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CSWIP 3.1 Welding Inspection
TWI Training & Examination Services
Course Reference WIS 5
Manual Metal Arc WeldingManual Metal Arc Welding
oror
Shielded Metal Arc WeldingShielded Metal Arc Welding
The MMA welding Process was first developed in the late 19th
century using bare wire consumables.
• MMA is a simple process in terms of equipment
• The 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
Manual Metal Arc Introduction
MMA - Principle of operation
MMA welding
Main features:
• Shielding provided by decomposition of flux covering
• Electrode consumable
• Manual process
Welder controls:
• Arc length
• Angle of electrode
• Speed of travel
• Amperage settings
Power sourcePower source
Holding ovenHolding oven
Inverter power Inverter power
sourcesource
Electrode holderElectrode holder
Power cablesPower cablesWelding visor Welding visor
filter glassfilter glass
Return leadReturn lead
ElectrodesElectrodes
Electrode Electrode
ovenoven
Control panel Control panel
(amps, volts)(amps, volts)
Manual Metal Arc Basic Equipment
Transformer:
• 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.
MMA Welding Plant
2020 80804040 6060 130130 140140120120100100 180180160160 200200
1010
6060
5050
4040
3030
2020
8080
7070
9090
100100
Normal Operating Normal Operating
Voltage RangeVoltage Range
Vo
lta
ge
Vo
lta
ge
AmperageAmperage
O.C.V. Striking voltage O.C.V. Striking voltage
(typical) for arc (typical) for arc
initiationinitiation
Constant Current Power Source(Drooping Characteristic)
2020 80804040 6060 130130 140140120120100100 180180160160 200200
1010
6060
5050
4040
3030
2020
8080
7070
9090
100100
Normal Operating Normal Operating
Voltage RangeVoltage Range
Large voltage variation, e.g. Large voltage variation, e.g. + +
10v (due to changes in arc 10v (due to changes in arc
length) length)
Small amperage change Small amperage change
resulting in virtually constant resulting in virtually constant
current e.g. current e.g. ++ 5A. 5A.
Vo
lta
ge
Vo
lta
ge
AmperageAmperage
Required for: MMA, TIG, Plasma Required for: MMA, TIG, Plasma
arc and SAW > 1000 AMPSarc and SAW > 1000 AMPS
O.C.V. Striking voltage (typical) for O.C.V. Striking voltage (typical) for
arc initiationarc initiation
Constant Current Power Source(Drooping Characteristic)
The static characteristic:
Current (A)Current (A)
Voltage (V)Voltage (V)
Short arcShort arc
Long arcLong arc
Arc voltageArc voltage
OCVOCV Large variation in voltage Large variation in voltage lead to lead to
small variation for welding small variation for welding
current current �������� consistent penetrationconsistent penetration
Welding Welding
currentcurrent
Constant Current Power Source(Drooping Characteristic)
Constant Current Power Source(Drooping Characteristic)
Factors to be considered when selecting a MMA power source:
• type of current required
• amperage range required
• the primary type of power available
• the position in which welding will be done
Current (A)Current (A)
Voltage (V)Voltage (V)
Normal arc lengthNormal arc length
Short arcShort arc
Long arcLong arc
Arc Arc
voltagevoltage
1515
4040
Low OCVLow OCV
High OCVHigh OCV
Greater change in current Greater change in current
improves arc starting and improves arc starting and
avoid avoid ““stubbingstubbing”” for low for low
amperage applicationsamperage applications
Constant Current Power Source(Drooping Characteristic)
Combined characteristic - the “arc force” option
Current (A)Current (A)
Voltage (V)Voltage (V)
OCVOCV Combined characteristic assist starting Combined characteristic assist starting
and avoid electrode stubbing should a and avoid electrode stubbing should a
welder use too short an arc lengthwelder use too short an arc length
Constant Constant
currentcurrent
Constant Constant
voltagevoltage
Trigger Trigger
voltagevoltage
VoltageVoltage• The arc voltage in the MMA process is measured as close to the
arc as possible. It is variable with a change in arc length
O.C.V.O.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 volts
CurrentCurrent• The 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• Polarity is generally determined by operation and electrode type
e.g DC +ve, DC –ve or AC
MMA Welding Variables
OCV open circuit voltsOCV open circuit volts
A check should be made to ensure that the equipment can produce the OCV required by the consumable and that any voltage selectorhas been moved to the correct position
Current & polarityCurrent & polarity
A check should be made to ensure the current type and range is as detailed on the WPS
Other variablesOther variables
Checks should be made for correct electrode angle, arc gap and travel speed
SafetySafety
Check 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 Checks
The welding equipmentThe welding equipment
A visual check should be made on the equipment to ensure it is in good working order
The electrodesThe electrodes
Checks 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 Basic fluxed electrodes ensure the correct heat treatment is being
carried out before use
RutileRutile electrodes do not require pre-baking but should be stored in
a dry condition.
