Classification of common welding methods
There are many welding methods. According to the characteristics of welding process, they can be divided into three categories: fusion welding, pressure welding and brazing.
Fusion welding: Generally speaking, two welded workpieces are locally heated to a molten state, and filler metal is added (or not) at the same time to form a molten pool, and a firm joint is formed after cooling. This is a common welding method, including manual arc welding and gas welding.
Pressure welding: it is a connection method that applies a certain pressure during welding to combine the metals at the contact of two welding parts. This kind of welding can be divided into two forms according to whether it is heated during welding: one is to locally heat the contact part of the welded metal to a plastic state or a local melting state, and then apply a certain pressure to bond the metals together; The other form is not heating, but applying enough pressure on the metal contact surface, and making the atoms close to each other by means of plastic deformation caused by pressure to obtain a firm extrusion weld. Forging welding, contact welding and friction welding belong to the former; Belonging to the latter are cold pressure welding and explosive welding.
Brazing: the brazing filler metal with a melting point lower than that of the weldment is heated together with the weldment. When the weldment does not melt but the solder melts, the two materials diffuse with each other to form a brazed joint. Brazing can be divided into brazing and soft fiber welding. The brazing heating temperature is low, the deformation is small, and the joint is smooth and flat.
In geological exploration and drilling construction, the commonly used welding methods are manual arc welding (also known as electric welding) and gas welding and gas cutting.
(1) welding
As shown in Figure 4-38, it is a schematic diagram of manual arc welding process; 1 is an electric welding machine, 2 is an electrode holder, 3 is a covered electrode, and 4 is a workpiece to be welded. When working, the metal electrode is clamped in the electrode clamp and connected to one pole of the power supply, and the workpiece is connected to the other pole of the power supply. At work, covered electrode instantly contacts the workpiece to form a short circuit, and then covered electrode is lifted to make it 2 ~ 4 mm away from the workpiece, thus igniting the arc. The workpiece to be welded and covered electrode are melted under the heating of electric arc to form a molten pool 5. As the arc moves along the weld, a new molten pool is continuously formed, and the original molten pool cools and solidifies, forming a firm connecting weld. The arrow a in the figure indicates the covered electrode feeding movement required for continuous melting of covered electrode.
Figure 4-38 Manual arc welding
1- welding machine; 2— Electrode holder; 3- covering electrode; 4- Welded parts; 5- molten pool
1. Manual arc welding process
Manual arc welding technology includes three aspects: welding joint, spatial position of welding seam and welding specification.
(1) welded joint
The place where two steel plates are connected by welding is called a welded joint.
The welded joint consists of weld, fusion zone and heat affected zone. Weld refers to an assembly welded by weldments. The heat affected zone refers to the area where the metallographic structure and mechanical properties of the weldment change due to the influence of heat (but not melting). The fusion zone is the transition zone from the weld to the heat affected zone. In order to ensure the reliable penetration and good formation of weld, the molten pool has good crystallization conditions; Before welding, the workpiece to be welded is processed into a groove with a certain geometric shape, which is called a groove.
According to the structural shape, thickness and working conditions of welded parts, the requirements for joint quality are different. There are butt joints, lap joints, T-joints, corner joints and hemmed joints.
1) docking. As shown in Figure 4-39; Butt joint is a joint where the end faces of two weldments are relatively parallel. It is the most widely used joint form in various structures because of its good stress condition and low stress concentration. There are many forms of joint groove, and the commonly used ones are: ① I-shaped groove. As shown in figure 4-39a. Generally suitable for butt joint of steel plates with thickness less than 6mm. One-sided welding or double-sided welding can be used for penetration. In order to make the arc go deep into the metal for heating and ensure penetration, a gap of 0 ~ 2.5 mm can be left between the edges. When the thickness of the welded workpiece increases, it is necessary to increase the gap accordingly, otherwise it may cause incomplete penetration. This joint has the advantages of convenient preparation and assembly, less covered electrode consumption and high welding productivity. ②Y-shaped groove. As shown in Figure 4-39b. Suitable for plate thickness of 3 ~ 26 mm. ③ Double Y-shaped groove. As shown in figure 4-39c. Suitable for U-shaped groove with thickness 12 ~ 60mm④ blunt edge. As shown in figure 4-39d. Suitable for 20 ~ 60 mm thickness ⑤ blunt edge double U-shaped groove. As shown in figure 4-39e. Suitable for all kinds of grooves with thickness greater than 30 mm, such as groove angle, root gap, blunt edge (height of straight edge) and root radius r (Figure 4-39).
Figure 4-39 Butt joint (unit: mm)
A-I tank; B-y groove; C-shaped double y-shaped groove; D-u-shaped groove with blunt edge; E— Double U-shaped groove with blunt edge.
