Liminalizar el currículo escolar para arriesgar el yo

Forging can be a very effective way of redistributing the metal in a billet into some desired shape, but things don't always go according to plan. In this section we'll look at a veritable rogue's gallery of defects that may arise as result of the forging process.

1. Laps - A lap is a surface defect where metal has folded over upon itself during forging, but was insufficiently worked so that is did not become welded to the bulk of the forging. Laps often occur at corners or other sharp transitions. They may first appear to be cracks because only the edge of the fold may be visible during inspection. Laps can generally be prevented by proper die design.

2. Surface Cracks - Surface cracks may come from many different sources during forging. Among these are poor die design, poor lubrication, insufficient starting stock, strain hardening, too heavy a reduction in cross section, and improper forging temperature.

3. Bursts - Bursts, or center cracks, occur in forgings and extrusions as a result of poor lubrication, too heavy a reduction in cross section, and poor die design. When metal is squeezed in a die, its cross section will be reduced and it will flow in a direction 90( to the applied force. If the frictional force between the work piece and the surface of the die is too high, the flow of metal at the interface will be constrained. The

squeezing action of the dies will generate a hydrostatic pressure within the work piece that may lead to tensile stresses sufficiently large to cause the center of the forging to rupture.

4. Raised Parting Line - This ridge of metal around a closed die forging is the result of a poor trimming job of the flash.

CHAPTER VII

& POWDER METALLURGY

5. Underfill - This describes the condition where metal has failed to completely fill a die during forging. This may be due to insufficient starting stock, improper forging temperature, or poor die design. 6. Mismatch - Mismatch is a gross dimensional error resulting from the

misalignment of the upper and lower dies during closed die forging.

ADVANTAGES AND LIMITATIONS OF FORGING

A properly forged part will have a wrought structure. A wrought structure means that the part has been sufficiently hot worked so that the starting ingot's cast structure has been completely broken up. The high temperature and pressure during forging will cause any cracks, porosity, shrinkage, gas voids, etc. that may be present in the starting material to be compressed and welded shut. Alloys solidify over a range of temperatures. There is a difference in the composition of the metal that first freezes compared to the metal that freezes last. This variation in chemical composition is known as segregation. The forging process can help to homogenize the material by breaking up areas of segregation. The high temperature at which parts are forged facilitates the diffusion of atoms of a given element from areas of high concentration to those of low so that a more uniform distribution is obtained. The large, dendritic grains that so often characterize a cast structure are eliminated as the material is hot worked and recrystallized. A wrought structure is thus substantially free of the discontinuities, chemical segregation, and large grain size that are frequently encountered in castings. The more homogeneous nature of a forging optimizes mechanical properties

(toughness and ductility in particular) and may also improve corrosion resistance. Forging may be used to cold work a material that cannot otherwise be

strengthened. There are some limitations to this because it is not always possible to uniformly cold work thick parts. Naturally the center of a thick part will see less work than the outer portions. This may result in a significant variation of strength throughout the cross section of a large part.

Forgings are often cleaner than many types of castings. In many casting

processes there is always a possibility that the molten metal may break off particles of mold material as it is poured into the mold and thus create non-metallic inclusions. These inclusions can decrease ductility and toughness.

Metal flows during a forging process. The metal grains will tend to align

themselves parallel to the direction of greatest metal-flow, the longitudinal direction. A transverse direction is 90( to the direction of the greatest metal-flow. Mechanical properties, toughness in particular, often vary by direction in a wrought material. A Charpy impact specimen removed from a forging such that its longitudinal axis is parallel to the direction of greatest metal-flow (the longitudinal orientation) will have a significantly higher toughness value than a specimen taken perpendicular to the

direction of greatest metal flow (the transverse orientation). Forgings can often be made such that the grain flow provides the optimum properties in the most highly stressed

CHAPTER VII

& POWDER METALLURGY

For an extrusion, the longitudinal direction is parallel to the centerline of the part. There is often no single, longitudinal direction in a closed die forging: grain flow will generally follow a direction parallel to the surface of the die.

How much hot work is necessary to convert a cast structure into a wrought one? We obviously don't want to hot work a metal any more than we have to because of increased costs due to die and equipment wear, furnace time, etc. There is no one answer to this question. A lot will depend on the size and condition of the starting material as well as the particular forging process itself. Many forge shops use a rule of thumb that calls for a reduction of area of 4 to 1 or sometimes 3 to 1. In other words, if the starting ingot's cross sectional area has been reduced by a factor of 4 (or whatever factor is selected) during forging, a wrought structure is assured. A 24" diameter ingot, for example, must be reduced to 12" diameter product. The reduction of area that we require in our specifications often varies by starting material. A VAR ingot, for example, requires much less hot work than a air melt ingot because it is already essentially free from porosity, shrinkage, blow holes, etc. A particular forging process, such as closed die forging, may not always provide sufficient reduction to meet a 4 to 1 criteria. In these cases, the starting billet material must already be in the wrought condition. The conversion of cast to wrought structure takes place during primary forging: the closed die forging process is utilized primarily to obtain the desired shape.

