Welding Technology

(1)

ISF – Welding Institute

RWTH – Aachen University

Lecture Notes

Welding Technology 1

Welding and Cutting Technologies

(2)

Chapter

Subject

Page

0. Introduction

1

1. Gas Welding

3

2. Manual Metal Arc Welding

13

3. Submerged Arc Welding

26

4. TIG Welding and

Plasma Arc Welding

43

5. Gas– Shielded Metal Arc Welding

56

6. Narrow Gap Welding,

Electrogas - and

Electroslag Welding

73

7. Pressure Welding

85

8. Resistance Spot Welding,

Resistance Projection Welding

and Resistance Seam Welding

101

9. Electron Beam Welding

115

10. Laser Beam Welding

129

11. Surfacing and Shape Welding

146

12. Thermal Cutting

160

13. Special Processes

175

14. Mechanisation and Welding Fixtures

187 15. Welding Robots 200

16. Sensors 208

(3)

2003

0.

(4)

Welding fabrication processes are classified in accordance with the German

Stan-dards DIN 8580 and DIN 8595 in main group 4 “Joining”, group 4.6 “Joining by Welding”, Figure 0.1.

Welding: permanent, positive joining method. The course of the strain lines is almost ideal. Welded joints show therefore higher strength prop-erties than the joint types depicted in Figure 0.2. This is of advantage, especially in the case of dynamic stress, as the notch effects are lower. 4.6.2 Fusion welding 1 Casting 5 Coating Changing of materials properties 6 2 Forming 3 Cutting 4 Joining 4.4 Joining by casting 4.1 Joining by composition 4.7 Joining by soldering 4.6 Joining by welding 4.3 Joining by pressing 4.2 Joining by filling 4.8 Joining by adhesive bonding 4.6.1 Pressure welding 4.5 Joining by forming

Production Processes acc. to DIN 8580

br-er0-01.cdr Figure 0.1 © ISF 2002 Connection Types Screwing Riveting Adhesive bonding Soldering Welding br-er0-02.cdr Figure 0.2

(5)

Figures 0.3 and 0.4 show the further subdivision of the different welding methods according to DIN 1910. Production processes 4 Joining 4.6 Joining by welding 4.6.2 Fusion welding 4.6.1 Pressure welding 4.6.1.1 Welding by solid bodies Heated tool welding 4.6.1.2 Welding by liquids Flow welding 4.6.1.3 Welding by gas Gas pressure-/ roll-/ forge-/ diffusion welding 4.6.1.4 Welding by electrical gas discharge Arc pressure welding 4.6.1.6 Welding by motion Cold pressure-/ shock-/ friction-/ ultrasonic welding 4.6.1.7 Welding by electric current Resistance pressure welding

Joining by Welding acc. to DIN 1910 Pressure Welding © ISF 2002 br-er0-03.cdr Figure 0.3 Production processes 4 Joining 4.6 Joining by welding 4.6.2 Fusion welding 4.6.1 Pressure welding 4.6.2.2 Welding by liquids 4.6.2.3 Welding by gas 4.6.2.5 Welding by beam 4.6.2.4 Welding by electrical gas discharge 4.6.2.7 Welding by electric current

Cast welding Gas welding Arc welding Beam welding Resistance welding

Joining by Welding acc. to DIN 1910 Fusion Welding

br-er0-04.cdr

(6)

2003

1.

(7)

Although the oxy-acetylene process has been introduced long time ago it is still applied for its flexibility and mo-bility. Equipment for oxyacetylene

welding consists of just a few

ele-ments, the energy necessary for weld-ing can be transported in cylinders, Figure 1.1.

Process energy is obtained from the exothermal chemical reaction between oxygen and a combustible gas, Figure 1.2. Suitable combustible gases are C2H2, lighting gas, H2, C3H8 and

natu-ral gas; here C3H8 has the highest

calorific value. The highest flame in-tensity from point of view of calorific value and flame propagation speed is,

however, obtained with C2H2.

acetylene hose

oxygen cylinder with pressure reducer

welding rod oxygen hose

welding nozzle welding torch

acetylene cylinder with pressure reducer

welding flame workpiece 1 9 7 2 6 4 5 3 8 1 9 7 2 6 4 5 3 8 br-er1-01.cdr Figure 1.1 © ISF 2002 2770 2850 3200 0 200 400 600645 0 ignition temperature [ C]O o x y g e n a ir 0.5 1.0 1.5 2.0 2.5 0

density in normal state [kg/m ]3

p ro p a n e 2.0 0.9 o x y g e n 1.43 1.17 a ir 1.29 300335 510 490 645

flame temperature with O2

flame efficiency with O 2

flame velocity with O 2

KW /cm 2 cm /s 43 10.3 8.5 1350 370 330 br-er1-02.cdr n a tu ra l g a s p ro p a n e °C k Figure 1.2

(8)

C2H2 is produced in acetylene gas

generators by the exothermal

trans-formation of calcium carbide with wa-ter, Figure 1.3. Carbide is obtained from the reaction of lime and carbon in the arc furnace.

C2H2 tends to decompose already at

a pressure of 0.2 MPa. Nonetheless, commercial quantities can be stored

when C2H2 is dissolved in acetone

(1 l of acetone dissolves approx. 24 l

of C2H2 at 0.1 MPa), Figure 1.4.

Acetone disintegrates at a pressure of more than 1.8 MPa, i.e., with a filling pressure of 1.5 MPa the storage of 6m³

of C2H2 is possible in a standard

cylin-der (40 l). For gas exchange (storage and drawing of quantities up to 700 l/h) a larger surface is necessary, therefore the gas cylinders are filled with a po-rous mass (diatomite). Gas consump-tion during welding can be observed from the weight reduction of the gas cylinder. © ISF 2002 Acetylene Generator loading funnel material lock gas exit feed wheel grille sludge to sludge pit br-er1-03.cdr © ISF 2002 Storage of Acetylene acetone acetylene porous mass acetylene cylinder filling quantity : acetone quantity : acetylene quantity : ~13 l 6000 l 15 bar up to 700 l/h cylinder pressure : br-er1-04.cdr

N

Figure 1.3 Figure 1.4

(9)

Oxygen is

pro-duced by

frac-tional distillation

of liquid air and stored in cylinders with a filling pres-sure of up to 20 MPa, Figure 1.5. For higher oxygen consumption, stor-age in a liquid state and cold gasifica-tion is more profit-able.

The standard cylinder (40 l) contains, at a filling pressure of 15 MPa, 6m³ of

O2 (pressureless state), Figure 1.6.

Moreover, cylinders with contents of 10 or 20 l (15 MPa) as well as 50 l at 20 MPa are common. Gas consump-tion can be calculated from the pres-sure difference by means of the gen-eral gas equation.

