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· Read chapter 05 of Kalpakjian, Serope. Manufacturing Engineering and Technology
Englewood Cliffs, NJ: Prentice Hall – Fourth Edition, 2001. ISBN # 0-201-36131-0
· Complete the homework assignment below.
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ManufacturingProcesses - INMT-1248 - Assignment 05
QUESTIONS
5.1 What are the major categories of ferrous alloys?
Answer (from page #__137___):
Carbon and alloy steels, stainless steels, tool and die steels, cast irons, and cast steels.
5.2 List the basic raw materials used in making iron and steel, and explain their functions.
Answer (from page #__138___):
The basic raw materials are Iron ore, limestone, and coke.
The principal iron ores are taconite (a black flintlike rock), hematite (an iron oxide mineral), and limonite (an iron oxide containing water).
The function of limestone (calcium carbonate) is to remove impurities from the molten iron.
Coke has several functions in steelmaking. One is to generate the high level of heat required for the chemical reactions in iron making to take place. A second is to produce carbon monoxide (a reducing gas, meaning that it removes oxygen), which is then used to reduce iron oxide to iron.
5.3 List the types of furnaces commonly used in steelmaking and describe their characteristics.
Answer (from page #__139, 140___):
The molten metal from the blast furnace is transported into one of the three types of furnaces: Open-hearth, electric, or basic oxygen.
Open-hearth derives from the shallow hearth shape that is directly open to the flames that melt the metal.
Electric furnace: The source of heat is continuous electric arc that is formed between the electrodes and the charged metal.
Basic-oxygen furnace: is the newest and fastest steel-making process. Typically, 200 tons of molten pig iron and 90 tons of scrap are charged (fed) into a vessel. Pure oxygen is then blown into the furnace for about 20 minutes through a water-cooled lance (which is a long tube), under pressure of about 1250 Kpa (180 psi). The BOF process is capable of refining 250 tons of steel in 35 to 50 minutes.
5.4 List and explain the characteristics of the types of steel ingots.
Answer (from page #__141, 142___):
Three types of steel ingots can be produced: Killed, semi-killed, and rimmed.
Killed steel is fully deoxidized steel; that is, oxygen is removed and porosity is thus eliminated.
Semi-killed steel is partially deoxidized steel. It contains some porosity (generally in the upper central section of the ingot), but it has little or no pipes; as a result, scrap is reduced. Semi-killed steels are economical to produce.
Rimmed steel, which is generally has a low carbon content (less than 0.15%), then evolved gases are “killed” (or controlled) only partially, by the addition of elements such a aluminum. The gases produce blowholes along the outer rim of the ingot-hence the term rimmed.
5.5 What does refining mean? How is it done?
Answer (from page #_142____):
The removal of impurities is known as refining; much of its done in melting furnaces or in ladles, by means of the addition of various elements.
5.6 What advantages does continuous casting have over casting into ingots?
Answer (from page #_142,143____):
Continuous-casting processes, which produces higher-quality steels at reduced cost. In additions to costing less, continuously cast metals have more uniform compositions and properties than those obtained by ingot casting.
5.7 Name the four alloying elements that have the greatest effect on the properties of steels.
Answer (from page #__144___):
Boron improves hardenability, without the loss of (or even with some improvement in) machinability and formability.
Calcium deoxidizes steels, improves toughness, and may improve formability and merchantability.
Carbon improves hardenability, strength, hardness, and wear resistance; it reduces ductility, weldability, and toughness.
Cerium controls the shape of inclusions and improves toughness in high-strength, low-alloy steels; it deoxidizes steels.
5.8 What are trace elements?
Answer (from page #__145___):
Residual elements are also unknown as trace elements.
5.9 What are the percentage carbon contents of lowcarbon, mediumcarbon, and highcarbon steels?
Answer (from page #_146____):
Low-carbon, also called mild steel has less than 0.30% carbon.
Medium-carbon steel has 0.30% to 0.60% carbon.
High-carbon steel has more than 0.60% carbon.
5.10 How do stainless steels become stainless?
Answer (from page #_148____):
Primarily their corrosion resistance, high strength and ductility, and high chromium content characterize stainless steels. They are called stainless because in the presence of oxygen (air) they develop a thin, hard adherent film of chromium oxide that protects the metal from corrosion (passivation). This protective film builds up again in the event that the surface is scratched.
5.11 List the types of stainless steels.
Answer (from page #_148, 149____):
A. Austenitic (200 and 300 series).
B. Ferritic (400 series).
C. Martensitic (400 and 500 series).
D. Precipitation-hardening (PH).
E. Duplex structure.
5.12 What are the major alloying elements in too] and die steels and highspeed steels?
Answer (from page #_150, 151____):
Molybdenum and tungsten.
5.13 How does chromium affect the surface characteristics of stainless steels?
Answer (from page #__148___):
Chromium protects the metal from corrosion (passivation). This protective film builds up again in the event that the surface is scratched.
5.14 What kind of furnaces are used to refine steels?
Answer (from page #__139___):
The molten metal from the blast furnace is transported into on of three types of furnaces: open-hearth, electric, or basic oxygen.
5.15 How is iron extracted from ores?
Answer (from page #_138____):
The ore is crushed into fine particles, the impurities are removed by various means such as magnetic separation, and the ore is formed into pellets, balls, or briquettes, using binders and waters.
5.16 What is highspeed steel?
Answer (from page #__151___):
High-speed steels (HSS) are the most highly alloyed tool and die steels. They maintain their hardness and strength at elevated operating temperatures. There are two basic types of high-speed steels: the molybdenum type (M series) and the tungsten type (T series).
5.17 What purpose does a tundish serve in continuous casting?
Answer (from page #__142___):
In the continuous casting the metal is poured into a refractory-lined intermediate pouring vessel (tundish), where impurities are skimmed off.
5.18 Why is limestone used in iron production?
Answer (from page #_138____):
The function of a limestone (calcium carbonate) is to remove impurities from the molten iron. The limestone reacts chemically with impurities, acting like a flux (meaning to flow as a fluid) that causes the impurities to melt at low temperature.
5.19 Where does the term "pig iron" come from?
Answer (from page #__138___):
Use of word pig comes from the early practice of pouring the molten iron into small sand molds, arranged like a litter of small pigs around a main channel.
5.20 What are the advantages of a high carbon content in steel? What is the advantage of a low carbon content?
Answer (from page #__144___):
Higher the carbon content, the greater the hardenability of the steel and the greater its strength, hardness, and wear resistance. On the other hand, ductility, weldability, and toughness are reduced with increasing carbon content.
======End of Homework Assignment ======
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