There are, of course, some limitations to forging. For small numbers of parts, it may be uneconomical because of the high cost of dies. Maintenance of the dies can be expensive. Available press or hammer forge sizes or roll sizes can limit the size of product that can be made on a given piece of equipment. Extremely large parts may not be capable of being forged because they exceed the power capacity of any forging equipment. It is extremely difficult and many times impossible to forge internal cavities, yet cavities are easily made in castings. Finally there are many alloys that cannot be forged because of their brittle nature or their propensity to work harden rapidly. Virtually all metals can be cast.

CASTING

Casting is a process for making a part by pouring molten metal into a mold having a cavity of the desired shape and then allowing it to solidify. The cavity will be slightly larger than the finished part because the metal contracts as it solidifies and cools. Casting processes are generally classified by the types of molds that are utilized. We'll examine four different casting processes used in making our products.

1. Sand Casting - In sand casting, sand mixed with water and suitable binders is the molding material. The molding material is packed around a pattern, a form (often made of wood) having the desired shape and slightly larger than the finished part. When the pattern is removed from the molding material, a cavity having the same shape as the pattern remains. Green sand molds are the most common type. "Green" means that the sand mixture is used damp: it is not dried in an oven

CHAPTER VII

& POWDER METALLURGY

Figure 7: Connecting Rod

before molten metal is poured in. Let's look at how a typical green sand mold is made. We'll make a cast connecting rod as an example (see Figure 7). Of course, we are not in the connecting rod business, but it is a whole lot easier for me to draw a connecting rod rather than a gate valve body.

The first step is to make a pattern corresponding to the shape of the casting we want. Due allowances must be made for the fact that molten metal shrinks as it solidifies, consequently our pattern will be slightly larger than the finished part. Because we plan on making a lot of these connecting rods, we are going to make a cope and drag pattern. This is a split pattern with each part mounted separately on plates. We are going to make our sand mold in two parts: a top half and a bottom half. These will be connected together when we pour the metal. The top half of the mold is called the cope and the bottom half is the drag.

We need a hole in our mold in which to pour the molten metal and we are going to need some channels to direct the molten metal into the mold cavity. We also want to have a reservoir of molten metal next to the cavity so that when the molten metal in the cavity solidifies and consequently shrinks, any voids will be immediately filled with more molten metal. The hole that we pour the molten metal in is called the pouring basin. The molten metal is directed down to the channels or runners by the sprue. The runners will channel the liquid metal into the reservoirs (or risers). The place where the molten metal enters the mold cavity is called the gate. We will incorporate the runners, risers, and gating system into our pattern. We will worry about the sprue and pouring basin later.

We will start making our green sand mold by taking a metal flask (basically a retaining wall) and putting it over the drag portion of our

CHAPTER VII

& POWDER METALLURGY

steps designed to insure uniform density. When the flask is filled we will invert it and remove the pattern, leaving a replica of its shape in the sand. We will repeat the process for the cope half of the mold. The flasks are positioned over the cope and drag pattern plates by locating pins. This is necessary in order to insure that the cavities in the cope and the drag match up when the two halves are brought together. After removing the cope pattern, we are going to poke a hole all the way through the cope half of the mold where we want the sprue to be. Then we will invert it and cut out an area for the pouring basin. Our connecting rod has a small hole in each end. It would be very difficult to use a pattern having corresponding holes in it and trying to compact the molding sand in the holes so that is had sufficient

strength. When we pull the pattern out of the flask we would probably damage the compacted sand in the holes anyway. Instead, we won't put the holes in our pattern. To get the holes in our casting, we will use cylindrical cores. A core is a sand (or some other material) preform that is inserted into the mold wherever you want an internal surface. Some cores may required the use of chaplets, metal supports, to hold them in place. Chaplets, if used, will fuse in contact with the molten metal and become part of the finished casting. After inserting the two cylindrical cores into the drag where we want the holes to be, we are ready to put the cope and the drag together with the aid of alignment pins.

Our mold is now complete and should look something like Figure 8. Metal of the desired composition is heated well above its melting point in order to insure good fluidity. If this is not done, the molten metal may turn "slushy" when it is poured into the cold mold and consequently be unable to completely fill it before it solidifies. Many casting grades of steels will have a higher specified amount of silicon than the

corresponding wrought grades because silicon also helps to increase the fluidity of the molten metal. The molten metal is poured into the pouring basin, runs down the sprue, and is channeled into the riser. Runners distribute the molten metal to the gating system where it is then fed into the mold cavity.

