毕业论文翻译.doc

翻译：The intense heat involved in the welding process influences the microstructure of both the weld metal and the parent metal close to the fusion boundary (the boundary between solid and liquid metal). As such, the welding cycle influences the mechanical properties of the joint.The molten weld pool is rapidly cooled since the metals being joined act as an efficient heat sink. This cooling results in the weld metal having a chill cast microstructure. In the welding of structural steels, the weld filler metal does not usually have the same composition as the parent metal. If the compositions were the same, the rapid cooling could result in hard and brittle phases, e.g. martensite, in the weld metal microstructure. This problem is avoided by using weld filler metals with a lower carbon content than the parent steel. 资料来源 360毕业设计网 The parent metal close to the molten weld pool is heated rapidly to a temperature which depends on the distance from the fusion boundary. Close to the fusion boundary, peak temperatures near the melting point are reached, whilst material only a few millimetres away may only reach a few hundred degrees Celsius. The parent material close to the fusion boundary is heated into the austenite phase field. On cooling, this region transforms to a microstructure which is different from the rest of the parent material. In this region the cooling rate is usually rapid, and hence there is a tendency towards the formation of low temperature transformation structures, such as bainite and martensite, which are harder and more brittle than the bulk of the parent metal. This region is known as the heat affected zone (HAZ).The microstructure of the HAZ is influenced by three factors:1. The chemical composition of the parent metal. 2. The heat input rate during welding. 3. The cooling rate in the HAZ after welding.The chemical composition of the parent metal is important since it determines the hardenability of the HAZ. The heat input rate is significant since it directly affects the grain size in the HAZ. The longer the time spent above the grain coarsening temperature of the parent metal during welding, the coarser the structure in the HAZ. Generally, a high heat input rate leads to a longer thermal cycle and thus a coarser HAZ microstructure. It should be noted that the heat input rate also affects the cooling rate in the HAZ. As a general rule, the higher the heat input rate the lower the cooling rate. The value of heat input rate is a function of the welding process parameters: arc voltage, arc current and welding speed. In addition to heat input rate, the cooling rate in the HAZ is influenced by two other factors. First, the joint design and thickness are important since they determine the rate of heat flow away from the weld during cooling. Secondly, the temperature of the parts being joined, i.e. any pre-heat, is significant since it determines the temperature gradient which exists between the weld and parent metal.The intense heat associated with welding causes the region of the weld to expand. On cooling contraction occurs. This expansion and subsequent contraction is resisted by the surrounding cold material leading to a residual stress field being set up in the vicinity of the weld. Within the weld metal the residual stress tends to be predominantly tensile in nature. This tensile residual stress is balanced by a compressive stress induced in the parent metal 2. A schematic view of the residual stress field obtained for longitudinal weld shrinkage is shown in Figure 3. The tensile residual stresses are up to yield point in magnitude in the weld metal and HAZ. It is important to note that the residual stresses arise because the material undergoes local plastic strain. This strain may result in cracking of the weld metal and HAZ during welding, distortion of the parts to be joined or encouragement of brittle failure during service.Transverse and longitudinal contractions resulting from welding can lead to distortion if the hot weld metal is not symmetrical about the neutral axis of a fabrication 2. A typical angular rotation in a single V butt weld is shown in Figure 4a. The rotation occurs because the major part of the weld is on one side of the neutral axis of the plate, thus inducing greater contraction stresses on that side. This leads to a distortion known as cusping in a plate fabrication, as shown in Figure 4b. Weld distortion can be controlled by pre-setting or pre-bending a joint assembly to compensate for the distortion or by restraining the weld to resist distortion. Examples of both these methods are shown in Figure 5. Distortion problems are most easily avoided by using the correct weld preparation. The use of non-symmetrical double sided welds such as those shown in Figure 2e and 2i accommodates distortion. The distortion from the small side of the weld (produced first) is removed when the larger weld is put on the other side. This technique is known as balanced welding.在焊接过程介入的酷热影响焊接金属和接近融合界限(在坚实和液体金属之间的界限的基体金属)微结构。 同样地，焊接周期影响联接的机械性能。溶解的焊接水池，因为被加入的金属作为一台高效率的吸热器，迅速地冷却。 这在有的焊接金属的冷却的结果冷颤熔铸了微结构。 在结构钢焊接，焊接填充金属通常没有构成和基体金属一样。 如果构成是相同的，急流冷却可能导致坚硬和易碎的阶段，即马氏体，在焊接金属微结构。 这个问题比父母钢被避免通过使用焊接与一种更低的含炭成分的填充金属。接近溶解的焊接水池的基体金属迅速地被加热对取决于从融合界限的距离的温度。 接近融合界限，材料外仅一些毫米也许只到达几百摄氏度，在熔点附近的高峰温度被到达。 接近融合界限的原材料是激昂入奥氏体阶段领域。 在冷却，这个区域变换对是与原材料的其余不同的微结构。 在这个区域冷却的率通常是迅速的，并且有往低温变革结构的形成的一个倾向，例如白氏体和马氏体，比基体金属的大多数坚硬和易碎。 这个地区叫作热影响区(HAZ)。三个因素影响HAZ的微结构：1. 基体金属的化学成分。 2. 在焊接期间的供热率。 3. 在HAZ的冷却的率在焊接以后。因为它确定HAZ的淬硬性，基体金属的化学成分是重要的。 因为它直接地影响在HAZ的粒度供热率是重大的。 越长在基体金属之上在焊接期间，越粗糙的五谷变粗的温度花费的时间在HAZ的结构。 通常，高温输入率导致一个更长的热量周期和因而一个更加粗糙的HAZ微结构。 值得注意的是，供热率也影响在HAZ的冷却的率。 通常，越高供热率越低冷却的率。 供热率的价值是焊接过程参数的作用： 弧电压，形成弧光潮流和焊接速度。 除供热率之外，其他二个因素影响在HAZ的冷却的率。 首先，因为他