Cellulosic Cellulosic electrodes should not be dried
MMA Welding Checks
MMA electrode holder
Collet or twist typeCollet or twist type““TongsTongs”” type with type with
springspring--loaded jawsloaded jaws
MMA welding parameters
Welding current
– approx. 35 A/mm of diameter
– governed by thickness, type of joint and welding position
Welding Welding
currentcurrent Too highToo highToo lowToo low
••poor startingpoor starting
••slag inclusionsslag inclusions
••weld bead contour too weld bead contour too
highhigh
••lack of lack of
fusion/penetrationfusion/penetration
••spatterspatter
••excess excess
penetrationpenetration
••undercutundercut
••burnburn--throughthrough
MMA welding parameters
Arc length = arc voltage
Arc Arc
voltagevoltage Too highToo highToo lowToo low
••arc can be extinguishedarc can be extinguished
••““stubbingstubbing””
••spatterspatter
••porosityporosity
••excess excess
penetrationpenetration
••undercutundercut
••burnburn--throughthrough
• Polarity: DCEP generally gives deeper penetration
MMA welding parameters
Travel speed
Travel Travel
speedspeed Too highToo highToo lowToo low
••wide weld bead contourwide weld bead contour
••lack of penetrationlack of penetration
••burnburn--throughthrough
••lack of root fusionlack of root fusion
••incomplete root incomplete root
penetrationpenetration
••undercutundercut
••poor bead profile, poor bead profile,
difficult slag removaldifficult slag removal
MMA welding parameters
Type of current:
• voltage drop in welding cables is lower with AC
• inductive looses can appear with AC if cables are coiled
• cheaper power source for AC
• no problems with arc blow with AC
• DC provides a more stable and easy to strike arc, especially with low current � better positional weld, thin sheet applications
• welding with a short arc length (low arc voltage) is easier withDC � better mechanical properties
• DC provides a smoother metal transfer � less spatter
MMA - Troubleshooting
MMA quality (left to right)MMA 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:Typical Welding Defects:
•Slag inclusions
•Arc strikes
•Porosity
•Undercut
•Shape defects (overlap, excessive root penetration, etc.)
MMA welding typical defects
Advantages:Advantages:
• Field or shop use
• Range of consumables
• All positions
• Portable
• Simple equipment
Disadvantages:Disadvantages:
• High welder skill required
• High levels of fume
• Hydrogen control (flux)
• Stop/start problems
• Comparatively uneconomic when compared with some other processes i.e MAG, SAW and FCAW
Manual Metal Arc Welding (MMA)
Any QuestionsAny Questions
QU 1.QU 1. State the main welding parameters of the MMA welding process
QU 2.QU 2. What type of power source characteristic is considered essential for a MMA welding plant?
QU 3.QU 3. Give the main advantages of the MMA welding process Give the main advantages of the MMA welding process
when compared to the MAG welding processwhen compared to the MAG welding process
QU 4.QU 4. State the four criteria that will govern the number of State the four criteria that will govern the number of
weld weld passes in a MMA welded jointpasses in a MMA welded joint
QU 5.QU 5. State two types of electrical supply and give the State two types of electrical supply and give the
advantages of eachadvantages of each
Questions
Manual Metal Arc Welding