10. Testing of Welded Joints - hcmut.edu.vndantn/WT/WT2-c10.pdf · 10. Testing of Welded Joints 127 The basic test for determination of material behaviour is the tensile test. Generally,

10.

Testing of Welded Joints

10. Testing of Welded Joints 127

The basic test for determination of material

behaviour is the tensile test.

Generally, it is carried out using a round

specimen. When determining the strength of

a welded joint, also standardised flat speci-

mens are used. Figure 10.1 shows both stan-

dard specimen shapes for that test. A

specimen is ruptured by a test machine while

the actual force and the elongation of the

specimen is measured. With these measure-

ment values, tension σ and strain ε are calcu-

lated. If σ is plotted over ε, the drawn diagram

is typical for this test, Figure 10.2.

Normally, if a steel with a bcc lattice structure

is tested, a curve with a clear yield point is

obtained (upper picture). Steels with a fcc

lattice structure show a curve without yield

point.

The most important characteristic values

which are determined by this test are: yield

stress ReL, tensile strength Rm, and elongation

A.

To determine the deformability of a weld, a

bending test to DIN EN 910 is used, Figure

10.3. In this test, the specimen is put onto two

supporting rollers and a former is pressed

through between the rollers. The distance of

the supporting rollers is Lf = d + 3a (former

diameter + three times specimen thickness).

The backside of the specimen (tension side)

is observed. If a surface crack develops, the

test will be stopped and the angle to which

the specimen could be bent is measured. The

Flat and Round Tensile Test Specimento EN 895, EN 876, and EN 10 002

S S

LO

LC

Lt

r

d d1

d = specimen diameterd = head diameter depending

on clamping deviceL = test length = L +

1

C 0 d/2

r = 2 mm

L = measurement length

(L = k with k = 5,65)

L = total length

S = initial cross-section within

test length

0

0 0

t

0

ÖS

in test area

in test area

S

S

S

S

S

S

S

S

a

L0

Lc

Lt

b1b

r

Ls

total length Lt depends on test unit

head width b1 b + 12

width of parallel length plates b12 with a £ 2

25 with a > 2

tubes b

6 with D £ 50

12 with 50 < D £168,3

parallel length1)

2) Lc ³ LS + 60

radius of throat r ³ 251) for pressure welding and beam welding, LS = 0.

2) for some other metallic materials (e.g.aluminium, copper and their alloys)

__ Lc ³ LS +100 may be required

© ISF 2002br-er10-01.cdr

Figure 10.1

ALud

Rm

Ag

A

e

ReH

Rel

sf

s

Ag

Stress-Strain Diagram Withand Without Distinct Yield Point

s

Rm

sf

e0,2 %

Ag

0,01 %

A

RP0,2

RP0,01


Figure 10.2


test result is the bending angle and the diameter of the used former. A bending angle of 180°

is reached, if the specimen is pressed through the supporting rollers without development of

a crack. In Figure 10.3 specimen shapes of this test are shown. Depending on the direction

the weld is bent, one distinguishes (from top to bottom) transverse, side, and longitudinal

bending specimen. The ten-

sion side of all three speci-

men types is machined to

eliminate any influences on

the test through notch ef-

fects. Specimen thickness of

transverse and longitudinal

specimens is the plate

thickness. Side bending

specimens are normally only

used with very thick plates,

here the specimen thickness

is fixed at 10 mm.

A determination of the toughness of a material or welded joint is carried out with the notched

bar impact test. A cuboid specimen with a V-notch is placed on a support and then hit by a

pendulum ram of the im-

pact testing machine (with

very tough materials, the

specimen will be bent and

drawn through the sup-

ports). The used energy is

measured. Figure 10.4

represents sample shape,

notch shape (Iso-V-

specimen), and a sche-

matic presentation of test

results.

