10-MMA Welding 2006

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CSWIP 3.1 Welding Inspection TWI Training & Examination Services Course Reference WIS 5 Manual Metal Arc Welding Manual Metal Arc Welding or or Shielded Metal Arc Welding Shielded Metal Arc Welding

Transcript of 10-MMA Welding 2006

Page 1: 10-MMA Welding 2006

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

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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

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MMA - Principle of operation

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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

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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

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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

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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)

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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)

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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)

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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

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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

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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

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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

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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

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MMA electrode holder

Collet or twist typeCollet or twist type““TongsTongs”” type with type with

springspring--loaded jawsloaded jaws

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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

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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

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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

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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

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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

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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

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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)

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Any QuestionsAny Questions

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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