MAE 4500 Exam 1

Why are compression tests subject to inaccuracy?

B/c of the presence of friction and the resulting barreling of the specimen

Cold working

Deformation at room temperature. Results in higher strength, reduced ductility, generally causes anisotropy (preferred orientation or mechanical fibering) whereby the properties are different in different directions.

Physical properties

Density, specific heat, thermal expansion and conductivity, melting point, electrical magnetic properties

Impact tests

Determine the energy required to completely break a specimen (impact toughness of the material)

Important factors in extrusion

Die design, extrusion ratio, billet temperature, lubrication, extrusion speed

Common defects/imperfections in bcc, fcc, hcp structures

Dislocations, vacancies, impurities, inclusions, grain boundaries

Please explain the functions of the additives (carbon, lime, oxygen, and other metal elements) to the pig irons when they are used for producing steels from the pig irons.

Ex- Limestone: (calcium carbonate) is used to remove impurities from the molten iron. Limestone reacts chemically with impurities, acting as a flux that causes impurities to melt at a low temperature. It combines with impurities and forms a slag (light) which floats over the molten metal, and, subsequently, is removed.

Expendable-mold, permanent-pattern processes

Include sand, shell-mold, plaster-mold, and ceramic-mold casting. These processes require the destruction of the mold for each casting produced, but mold production is facilitated by reusable patterns

Annealing

Includes normalizing, process annealing, stress relieving, tempering, astempering, and martempering, each with the purpose of enhancing the ductility and toughness of heat-treated parts

Fatigue tests

Indicate the endurance limit or fatigue limit of materials, i.e. the maximum stress to which a material can be subjected without fatigue fail

High-strength low-alloy steels

Low carbon content, consist of fine-grained ferrite as one phase and a second phase of martensite and austenite

Ductile fracture

Plastic deformation precedes fracture; a considerable amount of energy is required

Explain why the strength of a polycrystalline metal at room temperature decreases as its grain size increases.

Strength increases as more entanglements of dislocations occur with grain boundaries. Metals with larger grains have less grain-boundary area per unit volume, and hence will not be as able to generate as many entanglements at grain boundaries, thus the strength will be lower.

Mechanical properties

Strength, toughness, ductility, hardness, elasticity, fatigue, creep

Desirable properties of nonferrous alloys

Strength, toughness, hardness, and ductility; resistance to high temp, creep, and oxidation; a wide range of physical, thermal and chemical properties; high strength-to-weight and stiffness-to-weight ratios

Two parts have been made of the same material, but one was formed by cold working and the other by hot working. Explain the differences you might observe between the two.

The cold worked material will have a higher strength and hardness than the hot worked material. A cold worked material will have a lower recrystallization temperature than a hot worked material.

Nonferrous metals and alloys

Most common: aluminum, magnesium, and copper and their alloys. For high temps: nickel, titanium, refractory alloys and superalloys. Low-melting alloys: lead, zinc, tin.

Tensile test

Most commonly used to determine mechanical properties; true stress-true strain curves can be constructed that are needed to determine strength coefficient K, strain hardening exponent n, strain-rate sensitivity exponent m, and toughness

Brittle fracture

Not preceded by plastic deformation and can be catastrophic; requires much less energy than ductile fracture

Chemical properties

Oxidation, corrosion, toxicity, flammability

Creep

Permanent deformation of a component under a static load maintained for a period of time; in tension, the specimen eventually fails by rupture (necking and fracturing)

Describe the characteristics of (a) an alloy, (b) pearlite, (c) austenite, (d) martensite, and (e) cementite.

1. Alloy: Composed of two or more elements, at least one of which is a metal. The alloy may be a solid solution or it may form intermetallic compounds. 2. Pearlite: A two-phase aggregate consisting of alternate lamellae of ferrite and cementite. The closer the pearlite spacing of the lamellae, the harder the steel will be. 3. Austenite: Called gamma iron, it has a face-centered cubic structure. The fcc structure allows for higher solubility of carbon in the crystal lattice. This structure also possesses a high level of ductility, which increases the steel's formability. 4. Martensite: Forms by quenching austenite. It has a body-centered tetragonal (bct) structure, and carbon atoms in interstitial positions impart high strength to the structure. It is very brittle and hard. 5. Cementite, also known as iron carbide (Fe3C). Cementite is a hard and brittle phase

List the general recommendations you would make for forging materials with limited ductility.

1. Keep shapes simple, and minimize total strain. 2. Forge at high temperatures to improve ductility. 3. Demanding deformation can be attained by maintaining a high hydrostatic compressive stress; this means large and thin flash. 4. Forge slowly.

