Weld test

CRACKS
and
WELD TEST
By
Dr. V.RAVISANKAR
Prof. of MANUFACTURING
ENGG., AU

Hot Cracking Tests
A number of tests have been devised to check the
hot cracking tendency of weldments; some of the
well known amongst them include the following.
Murex test
Houldcroft test
Varestraint test
Ring weldability test
Hot ductility test

Murex Text
In this test a filet weld is made between two
test plates 10-15 mm thick and of dimensions
approximately 50mm x 70mm.
These plates are rigidly fixed on two supports
one of which can be rotated about an axis in
the root of the test weld, as shown in Fig. 6.
Rotation of the movable clamp starts 5
seconds after welding begins. Although rotating
speed is of the order of 1°/Sec, but various
rotational speeds are available.

Set-Up for Murex Hot Cracking Test

The drive mechanism of the machine should be such that it is not
affected by the deformation resistance of the weld metal.
Susceptibility to hot cracking is indicated by the extent to which the
weld metal cracks at various rates of applied strain during
solidification.
The strain developed is made proportional to the speed of rotation
and is a function of the V-angle, which is always initially 90°.
The maximum angle that can be obtained is 120°. Longitudinal cracks
occur during rotation however the length of a crater crack, if present,
is not included in measurement.
This test has been mainly used for assessing cracking sensitivity of
carbon and low alloy steel weld deposits made with coated electrodes.

Houldcroft Fishbone Test
Houldcroft Fishbone test is often used for evaluating the
solidification cracking susceptibility of sheet materials.
This test is mainly for application to TIG welding of sheet
but may be applied to other similar process.
The test can be applied to any thin plate or sheet material on
which the process is likely to be used.
A bead-on-plate weld, without filler rod, is made under
controlled conditions along the centerline of the test piece,
beginning from the end with the shortest slots and
proceeding right to the opposite end.

As tension builds up at the beginning of the weld a crack is likely
to start and will travel along the weld until the head build-up in
the relatively narrow plate, as represented by the distance
between the inner ends of the pairs of slots, causes the stress
intensity to fall below the critical value, as shown in Fig. 7a.
The slot at which the crack is arrested gives a measure of
weldability in a range of ten conditions (nine sets of slots)
ranging from zero weldability with cracking along the full length
to full weldability with no cracking.
The crack length from the starting edge of the test piece is an
index of solidification cracking sensitivity. The shaded area in
Fig. 7(C) is used only for starting purposes and is ignored in
estimating the result.

A Typical Testpiece Showing 50% Weldability by
Houldcroft Fishbone Test for Tig Welding an Aluminimum Alloy

Transverse Stress Distribution in the
Central Plane of a Progressive Fusion Butt Weld

The test makes use of the transverse stress pattern (Fig. 7b)
built up by a progressive fusion weld, to find the critical
heat flow condition for a particular material in a particular
thickness of sheet.
A test piece shown in (Fig 7c) is made from one piece of
material and has a series of symmetrically arranged slots of
progressively increasing depth of cut in pairs into the
opposite side of the material.
The test piece is laid flat on a sheet of carbon which rests on
a water-cooled copper block and is held in position by
Beryllium Copper clamping strip as shown in Fig. 7d.

9 slots, 0.8mm wide,
7.5 mm apart
Test piece Dimensions for Houldcroft Weldability Test

Schematic Representation of a
Set-Up for Houldcroft Fishbone Test

The size of the test piece for any given material depends partly
on the thermal conductivity of the material. Also a larger test
piece is used for thicker material. Specimen width is the most
critical dimension and this should be chosen so that in a strip
of material without slots the crack would follow the entire
length of the weld bed. It is important when designing the test
piece to have some material is known crack sensitivity available
so that the right severity of test may be obtained.

In this test the degree of restraint diminishes from start to
finish of the weld due to the increasing depth of saw cut, so
that the length of crack has a relation to the degree of
restraint required for crack propagation.
The test is suitable for investigating the effect of parent metal
and filler metal composition on hot cracking and takes no
account whatsoever of the effect of welding variables on hot
cracking.

