Fusion welding, is one of the most common welding methods.
The so-called fusion welding refers to a welding method in which the metal at the joint is completed in a molten state by a high temperature or the like during the welding process, and a welded joint can be formed. Since the workpieces to be welded are closely attached together, under the action of temperature field, gravity, etc., without the pressure, the melt melted by the two workpieces may be mixed. After the temperature is lowered, the molten portion is condensed, and the two workpieces are firmly welded together to complete the welding method. Due to the high temperature phase transition process inherent in the welding process, heat affected zones are created in the weld zone. Solid-state welding and fusion welding are the opposite. Solid-state welding does not melt metal.
Welding classification
1, gas welding
The combustible gas used in gas welding is the same as gas cutting, mainly including acetylene, liquefied petroleum gas (propane, butane, propylene, etc.) and hydrogen, and oxygen is a combustion-supporting gas.
The welding wire for gas welding functions as a filler metal, and together with the molten base metal, forms a weld metal during welding. Therefore, the wire of the corresponding composition or properties should be selected according to the chemical composition and mechanical properties of the workpiece, and sometimes the strip cut from the plate to be welded can be used as the welding wire.
When welding non-ferrous metals, cast iron and stainless steel, solder powder (flux) should also be used to eliminate refractory oxide film and other impurities covering the surface of the welding consumables and the molten pool, and form a layer of slag on the surface of the molten pool. The protective molten pool metal is not oxidized, the gases, oxides and other impurities in the molten pool are excluded, the fluidity of the molten metal is improved, the welding is smooth, and the quality and forming are ensured.
Gas welding is mainly used for welding of thin steel plates, low melting point materials (nonferrous metals and alloys thereof), cast iron parts and cemented carbide tools, as well as wear and tear, repair welding of scrapped parts, and flame correction of component deformation.
The advantage of gas welding is that the equipment is simple (oxygen cylinder, acetylene bottle, tempering fuse, welding torch, pressure reducer, oxygen, acetylene, conveying pipe, etc.) flexible; it has good adaptability to the welding of cast iron and some non-ferrous metals. Gas welding can play a greater role when welding is required where power is insufficient. The disadvantage is that the production efficiency is low; the deformation and heat affected zone of the workpiece after welding are large; it is difficult to achieve automation.
2, arc welding
Arc welding is the most widely used welding method in industrial production. Its principle is to use the heat generated by arc discharge (commonly known as arc combustion) to melt the electrode and the workpiece and form a weld after condensation, so as to obtain a welding process of the firm joint. When arc welding low carbon steel or low alloy steel, the temperature of the central part of the arc can reach 6000~8000 °C, and the temperature of the two electrodes can reach 2400~2600 °C.
1) Hand arc welding
Manual arc welding can perform multi-position welding such as flat welding, vertical welding, horizontal welding and overhead welding. In addition, because the arc welding equipment is light and flexible, it can be used for short-slit welding in maintenance and assembly in any place with power supply. It is especially suitable for welding of hard-to-reach parts. Suitable for welding of various metal materials, various thicknesses and various structural shapes. Such as industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and alloys.
2) Submerged arc welding
Submerged arc welding is also a welding method that uses an electric arc as a heat source. In submerged arc welding, the arc is burned under the cover of a layer of granular meltable flux, and the arc light is not exposed. Submerged arc has two modes: automatic submerged arc welding and semi-automatic submerged arc welding. The former's wire feed and arc movement are automatically completed by a special handpiece, the latter's wire feed is done mechanically, and the arc movement is manually performed. The main advantages of submerged arc welding are:
1) The thermal efficiency is high, the penetration depth is large, the groove of the workpiece can be small, and the amount of filler metal is reduced;
2) The welding speed is high. When the steel plate with the thickness of 8~10mm is welded, the submerged arc welding speed of the single wire can reach 50~2000px/min;
3) The presence of the flux not only separates the contact of the molten metal with the air, but also causes the molten pool metal to solidify slowly, reducing the possibility of defects such as pores and cracks in the weld.