Bending Specimens to EN 910

supporting rollerbending specimen

former

d

D

a

lLt

Lf

section A-Btension side

ar

r A

BLt

b

A

BLt

section A-Br

r

tension side

a

b

rtension side

r

a Lt

b

l L specimen lengthd former diameterD supporting roller diameter: 50 mma specimen thicknessr radius of specimen edgeb specimen width

distance of supporting rollerst

br-er10-03.cdr

Figure 10.3

Charpy Impact Test Specimen and SchematicRepresentation of Test Results

Dimensionslength 55 mm ± 0,60 mmwidth 10 mm ± 0,11 mmhight 10 mm ± 0,06 mmnotch angle 45° ± 2°

thickness in notch groove 8 mm ± 0,06 mmnotch rad ius 0,25 mm ± 0,025 mmnotch d is tance from endof specimen 27,5 mm ± 0,42 mmangle between notch axisand long itudinal axis 90° ± 2°

Nominal s ize To lerance

40

55 10r = 0,25

45°

10 8

average valuesmaximaum minimum

valuesvalues

45

40

35

30

25

20

15

10-80 -60 -40 -20 0 20 40

J

Ch

arpy

impa

ct e

nerg

y

°CTemperaturebr-er10-04.cdr

Figure 10.4


Three specimens are tested at each test tem-

perature, and the average values as well as

the range of scatter are entered on the impact

energy-temperature diagram (AV-T curve).

This graph is divided into an area of high im-

pact energy values, a transition range, and an

area of low values. A transition temperature is

assigned to the transition range, i.e. the rapid

drop of toughness values. When the tempera-

ture falls below this transition temperature, a

transition of tough to brittle fracture behaviour

takes place.

As this steep drop mostly extends across a

certain area, a precise assignment of transi-

tion temperature cannot be carried out. Fol-

lowing DIN 50 115, three definitions of the

transition temperature are useful, i.e. to fix TÜ

to:

1.) a temperature where the level of impact values is half of the level of the high range,

2.) a temperature, where the fracture area of the specimen shows still 50% of tough fracture

behaviour

3.) a temperature with an impact energy value of 27 J.

Figure 10.5 illustrates a specimen position and notch position related to the weld according to

DIN EN 875. By modifying the notch position, the impact energy of the individual areas like

HAZ, fusion line, weld metal, and base metal can be determined in a relatively accurate way.

Figure 10.6 presents the influence of various alloy elements on the AV-T - curve. Three basi-

cally different influences can be seen. Increasing manganese contents increase the impact

values in the area of the high level and move the transition temperature to lower values. The

values of the low levels remain unchanged, thus the steepness of the drop becomes clearer

with increasing Mn-content. Carbon acts exactly in the opposite way. An increasing carbon

content increases the transition temperature and lowers the values of the high level, the steel

becomes more brittle. Nickel decreases slightly the values of the high level, but increases the

Position of Charpy-V Impact TestSpecimen in Welded Joints to EN 875

a

b

b

a

b

Dicke

RL

bDicke

a

b

RL

VWS a/b

VWS a/b(fusionweld)

VHT 0/b

VWT a/b VHT a/b

V = Charpy-V notchW = notch in weld metal; reference line is centre line of weldH = notch in heat affected zone; reference line is fusion line or bonding zone

(notch should be in heat affected zone)S = notched area parallel to surfaceT = notch through thicknessa = distance of notch centre from reference line (if a is on centre line of weld, a = 0 and

should be marked)b = distance between top side of welded joint and nearest surface of the specimen

(if b is on the weld surface, then b = 0 and should be marked)

VWT 0/b

VWT a/b VHT a/b

Designation Weld centre Designation Fusion line/bonding zone

VWT 0/b VHT a/b

a

b

b

RL a

b

RL

b

a

b

a

a

RLRL


Figure 10.5


values of the low level with increasing con-

tent. Starting with a certain Nickel content

(depends also from other alloy elements), a

steep drop does not happen, even at lowest

temperature the steel shows a tough fracture

behaviour.

In Figure 10.7, the AV-T – curves of some

commonly used steels are collected. These

curves are marked with points for impact en-

ergy values of AV = 27 J as well as with points

where the level of impact energy has fallen to

half of the high level. It can clearly be seen

that mild steels have the lowest impact en-

ergy values together with the highest transi-

tion temperature. The development of fine-

grain structural steels resulted in a clear im-

provement of impact energy values and in

addition, the application of such steels could be extended to a considerably lower tempera-

ture range.

With the example of the steels St E 355 and St E 690 it is clearly visible that an increase of

strength goes mostly hand in hand with a decrease of the impact energy level. Another im-

provement showed the application of a thermomechanical treatment (controlled rolling during

heat treatment). The appli-

cation of this treatment re-

sulted in an increase of

strength and impact energy

values together with a paral-

lel saving of alloy elements.

