CSWIP 3.1 Welding InspectionTWI Training & Examination ServicesCourse Reference WIS 5Manual Metal Arc WeldingorShielded Metal Arc Welding

Manual Metal Arc IntroductionThe MMA welding Process was first developed in the late 19th century using bare wire consumables. MMA is a simple process in terms of equipmentThe process can by used in AC, DC+ or DC-The process is a manual process and demands a high skill from the welder.The process is widely used throughout the welding industry both for shop and site working conditions

MMA - Principle of operation

MMA weldingMain features: Shielding provided by decomposition of flux coveringElectrode consumableManual process

Welder controls:Arc lengthAngle of electrodeSpeed of travelAmperage settings

Manual Metal Arc Basic EquipmentPower sourceHolding ovenInverter power sourceElectrode holderPower cablesWelding visor filter glassReturn leadElectrodesElectrode ovenControl panel (amps, volts)

MMA Welding PlantTransformer:Changes mains supply voltage to a voltage suitable for welding. Has no moving parts and is often termed static plant.Rectifier:Changes a.c. to d.c., can be mechanically or statically achieved.Generator:Produces welding current. The generator consists of an armature rotating in a magnetic field, the armature must be rotated at a constant speed either by a motor unit or, in the absence of electrical power, by an internal combustion engine.Inverter:An inverter changes d.c. to a.c. at a higher frequency.

Constant Current Power Source (Drooping Characteristic)

Constant Current Power Source (Drooping Characteristic)

Constant Current Power Source (Drooping Characteristic)The static characteristic:

Constant Current Power Source (Drooping Characteristic)Factors to be considered when selecting a MMA power source:type of current requiredamperage range requiredthe primary type of power availablethe position in which welding will be done

Constant Current Power Source (Drooping Characteristic)Combined characteristic - the arc force option

MMA Welding VariablesVoltageThe arc voltage in the MMA process is measured as close to the arc as possible. It is variable with a change in arc lengthO.C.V.The open circuit voltage is the voltage required to initiate, or re-ignite the electrical arc and will change with the type of electrode being used e.g 70-90 voltsCurrentThe current used will be determined by the choice of electrode, electrode diameter and material type and thickness. Current has the most effect on penetration.PolarityPolarity is generally determined by operation and electrode type e.g DC +ve, DC ve or AC

MMA Welding ChecksOCV open circuit voltsA check should be made to ensure that the equipment can produce the OCV required by the consumable and that any voltage selector has been moved to the correct positionCurrent & polarityA check should be made to ensure the current type and range is as detailed on the WPSOther variablesChecks should be made for correct electrode angle, arc gap and travel speedSafetyCheck should be made on the current carrying capacity, or duty cycle of the equipment and all electrical insulation is sound and in place. Correct extraction systems should be in use to avoid exposure to toxic fume.

MMA Welding ChecksThe welding equipmentA visual check should be made on the equipment to ensure it is in good working orderThe electrodesChecks should be made to ensure that the correct specification of electrode is being used, the electrode is the correct diameter and in good condition. Basic fluxed electrodes ensure the correct heat treatment is being carried out before use Rutile electrodes do not require pre-baking but should be stored in a dry condition.Cellulosic electrodes should not be dried

MMA electrode holderCollet or twist typeTongs type with spring-loaded jaws

MMA welding parametersWelding current approx. 35 A/mm of diametergoverned by thickness, type of joint and welding position Welding currentToo highToo lowpoor startingslag inclusionsweld bead contour too highlack of fusion/penetrationspatterexcess penetrationundercutburn-through

MMA welding parametersArc length = arc voltage Arc voltageToo highToo lowarc can be extinguishedstubbingspatterporosityexcess penetrationundercutburn-throughPolarity: DCEP generally gives deeper penetration

MMA welding parametersTravel speed Travel speedToo highToo lowwide weld bead contourlack of penetrationburn-throughlack of root fusionincomplete root penetrationundercutpoor bead profile, difficult slag removal

MMA welding parametersType of current:

voltage drop in welding cables is lower with ACinductive looses can appear with AC if cables are coiledcheaper power source for ACno problems with arc blow with ACDC provides a more stable and easy to strike arc, especially with low current better positional weld, thin sheet applicationswelding with a short arc length (low arc voltage) is easier with DC better mechanical propertiesDC provides a smoother metal transfer less spatter

MMA - TroubleshootingMMA quality (left to right) - current, arc length and travel speed normal; current too low; current too high; arc length too short; arc length too long; travel speed too slow; travel speed too high