Principle of Oxygen Extraction

air cooling nitrogen gaseous cylinder bundle oxygen oxygen liquid air nitrogen vaporized liquid tank car pipeline

cleaning compressor separation

br-er1-05.cdr supply Figure 1.5 br-er1-06.cdr Storage of Oxygen 50 l oxygen cylinder protective cap cylinder valve take-off connection gaseous

p = cylinder pressure : 200 bar V = volume of cylinder : 50 l Q = volume of oxygen : 10 000 l content control Q = p V foot ring user gaseous still liquid vaporizer manometer safety valve

filling connection

liquid

N

(10)

In order to prevent mistakes, the gas cylinders are colour-coded. Figure 1.7 shows a survey of the present colour code and the future colour code which is in accordance with DIN EN 1089.

The cylinder valves are also of different designs. Oxygen cylinder connections show a right-hand

thread union nut. Acetylene cylinder valves are equipped with screw clamp retentions. Cylinder valves for other combustible gases have a left-hand thread-connection with a circumferen-tial groove.

Pressure regulators reduce the cylinder pressure to the requested working

pres-sure, Figures 1.8 and 1.9.

Gas Cylinder-Identification according to DIN EN 1089

br-er1-07.cdr

actual condition DIN EN 1089 oxygen techn. white blue (grey) blue acetylene brown yellow nitrogen darkgreen darkgreen black argon dark green grey grey

actual condition DIN EN 1089

grey grey brown helium carbon-dioxide grey grey grey grey argon-carbon-dioxide mixture vivid green hydrogen red © ISF 2002

Single Pressure Reducing Valve during Gas Discharge Operation

br-er1-08.cdr

cylinder pressure working pressure

Figure 1.7

(11)

At a low cylinder pressure (e.g. acetylene cylinder) and low pressure fluctuations, single-stage regulators

are applied; at higher cylinder pressures normally two-stage pressure regulators are used.

The requested pressure is set by the adjusting screw. If the pres-sure increases on the low pressure side, the throttle valve closes the increased pressure onto the mem-brane.

The injector-type torch consists of a

body with valves and welding cham-ber with welding nozzle, Figure 1.10. By the selection of suitable welding chambers, the flame intensity can be adjusted for welding different plate thicknesses.

Single Pressure Reducing Valve, Shut Down

br-er1-09.cdr

discharge pressure locking pressure

Welding Torch

br-er1-10.cdr

welding torch injector or blowpipe

coupling nut hose connection for oxygen A6x1/4" right mixer tube mixer nozzle oxygen valve

injector

pressure nozzle suction nozzle

fuel gas valve welding nozzle

hose connection for fuel gas A9 x R3/8” left welding torch head torch body

Figure 1.9

(12)

The special form of the mixing chamber guarantees highest possible safety against

flashback, Figure 1.11. The high outlet speed of the escaping O2 generates a

nega-tive pressure in the acetylene gas line, in consequence C2H2 is sucked and drawn-in.

C2H2 is therefore available with a very low pressure of 0.02 up to 0.05 MPa

-compared with O2 (0.2 up to 0.3 MPa).

A neutral flame adjustment allows the differentiation of three zones of a chemical

reaction, Figure 1.12:

0. dark core: escaping gas mixture

1. brightly shining centre cone: acetylene decomposition

C2H2 -> 2C+H2

2. welding zone: 1st stage of combustion

2C + H2 + O2 (cylinder) -> 2CO + H2

3. outer flame: 2nd stage of combustion

4CO + 2H2 + 3O2 (air) -> 4CO2 + 2H2O complete reaction: 2C2H2 + 5O2 -> 4CO2 + 2H2O © ISF 2002 Injector-Area of Torch br-er1-11.cdr acetylene oxygen acetylene

welding torch head injector nozzle pressure nozzle

coupling nut torch body

(13)

By changing the mixture ratio of the

volumes O2:C2H2 the weld pool can

greatly be influenced, Figure 1.13. At a

neutral flame adjustment the mixture

ratio is O2:C2H2 = 1:1. By reason of the

higher flame temperature, an excess

oxygen flame might allow faster

weld-ing of steel, however, there is the risk of oxidizing (flame cutting).

Area of application: brass

The excess acetylene causes the carburising of steel materials.

Area of application: cast iron

centre cone outer flame

3200°C 2500°C 1800°C 1100°C 400°C 2 - 5 Figure 1.12 © ISF 2002 excess of acetylene normal (neutral) excess of oxygen welding flame ratio of mixture

effects in welding of steel

sparking foaming spattering reducing oxidizing consequences: carburizing hardening

Effects of the Welding Flame Depending on the Ratio of Mixture

br-er1-13.cdr

(14)

By changing the gas mixture outlet speed the flame can be adjusted to the heat requirements of the welding job, for example when welding plates (thickness: 2 to 4 mm) with the weld-ing chamber size 3: “2 to 4 mm”, Fig-ure 1.14. The gas mixtFig-ure outlet speed is 100 to 130 m/s when using a medium or normal flame, applied to at, for example, a 3 mm plate. Using a

soft flame, the gas outlet speed is

lower (80 to 100 m/s) for the 2 mm plate, with a hard flame it is higher (130 to 160 m/s) for the 4 mm plate.

Depending on the plate thickness are the working methods “leftward weld-ing” and “rightward weldweld-ing” applied, Figure 1.15. A decisive factor for the designation of the working method is the sequence of flame and welding rod as well as the manipulation of flame and welding rod. The welding direction itself is of no importance. In leftward

welding the flame is pointed at the

open gap and “wets” the molten pool; the heat input to the molten pool can be well controlled by a slight move-ment of the torch (s = 3 mm).

discharging velocity and weld heat-input rate: low nozzle size: for plate thickness of 2-4 mm balanced (neutral) flame

welding flame

2

soft flame

moderate flame

hard flame

discharging velocity and weld heat-input rate: middle

discharging velocity and weld head-input rate: high

3

4

br-er1-14.cdr

Effects of the Welding Flame Depending on the Discharge Velocity

weld-rod flame

Rightward welding ist applied to a plate thickness of 3mm upwards. The wire circles, the torch remains calm. Advantages:

- the molten pool and the weld keyhole are easy to observe - good root fusion

- the bath and the melting weld-rod are permanently protected from the air

- narrow welding seam - low gas consumption

Leftward welding is applied to a plate thickness of up to 3 mm. The weld-rod dips into the molten pool from time to time, but remains calm otherwise. The torch swings a little. Advantages:

easy to handle on thin plates

Flame Welding

br-er1-15e.cdr Figure 1.14

(15)

In rightward welding the flame is di-rected onto the molten pool; a weld keyhole is formed (s = 3 mm).

Flanged welds and plain butt welds can be applied to a plate thickness of approx. 1.5 mm without filler material, but this does not apply to any other plate thickness and weld shape, Fig-ure 1.16.

