《机械工程专业英语》李光布 饶锡新 Lesson 2 Carbon and Alloy Steel 碳钢合金钢.docVIP

Lesson 2 Carbon and Alloy Steel TEXT Steel is probably the most widely used material for machine elements because of its properties of high strength, high stiffness, durability and relative ease of fabrication. The term steel refers to and alloy of iron, carbon, manganese and one or more other significant elements. Carbon has a very strong effect on the strength, hardness and ductility of any steel alloy. The other elements affect hardenability, toughness, corrosion resistance, machinability and strength retention at high temperatures. The primary alloying elements present in the various alloy steels are sulfur, phosphorus, silicon, nickel, chromium, molybdenum and vanadium. Importance of Carbon Although most steel alloys contain less than 1.0% carbon, it is included in the designation because of its effect on the properties of steel. As Figure 1.2 illustrates, the last tow digits indicate carbon content n hundredths of a percent. As carbon content increases, strength and hardness also increase under the same conditions of processing and heat treatment. Since ductility decreases with increasing carbon content, selecting suitable steel involves some compromise between strength and ductility. As a rough classification scheme, a low-carbon steel is one having fewer than 30 points of carbon (0.30%). These steels have relatively low strength but good formability. In machine element applications where high strength is not required, low-carbon steels are frequently specified. If wear is a potential problem, low-carbon steels can be carburized to increase the carbon content in the very outer surface of the part and to improve the combination of properties. Medium-carbon steels contain 30 to 50 points of carbon (0.30%-0.50%). Most machine elements having moderate to high strength requirements with fairly good ductility and moderate hardness requirements come from this group. High-carbon steels have 50 to 95 points of carbon (0.50%-0.95%). The high carbon content provides