The main advantage of the casting process is its versatility. Castings are best suited for complex geometries that cannot be easily produced by the forging process.


The principal difference between a casting and a forging is that the final part shape is created when the molten metal solidifies in the mold. Since the sand mold produces the desired finished shape, all that remains is to process the casting through various finishing operations in the foundry. This processing does not alter the directionality of the casting. A steel casting is homogenous. This means that the mechanical properties of a casting are the same regardless of the direction of applied stresses.


It is very important to understand the underlying principles that dictate how a casting solidifies. As steel cools in the mold it naturally changes from a liquid to a solid, resulting in volumetric contraction. Additional feed metal in the form of risers must be supplied to the casting to make up for this loss in volume. There also needs to be a pathway for the additional metal to feed the casting as it solidifies. If a region of a casting is isolated from the riser, a shrinkage cavity will form. In this case it is necessary to add material to allow the molten metal to be properly fed from the molten riser.