Download
1 / 140
1.4k likes | 1.43k Views
QETA/018 Manual Metal Arc (MMA) Welding. 501/1130/9 EAL Level 3 Diploma in Engineering Technology (QCF). Session Aims. 18.1 Understand the principles of Manual Metal Arc (MMA) Welding 18.2 Understand Metallurgy associated with welding 18.3 Understand welding health and safety
E N D
QETA/018 Manual Metal Arc (MMA) Welding 501/1130/9 EAL Level 3 Diploma in Engineering Technology (QCF)
Session Aims 18.1 Understand the principles of Manual Metal Arc (MMA) Welding 18.2 Understand Metallurgy associated with welding 18.3 Understand welding health and safety 18.4 Understand equipment associated with Manual Metal Arc (MMA) Welding 18.5 Understand the consumables used in Manual Metal Arc (MMA) welding 18.6 Understand welding procedures and methods of testing applied to Manual Metal Arc Welding
18.1Understand the principles of Manual Metal Arc (MMA) Welding You will be able to: • Describe electrical/electric arc theory • Describe fusion arc welding principles • Describe specific principles applicable to MMA Welding • Identify types of welds and joints
The Nature of Electricity (the Flow of Electrons) Electron flow is what we think of as electrical current. We are familiar with two types of electron flow, Direct Current, or DC, and Alternating Current, or AC. Direct Current is the kind of electrical flow we get from batteries and solar cells, when electrons travel in only one direction. An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma. The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of one coulomb per second. Electric current is measured using a device called an ammeter.
The Nature of Electricity (the Flow of Electrons) Electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors, inductors and generators. The particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom, and can move freely about within the metal. These conduction electrons are the charge carriers in metal conductors.
AC and DC Current AC and DC If the current flows in only one direction it is called direct current, or DC. Batteries and solar cells supply DC electricity. A typical battery may supply 1.5V. The diagram shows an oscilloscope screen displaying the signal from a DC supply. If the current constantly changes direction it is called alternating current, or AC. Mains electricity is an AC supply. The UK mains supply is about 230V. It has a frequency of 50Hz (50 hertz), which means that it changes direction and back again 50 times a second. The diagram shows an oscilloscope screen displaying the signal from an AC supply.
Sinusoidal Waveform and Polarity Sinusoidal Waveform (or Sine Wave) The sine wave or sinusoid is a mathematical curve that describes a smooth repetitive oscillation. It is named after the function sine, of which it is the graph. It occurs often in pure and applied mathematics, as well as physics, engineering, signal processing and many other fields. The sine wave is important in physics because it retains its wave shape when added to another sine wave of the same frequency and arbitrary phase and magnitude. It is the only periodic waveform that has this property. https://en.wikipedia.org/wiki/File:ComplexSinInATimeAxe.gif
Electrical Polarity Electrical polarity (positive and negative) is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current (DC) circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only.
Voltage, Current, Resistance, Power and Energy Voltage Voltage, also called electromotive force, is a quantitative expression of the potential difference in charge between two points in an electrical field. The greater the voltage, the greater the flow of electrical current (that is, the quantity of charge carriers that pass a fixed point per unit of time) through a conducting or semiconducting medium for a given resistance to the flow. Voltage is symbolized by an uppercase italic letter V or E. The standard unit is the volt, symbolized by a non-italic uppercase letter V. One volt will drive one coulomb (6.24 x 1018) charge carriers, such as electrons, through a resistance of one ohm in one second.
Voltage, Current, Resistance, Power and Energy Current Current is a flow of electrical charge carriers, usually electrons or electron-deficient atoms. The common symbol for current is the uppercase letter I. The standard unit is the ampere, symbolized by A. One ampere of current represents one coulomb of electrical charge (6.24 x 1018 charge carriers) moving past a specific point in one second. Physicists consider current to flow from relatively positive points to relatively negative points; this is called conventional current or Franklin current. Electrons, the most common charge carriers, are negatively charged. They flow from relatively negative points to relatively positive points. Electric current can be either direct or alternating. Direct current (DC) flows in the same direction at all points in time, although the instantaneous magnitude of the current might vary. In an alternating current (AC), the flow of charge carriers reverses direction periodically. The number of complete AC cycles per second is the frequency, which is measured in hertz. An example of pure DC is the current produced by an electrochemical cell. The output of a power-supply rectifier, prior to filtering, is an example of pulsating DC. The output of common utility outlets is AC.
Voltage, Current, Resistance, Power and Energy Resistance Resistance is the opposition that a substance offers to the flow of electric current. It is represented by the uppercase letter R. The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega. When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm. (For more discussion of the relationship among current, resistance and voltage, see Ohm's law.)