To make a comparison, the

AV-T - curve of the cryogenic

and high alloyed steel X8Ni9

was plotted onto the dia-

gram. The material is tested A -T Curves of Various Steel AlloysV

X8Ni9

S355N

S690N

S235J2G3

Cha

rpy

impa

ct e

nerg

y A

V

Temperature-150 -100 -50 0 50 100°C

300

200

100

27

S460M

S355J2G3

J

specimen position: weld centre, notch parallel to surfacespecimen shape: standard specimen with V-notch

br-er10-07.cdr

Figure 10.7

Influence of Mn, Ni, and Con the A -T-Curvev

Ch

arp

y im

pa

ct

en

erg

yA

V

-150 -100 -50 0 50 100°C

Temperature

200

100

27

0% Ni2% Ni5%

8,5%

200

100

27

13% Ni

3,5%

2% Mn

1% Mn

0,5% Mn

0% Mn27

100

200

300

J

J

J

specimen position:core longitudinal

specimen shape:ISO V

0,1% C

0,4% C

0,8% C


Figure 10.6


under very high test speed in the impact en-

ergy test, thus there are no reliable findings

about crack growth and fracture mechanisms.

Figure 10.8 shows two commonly used

specimen shapes for a fracture mechanics

test to determine crack initiation and crack

growth. The lower figure to the right shows a

possibility how to observe a crack propaga-

tion in a compact tensile specimen. During

the test, a current I flows through the speci-

men, and the tension drop above the notch is

measured.

As soon as a crack propagates through the

material, the current conveying cross section

decreases, resulting in an increased voltage

drop. Below to the left a measurement graph

of such a test is shown. If the force F is plotted across the widening V, the drawn curve does

not indicate precisely the crack initiation. Analogous to the stress-strain diagram, a decrease

of force is caused by a reduction of the stressed cross-section. If the voltage drop is plotted

over the force, then the start of crack initiation can be determined with suitable accuracy, and

the crack propagation can

be observed.

Another typical characteris-

tic of material behaviour is

the hardness of the work-

piece. Figure 10.9 shows

hardness test methods to

Brinell (standardised to

DIN 50 351) and Vickers

(DIN 50 133). When testing

to Brinell, a steel ball is

pressed with a known load Hardness Testing to Brinell and Vickers

F

h

F

d1 d 2

d

br-er-10-09.cdr

Figure 10.9

Fracture Mechanics TestSample Shape and Evaluation

0,5

5h

± 0

,25

1,2

h ±

0,2

5

P

P

C

C

a

L

h

1,25h ± 0,13

b CT - specimen

specimen width b specimen height h = 2b ± 0,25total crack length a = (0,50 ± 0,05)h

2,1h 2,1h

S

b

a

h

test load P

SENB3PB

-specimen

specimen width b bearing distance S = 4h

sample height h = 2b ± 0,05 total crack length a = (0,50 ± 0,05)h

UO

U

F

U ,aE E

crack initiationF,U

V

V

U


Figure 10.8


to the surface of the tested workpiece. The diameter of the resulting impression is measured

and is a magnitude of hardness. The hardness value is calculated from test load, ball diame-

ter, and diameter of rim of the impression (you find the formulas in the standards). The hard-

ness information contains in addition to the hardness magnitude the ball diameter in mm,

applied load in kp and time of influence of the test load in s. This information is not required

for a ball diameter of 10 mm, a test load of 3000 kp (29420 N), and a time of influence of 10

to 15 s. This hardness test method may be

used only on soft materials up to 450 BHN

(Brinell Hardness Number).

Hardness testing to Vickers is analogous.

This method is standardised to DIN 50133.

Instead of a ball, a diamond pyramid is

pressed into the workpiece. The lengths of

the two diagonals of the impression are

measured and the hardness value is calcu-

lated from their average and the test load.

The impressions of the test body are always

geometrically similar, so that the hardness

value is normally independent from the size

of the test load. In practice, there is a hard-

ness increase under a lower test load be-

cause of an increase of the elastic part of the

deformation.

Hardness testing to Vickers is almost universally applicable. It covers the entire range of ma-

terials (from 3 VHN for lead up to 1500 VHN for hard metal). In addition, a hardness test can

be carried out in the micro-range or with thin layers.