However, due to the use of granular flux, this welding method is generally only suitable for the flat welding position, and the relative position of the arc and the groove cannot be directly observed, and the welding bias is easy. In addition, it is not suitable for welding thin plates with a thickness of less than 1 mm.
Because submerged arc welding has a large penetration depth, high production efficiency, and high degree of mechanized operation, it is suitable for welding long welds of medium and heavy plate structures. It is widely used in the manufacturing sectors of shipbuilding, boilers and pressure vessels, bridges, cranes, railway vehicles, construction machinery, heavy machinery and metallurgical machinery, nuclear power plant structures and marine structures. It is the most commonly used welding method in today's welding production. one. Submerged arc welding can be developed from carbon structural steel to low alloy structural steel, stainless steel, heat resistant steel, and some non-ferrous metals such as nickel-based alloys, titanium alloys, and copper alloys.
q=IU/v
Where I--welding current (A);
U--Arc voltage (V);
V--welding speed (mm/s);
Q--heat input (J/mm).
For example, a low carbon steel plate with a thickness of 12 mm is double-sided submerged arc welding. The welding parameters are wire diameter 4 mm, welding current 600 A, arc voltage 38 V welding speed 8 mm/s, and the heat input is
q=(600A×38V)÷8mm/s=22800J/s÷8mm/s=2600J/mm
The heat input combines three welding parameters: welding current, arc voltage and welding speed. When the heat input is increased, the width of the heat-affected zone is increased, the area heated to a high temperature is widened, the residence time of the weldment at a high temperature is increased, and the cooling rate is slowed down.
3. Gas shielded arc welding
Arc welding using an applied gas as an arc medium and protecting the arc and the weld zone is referred to as gas shielded arc welding, referred to as gas welding.
Compared with other welding methods, gas welding has the following characteristics:
1) The visibility of the arc and the molten pool is good, and the welding parameters can be adjusted according to the condition of the molten pool during the welding process;
2) The welding process is convenient to operate, there is no slag or very little slag, and basically no slag is needed after welding;
3) The arc is concentrated under the compression of the protective airflow, the welding speed is faster, the molten pool is smaller, the heat affected zone is narrow, and the weldment is less deformed after welding;
4) facilitating mechanization and automation of the welding process, especially mechanized welding at spatial locations;
5) It is possible to weld magnesium, aluminum, titanium and alloys thereof which are chemically active and easily form a high melting point oxide film;
6) The thin plate can be welded;
7) When working outdoors, it is necessary to install a windshield, otherwise the gas protection effect is not good or even poor;
8) The optical radiation of the arc is very strong;
9) Welding equipment is more complicated and expensive than electrode arc welding equipment.
Gas electric welding is generally divided into non-melting pole (tungsten) inert gas shielded welding and gas metal arc welding, oxidizing mixed gas protective welding, CO2 gas shielded welding and tubular welding wire gas shielded welding according to whether the electrode is melted or protected.
A. Tungsten (non-melting pole) inert gas shielded welding.
Tungsten inert gas shielded welding is a welding method that uses an arc heat generated between a tungsten electrode and a workpiece to melt the base metal and fill the wire (if a filler wire is used) under the protection of an inert gas. When welding, a gas barrier is formed to insulate the air to prevent harmful effects on the tungsten electrode, the molten pool and the adjacent heat-affected zone, so that a good weld seam can be obtained, and the inert gas is mainly argon gas.
Tungsten argon arc welding operation modes are divided into three types: manual welding, semi-automatic welding and automatic welding. Tungsten inert gas shielded welding has the following advantages: it does not react with metals, and automatically removes the oxide film on the surface of the workpiece. It can weld chemically active non-ferrous metals, stainless steel, heat-resistant steel, etc. and various alloys; Ultra-thin plate material welding; welding in various positions is also an ideal method for double-sided forming of single-sided welding. The disadvantages are shallow penetration, low deposition rate and low productivity; the particles may enter the molten pool, causing pollution (tungsten); inert gases (argon, helium) are expensive and the production cost is high.
