【正文】 the letter L between the second and third numerals (for example, 10L45). (Note that in stainless steels, similar use of the letter L means “l(fā)ow carbon,” a condition that improves their corrosion resistance.) However, because lead is a wellknown toxin and a pollutant, there are serious environmental concerns about its use in steels (estimated at 4500 tons of lead consumption every year in the production of steels). Consequently, there is a continuing trend toward eliminating the use of lead in steels (leadfree steels). Bismuth and tin are now being investigated as possible substitutes for lead in steels. CalciumDeoxidized Steels. An important development is calciumdeoxidized steels, in which oxide flakes of calcium silicates (CaSo) are formed. These flakes, in turn, reduce the strength of the secondary shear zone, decreasing toolchip interface and wear. Temperature is correspondingly reduced. Consequently, these steels produce less crater wear, especially at high cutting speeds. Stainless Steels. Austenitic (300 series) steels are generally difficult to machine. Chatter can be s problem, necessitating machine tools with high stiffness. However, ferritic stainless steels (also 300 series) have good machinability. Martensitic (400 series) steels are abrasive, tend to form a builtup edge, and require tool materials with high hot hardness and craterwear resistance. Precipitationhardening stainless steels are strong and abrasive, requiring hard and abrasionresistant tool materials. The Effects of Other Elements in Steels on Machinability. The presence of aluminum and silicon in steels is always harmful because these elements bine with oxygen to form aluminum oxide and silicates, which are hard and abrasive. These pounds increase tool wear and reduce machinability. It is essential to produce and use clean steels. Carbon and manganese have various effects on the machinability of steels, depending on their position. Plain lowcarbon steels (less than % C) can produce poor surface finish by forming a builtup edge. Cast steels are more abrasive, although their machinability is similar to that of wrought steels. Tool and die steels are very difficult to machine and usually require annealing prior to machining. Machinability of most steels is improved by cold working, which hardens the material and reduces the tendency for builtup edge formation. Other alloying elements, such as nickel, chromium, molybdenum, and vanadium, which improve the properties of steels, generally reduce machinability. The effect of boron is negligible. Gaseous elements such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel. Oxygen has been shown to have a strong effect on the aspect ratio of the manganese sulfide inclusions。機(jī)械工業(yè)出版社 2004年3月第1版 MACHINABILITY The machinability of a material usually defined in terms of four factors: Chip control. Thus, good machinability good surface finish and integrity, long tool life, and low force And power requirements. As for chip control, long and thin (stringy) cured chips, if not broken up, can severely interfere with