Figure 10.10 illustrates a hardness test to Rockwell. In DIN 50103 are various methods stan-

dardised which are based on the same principle.

With this method, the penetration depth of a penetrator is measured.

Hardness Test toRockwell

0,2

00 m

m

1000

26

3

7

4

35

8

3

10

1

hard

ness

scale

0,2

00 m

m

100

0

6

78,9

10

specimen surface

referencelevel formeasurement

hard

ness

scale

0,2

00 m

m

0

30

130

specimen surface

referencelevel formeasurement

6

78,9

10

0,2

00 m

m

0

31

6 7

43

5

8

3

10

30130

Abbreviation

1 - cone angle = 120° ball diameter = 1,5875 mm (1/16 inch)

2 -

3 F0

4 F1

5 F

6 t0

7 t1

8 tb

9 e

10HRC

HRARockwell hardness = 100 - e

HRB

HRFRockwell hardness = 130 - e

resulting penetration depth, expressed in units of 0,002 mm: e =

tb / 0,002

radius of curvature of cone tip = 0,200 mm

test preload

test load

total test load = F0 + F1

Terms

penetration depth in mm under test preload F0. This defines the reference level

for measurement of tb.

total penetrationn depth in mm under test load F1

resulting penetration depth in mm, measured after release of F1 to F0


Figure 10.10


At first, the penetrator is put on the workpiece by application of a pre-test load. The purpose

is to get a firm contact between workpiece and penetrator and to compensate for possible

play of the device.

Then the test load is applied in a shock-free way (at least four times the pre-force) and held

for a certain time. Afterwards it is released to reach minor load. The remaining penetration

depth is characteristic for the hardness. If the display instrument is suitably scaled, the hard-

ness value can be read-out directly.

All hardness test methods to Rockwell use a ball (diameter 1.5875 mm, equiv. to 1/16 Inch)

or a diamond sphero-conical penetrator (cone angle 120°) as the penetrating body. There are

differences in size of pre- and test load, so different test methods are scaled for different

hardness ranges. The most commonly used scale methods are Rockwell B and C. The most

considerable advantage of these test methods compared with Vickers and Brinell are the low

time duration and a possible fully-automatic measurement value recognition. The disadvan-

tage is the reduced accuracy in contrast to the other methods. Measured hardness numbers

are only comparable under identical conditions and with the same test method. A comparison

of hardness values which were determined

with different methods can only be carried out

for similar materials. A conversion of hardness

values of different methods can be carried out

for steel and cast steel according to a table in

DIN 50150. A relation of hardness and tensile

strength is also given in that table.

All the hardness test methods described above

require a coupon which must be taken from the

workpiece and whose hardness is then deter-

mined in a test machine. If a workpiece on-site

is to be tested, a dynamical hardness test

method will be applied. The advantage of these

methods is that measurements can be taken

on completed constructions with handheld

units in any position. Figure 10.11 illustrates a Poldi - Hammer

specimen

reference bar

piston


Figure 10.11


hardness test using a Poldi-Hammer. With this (out of date) method, the measurement is car-

ried out by a comparison of the workpiece hardness with a calibration piece. For this purpose

a calibration bar of exactly determined hardness is inserted into the unit, which is held by a

spring force play-free between a piston and a penetrator (steel ball, 10 mm diameter). The

unit is put on the workpiece to be tested. By a hammerblow to the piston, the penetrator

penetrates the workpiece and the calibration pin simultaneously. The size of both impres-

sions is measured and with the known hardness of the calibration bar the hardness of the

workpiece can be determined. However, there are many sources of errors with this method

which may influence the test result, e.g. an inclined resting of the unit on the surface or a

hammerblow which is not in line with the device axis. The major source of errors is the

measurement of the ball impression on the workpiece. On one hand, the edge of the impres-

sion is often unsharp because of the great ball diameter, on the other hand the measurement

of the impression using magnifying glasses is subjected to serious errors.

Figure 10.12 shows a modern measurement method which works with ultrasound and com-

bines a high flexibility with easy handling and high accuracy. Here a test tip is pressed manu-

ally against a workpiece. If a defined test load is passed, a spring mechanism inside the test

tip is triggered and the measurement starts.