By the specific heat input of the differ-ent welding methods all welding posi-tions can be carried out using the oxyacetylene welding method, Figures 1.17 and 1.18

When working in tanks and confined

spaces, the welder (and all other

per-sons present!) have to be protected against the welding heat, the gases produced during welding and lack of

oxygen ((1.5 % (vol.) O2 per 2 % (vol.)

C2H2 are taken out from the ambient

atmosphere)), Figure 1.19. The addi-tion of pure oxygen is unsuitable (ex-plosion hazard!).

gap

preparations denotation sym-_bol plate thickness range s [mm] from to 1,5 1,0 1,0 4,0 3,0 12,0 1,0 8,0 1,0 8,0 1,0 8,0 flange weld plain butt weld V - weld corner weld lap seam fillet weld 1 - 2 1 - 2

Gap Shapes for Gas Welding

s + 1 ~ ~ r = s br-er1-16.cdr © ISF 2002 PA PB PF PG PC PE PD butt-welded seams in gravity position

gravity fillet welds

horizontal fillet welds vertical fillet and butt welds vertical-upwelding position vertical-down position

horizontal on vertical wall

overhead position

horizontal overhead position

Welding Positions I br-er1-17.cdr f s Figure 1.16 Figure 1.17

(16)

A special type of autogene method is

flame-straightening, where specific

lo-cally applied flame heating allows for shape correction of workpieces, Figure 1.20. Much experience is needed to carry out flame straightening processes. The basic principle of flame straightening depends on locally applied heating in connection with prevention of expansion. This process causes the appearance of a heated zone. During cooling, shrinking forces are generated in the heated zone and lead to the desired shape correction.

5. after welding: Removing the equipment from the tank 4. illumination and electric machines: max 42volt 3. second person for safety reasons

2. extraction unit, ventilation

1. requirement for a permission to enter protective measures / safety precautions

Hazards through gas, fumes, explosive mixtures, electric current

Safety in welding and cutting inside of tanks and narrow rooms

br-er1-19e.cdr

Gas Welding in Tanks and Narrow Rooms

welded parts

first warm up both lateral plates, then belt

butt weld 3 to 5 heat sources close to the weld-seam

double fillet weld 1,3 or 5 heat sources Flame straightening Flame Straightening br-er1-20.cdr © ISF 2002 br-er1-18.cdr PA PB PC PD PE PG PF Welding Positions II Figure 1.18 Figure 1.19 Figure 1.20

(17)

2003

2.

(18)

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 cISF 2002

Weld Point

Figure 2.1

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 ignition b) increase of arc stability

a) influence the transferred metal droplet b) shield the droplet and the weld pool against atmosphere

c) form weld bead a) organic components b) carbides

Task of Electrode Coating

br-er2-02.cdr

(19)

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.

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 -TiO2 to increase slag viscosity,_{good re-striking}

magnetite - Fe O_{3 4} to refine transfer of droplets through the arc calcareous spar -CaCO₃ to reduce arc voltage, shielding gas

emitter and slag formation

fluorspar - CaF₂ to 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 O_{2 3} ₂ ₂ lubricant potassium water glass

K SiO / Na SiO₂ ₃ ₂ ₃ bonding agent

Figure 2.3

1 2 3 raw wire

storage _{wire drawing machine} and cutting system

inspection to the pressing plant electrode compound raw material storage

for flux production

jaw crusher magnetic separation cone crusher for pulverisation sieving

to further treatment like milling, sieving, cleaning and weighing

sieving system weighing and mixing inspection wet mixer descaling inspection example of a three-stage wire drawing machine

drawing plate

Ø 6 mm Ø 5,5 mm Ø 4 mm 3,25 mm

Stick Electrode Fabrication 1

br-er2-04.cdr

(20)

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

drying stove TO DELIVERY packing inspection electrode-press compound nozzle convey-ing belt wire magazine wire feeder pressing head Figure 2.5 core rod coating pressing nozzle pressing cylinder pressing cylinder

pressing mass core rod guide

Production of Stick Electrodes

br-er2-06.cdr

(21)

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.

Characteristic Features of Different Coating Types

br-er2-07.cdr

cellulosic type acid type rutile type basic typ cellulose

rutile quartz Fe - Mn

potassium water glass 40 20 25 15 magnetite quartz calcite Fe - Mn

potassium water glass 50 20 10 20 rutile magnetite quartz calcite Fe - Mn

potassium water glass TiO₂ SiO₂ Fe O SiO CaCO 3 4 2 3 TiO Fe O SiO CaCO 2 3 4 2 3 fluorspar calcite quartz Fe - Mn

potassium water glass 45 10 20 10 15 45 40 10 5 CaF CaCO SiO 2 3 2 almost no slag slag solidification time: long slag solidification time: medium slag solidification time: short droplet transfer : toughness value: medium- sized droplets

good normal good very good

fine droplets to 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

Characteristics of Different Coating Types

br-er2-08.cdr coating type symbol gap bridging ability current type/polarity welding positions sensitivity of cold cracking weld appearance slag detachability characteristic features cellulosic type C acid type A rutile type R basic type B

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 fume formation high burn-out losses universal application low burn-out losses hygroscopic predrying!! ~ / + ~ / + ~ / + = / + Figure 2.8

(22)

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 c o n d u c ti v it y temperature reignition threshold high rutile-con taining slag semiconducto r acid sla g high -tem pera ture cond ucto r basi c 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 deposit butt weld: gravity position

fillet weld: gravity position

suitable for direct and alternating current recovery between 125% and 160% basic thick-coated electrode

chemical composition 1,4% Mn and approx. 1% Ni minimum impact 47 J in -30 C

minimum weld metal deposit yield strength: 460 N/mm distinguishing letter for manual electrode stick welding

3

o

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

(23)

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.

Characteristic Key Numbers of Yield Strength, Tensile Strength and Elongation

br-er2-11.cdr

key number minimum yield strength

N/mm2 tensile strength N/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 Z A 0 2 3 4 5 6 7 8

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

Figure 2.11

(24)

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.

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

br-er2-13.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

(25)

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.

