2. Manual Metal Arc Weldingdantn/WT/WT1-c2.pdf · quartz - SiO 2 to raise current-carrying ......

2003

2.

Manual Metal Arc Welding

2. Manual Metal Arc Welding 13

Figure 2.1 describes the burn-off of a

covered stick electrode. The stick

electrode consists of a core wire with

a mineral covering. The welding arc

between the electrode and the work-

piece melts core wire and covering.

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 influ-

ences of the surrounding atmosphere.

Covered stick elec-

trodes have re-

placed the initially

applied metal arc

and carbon arc

electrodes. The

covering has taken

on the functions

which are described

in Figure 2.2.

br-er2-01.cdr ISF 2002c

Weld Point

Figure 2.1

© ISF 2002

1. Conductivity of the arc plasma is improved by

2. Constitution of slag, to

3. Constitution of gas shielding atmosphere of

4. Desoxidation and alloying of the weld metal

5. Additional input of metallic particles

a) ease of ignitionb) increase of arc stability

a) influence the transferred metal dropletb) shield the droplet and the weld pool against atmospherec) form weld bead

a) organic componentsb) carbides

Task of Electrode Coating

br-er2-02.cdr

Figure 2.2


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 mate-

rials are used as protection for the core wire which has been drawn to finished di-

ameter and subsequently cut to size, Figure 2.4.

© ISF 2002

Influence of the Coating Constituents on Welding Characteristics

br-er2-03.cdr

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 machinedrawing plate

Ø 6 mm Ø 5,5 mm 3,25 mmØ 4 mm

© ISF 2002

Stick Electrode Fabrication 1

br-er2-04.cdr

Figure 2.4


The core wires are coated with the

covering material which contains bind-

ing agents in electrode extrusion

presses. The defect-free electrodes

then pass through a drying oven and

are, after a final inspection, automati-

cally packed, Figure 2.5.

Figure 2.6 shows how the moist ex-

truded covering 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

inspectioninspection

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

br-er2-06.cdr

Figure 2.6


Stick electrodes are, according to their covering compositions, categorized into

four different types, Figure 2.7. with concern to burn-off characteristics and achiev-

able 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

br-er2-07.cdr

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 42

3

TiOFe OSiOCaCO

23 4

23

fluorsparcalcitequartzFe - Mnpotassium water glass

4510201015

4540105

CaFCaCOSiO

23

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

br-er2-08.cdr

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


The dependence on temperature of the slag’s electrical conductivity determines

the reignition behaviour of a stick electrode, Figure 2.9. The electrical conductivity for

a rutile stick elec-

trode lies, also at

room temperature,

above the thresh-

old value which is

necessary for reig-

nition. Therefore,

rutile electrodes

are given prefer-

ence in the

production of tack

welds where reig-

nition occurs fre-

quently.

The complete des-

ignation for filler

materials, following

European Stan-

dardisation, in-

cludes details–

partly as encoded

abbreviation –

which are relevant

for welding, Figure

2.10. The identifica-

tion letter for the

welding process 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

br-er2-09.cdr

cond

uctiv

ity

temperature

reignition threshold

high rutile-containing slag

semiconductor

acid

slag

high-

tempe

ratur

e

cond

uctor ba

sic sl

ag

high

-tem

pera

ture

cond

ucto

r

Figure 2.9

© ISF 2002

Designation Example for Stick Electrodes

br-er2-10.cdr

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


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 parame-

ter for the weld metal toughness – are shown in Figure 2.12.

© ISF 2002

Characteristic Key Numbers of Yield Strength, Tensile Strength and Elongation

br-er2-11.cdr

key number minimum yield strengthN/mm2

tensile strengthN/mm2

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

Characteristic Key Numbers for Impact Energy

br-er2-12.cdr

characteristic figure minimum impact energy 47 J [ C]0

no demands +20 0 -20 -30 -40 -50 -60 -70 -80

ZA02345678

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 2.11

Figure 2.12


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 thick-

ness and the covering type. Both de-

tails are determined by the identifica-

tion letter for the electrode covering,

Figure 2.14.

© ISF 2002

Alloy Symbols for Weld MetalsMinimum Yield Strength up to 500 N/mm2

br-er2-13.cdr

© ISF 2002br-er2-14.cdr

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 2.13

Figure 2.14


Figure 2.15 ex-

plains the additional

identification figure

for electrode recov-

ery and applicable

type of current.

The subsequent

identification figure

determines the ap-

plication possibili-

ties for different

welding positions:

1- all positions

2- all positions, except vertical down position

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

The last detail of the European Standard designation determines the maximum hy-

drogen content of the weld metal in cm³ per 100 g weld metal.