Voltage, Current, Resistance, Power and Energy Power Electrical power is the rate at which electrical energy is converted to another form, such as motion, heat, or an electromagnetic field. The common symbol for power is the uppercase letter P. The standard unit is the watt, symbolized by W. In utility circuits, the kilowatt (kW) is often specified instead;1 kW = 1000 W. One watt is the power resulting from an energy dissipation, conversion, or storage process equivalent to one joule per second. When expressed in watts, power is sometimes called wattage. The wattage in a direct current (DC) circuit is equal to the product of the voltage in volts and the current in amperes. This rule also holds for low-frequency alternating current (AC) circuits in which energy is neither stored nor released. At high AC frequencies, in which energy is stored and released (as well as dissipated or converted), the expression for power is more complex.
Voltage, Current, Resistance, Power and Energy Energy • Energy causes things to happen around us. Look out the window. • During the day, the sun gives out light and heat energy. At night, street lamps use electrical energy to light our way. • When a car drives by, it is being powered by gasoline, a type of stored energy. • The food we eat contains energy. We use that energy to work and play. Thedefinition of energy in the introduction: "Energy Is the Ability to Do Work." Energy can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal energy), light (radiant energy), mechanical energy, and nuclear energy Energy makes everything happen and can be divided into two types: • Stored energy is called potential energy. • Moving energy is called kinetic energy.
Voltage, Current, Resistance, Power and Energy Energy is measured in many ways. One of the basic measuring blocks is called a Btu. This stands for British thermal unit and was invented by, of course, the English. Btu is the amount of heat energy it takes to raise the temperature of one pound of water by one degree Fahrenheit, at sea level. One Btu equals about one blue-tip kitchen match. One thousand Btus roughly equals: One average candy bar or 4/5 of a peanut butter and jelly sandwich. It takes about 2,000 Btus to make a pot of coffee. Energy also can be measured in joules. Joules sounds exactly like the word jewels, as in diamonds and emeralds. A thousand joules is equal to a British thermal unit. 1,000 joules = 1 Btu
Voltage, Current, Resistance, Power and Energy A piece of buttered toast contains about 315 kilojoules (315,000 joules) of energy. With that energy you could: • Jog for 6 minutes • Bicycle for 10 minutes • Walk briskly for 15 minutes • Sleep for 1-1/2 hours • Run a car for 7 seconds at 80 kilometres per hour (about 50 miles per hour) • Light a 60-watt light bulb for 1-1/2 hours • Or lift that sack of sugar from the floor to the counter 21,000 times!
Nature and characteristics of the Arc (AC & DC) An electric arc or arc discharge is an electrical breakdown of a gas that produces an ongoing plasmadischarge, resulting from a current through normally nonconductive media such as air. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on thermionic emission of electrons from the electrodes supporting the arc. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp". A drawn arc can be initiated by two electrodes initially in contact and drawn apart; this can initiate an arc without the high-voltage glow discharge. This is the way a welder starts to weld a joint, momentarily touching the welding electrode against the workpiece then withdrawing it till a stable arc is formed. Another example is separation of electrical contacts in switches, relays and circuit breakers; in high-energy circuits arc suppression may be required to prevent damage to contacts.
Nature and characteristics of the Arc (AC & DC) Industrially, electric arcs are used for welding, plasma cutting, for electrical discharge machining, as an arc lamp in movie projectors and follow spots in stage lighting. Electric arc furnaces are used to produce steel and other substances. Calcium carbide is made in this way as it requires a large amount of energy to promote an endothermicreaction (at temperatures of 2500 °C). Spark plugs are used in internal combustion engines of vehicles to initiate the combustion of the fuel in a timed fashion. Spark gaps are also used in electric stove lighters (both external and built-in).
Arc Power and Energy (Arc Voltage and Heat Input) Arc Voltage It is the voltage that appears across the contacts of the circuit breaker during the arcing period. As soon as the contacts of the circuit breaker separate, an arc is formed. The voltage that appears across the contacts during arcing period is called the arc voltage. Its value is low except for the period the fault current is at or near zero current point. At current zero, the arc voltage rises rapidly to peak value and this peak voltage tends to maintain the current flow in the form of arc. Heat Input Heat input (the present best practice term, as it provides a more relevant way of comparing arc welding processes) considers the effect process efficiency has on the energy that actually reaches the workpiece to form the weld. HI is given by the following relationship to Arc Energy or AE. The heat source of an Arc can get up to 6000 degrees Celsius.