The thickness range of the plate to be welded by the tungsten inert gas shielded welding is preferably 3 mm or less from the viewpoint of productivity. For some thick-walled important components of black and non-ferrous metals (such as pressure vessels and pipes), tungsten inert gas shielded welding is also used to ensure high weld quality.
B. MIG welding.
This method also uses a continuous arc between the welding wire and the workpiece as a heat source, and the gas ejected from the torch nozzle protects the arc for welding. Unlike tungsten gas shielded welding, the wire as a pole is melted into a liquid metal during the welding process and filled in the weld. Therefore, in addition to the main advantages of non-melting gas shielded welding (can be welded at various locations; suitable for welding of most metals such as non-ferrous metals, stainless steel, heat-resistant steel, carbon steel, alloy steel), it also has The welding speed is faster and the welding efficiency is higher.
C. Carbon dioxide gas shielded welding.
Carbon dioxide gas shielded welding is a gas metal arc welding, which has the characteristics of high production efficiency, small welding deformation and wide application range. The arc is arc welding during welding, and the visibility is good. The semi-automatic welding method is convenient for welding the curved weld and the spatial position weld, and the operation is simple and easy to grasp, but the disadvantage is that the welding spatter is large and the wind resistance is poor. CO2 gas shielded welding is a widely used arc welding method, mainly used for welding metal structures such as automobiles, ships, pipelines, rolling stock, containers, mining and engineering machinery, power station equipment and buildings. From the material of the welded part, CO2 gas shielded welding can weld carbon steel and low alloy steel; from the thickness of the workpiece, the thin wire transition can be used to weld the thin plate; the thick wire droplet transfer method can be used for welding. , thick plate; from the welding position, it can be used for all-position welding, but also for flat welding, horizontal fillet welding and other spatial positions.
Plasma arc welding is also a non-melting arc welding, in which the plasma arc is compressed by a free arc, called a transfer arc. The ionic gas is argon, nitrogen, helium or a mixture of the two. The energy of the plasma arc is concentrated, the temperature is high, and the flame flow speed is large. These characteristics make plasma arcs widely used in welding, spraying and surfacing.
4, plasma arc welding
Compared with tungsten inert gas shielded welding, plasma arc welding has the following characteristics:
1) The energy of the plasma arc is concentrated and the temperature is high. For most metals, the small hole effect can be obtained within a certain thickness range, and the weld can be fully penetrated and the back surface can be uniformly formed;
2) The arc stiffness is good. The diffusion angle of the plasma arc is only about 5°, which is basically cylindrical. The change of arc length has little effect on the heating area and current density on the workpiece. Therefore, the influence of the arc length change of the plasma arc welding on the weld formation is not obvious;
3) The welding speed is faster than tungsten inert gas shielded welding;
4) It can weld thinner and thinner workpieces (such as welding of very thin metal below 1mm);
5) The equipment is complicated, the cost is high, and the adjustment of the process parameters is also complicated.
5. Electroslag welding: Electroslag welding is a method of welding by using the resistance heat generated when current flows through liquid slag.
6. Laser welding: Laser welding is a method in which a focused laser beam is used as an energy source to bombard the heat generated by the weldment.
7. Electron beam welding: Electron beam welding is a method of welding by using the accelerated and concentrated electron beam to bombard the heat generated by a weldment placed in a vacuum or non-vacuum.
Gas
1. During the welding process, the welding area is filled with a large amount of gas.
When welding with acid electrode, the main gas components are CO, H2, H2O; when welding with alkaline electrode, the main gas components are CO and CO2; when submerged arc welding, the main gas components are CO and H2.
The gas in the welding zone is mainly derived from the following aspects: First, in order to protect the welding area from air intrusion, artificially add a protective gas in the welding area, such as gas generating agent in the coating (starch, wood flour, marble). Etc.) Gas generated by thermal decomposition, protective gas (CO2 gas, Ar gas) used for gas shielded welding, and the like, followed by gas, poorly protected air, welding wire, and the like when welding with a wet electrode or flux. The impurities (oil, rust, paint, etc.) on the surface of the base metal are heated by the heat, and the gases generated by the high temperature evaporation of the metal and slag.
