Heat Treatment& Hot Working of Metals,Unit 4,金属的热处理和热加工,Heat Treatment and Working Equipment,Testing Before Class：,What the meaning of “Shaping”? (radio),Heat Treatment& Hot Working of Metals,Unit 4 Heat Treatment& Hot Working of Metals We can alter the characteristics of steel in various ways. In the first place, steel which contains very little carbon, will be milder than steel, which contains a higher percentage of carbon, up to the limit of about 1.5. Secondly, we can heat the steel above a certain critical temperature, and then allow it to cool at different rates. At this critical temperature, changes begin to take place in the molecular structure of the meta1. In the process known as annealing we heat the steel above the critical temperature and permit it to cool very slowly. This causes the metal to become softer than before, and much easier to machine.,Annealing has a second advantage. It helps to relieve any internal stresses which exist in the metal. These stresses are liable to occur through hammering or working the metal, or through rapid cooling. Metal which we cause to cool rapidly contracts more rapidly on the outside than on the inside. This produces unequal contraction, which may give rise to distortion or cracking. Metal which cools slowly is less liable to have these internal stresses than metal, which cools quickly.,On the other hand, we can make steel harder by rapid cooling. We heat it up beyond the critical temperature, and then quench it in water or some other liquid. The rapid temperature drop fixes the structural change in the steel, which occurred at the critical temperature, and makes it very hard. But a bar of this hardened steel is more liable to fracture than normal steel. We therefore heat it again to a temperature below the critical temperature, and cool it slowly. This treatment is called tempering. It helps to relieve the internal stresses, and makes the steel less brittle than before. The properties of tempered steel enable US to use it in the manufacture of tools, which need a fairly hard steel. High carbon steel is harder than tempered steel, but it is much more difficult to work.,These heat treatments take place during the various shaping operations. We can obtain bars and sheets of steel by rolling the metal through huge rolls in a rolling mill. The roll pressures must be much greater for cold rolling than for hot rolling, but cold rolling enables the operators to produce rolls of great accuracy and uniformity, and with a better surface finish. Other shaping operations include drawing into wire, casting in moulds, and forging.,The mechanical working of metal is the shaping of metal in either a cold or a hot state by some mechanical means. This does not include the shaping of metals by machining or grinding, in which processes metal is actually machined off, nor does it include the casting of molten metal into some form by use of molds. In mechanical working processes, the metal is shaped by pressure-acutely forging, bending, squeezing, drawing, or shearing it to its final shape. In these processes the metal may be either cold or hot worked. Although normal room temperatures are ordinarily used for cold working of steel, temperatures up to the re-crystallization range are sometimes used. Hot working of metals takes place above the re-crystallization or workhardening range.,For steel, re-crystallization starts around 650 to 700C, although most hot work on steel is done at temperatures considerably above this range. There is no tendency for hardening by mechanical work until the lower limit of the re-crystalline range is reached. Some metals, such as lead and tin, have a low re-crystalline range and can be hot worked at room temperature, but most commercial metals require some heating. Alloy composition has a great influence upon the proper working range, the usual result being to raise the re-crystalline range temperature. This range may also be increased by prior cold working.,molecular mlekjl a. 由分子组成的, 分子的 annealing ni:li n.(低温)退火，焖火，磨炼 relieve rili:v v.解除，减轻，使不单调乏味 liable laibl a.有责任的，有义务的，应受罚的，有倾向的 contract kntrkt v. 收缩，缩紧，感染，订约 rise raiz v. 上升，增强，起立，高耸 shaping eipi n. 成形，造型，塑造 mechanical miknikl a. 机械的，呆板的，力学的 machining mi:ni n. 切肖，制造，机械加工 grinding raindi n. 磨肖力工 machined mi:nd a. 机械加工的 pressure pre n. 压力，压强 bend bend v. 使弯曲，使屈服，使致力，使朝向 bending bendi n. 弯曲 (度)，扭曲(度),New words and phrases,squeeze skwi:z v.挤 (压，干)，压榨，勒索，紧握 prior prai a. 在先的，在前的，优先的 distortion dist:n n. 变形，扭曲，曲解，失真 crack krk v. &n破裂，裂纹，打开 beyond bijnd prep.超过的范围, 超过, 越过 quench kwent v. &n熄灭，机 淬火, 解渴； fix fiks v. & n. 安装, 修理，使固定，准备 tempering tempri v. 回火 rolls ruls n. 薄板卷，泛指钢材 draw dr: v.拉, 画, 吸引 drawing dr:i n. 冲压成形 shear i v. 剪断，切断，修剪，剥夺 recrystallization ri:,kristlaizein n.再结晶 hardening h:dni n.硬化，淬火, 锻炼 crystalline kristlain a. 结晶的凡结晶体 influence influ