Magnetic Arc Blow Magnetic arc blow or "arc wander" is the deflection of welding filler material within an electric arc deposit by a build up of magneticforce surrounding the weld pool. Magnetic arc blow can occur because of: • Workpiece connection • Joint design • Poor fit-up • Improper settings • Atmospheric conditions Arc blow tends to occur if the material being welded has residual magnetism at a certain level, particularly when the weld root is being made, and the welding current is direct current (DC positive or negative).
Magnetic Arc Blow Magnetic arc blow is popularly attributed to a change in the direction of current as it flows into and through the workpiece. Magnetic arc blow is known to begin at field densities as low as 10 gauss and becomes severe at densities of, equal to or greater than, 40 gauss; it is directional and can be classified as forward or backward moving along the joint, but can occasionally occur to the sides depending on the orientation of the poles to the workpiece. Magnetic arc blow is more common in DC welding than in AC welding.
Fusion Arc Welding Principles Weld Formation To strike the electric arc, the electrode is brought into contact with the workpiece by a very light touch with the electrode to the base metal then is pulled back slightly. This initiates the arc and thus the melting of the workpiece and the consumable electrode, and causes droplets of the electrode to be passed from the electrode to the weld pool. As the electrode melts, the flux covering disintegrates, giving off shielding gases that protect the weld area from oxygen and other atmospheric gases. In addition, the flux provides molten slag which covers the filler metal as it travels from the electrode to the weld pool. Once part of the weld pool, the slag floats to the surface and protects the weld from contamination as it solidifies. Once hardened, it must be chipped away to reveal the finished weld. As welding progresses and the electrode melts, the welder must periodically stop welding to remove the remaining electrode stub and insert a new electrode into the electrode holder..
Fusion Arc Welding Principles This activity, combined with chipping away the slag, reduces the amount of time that the welder can spend laying the weld, making SMAW one of the least efficient welding processes. The actual welding technique utilized depends on the electrode, the composition of the workpiece, and the position of the joint being welded. The choice of electrode and welding position also determine the welding speed. Flat welds require the least operator skill, and can be done with electrodes that melt quickly but solidify slowly. This permits higher welding speeds
‘Metal Arc’ An electric current, in the form of either alternating current or direct current from a welding power supply, is used to form an electric arc between the electrode and the metals to be joined. The workpiece and the electrode melts forming the weld pool that cools to form a joint. As the weld is laid, the flux coating of the electrode disintegrates, giving off vapours that serve as a shielding gas and providing a layer of slag, both of which protect the weld area from atmospheric contamination.
Function of Electrodes Welding electrodes in arc welding an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding or shielded metal arc welding, or non-consumable, such as in gas tungsten arc welding.
Function of Electrodes The choice of electrode for SMAW depends on a number of factors, including the weld material, welding position and the desired weld properties. The electrode is coated in a metal mixture called flux, which gives off gases as it decomposes to prevent weld contamination, introduces deoxidizers to purify the weld, causes weld-protecting slag to form, improves the arc stability, and provides alloying elements to improve the weld quality.Electrodes can be divided into three groups—those designed to melt quickly are called "fast-fill" electrodes, those designed to solidify quickly are called "fast-freeze" electrodes, and intermediate electrodes go by the name "fill-freeze" or "fast-follow" electrodes. Fast-fill electrodes are designed to melt quickly so that the welding speed can be maximized, while fast-freeze electrodes supply filler metal that solidifies quickly, making welding in a variety of positions possible by preventing the weld pool from shifting significantly before solidifying. The composition of the electrode core is generally similar and sometimes identical to that of the base material. But even though a number of feasible options exist, a slight difference in alloy composition can strongly impact the properties of the resulting weld. This is especially true of alloy steels such as HSLA steels.
Function of Electrodes Likewise, electrodes of compositions similar to those of the base materials are often used for welding nonferrous materials like aluminium and copper.However, sometimes it is desirable to use electrodes with core materials significantly different from the base material. For example, stainless steel electrodes are sometimes used to weld two pieces of carbon steel, and are often utilized to weld stainless steel workpieces with carbon steel workpieces. Electrode coatings can consist of a number of different compounds, including rutile, calcium fluoride, cellulose, and iron powder. Rutile electrodes, coated with 25%–45% TiO2, are characterized by ease of use and good appearance of the resulting weld. However, they create welds with high hydrogen content, encouraging embrittlement and cracking. Electrodes containing calcium fluoride (CaF2), sometimes known as basic or low-hydrogen electrodes, are hygroscopic and must be stored in dry conditions. They produce strong welds, but with a coarse and convex-shaped joint surface. Electrodes coated with cellulose, especially when combined with rutile, provide deep weld penetration, but because of their high moisture content, special procedures must be used to prevent excessive risk of cracking. Finally, iron powder is a common coating additive that increases the rate at which the electrode fills the weld joint, up to twice as fast.