Welding current

amperage and

core wire diame-

ter of the stick

electrode are de-

termined by the

thickness of the

workpiece to be

welded. Fixed stick

electrode lengths

are assigned to

each diameter,

Figure 2.16.

© ISF 2002

Additional Characteristic Numbers for Deposition Efficiency and Current Type

br-er2-15.cdr

Figure 2.15

© ISF 2002

Size and Welding Currentof Stick Electrodes

br-er2-16.cdr

Figure 2.16


Figure 2.17 shows

the process princi-

ple of manual

metal arc welding.

Polarity and type of

current depend on

the applied elec-

trode 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 con-

stant, a steeply descending power

source is used. Different arc lengths

lead therefore to just minimally altered

weld current intensities, Figure 2.18.

Penetration remains basically unal-

tered.


U

1

2

2 1 I

A2 A1

A2

A1

characteristic of the arc

power source characteristic

© 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

Figure 2.18


Simple welding transformers are

used for a.c. welding. For d.c. welding

mainly converters, rectifiers and se-

ries regulator transistorised power

sources (inverters) are applied. Con-

verters are specifically suitable for

site welding and are mains-

independent when an internal com-

bustion engine is used. The advan-

tages of inverters are their small size

and low weight, however, a more

complicated electronic design is nec-

essary, Figure 2.19.

Figure 2.20 shows the standard weld-

ing parameters of different stick elec-

trode 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 percent. Electrode recovery can

reach values of up to 220% with metal

covering components in high-efficiency

electrodes.


medium weld current

med

ium

wel

d vo

ltage

B15

B53

RA12RR12

RA73

RR73

100 200 300 400

6

3,2545

====

20

25

30

35

40

45

A

V


arc welding converter

transformer

rectifier

invertertype

Figure 2.19

Figure 2.20


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 ap-

plications and also explains the wide application range of the method.

In d.c. welding, the

concentration of the

magnetic arc-blow

producing forces can

lead to the deflection

of the arc from power

supply point on the

side of the workpiece,

Figure 2.23. The ma-

terial transfer also

does not occur at the

intended point.


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

rate

at 1

00%

dut

y cy

cle

welding amperage

kg/h

100 200 3000 400 500A

= RR12 - 5 mmX = RR73 - 5 mm

thick-co

ated

thin-coated

220%

dep

ositio

n ef

ficie

ncy

160%

dep

ositio

n ef

ficien

cyX

c

b

a

Figure 2.21


constructional steelsshipbuilding steelshigh-strength constructional steelsboiler and pressure vessel steelsaustenitic steelscreep resistant steelsaustenitic-ferritic steels (duplex)scale resistant steelswear resistant steelshydrogen resistant steelshigh-speed steelscast steelscombinations of materials (ferritic/ austenitic)

steel:

cast iron: cast iron with lamella graphitecast iron with globular graphite

nickel: pure nickelNi-Cu-alloysNi-Cr-Fe-alloysNi-Cr-Mo-alloys

copper: electrical grade copper (ETP copper)bronzes (CuSn, CuAl)gunmetal (CuSnZnPb)Cu-Ni-alloys

aluminium: pure aluminiumAlMg-alloysAlSi -alloys

Figure 2.22

Arc Blow Effect through Concentrationof Magnetic Fields

br-er2-23e.cdr

Figure 2.23


Arc deflection may also occur at

magnetizable 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

additional, opposite placed steel

masses as well as by skilful transfer


tilting of electrode

the weldingsequence

great number of tacks

tacks


through additional blocks of steel

through relocating the current-connection (rarely used)

through using a welding transformer alternating current (not applicable for all types of electrodes)


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

Figure 2.25 Figure 2.26


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 influ-

ence of the magnetic arc blow.

Depending on the electrode covering,

the water absorption of a stick elec-

trode may vary strongly during stor-

age, Figure 2.27. The absorbed hu-

midity leads during subsequent weld-

ing frequently to an increased hydro-

gen content in the weld metal and,

thus, increases cold cracking suscep-

tibility.

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 speci-

fied by the manu-

facturer. Baking is

carried out in spe-

cial ovens; in damp

working conditions

and only just before

welding are elec-

trodes taken out

from electrically

heated receptacles.


Time of storage

Wat

er c

onte

nt o

f the

coa

ting

1 10 100Tage00

1,0

2,0

3,0

4,0

%

20°C / 70% RF

© 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

0,28AWS A5.5

Wat

er c

onte

nt o

f the

coa

ting

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 2.27

Figure 2.28

2. Manual Metal Arc Weldingdantn/WT/WT1-c2.pdf · quartz - SiO 2 to raise current-carrying ......

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