结构钢的焊接性外文文献翻译@中英文翻译@外文翻译

1 2012届毕业设计外文翻译 Weldability of Structural Steels 焊接结构钢 学生姓名： 侯林珠 指导教师： 崔晓东、任芝兰 职 称： 工程师、副教授 专 业： 机械设计制造及其自动化 班 级： 机本 0804 班 完成时间： 2012 年 5 月 2 附录 1 英文原文 Lecture 2.6: Weldability of Structural Steels The lecture briefly discusses the basics of the welding process and then examines the factors governing the weldability of structural steels. SUMMARY The fundamental aspects of the welding process are discussed. The lecture then focuses on the metallurgical parameters affecting the weldability of structural steels. A steel is considered to exhibit good weldability if joints in the steel possess adequate strength and toughness in service. Solidification cracking, heat affected zone - liquation cracking, hydrogen-induced cracking, lamellar tearing, and re-heat cracking are described. These effects are detrimental to the performance of welded joints. Measures required to avoid them are examined. 1. INTRODUCTION 1.1 A Brief Description of the Welding Process Welding is a joining process in which joint production can be achieved with the use of high temperatures, high pressures or both. In this lecture, only the use of high temperatures to produce a joint is discussed since this is, by far, the mo st common method of welding structural steels. It is essentially a process in which an intense heat source is applied to the surfaces to be joined to achieve local melting. It is common for further filler metal to be added to the molten weld pool to brid ge the gap between the surfaces and to produce the required weld shape and dimensions on cooling. The most common welding processes for structural steelwork use an electric arc maintained between the filler metal rod and the workpiece to provide the intense heat source. If unprotected, the molten metal in the weld pool can readily absorb oxygen and nitrogen from the atmosphere. This absorption would lead to porosity and brittleness in the solidified weld metal. The techniques used to avoid gas absorption in the weld pool vary according to the welding process. The main welding processes used to join structural steels are considered in more detail below. 1.2 The Main Welding Processes 3 a. Manual Metal Arc welding (MMA) In this process, the welder uses a metal stick electrode with a fusible mineral coating, in a holder connected to an electrical supply. An arc is struck between the electrode and the weld area which completes the return circuit to the electricity supply. The arc melts both the electrode and the surface region of the workpiece. Electromagnetic forces created in the arc help to throw drops of the molten electrode onto the molten area of the workpiece where the two metals fuse to form the weld pool. The electrode coating of flux contributes to the content of the weld pool by direct addition of metal and by metallurgical reactions which refine the molten metal. The flux also provides a local gaseous atmosphere which prevents absorption of atmospheric gases by the weld metal. There are many types of electrodes. The main differences between them are in the flux coating. The three main classes of electrode are shown below: 1. Rutile: General purpose electrodes for applications which do not require strict control of mechanical properties. These electrodes contain a high proportion of titanium oxide in the flux coating. 2. Basic: These electrodes produce welds with better strength and notch toughness than rutile. The electrodes have a coating which contains calcium carbonate and other carbonates and fluorspar. 