Additional Characteristic Numbers for Deposition Efficiency and Current Type

br-er2-15.cdr

Figure 2.15

Size and Welding Current of Stick Electrodes

br-er2-16.cdr

(26)

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 2002 br-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

br-er2-17.cdr work piece arc stick electrode electrode holder power source = or ~ - (+) + (-) Figure 2.17 Figure 2.18

(27)

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 elecelec-trode 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

m e d iu m w e ld v o lt a g e B15 B53 RA12 RR12 RA73 RR73 100 200 300 400 6 3,25 4 5 = = = = 20 25 30 35 40 45 A V © ISF 2002 br-er2-19.cdr arc welding converter transformer rectifier inverter type Figure 2.19 Figure 2.20

(28)

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 electrodes

b = basic-coated electrodes, recovery <125%

a = A- and R- coated electrodes, recovery 105%

0 1 2 3 4 5 6 7 b u rn -o ff r a te a t 1 0 0 % d u ty c y c le welding amperage kg/h 100 200 300 0 400 A 500 = RR12 - 5 mm X= RR73 - 5 mm thic k-co ated thin-c oate d 220 % d epos ition effi cien cy 160% dep ositi on e ffici ency X c b a Figure 2.21 © ISF 2002 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

Arc Blow Effect through Concentration of Magnetic Fields

br-er2-23e.cdr

(29)

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 welding sequence

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 Effect on Steel Parts

inwards at the edges

close to current-connection

close to large workpiece masses

in gaps towards the weld

Figure 2.24

(30)

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 2002 br-er2-27.cdr Time of storage W a te r c o n te n t o f th e c o a ti n g 1 10 Tage 100 0 0 1,0 2,0 3,0 4,0 % 20°C / 70% RF © ISF 2002

Water Content of the Coating after Storage and Baking

br-er2-28.cdr

basic-coated electrode (having been stored at 18 - 20°C for one year)

storage and baking

0,74 0,39 0,28 AWS A5.5 W a te r c o n te n t o f th e c o a ti n g 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 30 40 50 60 70 % 80 % Figure 2.27 Figure 2.28

(31)

2003

3.

(32)

In submerged arc welding a mineral weld flux layer protects the welding point and the freezing weld from the influence of the surrounding atmosphere, Figure 3.1. The arc burns in a cavity filled with ionised gases and vapours where the droplets from the continuously-fed wire electrode are transferred into the weld pool. Un-fused flux can be extracted from be-hind the welding head and subse-quently recycled.

Main components of a submerged arc welding unit are:

the wire electrode reel, the wire feed motor equipped with grooved wire feed rolls which are suitable for the demanded wire diameters, a wire straigthener as well as a torch head for current transmission, Figure 3.2.

Flux supply is car-ried out via a hose from the flux con-tainer to the feeding hopper which is mounted on the torch head. De-pending on the de-gree of automation it is possible to in-stall a flux excess pickup behind the torch. Submerged

Process Principle of Submerged Arc Welding

br-er3-01e.cdr electrode contact piece flux hopper Figure 3.1 © ISF 2002

Assembly of a SA Welding Equipment

br-er3-02e.cdr

AC or DC current supply wire straightener wire feed rolls flux supply indicators wire reel

power source welding machine holder

(33)

arc welding can be operated using either an a.c. power source or a d.c. power source where the electrode is normally connected to the positive terminal.

Welding advance is provided by the welding machine or by workpiece movement.

Identification of wire electrodes for

submerged arc welding is based on the average Mn-content and is carried out in steps of 0.5%, Figure 3.3. Standardisation for welding filler ma-terials for unalloyed steels as well as for fine-grain structural steels is con-tained in DIN EN 756, for creep

resis-tant steels in DIN pr EN 12070 (previ-ously DIN 8575) and for stainless and heat resistant steels in DIN pr EN 12072 (previously DIN 8556-10).

The proportions of additional alloying elements are dependent on the mate-rials to be welded and on the me-chanical-technological demands which emerge from the prevailing operating conditions, Figure 3.4. Connected to this, most important alloying

ele-ments are manganese for strength,

molybdenum for high-temperature strength and nickel for toughness.

Properties and application

S1 1.0351 C Si Mn = 0,08 = 0,09 = 0,50 C Si Mn = 0,11 = 0,15 = 1,50 C Si Mn = 0,10 = 0,30 = 1,00 C Si Mn Mo = 0,10 = 0,15 = 1,00 = 0,50 C Si Mn Ni = 0,09 = 0,12 = 1,00 = 1,20 C Si Mn Ni = 0,10 = 0,12 = 1,00 = 2,20 C Si Mn Mo Ni = 0,12 = 0,15 = 1,00 = 0,50 = 1,00

For lower welding joint quality requirements;in: boiler and tank construction, pipe production, structural steel engineering, shipbuilding = 0,10 = 0,10 = 1,00 C Si Mn S2 1.5035 S3 1.5064 S2Si 1.5034 S2Mo 1.5425 S2Ni1 S2Ni2 S3NiMo1

For higher welding joint quality requirements; in: pipe production, boiler and tank construction, sructural steel engineering, shipbuilding. Fine-grain structural steels up to StE 380. For high-quality welds with medium wall-thicknesses.

Fine-grain structural steels up to StE 420. Especially suitable for welding of pipe steels, no tendency to porosity of unkilled steels. Fine-grain structural steels up to StE 420.

For welding in boiler and tank construction and pipeline production with creep-resistant steels. Working temperatures of up 500 °C. Suitable for higher-strength fine-grain structural steels.

For welding low-temperature fine-grain structural steels.

Non-ageing.

Especially suitable for low-temperature welds. Non-ageing.

For quenched and tempered fine-grain structural steels.

Suitable for normalising and/or re-quenching and tempering.

commercial wire electrodes

main alloying elements

Mn Ni Mo Cr V alloy type Mn MnMo Ni NiMo NiV NiCrMo S1 S2 S3 S4 0,5 1,0 1,5 2,0 S2Mo S3Mo S4Mo 1,0 1,5 2,0 0,5 0,5 0,5 S2Ni1 S2Ni2 1,0 1,0 1,0 2,0 S2NiMo1 S3NiMo1 1,0 1,5 1,0 1,0 0,5 0,5 S3NiV1 1,5 1,0 0,15 S1NiCrMo2,5 S2NiCrMo1 S3NiCrMo2,5 0,5 1,0 1,5 2,5 1,0 2,5 0,6 0,6 0,6 0,8 0,5 0,8

From a diameter of 3 mm upwards all wire electrodes have to be marked with the following symbols:

S1 Si Mo S6: : : I IIIIII _ Example:_S2Si: _ S3Mo: II III Figure 3.3 Figure 3.4

(34)

The identification

of wire electrodes

for submerged arc welding is stan-dardised in DIN EN 756, Figure 3.5.

During manufacture of fused welding fluxes the individual mineral constituents are, with regard of their future

compo-sition, weighed and subsequently fused in a cupola or electric furnace, Figure 3.6. In the dry granulation proc-ess, the melt is poured stresses break the crust into large fragments. During water granulation the melt hardens to form small grains with a diameter of approximately 5 mm.

As a third variant, compressed air is additionally blown into the water tank resulting in finely blistered grains with

low bulk weight. The fragments or

grains are subsequently ground and screened – thus bringing about the desired grain size.

Identification of a Wire Electrode in Accordance with DIN EN 756

br-er3-05e.cdr

W i r e e l e c t r o d e DIN EN 756 - S2Mo

DIN main no.

