Forge Ability 4p631b

Forgeability Material selection must be considered carefully in forging manufacture. The ability of a metal to experience deformation without failure or cracking is an important characteristic to consider in its selection as a material for a forging process. In metal forming industry several tests have been developed to try and quantify this ability. The amount of deformation a particular metal can tolerate without failure is directly related to that metal's forgeability. The higher the amount of deformation, the higher the forgeability. One popular test involves compressing a cylindrical work piece between two flat die. This is called upsetting the work, and thus this test is called the upsetting test. In an upsetting test, the part is compressed in open flat die, reducing it in height until cracks form. The amount of reduction can be considered a measurement of forgeability. Upsetting tests can be performed at different temperatures and different compression speeds. Testing various temperatures and strain rates will help determine the best conditions for the forging of a particular material. Another common test used in modern industry is called the hot twist test. In a hot twist test, a round bar is twisted in one direction until material failure occurs. The amount of rotation is taken as a quantitative measurement of forgeability. This test is often conducted on a material at several different temperatures. Other tests are also used in industrial forging manufacture. Impact testing is sometimes used to gauge the forgeability of a material. Cracks in the metal are the common criteria for failure for most tests; however, forgeability tests can also determine other negative effects that a material may exhibit under different conditions of stress, strain rate, and temperature.

Lubrication in Industrial Forging Manufacture Frictional forces within the mold, between the work and the surfaces of the die cavity, have a large influence over the flow of material in a forging operation. Lubricants are used in industrial forging production in order to lower frictional forces, and enact a smoother flow of metal through the mold. In addition they are used to slow the cooling of the work and reduce temperature gradients in hot forging manufacture, serving as a thermal barrier between the metal and the mold. Lubricants also help keep the metal and die surfaces from sticking together and assist in the removal of the forging from the die. Common lubricants used in modern forging industry include, water, mineral oil, soap, saw dust, graphite, molybdenum disulfide, and liquid glass.

Forging Die Material The exact material used to make a forging die is dependent upon all the details of that particular forging process. In general a forging die must be tough, possess high strength and hardness at elevated temperatures, good shock resistance, resistance to thermal gradients, hardenability and ability to withstand abrasive wear. During the manufacture of a hot forged part the mold is usually preheated before the operation begins. Preheating die reduces thermal cycling that can cause cracks in the die. Forging die are hardened and tempered. Mold dimensions must for shrinkage of the work, as well as extra material allowances for the finishing of the part. The abrasive wear present in hot forging operations is due largely to the scale on the work piece. Much of the scale can be removed from the blank immediately after heating in the furnace, prior to the forging of the part. Adequate lubrication can also greatly mitigate wear. Sometimes a mold may be assembled using different sections. These sections, called die inserts are manufactured separately and may be of different materials. Complex cavities can be produced easier with die inserts; also different sections of the mold can be individually replaced. Some factors to consider when determining the material composition of a forging mold are, type of metal forming operation, number of forgings desired, size of forged part, complexity of forged part, type of machinery to be used, temperature that the part will be forged at, and the cost of materials. Forging die are made from tool steels, that depending upon process criteria are alloyed with various levels of one or more of these materials, chromium, molybdenum, vanadium, and nickel. Die blocks are cast from the alloy, metal formed, then machined, and finished.

Forging Die Design Forging die design will always depend on the factors and requirements of the manufacturing process. However, there are some general principles to consider for good forging die design. During the forging process metal is flowing under pressure to fill the impression within the mold. Similar to the metal casting process of die casting, in forging, an increase in pressure on the metal within the mold will increase the ability to fill the mold completely. One main difference being that in die casting the metal is liquid, while in forging, the work is a solid metal above or below its recrystallization temperature. Smaller, thinner, longer, and more complex sections can be produced with more pressure, but too much pressure within the mold is bad because it can damage the die and machinery. The formation of flash is an important part of impression die forging manufacture. First, flash provides a way for excess material from the work piece to exit the mold. If this material could not escape during

compression, the buildup of pressure as the volume of work metal exceeded the volume of the mold could easily crack the die. Flash, while allowing material to escape does increase the pressure within the die cavity (mold). Flash must travel through a narrow age called land before it opens up into a gutter. As it flows through land, the friction between the flash and the mating surfaces resists further flow of material out of the mold, increasing pressure within the mold. In addition the cooling of the flash from the mating surfaces increases resistance to flow of material out of the mold, thus also increasing pressure within the die cavity. A longer land will cause the flash to have to flow further under resistance increasing the mold pressure. Decreasing the width of land will increase the cooling rate of the flash, as the temperature goes down the metals resistance to flow goes up. More resistance to flow will cause a thinner land to have higher mold pressure. The pressure within the die cavity is often controlled by varying the width of land.