Describe the differences of killed, semi-killed, and rimmed steels. Please give at least two possible manufacturing processes that can eliminate the porosity in ingots.

1. Killed Steel. The term killed comes from the fact that the steel lies quietly after being poured into the mold. Killed steel is a fully deoxidized steel; that is, oxygen is removed and the associated porosity is thus eliminated. In the deoxidation process, the oxygen dissolved in the molten metal is made to react with elements such as aluminum, silicon, manganese, and vanadium that have been added to the melt. These elements have an affinity for oxygen and form metallic oxides. If aluminum is used, the product is called aluminum-killed steel (see Table 16.4). If they are sufficiently large, the oxide inclusions in the molten bath float out and adhere to, or are dissolved in, the slag. A fully killed steel thus is free of any porosity caused by gases; it also is free of any blowholes (large spherical holes near the surfaces of the ingot). Consequently, the chemical and mechanical properties of a killed-steel ingot are relatively uniform throughout. Because of shrinkage during solidification, however, an ingot of this type develops a pipe at the top (also called a shrinkage cavity); it has the appearance of a funnel-like shape. This pipe can take up a substantial volume of the ingot, and it has to be cut off and scrapped. 2. Semi-killed Steel. Semi-killed steel is a partially deoxidized steel. It contains some porosity (generally in the upper central section of the ingot), but it has little or no pipe. Although the piping in semi-killed steels is less, this advantage is offset by the presence of porosity in that region. Semi-killed steels are economical to produce. 3. Rimmed Steel. In a rimmed steel, which generally has a carbon content of less than 0.15%, the evolved gases are only partially killed (or controlled) by the addition of other elements, such as aluminum. The gases produce blowholes along the outer rim of the ingot—hence the term rimmed. These steels have little or no piping, and they have a ductile skin with good surface finish; however, if not controlled properly, blowholes may break through the skin. Furthermore, impurities and inclusions tend to segregate tow ard the center of the ingot. Products made from this steel may thus be defective, hence thorough inspection is essential.

Swaging

A type of rotary forging in which a solid rod or a tube is reduced in diameter by the reciprocating radial movement of a set of two or four dies; suitable for producing short or long lengths of bar or tubing with various internal or external profiles

Forging

A family of metalworking processes in which deformations of the workpiece is carried out by compressive forces applied through a set of dies. Capable of producing a wide variety of structural parts, with favorable characteristics such as higher strength, improved toughness, dimensional accuracy, and reliability in service

Please explain aluminum alloys with AISI designation of 2024-T4 and 6061-T6 in term of their composition and the ways how they are processed. (8 points) Please give a possible application for each of them.

Aluminum 2024 alloy is an age-hardening and is strengthened during the heat treatment process. T4 condition is obtained when this alloy is heated at 493°C (920°F) followed by cold water quenching and finally aging at room temperature. 2024 - Truck wheels, screw machine products, aircraft structures. 6061-T6 aluminum is a precipitation-hardened aluminum. Precipitation hardening uses high temperatures to increase the yield strength of aluminum. Precipitation hardening lowers the plasticity and hardens aluminum into 6061-T6. 6061 applications: Heavy-duty structures where corrosion resistance is needed; truck and marine structures, railroad cars, furniture, pipelines, bridge railings, hydraulic tubing.

The effects of cold working can be reversed by...

Annealing the metal; i.e. heating it within a certain temperature range for a given period of time, thereby allowing the successive process of recovery, recrystallization, and grain growth to take place.

Please explain why the fatigue induced failure is like brittle fracture. Please why surface finishing is important to fatigue lifetime. Please give a few methods to improve fatigue lifetime of parts.

Because Fatigue fractures begin as a microscopic crack or cracks that grow as force is applied repeatedly to a part, with good surface finishes the likelihood of cracks forming on the surface decreases, thus improving the lifetime of the part. Some of the methods to improve fatigue life include: 1. Inducing compressive residual stresses on surfaces— for example, by shot peening or by roller burnishing 2. Case hardening (surface hardening) by various means 3. Providing a fine surface finish, thereby reducing the detrimental effects of notches and other surface imperfections. 4. Selecting appropriate materials and ensuring that they are free from significant amounts of inclusions, voids, and impurities. Conversely, the following factors and processes can reduce fatigue strength: 1. Tensile residual stresses on the surface 2. Decarburization 3. Surface pits (such as due to corrosion), that act as stress raiser 4. Hydrogen embrittlement 5. Galvanizing 6. Electroplating

Please give a few reasons why there can be so much variation in the strength and elongation in a class of metal alloys? (Refer to Table 2.2)

Because alloy classes have a wide range of different elements and concentrations which affect the material properties of the final product.