3. Cellulosic: The arc produced by this type of electrode is very penetrating. These electrodes have a high proportion of combustible organic materials in their coating. b. Submerged Arc Welding (SAW) This process uses a bare wire electrode and a flux added separately as granules or powder over the arc and weld pool. The flux protects the molten metal by forming a layer of slag and it also stabilises the arc. The process is used mainly in a mechanical system feeding a continuous length of wire from a coil whilst the welding lead is moved along the joint. A SAW machine may feed several wires, one behind the other, so that a multi-run weld can be made. Submerged arc welding produces more consistent joints than manual welding, but it is not suitable for areas of difficult access. c. Gas shielded welding In this process, a bare wire electrode is used and a shielding gas is fed around the 4 arc and weld pool. This gas prevents contamination of the electrode and weld pool by air. There are three main variations of this process as shown below: 1. MIG (metal-inert gas) welding - Argon or helium gas is used for shielding. This process is generally used for non-ferrous metals. 2. MAG (metal-active gas) welding - Carbon dioxide (usually mixed with argon) is used for shielding. This process is generally used for carbon and carbon-manganese steels. 3. TIG (tungsten-inert gas) - Argon or helium gas is used for shielding and the arc struck between the workpiece and a non-consumable tungsten electrode. This process is generally used for thin sheet work and precision welding. 1.3 Welded Joint Design and Preparation There are two basic types of welded joints known as butt and fillet welds 1. Schematic views of these two weld types are shown in Figure 1. The actual shape of a weld is determined by the preparation of the area to be joined. The type of weld preparation depends on the welding process and the fabrication procedure. Examples of different weld preparations are shown in Figure 2. The weld joint has to be located and shaped in such a way that it is easily accessible in terms of both the welding process and welding position. The detailed weld shape is designed to distribute the available heat adequately and to assist the control of weld metal penetration and thus to produce a sound joint. Operator induced defects such as lack of penetration and lack of fusion can be difficult to avoid if the joint preparation and design prevent good access for welding. 5 6 1.4 The Effect of the Welding Thermal Cycle on the Microstructure The intense heat involved in the welding process influences the microstructure of 7 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. 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: The chemical composition of the parent metal. The heat input rate during welding. 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 8 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. 1.5 Residual Welding Stresses and Distortion 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. 9 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. 10 It is not possible to predict accurately the distortion in a geometrically complicated fabrication, but one basic rule should be followed. This rule is that welding should preferably be started at the centre of a fabrication and all succeeding welds be made from the centre out, thus encouraging contractions to occur in the free condition. 11 If distortion is not controlled, there are two methods of correcting it； force and heat. The distortion of light sections can be eliminated simply by using force, e.g. the use of hydraulic jacks and presses. In the case of heavier sections, local heating and cooling is required to induce thermal stresses counteracting those already present. 1.6 Residual Stress Relief The most common and efficient way of relieving residual stresses is by heating. Raising the temperature results in a lower yield stress and allows creep to occur. Creep relieves the residual stresses through plastic deformation. Steel welded treatments. The heating and cooling rates during this thermal stress relief must be carefully controlled otherwise further residual stress patterns may be set up in the welded component. There is a size limit to the structures which can be thermally stress relieved both because of the size of the ovens required and the possibility of a structure distorting under its own weight. It is possible, however, to heat treat individual joints in a large structure by placing small ovens around the joints or by using electric heating elements. Other methods of stress relief rely on thermal expansion providing mechanical forces capable of counteracting the original residual stresses. This technique can be applied in-situ but a precise knowledge of the location of the compressive residual stresses is vital, otherwise the level of residual stress may be increased rather than decreased. Purely mechanical stress relief can also be applied provided sufficient is available to accommodate the necessary plastic deformation. 