Symbols of the chemical composition:

S0, S1...S4, S1Si, S2Si, S2Si2, S3Si, S4Si, S1Mo,..., S4Mo, S2Ni1, S2Ni1.5, S2Ni2, S2Ni3, S2Ni1Mo, S3Ni1.5, S3Ni1Mo, S3Ni1.5Mo

lime quarz rutile bauxite magnesite

silos balance roasting kiln coke coke air raw material molten metal tapping coal-burning stove electrical furnace granulation tub foaming air screen balance cylindrical crusher drying oven Figure 3.5 Figure 3.6

(35)

During manufacture of

agglomer-ated weld fluxes the raw materials

are very finely ground, Figure 3.7. After weighing and with the aid of a suitable binding agent (waterglass) a pre-stage granulate is produced in the mixer.

Manufacture of the granulate is fin-ished on a rotary dish granulator where the individual grains are rolled up to their desired size and consoli-date. Water evaporation in the drying oven hardens the grains. In the an-nealing furnace the remaining water is subsequently removed at tempera-tures of between 500°C and 900°C, depending on the type of flux.

The fused welding fluxes are charac-terised by high homogeneity, low sen-sitivity to moisture, good storing prop-erties and high abrasion resistance. An important advantage of the ag-glomerated fluxes is the relatively low manufacturing temperature, Figure 3.8. The technological properties of the welded joint can be improved by the addition of temperature-sensitive deoxidation and alloying constituents to the flux. Agglomerated fluxes have, in general, a lower bulk weight (lower consumption) which allows the use of components which are reacting among

rutile Mn - ore fluorspar magnesite alloys

sintering furnace silos ball mill balance mixer dish granulator gas drying oven

heat treatment furnace

cooling pipe screen balance Figure 3.7 © ISF 2002 br-er3-08e.cdr Properties uniformity of grain size distribution grain strength homogeneity susceptibility to moisture storing properties resistance to dirt current carrying capacity slag removability high-speed welding properties multiple-wire weldability flux consumption 1)

assessment : -- bad, - moderate, + good, ++ very good

2)_{core agglomerated flux}

Fused fluxes1) Agglomerated

fluxes1) +/++ +/++ +/++ +/++ +/++ +/++ -/++ -/+ -/++ -/++ -/++ -- /++2) +/++ +/++ +/++ +/++ +/++ --/+ -/+ -/+ -/++ +/++ Figure 3.8

(36)

themselves during the melting proc-ess. However, the higher susceptibil-ity to moisture dur-ing storage and-processing has to be taken intocon-sideration.

The SA welding fluxes are, in accordance with their mineralogical constituents, clas-sified into nine groups, Figure 3.9. The composition of the individual flux groups is to be considered as in principle, as fluxes which belong to the same group may differ substantially with regards to their welding or weld metal properties. In addition to the groups mentioned above there is also the Z-group which allows free compositions of the flux.

The calcium silicate fluxes are rec-ognized by their effective silicon pickup. A low Si pickup has low crack-ing tendency and liability to rust, on the other hand the lower current car-rying capacity of these fluxes has to be accepted. A high Si pickup leads to a high current currying capacity up to 2500 A and a deep penetration.

Alu-minate-basic fluxes have, due to the higher Mn pickup, good mechanical

Different Welding Flux Types According to DIN EN 760 br-er3-09e.cdr MS CS ZS RS AR AB AS AF FB Z MnO + SiO CaO 2 min. 50% max. 15% manganese-silicate CaO + MgO + SiO

CaO + MgO

2 min. 55%

min.15% calcium-silicate ZrO + SiO + MnO

ZrO 2 2 2 min. 45% min. 15% zirconium-silicate TiO + SiO TiO 2 2 2 min. 50% min. 20% rutile-silicate Al O + TiO2 3 2 min. 40% aluminate-rutilel

Al O + CaO + MgO Al O CaF 2 3 2 3 2 Al O + SiO + ZrO CaF + MgO ZrO 2 3 2 2 2 2 Al O + CaF2 3 2

CaO + MgO + CaF + Mo SiO CaF 2 2 2 min. 40% min. 20% max. 22% aluminate-basic min. 40% min. 30% min. 5% aluminate-silicate min. 70% aluminate-fluoride-basic min. 50% max. 20% min. 15% fluoride-basic other compositions Figure 3.9 © ISF 2002 br-er3-10ae.cdr

MS - high manganese and silicon pickup - restricted toughness values

- high current carrying capacity/ high weld speed - unsusceptible to pores and undercuts - unsuitable

- suitable for high-speed welding and fillet welds for thick parts

CS acidic types

basic types

- highest current carrying capacity of all fluxes - high silicon pickup

- suitable for welding by the pass/ capping method of thick parts with low requirements

- low silicon pickup

- suitable for multiple pass welding

- current carrying capacity decreases with increaseing basicity

ZS - high-speed welding of single-pass welds

RS - high manganese pickup/ high silicon pickup - restricted toughness values of the weld metal - suitable for single and multi wire welding - typical: welding by the pass/ capping pass method

AR - average manganese and silicon pickup - suitable for a.c. and d.c.

- single and multi wire welding

- application fields: thin-walled tanks, fillet welds for structural steel construction and shipbuilding

(37)

properties. With the application of wire electrodes, as S1, S2 or S2Mo, a low cracking tendency can be obtained.

Fluoride-basic fluxes are

character-ised by good weld metal impact val-ues and high cracking insensitivity. Figures 3.10a and 3.10b show typical properties and application areas for the different flux types.

Figure 3.11 shows the identification

of a welding flux according to DIN

EN 760 by the example of a fused calcium silicate flux. This type of flux is suitable for the welding of joints as well as for overlap welds. The flux can be used for SA welding of unalloyed and low-alloy steels, as, e.g. general structural steels, as well as for welding high-tensile and creep resistant steels. The silicon pickup is 0.1 – 0.3% (6), while the manganese pickup is expected to be 0.3 – 0.5% (7). Either d.c. or a.c. can be used, as, in principle, a.c.

weldability allows also for d.c. power source. The hydro-gen content in the clean weld metal is lower than the 10 ml/100 g weld metal.

Identification of a Welding Flux According to DIN EN 760

br-er3-11e.cdr

w e l d i n g f l u x D I N EN 760-SF CS 1 67 AC H10

DIN main no. flux/SA welding method of manufacture

F fused A agglomerated

M mechanically mixed flux

flux type (figure 3.9) flux class 1-3 (table 1) metallurgical behaviour (table 2) hydrogen content (table 4) type of current © ISF 2002 br-er3-10be.cdr

AB - medium manganese pickup

- good weldability

- good toughness values in welding by the pass/ capping pass method

- application field:unalloyed and low alloyed structural steels - suitable for a.c. and d.c.