The measurement principle is based on a

measurement of damping characteristics in

the steel. The measurement tip is excited to

emit ultrasonic oscillations by a piezoelectric

crystal. The test tip (diamond pyramid) pene-

trates the workpiece under the test pressure

caused by the spring force. With increasing

penetration depth the damping of the ultra-

sonic oscillation changes and consequently

the frequency. This change is measured by

the device. The damping of the ultrasonic os-

cillation depends directly on penetration depth

thus being a measure for material hardness.

The display can be calibrated for all com-

monly used measurement methods, a meas-

urement is carried out quickly and easily.

- little work on surface preparation of specimens (test force 5 kp)- Data Logger for storage of several thousands of measurement points- interfaces for connection of computers or printers- for hardness testing on site in confined locations

5.0

4.0

3.0

2.0

5 kp

Test force

kp

Federweg


Figure 10.12


Measurements can also be carried out in confined spaces. This measurement method is not

yet standardised.

To test a workpiece under oscillating stress, the fatigue test is standardised in DIN 50100.

Mostly a fatigue strength is determined by the Wöhler procedure. Here some specimens

(normally 6 to 10) are exposed to an oscillating stress and the number of endured oscillations

until rupture is determined (endurance number, number of cycles to failure). Depending on

where the specimen is to be stressed in the range of pulsating tensile stresses, alternating

stresses, or pulsating compressive stresses, the mean stress (or sub stress) of a specimen

group is kept constant and the stress amplitude (or upper stress) is varied from specimen to

specimen, Figure 10.13. In this way, the stress amplitude can be determined with a given

medium stress (prestress) which can persist for infinite time without damage (in the test: 107

times). Test results are presented in fatigue strength diagrams (see also DIN 50 100). As an

example the extended Wöhler diagram is shown in Figure 10.13. The upper line, the Wöhler

line, indicates after how many cycles the specimen ruptures under tension amplitude σa. The

crack is free, surface is clean

crack and surfacewith penetrantliquid

cleaned surface,dye penetrant liquid in crack

surface withdeveloper showsthe crack by coloring

all materials with surface cracks

Dye penetrant method

Description Application

Magnetic particle testing

A workpiece is placedbetween the poles ofa magnet or solenoid.Defective parts disturbthe power flux. Iron particles are collected.

Surface cracks andcracks up to 4 mm below surface.

Only magnetizablematerials and only for cracks perpendicular to power lines

However:


Figure 10.14 Figure 10.13

Fatigue Strength Testing

I area of overload with material damageII III area of load below fatigue strength limit

area of overload without material damage

III

III

failure line

Wöhler line

1 10 102 103 104 105 106 107

0

Str

ess

σ

Fatigue strength (endurance) number lg N

σD

σσ

ma

>

σσ

ma

=

σσ

ma

<

σσ

ma

<

σσ

ma

=

σσ

ma

>

σm =

0

pulsation range(compression) alternating range

pulsation range(tension)

com

pre

ssio

n -

+ te

nsio

n

time



damage line indicates analogously, when a

damage to the material starts in form of

cracks. Below this line, a material damage

does not occur.

Test methods described above require

specimens taken out of the workpiece and a

partly very accurate sample preparation. A

testing of completed welded constructions is

impossible, because this would require a de-

struction of the workpiece. This is the reason

why various non-destructive test methods

were developed, which are not used to de-

termine technological properties but test the

workpiece for defects. Figure 10.14 shows

Non-Destructive Test MethodsRadiographic Testing

Description

X-ray or isotope radiation penetratea workpiece. The thicker the work-piece, the weaker the radiationreaching the underside.

Application

Mainly for defects with orientationin radiation direction.

¬−®¯°

radiation source

workpiece

film (displayed in distance from workpiece)

defect in radiation direction; difficult to identify (flank lack of fusion)