Fig. 8(A) A Schematic Representation of the Set-Up for
Vasrestraint Test
The Varestraint test

The varestraint test i.e., the variable restraint test requires a
metal plate 275 mm x 100 mm having a thickness of 6 to 13
mm which is fixed on one side in the welding jig as shown
in Fig. 8(a). The weld metal is then deposited right across
the middle of the plate or alternatively TIG welding process
may be employed without the use of filter.
This test like Murex test utilizes external loading to impose
plastic deformation on the test piece while weld bead is
being deposited. The load F is suddenly applied by
actuating the loading yoke, as the center of the arc passes
the point of tangency between the curved surface of the die
block and the fixed end of the specimen i.e., point A.

In order that the bend radius is not dependent on
the geometrical factors of the weld, the test piece is
placed on a bending block (B) of radius R.
Bending of the test plate, with the weld bead laid
on it, produces distortion of the upper fibres of the
weld bead.
The severity of deformation causing cracking can
be determined as,
The applied augemented strain,
where, t = thickness of the test piece,
R = radius of the bend.
A change in the bend radius will also cause a change
in the size of the distortion o
f the upper part of the weld bead. If the applied strain
is sufficient, cracks are formed in the weld bead.

Hot cracking can occur both inside and outside the weld
metal. Both, the amount of the applied strain and the crack
length (either the total length of all cracks or the maximum
crack length) serve as an index of crack sensitivity i.e., the
evaluation criterion of the test may be given by the overall
length of the cracks as a function of maximum deformation,
ε.
The relationship between ε and crack length also allows
determination of the threshold value of deformation, εcrit, at
which cracks are initiated and by virtue of this criterion
comparison of various steels or welds can be made. The
higher the value of εcrit, the more resistance, to the cracks, is
steel.

b) The Trans-Varestraint Test
It is a modified form of the varestraint test. While in the
varestraint test, the axis of the bend is perpendicular to the
direction of the weld and cracks occur vertically to the weld,
in the Trans-Varestraint test this axis runs parallel to the
weld, thus keeping cracks inside the weld metal resulting in
centerline cracks. The schematic representation of the set-
up for trans-varestraint test with a bilateral bend is shown in
Fig. 8b.

Trans-Varestraint Test Set-Up With Bilateral Bending of
Test Piece
The evaluation criterion is based on the unit of crack
susceptibility (UCS), where UCS = 10 (3 – R)
Higher values of UCS indicate low weldability.

Ring Weldability Test
⮚The circular patch or ring weldability test is used for studying
hot cracking in the weld metal or partially melted zone,
because it imposes a relatively high restraint in the weld zone
transverse to the weld.
⮚The stresses causing cracking are not precisely known but a
material has to be very weldable to pass the test without
cracking.
⮚Ignorance about the values of stress is of not much
consequence because in practical applications the usual
concern is not whether one weld metal composition in much
worse or only slightly worse than another, but whether or not it
cracks.

The test piece is made up of a square piece of sheet or plate,
with sides about 150-450 mm, out of the center of what is cut
a disc of diameter about one-third of the side of the square.
If the disc can be cut out with a very narrow cut then it can
be used in the test, but if the gap is too wide to give
reasonable representation of a weld gap then a separate
slightly larger disc has to be prepared.
Fig. 9a shows the schematic of a set-up for such a test jig
while Fig. 9b shows a set-up with zero gap for use with TIG
welding.

Fig. 9(A) Schematic Illustration of the Set-Up for Ring
Weldability Test

To make the test, the disc is lightly tacked into the test plate
at, say, 4 positions at regular interval so that the gap is
uniform and representative of an open square butt
preparation.
If required, single or double Vee edge preparation may also be
used. Welding is started at one of the tacks and is carried on
to complete the circle.
On welding the patch the radial and circumferential strains
imposed on the solidifying weld pool will increase with the
increase in length of the deposited weld and after some
interval both types of strain will reach a maximum, although
not necessarily simultaneously.
Thus, during welding a point is reached when, because of the
weakness of the material, the strains are sufficient to initiate a
crack. Further, it is assumed that at any instant the
circumferential strains are larger than the radial strains.

