chapter2 - manual metal arc welding

2.

Manual Metal Arc Welding

2. Manual Metal Arc Welding 18

2005

Figure 2.1 describes the burn-off of a cov-

ered stick electrode. The stick electrode

consists of a core wire with a mineral cover-

ing. The welding arc between the electrode

and the workpiece melts core wire and cover-

ing. Droplets of the liquefied core wire mix

with the molten base material forming weld

metal while the molten covering is forming

slag which, due to its lower density, solidifies

on the weld pool. The slag layer and gases

which are generated inside the arc protect the

metal during transfer and also the weld pool

from the detrimental influences of the sur-

rounding atmosphere.

Covered stick electrodes

have replaced the initially

applied metal arc and car-

bon arc electrodes. The

covering has taken on the

functions which are de-

scribed in Figure 2.2.

br-er2-01.cdr ISF 2002c

Weld Point

electrode core

electrode coating

air(O , N , etc.)2 2

liquid slag

solid slag

Smoke and gas

Figure 2.1

Figure 2.2


2005

The covering of the stick electrode consists of a multitude of components which are mainly

mineral, Figure 2.3.

For the stick electrode manufacturing mixed ground and screened covering materials are

used as protection for the core wire which has been drawn to finished diameter and subse-

quently cut to size, Figure 2.4.

© ISF 2002

Influence of the Coating Constituents on Welding Characteristics

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coating raw material effect on the welding characteristics

quartz - SiO2 to raise current-carrying capacity

rutile -TiO2to increase slag viscosity,good re-striking

magnetite - Fe O3 4 to refine transfer of droplets through the arc

calcareous spar -CaCO3to reduce arc voltage, shielding gas emitter and slag formation

fluorspar - CaF2to increase slag viscosity of basic electrodes,decrease ionization

calcareous- fluorspar -K O Al O 6SiO2 2 3 2

easy to ionize, to improve arc stability

ferro-manganese / ferro-silicon deoxidant

cellulose shielding gas emitter

kaolin -Al O 2SiO 2H O2 3 2 2

lubricant

potassium water glassK SiO / Na SiO2 3 2 3

bonding agent

Figure 2.3

1 2 3

raw wirestorage wire drawing machine

and cutting system

inspection

to the pressing

plant

electrodecompound

raw material storagefor flux production

jawcrusher

magneticseparation

cone crusherfor pulverisation

sieving

to further treatment like milling, sieving, cleaning and weighing

sieving system

weighingand

mixing

inspection

wet mixer

descaling

inspection

example of a three-stage wire drawing machine

drawing plate

Ø 6 mm Ø 5,5 mm 3,25 mmØ 4 mm

© ISF 2002

Stick Electrode Fabrication 1

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Figure 2.4


2005

The core wires are coated

with the covering material

which contains binding

agents in electrode extru-

sion presses. The defect-

free electrodes then pass

through a drying oven and

are, after a final inspection,

automatically packed, Fig-

ure 2.5.

Figure 2.6 shows how the moist extruded cov-

ering is deposited onto the core wire inside an

electrode extrusion press.

Stick Electrode Fabrication 2

© ISF 2002br-er10-33e.cdr

core wiremaga-

zine

electrodecompound

inspection

inspection inspection

inspection

inspection

the pressing plant

drying stove

TODELIVERY

packinginspection

electrode-press

compound

nozzleconvey-ingbelt

wiremagazine

wirefeeder

pressinghead

Figure 2.5

core rodcoatingpressing nozzlepressing cylinderpressing cylinder

pressing mass core rod guide

Production of Stick Electrodes

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Figure 2.6


2005

Stick electrodes are, according to their covering compositions, categorized into four differ-

ent types, Figure 2.7. with concern to burn-off characteristics and achievable weld metal

toughness these types show fundamental differences.

The melting characteristics of the different coverings and the slag properties result in further

properties; these determine the areas of application, Figure 2.8.

© ISF 2002

Characteristic Features of Different Coating Types

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cellulosic type acid type rutile type basic typ

celluloserutilequartzFe - Mnpotassium water glass

40202515

magnetitequartzcalciteFe - Mnpotassium water glass

50201020

rutilemagnetitequartzcalciteFe - Mnpotassium water glass

TiO2SiO2

Fe OSiOCaCO

3 4

2

3

TiOFe OSiOCaCO

2

3 4

2

3

fluorsparcalcitequartzFe - Mnpotassium water glass

4510201015

4540105

CaFCaCOSiO

2

3

2

almostno slag

slag solidification time: long

slag solidificationtime: medium

slag solidification time: short

droplet transfer :

toughness value:

medium- sizeddroplets

good normal good very good

fine dropletsto sprinkle

medium- sized to fine droplets

medium- sized to big droplets

droplet transfer : droplet transfer : droplet transfer :

toughness value: toughness value: toughness value:

