Chapter 9 welding defects

Defect Class: Shape Defects undercut open end crater weld reinforcement too small throat thickness start defects excessive seam width burn through undercut, continuous unfused longitudin

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9

Welding Defects

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Figures 9.1 to 9.4 give a rough survey about the classification of welding defects to DIN

8524 This standard does not classify existing welding defects according to their origin but only to their appearance

Defect Class: Shape Defects

undercut

open

end crater

weld reinforcement

too small throat

thickness

start defects

excessive

seam width

burn through

undercut, continuous

unfused longitudinal seam edge

end crater with reduction

of weld cross section

nominal

surface defects at a start point

weld is too wide

through-going hole

in or at the edge

of the seam

Figure 9.1

Defect Class:

Cracks and Cavities

longitudinal crack

transverse crack

star shaped crack

porosity pore

nest of pores

line of pores

worm hole

in the unaffected base metal

in weld metal

in the HAZ

in weld metal

in the HAZ

in weld metal

globular gas inclusion

many, mainly evenly distributed pores

locally repeated pores

pores arranged in a line

elongated gas inclusion

in weld direction

Figure 9.2

Defect Class: Lack of Fusion, Insufficient Through-Weld

insufficiently

welded root

one or two longitudinal edges of the groove are unfused

insufficient

through weld

insufficiently welded cross section

lack of fusion

between passes

root lack of fusion

flank lack of fusion

lack of fusion between weld passes or weld beads

lack of fusion in the area of weld root

lack of fusion between weld and base metal

Figure 9.3

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A distinction of arising defects by their origin is shown in Figure 9.5 The development of the most important welding defects is explained in the following paragraphs

Lack of fusion is defined

as unfused area between weld metal and base mate-rial or previously welded layer This happens when the base metal or the pre-vious layer are not

molten Figure 9.6 explains the influence of welding parameters on the devel-opment of lack of fusion In the upper part, arc charac-teristic lines of MAG

and mixed gas The weld-ing voltage depends on welding current and is se-lected according to the joint type With present tension, the welding cur-rent is fixed by the wire feed speed (thus also melting rate) as shown in the middle part of the fig-ure

Melting rate (resulting from selected welding parameters) and welding speed define the heat input As it can be changed within certain limits, melting rate and welding speed do not limit each other, but a working range is created (lower part of the figure) If the heat input is too low, i.e too high welding speed, a definite melting of flanks cannot be ensured Due to the

Defect Class: Solid Inclusions

slag line

single

slag inclusions

pore nest

stringer type inclusions

different shapes and directions

irregular slag inclusions

locally enriched

Figure 9.4

Welding Defects

spatters and

start points

undercuts

seam shape

defects

lacks of fusion

slag inclusions mechanical pore formation

solidification cracks remelt cracks

hydrogen cracks hardening cracks lamellar cracks precipitation cracks

metallurgical pore formation crater formation

welding joint defects

external weld defects internal weld defects hot cracks cold cracks cavities with weld metal

welding defects due to material welding defects due to manufacture

Figure 9.5

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poor power, lack of fusion is the result With too high heat input, i.e too low welding speed, the weld pool gets too large and starts to flow away in the area in front of the arc This effect prevents a melting of the base metal The arc is not directed into the base metal, but onto the weld pool, and flanks are not entirely molten Thus lack of fusion may occur in such areas

Figure 9.7 shows the influence of torch position on the development of weak fusion The up-per part of the figure explains the terms neutral, positive and negative torch angle Compared with a neutral position, the seam gets wider with a positive inclination together with a slight reduction of penetration depth A negative inclination leads to narrower beads The second part of the figure shows the torch orientation transverse to welding direction with multi-pass welding To avoid weak fusion between layers, the torch orientation is of great importance, as

it provides a reliable melting and a proper fusion of the layers The third figure illustrates the influence of torch orientation during welding of a fillet weld

With a false torch orientation, the perpendicular flank is insufficiently molten, a lack of fusion occurs When welding an I-groove in two layers, it must be ensured that the plate is

com-Influence of Welding Parameters

on Formation of Lack of Fusion

mixed gas

CO 2

Welding current

Wire feed Melting rate

lack of fusion

due to too

low performance

lacks of fusion due to preflow

br-er09-06.cdr

Figure 9.6

Influence of Torch Position

on Formation of Lack of Fusion

false correct

1 2

positive torch angle neutral negative torch angle

welding direction

torch axes

correct

torch axes

false

90°

approx 45°

Figure 9.7

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pletely fused A false torch orientation may lead to lack of fusion between the layers, as shown in the lower figure