This implies that transverse cracks should appear before
centerline cracks; assuming that the weaknesses are uniform
throughout the weld bead.
Once a centerline crack has started it should continue until
the welding is terminated.
This is because the increased strain brought about by the
notch effect at the root of the crack will help its continuance.
A measure of the crack resistance is the angle θr, subtended
between the start of the weld and the point where the crack
first began, as shown in Fig. 9c.
Likewise the crack susceptibility is equal to
(360-θr).
Cracks at the beginning should be ignored except where it is
obvious that they from part of the main weld, since the region
at the start of the weld is not representative of the welding
conditions obtained in the main part of the weld.

Fig. 9(B) Jig For Ring Weldability Test in Sheet Material Using Tig
Welding Process

Another criterion of crack sensitivity is the occurrence of
transverse cracks; in some cases there are the only types of
cracks observed.
To a first approximation, a measure of the crack
susceptibility is the interval, measured in degrees,
between individual cracks.
This interval is in some way a function of the strain
necessary to cause cracking; the smaller the interval, the
more crack susceptible is the material.
Restrain can be varied in a given plate thickness by
adjusting the plate size and patch diameter.
Also, by using a patch on one material and a plate of
another the cracking tendencies of dissimilar metal joints
can be studied.

Fig. 9(C) Typical Test Result for Patch
Test in Stainless Steel

Fig. 9(D) Welding Sequence to Help Eliminate
distortion and Cracking in a Circular Weld

Ring weldability test is used in aluminium alloys and low
alloy steels for establishing correct welding procedures and
sequences to avoid restraint cracking.
Fig. 9d shows an example of welding sequence to help
eliminate to help estimate distortion and cracking in a
circular weld.
This may be used for comparison if the patch test shows
cracks.

Hot Ductility Test
The thermal cycles experienced by the workpieces during
welding can be duplicated in small specimens conveniently for
mechanical testing, by using weld thermal simulator.
By performing high-speed tensile testing during weld thermal
simulation, the elevated temperature ductility (and strength)
of metals can be evaluated. This is called the hot ductility test
and is illustrated in Fig 10a. This test can be used to
investigate the hot cracking tendency of the partially melted
HAZ in a weld. It has been used extensively for evaluating the
hot cracking susceptibility of nickel-base alloys.

Cold Cracking Test
Weldability is also assessed by the cold cracking
susceptibility of a weldment. Like for hot cracking, there are
a large number of tests developed to determine the cold
cracking tendency; some of the more popular amongst them
include the following.
⮚Controlled thermal severity (CTS) test.
⮚Tekken test.
⮚Lehigh restraint test.
⮚Longitudinal bead-weld test.
⮚Implant test.

Fig. 10(A) Illustration of Hot Ductility Test Results: (A) Haz Thermal
Cycle, and (B) Temperature Vs. Ductility Curve

CTS Test
The CTS test is based on the principle of the fillet welded
joint particularly for assessing weldability in relation to
steels welded by arc welding processes for establishing safe
welding procedures for low alloy steels. Under appropriate
conditions the test can assess a steel parent metal, a weld
deposit, or a process in terms of a critical cooling condition
related to the number of units of 6.25 mm (1/4 inch) of
thickness of material conducing heat away from the weld.
The test pieces of the dimensions shown in Fig. 11a are
bolted together.
Welds A and B are anchor welds and the test welds are fillet
welds of standard size laid at C and D under controlled
conditions.

The test piece is set up in a convenient fixture so that the test
weld can be made in the open flat position i.e., with the V in the
upright position and the line of welding horizontal; the whole
set-up is exposed only to normal still air cooling.
Each test weld is begun with the test piece at room
temperature. Thus, weld C is made under bithermal
conditions, with two thicknesses of plate conducting away the
heat, whilst weld D is made under tri-thermal conditions, with
three thicknesses conducting away the heat, whilst weld D is
made under trithermal conditions, with three thicknesses
conducting away the heat (the bottom plate conducts in two
directions).

