General information about welding

 

 FUNDAMENTALS OF WELDING PRODUCTION

 General information about welding

Welding is the technological process of forming permanent joints due to the formation of atomic-molecular bonds between the particles of the mating parts after melting, local plastic deformation and diffusion of atoms.

 Welding of metals and their alloys is of the greatest industrial importance, but welding of non - metallic materials is also possible- glass, plastic, ceramics, etc.

 The beginning of the development of welding technology coincides with the border of the XIX and XX centuries. At first, gas welding based on oxygen and acetylene was of primary importance. Methods of their storage and transportation were developed. Electric arc fusion welding was proposed in 1882. Russian inventor N. N. Benardos, using an electric arc with a carbon electrode (he dreamed up and received the grand Prix at an exhibition in Brussels). The use of a melting electrode for welding, which served as a filler material at the same time, was proposed in 1888 by the Russian engineer N. G. Slavyanov. At that time, all-in-one joints were in the form of rivets, which quickly collapsed, and the workers became capercaillie.

 In 1935-36. Paton E. O. (father) developed a method of semi-automatic submerged welding. Now in Ukraine, the Institute of electric welding under the leadership of Academician B. E. Paton, created by E. O. Paton, is engaged in welding issues. Automatic submerged, electroslag welding for welding vertical seams of unlimited metal thickness, etc. The development of welding production went in two directions :

 - development of new welding methods;

 - development of welding materials science that ensures high quality of the seam metal.

 The use of new welding methods ensures the production of welded structures with specified dimensions that do not require mechanical processing.

 Welded structures have many advantages compared to riveted ones. Welding reduces the mass of structures by 10-20%, reduces the labor intensity and production time. During welding, tight seams are formed that ensure the tightness of tanks, boilers, tanks, wagons, pipelines, car hulls, tractors, ships, etc. Welding allows you to connect elements with different thicknesses and simplify the manufacturing technology of complex components and structures. The possibility of mechanization and automation of production processes, high quality of welded joints and rational use of metal have made welding a progressive, high-performance and economically profitable technological process.

 b) The essence of welding and the classification of its methods.

 The formation of an integral joint during welding occurs due to atomic-molecular bonds between the contacting surfaces, for which it is necessary to bring these molten surfaces closer to an atomic distance and give them energy.

 Depending on the activation method, the formation of bonds can occur in the solid or liquid phases. In accordance with this, all welding methods are divided into two main groups:

 - welding by plastic deformation (pressure);

 - fusion welding.

 During pressure welding, the convergence of atoms and molecules and the activation of the surface of the joined materials are achieved as a result of joint elastic-plastic deformation. Pressure welding is carried out either without heating, or by heating to the flow temperature, or by explosion, ultrasound, friction, etc.

 When welding by melting, the parts are connected by local melting of the metal along the edges of the base metal and the electrode (or filler metal).

 The metal melted during welding forms a common welding bath, while the destruction of the oxide films covering the surface of the connected elements is achieved, and the atoms converge to a distance at which bonds between them arise. After the crystallization of the metal, a weld with a cast structure is formed. For the melting of the base and filler metal, heat sources with a temperature of at least 3000 °C are used. Depending on the nature of the heat source, electric and chemical fusion welding are distinguished.

 In electric fusion welding, the heat source is an electric current. In chemical fusion welding, an exothermic combustion reaction of gases (gas welding) or a powdered combustible mixture (thermite welding) is used as a source of heat.Gorenje is a chemical fusion welding process. Melting welding includes: - arc, - plasma-arc; - electron beam; - electroslag; - laser, - gas; - termite. For pressure welding: - contact; - induction; - diffusion; - thermocompressor; - cold, - by friction, - by explosion, - ultrasonic.

Basics of manual arc welding

 In arc welding, the source of heat is an electric arc burning between the welded (base) metal and one or two electrodes. The electrode rod melts and the molten metal drips into the welding bath. Together with the rod, the electrode coating melts, forming a gas or gas-slag protection of the arc and the welding bath, which isolates them from the air. As the arc moves, the metal of the welding bath hardens and a welding seam is formed. As the liquid slag cools, it forms a slag crust on the seam surface.

 During the welding process, as a result of heating and cooling, the structure and properties change in the sections of the base metal adjacent to the seam. The entire zone of the base metal, in which the structure and properties change as a result of heating and cooling, is called the zone of thermal influence. The structure of the thermal influence zone for structural steel is shown in Fig. 19.2.

 I. The section of full-melting (seam metal) has a coarse-grained cast structure when cooling.

 II. The area of incomplete melting is a transition from the deposited metal to the main one and is hummed by the _ fusion zone.

 III. The superheat part is heated to 1100-1300°With and is characterized by grain. Zones II-III are called the near-wave zone. In this zone, as a result of heating and cooling, the structure and properties of the base metal that determine weldability change most sharply, ductility and toughness decrease.

 IV. The normalization site is heated above point A3 and is characterized by grain grinding and increased mechanical properties.

 V. The site of incomplete recrystallization is characterized by heating from A1 to A3. Structural changes in this zone have little effect on the properties of welded joints.

 VI-VII _particles - zones of recrystallization and aging.

 When welding with a direct arc, heat is released in the arc between the metal and the electrode (a), and when welding with an indirect arc (b), the metal melts in the arc column between the two electrodes. Use alternating and direct current, or three-phase current.