Figure 9.8 shows the influ-ence of the torch orienta-tion during MSG welding of

a rotating workpiece As

an example, the upper fi-gure shows the desired torch orientation for usual welding speeds This ori-entation depends on pa-rameters like workpiece diameter and thickness,

rate, and welding speed The lower figure illustrates variations of torch orientation on seam formation A torch orientation should be chosen in such a way that a solidification of the melt pool takes place in 12 o'clock position, i.e the weld pool does not flow in front or behind of the arc Both may cause lack of fusion

In contrast to faulty fusion, pores in the weld metal due to their globular shape are less criti-cal, provided that their size does not exceed a certain value Secondly, they must occur iso-lated and keep a minimum

distance from each other

There are two possible

mechanisms to develop

cavities in the weld metal:

the mechanical and the

metallurgical pore

causes of a mechanical

pore formation as well as

possibilities to avoid them

To over-weld a cavity (lack

Influence of Torch Position

on Formation of Lacks of Fusion

9 Uhr

12

1 2 3

6

9 Uhr

12

1 2 3

6

9 Uhr

12

1 2 3

6

Figure 9.8

Figure 9.9

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of fusion, gaps, overlaps etc.) of a previous layer can be regarded as a typical case of a me-chanical pore formation

The welding heat during

welding causes a strong

expansion of the gasses

contained in the cavity and

consequently a

develop-ment of a gas bubble in

the liquid weld metal If the

solidification is carried out

so fast that this gas bubble

cannot raise to the surface

of the weld pool, the pore

will be caught in the weld

metal

Figure 9.10 shows a X-ray

photograph of a pore which developed in this way, as well as a surface and a transverse

sec-gas/gas developing material

air too low shielding gas flow through:

regulator too low supply pressure for pressure

regulator

Pressure of bottles or lines must meet the required supply pressure of the pressure regulator

insufficient gas shield through:

insufficient gas flow at start and at completion of welding

suitable gas pre- and post-flow time

tube

type false gas nozzle position (with

decentralised gas supply)

position gas nozzel behind torch - if possible

turbulences through:

spatters on gas nozzle or contact tube clean gas nozzle and contact tube

increase voltage, if wire electrode splutters, ensure good current transition

in contact tube, correct earth connection, remove slag of previously welded layers

thermal current - possibly increased by chimmney effects with one-sided welding

weld on backing or with root forming gas

gas line, avoid visible gas nozzle slots

hose after ingress of water

power or increasing welding speed

Metallurgical Pore Formation

Figure 9.11

Growth and Brake Away of Gas Cavities

at the Phase Border

a) low crystallisation speed

b) high crystallisation speed

Figure 9.12 Figure 9.10

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tion This pore formation shows its typical pore position at the edge of the joint and at the fu-sion line of the top layer

Figure 9.11 summarises causes of and

meas-ures to avoid a metallurgical pore formation

Reason of this pore formation is the

conside-rably increased solubility of the molten metal

compared with the solid state

During solidification, the transition of liquid to

solid condition causes a leapwise reduction of

gas solubility of the steel As a result, solved

gasses are driven out of the crystal and are

enriched as a gas bubble ahead of the

solidifi-cation front With a slow growth of the

crystalli-sation front, the bubbles have enough time to

raise to the surface of the weld pool, Figure

9.12 upper part Pores will not be developed

However, a higher solidification speed may

lead to a case where gas bubbles are passed

by the crystallisation front and are trapped as

pores in the weld metal, lower part of the figure

Figure 9.13 shows a X-ray photograph, a surface and a transverse section of a seam with metallurgical pores The evenly distributed pores across the seam and the accumulation of

pores in the upper part of the seam (transverse sec-tion) are typical

Figure 9.14 shows the ways of ingress of gasses into the weld pool as an example during MAG weld-ing A pore formation is mainly caused by hydrogen and nitrogen Oxygen is

Figure 9.13

Figure 9.14

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bonded in a harmless way when using universal electrodes which are alloyed with Si and Mn Figure 9.15 classifies cracks to DIN 8524, part 3 In contrast to part 1 and 2 of this standard, are cracks not only classified by their appearance, but also by their development

Figure 9.15

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Figure 9.16 allocates cracks according to their

welding heat cycle Princi-pally there is a distinction between the group 0010 (hot cracks) and 0020 (cold cracks)

