結(jié)構(gòu)鋼的焊接性外文文獻(xiàn)翻譯@中英文翻譯@外文翻譯

1 2012屆畢業(yè)設(shè)計(jì)外文翻譯 Weldability of Structural Steels 焊接結(jié)構(gòu)鋼 學(xué)生姓名： 侯林珠 指導(dǎo)教師： 崔曉東、任芝蘭 職 稱： 工程師、副教授 專 業(yè)： 機(jī)械設(shè)計(jì)制造及其自動(dòng)化 班 級(jí)： 機(jī)本 0804 班 完成時(shí)間： 2012 年 5 月 2 附錄 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 譯文： 結(jié)構(gòu)鋼的焊接性 演講簡(jiǎn)要討論焊接工藝的基礎(chǔ)，然后測(cè)試決定結(jié)構(gòu)鋼焊接性的因素。 摘要 焊接的基本過(guò)程方面在這里被討論。然后把重點(diǎn)放在冶金參數(shù)對(duì)結(jié)構(gòu)鋼的焊接性的影響。一種鋼如果被認(rèn)為有良好的焊接性，如果焊接處有足夠的強(qiáng)度和韌性。 凝固裂紋，熱影響區(qū)液化開裂氫致開裂，層狀撕裂，再 熱裂解在這里被描述。這些是焊點(diǎn)不利影響的表現(xiàn)。采取的減少這些影響的措施被測(cè)試。 1 .導(dǎo)言 1.1焊接工藝簡(jiǎn)介 焊接是材料加入過(guò)程，焊縫可以通過(guò)高溫、高壓或兩者共同產(chǎn)生。在本文中，只討論高溫產(chǎn)生焊縫。因?yàn)檫@是到目前為止最常用焊接結(jié)構(gòu)鋼的方法。這基本上是這樣一個(gè)過(guò)程：激烈的熱源用于工件表面以實(shí)現(xiàn)熔化。同時(shí)將“料”添加到熔融熔池，以連接之間的縫，生產(chǎn)所需的焊縫形狀和尺寸并冷卻。最常見的焊接工藝為鋼結(jié)構(gòu)使用電弧，保持焊棒和工件產(chǎn)生強(qiáng)烈的熱源。 如果得不到很好的保障，熔融金屬在熔池隨時(shí)可以接觸大氣中中的氧氣和氮?dú)猓?這樣會(huì)導(dǎo)致凝固焊縫金屬中間有孔和脆性。這種技術(shù)被用于避免融池吸收空氣，主要用于焊接工藝加入結(jié)構(gòu)鋼在下面更詳細(xì)的介紹。 1.2 主要焊接工藝 A.手動(dòng)材料電弧焊接 在這個(gè)過(guò)程中，焊機(jī)采用了金屬電極棒與熔礦物涂層，在持有人連接到電力供應(yīng)。一個(gè)電弧在電極和焊點(diǎn)區(qū)域產(chǎn)生，形成回路，電極表面區(qū)域和工件都是電弧熔體。電磁力產(chǎn)生電弧，幫助失液電極上熔融面積工件的情況下兩個(gè)金屬保險(xiǎn)絲，形成熔池。 電極涂層的焊劑貢獻(xiàn)直接熔池，防止了金屬反應(yīng)，其中完善熔化金屬。焊劑 16 也提供了一個(gè)氣態(tài)的氣氛阻止吸收大氣中的氣體由焊縫金屬。 有有很多 類型的電極。主要不同點(diǎn)是在焊劑涂層。三個(gè)主要類別的電極如下所示： 1. 金紅石型：通用電極，應(yīng)用在不需要嚴(yán)格控制的機(jī)械性能的場(chǎng)合。這些電極含有高比例的二氧化鈦在焊劑涂層。 2. 基本型：這些電極產(chǎn)生比金紅石型焊縫更好的強(qiáng)度和韌性。電極有一個(gè)涂層，其中包含碳酸鈣和其他碳酸鹽巖和螢石。 3. 纖維素型：這種的電極類型所產(chǎn)生的電弧是非常精確的。這些電極在他們的涂層有很高比例的可燃有機(jī)材料。 B.埋弧焊（ saw） 這個(gè)過(guò)程中采用了裸絲電極和焊劑的補(bǔ)充分被加入以顆?；蚍勰顟B(tài)加入電弧和熔池。焊劑保護(hù)熔融金屬形成一層爐渣和它也使電弧穩(wěn)定 。 這一過(guò)程主要是用于一個(gè)機(jī)械系統(tǒng)的焊接連續(xù)長(zhǎng)度的焊絲從一個(gè)線圈，而焊接鉛是沿著焊縫，一個(gè)埋弧焊機(jī)可以吃幾條焊絲。一個(gè)接著另一個(gè)，所以一個(gè)多線運(yùn)行焊縫可以做出。埋弧焊比手工焊接產(chǎn)生更一致的焊點(diǎn)，但它是不適合難以進(jìn)入的領(lǐng)域。 C.氣體保護(hù)焊 在這個(gè)過(guò)程中，裸絲電極被使用，保護(hù)氣體充滿電弧和熔池周圍。這種氣體，防止由空氣污染電極和熔池。這個(gè)工藝過(guò)程中有三個(gè)主要變化，如下所示： 1. MIG(金屬惰性氣體 )焊接， 氬氣或氦氣用來(lái)作為屏蔽氣體。