defect ; easy to identifyin radiation direction

¬

−

®

¯

°


Figure 10.15

Determination of Picture Qualityto DIN 54105Number

Tolerated

deviation

mm mm

3,2 1

2,5 2

2 3

1,6 4

1,25 5

1 6

0,8 7

0,63 8

0,5 9

0,4 10

0,32 11

0,25 12

0,2 13

0,16 14

0,125 15

0,1 16± 0,005

W ire diameter

W ire number

± 0,03

± 0,02

± 0,01

AbbreviationWire length

mmWire material

Material groups

to be tested

FE 1/7 1 to 7 50

FE 6/12 6 to 12 50 or 25

FE 10/16 10 to 16 50 or 25

CU 1/7 1 to 7 50

CU 6/12 6 to 12 50

CU 10/16 10 to 16 50 or 25

AL 1/7 1 to 7 50

AL 6/12 6 to 12 50

AL 10/16 10 to 16 50 or 25

Wire number to

Table 1

iron materials

copper, zink, tin

and its alloys

aluminium

and its alloys

mild

steel

copper

aluminium


Figure 10.16

Non-Destructive Test MethodsUltrasonic Testing II

Mainly for defects with an orientationtransverse to sound input direction.

US-head generates high-frequency soundwaves, which are transferred via oil couplingto the workpiece. Sound waves are reflectedon interfaces (echo).

Description Application

ÀÁÂÃÄÅÆ³

sound head

oil coupling

workpiece

defect

ultrasonic test device

radiation pulse

defect echo

backwall echo

Ã

À

Á

Â

Ä

Å Æ ³


Figure 10.17


two methods to test a workpiece for surface defects.

Figure 10.15 illustrates the principle of radio-

graphic testing which allows to identify also de-

fects in the middle of a weld. The size of the

minimum detectable defects depends greatly

on the intensity of radiation, which must be

adapted to the thickness of the workpiece to be

radiated. As the film with documented defects

does not permit an estimation of the plate

thickness, a scale bar must be shown for esti-

mation of the defect size.

For that purpose, a plastic template is put on

the workpiece before radiation which contains

metal wires with different thickness and incor-

porated metallic marks, Figure 10.16. The size

of the thinnest recognisable wire indicates the

size of the smallest visible defect. Radiation

testing provides information about the defect

position in the plate plane, but not about the position within the thickness depth. A clear ad-

vantage is the good documentation ability of defects.

An information about the

depth of the defect is pro-

vided by testing the work-

piece with ultrasound. The

principle is shown in Fig-

ures 10.17 and 10.18

(principle of a sonar).

The display of original

pulse, backwall and defect

echo is carried out with an

oscilloscope.

Ultrasonic Testing of Fillet Welds

br-er10-19.cdr

Figure 10.19

Figure 10.18


This method provides not only a perpendicular sound test, but also inaccessible regions can

be tested with the use of so called angle testing heads, Figure 10.19.

Figures 10.20 and 10.21

show schematically the

display of various defects

on an oscilloscope. A cor-

rect interpretation of all the

signals requires great ex-

perience, because the

shape of the displayed sig-

nals is often not so clear.

Figure 10.22 illustrates the

potential of metallographic

examination. Grinding and

Defect Identification With Ultrasound

Wall thickness is below 40 mm. The roughness provides smallerand wider echos.

The perpendicular crack penetrating the material does not provide a displaybecause the reflecting surface (tip of crack) is too small.

The oblique backwall reflectsthe soundwaves against thecrack. this is the reason why an ‘impossible’ depth of 65 mm is displayed.


Figure 10.21

Defect Identification with Ultrasound

Pores between 10 and 20 mmdepth provide an unbroken echo sequence across the entire display starting from 10mm. The backwall echo sequence of 30mm is not yet visible.

Echo sequence of 20 mm depth.The backwall is completely screened.

The oblique and rough defectfrom 20 to 30 mm provides awide echo of 20 to 30 mm. Starting with SKW 4,

echo sequence follows.The inclination of the reflector is recornised by a change of the 1st echo when shifting thetest head.

an un-broken

Echo sequence of 10 mm depth.The reflector in 30 mm depth iscompletely screened.

40

30


Figure .10.20

Metallographic Examination of a Weld

2,5 mm

50 µ

macro section

base material ferrite+ perlite

fine grain zone ferrite+ perlite

coarse grain zone bainite

weld metal cast structure

fusion line bainite

Steel: S355N (T StE 355)

br-er10-22.cdr

Figure 10.22


etching with an acid makes the microstructure

visible. The reason is that depending on

structure and orientation, the individual grains

react very differently to the acid attack thus

reflecting the light in a different way. The

macrosection, i.e. without magnification, gives

a complete survey about the weld and fusion

line, size of the HAZ, and sequence of solidi-

fication. Under adequate magnification, these

areas can still not be distinguished precisely,

however, an assessment of the developed

microstructure is possible.