2. The effect and influence of nitrogen, hydrogen and oxygen on weld metal
(1) Nitrogen Nitrogen mainly comes from the air around the weld area. In hand arc welding, the weld overlay metal contains approximately 0.025% nitrogen. Nitrogen is an element that increases the strength of weld metal, reduces ductility and toughness, and is one of the main causes of porosity in welds.
(2) Hydrogen Hydrogen is mainly derived from the electrode coating, the moisture in the flux, the organic matter in the coating, the dirt (rust, oil) on the surface of the weldment and the wire, and the moisture in the air. All kinds of welding methods increase the hydrogen in the weld, but the degree of hydrogen increase is different: the weld seam welded with the cellulose coating electrode in hand arc welding has 70 times higher hydrogen content than the base material; only the low hydrogen type electrode is used. When welding, the hydrogen content of the weld is relatively low; while with CO2 gas shielded welding, the hydrogen content is the lowest.
Hydrogen severely reduces the plasticity of the weld metal, causing pores and time-delayed cracks in the welded joint, and also forming white spots on the section of the tensile specimen.
(3) Oxygen Oxygen is mainly derived from oxides, moisture and oxides on the surface of solder materials in air, coating and flux. As the oxygen content in the weld increases, its strength, hardness and plasticity will decrease significantly, which will also cause hot brittle, cold brittle and age hardening of the metal, and also the main cause of the formation of pores (CO pores) in the weld. One.
In short, nitrogen, hydrogen, and oxygen entering the weld metal are all harmful elements.
3. Protection of the welding area The welding purpose of the welding area is to prevent air from intruding into the droplets and the molten pool, and to reduce the nitrogen and oxygen content in the weld metal. There are three ways to protect:
(1) Gas protection For example, gas shielded welding uses a shielding gas (CO2, H2, Ar) to isolate the welded area from the air.
(2) Slag protection The molten metal surface is covered with a layer of molten slag to separate it from the air, such as electroslag welding and submerged arc welding.
(3) Gas-slag joint protection The protective gas and slag are simultaneously protected by molten metal, such as hand arc welding.
4. Reduce the oxygen content in the weld metal
Protecting the weld area and preventing the air from coming into contact with the molten metal is an important measure to control the oxygen content in the weld metal, but it cannot solve the problem fundamentally, because oxygen can also enter the weld through many other channels, and these channels should be completely blocked. In fact, it is impossible, so only measures can be taken to deoxidize the oxygen that has entered the molten metal.
5, the common deoxidation method for weld metal
The use of slag or core (wire) metal to interact with molten metal for deoxidation is a common deoxidation method for weld metal.
(1) Diffusion deoxidation When the temperature drops, the FeO originally melted in the molten pool will continuously diffuse to the slag, thereby reducing the oxygen content in the weld. This deoxidation method is called diffusion deoxidation.
If there is a strong acidic oxide SiO2, TiO2, etc. in the slag, they will form a complex with FeO, and the reaction formula is
(SiO2+FeO)= FeO·SiO2
(TiO2+FeO)= FeO·TiO2
As a result of the reaction, the free FeO in the slag is reduced, which causes the [FeO] in the molten pool metal to continuously diffuse into the slag, and the content in the weld metal is thus reduced.
The acidic slag (such as the slag formed by melting the electrode J422 and the flux HJK431) contains a large amount of SiO2 and TiO, so the deoxidation method is mainly diffusion deoxidation. However, under the welding conditions, the molten pool has a fast cooling rate, the interaction time between the slag and the liquid metal is short, and the diffusion deoxidation is insufficient. Therefore, the weld seam welded by the acid electrode (agent) has an oxygen content. Higher, the weld metal has lower plasticity and toughness.
6. Deoxidation with a deoxidizer adds an element to the core, the coating or the wire to cause it to be oxidized during the welding process, thereby ensuring that the metal to be welded and its alloy elements are not oxidized or the metal that has been oxidized is reduced. This element for deoxidation is called a deoxidizer. Commonly used deoxidizers are carbon, manganese, silicon, titanium and aluminum.
