机电技术专业英语 教学课件 作者 徐起贺 张庆良pata b lesson three.pptVIP

* 机电技术专业英语 Lesson Three Text A machinist will often be required to machine steels that have been previously heattreated. It is important, therefore, that they be able to recognize that the steel has been heat treated and to understand how the treatment changed the steel. Armed with this knowledge the machinist can select the best machinery, cutting tool, and cutting conditions. If a machinist works as a tool and die maker they will be working quite a bit with tool steels. These metals are machined in a soft condition, heattreated to gain hardness, and then finish machined with a grinding or lapping process that can handle the hardness1. Heat Treatments Lesson Three Other times a machinist may need to work on a weldment that (due to the welding process) has many internal stresses. They must stress relieve the weldment prior to machining to prevent distortion after machining. In this case the heat treating process is not to make the workpiece harder. It is to make the workpiece more machinable and, in truth, makes the metal a bit softer. Regardless of the reason for heat treating, the basic process is the same and has three steps: (1) Heat the metal to a specific temperature. (2) Hold the metal at that temperature for a specific amount of time. (3) Cool the metal in a specific manner. Lesson Three New Words and Expressions Lesson Three Lesson Three Notes 1. These metals are machined in a soft condition, heattreated to gain hardness, and then finish machined with a grinding or lapping process that can handle the hardness. 这些金属在较软的状态下加工，并且通过热处理提高硬度，然后由能够加工这个硬度的磨削或研磨工艺完成对金属的加工。 在这个句子中，“are machined”用的是被动语态，后面的“heattreated”与“machined”是相同的结构和成分，句子在“heattreated”前省略了连词and和系动词are。that引导了一个定语从句，process为定语从句的先行词。 2. A material can be stress relieved by heating it to a specific temperature that is lower than that of annealing or normalizing and letting it cool to room temperature inside or outside of the oven.