An assessment of the distribution of alloy

elements across the welded joint can be car-

ried out by the electron beam micro-analysis.

An example of such an analysis is shown in

Figure 10.23. If a solid body is exposed to a

focused electron beam of high energy, its atoms are excited to radiate X-rays. There is a

simple relation between the wave length of this radiation and the atomic number of the

chemical elements. As the intensity of the radiation depends on the concentration of the ele-

ments, the chemical composition of the solid body can be concluded from a survey of the

emitted X-ray spectrum

qualitatively and quantita-

tively. A detection limit is

about 0.01 mass % with this

method. Microstructure ar-

eas of a minimum diameter

of about 5 µm can be ana-

lysed. If the electron beam is

moved across the specimen

(or the specimen under the

beam), the element distribu-

tion along a line across the Strauß - Test

weldweld

1. weld2. weld

axis ofbending former

axis ofbending former

5050

20 20

100

50 5020 20

Agents:- electrolytic copper in the form of chips (min. 50 g/l test solution)- 100 ml H SO diluted with 1 l water and then 110 g CuSO 5 H O are added

Test:The specimens remain for 15 h in the boiling test solution. Then the specimens are bent across a former up to an angle of 90° and finally examined for grain failure under a 6 to 10 times magnification.

2 4

2.

br-er-10-24.cdr

Figure 10.24

Micro-Analysis of the Transition ZoneBase Material - Strip Cladding

0

2

4

6

8

10

% Ni

0

5

10

15

20

25

200 100 0 100mm

0

20

40

60

80

100

% Fe

Distance from fusion line

% Cr

Ni

Fe

Cr


Figure 10.23


solid body can be determined. Figure 10.23 presents the distribution of Ni, Cr, and Fe in the

transition zone of an austenitic plating in a ferritic base metal. The upper part shows the re-

lated microsection which belongs to the analysed part. This microanalysis was carried out

along a straight line between two impressions of a Vickers hardness test. The impressions

are also used as a mark to identify precisely the area to be analysed.

The so called Strauß test is

standardised in DIN 50

914. it serves to determine

the resistance of a weld

against intergranular corro-

sion. Figure 10.24 shows

the specimen shape which

is normally used for that

test. In addition, some de-

tails of the test method are

explained.

Figure 10.25 presents a specimen shape for testing the crack susceptibility of welding con-

sumables. For this test, weld number 1 is welded first. The 2. weld is welded not later than 20

s in reversed direction after completion of the first weld. Throat thickness of weld 2 must be

20% below of weld 1. After

cooling down, the beads

are examined for cracks. If

cracks are found in weld 1,

the test is void. If weld 1 is

free from cracks, weld 2 is

examined for crack with

magnifying glasses. Then

weld 1 is machined off and

weld 2 is cracked by bend-

ing the weld from the root.

Test results record any

aa

a aa

aa

a

base plate

weld2 weld1

tack welds

web

measurement points8012

12

80

20 40 40 20

120

a

Test of Crack Susceptibility of WeldingFiller Materials to DIN 50129

br-er-10-25.cdr

Figure 10.25

Tensioning Specimen for Crack Susceptibility Test

tensioning bolt

hexagon nutmin. M12 DIN 934

tensioning plate

specimen

base body

a

guidance plates

br-er-10-26.cdr

Figure 10.26


surface and root cracks together with information about position, orientation, number, and

length. The welding consumable is regarded as 'non-crack-susceptible' if the welds of this

test are free from cracks.

Figure 10.26 presents two proposals for self-stressing specimens for plate tests regarding

their hot crack tendency. Such tests are not yet standardised to DIN.

There are various tests to examine a cold crack tendency of welded joints. The most impor-

tant ones are the self-stressing Tekken test and the Implant test where the stress comes from

an external source.

In the Tekken test which is standardised in Japan, two plates are coupled with anchor joints

at the ends as a step in joint preparation see Figure 10.27. Then a test bead is welded along

the centre line. After storing the specimen for 48 hours, it is examined for surface cracks. For

a more precise examination, various transverse sections are planned. The value to be de-

termined is the minimum working temperature at which cracks no longer occur. The speci-

men shape simulates the conditions during welding of a root pass.