Open Circuit and Arc Voltage The highest voltage is the open circuit voltage of the power source. Once the arc is struck the voltage rapidly falls as the gases in the arc gap become ionised and electrically conductive, the electrode heats up and the size of the arc column increases. The welding current increases as the voltage falls until a point is reached at which time the voltage/current relationship becomes linear and begins to follow Ohms Law. What is important to note from Fig. 1 is that as the arc length changes both the voltage and welding current also change – a longer arc giving higher voltage but with a corresponding drop in welding current and vice versa.
ISO Weld Symbols ISO Weld Symbols Please refer to the following links: http://www.draftsperson.net/images/6/63/Weld_Symbols.png http://triblab.teipir.gr/files/Welding/Lab/CH3_1_Welding_joint_symbols.pdf Refer to the different types of welded joints such as Butt, Tee, Lap and Corner.
Fillet and Butt Weld Characteristics Fillet Weld Characteristics: • Leg Length • Throat Thickness • Penetration • Number of Runs • Surface Finish • Weld Toes and Weld Profile Butt Weld Characteristics: • Types of Preparation • Number of Runs • Excess weld metal • Penetration • Surface Finish
18.2 Understand metallurgy associated with welding • Describe the effects of heating and cooling metals • Describe the effects of welding on metals • Describe how cracking occurs in welds • Describe residual stress
The Effects of Heating and Cooling Metals Heating Metals and Colling slowly and rapidly Heat treating is a group of industrial and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, normalizing and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding. With the exception of stress-relieving, tempering, and aging, most heat treatments begin by heating an alloy beyond the upper transformation temperature. This temperature is referred to as an "arrest" because, at the upper transformation temperature nothing happens. Therefore, the alloy must be heated above the temperature for a transformation to occur. The alloy will usually be held at this temperature long enough for the heat to completely penetrate the alloy, thereby bringing it into a complete solid solution.
The Effects of Heating and Cooling Metals Because a smaller grain size usually enhances mechanical properties, such as toughness, shear strength and tensile strength, these metals are often heated to a temperature that is just above the upper critical temperature, in order to prevent the grains of solution from growing too large. For instance, when steel is heated above the upper critical temperature, small grains of austenite form. These grow larger as temperature is increased. When cooled very quickly, during a martensite transformation, the austenite grain-size directly affects the martensitic grain-size. Larger grains have large grain-boundaries, which serve as weak spots in the structure. The grain size is usually controlled to reduce the probability of breakage. The diffusion transformation is very time-dependent. Cooling a metal will usually suppress the precipitation to a much lower temperature. Austenite, for example, usually only exists above the upper critical temperature. However, if the austenite is cooled quickly enough, the transformation may be suppressed for hundreds of degrees below the lower critical temperature. Such austenite is highly unstable and, if given enough time, will precipitate into various microstructures of ferrite and cementite. The cooling rate can be used to control the rate of grain growth or can even be used to produce partially martensitic microstructures.However, the martensite transformation is time-independent. If the alloy is cooled to the martensite transformation (Ms) temperature before other microstructures can fully form, the transformation will usually occur at just under the speed of sound
Microstructures of Welded Joints Unlike in casting, during welding, where the molten pool is moved through the material, the growth rate and temperature gradient vary considerably across the weld pool. Geometrical analyses have been developed that relate welding speed to the actual growth rates of the solid at various locations in the weld pool.Along the fusion line the growth rate is low while the temperature gradient is steepest. As the weld centre line is approached, the growth rate increases while the temperature gradient decreases. Consequently, the microstructure that develops varies noticeably from the edge to the centre line of the weld. Most of these microstructural features can be interpreted by considering classical theories of nucleation and growth.
