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1、焊接工艺参数（The welding process parameters）The choice of the welding parametersHttp:/Manual electric arc welding parameters include electrode diameter, welding current, arc voltage, welding layers, power type and polarity etc.1. electrode diameterSelect the diameter of electrode mainly depends on the thi
2、ckness of welding joint form, welding position and welding level etc. In general, according to table 6-4 according to the welding thickness choice of electrode diameter, and tend to choose the larger diameter of the electrode. In addition, in the flat, large diameter; welding, the electrode diameter
3、 is less than 5mm; horizontal welding and overhead welding, with a diameter of not more than 4mm; groove multilayer welding, in order to prevent the defects of lack of penetration, the first welding seam should adopt 3.2mm diameter electrode.Table 6-4 mm relationship between the diameter of electrod
4、e and welding pieces of thicknessWelding thicknessLess than or equal to 23451212Electrode diameterTwoThree point two45More than 152. welding currentThe welding current is too large or too small will affect the welding quality, so the choice should be based on the electrode type, diameter, thickness,
5、 welding joints, weld location and other factors to consider, the electrode diameter and weld position is the key. In general the welding steel structure, welding current and electrode diameter relationship formula was selected following experience available:I=10d2 (6-1)TypeI - welding current (A);D
6、 - electrode diameter (mm).In addition, welding, welding current should be higher than the small 15% 20%; horizontal welding and overhead welding, welding current should be higher than the current 10% to 15%.3. arc voltageAccording to the characteristics of the power supply, welding current and arc
7、voltage decided by the corresponding. In addition, the arc voltage and arc length. Long arc arc voltage, arc short arc voltage low. General requirements for arc length is less than or equal to the diameter of electrode, short arc welding. In acidic welding when used to preheat the parts or reduce th
8、e temperature of the molten pool, sometimes will be slightly elongated arc welding, which is called arc welding.4. welding layersWelding layers should be considered depending on the thickness of welding pieces. In addition to sheet are generally used, multi-layer welding. The number of welding seam
9、thickness of each layer is too small, too large, has an adverse effect on the ductility of the weld metal. The thickness of each layer of weld construction should not be greater than 4 5mm.5. types of power supply and polarityThe DC power supply due to stable arc, small splash, good welding quality,
10、 generally used in important welding structure or plate stiffness structure. In other cases, we should first consider the AC welder.According to the characteristics of the welding electrode form and different characteristics of anodic arc in high temperature than the cathode, with different polariti
11、es to weld various components. With basic electrode or welding of thin plates, using DCRP (the workpiece is connected to the anode by acid electrodes); and, (usually used is then connected with the anode workpiece).6. the weldability of steelThe difference of different kinds of steel welding perform
12、ance is used to represent the weldability. The weldability of steel refers to the appropriate design and operating conditions, and the welding material is easy to meet the structural performance level. General weldability of specific performance in the following aspects:(1) the welding operation eas
13、ier;(2) crack and other harmful defects do not occur when welding;(3) the parent material and mechanical, chemical and physical properties of the welded joint;(4) notch toughness of base metal is excellent;(5) of welded joints plasticity and toughness enough.Weldability is often affected by the chem
14、ical composition of the steel, rolling method and plate thickness factor. In order to evaluate the effect of chemical composition on welding, general use of carbon equivalent (Ceq) said.Ceq is the chemical composition of the welding heat affected zone of the highest hardness, the International Insti
15、tute of welding of carbon equivalent formula is recommended:According to the experience:Ceq0.6%, quench hardening trend, difficult welding, welding should be carefully before preheating, and take control of welding process and other measures.1.4 welding parameters1.4 welding parametersWelding parame
16、ter refers to welding, various physical quantities are selected to ensure the welding quality (such as: welding current, arc voltage, welding speed, heat input). SMAW welding parameters including electrode diameter, welding current, arc voltage, welding speed and preheating temperature etc.1.4.1 ele
17、ctrode diameterThe electrode diameter is according to the thickness of welding, welding position, welding joints, layer selection.The thickness of the weldment is larger, overlapping and T joint weld should be used for larger diameter electrode. For small groove welding, in order to ensure the botto
18、m of the penetration, should adopt thinner electrode, 3.2mm electrode or Phi Phi 2.5mm is generally used as backing welding. Different welding position, electrode diameter selection is different, usually flat with thick diameter (4 6) mm welding rod, welding and overhead welding with phi (3.2 4) mm
19、electrode; transverse welding with phi (3.2 5) mm electrode. For the special needs of small steel, the welding parameters can be used for small diameter electrode.According to the workpiece thickness selection, refer to table 3-20. For the important structure should be based on the provisions of the
20、 scope of the welding current (determined according to the heat input) refer to table 3 - 21 the diameter of the welding current and the relationship between electrode to determine the diameter of electrode.1.4.2 welding currentThe welding current is the main process parameters of arc welding, the w
21、elder is only adjusting welding current needs in the process of operation, and the welding speed and arc voltage are controlled by the welder. The welding current selection directly affects the welding quality and productivity.The increase of welding current, weld depth is bigger, melting fast, high
22、 welding efficiency, but the current is too large when welding spatter and smoke, electrode tail redness, partial coating to failure or collapse, but also prone to undercut, burn and other defects, increase welding deformation, will make joint coarse grain heat affected zone, reduce welding joint to
23、ughness; welding current is too small, it is difficult to arc, welding rod adhesion on the workpiece, the arc is stable, easy to produce welding incomplete fusion defects such as porosity and slag, and low productivity.Therefore, the choice of welding current, should be based on the electrode type,
24、electrode diameter and thickness of welding joints, welding position and welding layers to consider. Should first ensure the welding quality, secondly should be used in the large current, to improve production efficiency. Compared with the plate thickness, T joint and lap joint, the welding environm