Tekken Test

groove shape

specimen shape

crack coefficient

cross-section

60°weldmetal

2

HcH

60°

2

Wd.Wd./2

Wd./2

start end crater

150

60 80 60

anchor weld test weld anchor weld

1 2 3 4 5

sections


C = x 100 (in %)c

Figure 10.27

Implant Test

thermo coupleelectrode

welding direction

support plate

implant

load

temperature in °Cload in N

rupture timetime in st8/5

800

500

150100

Tmax


Figure 10.28


The most commonly used cold crack test is the Implant test, Figure 10.28. A cylindrical body

(Implant) is inserted into the bore hole of a support plate and fixed by a surface bead. After

the bead has cooled down to 150°C the implant is exposed to a constant load. The time is

measured until a rupture or a crack occurs (depending on test criterion 'rupture' or 'crack').

Varying the load provides the possibility to determine the stress which can be born for 16

hours without appearance of a crack or rupture. If a stress is specified to be of the size of the

yield point as a requirement, a preheat temperature can be determined by varying the work-

ing temperature to the point at which cracks no longer appear.

As explained in chapter 'cold cracks' the hydrogen content plays an important role for cold

crack development. Figure 10.29 shows results of trials where the cold crack behaviour was

examined using the Tekken and Implant test. Variables of these tests were hydrogen content

of the weld metal and preheat temperature. The variation of the hydrogen content of the weld

metal was carried out by different exposure to humidity (or rebaking) of the used stick elec-

trodes. Based on the hydrogen content, the preheat temperature was increased test by test.

Consequently, the curves of Figure 10.29 represent the limit curves for the related test.

Specimens above these

curves remain free from

cracks, below these curves

cracks are present. Evi-

dent for both graphs is that

with increased preheat

temperature considerably

higher hydrogen contents

are tolerated without any

crack development be-

cause of the much better

hydrogen effusion.

If both graphs are com-

pared it becomes obvious that the tests produce slightly different findings, i.e. with identical

hydrogen content, the determined preheat temperatures required for the avoidance of crack-

ing, differ by about 20°C.

Test Result Comparison ofImplant and Tekken Test

Pre

hea

t te

mpe

ratu

re

20

50

100

150

°C

Pre

hea

t te

mpe

ratu

re

20

50

100

150

°C

Implant-Test Tekken-Test

fractured starting crackscrack-free

starting crackscrack-free

0 10 20 30 40ml/100 g

0 10 20 30 40ml/100 g

Diffusible hydrogen content

R = R = 358 N/mm²cr p0,2

heat input: 12 kJ/cmbasic coated stick electrodeplate and support plate thickness: 38 mm

br-er-10-29.cdr

Figure 10.29


Figure 10.30 illustrates a method to measure the diffusible hydrogen content in welds which

is standardised in DIN 8572. Figure a) shows the burette filled with mercury before a speci-

men is inserted. The coupons are inserted into the opened burette and drawn with a magnet

through the mercury to the capillary side (density of steel is lower than that of mercury, cou-

pons surface). Then the burette is closed and evacuated. The hydrogen, which effuses of the

coupons but does not diffuse through the mercury, collects in the capillary. The samples re-

main in the evacuated burette 72 hours for degassing. To determine the hydrogen volume

the burette is ventilated and the coupons are removed from the capillary side. The volume of

the effused hydrogen can be read out from the capillary; the height difference of the two mer-

cury menisci, the air pres-

sure, and the temperature

provide the data to calculate

the norm volume under

standard conditions. This

volume and the coupons

weight are used to calculate,

as measured value, the hy-

drogen volume in ml/100 g

weld metal. This is the most

commonly used method to

determine the hydrogen

content in welded joints.

Burettes for Determination ofDiffusible Hydrogen Content

to pump

capillary side

mercury

a) starting condition

hydrogenunder redu-

ced pressure

coupons

evacuated

VTair pressure B

M

meniskus1

meniskus2

c) ventilated after degassingb) during degassing

br-er-10-30.cdr

Figure 10.30

Documents

10. Testing of Welded Joints - hcmut.edu.vndantn/WT/WT2-c10.pdf · 10. Testing of Welded Joints 127 The basic test for determination of material behaviour is the tensile test. Generally,