Fig. 11(A) Controlled Thermal Severity Test, (A And B) Set-Up, and ©
Location of Test Pieces for Microstructure Study

The cooling rate is designated by means of a thermal severity
number (T.S.N). TSN1 is the thermal severity corresponding
to heat flow along a single steel plate 6.25mm thick.
TSN2 is obtained in a butt weld between two 6.25 mm plates
whilst in a 6.25mm T-joint, where there are three heat flow
paths, the thermal severity number is 3.
The TSN is also increased in proportion to the plate
thickness, so that CTS test pieces, which also have three heat
flow paths, in 12.5 mm plate would have TSN6.
The thicknesses t and b, of top and bottom plates
respectively, can be changed as required for successive tests,
the thermal severity number being calculated from the units
of 6.25 mm of thickness.

Thus, in general,
i) for bithermal welds,
(TSN)B = 1 / 6.25 (t + b)
and (ii) for tri-thermal welds,
(TSN)T = 1 / 6.25 (t + 2b)
where t and b are in mm.
Susceptibility to cracking grows with the increasing gap
between the plates, therefore a shim is often placed between
the two plates to increase the gap in the fillet weld as shown
in Fig. 11(b).
Apart from thickness of plate, the severity of the test may
also be varied by the hydrogen level in the test welds and the
composition of the weld metal. With this test, cracks occur
in the underbead zone or the weld metal.

The assembly is allowed to stand for a period of 72 hours
after which the welds are sectioned to prepare three test
pieces from the transverse sections, for macrostructure
study, as shown in Fig. 11 (c).
Standard known quality plates can be used to assess
weldability of particular electrode deposits and standard
weld deposits can be used to indicate weldability of
particular sheet or plate material.
Some authorities do not regard the bithermal and tri-thermal
conditions as giving any sufficiently significant differences in
information so make the test plates of a size to give two
bithermal welds instead of one of each type.
For effective location of the critical TSN it is necessary to
make several test welds under different conditions, so a
complete assessment will take several days to finish.

Tekken Test
This is a simple butt welding test and has found wide application
in determining cold cracking tendency of welds made by arc
welding processes including submerged arc welding.
The thickness of test plates of the dimensions shown in Fig. 7.14
are prepared for butt welding with Y-edge preparation having 2 mm
root gap, and a 60° groove angle. First symmetrical auxiliary welds
are made on both ends and then the test weld of length about 75
mm is made in the central section.
When SAW process is used, the auxiliary weld on one side is left
incomplete so as to leave room for the test weld proper. It is
reported by Japanese research workers that the intensity of the
restraint is not much affected by the dimensions of the test pieces.
Tekken test is used in selecting welding parameters for the root
run of butt joints. In this test three types of cracks, viz. a, b and c
may be noted, as shown schematically in Fig. 7.15. These are,

Fig. 12(A) Test Piece and Edge
Preparation Details for Tekken
Test,
The cracks of the type (a) initiating
from the fusion boundary zone of the
bottom part of the root run on the
double-bevel side of the weld edge.
These are typical cold cracks
extending to HAZ and then turn back
to extend right into the weld metal.
Fig. 12 (B) Schematic Representation of
Types Of Cracks (A, B, C) Observed in a
Tekken Test Specimen

Cracks of the types (b) and (c) initiate in the weld metal and
may join to become a single crack.
The test procedure involves metallographic analysis on 5
section, two of which are obtained by cutting through the
initial and final weld craters. From these analyes it is possible
to determine the percentage of crack incidence in relation to
welding parameters. For example, Fig. 12 (c) shows the
relationship between preheating temperature and percentage
of cracks. The test weld is cut for examination atleast 48
hours after welding. Tekken test is suitable for comparing the
cold cracking susceptibilities of parent materials.
Fig. 12(C) Tekken Test Data
Represented to Correlate Preheat
Temperature to Percentage of Weld
Cracks

Lehigh Restraint Test
The characteristic feature of this test for thick plates
is that the degree of restraint is varied by freeing the
edges of the test pieces with a series of sawcuts
extending inwards over a distance X' from the edge; the
depth of the sawcut determines the restraint.
Fig. 13(A) Dimensioned Sketch of the Test Piece for Lehigh
Weldability Test

