1、毕 业 设 计（论 文）外 文 参 考 译文题目：Principle of Heat Treatment of Steal 钢的热处理原则学生姓名： 学 号： 专 业： 所在学院： 指导教师： 职 称： 20xx年 2月 27日Principle of Heat Treatment of StealThe role of heat treatment in modern mechanical engineering cannot be overestimated. The changes in the properties of metals due to heat treatment are
2、 of extremely great significance.1、 Temperature and TimeThe purpose of any heat treating process is to produce the desired changes in the structure of metal by heating to a specified temperature and by subsequent cooling.Therefore, the main factors acting in heat treatment are temperature and time,
3、so that any process of heat treatment can be represented in temperaturetime () coordinates.Heat treatment conditions are characterized by the following parameters: heating temperature, i.e. the maximum temperature to which an alloy metal is heated；time of holding at the heating temperature; heating
4、rate and cooling rate.If heating (or cooling) is made at a constant rate, the temperature-time relationship will be described by a straight line with a respective angle of incline.With a varying heating (or cooling) rate, the actual rate should be attributed to the given temperature, more strictly,
5、to an infinite change of temperature and time: that is the first derivative of temperature in time:.Heat treatment may be a complex process, including multiple heating stages, interrupted or stepwise heating (cooling), cooling to subzero temperature, etc. Any process of heat treatment can be describ
6、ed by a diagram in temperature-time coordinates.2、Formation of AusteniteThe transformation of pearlite into austenite can only take place at the equilibrium critical point on a very slow heating as follows from the Fe-C constitutional diagram. Under common conditions, the transformation is retarded
7、and results in overheating, i.e. occurs at temperatures slightly higher than those indicated in the Fe-C diagram.When overheated above the critical point, pearlite transforms into austenite, the rate of transformation being dependent on the degree of overheating.The time of transformation at various
8、 temperatures (depending on the degree of overheating) shows that the transformation takes place faster (in a shorter time) at a higher temperature and occurs at a higher temperature on a quicker heating.For instance, on quick heating and holding at 780, the pearlite to austenite transformation is c
9、ompleted in 2 minutes and on holding at 740, in 8 minutes.The end of the transformation is characterized by the formation of austenite and the disappearance of pearlite (ferrite + cementite). This austenite is however inhomogeneous even in the volume of a single grain. In places earlier occupied by
10、lamellae (or grains) of a pearlite cementite, the content of carbon is greater than in places of ferritic lamellae. This is why the austenite just formed is inhomogeneous.In order to obtain homogeneous austenite, it is essential on heating not only to pass through the point of the end of pearlite to
11、 austenite transformation, but also to overheat the steel above that point and to allow a holding time to complete the diffusion processes in austenitic grains.The rate of homogenization of austenite appreciably depends on the original structure of the steel, in particular on the dispersion and part
12、icle shape of cementite. The transformations described occur more quickly when cementite particles are fine and, therefore, have a large total surface area.3、Coarsening of Austenite GrainsAt the beginning of pearlite to austenite transformation, the grains of austenite form at the boundaries between
13、 the ferrite and cementite the two structural constituents of pearlite. Since these boundaries are very developed, the transformation starts from formation of a multitude of fine grains. Therefore at the end of the transformation the austenite will be composed of a great multitude of fine grains who
14、se size characterize what is called the original austenitic grains size.Further heating (or holding) upon the transformation will cause coarsening of austenitic grains. The process of grain coarsening is spontaneous, since the total surface area of grains diminishes (the surface energy decreases) an
15、d a high temperature can only accelerate the rate of this process.In that connection, two types of steels are distinguished: inherent fine grained and inherent coarse grained, the former being less liable to grain coarsening than the latter. The size of grains formed in a steel by heat treatment is called the actual grain size.Thus, a distinction should be made between: (1) original grain, i.e. the size of austenitic grains immediately after the pearlite to austenite transformation; (2) inherent (natural) grain, i.e. the liability of austenite to grain coarseni