Figure 2.7

© ISF 2002

Characteristics of Different Coating Types

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coating typesymbol

gap bridging ability

current type/polarity

welding positions

sensitivity ofcold cracking

weld appearance

slag detachability

characteristic features

cellulosic typeC

acid typeA

rutile typeR

basic typeB

very good moderate good good

PG,(PA,PB,PC,PE,PF)

PA,PB,PC,PE,PF,PG

PA,PB,PC,PE,PF,(PG)

PA,PB,PC,PE,PF,PG

low high low very low

moderate good good moderate

good very good very good moderate

spatter,little slag,

intensive fumeformation

high burn-outlosses

universalapplication

low burn-out losses

hygroscopic predrying!!

~ / + ~ / +~ / + = / +

Figure 2.8


2005

The dependence on

temperature of the slag’s

electrical conductivity

determines the reignition

behaviour of a stick elec-

trode, Figure 2.9. The

electrical conductivity for a

rutile stick electrode lies,

also at room temperature,

above the threshold value

which is necessary for

reignition. Therefore, rutile

electrodes are given pref-

erence in the production of

tack welds where reigni-

tion occurs frequently.

The complete designation

for filler materials, follow-

ing European Standardi-

sation, includes details–

partly as encoded abbre-

viation – which are rele-

vant for welding, Figure

2.10. The identification

letter for the welding proc-

ess is first:

E - manual electrode welding G - gas metal arc welding

T - flux cored arc welding W - tungsten inert gas welding

S - submerged arc welding

© ISF 2002

Conductivity of Slags

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co

nd

uctivity

temperature

reignition threshold

high rutile-containing slag

semiconductor

acid sla

g

high

-tem

pera

ture

cond

ucto

r

basi

c slag

high

-tem

pera

ture

cond

ucto

r

Figure 2.9

© ISF 2002

Designation Example for Stick Electrodes

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The mandatory part of the standard designation is: EN 499 - E 46 3 1Ni B

hydrogen content < 5 cm /100 g welding depositbutt weld: gravity positionfillet weld: gravity positionsuitable for direct and alternating currentrecovery between 125% and 160%basic thick-coated electrodechemical composition 1,4% Mn and approx. 1% Niminimum impact 47 J in -30 Cminimum weld metal deposit yield strength: 460 N/mmdistinguishing letter for manual electrode stick welding

3

o

2

DIN EN 499 - E 46 3 1Ni B 5 4 H5

Figure 2.10


2005

The identification numbers give information about yield point, tensile strength and elongation

of the weld metal where the tenfold of the identification number is the minimum yield point

in N/mm², Figure 2.11.

The identification figures for the minimum impact energy value of 47 J – a parameter for the

weld metal toughness – are shown in Figure 2.12.

© ISF 2002

Characteristic Key Numbers of Yield Strength, Tensile Strength and Elongation

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key number minimum yield strengthN/mm

2

tensile strengthN/mm

2

minimum elongation*)%

35

38

42

46

50

355

380

420

460

500

440-570

470-600

500-640

530-680

560-720

22

20

20

20

18

*) L = 5 D0 0

Figure 2.11

Characteristic Key Numbers for Impact Energy

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characteristic figure minimum impact energy 47 J [ C]0

no demands

+20

0

-20

-30

-40

-50

-60

-70

-80

Z

A

0

2

3

4

5

6

7

8

The minimum value of the impact energy allocated to the characteristicfigures is the average value of three ISO-V-Specimen, the lowest value of whitch amounts to 32 Joule.

Figure 2.12


2005

The chemical composition

of the weld metal is shown

by the alloy symbol, Figure

2.13.

The properties of a stick

electrode are characterised

by the covering thickness

and the covering type. Both

details are determined by

the identification letter for

the electrode covering,

Figure 2.14.