What factors other than mechanical strength should be considered in selecting metals and alloys for high-temperature applications? Explain.

Because high temperatures tend to increase corrosion rates, the alloy should have good high-temperature corrosion resistance. Also, creep resistance should be high, since high temperatures promote creep. (Section 2.8). If the particular application requires cycling through temperature ranges, the alloy should also possess thermal-fatigue resistance.

What type of cast iron would be suitable for heavy-machine bases, such as presses and machine tools? Why?

Because of its relatively high strength and excellent castability (which generally means low cost), a pearlitic gray cast iron would probably be most suitable for this application. Note that, as no significant ductility is required for this application, the low ductility of gray irons is of little consequence. An important further advantage is the damping capacity of these cast irons, especially for machine tools.

Please explain why the processing speed must be slow when rolling steel. Please why the hot rolling is needed when the strain is large. (Hint: Fig. 2.7)

Because with increasing strain rate comes strain hardening. Hot rolling is done about recrystallization temperature of steel, making it more malleable and thus it is more appropriate when the strain is large.

Three basic crystal structures in metals

Body-centered cubic (bcc), face-centered cubic (fcc), hexagonal close-packed (hcp)

Which of the following considerations are important for a riser to function properly? Must it: (a) have a surface area larger than the part being cast, (b) be kept open to atmospheric pressure, and/or (c) solidify first? Explain.

Both (a) and (c) would result in a situation contrary to a riser's purpose, that is, if a riser solidifies first, it cannot feed the mold cavity to avoid shrinkage in the part. Concerning (b), when the molten metal enters the mold cavity, the air which was in the mold has to be forced out. If a riser is not open to the atmosphere, either the gas will become dissolved into the metal (due to the increased pressure and depending on solubility), or sufficient pressure will build up which may crack the mold. Thus, a riser should be kept open to atmospheric pressure in order for it to function properly.

Please comment on both the structural and mechanical properties (hardness and toughness) evolution of the part by end quench according to the distance from the quenched end (Figure 4.18). (Hint: you may choose three main regions.)

By heating the steel to the fcc structure and quenching, it develops into martensite, which is a very hard, hence strong, structure. Steels are rarely used in their as-quenched condition because they are very brittle and thus lack toughness. These detrimental conditions are overcome by tempering the steel, which restores toughness. Quenching increases hardness, but results in a brittle metal.

Hardenability

Capability of an alloy to be hardened by heat treatment

Major categories of ferrous metals and alloys

Carbon steels, alloy steels, stainless steels, tool and die steels

Stainless steels

Chromium is the major alloying element; they are called stainless b/c they form a passivating chromium-oxide layer on their surface

Why are alloys used in engineering applications?

Commercially pure metals generally do not have sufficient strength for most engineering applications, so they must be alloyed with various elements which alter their structures and properties

Please give advantages of continuous casting. What would happen if the speed of the continuous-casting process shown in Fig. 5.4a is (a) higher or (b) lower than that indicated, typically 25 mm/s.

Continuous casting (Section 5.4) eliminates processing of individual ingots and also eliminates most of the porosity, elemental segregation, and shrinkage associated with ingot casting. Continuously-cast bars can be made in a variety of shapes and sizes, significantly reducing the number of subsequent rolling operations. These major benefits make continuous casting capable of producing higher quality steels at lower cost than individual ingot processing. (a) If the speed of the continuous casting process is higher, the metal may not have sufficient time to completely solidify before it leaves the mold area. Liquid metal will eventually start spilling out of the mold. (b) Lower speeds, on the other hand, adversely affect the economics of the process, and are obviously unnecessary.

According to Figure 4.16, please compare the properties (hardness and toughness) of ferrite+pearlite, coarse and fine pearlite, spheroidite steels. Please explain what heat treatment conditions are used to form fine or coarse pearlite and spheroidite steels.

Ferrite + pearlite have a higher hardness, but lesser toughness than spheriodite. Spheroidite is formed by tempering the steel at 700°C for a day. Pearlite. If the ferrite and cementite lamellae in the pearlite structure of the eutectoid steel, shown in Fig. 4.9, are thin and closely packed, the microstructure is called fine pearlite; if they are thick and widely spaced, it is called coarse pearlite. The difference between the two depends on the rate of cooling through the eutectoid temperature, which is the site of a reaction in which austenite is transformed into pearlite. If the rate of cooling is relatively high (as in air), fine pearlite is produced; if cooling is slow (as in a furnace), coarse pearlite is produced.