Microstructures of Welded Joints In welds, weld pool solidification often occurs without a nucleation barrier. Therefore, no significant undercooling of the liquid is required for nucleation of the solid. Solidification occurs spontaneously by linear growth on the partially melted grains. This is the case during autogenous welding. In certain welds, where filler metals are used, inoculants and other grain-refining techniques are used in much the same way as they are in casting practices. In addition, dynamic methods for promoting nucleation such as weld-pool stirring and arc oscillation have been used to refine the weld metal solidification structure.Although the mechanisms of nucleation in weld metal are reasonably well understood, not much attention is given to modelling this phenomenon. Often, weld solidification models assume epitaxial growth and for most of the cases the assumption seems to be appropriate. However, to describe the effects of inoculants, arc oscillations, and weld pool stirring, heat and mass transfer models have to be coupled with either probabilistic models such as cellular automataor deterministic models using the fundamental equations of nucleation as described elsewhere
Microstructures and the Changes produced by welding During growth of the solid in the weld pool, the shape of the solid-liquid interface controls the development of microstructural features. The nature and the stability of the solid-liquid interface is mostly determined by the thermal and constitutional conditions (constitutional supercooling) that exist in the immediate vicinity of the interface. Depending on these conditions, the interface growth may occur by planar, cellular, or dendritic growth. Dendritic growth of the solid, with its multiple branches, is shown in Figure 3. Another example of changes in solidification morphology directly related to welding conditions is shown in Figure 4. This figure shows a spot weld on a nickel-based superalloy in which the morphology changes from cellular to dendritic as the growth velocity increases toward the centre of the spot weld after the spot weld arc is extinguished. The micrograph also shows the elimination of a poorly aligned dendrite
Cracking in Welding Definition - What does Cold Cracking mean? • Cold cracking is cracking that occurs as the result of hydrogen dissolving in the weld metal and then diffusing into the heat affected zone (HAZ). Cold cracks mostly develop long after the weld metal solidifies, but sometimes appear sooner. Cold weld cracking occurs at temperatures well below 600°F. It is considered a serious welding defect because it can significantly affect the integrity of infrastructure. • Cold cracking is also known as hydrogen-induced cracking, delayed cracking or underbead cracking.
Cracking in Welding • What exactly is Cold Cracking?? • Cold cracking is especially common in thick materials, as they tend to create areas of high restraint and can serve as a heat sink that leads to fast cooling rates. Rapid cooling causes the microstructure in the HAZ to form a new crystalline microstructure called martensite, which very hard, very brittle and lacks ductility. Martensite provides a location for diffusible hydrogen to gather, which causes residual stresses to build in the HAZ. Once these residual stresses reach a critical level, cold cracking occurs. • Cold cracking may occur for the following reasons: • There is hydrogen in the weld material or atmosphere • Susceptible microstructure (martensite) • Mechanical stresses (thermal or residual stresses) • The following can be done to prevent cold cracking of a metal: • Pre-heating the base material in order to reduce the speed of cooling, preventing the formation of martensite on the weld and allowing the hydrogen to be removed from the weld • Reducing tension concentration, avoiding discontinuities on the weld or carefully selecting the disposition of the welds and the assembly sequence of the structure • Using welding consumables with low hydrogen to minimize the hydrogen diffusion on the weld • Selecting the appropriate welding process
How Hydrogen appears in Cold Cracking Weld metal hydrogen content The principal source of hydrogen is moisture contained in the flux, i.e. the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. The amount of hydrogen generated is influenced by the electrode type. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes. It is important to note that there can be other significant sources of hydrogen, e.g. from the material, where processing or service history has left the steel with a significant level of hydrogen or moisture from the atmosphere. Hydrogen may also be derived from the surface of the material or the consumable. • Sources of hydrogen will include: • oil, grease and dirt • rust • paint and coatings • cleaning fluids
Lamellar Tearing As lamellar tearing is associated with a high concentration of elongated inclusions oriented parallel to the surface of the plate, tearing will be transgranular with a stepped appearance. Causes • It is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur: • Transverse strain - the shrinkage strains on welding must act in the short direction of the plate ie through the plate thickness • Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions • Material susceptibility - the plate must have poor ductility in the through-thickness direction • Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen • Factors to be considered to reduce the risk of tearing • The choice of material, joint design, welding process, consumables, preheating and buttering can all help reduce the risk of tearing.
Hot Cracking (1) Solidification cracks can appear in several locations, and orientations, but most commonly are longitudinal centreline cracks (coincident with the intersection of grains growing from opposite sides of the weld), or 'flare' cracks, again longitudinal, but at an angle to the through-thickness direction ( Fig.1). Where there is a central segregate band in the plate, cracking may extend from this position at the fusion boundary ( Fig.2). The cracks in all locations can be buried (Fig.3) or surface-breaking.
Hot Cracking (2) Cracking is associated with impurities, particularly sulphur and phosphorus, and is promoted by carbon whereas manganese and silicon can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon-manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%. Weld metal composition is dominated by the consumable and as the filler is normally cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Plate composition assumes greater importance in high dilution situations such as when welding the root in butt welds, using an autogenous welding technique like TIG, or a high dilution process such as submerged arc welding.
