Chapter2 - Manual Metal Arc Welding
-
Upload
akhil-venugopal -
Category
Documents
-
view
29 -
download
4
Transcript of 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
2. Manual Metal Arc Welding 19
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
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 machine
drawing 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
2. Manual Metal Arc Welding 20
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
br-er2-06.cdr
Figure 2.6
2. Manual Metal Arc Welding 21
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
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 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
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
2. Manual Metal Arc Welding 22
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
br-er2-09.cdr
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
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
2. Manual Metal Arc Welding 23
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
br-er2-11.cdr
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
br-er2-12.cdr
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
2. Manual Metal Arc Welding 24
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
br-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 2.14
2. Manual Metal Arc Welding 25
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.
alternating and direct current
direct current
alternating and direct current
direct current
alternating and 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
2. Manual Metal Arc Welding 26
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
br-er2-18e.cdr
U
1
2
2 1 I
A2 A1
A2
A1
characteristicof the arc
power sourcecharacteristic
Figure 2.18
© ISF 2002
Power Sourcesfor MMAW
br-er2-19.cdr
arc weldingconverter
transformer
rectifier
invertertype
Figure 2.19
2. Manual Metal Arc Welding 27
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
br-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 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
br-er2-22.cdr
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
2. Manual Metal Arc Welding 28
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
2. Manual Metal Arc Welding 29
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
br-er2-25.cdr
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
2. Manual Metal Arc Welding 30
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