Components in iron-carbon system

Ferrite, austenite, cementite

Ceramics & glasses

Glass ceramics, graphite, diamond, and diamond-like materials

Basic types of cast irons

Gray iron, ductile (nodular) iron, white iron, malleable iron, compacted-graphite iron

What is the difference between an interstitial atom and a substitutional atom?

In a substitutional alloy, the atoms from each element can occupy the same sites as their counterpart. In interstitial alloys, the atoms do not occupy the same sites.

Expendable-mold, expendable-pattern processes

Include lost-foam and investment casting. In these processes, a pattern is consumed for each mold produced and the mold is destroyed after each casting.

Describe the factors involved in precision forging.

Precision forming is outlined in general terms in Table 14.1, which also identifies the important design considerations and manufacturing process variables. Precision forming involves high tolerances and detailed geometries; these can only be achieved with intricate dies (so that machining and finishing costs of the dies will be high) and high forging forces (which have adverse effects on die life). Precision forming is usually done cold, so that there is no thermal strain-induced warping, and this also means forging forces will be high. Also, effective lubrication is a concern, since a thick lubricant film may result in a part not achieving the die shape, and also may result in orange peel.

Solidification of pure metals vs alloys

Pure metals: takes place at a constant temperature. Alloys: occurs over a range of temperatures

Describe the importance of controlling roll speeds, roll gaps, temperature, and other process variables in a tandem-rolling operation, as shown in Fig. 13.12. Explain how you would go about determining the distance between the stands.

Referring to the tandem rolling operation shown in Fig. 13.12, note that mass continuity has to be maintained during rolling. Thus, if the roll speed is not synchronized with the strip thickness in a particular stand, excessive tensions or slack may develop between the stands; some rolls may slip. Also, if the temperature is not controlled properly, strip thickness will change, thus affecting reduction per pass and, consequently, the roll forces involved. This, in turn, will also affect the actual roll gap and roll deflections. Complex control systems have been developed for monitoring and controlling such operations at high rolling speeds.

Composite (engineered) materials

Reinforced plastics, metal-matrix, ceramic-matrix composites

Residual stresses

Remain in a workpiece after it has been plastically deformed and then has had all external forces removed

Which type of hardness test, Brinell or Rockwell, does have sensitive to the surface roughness? Why?

Rockwell is more sensitive to the surface roughness due to the fact that each Rockwell point = 0.002 mm, Rockwell results are measured in 0.1 Rockwell points, meaning an excessively rough surface would affect the results significantly.

Hot rolling

Rolling carried out at elevated temperatures

Cold rolling

Rolling carried out at room temperature

Please explain the type of SAE 4140 steel and its composites. (5 points) Please give two possible applications for this type of steel.

SAE 4140 steel is Carbon and Alloy Steel. It contains low amounts of chromium-, molybdenum-, and manganese-containing. It has high fatigue strength, abrasion and impact resistance, toughness, and torsional strength. Applications include: Aircraft forgings, tubing, fittings, landing gear

Nanomaterials

Shape-memory alloys, amorphous alloys, semiconductor, and superconductors

How does grain size affect the strength of a metal?

Smaller size = stronger metal, larger size = more ductile metal. However, excessively large grains are associated with brittle behavior.

Two forms of alloys

Solid solutions (may be substitutional or interstitial) and intermetallic compounds

Casting

Solidification process in which molten metal is poured into a mold and allowed to cool. The metal may flow through a variety of passages (pouring basins, sprues, runners, risers, and gating systems) before reaching the mold cavity.

Design a heat-treating cycle for carbon steel, including temperature and exposure times, to produce (a) pearlite-martensite steels, (b) bainite-martensite steels, and c) austenite-pearlite (coarse and fine). You may draw the process on the following figure and then give brief explanation.

