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1、焊接工艺参数（Welding process parameter）Selection of welding process parametersHttp:/ welding parameters of manual arc welding include welding rod diameter, welding current, arc voltage, welding layer number, power supply type and polarity.1. electrode diameterThe choice of electrode diameter mainly depend
2、s on the thickness of weldment, the form of joint, the position of weld and the level of welding. In general, the diameter of the electrode can be selected according to the thickness of the weldment according to table 6-4, and the electrode with larger diameter is preferred. In addition, in the flat
3、, large diameter; welding, the electrode diameter 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
4、 relationship between welding rod diameter and weldment thickness mmWeldment 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
5、electrode type, diameter, thickness, welding joints, weld location and other factors to consider, the electrode diameter and weld position is the key. In the welding of the general steel structure, the relationship between the welding current and the diameter of the electrode can be selected by the
6、following empirical formula:I=10d2 (6-1)TypeI - welding current (A);D - 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 voltageThe arc v
7、oltage is determined by the welding current according to the characteristics of the power supply. In addition, arc voltage is also related to the length of arc. The arc length is higher than the arc voltage, and the arc voltage is low when the arc is short. Generally, the arc length is less than or
8、equal to the diameter of the welding rod, that is, short arc welding. In the use of acid welding electrode, in order to preheat parts or reduce the pool temperature, sometimes also slightly elongated arc welding, that is called long arc welding.4. welding layersThe number of welds shall be determine
9、d according to the thickness of the weldment. In addition to the thin plate, multilayer welding is usually adopted. The number of welded layers is too small, and the thickness of each layer is too large, which has a detrimental effect on the plasticity of weld metal. The thickness of each seam shall
10、 not be greater than 4 to 5mm during construction.5. types and polarity of power suppliesThe DC power supply due to stable arc, small splash, good welding quality, generally used in important welding structure or plate stiffness structure. In other cases, AC welder shall be considered first.Accordin
11、g to the different forms of welding rod and welding characteristics, the polarity of the arc is higher than the cathode, so different polarity is used to weld various components. When using alkaline welding rod or welding plate, the direct reverse connection (workpiece is connected to negative elect
12、rode) is adopted; and the positive electrode is used when the acid welding rod is used (the workpiece is connected to the positive electrode).6. weldability of steelThe difference in weldability of various steels is indicated by weldability. The weldability of steels refers to the degree of ease of
13、welding and the performance of structures under proper design and working conditions. General weldability is expressed in the following aspects:(1) welding operation is easy;(2) cracks and other harmful defects are not occurred during welding;(3) the mechanical, chemical and physical properties of b
14、ase metal and welded joint are good;(4) the notch toughness of base metal is excellent;(5) welded joints have sufficient plasticity and toughness.Weldability is often affected by the chemical composition, rolling method, and thickness of the steel. In order to evaluate the influence of chemical comp
15、osition on solderability, carbon equivalent (Ceq) is commonly used.Ceq is the effect of the chemical composition on the highest hardness of the weld heat affected zone. The formula recommended by the international society of welding is the equivalent of carbon equivalent:Based on experience:Ceq0.6%,
16、 the tendency of hardening is large, it is difficult to weld. The welding needs to be preheated carefully before welding, and the welding process is strictly controlled.1.4 welding process parameters1.4 welding process parametersWelding process parameters refer to the physical quantities (such as we
17、lding current, arc voltage, welding speed, heat input, etc.) selected to ensure the quality of welding when welding. The welding parameters of electrode arc welding include electrode diameter, welding current, arc voltage, welding speed and preheating temperature, etc.1.4.1 electrode diameterThe dia
18、meter of the electrode is selected according to the thickness of the weldment, the position of welding, the form of joint and the number of welding layers.Larger thickness of the welding, lap and T joint weld should be used in larger diameter welding rod. For small groove welding, in order to ensure
19、 the penetration of the lower layer, it is better to use a thinner electrode, such as 2.5mm or Phi 3.2mm welding rod. 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 electro
20、de; transverse welding with phi (3.2 5) mm electrode. For special steel, small process parameters are needed, and small diameter welding rod can be used in welding.According to the thickness of the workpiece, refer to table 3-20. For an important structure, the diameter of the electrode shall be det
21、ermined according to the specified range of welding current (according to the heat input), and the relationship between the welding current and the diameter of the electrode according to table 3 - 21.1.4.2 welding currentWelding current is the main process parameter of arc welding, and only welding
22、current needs to be regulated in the course of operation, while welding speed and arc voltage are controlled by welders. The choice of welding current directly affects welding quality and labor productivity.The increase of welding current, weld depth is bigger, melting fast, high welding efficiency,
23、 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 toughness; welding cur
24、rent 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 selection of welding current should be based on the type of electrode, electrode diame
