D.M.L. Materials/Machinability Exam 1 Latest Updated

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By what universal coding system are carbon and alloy steel designated? - ANSWER Carbon and alloy steel are designated by the numerical SAE or AISI system. What are three basic types of stainless st ... eels, and what is the number series assigned to them? What are their basic differences? - ANSWER Martensitic (hardenable), ferritic (nonhardenable), both magnetic and of the 400 series, and austenitic (nonmagnetic and nonhardenable, except by work hardening) of the 300 series A small part has obviously been made by a casting process. Using an inexpensive testing method, how can you determine whether it is a ferrous or a nonferrous metal, or if it is steel or white or gray cast iron? - ANSWER A magnetic test can quickly determine whether it is a ferrous metal or perhaps nickel. If the metal is white in color, a spark test will be needed to determine whether it is a nickel casting or one of the white cast iron, because they are similar in appearance. If a small piece can be broken off, the fracture will show whether it is white or gray cast iron. Gray cast iron will leave a black smudge on the finger. If it is cast steel, it will be more ductile than cast iron, and a spark test should reveal a smaller carbon content. What is the meaning of the symbols O1 and W1 when applied to tool steels? - ANSWER O1 refers to an alloy type oil hardening (oil-quench) tool steel. W1 refers to a water hardening (water-quench) tool steel. 2 7/16-in. diameter steel shaft weighs 1.32 lb per linear inch, as taken from a table of weights of steel bars. A 40-in. length is needed for a job. At 30 cents per pound, what would the shaft cost? - ANSWER Cost = 1.322 x 40 x .30 = $15.86 When you check the hardness of a piece of steel with the file test, the file slides over the surface without cutting. a.) Is the steel piece readily machinable? b.) What type of steel is it most likely to be? - ANSWER a.) No b.) Hardened tool steel or case hardened steel. Steel that is nonmagnetic is called ___. - ANSWER Austenitic (having a face centered cubic unit cell in its lattice structure). Which nonferrous metal is magnetic? - ANSWER Nickel is a nonferrous metal that has magnetic properties. List at least four properties of steel that should be kept in mind when you select the material for a job. - ANSWER Strength, machinability, hardenability, weldability, fatigue resistance, corrosion resistence What advantages do aluminum and its allows have over steel alloys? What disadvantages? - ANSWER Because aluminum is about 1/3 less dense than steel, it is used extensively in aircraft. It also forms an oxide not the surface that resists further corrosion. The initial cost is much greater. Some higher-strength aluminum alloys cannot be welded. What is the meaning of the letter H when it follows the four-digit number that designates an aluminum alloy? The meaning of the letter T? - ANSWER The letter H following the four-digit number always designates strain or work-hardening. The letter T refers to heat treatment. Name two ways in which magnesium differs from aluminum. - ANSWER Magnesium is approximately 1/3 less dense that aluminum and is approximately 1/4 as dense as steel. Magnesium will burn in air when finely divided. What is the major use of copper? How can copper be hardened? - ANSWER Copper is most extensively used in the electrical industries because of its low resistance to the passage of current when it is unalloyed with other metals. Copper can be strain hardened or work hardened, and certain allows may be harder by a solution heat-treat and aging process. What is the basic difference between brass and bronze? - ANSWER Bronze is basically copper and tin. Brass is basically copper and zinc. Name two uses for nickel. - ANSWER Nickel is used to electroplate surfaces of metals for corrosion resistance, and as an alloying element with steels and nonferrous metals. Lead, tin, and zinc all have one useful property in common. What is it? - ANSWER All three resist deterioration from corrosion. Molybdenum and tungsten are both used in ___ steels. - ANSWER Alloy Babbitt metals, used for bearings, are made in what major basic types? - ANSWER Tin, lead, and cadmium What type of metal can be injected under pressure into a permanent mold? - ANSWER Die-cast metals, sometimes called "pot metals" If you heated AISI C1080 steel to 1200F and quenched it in water what would happen? - ANSWER No hardening would result. 1200 F is less than the lower critical point, and no dissolving of carbon has take place. If you heated AISI C1020 steel to 1500 F and quenched it in water what would happen? - ANSWER There would be almost no change. For all practical purposes in the shop these low-carbon steels are not considered hardenable. List as many problems encountered with water-hardening steels as you can think of. - ANSWER They are shallow hardening and liable to distortion and quench cracking because of the severity of the water quench. List advantages of using air- and oil-hardening tool steels. - ANSWER Air- and oil-hardening steels are not so subject to distortion and cracking as W1 steels, and they are deep hardening. What is the correct temperature for quenching AISI C1095 tool steel? For any carbon steel? - ANSWER 1450F, 50F above the upper critical limit Why is steel tempered after it is hardened? - ANSWER Tempering is done to remove the internal stresses in martensite, which is brittle. the temperature used gives the best compromise between hardness and toughness or ductility. What factors should you consider when you choose the tempering temperature for a tool? - ANSWER Tempering temperature should be specified according to the hardness, strength, and ductility desired. Mechanical properties charts give these data. The approximate temperature for tempering a center punch should be ___. The oxide color will be ___. - ANSWER 525F, purple If a cold chisel became blue when the