The heat-treat cycle for these conditions can be obtained from Fig. 4.17b. For part (a), it is desired to produce a pearlite-martensite steel, so it is important that the cooling rate be maintained between 140◦ and 35◦C/s when cooling the material from the eutectoid temperature. Such a cooling rate can be achieved with a salt or oil quench, where the bath temperature will determine the cooling rate and the ultimate percentage of pearlite and martensite. For part (b), it is desired to have bainite, which forms under very rapid cooling. Thus the two heat-treat cycles desired can be sketched as shown below. (fig. in solution)

Extrusion

The process of pushing a billet through a die, to reduce its cross-section or to produce various solid or hollow cross-sections; generally carried out at elevated temperatures in order to reduce the extrusion force and improve ductility of the material

Rolling

The process of reducing the thickness or changing the cross-section of a long strip by compressive forces applied through a set of rolls. Shape rolling is used to make products with various cross-sections. Other rolling operations: ring rolling, thread rolling

How does the shape of graphite in cast iron affect its properties? (hint: four types of shapes)

The shape of graphite in cast iron has the following basic forms: 1. Flakes: Have sharp edges which act as stress raisers. The shape makes cast iron low in tensile strength and ductility, but it still has good compressive strength. The flakes also act as vibration dampers. 2. Nodules: Spheroids formed by graphite when magnesium or cerium is added to the melt. This form has increased ductility, strength, and shock resistance over flakes, but the damping capacity is reduced. 3. Clusters: Much like nodules, except they form from the breakdown of white cast iron upon annealing. Clusters have properties similar to flakes. 4. Compacted flakes: Short thick flakes with rounded edges. This form has properties that are between nodular and flake graphite

Please explain what would happen if the sprue has the constant cross-section area. Explain why this consequence is bad to the casting. Please give two methods for mitigating this problem.

The sprue is a tapered vertical channel through which the molten metal flows downward in the mold. The cross-sectional area of the stream decreases as the liquid gains velocity downward. Thus, if a sprue has a constant cross-sectional area and molten metal is poured into it, regions can develop where the liquid loses contact with the sprue walls. As a result, aspiration (a process whereby air is drawn in or entrapped in the liquid) may take place. One of two basic alternatives is used to prevent aspiration: (a) A tapered sprue is used to prevent molten metal separation from the sprue wall or (b) straight-sided sprues are supplied with a choking mechanism at the bottom, consisting of either a choke core or a runner choke, as shown in Fig. 11.3. The choke slows the flow sufficiently to prevent aspiration in the sprue.

According to Figure 4.19 and Figure 4.20, please explain the process for the precipitation hardening and how temperature affects the final hardness.

The structure obtained in A in Fig. 4.19b can be made stronger by precipitation hardening. In this process, the alloy is first reheated to an intermediate temperature and then held there for a period of time, during which precipitation takes place. The copper atoms diffuse to nucleation sites and combine with aluminum atoms. This process produces the theta phase, which forms as submicroscopic precipitates (shown in B by the small dots within the grains of the kappa phase). The resulting structure is stronger than that in A, although it is less ductile. The increase in strength is due to increased resistance to dislocation movement in the region of the precipitates. The lower the precipitation hardening temperature is, the higher the hardness of the metal is going to be, though it would require a much longer aging time as a result.

List the possible consequences of rolling at (a) too high of a speed and (b) too low of a speed.

There are advantages and disadvantages to each. Rolling at high speed is advantageous in that production rate is increased, but it has disadvantages as well, including: 1. The lubricant film thickness entrained will be larger, which can reduce friction and lead to a slick mill condition where the rolls slip against the workpiece. This can lead to a damaged surface finish on the workpiece. 2. The thicker lubricant film associated with higher speeds can result in significant oil peel, or surface roughening. 3. Because of the higher speed, chatter may occur, compromising the surface quality or process viability. 4. There is a limit to speed associated with the motor and power source that drive the rolls. Rolling at low speed is advantageous because the surface roughness in the workpiece can match that of the rolls (which can be polished). However, rolling at too low a speed has consequences such as: 1. Production rate will be low, and thus the cost per unit weight will be higher. 2. Because a thick lubricant film cannot be developed and maintained, there is a danger of transferring material from the workpiece to the roll (pickup), thus compromising surface finish. 3. The workpiece may cool excessively before contacting the rolls. This is because a long billet that is rolled slowly loses some of its heat to the environment and also through conduction through the roller conveyor.

Is it possible for two pieces of the same metal to have different recrystallization temperatures? Is it possible for recrystallization to take place in some regions of a part before it does in other regions of the same part? Explain.

Two pieces of the same metal can have different re-crystallization temperatures if the pieces have been cold worked to different amounts. The piece that was cold worked to a greater extent (higher strains), will have more internal energy (stored energy) to drive the re-crystallization process, hence its re-crystallization temperature will be lower.

Permanent-mold processes

Use molds or dies that can be used to produce castings at high production rates; common processes include slush casting, pressure casting, die casting and centrifugal casting

Explain four types of possible defects in metal rolling process and their mitigation strategies.