25、ter, weldment thickness, joint form, weld location and number of welded layers to consider. First of all, welding quality should be guaranteed. Secondly, larger current should be adopted to improve production efficiency. When the welding temperature is low, the welding current is greater because of
26、the lower thickness of the plate, the T - shaped joint and the lap joint head. But mainly consider the electrode diameter, welding position and bead level and other factors.1) considering the diameter of electrode electrode diameter is thicker, melting the electrode required more heat, to increase t
27、he 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 welding with carbon steel electrode, the welding current can be calculated by the following empirical formula according to t
28、he diameter of the selected electrode:I=dKType 1: I welding current (A):D - electrode diameter (mm):K - empirical coefficient (A / CRA), see table 3-20.Table 3-20 relationship between welding current empirical coefficient and electrode diameter 9Electrode diameter D / mm 1.62 to 2.5, 3.24 to 6Empiri
29、cal coefficients K ranged from 202525 to 30304040 to 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 la
30、yer usually, in order to ensure the quality 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.Welding
31、 current - usually in accordance with the diameter of the electrode for preliminary selection, after the initial selection of welding current, after passing the test welding, check the weld forming and defects can be determined. For mechanical properties, such as boilers, pressure vessels and other
32、important structures, after the welding process qualification, in order to finalize the welding current and other process parameters.1.4.3 arc voltageWhen the welding current is adjusted, the external characteristic curve of the welding machine is decided. In fact, arc voltage is mainly determined b
33、y arc length. Arc length, arc voltage is high, otherwise it is low. Welding process, the arc should not be too long, otherwise there will be unstable arc combustion, splash, melting depth and the creation of undercut, blowhole and other defects: if the arc is too short, easy to stick welding rod. Ge
34、nerally, the arc length is equal to 0.5 to 1 times of the diameter of the welding rod, and the corresponding arc voltage is 16 - 25V. The arc length of the basic electrode should not exceed the diameter of the electrode. It is better to choose half arc welding as much as possible. The arc length of
35、the acid welding rod shall be equal to the diameter of the welding rod.1.4.4 welding speedThe welding speed of electrode arc welding refers to the speed of welding rod moving along the welding direction in the process of welding, that is, the length of welding seam completed in unit time. The weldin
36、g speed is too high, it will cause the weld seam to be narrow, and it will make the undercut and weld waveform become sharp. The welding speed will be too slow, the weld width will be increased, the remaining height will be increased, and the effect will be reduced. Welding speed also directly deter
37、mines the amount of heat input, usually based on the hardening tendency of the steel to choose.1.4.5 weld seam numberThick plate welding, generally bevel, and use multi-layer welding or multi-layer welding. The microstructure of multi layer welding and multi channel welding joint is fine and the hea
38、t affected zone is narrower. The former bead acts as a preheating to the latter bead, and the latter bead acts as a heat sink on the previous bead. Therefore, the joint ductility and toughness are better. Especially for the easy quenched steel, the tempering effect of the back bead to the front bead
39、 can improve the microstructure and properties of the joint.For low alloy high strength steel and other steel grades, the number of weld layers has an obvious influence on the joint properties. The ductility and toughness of the welded joint will be reduced due to the coarsening of the weld seam whe
40、n the weld seam is too small and the thickness of each layer is too large.1.4.6 heat inputWhen welding, the heat input from the welding energy to the unit length weld is called heat input. The formula is as follows:Q=NLU/uType Q heat input of unit length weld (J / cm)I - welding current (A);U - arc
41、voltage (V);U - welding speed (cm / s)N - the coefficient of thermal efficiency, welding rod arc welding is 0.7 0.8.Heat input has little effect on the properties of low carbon steel welded joints. Therefore, for low carbon steel electrodes, arc welding generally does not specify heat input. For low
42、 alloy steel and stainless steel, when the heat input is too large, the performance of the joint may be reduced: the heat input is too long, and some steel welds may crack when welding. Therefore, the welding process specifies the heat input. After welding current and heat input are specified, the a
43、rc voltage and the welding speed of the arc welding are indirectly approximately determined.Generally, the heat input range which can not produce welding cracks and guarantee the performance of the joint is determined by experiments.The greater the allowable heat input range, the easier the welding
44、operation.1.4.7 preheating temperaturePreheating is the process of proper heating of all or part of the weldment prior to the start of welding. Preheating can reduce the cooling rate after welding, avoid hardening structure, and reduce welding stress and deformation. It is an effective way to preven
45、t cracks. It is generally not necessary to warm up the general structure of rigid low carbon steel and low strength low-alloy high-strength steel. But the structure is easy to crack because of its rigidity or weldability, so it needs preheating before welding.According to the comprehensive considera
46、tion of the preheating temperature of the chemical composition, base material and welding performance, thickness, welding joints and welding restraint degree of ambient temperature and related products, technical standards and other conditions, the minimum preheat temperature of important structure
47、after cracking test for uncertain crack. Choose the preheating temperature is much higher, the better effect 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. The whole preheat is usually heated by various furnaces. Partial preheating usually adopts gas flame heating or infrared heating. Measurement of preheating temperature