Most welding defects in MMA are caused by a lack of welder skill (not an easily controlled process), the incorrect settings of the equipment, or the incorrect use, and treatment of electrodes

Typical Welding Defects:Slag inclusionsArc strikes PorosityUndercutShape defects (overlap, excessive root penetration, etc.)MMA welding typical defects

Manual Metal Arc Welding (MMA)Advantages:Field or shop useRange of consumablesAll positionsPortableSimple equipment Disadvantages:High welder skill requiredHigh levels of fumeHydrogen control (flux)Stop/start problemsComparatively uneconomic when compared with some other processes i.e MAG, SAW and FCAW

QU 1.State the main welding parameters of the MMA welding processQU 2.What type of power source characteristic is considered essential for a MMA welding plant?QU 3.Give the main advantages of the MMA welding process when compared to the MAG welding processQU 4.State the four criteria that will govern the number of weld passes in a MMA welded jointQU 5.State two types of electrical supply and give the advantages of eachQuestionsManual Metal Arc Welding

After the discovery of the electric arc in 1800 by Humphry Davy, arc welding began to develop slowly, and by the end of the 19th century, an early welding process called carbon arc welding was developed. Nikolai N. Benardos and Stanislaus Olszewski were awarded patents in the 1880s showing a rudimentary electrode holder, and later, in 1890 Manual metal arc welding is the most versatile of the welding processes, suitable for almost all thicknesses and types of ferrous and most non-ferrous metals. Welding can be carried out in all positions relatively economically with reasonable ease of use, although the eventual weld quality is dependent mainly upon the skill of the welder.Manual metal arc welding is an arc welding process, the heat being provided by an electric arc which is itself formed between a flux coated consumable electrode and the metal being welded. The arc has an average temperature of around 6,000C which is more than sufficient to melt the parent metal, consumable electrode and flux.To strike the electric arc, the electrode is brought into contact with the workpiece in a short sweeping motion and then pulled away 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 vapors 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. 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. Sloped, vertical or upside-down welding requires more operator skill, and often necessitates the use of an electrode that solidifies quickly to prevent the molten metal from flowing out of the weld pool. However, this generally means that the electrode melts less quickly, thus increasing the time required to lay the weld.[7]Shielded metal arc welding equipment typically consists of a constant current welding power supply and an electrode, with an electrode holder, a work clamp, and welding cables (also known as welding leads) connecting the two Give handout to classThe power supply used in SMAW has constant current output, ensuring that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of SMAW are manual, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult. However, because the current is not maintained absolutely constant, skilled welders performing complicated welds can vary the arc length to cause minor fluctuations in the current.[13]Large voltage variation, e.g. + 10v (due to changes in arc length) Small amperage change resulting in virtually constant current e.g. + 5A. Required for: MMA, TIG, Plasma arc and SAW > 1000 AMPS

The power source must provide:An Open Circuit Voltage (OCV) to initiate the arc. Between 50 and 90 volts.Welding Voltage to maintain the arc during welding. Between 20 and 30 voltsA suitable current range. Typically 30 35oampsA stable arc. Rapid arc recovery or arc re-ignition without current surge.A constant welding current. The arc length may change during welding, but consistent electrode burn off rate and weld penetration characteristics must be maintained during welding.10.3POWER REQUIREMENTSManual Metal Arc welding can be carried out using either direct (DC) or alternating (AC) current. With DC welding current either positive (+ve) or negative (-ye) polarity can be used, so current is flowing in one direction. AC welding current flows from negative to positive, and is two directional.Power sources for MMA welding are, either transformers (which transforms main AC to AC suitable for welding), transformer-rectifiers (which rectifies AC to DC), diesel or petrol driven generators (preferred for site work) or inverters (a more recent addition to welding power sources). For MMA welding a power source with A Constant Current (drooping) output characteristic must be used.There will be a range of voltage in the WPS.E6010 Electrode will only run in DCRP an E7018 electrode will run in DCRP and ACOpen-Circuit Voltage (OCV) - As the name implies, no current is flowing in the circuit because the circuit is open. The voltage is impressed upon the circuit, however, so that when the circuit is completed, the current will flow immediately Are all the connections tight, including the earth ground?OSHA regulations require output terminals to be insulated.Rubber boots are available for that purpose.Are the electrode holder and welding cable well insulated and in good condition?Are the settings correct for the job youre about to begin?Fumes can cause symptoms such as nausea, headaches, dizziness, and metal fume fever.