2) Lapping. As shown in figure 4-40. Overlapping two steel plates, welding along the edge of one plate or two plates, or opening holes in the upper steel plates, and welding the two steel plates together by plug welding are called lap joints. In Figure 4-40, the spacing between L, C and plug solder joints is determined by the design. Lapping is generally used for welding thin plates with a thickness of 10 ~ 20 mm, and the lap length is generally 3 ~ 5 times of the thickness. It must be welded on both sides, and the general bearing capacity is not high. This kind of joint consumes more steel plates and increases the weight of the structure. When subjected to external force, because the two workpieces are not on the same plane, great torque can be generated, which makes the stress on the weld complex, so the bearing capacity of the joint is low, so lap joint should be avoided as far as possible in structural design.
Figure 4-40 Lap joint (unit: mm)
3) T-joint. As shown in figure 4-4 1. A T-joint composed of two steel plates is called a T-joint. Some people also call it a T-joint. T-joints can also be I-shaped, single-sided V-shaped blunt edges, double-sided V-shaped blunt edges and double J-shaped grooved blunt edges. When the steel plate thickness of T-joint is 2 ~ 30 mm, I-groove can be used (Figure 4-41a); Generally, there is no need for penetration, but it is necessary to ensure that the welding feet K on both sides are equal to the thickness of the workpiece. When the riser is thick or important welding needs to be penetrated, the groove in the form shown in Figure 4-4 1b, Figure 4-4 1c and Figure 4-4 1d shall be adopted.
Figure 4-4 1 tee joint (unit: mm)
4) Corner joint. As shown in figure 4-42. It is a connecting joint with an included angle of 30 ~ 150 degrees between the ends of two steel plates. Similarly, according to the thickness and strength requirements of the workpiece, it can be divided into I-groove, single-sided blunt-edged V-groove, double-sided V-groove and Y-groove. General weldments can take the form as shown in Figure 4-42a. If the thickness of the workpiece is above 10mm, two workpieces can overlap by 3 ~ 5 mm to ensure the penetration depth (Figure 4-42b). If the operation is convenient, a gap of L ~ 2mm can be left between two workpieces for welding (Figure 4-42c).
Figure 4-42 Angle joint (unit: mm)
5) Crimp joint. As shown in figure 4-43. Generally suitable for thin metal plates with a thickness below 2mm. Before welding, press the edge of the joint with a bender or by hand; When welding, there is no need to add filler metal, and the edge can be melted by arc, and the weld will be formed after the metal solidifies. Crimped connection is characterized by convenient preparation and assembly, high productivity, but low bearing capacity, and can only be used for thin shell structures with small load.
Figure 4-43 hemming seam
(2) the position of weld in space
According to the position of weld in space, welding can be divided into flat welding, vertical welding, horizontal welding and overhead welding. As shown in Figure 4-44a, the form is flat welding; As shown in Figure 4-44b, there are horizontal welding and vertical welding. As shown in Figure 4-44c, the form is overhead welding. The flat welding operation is convenient, the quality is easy to ensure and the overhead welding process is poor.
Figure 4-44 Spatial position of weld.
(3) Welding specification
Welding specifications include covered electrode diameter, welding current and welding speed. It is an important factor affecting welding quality and productivity. Because the welding speed depends on the covered electrode diameter and welding current. Therefore, the welding specification mainly refers to covered electrode diameter and welding current.
The selection of covered electrode diameter is based on the thickness of the workpiece and the joint form. In principle, on the premise of ensuring the welding quality, try to choose covered electrode with large diameter, which can improve the productivity.
2. Welding equipment and tools
(1) welding machine
At present, there are three kinds of welding equipment used in China: DC arc welder, AC arc welder and welding rectifier. AC arc welding machine is commonly used in the construction site (Figure 4-45). Its main body is a special step-down transformer. The no-load voltage is 60 ~ 70 V, the working voltage is 30V, and the current adjustment range is 50 ~ 450 A. The AC arc welding machine has simple structure, convenient maintenance and low price, but the arc stability is poor.
Fig. 4-45 BX 1-330 AC arc welding machine
1- primary winding; 2,3-secondary winding; 4- moving iron core; 5— Static iron core; 6 terminal board; Seven crank
Generally, electric welding equipment must meet the following requirements:
1) should have a higher no-load voltage for arc striking and ensure working safety at the same time, so it is generally controlled between 50 ~ 90V.
2) The short-circuit current should not be too large to prevent damage to the equipment.
3) The electric welding machine should have special performance to ensure the stability of the arc.
4) The welding current can be adjusted to adapt to the change of weldment thickness.
