2. manual metal arc weldingdantn/wt/wt1-c2.pdf · quartz - sio 2 to raise current-carrying ......
TRANSCRIPT
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.
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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
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Figure 2.2
2. Manual Metal Arc Welding 14
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
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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 machinedrawing 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
2. Manual Metal Arc Welding 15
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
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Figure 2.6
2. Manual Metal Arc Welding 16
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
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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 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
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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
2. Manual Metal Arc Welding 17
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
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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
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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
2. Manual Metal Arc Welding 18
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
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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
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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
2. Manual Metal Arc Welding 19
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
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© 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
2. Manual Metal Arc Welding 20
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
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Figure 2.15
© ISF 2002
Size and Welding Currentof Stick Electrodes
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Figure 2.16
2. Manual Metal Arc Welding 21
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.
© ISF 2002br-er2-18.cdr
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
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work piece
arc
stick electrode
electrode holder
power source= or ~
- (+)
+ (-)
Figure 2.17
Figure 2.18
2. Manual Metal Arc Welding 22
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.
© ISF 2002br-er2-20.cdr
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
© ISF 2002br-er2-19.cdr
arc welding converter
transformer
rectifier
invertertype
Figure 2.19
Figure 2.20
2. Manual Metal Arc Welding 23
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.
© ISF 2002br-er2-21.cdr
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
© ISF 2002br-er2-22.cdr
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
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Figure 2.23
2. Manual Metal Arc Welding 24
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
© ISF 2002br-er2-25.cdr
tilting of electrode
the weldingsequence
great number of tacks
tacks
© ISF 2002br-er2-26.cdr
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)
© 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
Figure 2.25 Figure 2.26
2. Manual Metal Arc Welding 25
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.
© ISF 2002br-er2-27.cdr
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
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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