- applicable for multilayer welding or welding by the pass/ capping pass method

AS - mainly neutral metallurgical behavior - manganese burnoff possible

- good weld appearance and slag removability - to some degree suitable for d.c.

- recommended for multi layer welds for high toughness requirements

- application field: high-tensile fine grain structural steels, pressure vessels, nuclear- and offshore components

- mainly neutral metallurgical behaviour - however, manganese burnoff possible - highest toughness values right down to very low temperatures

- limited current carrying capacity and welding speed - recommended for multi layer welds

- application field: high-tensile fine-grain structural steeels FB

AF - suitable for welding stainless steels and nickel-base alloys - neutral behaviour as regards Mn, Si and other

constituents

Z - all other compositions

Figure 3.10b

(38)

The flux classes 1-3 (table 1) explain the suitability of a flux for welding certain ma-terial groups, for welding of joints and for overlap welding. The flux classes also characterise the metallurgical material behaviour. In table 2 defines the identification figure for the pickup or burn-off behaviour of the respective ele-ment. Table 4 shows the grada-tion of the diffus-ible hydrogen content in the weld metal, Fig-ure 3.12.

Figure 3.13 shows the identification of a wire-flux combination and the resultant weld metal. It is a case of a combination for multipass SA welding where the weld metal shows a minimum yield point of 460 N/mm² (46) and a mini-mum metal impact value of 47 J at – 30°C (3). The flux type is aluminate-basic (AB) and is used with a wire of the quality S2.

Parameters for Flux Identification According to DIN EN 760 br-er3-12e.cdr unalloyed and low-alloyed steel general structural steel high-tensile & creep resistant steels

welding of joints

hardfacing stainless and heat resistant steels Cr- & CrNi steels

pickup of elements as C, Cr, Mo flux class 1 2 3 table 1 table 2 metallurgial behaviour identification figure proportion flux in all-weld metal % 1 2 3 4 over 0,7 0,5 up to 0,7 0,3 up to 0,5 0,1 up to 0,3 burnoff 5 pickup or burnoff 0 up to 0,1 6 7 8 9 0,1 up to 0,3 0,3 up to 0,5 0,5 up to 0,7 over 0,7 pickup table 4 identification hydrogen content ml/100g all-weld metal max. H5 H10 H15 5 10 15 Figure 3.12

Identification of a Wire-Flux Combination According to DIN EN 756

br-er 3-13e.cdr

chemical composition of the wire electrode w i r e - f l u x c o m b i n a t i o n

D I N E N 7 5 6 - S 4 6 3 A B S 2 standard no.

wire electrode and/or wire-flux combination for submerged arc welding strength and fracture strain (table1 and 2) impact energy (table 3) type of flux (figure 3.10) Figure 3.13

(39)

The tables for the identification of the tensile properties as well as of the impact en-ergy are combined in Figure 3.14.

The chemical composition of the weld metal and the structural constitution are dependent on the different

metal-lurgical reactions during the welding

process as well as on the used

mate-rials, Figure 3.15. The welding flux

influences the slag viscosity, the pool motion and the bead surface. The different combinations of filler material and welding flux cause, in direct de-pendence on the weld parameters (current, voltage), a different melting behaviour and also different chemical reactions. The dilution with the base metal leads to various strong weld pool reactions, this being dependent on the weld parameters.

The diagram of the

characteristics for 3 different welding fluxes assists, in

dependence of the used wire elec-trodes, to determine the pickup and burn-off behaviour of the element manganese, Figure 3.16. For example: A welding flux with

table 2 identifi-cation

minimum base metal yield strength N/mm2 minimum tensile strength N/mm2 2T 3T 4T 5T 275 355 420 500 370 470 520 600 Identification for strength properties of welding by the pass/ capping pass method welded joints

identification minimum yield point

n/mm2 tensile strength_N/mm2 minimum fracture strain_%

440 up to 570 470 up to 600 500 up to 640 530 up to 680 560 up to 720 355 380 420 460 500 35 38 42 46 50 22 20 20 20 18 table 1 Identification for strength properties of multipass weld joints

table 3 Identification for the impact energy of clean all-weld metal or of welding by _{the pass/ capping pass method welded joints} Z no demands A 0 2 3 4 5 6 7 8 -80 -70 -60 -50 -40 -30 -20 0 +20 identification

temp. for minimum impact energy 47J

°C

Figure 3.14

Metallurgical Reactions During Submerged Arc Welding

br-er 3-15e.cdr

droplet reaction

dilution weld pool reaction

welding flux welding filler metal

slag weld metal base metal welding data welding data welding data Figure 3.15

(40)

the mean charac-teristic and when a wire electrode S3 is used, has a neu-tral point where neither pickup nor burn-off occur.

The pickup and burn-off behaviour is, besides the filler material and the welding flux, also directly dependent on the welding amperage and welding voltage, Figure 3.17. By the example of the selected flux a higher welding voltage causes a more steeply descending manganese

char-acteristic at a constant neutral point. Silicon pickup increases with the in-creased voltage. The influence of cur-rent and voltage on the carbon content is, as a rule, negligible.

Inversely proportional to the voltage is the rising characteristic as regards manganese in dependence on the welding current, Figure 3.18. Higher currents cause the characteristic curve to flatten. As the welding voltage, the welding current also has practically no influence on the location of the neutral point. Silicon pickup decreases with increasing current intensity.

Manganese-Pickup and Manganese-Burnoff During Submerged Arc Welding

br-er 3-16e.cdr S1 1,0% 2,0% 3,0% Mn in wire Mn-burnoff Mn-pickup S2 S3 S4 S5 S6 Figure 3.16 © ISF 2002 br-er3-17e.cdr weld flux LW 280 current intensity 580 A welding speed 55 cm/min

neutral point % Mn wire % Si wire % C wire p ic k u p / b u rn o ff X i n w e ig h t % r Figure 3.17

(41)

The Mn-content of the weld metal can be determined by means of a welding flux

diagram, Figure 3.19.

In this example, the two points on the axis which determine the flux characteris-tic are defined for the parameters 600A welding current and 29V welding voltage, with the aid of the auxiliary straight line

and the neutral point curve (MnNP). In this

case, the two points are positioned at

0.6% ∆Mn and 1.25% MnSZ. Dependent

on the manganese content of the used filler material, the pickup or burn-off con-tents can be recognized by the reflection with respect to the characteristic line

(0.38% Mn-pickup with a wire contain-ing 0.5%Mn, 0.2% Mn-burnoff with a wire containing 1.75%Mn).