After the metal has solidified, the finished casting can be shaken or knocked out of the mold (which can be used only once). There will be excess metal attached to the cast part where the molten metal froze in the riser, runners, and sprue, etc., but this is easily knocked off while the casting is still hot or may be cut off. Cores will be broken out. Sand castings represent the greatest tonnage of castings. Virtually any size casting can be produced. Extremely large sand castings have been made by fabricating a sand mold in an excavation in the ground. Large sand castings are most often made using a dry sand mold. Dry sand molds are made similar to green sand molds except a refractory

CHAPTER VII

& POWDER METALLURGY

Figure 8: Green Sand Mold

paste is used to coat the surface of the mold cavity and then the mold is allowed to dry. The refractory paste strengthens the surface of the mold and prevents mold material from breaking off.

The advantages of sand castings in comparison to other casting processes include low cost, high flexibility, and the fact that it is a simple, expedient way to make a wide size range of castings. It is generally not economical for very small castings and may not be suitable for castings having long, thin sections. Other disadvantages are that the tolerances are not as tight as some of the other processes and the possibility of grains of sand being torn off the mold and

becoming inclusions in the cast metal. We use sand molding for some gate valve bodies, handwheels, firesafe clamps, etc.

2. Investment Casting - Investment casting is also known as the "lost wax" process or as "precision casting." The term investment means an outer layer of covering, in this case a refractory mold, surrounding a refractory-covered wax pattern. The detailed steps of the process are as follows:

A. A metal die is made for casting the wax patterns in. Dies are blocks of metal that have cavities machined into them that correspond to the shape of the desired part.

B. Melted wax is injected into the die under pressure and allowed to solidify.

C. The wax pattern is removed from the die and then dipped into a slurry of refractory coating material. After dipping, the refractory coated wax pattern is sprinkled with fine silica sand and the assembly allowed to dry.

D. The assembly is then invested in the mold. This consists of inverting the assembly onto a flat surface, placing a paper lined metal tube over it, and filling up the tube with the molding

CHAPTER VII

& POWDER METALLURGY

E. The mold is allowed to air set until it is sufficiently hardened. The wax is removed by heating the mold to 200-300(F while it is in an inverted position. The wax melts and runs out of the mold leaving a cavity having the same shape as the desired part. The mold is then further heated until it reaches a temperature compatible with the pouring temperature of the particular metal being cast. This serves to insure that any wax trapped in the mold is burnt out (if it remained, the wax would cause gas pockets) and slows down the cooling rate of the molten metal so that it stays molten longer and consequently can fill all the nooks and crannies of the mold before it solidifies. F. The molten metal is poured in and allowed to solidify.

G. The cast part is removed from the mold and any flashing cleaned off.

Investment casting has many advantages. Extremely fine detail can be reproduced and close dimensional tolerances can be held. This means that many investment castings often require little or no machining. Surface finishes are superior to many other casting or forging pro- cesses. There is no parting line as with sand castings. Investment casting is readily adapted for use in a vacuum or in an inert gas atmosphere (some molten metals cannot be exposed to oxygen, hydrogen, etc. found in air because of potential embrittlement problems). The main limitation of the process is that the size and weight of castings is restricted by both physical and economic

constraints. Most investment castings are under 10 pounds although castings as large as several hundred pounds have been made. We have used investment castings for gate valve seat rings as well as for wellhead seals and other small parts.

3. Centrifugal Casting - A centrifugal casting is made by pouring molten metal into a horizontal, cylindrically shaped carbon, metal, or refractory mold that is rotating about its own axis. The spinning action of the mold creates a centrifugal force on the molten metal that throws it up against the mold surface. Porosity and slag inclusions migrate towards the center of the mold as they are displaced by the heavier metal. The metal against the side of the wall will solidify first and freezing

proceeds towards the center. The hollow shaped casting must be bored out to remove the porosity, inclusions, etc. that have

accumulated near the casting's ID.

Centrifugal castings offer several significant advantages over static castings. They are generally cleaner because most inclusions are removed in the boring operation.

CHAPTER VII

& POWDER METALLURGY

Shrinkage voids are virtually eliminated because of the pressure that the molten metal exerts against the solidifying metal as solidification progresses inwardly from the ID surface of the mold. Centrifugal castings have a finer, more randomly orientated, and more uniform grain structure than static castings. This results in better and more homogenous properties.

We currently use centrifugal castings in the manufacture of some gate valve and ball valve seats.

4. Shell Casting - Shell casting makes use of a mold that is made by compacting a thin layer of a thermosetting resin/sand mixture onto a heated metal pattern. Depending on the type of resin utilized, the pattern may be heated to 300-600(F. The resin will set when the mold mixture is brought into contact with the hot pattern thus binding the sand particles together. The relatively thin mold (the "shell") is

removed from the pattern and placed upside down in a flask. Sand (or