A longitudinal 17-groove is milled in a plate about 300 mm x
200 mm; the shape of the groove is shown in Fig. 13(a) and its
length (L) is varied in accordance with the plate thickness for a
plate less than 25 mm thick, L = 90 mm and for a plate more than
25 mm thick, L = 140 mm.
The test piece thickness varies between 12—50 mm and the
root gap of the shaped groove measures about 1.6—2 mm.
For plates of thickness up to 25 mm, a single U-edge groove is
cut, while for thicker plates double-U groove is used. Weld metal is
deposited in the grooved preparation and explored for cracking by visual
examination or by magnetic particle testing or radiography, etc.
For steels with 0.30% C, cracks occur practically exclusively in the weld
metal but may be initiated in the root or the upper part whereas crack
sensitive higher carbon or alloy steels show cracking in the HAZ extending
into the weld metal. This test is recommended for the selection of
electrodes for use with arc welding processes.

Longitudinal Bead Weld Cracking Test
This test uses a bead-on-plate weld deposited on a steel test plate of the
size 150 mm x 75 mm x 25 mm, partly immersed in water to 6.25 mm of its
top surface, with the length dimension in the direction of rolling.
A bead 100 mm long is deposited in the central part of the test plate, as
shown in Fig. 14(a), with a 3.15 mm diameter electrode at a welding current
of 100 Amp and an arc voltage of 24—26 volts and a travel speed of 25.4
cm/min. ; E6010 type electrode is used for welding to provide a high
potential of hydrogen in the arc atmosphere.
Fig. 14 Longitudinal
Bead Weld Cold
Cracking Test; (A) Test
Plate Dimensions, (B)
Test Plate Cut
Longitudinally Along
Weld Axis, (C And D)
Cut and Ground Test
Pieces for Underbead
and Toe Cracks

The specimen is then cut longitudinally along the axis of the
weld bead, and the cross-section of one-half the test plate is
ground (Fig. 14 (b), (c)) and tested for underbead cracks by the
magnetic particle or metallographic technique.
The weld bead on the second half of the test plate is ground
flush with the plate surface (Fig. 14(d)) and then examined in
the same way for toe cracks around the edge of the weld bead.
Cracking is measured after the specimen is age 4 for 24 hours
at 15°C, and is then thermally stress relieved at 595°C for one
hour to avoid possible grinding cracks.
Results are expressed as total length of crack(s) as a
percentage of the test weld length; underbead and toe cracks
are reported separately. For a reasonable evaluation atleast ten
tests have to be conducted.

Implant Test
While the four cold
cracking tests described
above are self-restraint tests,
the Implant test is a forced
restraint test. In this test, a
cylindrical specimen 6 — 8
mm diameter, and the other
dimensions as shown in Fig.
15 (a), is notched and
inserted in a hole in a 20—30
mm thick plate made of the
same or similar material as
the cylindrical Component.
The detailed dimensions of
notch are also given in
Fig. 15 (a).
Fig. 15 Dimensions of the Test
Components for Implant Cold
Cracking Test

Fig. 15(C) The Notch in the
Cylindrical Component and its
Location in Haz in the Implant Test
A weld run is made
over the specimen, with
an electrode of the type
that is to be used in
actual fabrication,
which is located in such
a way that its top
becomes a part of the
fusion zone and the
notch lies in the HAZ, as
shown in Fig. 15(c).

After welding, when the temperature falls below 100°C, a load is applied to
the cylindrical specimen, and the time to failure is determined.
A plot of stress versus time to failure gives an assessment of hydrogen
cracking susceptibility. Fig.15(d) shows such plots for high strength low
alloy (HSLA) pipeline steel. In this case loading was applied to the
specimen when the weld cooled down to 125°C.
It is evident from the plots that the welds made with low-hydrogen
electrodes (E7018-Basic Coated) are less susceptible to hydrogen
cracking than the welds made with high hydrogen electrode (E7010—
cellulosic coating).
The GMA weld made with at + 2% O2 as the shielding gas was least
susceptible to cracking as the only hydrogen encountered was the residual
hydrogen in the base metal, test pieces, and the welding wire. Thus, this
test also permits the critical stress, Scrit, to be determined at which no
fracture or crack initiation will occur anymore.