Wavy edges on sheets (Fig. 13.8a) are due to roll bending. The strip becomes thinner along its edges than at its center (see Fig. 13.4a), thus, because of volume constancy in plastic deformation, the edges have to elongate more than the material at the center. Consequently, the edges buckle because they are constrained by the central region from expanding freely in the longitudinal (rolling) direction. The cracks shown in Figs. 13.8b and c are usually the result of low material ductility at the rolling temperature. Because the quality of the edges of the sheet is important in subsequent forming operations, edge defects in rolled sheets may have to be removed by shearing and slitting operations (Section 16.2). Alligatoring (Fig. 13.8d) is typically caused by nonuniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material.

Give some applications for (a) glassy metals, (b) precious metals, (c) low-melting alloys, (d) super alloys.

a) Amorphous alloys exhibit excellent corrosion resistance, good ductility, high strength, and very low magnetic hysteresis (utilized in magnetic steel cores for transformers, generators, motors, lamp ballasts, magnetic amplifiers, and linear accelerators). b) The most important precious (costly) metals, also called noble metals, are the following: • Gold (Au, from the Latin aurum) is soft and ductile, and has good corrosion resistance at any temperature. Typical applications include jewelry, coinage, reflectors, gold leaf for decorative purposes, dental work, electroplating, and electrical contacts and terminals. • Silver (Ag, from the Latin argentum) is ductile and has the highest electrical and thermal conductivity of any metal (see Table 3.2); however, it develops an oxide film that adversely affects its surface characteristics and appearance. Typical applications for silver include tableware, jewelry, coinage, electroplating, solders, bearing linings, and food and chemical equipment. Sterling silver is an alloy of silver,7.5%copper. • Platinum (Pt) is a soft, ductile, grayish-white metal that has good corrosion resistance, even at elevated temperatures. Platinum alloys are used as electrical contacts; for spark-plug electrodes; as catalysts for automobile pollution control devices; in filaments and nozzles; in dies for extruding glass fibers (Section 18.3.4), in thermocouples; and in jewelry and dental work. c) Low-melting alloys are so named because of their relatively low melting points. The major metals in this category are lead, zinc, tin, and their alloys. Lead can be used for radiation shielding, zinc alloys can be used for fuel pumps, Tin alloys can be used in dental applications d) Major applications of superalloys are in jet engines and gas turbines; other applications are in reciprocating engines, rocket engines, tools and dies for hot working of metals, and in the nuclear, chemical, and petrochemical industries.

Select an appropriate hardness test for each of the following materials, and justify your answer. (Refer to Fig. 2.15) a. Cubic boron nitride; b. Cold-drawn 0.5% steel; c. Lead

a)Cubic boron nitride is very hard, and useful data can be obtained only from the Knoop and Mohs tests. The Mohs scale is qualitative and does not give numerical values for hardness, so the Knoop test is preferable. (b)Cold-drawn 0.5% steel. From Fig. 2.15, all of the hardness tests are suitable for this material. As discussed earlier, the best choice for this material will depend on a number of factors. (c)Lead. As shown in Fig. 2.14, lead is so soft that only the Brinell and Vickers tests yield useful data. Recognizing that lead is very soft, the lightest loads in these tests should be used. Consider the expected results in this test if a typical value of hardness is 4 HB or 4 HV. Math and stuff ... Vickers test is the best alternative for lead.

Explain the purpose of each feature (pouring cup, sprue, runner, riser, and gate) as shown in Figure 10.8 and the consequences of omitting the feature from the mold design.

• Pouring cup; this serves to accept the molten metal and provide for a large cross sectional area to provide controlled metal flow into the sprue; the absence of a pouring cup leads to more entrained air. • Sprue; this serves to transport material from the pouring cup, and is designed to provide a pressure head to assure proper filling of the mold. The absence of a sprue complicates mold design, as the casting must be fed by gravity. • Well; this can be used instead of a sprue or in combination with one; the well traps dross and also leads to low Reynolds numbers in the gating system; in the absence of a well, the flow may aspirate. Reynolds number, Re, is used to quantify turbulence aspect of fluid flow; it represents the ratio of the inertia to the viscous forces in fluid flow • A runner serves to transport molten metal from the sprue and well to the casting; without a runner, the metal would be transported directly from the sprue, leading to high Reynolds numbers, increased aspiration and a casting that is more difficult to separate from the other features. • Risers provide molten metal to account for shrinkage