這種工藝一般用于廢鐵結(jié)束的焊接。 2. MAG（ 金屬活性氣體 ）焊接， 二氧化碳（通常是混 合氬）用來(lái)作為屏蔽氣體。這種工藝一般用于碳鋼和碳錳鋼。 3. TIG（鎢惰性氣體）焊接，氬氣或氦氣用于屏蔽氣體以及電弧之間工件和非消耗品鎢電極。 這個(gè)工藝一般用于薄板的工作和精密焊接。 1.3 焊接縫的設(shè)計(jì)與準(zhǔn)備 有兩個(gè)基本類型的焊接縫稱為對(duì)接焊接縫和角焊縫 1。這兩個(gè)焊縫類型，如圖 1 所示。實(shí)際焊縫的形狀是由將要結(jié)合的形狀決定的。焊縫準(zhǔn)備的類型，要看焊接的工藝個(gè)制作的工藝。例如不同的焊接準(zhǔn)備工作正在如圖 2 所示；該焊縫要設(shè)置形成這樣一種方式：這是方便雙方的焊接工藝和焊接位置。詳細(xì)的焊縫形 17 狀的設(shè)計(jì)可用熱充分分配，并協(xié)助 控制焊縫金屬的滲透，從而產(chǎn)生一個(gè)完善的焊縫。操作者導(dǎo)致的缺陷，如缺乏滲透與融合，這些難以避免。如果焊縫籌備和設(shè)計(jì)良好的焊接條件可以防止這些。 18 1.4焊接熱循環(huán)對(duì)微觀結(jié)構(gòu)的影響 焊接過(guò)程中所涉及激烈的熱，影響焊縫金屬及原金屬和接近融合的邊界的微觀結(jié)構(gòu)（邊界之間的固體和液體金屬）。因此，焊接周期影響焊縫的力學(xué)性能。 熔融熔池迅速冷卻，由于金屬被加入作為一個(gè)有效率的散熱片。這冷卻的結(jié) 19 果，在焊縫金屬中有一個(gè)冷鑄態(tài)組織。在焊接結(jié)構(gòu)鋼中，焊接釬料通常不具有與母 材料金屬相同的成分。如果成分相同，快速冷卻可能會(huì)導(dǎo)致硬脆階，如馬氏體，在焊縫金屬的微觀結(jié)構(gòu)。這個(gè)問(wèn)題的避免方法是采用焊接釬料碳含量比較母質(zhì)底。 母板金屬接近熔化的熔池迅速加熱到達(dá)一個(gè)由融合邊界決定的溫度。接近融合的邊界，定點(diǎn)溫度接近熔點(diǎn)或已經(jīng)到達(dá)熔點(diǎn)。而材料，只有幾毫米的距離，可能只能達(dá)到幾百攝氏度。母質(zhì)接近融合邊界加熱到奧氏體相場(chǎng)。由于冷卻，這一地區(qū)的變換到一個(gè)不同于其余的母材微觀結(jié)構(gòu)。在這一區(qū)域的冷卻速度通常是快速，因此有一種向低溫結(jié)構(gòu)轉(zhuǎn)型傾向，如貝氏體，馬氏體，這比大部份的母金屬更硬，更脆。這一區(qū)域被 稱為熱影響區(qū)（ HAZ.） 焊接熱影響區(qū)的微觀結(jié)構(gòu)受以下三個(gè)因素影響： 1 母質(zhì)金屬的化學(xué)成分 2 焊接的熱輸入速率 3 熱影響區(qū)在焊后的冷卻速率 母質(zhì)金屬的化學(xué)成分是很重要的，因?yàn)樗鼪Q定了焊接熱影響區(qū)的淬透性。熱輸入速率的影響也是顯著的，因?yàn)樗苯佑绊懞附訜嵊绊憛^(qū)的晶粒尺寸。一般來(lái)說(shuō)，高熱烈輸入速率導(dǎo)致較長(zhǎng)的熱循環(huán)，從而使 焊接熱影響區(qū)的顯微結(jié)構(gòu)粗化。應(yīng)該指出的是，熱輸入速率，也影響到焊接熱影響區(qū)的冷卻速率。一般規(guī)則是，熱輸入速率越高冷卻速度越低。熱輸入率的價(jià)值是他是一個(gè)焊接工藝的參數(shù)：電弧電壓，電弧電流和焊接速度 。此外，焊接熱影響區(qū)的熱輸入速率，冷卻速率是受另外兩個(gè)因素影響的。第一，焊縫的設(shè)計(jì)和厚度是重要的，因?yàn)樗麄兇_定熱流遠(yuǎn)離焊縫冷卻過(guò)程中的速率。其次，被焊接部分的溫度，即任何原先已有的熱量，具有重要意義，因?yàn)樗鼪Q定了焊縫和母材之間存在的溫度梯度。 1.5 焊接殘余應(yīng)力和變形 焊接過(guò)程中焊接區(qū)域吸收強(qiáng)熱擴(kuò)張，冷卻過(guò)程中收縮發(fā)生。這中擴(kuò)張和收縮被周圍的冷物質(zhì)抵制，導(dǎo)致了在焊縫附近有殘余應(yīng)力場(chǎng)存在。焊縫金屬的殘余應(yīng)力主要是拉伸性質(zhì)的，發(fā)生在冷卻收縮。這拉伸殘余應(yīng)力時(shí)平衡的這在母質(zhì)金屬上誘導(dǎo)了一個(gè)壓應(yīng)力。 2.一幅鑒于 縱向焊縫收縮殘余應(yīng)力場(chǎng)的示意圖。如圖 3所示。 20 圖 3 縱向焊縫收