Figure 2.15 explains the additional identifica-

tion figure for electrode recovery and applica-

ble type of current. The subsequent identifi-

cation figure determines the application possi-

bilities for different welding positions:

1- all positions

2- all positions, except vertical down

postion

3- flat position butt weld, flat position fillet

weld, horizontal-, vertical up position

4- flat position butt and fillet weld

5- as 3; and recommended for vertical

down position

© ISF 2002

Alloy Symbols for Weld MetalsMinimum Yield Strength up to 500 N/mm

2

br-er2-13.cdr

alloy symbol chemical composition*)

%

Mn Mo Ni

without 2,0_ -

Mo

MnMo

1 Ni

2 Ni

3 Ni

Mn 1 Ni

1 Ni Mo

1,4

>1,4 - 2,0

1,4

1,4

1,4

>1,4 - 2,0

1,4

0,3 - 0,6

0,3 - 0,6

-

-

-

-

0,3 - 0,6

-

-

0,6 - 1.2

1,8 - 2,6

2,6 - 3,8

0,6 - 1,2

0,6 - 1,2

Z other specified compositions

*) companion elements: Mo 0,2; Ni 0,5; Cr 0,2; V 0,08; Nb 0,05; Cu 0,3; Al 2,0

(applies only to self-shielded flux-cored electrodes).

single values are maxima

£

Figure 2.13

© ISF 2002

Key Letters forElectrode Coatings

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key letter type of coating

A

B

acid coating

basic coating

C cellulose coating

R rutile coated(medium thick)

RR rutile coated (thick)

RA rutile acid coating

RB rutile basic coating

RC rutile cellulose coating

Figure 2.14


2005

The last detail of the Euro-

pean Standard designation

determines the maximum

hydrogen content of the

weld metal in cm³ per 100

g weld metal.

Welding current amper-

age and core wire diame-

ter of the stick electrode

are determined by the

thickness of the workpiece

to be welded. Fixed stick

electrode lengths are as-

signed to each diameter,

Figure 2.16.

Figure 2.17 shows the

process principle of man-

ual metal arc welding.

Polarity and type of current

depend on the applied

electrode types. All known

power sources with a de-

scending characteristic

curve can be used.

Since in manual metal arc welding the arc length cannot always be kept constant, a steeply

descending power source is used. Different arc lengths lead therefore to just minimally al-

tered weld current intensities, Figure 2.18. Penetration remains basically unaltered.

© ISF 2002

Additional Characteristic Numbersfor Deposition Efficiency and Current Type

br-er2-15.cdr

additional

characteristic number

1

2

3

4

5

6

7

8

deposition efficiency

%

current type*)

<105

<105

>160

>160

>105 125

>105 125

>125 160

>125 160

alternating and direct current

direct current

*) To prove the suitability for direct current,

the tests have to be run with a no-load voltage of max. 65 V.


direct current


direct current


direct current

Figure 2.15

© ISF 2002

Size and Welding Currentof Stick Electrodes

br-er2-16.cdr

diameter

length

current

2,0 2,5 3,25 4,0 5,0 6,0

250/300 350 350/450 350/450 450 450

40-80 50-100 90-150 120-200 180-270 220-360

20 x d 30 x d 35 x d

d

mm

l

mm

I

A

min.

max. 40 x d 50 x d 60 x d

rule-of -thumbfor current[A]

Figure 2.16


2005

Simple welding transformers are used for a.c. welding. For d.c. welding mainly converters,

rectifiers and series regulator transistorised power sources (inverters) are applied. Convert-

ers are specifically suitable

for site welding and are

mains-independent when

an internal combustion en-

gine is used. The advan-

tages of inverters are their

small size and low weight,

however, a more compli-

cated electronic design is

necessary, Figure 2.19. © ISF 2002

Principle Set-up of MMAW Process

br-er2-17.cdr

work piece

arc

stick electrode

electrode holder

power source

= or ~

- (+)

+ (-)

Figure 2.17

© ISF 2002

Operating Point atDifferent Arc Lengths

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U

1

2

2 1 I

A2 A1

A2

A1

characteristicof the arc

power sourcecharacteristic

Figure 2.18

© ISF 2002

Power Sourcesfor MMAW

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arc weldingconverter

transformer

rectifier

invertertype

Figure 2.19


2005

Figure 2.20 shows the standard welding pa-

rameters of different stick electrode diameters

and stick electrode types.

The rate of deposition of a stick electrode is,

besides the used current intensity, dependent

on the so-called “electrode recovery”, Figure

2.21. This describes the mass of deposited

weld metal / mass of core wire ratio in per-

cent. Electrode recovery can reach values of

up to 220% with metal covering components

in high-efficiency electrodes.

