- # PAGE #6# -ter 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, ferrite stainless steels (also 300 series) have good machinability. Martensitic (400 series) steels are abrasive, tend to form a built-up edge, and require tool materials with high hot hardness and crater-wear resistance. Precipitation-hardening stainless steels are strong and abrasive, requiring hard and abrasion-resistant 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 combine with oxygen to form aluminum oxide and silicates,which are hard and abrasive. These compounds 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 composition. Plain low-carbon steels (less than 0.15% C) can produce poor surface finish by forming a built-up 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 built-up edge formation. Otheralloying 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; the higher the oxygen content, the lower the aspect ratio and the higher the machinability. In selecting various elements to improve machinability, we should consider the possible detrimental effects of these elements on the properties and strength of the machined part in service. At elevated temperatures, for example, lead causes embitterment of steels (liquid-metal embitterment, hot shortness; see Section 1.4.3), although at room temperature it has no effect on mechanical properties. Sulfur can severely reduce the hot workability of steels, because of the formation of iron sulfide, unless sufficient manganese is present to prevent such formation. At room temperature, the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese sulfide inclusions (anisotropy). Rephosphorized steels are significantly less ductile, and are produced solely toimprove machinability. o machine than brass. Magnesium is very easy to machine, with good surface finish and prolonged tool life. However care should be exercised because of its high rate of oxidation and the danger of fire (the element is hydrophobic). Molybdenum is ductile and work-hardening, so it can produce poor surface finish. Sharp tools are necessary. Nickel-based alloys are work-hardening, abrasive, and strong at high temperatures. Their machinability is similar to that of stainless steels. Tantalum is very work-hardening, ductile, and soft. It produces a poor surface finish; tool wear is high. Titanium and its alloys have poor thermal conductivity (indeed, the lowest of all metals), causing significant temperature rise and built-up edge; they can bedifficult to machine. Tungsten is brittle, strong, and very abrasive, so its machinability is low, although it greatly improves at elevated temperatures. Zirconium has good machinability. It requires a coolant-type cutting fluid, however, because of the explosion and fire. To relieve these stresses, machined parts can be annealed for a period of time at temperatures ranging from C°80 to C°160 (F°175 to F°315), and then cooled slowly and uniformly to room temperature. Thermosetting plastics are brittle andsensitive to thermal gradients during cutting. Their machinability is generally similar to that of thermoplastics. Because of the fibers present, reinforced plastics are very abrasive and are difficult to machine. Fiber tearing, pulling,and edge delaminating are significant problems; they can lead to severe reduction in the load-carrying capacity of the component. Furthermore, machiningof these materials requires careful removal of machining debris to avoid contactwith and inhaling of the fibers. The machinability of ceramics has improved steadily with the development of nanoceramics (Section 8.2.5) and with the selection of appropriate processing parameters, such as ductile-regime cutting (Section 22.4.2). Metal-matrix and ceramic-matrix composites can be difficult tomachine, depending on the properties of the individual components, i.e., reinforcing or whiskers, as well as the matrix material. 因素的方式定義：1、分的表面光潔性和表面完整性。2、刀具的壽命。3、切削力和功率的需求。4、切屑控制。以這種方式，好的可機(jī)加工性指的是好的表面光潔性和完整性，長的刀具壽命，低的切削力和功率需求。關(guān)于切屑控制，細(xì)長的卷曲切屑，如果沒有被切割成小片，以在切屑區(qū)變的混亂，纏在一起的方式能夠嚴(yán)重的介入剪切工序。因?yàn)榧羟泄ば虻膹?fù)雜屬性，所以很難建立定量地釋義材料的可機(jī)加工性的關(guān)系。在制造廠里，刀具壽命和表面粗糙度通常被認(rèn)為是可機(jī)加工性中最重要的因素。盡管已不再大量的被使用，近乎準(zhǔn)確的機(jī)加工率在以下的例子中能夠被看到。，但也有磨蝕性。鑄造無中的 低 的可機(jī)加工性， 起刀具切屑 。 需要具有 性的工具。具有 的刀具材料的 鑄 和 性 是可加工的。 有磨蝕性 度加工 的。 要求的 具有 蝕性的刀具材料 有低的 刀和 度。盡管鑄 很 機(jī)加工，但因?yàn)? 的￠屑￡?成因￥? 很難機(jī)加工。? 很 機(jī)加工，§currency1是有'加的“? 。? ?? ?難機(jī)加工。fi很 機(jī)加工，fifl有很好的表面光潔性和長 的刀具壽命。–￥，因?yàn)?的? 度和?種的· ?這種?素 ??，因”? …‰§currency1小 使用 。? 長 加工 ，因” `成很′的表面光潔性。 的刀具是很?要的。? 加工 ，具有磨蝕性， 在 ˉ下?常 。 的可機(jī)加工性和不˙¨ ?。??常的加工 ，具有可 性 ??。 `成很′的表面光潔性 刀具磨ˇ?常大?！?的 性(的確，是所有 屬中最低的)，因” 起 的ˉ度 和￠屑￡。 是難機(jī)加工的。 ， ， 具有磨蝕性，因”盡管的性能在 ˉ下能大大 ，但 的機(jī)加工性 很低。 有很好的機(jī)加工性。–￥，因?yàn)橛?和?種的·性， 要求有一 性 好的切削?。