2. THE WELDABILITY OF STRUCTURAL STEELS 2.1 Introduction If weld preparation is good and operator induced defects (e.g. lack of penetration or fusion) are avoided, all the common structural steels can be successfully welded. However, a number of these steels may require special treatments to achieve a satisfactory joint. These treatments are not convenient in all cases. The difficulty in producing satisfactory welded joints in some steels arises from the extremes of heating, cooling and straining associated with the welding process combined with microstructural changes and environmental interactions that occur during welding. It is not possible for some structural steels to tolerate these effects without joint cracking occurring. The various types of cracking which can occur and the remedial measures which can be taken are discussed below. 12 2.2 Weld Metal Solidification Cracking Solidification of the molten weld pool occurs by the growth of crystals away from the fusion boundary and towards the centre of the weld pool, until eventually there is no remaining liquid. In the process of crystal growth, solute and impurity elements are pushed ahead of the growing interface. This process is not significant until the final stages of solidification when the growing crystals interlock at the centre of the weld. The high concentration of solute and impurity elements can then result in the production of a low freezing point liquid at the centre of the weld. This acts as a line of weakness and can cause cracking to occur under the influence of transverse shrinkage strains. Impurity elements such as sulphur and phosphorus are particularly important in this type of cracking since they cause low melting point silicides and phosphides to be present in the weld metal 3. A schematic view of solidification cracking is shown in Figure 6. Weld metals with a low susceptibility to solidification cracking (low sulphur and phosphorous) are available for most structural steels, but cracking may still arise in the following circumstances: 13 a. If joint movement occurs during welding, e.g. as a result of distortion. A typical example of this is welding around a patch or nozzle. If the weld is continuous, the contraction of the first part of the weld imposes a strain during solidification of the rest of the weld. b. If contamination of the weld metal with elements such a sulphur and phosphorus occur. A typical example of this is the welding of articles with a sulphur rich scale, such as a component in a sulphur containing environment. c. If the weld metal has to bridge a large gap, e.g. poor fit-up. In this case the depth to width ratio of the weld bead may be small. Contraction of the weld results in a large strain being imposed on the centre of the weld. d. If the parent steel is not suitable in the sense that the diffusion of impurity elements from the steel into the weld metal can make it susceptible to cracking. Cracking susceptibility depends on the content of alloying element with the parent metal and can be expressed in the following equation: Hot cracking susceptibility = Note: The higher the number, the greater the susceptibility. Solidification cracking can be controlled by careful choice of parent metal composition, process parameters and joint design to avoid the circumstances previously outlined. 