A survey of the material spectrum which is

suitable for manual metal arc welding is given

in Figure 2.22. The survey comprises almost

all metals known for technical applications and

© ISF 2002

Medium Weld Current andVoltages for Stick Electrodes

br-er2-20.cdr

medium weld current

mediu

m w

eld

voltage

B15

B53

RA12

RR12

RA73

RR73

100 200 300 400

6

3,25

4

5

=

=

=

=20

25

30

35

40

45

A

V

Figure 2.20

© ISF 2002

Burn-Off Rateof Stick Electrodes

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c = high-performance electrodesb = basic-coated electrodes, recovery <125%a = A- and R- coated electrodes, recovery 105%

0

1

2

3

4

5

6

7

burn

-off r

ate

at 100%

duty

cycle

welding amperage

kg/h

100 200 3000 400 500A

= RR12 - 5 mmX = RR73 - 5 mm

thick-

coate

d

thin

-coate

d

220%

depo

sitio

nef

ficie

ncy

160%

depo

sitio

nef

ficie

ncy

X

c

b

a

Figure 2.21

© ISF 2002

Suitable Materials forManual Metal Arc Welding

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constructional steels

shipbuilding steels

high-strength constructional steels

boiler and pressure vessel steels

austenitic steels

creep resistant steels

austenitic-ferritic steels (duplex)

scale resistant steels

wear resistant steels

hydrogen resistant steels

high-speed steels

cast steels

combinations of materials (ferritic/ austenitic)

steel:

cast iron: cast iron with lamella graphite

cast iron with globular graphite

nickel: pure nickel

Ni-Cu-alloys

Ni-Cr-Fe-alloys

Ni-Cr-Mo-alloys

copper: electrical grade copper (ETP copper)

bronzes (CuSn, CuAl)

gunmetal (CuSnZnPb)

Cu-Ni-alloys

aluminium: pure aluminium

AlMg-alloys

AlSi -alloys

Figure 2.22


2005

also explains the wide ap-

plication range of the

method.

In d.c. welding, the concen-

tration of the magnetic

arc-blow producing

forces can lead to the de-

flection of the arc from

power supply point on the

side of the workpiece, Fig-

ure 2.23. The material

transfer also does not oc-

cur at the intended point.

Arc deflection may also occur at magnetiz-

able mass accumulations although, in that

case, in the direction of the respective mass,

Figure 2.24.

Figures 2.25 and 2.26 show how by various

measures the magnetic arc blow can be

compensated or even avoided.

The positioning of the electrodes in opposite

direction brings about the correct placement of

the weld metal. Numerous strong tacks close

the magnetic flux inside the workpiece. By ad-

ditional, opposite placed steel masses as well

as by skilful transfer of the power supply point

the various reasons for arc deflection can be

eliminated. The fast magnetic reversal when

a.c. is used minimises the influence of the

magnetic arc blow.

Arc Blow Effect through Concentrationof Magnetic Fields

br-er2-23e.cdr

Figure 2.23

© ISF 2002br-er2-24.cdr

Arc Blow Effecton Steel Parts

inwards at the edges

close to current-connection

close to large workpiece masses

in gaps towards the weld

Figure 2.24


2005

Depending on the electrode covering, the wa-

ter absorption of a stick electrode may vary

strongly during storage, Figure 2.27. The ab-

sorbed humidity leads during subsequent

welding frequently to an increased hydrogen

content in the weld metal and, thus, increases

cold cracking susceptibility.

© ISF 2002

Remedy AgainstArc Blow Effect 1

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tilting of electrode

the weldingsequence

great number of tacks

tacks

Figure 2.25

© ISF 2002

Remedy AgainstArc Blow Effect 2

br-er2-26.cdr

through additional blocks of steel

through relocating the current-connection (rarely used)

through usinga welding transformeralternating current (notapplicable for alltypes of electrodes)

Figure 2.26

© ISF 2002

Water Absorption of DifferentBasic-Coated Electrodes

br-er2-27.cdr

Time of storage

Wa

ter

co

nte

nt

of

the

co

atin

g

1 10 100Days0,10

1,0

2,0

3,0

4,0

%

20°C / 70% RF

Figure 2.27


2005

Stick electrodes, particularly those with a basic, rutile or cellulosic cover have to be baked

before welding to keep the water content of the cover during welding below the permissible

values in order to avoid

hydrogen-induced cracks,

Figure 2.28. The baking

temperature and time are

specified by the manufac-

turer. Baking is carried out

in special ovens; in damp

working conditions and

only just before welding are

electrodes taken out from

electrically heated recepta-

cles.

© ISF 2002

Water Content of the Coatingafter Storage and Baking

br-er2-28.cdr

basic-coated electrode(having been stored at18 - 20°C for one year)

storage and baking

0,74

0,39

0,28

AWS A5.5

Wa

ter

conte

nt of th

e c

oatin

g

1,0

0,9

0,8

0,7

0,6

0,5

0,4

0,3

0,2

0,1

030 40 50 60 70 80%

%

Figure 2.28

chapter2 - manual metal arc welding

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