25、ent at low temperature, due to heat faster, so the welding current is bigger. But the main consideration of the electrode diameter, welding position and welding level etc.1) considering the diameter of electrode electrode diameter is thicker, melting the electrode required more heat, to increase the
26、 welding current, each electrode has a most suitable current range, table 3-21 welding current reference value of a variety of commonly used suitable electrode diameter.When using carbon steel electrode welding, also can be selected according to the electrode diameter, welding current is calculated
27、by empirical formula below:I=dKType: I - welding current (A):D - electrode diameter (mm):K - the empirical coefficient (A / CRA), see table 3-20.Table 3-20 current experience coefficient and diameter of electrode welding 9The electrode diameter D / mm 1.62 2.5 and 3.24 6Experience coefficient K 20 2
28、525 3030 4040 502) position of welding in flat position welding is considered, can choose more large welding current, non flat position welding, easy to control weld, the welding current is 10% 20% small flat position.3) consider welding backing bead welding layer usually, in order to ensure the qua
29、lity of the use of back bead, the welding current is smaller; the welding bead filling, in order to improve efficiency, ensure good fusion, the use of large welding current: cover weld, prevent undercut and ensure a bead shape, using the current smaller.The welding current can be selected according
30、to the preliminary - electrode diameter, welding current is selected by trial after welding, weld inspection and defect, it can be determined. For the requirements of mechanical properties such as boilers, pressure vessels and other important structures, to after the qualification of welding process
31、, can finally determine the welding parameters such as current.The arc voltage of 1.4.3When the welding current is adjusted, the characteristic curve of welding machine is decided. In fact, the arc voltage is mainly determined by the arc length. Arc length, arc voltage is high, and vice versa. In th
32、e process of welding, the arc should not be too long, otherwise there will be arc combustion instability, spatter, weld depth and undercut, porosity and other defects: if the arc is too short, easy to stick electrode. In general, the arc length is equal to the electrode diameter 0.5 1 times as well,
33、 the arc voltage corresponding to 16 - 25V. The arc length does not exceed the basic electrode electrode diameter, electrode diameter for half the better, as far as possible to choose short arc welding; arc length acid electrode should be equal to the diameter of electrode.1.4.4 welding speedSMAW we
34、lding speed refers to the welding process welding rod along the welding direction, moving speed, the length of the weld is completed per unit of time. The welding speed is too fast will cause serious weld narrowing, uneven, prone to undercut and weld waveform taper; welding speed will make the weld
35、width, residual height increase, reduce the effect. The welding speed also directly determines the heat input size, generally selected according to the steel hardened tendency.1.4.5 welding layersThick plate welding, general groove and multi-layer welding or welding. The microstructure of multi laye
36、r and multi pass welding head is fine, the heat affected zone is narrow. Before a bead to bead up after a preheating, then a bead has the effect of heat treatment on a bead. Therefore, the joint has good ductility and toughness. Especially for hardened steel, the weld bead after the tempering effect
37、, can improve the microstructure and mechanical properties were investigated.For high strength low alloy steel and other steel, welding layers have obvious influence on the performance of joint. Weld less layers, each layer of the weld thickness is too large, due to grain coarsening, will lead to a
38、decline in the ductility and toughness of welded joints.1.4.6 heat inputWhen welding, the welding energy input to the unit length of weld heat input is called heat. The formula is as follows:Q=NLU/uThe heat input type in Q - unit length of weld (J / cm)I - welding current (A);U - arc voltage (V);U -
39、 welding speed (cm / s)N - coefficient of thermal efficiency, SMAW is 0.7 0.8.The heat input is small, influence on properties of welded joint of low carbon steel for the low carbon steel electrode electric arc welding - generally does not require heat input. For low alloy steel and stainless steel,
40、 the heat input is too large, the joint performance may reduce the heat input is too small, there may be cracks in welding steel. Therefore, the welding procedure specification of heat input. After the welding current provisions of heat input and arc voltage arc welding and the welding speed is indi
41、rectly determined roughly.General should be determined through test without welding cracks and ensure the heat input range of joint performance qualified.The heat input range is bigger, more convenient for welding operation.1.4.7 preheating temperaturePreheating is welding on the welding workpieces
42、all or part of the proper heating before the start. Preheating can decrease of cooling speed of welded joints, avoid the quenched structure, reduce welding stress and deformation. It is the effective measures to prevent cracks. The general structure of low carbon steel and low alloy strength rigid s
43、mall low strength steel, generally without preheating. But for the rigid or welding the crack structure of the poor, need to be preheated before welding.According to the comprehensive consideration of the preheating temperature of the chemical composition, base material and welding performance, thic
44、kness, welding joints and welding restraint degree of ambient temperature and related products, technical standards and other conditions, the minimum preheat temperature of important structure after cracking test for uncertain crack. Choose the preheating temperature is much higher, the better effec
45、t to prevent cracks; but more than the preheating temperature required, will cause the metal grain coarsening near the fusion zone, reduce the welding quality, labor conditions will be worse. Usually with a variety of heating stove integral preheating. Local preheating gas used as flame heating or i
46、nfrared heating. The commonly used surface thermometer temperature.After 1.4.8 heat and heat treatment after weldingImmediately after welding welded pieces of all (or local) heating or insulation, the corrosion process measures called heat cold. After the heat is to avoid the formation of hard tissu
47、e, and the diffusion of hydrogen escaping the weld surface, so as to prevent the generation of cracks.Post weld heat treatment to improve the microstructure and properties of welded joints or eliminate the welding residual stress is called heat treatment after welding. The main effect of postweld heat treatment is to eliminate the welding residual stress, reduce the hardness of the weld zone, the diffusion of hydrogen evolution, stable orga