One of the main principles to when deg a forging die for a specific manufacturing process is that while deformation of the metal is occurring, the material will always flow in the direction of least resistance. Proper metal flow within the mold is important in ensuring a complete filling of the die cavity, preventing defects, and in effecting the grain structure of the forged part. Friction in the mold is an important consideration in forging manufacture. Friction will act to resist the movement of the material and increase the forces required to fill the die cavity during the process. More forces, in turn, mean more stress and wear on the mold and equipment.

Another critical factor in the movement of material within the die cavity during the forming of the part is the interior geometry of the die cavity. The size of a part, work material, complexity of a forging, size and thickness of different part features, and distance different areas are from the parting line, are some of the important factors concerning the structure of the mold. Basically thinner more complex features will be more difficult to fill completely, as would areas further from the parting line or out of the way of the predominant flow of material. Thin portions of a metal forging are called ribs and webs. A rib is a section that runs perpendicular to the forging plane as determined by the parting line. Long narrow ribs are harder to fill and require more forces, increasing the width of a long rib will better facilitate the filling of the rib with material during the process. A web is a portion of the metal forging that runs parallel to the forging plane. The thickness of webs can be minimized as much as practical. When deg a forging die web thickness should not be too small or else there may be trouble completely filling the web with metal. Webs that are too thin may also cool faster than the rest of the forging; the resulting shrinkage could cause tears or warping of the part.

As the work metal fills the die cavity, the flow of material will have to change directions depending upon the part's geometry. Smooth, large filleted turns will allow the material to change directions while adhering to the molds dimensions. If corners within the forging are too sharp then the material may not completely follow the path of those corners, resulting in vacancies, laps, or cold shuts. Sharp corners will also act as stress raisers in the mold. Good forging die design should provide adequate enough fillet and corner radius to allow for easy material flow.

Forging Process Design

In modern industry metal parts of complex geometry are often formed completely with the need for only minor finishing operations. These parts can not be manufactured with a single forging. The work piece is taken through a series of metal forming operations that, in steps, alter the geometric shape of the material until the final process creates the desired forging. In these types of forging sequences each operation must be planned in such a way as to prepare the work piece for the next forming process. Together, the series of metal forming operations that are required to create a part make a larger single process, and each individual forging operation has its place within the larger process. When deg a complex forging process great consideration should be taken with each step and how it relates to the final product. Also the chosen path for the redistribution of the work material from the start of the process to the end of the last step. Forging design in general should first accomplish a rough redistribution of the material, then the more detailed impression die forging operations, and finally finishing operations. In addition to providing a smooth transition of material, the forging processes as a whole should be designed to produce a controlled grain structure in the final product. When choosing a path for material redistribution, a metal forming design should consider how this particular method of metal deformation will effect and change the grain structure of the part. It is desirable that the final product contain a favorable grain orientation throughout the structure of its material. Such a grain structure should strengthen the part, particularly with respect to that parts application.

Open die forging often plays a roll in the early stages, providing a general mass redistribution of the work metal. Before the more detailed impression forgings can shape the work, metal must be formed in such a way as to place higher concentrations of material in regions that will require more material. Fullering and edging, discussed in the open die forging section is very important open die forging processes used to accomplish a rough transfer of material. Fullering and edging will squeeze more metal into some areas of the work, while causing other areas to have less depending on the needs of the process. Figure 170 shows two rough forms, one was subject to fullering the other to edging, the nature of the different processes should be apparent.

Impression die forging occurs after the rough form has been created. The closed die forgings will impart the geometric features of the part on the work piece. The flow of material must be carefully designed both before and during this phase. Finishing processes, such as sizing, impart less but accurate geometric change to the forging in the final stages of the metal forming manufacture. Figure 171 shows the different steps in the metal forming process used to manufacture a complex part.

Most all industrial forging products will be processed with further manufacturing operations that will impart higher tolerances and dimensional accuracy than forging manufacture alone. These operations, (such as machining), although more accurate than forging, do not produce the stronger material within the work that forming operations do. By combining different types of processes such as forging and machining a manufacturer can utilize the benefits of both processes, creating very accurate parts, good surface finish and superior mechanical properties.