1、附录 5 Ferrous Metals Metals are divided into two general groups: ferrous metals and nonferrous metals. Ferrous metals are those metals whose major element is iron. The major types of ferrous metals arc irons, carbon steels, alloy steels and tool steels. Iron The iron ore which we find in the earth is
2、 not pure. It contains some impurities which we must remove by smelting. The process of smelting consists of heating the ore in a blast furnace with coke and limestone, and reducing it to metal. Blasts of hot air enter the furnace from the bottom and provide the oxygen which is necessary for the red
3、uction of the ore, The ore becomes mohen, and its oxide combines with carbon kom the coke. The non-metallic constituents of there combine with the limestone to form a liquid slag. This floats on top of the molten iron, and passes out of the furnace through a tap. The metal which remains is pig-iron,
4、 and consists of approximately 93 percent iron, 5 percent carbon, and 2 percent impurities. Remehing pig iron and scrap iron in a furnace to remove some of the impurities produces cast iron. The type, or grade, of cast iron is determined by the extent of refining, the amounts of pig iron and scrap i
5、ron, and the methods used to cast and cool the metal. The three primary types of cast iron are gray cast iron, white cast iron, and malleable iron. Gray cast iron is primarily used for cast frames, automobile engine blocks, handwheel, and east housings. White cast iron is hard and wear resistant and
6、 is used for parts such as train wheels. Malleable cast iron is a tough material used for tools such as pipes and wrenches. Generally, cast irons have very good compressive strength, corrosion resistance, and good machinability. The main disadvantage of cast iron is its natural brittleness. Carbon S
7、teel Carbon steel is made from pig iron that has been refined and cleaned of most impurities. Most of the original carbon in the metal is burned out during the refining process. Measured amounts of carbon are then added to the molten metal to produce the exact grade of carbon steel desired. After th
8、e steel is poured into ingots and allowed to cool, it is usually sent to a rolling mill to be rolled and formed into specific shapes. The three principal types of carbon steel used in industry are low, medium, and high carbon steel. The percentage of carbon :is the most important factor in determini
9、ng the mechanical properties of each type of carbon steel. : Low carbon contains between 0. 05% and O. 30% carbon and is primarily used for parts that do not require great strength. Typical uses of low carbon steel include chains, bolts, screws, washers, nuts, pins, wire, shafting, and pipes. This m
10、etal is also known as machine steel, mild steel, and cold-rolled steel. Low carbon steel is tough, ductile material that is easily machined and welded. It is useful for parts that must be stamped or formed. Containing between 0. 30 and 0.50% carbon, medium carbon steel is used for parts that require
11、d great strength than is possible with low carbon steel, such as gears, crankshafts, machine parts and axles. Because this steel has higher carbon content, it can be heat-treated to increase both hardness and wear resistance. Medium carbon steel is a tough, hardenable metal that has good machinabili
12、ty and is easily welded. Containing between 0.50 and 1.70% carbon, high carbon steel is used for parts that require hardness and strength, such as files, knives, drills, razors, and woodworking tools. Due to their increased carbon content, high carbon steels can be heat-treated to make them harder a
13、nd more wear resistant than low or medium carbon steels. Due to their great hardness, high carbon steels are often brittle. Alloy Steels Alloy steels are basically carbon steels with elements added to modify of change the mechanical properties of the steel. All steels are alloy steels because each i
14、s a combination of elements, including carbon steel, a mixture of iron and carbon. To identify the two groups, one is called carbon or plain steel and the other is referred to as alloy steel. Alloying elements are added to the molten steel in measured amounts. The desired end product determines the
15、elements and amounts added. The primary alloying elements and their effect on the steel are as follows: Boron The hardenability of an alloy is increased by boron. Only very small amounts of boron are needed to increase the hardenability characteristics of the other elements in the alloy. Chromium Wh
16、en used in small amount, chromium increases the depth hardness of the metal. The more chromium used, the better the alloy resists corrosion. Chromium is a principal element in stainless steels. Cobalt Cobalt is added to an alloy to increase wear resistance and increase red hardness, which is the abi
17、lity of a metal to maintain a cutting edge at elevated temperature. Cobalt is a valuable addition to some high-speed tool steels. Lead By reducing the cutting friction, lead improves machinability. Leaded steels also have good weldability and formability. Manganese Impurities in alloy steels are con
18、trolled by using manganese as a purifier and scavenger. When added in larger amount ( 1 to 15 percent) , manganese produces good hardness and wear resistance. Molybdenum A tough alloy suitable for a wide range of high-strength applications, molybdenum steel permits good depth hardness and strength a
19、t elevated temperatures. Nickel High-strength alloys resistant to both elevated temperatures and corrosion are produced by nickel. When alloyed with molybdenum, nickel steel becomes a very tough alloy, which is often used for many aircraft parts. Larger amounts of nickel greatly add to the corrosion
20、 resistance of stainless steels. Phosphorus and Sulfur Free-machining carbon steels are produced with phosphorus and sulfur. When alloyed with carbon steels, phosphorus and suffer produce alloys with excellent machining characteristics. Tungsten When alloyed with steel, tungsten produces a variety o
21、f high-speed tool steels and adds hardenability and strength at elevated temperatures as well as high resistance to wear. Vanadium A tough, fine-grained steel that acts as a cleanser and purifier to eliminate many of the impurities of steel is produced by vanadium. Tool Steels Tool steels are a spec
22、ial grade of alloy steels used for making a wide variety of tools. Depending on their composition, tool steels are highly resistant to wear, shocks, and heat. These alloys gener ally contain more carbon, tungsten, and cobalt than do the standard alloy steels, i41 Another principal difference between most alloy steels and tool steels is the control with which elements are added.Tool steels are made with much closer quality controls than are other alloy steels.