The structure of the characteristic line for the determination of the silicon

pickup content, is, in principle, exactly

the same as described above, Figure 3.20. As silicon has only pickup prop-erties and therefore no neutral point exists, the second auxiliary straight line must be considered for the deter-mination of the second characteristic line point.

weld flux LW 280 arc voltage 29 V

welding speed 55 cm/min

p ic k u p / b u rn o ff X i n w e ig h t % r neutral point % Mn wire % Si wire % C wire 450 A Figure 3.18 © ISF 2002 br-er3-19e.cdr flux diagramm LW 280, manganese wire electrode 4 mm acc. to Prof. Thier

= 580 A U = 29 V Mn = 0.48 % Mn Mn = 1.69 % Mn example: I SZ1 SZ2 Ø Figure 3.19

(42)

Weld preparations for multipass fabrication are dependent on the thickness of the plates to be welded, Figure 3.21. If no root is planned during weld prepara-tion and also no support of the weld pool is made, the root pass must be welded using low energy input.

When welding very thick plates which are accessible from both sides, the

double-U butt weld may be applied,

Figure 3.22. Before the opposite side is welded, the root must be milled out (gouging/sanding). This type of weld cannot be produced by flame cutting and is, as milling is necessary, more

expensive, although exact weld

preparation and correct selection of the welding parameters lead to a high weld quality.

Another variation of heavy-plate welded joints is the so-called

“steep single-V butt weld”, Figure 3.23.

The very steep edges keep the welding vol-ume at a very low level. This technique, however, requires the application of special narrow-gap torches. The geometry during slag detachment and

flux diagramm LW 280, silicon

wire electrode 4 mm acc. to Prof. Thier

= 580 A U = 29 V Si = 0.16 % Si example: I SZ Ø auxiliary straight line auxiliary straight line Figure 3.20

Welding Procedure Sheets for Single-V Butt Welds, Single-Y Butt Welds with Broad Root Faces and Double-V Butt Welds

br-er 3-21e.cdr

preparation geometry weld buildup

manual metal arc welding

manual metal arc welding manual metal arc welding

and SA SA SA SA SA SA SA SA SA SA Figure 3.21

(43)

also during rework-ing weld-related defects may cause problems. Here, high demands are made on torch ma-nipulation and process control. Special narrow-gap welding fluxes fa-cilitate slag re-moval.

The most important welding parameters as regards weld bead formation are weld-ing current, voltage and speed, Figure 3.24. A higher weldweld-ing current causes higher deposition rates and energy input, which leads to reinforced beads and a deeper penetration. The weld width remains roughly constant. The increased welding voltage leads to a longer arc which also causes the bead to be wider. The change in welding speed causes - on both sides of an optimum - a decrease of the penetration depth. At lower weld speeds, the weld pool running ahead of the welding arc acts as a buffer between arc

and base metal. At high speeds, the energy per unit length decreases which leads, be-sides lower penetration, also to narrower beads.

Welding Procedure Sheet for Square-Edge Welds

Welding Procedure Sheet for Double-U Butt Welds

br-er3-22e.cdr

preparation geometry weld buildup

manual metal arc welding turning and sanding manual metal arc welding turn turn turn side 1 side 2 SA SA SA SA SA SA SA SA Figure 3.22 Figure 3.23

(44)

Weld flux consumption is dependent on the selected weld type, Figure 3.25. Due to

geometrical shape, the flux consumption of a fillet weld is significantly lower than that of a butt weld. Because of their lower bulk weight, the specific consumption of

ag-glomerated fluxes is lower than that of fused fluxes.

Two different control concepts allow the regulation of the arc length (the principle is shown in Figure 3.26). The applica-tion of the appropri-ate control system is © ISF 2002 br-er3-24e.cdr constant: plate thickness: wire electrode: flux: welding current ( )I constant:

arc voltage (U)

constant: welding speed (v) p e n e tr a ti o n d e p th t in m m p w e ld w id th b i n m m w tp I w tp I w te Figure 3.24 © ISF 2002 br-er3-25e.cdr 2,4 2,2 2,0 1,8 1,6 1,6 1,4 1,4 1,2 1,2 1,0 1,0 0,8 0,8 0,6 0,6 0,4 0,4 0,2 0,2 0 400 400 0 500 500 600 600 700 700 800 800 900 900 1000 1000 1100 1100 c o n s u m p ti o n k g f lu x / k g w ir e c o n s u m p ti o n k g f lu x / k g w ir e

A) flat weld - I square butt joint

current intensity (A)

current intensity (A) B) fillet weld

fused composition fluxes

agglomerated fluxes

Figure 3.25

Control of the Arc Length

br-er3-26e.cdr 1 2 3 direction of welding L1 L2 L3 Figure 3.26

(45)

dependent on the available power source characteristics.

The external regulation of the arc length by the control of the wire feed speed requires a power source with a steeply descending characteristic, Figure 3.27. In this case, the shorten-ing of the arc caused by some

process disturbance, entails a strong voltage drop at a low current rise. As a regulated quantity, this voltage drop reduces the wire feed speed. Thus, the initial arc length can be regulated at an almost constant deposition rate. In contrast, the internal regulation effects, when the arc is reduced, a strong current rise at a low voltage

drop (slightly descending characteris-tic). At a constant wire feed speed the initial arc length is independently regu-lated by the increased burn-off rate which again is a consequence of the high current.

The reaction of the internal

regula-tion to process disturbance is very

fast. This process is self regulating and does not require any machine ex-penditure.

In submerged arc welding of butt joints, it is, depending on the weld preparation, necessary to support the

external regulation (∆U-regulation)

A A´ U ∆ U ∆

internal self regulation (∆I-regulation)

Figure 3.27

br-er3-28e.cdr

Examples of Weld Pool Backups

backing flux

ceramic backing bar

flux copper backing

(46)

liquid weld pool with a backing, Figure 3.28. This is normally done with either a ce-ramic or copper backing with a flux layer or by a backing flux. Dependent on the shape of the backing bar, direct formation of the underside seam can be achieved. When welding circumferential tubes, the inclination angle of the

elec-trode has a direct influence onto the

formation of the weld bead, Figure 3.29. For external as well as for inter-nal tube welds, the best weld shapes may be obtained with an adjusted an-gular position of the torch. If the ad-vance is too low, the molten bath runs ahead and produces a narrow weld

with a medium-sized ridge, too high

an advance causes the flowback of

the molten bath and a wide seam with a formed trough in the centre. The processes described here for external tube welds are, the other way round, also applicable to internal tube welds.

To increase the

efficiency of sub-merged arc weld-ing, different

proc-ess variations are applied, Figure 3.30. In multiwire

welding, where up

to 6 wires are used, each welding torch is operated from a separate power source. In twin wire

single wire tandem

(47)

welding, two wire

electrodes are connected in one torch and supplied from one power source. Dependent on the application, the wires can be arranged in a parallel or in a tan-dem.

In submerged arc welding with iron powder addition can the deposition rate be substantially increased at constant electrical parameters, Figure 3.31. The increased deposition rate is realised by either the addition of a currentless wire (cold wire) or of a preheated filler wire (hot wire). The

use of a rectangular strip instead of a wire electrode allows a higher current carrying capacity and opens the SA method also for the wide application range of surfacing.