 A welding arc is a stationary electric discharge in gases and vapors between energized electrodes. The welding arc is characterized by a high temperature of gases and a large current in the discharge zone. The distance between the electrodes is the area of the arc discharge or the length of the arc. An electric arc is ionized air under the influence of voltage.

 The arc ignition during welding with a melting electrode begins with a short circuit of the electrode with the product. The short-circuit current instantly melts the metal at the contact point, resulting in a liquid bridge. When the electrode is removed from the product, the liquid bridge stretches, the metal overheats and its temperature reaches the boiling point, metal vapors and gases are ionized under the influence of thermo - and auto - electronic emission-an arc is excited.

An electric arc is a concentrated source of heat. DC power N = U I. (1)

 However, not all the arc power is spent on heating and melting the metal. The efficiency of h = Npol /N is " 50%. For automatic submerged arc welding h "0.9, for manual arc welding h" 0.8.

 The electric arc consists of three parts: cathode (Lc), arc column (Lc) and anode region (La). The length of the cathode region is ~ 10-5, the anode region is ~ 10-3 - 10-4 cm.

 The arc column can be considered as a gas plasma in thermodynamic equilibrium. Total arc voltage

 U = Uan + Ucp + Uk = Uk. a + Ec L, (2)

 where Uk. a = Uk + Ua is the total voltage drop in the cathode (Uk) and anode (Ua) regions, Ec is the field strength in the arc column of length Lc.

 The main parameters of the manual arc welding mode are the voltage U and the welding current Ic. The diameter of the de electrode is chosen depending on the thickness h of the metal being welded.

d, mm 1,5-2,5 3-4 4-5 > 5

h, mm 1-2 4-5 6-12 > 13

 The welding performance is determined by the amount of metal m deposited during time t and the welding current Ic:

 m = an ic t , (3)

 an is the surfacing coefficient, [an ] = (g/A) * hour. With manual arc welding, it is equal to 8-12 (g/A) * hour.

 The optimal value of the welding current Ic is set experimentally Ic = kdE, where dE is the diameter of the electrode, k is the coefficient depending on the type of material to be welded and its thickness. For electrodes with a rod made of low-carbon steel k = 30-60 A / mm, and of high-alloy steel k = 30-40 A / mm.

Stability of arc gorenje.

 An arc that burns evenly, without arbitrary breaks that require re-ignition, is called stable. If the arc burns unevenly, often breaks off and goes out, then such an arc is called unstable.

 The stability of the arc depends on many reasons, the main of which are:

 * current type and polarity;

 * composition and coating of the electrodes;

 · arc length.

 For coated electrodes with a diameter of 4...5 mm, the normal arc length is 5...6 mm. Such an arc is called short, it burns steadily and ensures the normal course of the welding process.

 An arc with a length greater than 6 mm is called a long one. The process of melting the metal proceeds unevenly at the same time. The resulting droplets of the electrode metal are more oxidized by air oxygen and enriched with nitrogen. The deposited metal turns out to be porous, the seam has an uneven surface, and the arc burns unstable. With a long arc, productivity decreases, metal splashing increases, places with non-steam are formed more often.

 In a welding arc, an arc column can be considered as a flexible conductor through which an electric current passes and which can change its shape under the influence of an electromagnetic magnetic field. The deflecting effect of magnetic fields on the welding arc is called magnetic blast.

 The strength of the magnetic field is proportional to the square of the current, so the magnetic blast is especially noticeable when welding with a direct current of a significant magnitude (over 300...400 A). When welding with alternating current with coated electrodes and submerged welding, the phenomenon of electromagnetic blowing has a weaker effect than with direct current and the use of bare or thin-coated electrodes. to reduce the deflecting effect of magnetic fields on the arc, welding should be carried out with as short an arc as possible.

 The main condition for the stable engorge of the welding arc is the compliance of the external (1) characteristics of the power supply _ with the static_(2) the characteristic of the arc (Fig. 19.4). The external characteristic is the relationship between the current strength in the welding circuit and the source voltage.

 Fig. 19.4. Unified energy system of external characteristics of the power supply (1) and the welding arc (2) The point A0 of the intersection corresponds to the steady-state mode of operation of U and I. The condition for the stability of the source - arc system is expressed in the following form:

 Cu = (dUu/dl - dUu/dl)Ip0>0 (4)

 Ku is the stability coefficient.

 For manual arc welding with low-density currents

 (dUu/dl)Ip 0 < (dUd/dl)Ip 0 < 0. (5)

 In the A0 mode, the arc length is automatically maintained constant if the feed rate of the electrode wire is constant. As the arc length changes, the current also changes. The melting rate decreases, and the arc length is restored at a constant wire feed rate. This property of the arc is called self-regulation.

 When passing through the arc gap, the drops of molten electrode metal are affected by:

 * gravity;

 * surface tension forces;

 * electromagnetic field forces;

 * internal pressure forces of gases.

 The electrical characteristics of the arc determine the requirements for the welded joint, for power sources. In the steady state, the dependence between the voltage U and the current I is expressed by the static volt-ampere characteristic of the arc (Fig. 19. 1).

 I zone - an increase in the electrical conductivity of the arc column – large-drop metal transfer; II zone-an increase in the cross - section of the arc column; W zone – the cross-section cannot increase (limit) - small-drop metal transfer. Therefore, in electric arc welding, the rigid part of the vol-ampere characteristic of the arc is used.