Another possible procedure
is to fix the applied stress
equal to the yield stress of
the test piece material and to
alter the thermal conditions
like preheat, heat input,
postweld heating. In this case
it is possible to determine the
critical cooling rate, Scrit,
between 800 and 500°C,
above which no cracking
takes place. Cracking
parameter (Pc) for steel with
0.8 to 2.5 % Mn, suggested by
Tanaka and Kitada, is given
by the following equation.
Fig. 15 (D) Implant Test
Results for an HSLA
Pipeline Steel

Another possible procedure is to fix the applied stress equal to
the yield stress of the test piece material and to alter the thermal
conditions like preheat, heat input, postweld heating. In this case
it is possible to determine the critical cooling rate, Scrit, between
800 and 500°C, above which no cracking takes place. Cracking
parameter (Pc) for steel with 0.8 to 2.5 % Mn, suggested by
Tanaka and Kitada, is given by the following equation.

Nick-Break Test
One of the methods of testing a
fusion weld is to cut a strip about 20
mm wide at right angle to the weld
axis and make a saw cut down the
centre line of the weld 3 to 6 mm
deep. By holding one-half of the
specimen in a vice, Fig. 16 (a) and
giving the other half a sharp blow
with hammer or by bending in a
machine as shown in Fig. 16 (b), the
weld is broken. This test which is
required by various boiler and
pressure vessel codes, shows up any
centreline defects, such as lack of
fusion, gas pockets, slag inclusion,
and the degree of porosity in the weld
bead. The defects are generally
examined by visual examination, and
should not have a length of more
than 3 mm individually.
Fig. 16 Different Forms of Nick-Break
Test: (A) for Butt-Welded Thin
Component, (B) for Butt-Welded Thick
Plates, and (C) for a Fillet Weld
Nick-Break Test

Fillet welds may be similarly tested by notching and bending as shown in Fig.
16 (c).
The standard length for a specimen in the fillet nick-break test is 100 to 150
mm.
The test is carried out by simply applying force in the back of the fillet,
crushing the angle flat.
One requirement of the fillet nick-break test is that there should be no tack
welds on the other side of the fillet weld.
One small tack weld will have enough holding power to invalidate the test.
Nick-Break Test: (C) for a Fillet Weld

Bend Tests
The quality of the weld, in
terms of ductility of the weld
metal and HAZ as well as tests
for opening of defects
particularly lack of side wall
fusion (side bend), root fusion,
and penetration of welded joint,
are most frequently checked by
means of a bend test. Such tests
are sub-divided into three types:
•Free bend test.
•Guided bend test.
•Controlled bend test.
Bend specimens may be longitudinal or
transverse to the weld axis and may be bent in
simple three-or four point free bending as shown in
Fig. 17. The test may be carried a step further by
flattening the arms together in a press to complete a
180 0 bend as shown in Fig. 17(c, d).
Fig. 17 Free Bend
Test; (A) Three-
Point Bending, (B)
Four Point
Bending, and (C)
Press Bending to
Achieve Final U-
Shape

Fig. 18 Guided Bend Test Fig. 19 Controlled Bend Test Details
Due to inhomogenity of the joint, there is a tendency for free
bend test specimens to take up an irregular shape, so that the
actual radius at various points differs from the specified value.
This defect is overcome to some extent in the guided bend test,
Fig. 18, but can be avoided most effectively by the use of the
controlled bend test as shown in Fig. 19.

Side Bend Test: Very thick welded plates are difficult to bend in the
normal manner, and under these circumstances it may be both
permissible and desirable to make a side bend test. A slice 3 to 6 mm
thick is cut at right angles to the plate surface and to the weld axis and
is then bent in the usual way as shown in Fig. 20.
Side bend testing shows up lack of side wall fusion very well but is
somewhat less sensitive to face and root defects than the normal type
of bend test. The relative locations for cutting face, root- and side-
bend test specimens in a thick butt welded plate are as shown in Fig.
20 (a).
Fig. 20 (A) Relatived
Location of Face, Root,
and Side Bend Test
Specimens in a Butt
Welded Thick Plate

Weld test

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