edge was ground on an abrasive wheel, to approximately what temperature was it raised? How would this temperature change affect the tool? - ANSWER 600F. It would be too soft for any cutting tool. How soon after hardening should you temper a part? - ANSWER Immediately. If you let it set for any length of time, it may crack from internal stresses. What is the advantage of using low-carbon steel for parts that are to be case-hardened? - ANSWER The low-carbon steel core does not harden when quenched from 1650F, so it remains soft and tough, but the case becomes hard. No tempering is therefore required, as the piece isn to brittle all the way through, as a fully hardened carbon steel would be. How can a deep case be made? - ANSWER A deep case can be made by pack carburizing or by a liquid bath carburizing. A relatively deep case is often applied by nitriding or by similar procedures. Are surface-hardened parts always case-hardened by carburizing? - ANSWER No. The base material must contain sufficient carbon to harden by itself without adding more for surface hardening. Name three methods by which carbon may be introduced into the surface of heated steel. - ANSWER Three methods of introducing carbon into heated steel are roll, pack, and liquid carbonizing. What method of case hardening uses ammonia gas? - ANSWER Nitriding. Name three kinds of furnaces used for heat-treating steels. - ANSWER Electric-, gas-, and oil-fired furnaces, and pot furnaces. What can happen to a carbon steel when it is heated to high temperatures in the presence of air (oxygen)? - ANSWER The surface decarburizes or loses surface carbon to the atmosphere as it combines with oxygen to form carbon dioxide. Why is it absolutely necessary to allow soaking time (which varies for different kinds of steels) before quenching the piece of steel? - ANSWER Dispersion of carbon atoms in the sold solution of austenite may be incomplete, so little or no hardening in the quench takes place as a result. Also, the center of a thick section takes more time to come to the austenitizing temperature. Why should the part or the quenching medium be agitated when you are hardening steel? - ANSWER Circulation or agitation breaks down the vapor barrier. This action allows the quench to proceed at a more rapid rate. Which method of tempering gives the heat treater the most control of the final product: by color or by furnace? - ANSWER By furnace. Describe two characteristics of quench cracking that would enable you to recognize them. - ANSWER The run from the surface toward the center of the piece. They fractured surfaces usually appear blackened. The surfaces have a fine crystalline structure. Name four or more causes of quench cracks. - ANSWER Overheating; wrong quench; wrong selection of steel; poor design; time delays between quench and tempering; wrong angle into quench; not enough material to grind off decarburization. In what ways can you avoid decarburizing a part when heating it in a furnace? - ANSWER Controlled atmosphere furnace; wrapping the piece in stainless feel foil; covering with cast iron chips Describe two types of surface failures of hardened steel when it is being ground. - ANSWER Changes in hardness of the surface area and the development of high internal stress during grinding When distortion must be kept to a minimum, which type of tool steel should be used? - ANSWER An air-hardened tool steel should be used when distortion must be kept to a minimum. When might normalizing be necessary? - ANSWER Medium-carbon steels that are not uniform and have hardened areas from welding or prior heat treating need to be normalized so they can be machined. Forgings, castings, and tool steel in the as-rolled condition are normalized before any further heat treatments or machining is done. At what approximate temperatures should you normalize .4 percent carbon steel? - ANSWER 1550F, 50F above the upper critical limit. What is the spheroidizing temperature of .8 percent carbon steel? - ANSWER The spheroidization temperature is quite close to the lower critical temperature line, about 1300F What is the essential difference between the full anneal and stress relieving? - ANSWER The full anneal brings carbon steel to its softest condition as all the grains are re-formed (recrystallized), and any hard carbide structures become soft pearlite as it slowly cools. Stress relieving will recrystallized only distorted ferrite grains and not the hard carbide structures or pearlite grains. When should you use stress relieving? - ANSWER Stress relieving should be used on severely cold-worked steels or for weldments. What kind of carbon steels would need to be spheroidized to give them free-machining qualities? - ANSWER High-carbon steels Explain process annealing. - ANSWER Process annealing is used by the sheet and wire industry, and is essentially the same as stress relieving. How should the piece be cooled for a normalizing heat treatment - ANSWER In still air. How should the piece be cooled for the full anneal? - ANSWER Slowly. Packed in insulating material or cooled in a furnace. What happens to machinability in low-carbon steels that are spheroidized? - ANSWER Low-carbon steels tend to become gummy when spheroidized, so the machinability is worse than in the as-rolled condition. Spheroidization sometimes is desirable when stress relieving weldments on low-carbon steels. What one specific category of the property of hardness do the Rockwell and Brinell hardness testers use and measure? How is it measured? - ANSWER Resistance to penetration is the one category utilized by the Rockwell and Brinell testers. The depth of penetration is measured when the mayo load is removed on the Rockwell tester, and the diameter of the impression is measure to determine a Brinell hardness number. State the relationship that exists between hardness and tensile strength. - ANSWER As the hardness of metal increases, the strength increases [Show More]

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