A model of remelting development and

solidifi-cation cracks is shown in Figure 9.17 The

up-per part illustrates solidification conditions in a

simple case of a binary system, under the

pro-vision that a complete concentration balance

takes place in the melt ahead of the

solidifica-tion front, but no diffusion takes place in the

crystalline solid When a melt of a composition

when the liquidus line is reached Its

concen-tration can be taken from the solidus line In

the course of the ongoing solidification, the rest

of molten metal is enriched with alloy elements

in accordance with the liquidus line As defined

in the beginning, no diffusion of alloy elements

in the already solidified crystal takes place,

thus the crystals are enriched with alloy

ele-ments much slower than in a case of the binary

system (lower line)

As a result, the concentration of the melt exceeds the maximum equilibrium concentration

Crack Formation During Steel Welding

0010 area of hot crack formation

0011 area of solidification crack formation

0012 area of remelting crack formation

0020 area of cold crack formation

0021 area of brittle crack formation

0022 area of shrinking crack formation

0023 area of hydrogen crack formation

0024 area of hardening crack formation

0025 area of tearing crack formation

0026 area of ageing crack formation

0027 area of precipitation crack formation

0028 area of lamella crack formation

0010 0011 0012 0020 0021

0022 0023 0024 0025

0026 0027

0028

T S

M S

1 10 10 2

10 3

10 4

10 5

10 6

10 7

Time

1600

1200

800

400

0

°C

s

Figure 9.16

Development of Remelting and Solidification Cracks

Tm A

Tm B

C 5

T

C B tension

tension

a

b

aaaaaaaaaa

tension

Figure 9.17

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point is considerably lower when compared with the firstly developed crystalline solid Such concentration differences between first and last solidified crystals are called segregations This model of segregation development is very much simplified, but it is sufficient to under-stand the mechanism of hot crack formation The middle part of the figure shows the

formation of solidification cracks Due to the segre-gation effects described above, the melt between the crystalline solids at the end of solidification has a

solidus temperature As indicated by the black ar-eas, rests of liquid may be trapped by dendrites If

(shrinking stress of the

welded joint), the liquid

ar-eas are not yet able to

trans-fer forces and open up

The lower part of the figure

shows the development of

remelting cracks If the base

material to be welded

con-tains already some

segrega-tions whose melting point is

lower than that of the rest of

the base metal, then these

zones will melt during

weld-ing, and the rest of the

ma-terial remains solid (black areas) If the joint is exposed to tensile stress during solidification, then these areas open up (see above) and cracks occur A hot cracking tendency of a steel is above all promoted by sulphur and phosphorus, because these elements form with iron very

Crystallisation of Various Bead Geometries

a: non-preferred

bead shape

b< 1

t

b: preferred bead shape

b > 1 t

c: non-preferred bead shape

b

Figure 9.18

Figure 9.19

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low melting phases (eutectic point Fe-S at 988°C) and these elements segregate intensely In addition, hot crack tendency increases with increasing melt interval

As shown in Figure 9.18, also the geometry of the groove is important for hot crack tendency With

crystallisation takes place

of all sides of the bead, entrapping the remaining melt in the bead centre With the occurrence of

cracks may develop In the case of flat beads as shown in the middle part of

Macrosection of a SA-Weld

Figure 9.21

Figure 9.20

Figure 9.22

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the figure, the remaining melt solidifies at the surface of the bead The melt cannot be trap-ped, hot cracking is not possible The case in figure c shows no advantage, because a remel-ting crack may occur in the centre (segregation zone) of the first layer during welding the second layer

The example of a hot crack in the middle of a SA weld is shown in Figure 9.19 This crack developed due to the unsuitable groove geometry

Figure 9.20 shows an example of a remelting crack which started to develop in a segrega-tion zone of the base metal and spread up to the bead centre

The section shown in Figure 9.21 is similar to case c in Figure 9.18 One can clearly see that an existing crack develops through the following layers during over-welding

Figure 9.22 classifies cold cracks depending

on their position in the weld metal area Such

a classification does not provide an explana-tion for the origin of the cracks

Figure 9.23 shows a summary of the three

main causes of cold crack formation and their

main influences As explained in previous

chapters, the resulting welding microstructure

depends on both, the composition of base

and filler materials and of the cooling speed

of the joint An unsatisfactory structure

com-position promotes very much the formation of

cold cracks (hardening by martensite)

Causes of Cold Crack Formation

structure

(hardness) hydrogen stresses

chemical

composition

(C-equivalent)

welding consumables humidity on welding edges

residual stresses (yield stress of steels and joints)

cooling rate

(t 8/5 )

cooling rate (t 8/1 )

additional stresses (production conditions)

Figure 9.23

Cold Cracks in the Heat Affected Zone and Weld Metal

section plane

crack in heat affected zone

0,2 mm

etching: HNO 3

5 mm

transverse cracks in weld metal

5 mm

Figure 9.24

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