However, the mentioned process

variations can be combined over

wide ranges, where the electrode dis-tances and positions have to be ap-propriately optimised, Figure 3.32. Current type, polarity, geometrical co-ordination of the individual weld heads and the selected weld parameters also have substantial influence on the weld result.

tandem welding

three-wire welding

three-wire, hot wire welding four-wire welding 1. WH 1. WH 1. WH 2. WH 2. WH 3. WH HW = = = ~ ~ ~ ~ ~ ~ 3. WH ~ ~ 2. WH ~ 65° 65° 12..16 12..16 35 10 10 15 35 12..16 75° 80° 12 15 18 Process Variations of Submerged-Arc Welding iron powder/ chopped wire hot wire cold wire strip © ISF 2002 br-er3-31e.cdr Figure 3.31 Figure 3.32

(48)

The description of these individual process variations of submerged arc welding shows that this method can be applied sensibly and economically over a very wide operating range, Figure 3.33. It is a high-efficiency

welding process with a deposition

rate of up to 100 kg/h. Due to large molten pools and flux application posi-tional welding is not possible.

When more than one wire is used in order to obtain a high deposition rate, arc inter-actions occur due

to magnetic arc

blow, Figure 3.34.

Therefore, the selection of the current type (d.c. or a.c.) and also sensible phase displacements between the indi-vidual welding torches are very important. © ISF 2002 br-er3-33e.cdr 0 500 1000 1500 2000 2500 A 3500 0 10 20 30 40 50 60 70 80 kg/h 100 d e p o s it io n r a te current intensity w e ld m e ta l voltage = 30 V speed = 40 cm/min wire protrusion = 10d length current intensity ∅3,0 mm ∅4,0 mm ∅5,0 mm 12 9 6 3 0 300 400 500 600 A 800 ~ ~ kg/h

single wire+ metal powder single wire+ hot wire

double wire single wire tandem three-wire four-wire Figure 3.33 © ISF 2002 br-er3-34e.cdr + + ( )_ ( )_ + _ + ~ _ _ ₊ ( ) _ _ ( ) elektrode arc workpiece + +

Magnetic Interaction of Arcs at SA Tandem Welding

(49)

2003

4. TIG Welding and

Plasma Arc Welding

(50)

TIG welding and plasma welding belong to the group of the gas-shielded tungsten arc welding processes, Figure 4.1. In all processes mentioned in Figure 4.1, the arc burns between a

non- consumable tungsten elec-trode and the

workpiece or, in plasma arc weld-ing, between the tungsten electrode and a live copper electrode inside the torch. Exclu-sively inert gases (Ar, He) are used as shielding gases.

The potential curve of the ideal arc, as shown in Figure 4.2, can be divided into three characteristic sectors:

1.cathode- drop region 2.arc

3. anode-drop region

In the cathode-drop region almost 50% of the total voltage drop oc-curs over a length of 10-4 mm. A similarly high voltage drop oc-curs in the anode-drop region, here, however, over a length of 0.5 mm. © ISF 2002

Classification of Gas-Shielded Arc Welding acc. to DIN ISO 857

br-er4-01e.cdr Plasma arc welding with semi-transferred arc Plasma arc welding with transferred arc Plasma arc welding with non-transferred arc CO welding2 Mixed gas

welding narrow-gap gas-shielded arc welding plasma metal arc welding electrogas welding Metal inert-gas welding MIG Metal active gas

welding MAG

Gas-shielded arc welding

Tungsten hydrogen welding Tungsten plasma welding with electrode Tungsten inert-gas welding TIG Gas-shielded metal arc welding

Arc Potential Curve

br-er4-02e.cdr U V 20 10 0 1 2 3 4 5 10-4 0,5 l US l mm K L A + -A: K: L: l:

anode spot (up to 4000°C) cathode spot (approx. 3600°C) arc column (4500-20000°C) arc length

arc potential curve (example)

(51)

The voltage drop on the remaining arc length is comparatively low. Main en-ergy conversion occurs accordingly in the anode-drop and cathode-drop re-gion.

Figure 4.3 shows the potential

dis-tribution by the example of a real TIG

arc under the influence of different

shielding gases. UA and UK have

dif-ferent values, the potential curve in the arc is not exactly linear. There is no discernible expansion of the cath-ode-drop and ancath-ode-drop region .

The electrical characteristics of the

arc differ, depending on the selected

shielding gas, Figure 4.4. As the ionisa-tion potential of helium in comparison with argon is higher, arc voltage must necessarily be higher. © ISF 2002 br-er4--03e.cdr X X 0 0 1 1 2 2 3 3 4 4 6 6 20 40 10 20 5 10 U U anode anode cathode cathode U = 6,5 VK U = 6,5 VK U = 3,5 VA U = 6,1 VA Argon 60 A Helium 60 A V V mm mm ARC ARC ARC ARC Figure 4.3 © ISF 2002 br-er4-04e.cdr a rc v o lt a g e 25 20 15 10 a rc l e n g th 4 2 4 2 heliu m argon weld current 50 100 150 200 250 350 0 mm V A Figure 4.4

(52)

The temperature

distribution of a

TIG arc is shown in Figure 4.5.

In TIG welding just approximately 30% of the input electrical energy may be used for melting the base metal, Fig-ure 4.6. Losses result from the arc ra-diation and heat dissipation in the workpiece and also from the heat con-version in the tungsten electrode.

Temperature Distribution in a TIG Arc (at I=100 A)

br-er4-05e.cdr TIG cathode 1 0 0 0 0 K 9 0 0 0 K 8 0 0 0 K x x x x x x x x x x x x x x x x x x x x x x anode spot weld pool 2 mm 4 6 8 2 mm 4 6 8 _{4 3 2 1 0 1 2 mm} ₄ © ISF 2002 br-er4-06e.cdr melting of wire welding direction radiation R.I2 P = U.I thermal conductivity [W/m K] fusion heat [kJ/kg] specific heat [kJ/kg K] Figure 4.5 Figure 4.6

(53)

Figure 4.7 describes the process principle of TIG welding.

Figure 4.8 explains by an example the code for a TIG welding wire, as stipulated in the drafts of the European Standardisations.

Tungsten Inert Gas Welding (TIG)

br-er4-07e.cdr

tungsten electrode electric contact

shielding gas shielding gas nozzle

Designation of a Tungsten Innert Gas Welding Wire to EN 1668

br-er4-08e.cdr

identification of filler rod as an individual product: W2 chemical composition table

rods and wires for tig-welding

minimum impact energy value 47 J at -30°C

minimum weld metal yield point: 460 N/mm2

identification letter for TIG-welding

W 46 3 W2