The deoxidizer of the alkaline electrode is added to the coating in the form of an iron alloy, such as ferromanganese, ferrosilicon, and the like. Submerged arc welding often uses alloy welding wire, such as H08MnA, H10MnSi and so on.
Deoxidation with deoxidizer is much better than diffusion deoxidation. Therefore, welds welded with alkaline electrodes have lower oxygen content than those with acid electrodes, and the plasticity and toughness are correspondingly improved. Therefore, basic welding rods are commonly used. To weld alloy steel and important welded structures.
7. Method for reducing hydrogen content in weld metal
Common measures to reduce the hydrogen content in weld metal are:
1) drying the flux of the electrode;
2) Remove the impurities on the weldment and the surface of the wire and keep the surface of the wire and the weldment as dry as possible;
3) Add appropriate amount of fluorspar (CaF2) and silica sand (SiO2) to the coating and flux, both of which have good dehydrogenation effect;
4) Immediately after welding, the weldment is heated and post-heat treated;
5) Low hydrogen type electrode, ultra low hydrogen type electrode and alkaline flux.
8. Harm of sulfur in weld metal
Sulfur is one of the most harmful elements in the weld. Sulfur can cause hot cracking in the weld metal, reduce impact toughness and corrosion resistance, and promote segregation. Sulfur can also cause lamellar tears when thick plates are welded.
Sulfur exists in the form of FeS in the liquid metal, and Mn, MnO, and CaO in the slag have a certain desulfurization effect; the reaction formula is as follows
[Mn]+[FeS] =[MnS]+[Fe]
[MnO]+[FeS]=[MnS]+[FeO]
[CaO]+[FeS] =[CaS]+[FeO]
The generated MnS and CaS all enter the slag. Since both MnO and CaO are alkaline oxides and have a large content in the alkaline slag, the desulfurization ability of the alkaline slag is stronger than that of the acidic slag.
9. Harm of phosphorus in weld metal.
Phosphorus is also one of the most harmful elements in the weld. Phosphorus increases the cold brittleness of the steel, greatly reduces the impact toughness of the weld metal, and increases the brittle transition temperature. Phosphorus also causes hot cracking in the weld metal when the austenitic steel or weld has a high carbon content.
Phosphorus exists in the liquid metal in the form of Fe2P, P2O5. The dephosphorization reaction can be carried out in two steps: the first step is to oxidize phosphorus to P2O5; the second step is to form a stable complex with the basic oxide CaO in the slag into the slag. Its reaction formula is
2[Fe2P]+5(FeO=P2O5+11[Fe]
P2O5+3(CaO)=(CaO)3·P2O5
P2O5+4(CaO)=(CaO)4·P2O5
Since the alkaline slag contains more CaO, the dephosphorization effect is better than the acidic slag.
However, in fact, whether it is alkaline slag or acidic slag, the final desulfurization and dephosphorization effects are still not satisfactory. Therefore, the method of controlling the sulfur and phosphorus content in the weld can only adopt the method of limiting the sulfur and phosphorus content of the raw materials (base metal, welding rod, welding wire).
10, alloying of weld metal
Alloying is the transition of the required alloying elements into the weld metal (or surfacing metal) through the welding material.
The purpose of alloying: 1) compensate for the loss of alloying elements due to oxidation, evaporation, etc. during the welding process; 2) improve the microstructure and properties of the weld metal; 3) obtain surfacing metal with special properties.
Commonly used alloying methods include: application of alloy welding wire; application of flux cored wire or flux cored electrode; application of alloy coating or bonding flux; application of alloy powder; application of displacement reaction between slag and metal.
11, the transition coefficient of alloying elements
During the welding process, some of the alloying elements are lost due to oxidation, evaporation, etc., and it is impossible to completely transition into the weld. The transition coefficient of the alloying element refers to the percentage of the alloying element in the welding material that transitions to the surfacing metal and its original content, ie
Η--the transition coefficient (%) of an alloying element;
CF--the content of an alloying element in the surfacing metal;
CT--the original total content of an alloying element in the electrode (wire, flux).