2.3 Heat Affected Zone (HAZ) Cracking 2.3.1 Liquation cracking (burning) The parent material in the HAZ does not melt as a whole, but the temperature close to the fusion boundary may be so high that local melting can occur at grain boundaries due to the presence of constituents having a lower melting point than the surrounding matrix. Fine cracks may be produced in this region if the residual stress is high. These cracks can be extended by fabrication stresses or during service 3. A schematic view of liquation cracking is shown in Figure 7. 14 In steels the low melting point grain boundary films can be formed from impurities such as sulphur, phosphorus, boron, arsenic and tin. As with solidification cracking, increased carbon, sulphur and phosphorous make the steel more prone to cracking. There are two main ways of avoiding liquation cracking. First, care should be taken to make sure that the sulphur and phosphorus levels in the parent metal are low. Unfortunately, many steel specifications permit high enough levels of sulphur and phosphorus to introduce a risk of liquation cracking. Secondly, the risk of liquation cracking is affected by the welding process used. Processes incorporating a relatively high heat input rate, such as submerged arc or electroslag welding, lead to a greater risk of liquation cracking than, for example, manual metal arc welding. This is the case since the HAZ spends longer at the liquation temperature (allowing greater segregation of low melting point elements) and there is a greater amount of thermal strain accompanying welding. 15 译文： 结构钢的焊接性 演讲简要讨论焊接工艺的基础，然后测试决定结构钢焊接性的因素。 摘要 焊接的基本过程方面在这里被讨论。然后把重点放在冶金参数对结构钢的焊接性的影响。一种钢如果被认为有良好的焊接性，如果焊接处有足够的强度和韧性。 凝固裂纹，热影响区液化开裂氢致开裂，层状撕裂，再 热裂解在这里被描述。这些是焊点不利影响的表现。采取的减少这些影响的措施被测试。 1 .导言 1.1焊接工艺简介 焊接是材料加入过程，焊缝可以通过高温、高压或两者共同产生。在本文中，只讨论高温产生焊缝。因为这是到目前为止最常用焊接结构钢的方法。这基本上是这样一个过程：激烈的热源用于工件表面以实现熔化。同时将“料”添加到熔融熔池，以连接之间的缝，生产所需的焊缝形状和尺寸并冷却。最常见的焊接工艺为钢结构使用电弧，保持焊棒和工件产生强烈的热源。 如果得不到很好的保障，熔融金属在熔池随时可以接触大气中中的氧气和氮气， 这样会导致凝固焊缝金属中间有孔和脆性。这种技术被用于避免融池吸收空气，主要用于焊接工艺加入结构钢在下面更详细的介绍。 1.2 主要焊接工艺 A.手动材料电弧焊接 在这个过程中，焊机采用了金属电极棒与熔矿物涂层，在持有人连接到电力供应。一个电弧在电极和焊点区域产生，形成回路，电极表面区域和工件都是电弧熔体。电磁力产生电弧，帮助失液电极上熔融面积工件的情况下两个金属保险丝，形成熔池。 电极涂层的焊剂贡献直接熔池，防止了金属反应，其中完善熔化金属。焊剂 16 也提供了一个气态的气氛阻止吸收大气中的气体由焊缝金属。 有有很多 类型的电极。主要不同点是在焊剂涂层。三个主要类别的电极如下所示： 1. 金红石型：通用电极，应用在不需要严格控制的机械性能的场合。这些电极含有高比例的二氧化钛在焊剂涂层。 2. 基本型：这些电极产生比金红石型焊缝更好的强度和韧性。电极有一个涂层，其中包含碳酸钙和其他碳酸盐岩和萤石。 3. 纤维素型：这种的电极类型所产生的电弧是非常精确的。这些电极在他们的涂层有很高比例的可燃有机材料。 B.埋弧焊（ saw） 这个过程中采用了裸丝电极和焊剂的补充分被加入以颗粒或粉末状态加入电弧和熔池。焊剂保护熔融金属形成一层炉渣和它也使电弧稳定 。 这一过程主要是用于一个机械系统的焊接连续长度的焊丝从一个线圈，而焊接铅是沿着焊缝，一个埋弧焊机可以吃几条焊丝。一个接着另一个，所以一个多线运行焊缝可以做出。埋弧焊比手工焊接产生更一致的焊点，但它是不适合难以进入的领域。 C.气体保护焊 在这个过程中，裸丝电极被使用，保护气体充满电弧和熔池周围。这种气体，防止由空气污染电极和熔池。这个工艺过程中有三个主要变化，如下所示： 1. MIG(金属惰性气体 )焊接， 氩气或氦气用来作为屏蔽气体。这种工艺一般用于废铁结束的焊接。 2. MAG（ 金属活性气体 ）焊接， 二氧化碳（通常是混 合氩）用来作为屏蔽气体。这种工艺一般用于碳钢和碳锰钢。 3. TIG（钨惰性气体）焊接，氩气或氦气用于屏蔽气体以及电弧之间工件和非消耗品钨电极。 这个工艺一般用于薄板的工作和精密焊接。 1.3 焊接缝的设计与准备 有两个基本类型的焊接缝称为对接焊接缝和角焊缝 1。这两个焊缝类型，如图 1 所示。实际焊缝的形状是由将要结合的形状决定的。焊缝准备的类型，要看焊接的工艺个制作的工艺。例如不同的焊接准备工作正在如图 2 所示；该焊缝要设置形成这样一种方式：这是方便双方的焊接工艺和焊接位置。详细的焊缝形 17 状的设计可用热充分分配，并协助 控制焊缝金属的渗透，从而产生一个完善的焊缝。操作者导致的缺陷，如缺乏渗透与融合，这些难以避免。如果焊缝筹备和设计良好的焊接条件可以防止这些。 18 1.4焊接热循环对微观结构的影响 焊接过程中所涉及激烈的热，影响焊缝金属及原金属和接近融合的边界的微观结构（边界之间的固体和液体金属）。因此，焊接周期影响焊缝的力学性能。 熔融熔池迅速冷却，由于金属被加入作为一个有效率的散热片。这冷却的结 19 果，在焊缝金属中有一个冷铸态组织。在焊接结构钢中，焊接钎料通常不具有与母 材料金属相同的成分。如果成分相同，快速冷却可能会导致硬脆阶，如马氏体，在焊缝金属的微观结构。这个问题的避免方法是采用焊接钎料碳含量比较母质底。 母板金属接近熔化的熔池迅速加热到达一个由融合边界决定的温度。接近融合的边界，定点温度接近熔点或已经到达熔点。而材料，只有几毫米的距离，可能只能达到几百摄氏度。母质接近融合边界加热到奥氏体相场。由于冷却，这一地区的变换到一个不同于其余的母材微观结构。在这一区域的冷却速度通常是快速，因此有一种向低温结构转型倾向，如贝氏体，马氏体，这比大部份的母金属更硬，更脆。这一区域被 称为热影响区（ HAZ.） 焊接热影响区的微观结构受以下三个因素影响： 1 母质金属的化学成分 2 焊接的热输入速率 3 热影响区在焊后的冷却速率 母质金属的化学成分是很重要的，因为它决定了焊接热影响区的淬透性。热输入速率的影响也是显著的，因为它直接影响焊接热影响区的晶粒尺寸。一般来说，高热烈输入速率导致较长的热循环，从而使 焊接热影响区的显微结构粗化。应该指出的是，热输入速率，也影响到焊接热影响区的冷却速率。一般规则是，热输入速率越高冷却速度越低。热输入率的价值是他是一个焊接工艺的参数：电弧电压，电弧电流和焊接速度 。此外，焊接热影响区的热输入速率，冷却速率是受另外两个因素影响的。第一，焊缝的设计和厚度是重要的，因为他们确定热流远离焊缝冷却过程中的速率。其次，被焊接部分的温度，即任何原先已有的热量，具有重要意义，因为它决定了焊缝和母材之间存在的温度梯度。 1.5 焊接残余应力和变形 焊接过程中焊接区域吸收强热扩张，冷却过程中收缩发生。这中扩张和收缩被周围的冷物质抵制，导致了在焊缝附近有残余应力场存在。焊缝金属的残余应力主要是拉伸性质的，发生在冷却收缩。这拉伸残余应力时平衡的这在母质金属上诱导了一个压应力。 2.一幅鉴于 纵向焊缝收缩残余应力场的示意图。如图 3所示。 20 图 3 纵向焊缝收