(2) Electric welding appliances
Need to be equipped with electrode holder, mask, welding cable, covered electrode box, sharp hammer, wire brush, brush, etc. In addition, when welding, workers must wear leather gloves, canvas overalls, foot covers and insulating rubber shoes to prevent electric shock and burns.
(2) Gas welding and gas cutting
1. Gas welding
Working principle of (1) gas welding
Gas welding is to use the heat generated by acetylene burning in air to melt the workpiece and welding wire for welding.
Because gas welding has the disadvantages of low welding temperature, slow heating, relatively dispersed heat, low productivity and easy deformation after welding. Therefore, gas welding is mainly suitable for welding thin steel plates, nonferrous metals and their alloys, tool steels and cast iron. Acetylene is a colorless gas with molecular formula C2H2, which is obtained by the reaction of calcium carbide (CaC2) with water.
CaC2+2H2O→Ca(OH)2 decac2h2
Combustion of acetylene in air can produce a temperature of 2200℃. When burning in pure oxygen, the high temperature of 3200℃ can be obtained.
(2) Equipment required for gas welding
1) oxygen cylinder. The container for storing oxygen has a maximum pressure of 150× 105Pa.
2) Pressure reducing valve container. It is used to reduce the hyperbaric oxygen in oxygen cylinder to the working pressure, which is about (3 ~ 4) × 105 Pa, and keep the pressure stable during welding.
3) Acetylene generator. As shown in Figure 4-46, it is a device that makes water contact with calcium carbide to produce acetylene. There are many kinds, the most common of which is submersible acetylene generator. The working principle of acetylene generator is to put calcium carbide into the calcium carbide basket connected to the buoy. When calcium carbide comes into contact with water in a steel cylinder, it reacts to release acetylene gas, which is stored in a buoy and led out through a conduit. With the continuous reaction, more and more acetylene is stored in the buoy, and the pressure is rising, which makes the buoy rise gradually. When the pressure of acetylene gas in the buoy exceeds the pressure required for work, the height of the buoy is just enough to make calcium carbide leave the water surface, thus stopping the reaction. When the pressure in the buoy drops, the buoy also drops to make calcium carbide contact with water, and the reaction continues and the pressure rises. So as to ensure the stability of pressure during welding. Acetylene from the buoy must first pass through the flashback preventer and then enter the acetylene pipeline. The function of anti-backfire device is to prevent acetylene flame from flowing back into acetylene generator and causing explosion. The reason of tempering is often that the nozzle of welding gun is blocked, which makes the injection speed of mixed gas less than the combustion speed.
Figure 4-46 Acetylene Generator
1-calcium carbide; 2-Buoy; 3- calcium carbide basket; 4- acetylene bottle
4) welding torch (also known as welding torch). As shown in figure 4-47. It is a tool for mixing acetylene and oxygen in a certain proportion to obtain gas welding flame. When in use, first slightly open the oxygen regulating valve, then open the acetylene regulating valve for ignition, then gradually open the oxygen regulating valve, properly adjust the flame, hold the welding torch in one hand and the welding wire in the other, and move along the weld seam for welding (Figure 4-48).
Figure 4-47 Structure of Spray-suction Welding Gun
1- acetylene control valve; 2- acetylene pipeline; 3- oxygen tube; 4- oxygen regulating valve; 5— Nozzle; 6- pat the straw; 7- mixed trachea; 8— Welding nozzle
2. Gas cutting
(1) working principle of gas cutting
Oxygen cutting is called gas cutting.
During gas cutting, firstly, the metal at the cutting place is heated to the ignition point by oxygen-acetylene flame, and then the metal is violently oxidized into slag by injecting hyperbaric oxygen airflow, which is blown off from the incision, thus separating the metal (Figure 4-49). When cutting, use a knife (Figure 4-50).
Figure 4-48 Gas welding
Figure 4-49 Gas cutting
Figure 4-50 Structure of Jet Suction Cutting Torch
1- oxygen inlet; 2- acetylene import; 3- acetylene control valve; 4- oxygen regulating valve; 5- hyperbaric oxygen valve; 6— Nozzle; 7- pat the straw; 8- mixed trachea; 9- hyperbaric oxygen trachea; 10- cutting nozzle
The process of gas cutting is to spray the mixed gas of oxygen and acetylene from the nozzle (Figure 4-50), use the ignited preheating flame to heat the metal at the cutting point, and then spray a high-pressure pure oxygen stream from the central outlet to blow away the slag.
(2) the scope of application of gas cutting
Gas cutting is generally only suitable for cutting medium and low carbon steel, and the cutting quality of high carbon steel is poor because the ignition point is close to the melting point. The melting point of cast iron is lower than its ignition point, so gas cutting cannot be used. Non-ferrous metals have good thermal conductivity and are easy to oxidize, so they cannot be cut by gas.