Integrated Experimental Procedures Assessing Hydrogen ... · PDF fileSING HYDROGEN INDUCE. D...
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INTEGRATED EXPERIMENTAL PROCEDURES ASSESSING HYDROGEN INDUCED CRACKING SUSCEPTIBILITY
Ahmed Fotouh Ph.D., Quality Department, KBR
Industrial Canada Co. Edmonton, AB, Canada
R. El-Hebeary Ph.D., Professor Emeritus,
Mechanical Design and Production Engineering
Department, Cairo University Giza, Egypt
M. El-Shennawy* Ph.D., Associate professor;
Mechanical Engineering Department, Engineering
College, Taif University, Taif, Saudi Arabia
David Tulloch Subject Matter Expert (SME)-
Welding Specialist, KBR Canada.
Edmonton, AB, Canada
Jason Davio Sr. Manager, Quality Assurance,
KBR Industrial Canada Co. Edmonton, AB, Canada
Rob Reid Director of Quality, KBR
Canada. Edmonton, AB, Canada
Abstract* This study proposes a complete set of integrated
experimental procedures to assess the risk of Hydrogen Induced
Cracking (HIC) using implant test. The proposed experimental
procedures assess HIC susceptibility in base metals using two
measures: the implant static fatigue limit stress (σimp); and Heat
Affected Zone (HAZ) maximum hardness (HV10MAX). The base
metal susceptibility to HIC was evaluated by examining the
effect of three welding factors: the critical cooling time
between 800 C and 500 C (t800/500); the base metal carbon
equivalent (CE); and the diffusible Hydrogen content (H). A 3-
D mapping technique was used to demonstrate the interactive
integrated relationships among the three examined welding
factors (i.e. t800/500, CE and H) and the susceptibility of the base
metal to HIC. Using the 2-D projection of the developed 3-D
mapping, it was proven that the diffusible hydrogen content (H)
had more effect on the HIC susceptibility of High Strength Low
Alloy (HSLA) steel compared to the effect of H on the HIC
susceptibility of Carbon-Manganese (C-Mn) steel.
Introduction Welding applications are widely varied in steel structured
products, as welding is considered to be the most economical
joining process for steel [1]. Hydrogen Induced Cracking (HIC)
*On a leave from Mechanical Engineering Department, Helwan University,
Helwan, Cairo, Egypt.
in steel weldments is considered as a delayed crack that is
formed after solidification of the fusion weld, and it is always
associated with hydrogen embrittlement [1, 2].
Most likely for steels with approximate yield strengths
between 350 and 600 MPa, HIC in HAZ is the major type of
cracks formed by the hydrogen embrittlement, especially with
using welding electrodes with low carbon contents [1, 3-6].
However, in case of thick plate weldments, transverse weld
metal cracks may occur [7]. On the other hand, for extra-high
strength steels (i.e. yield strength higher than 600 MPa) with
weld metals that have matching or overmatching strength, weld
metal crack could become the dominating type of cold cracks
[3, 8].
Generally, there are some main welding factors affecting
the steel weldment susceptibility to HIC [3, 9-11]: 1) the
susceptible HAZ microstructure such as martensitic and
bainitic microstructures that are controlled by the critical
cooling rate between 800 oC and 500
oC (t800/500) and the base
metal carbon equivalent (CE) and carbon content; 2) the
amount of diffusible hydrogen content (H) associated with the
used welding process. Furthermore, the amount of welding
residual stresses, resulting from both microstructure phase
transformations (internal) and welded structure dimensions and
constraints (external), play another role in the susceptibility to
HIC.
In this study, integrated experimental procedures were
developed using currently applicable developed standards to
Proceedings of the ASME 2015 Pressure Vessels and Piping Conference PVP2015
July 19-23, 2015, Boston, Massachusetts, USA
PVP2015-46011
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evaluate the integrated effect of the previous mentioned
welding factors (i.e. the critical cooling rate between 800 oC
and 500 oC (t800/500), the base metal carbon equivalent (CE) and
the diffusible hydrogen content (H)) on the susceptibility to
HIC in HAZ. The susceptibility to HIC in HAZ was assessed
using: the implant static fatigue limit stress (σimp); and the
maximum hardness of HAZ coarsened grain region (HV10MAX).
Integrated Experimental Procedures This section attempts to establish experimental procedures
using current developed standards to evaluate HIC
susceptibility in single bead weldments. The proposed
experimental procedures were used to assess the interactive
integrated effect of three main welding factors, t800/500, CE and
H) on HAZ, coarsened grain region susceptibility to HIC;
therefore, the proposed experimental procedures can be referred
to as integrated experimental procedures.
The implant test was selected to asses the susceptibility to
HIC, as it was mainly developed to determine the maximum
amount of welding stresses (internal and external) before
failure under certain welding conditions including the amount
of diffusible hydrogen content (H). On the other hand, the
maximum hardness value of HAZ coarsened grain region was
used to give a quantitative measure for how much the
developed microstructure in HAZ coarsened region is
susceptible to hydrogen embrittlement.
Table 1 — Chemical Composition and Carbon Equivalent for Tested Base Metals
Base Metal
Sample
Steal
Category
Steel
Designation
Chemical Composition, %
CE, % C Si Mn P S Cr Mo Ni V Cu AL
A
C-Mn
DIN: 17Mn4 0.130 0.241 1.400 0.017 0.012 0.034 0.010 0.015 0.001 0.018 0.038 0.38
B DIN: St 52-3N 0.148 0.266 1.380 0.018 0.012 0.029 0.009 0.130 0.005 0.324 0.049 0.42
C MSZ: E420C 0.210 0.450 1.250 0.020 0.020 0.120 0.060 0.050 0.070 0.120 0.018 0.48
D
HSLA
MSZ:KL3 0.220 0.210 1.260 0.020 0.012 - - 0.700 0.130 0.150 0.008 0.52
E ASTM:387-
G11 0.118 0.505 0.528 0.011 0.003 1.32 0.536 0.047 0.01 0.02 0.021 0.58
F DIN:20CrMo5 0.198 0.188 1.060 0.017 0.016 1.250 0.215 0.115 0.010 0.185 0.020 0.69
Base Metals
The carbon equivalent (CE) was calculated using the
International Institute of Welding (IIW) formula, O’Neill
equation, that is applicable for steel with carbon content (C%)
above 0.12% [12, 13]. By applying O’Neill equation to
measure CE, the risk of susceptibility to HIC should be
considered for CE higher than 0.35% [12]. O’Neill equation
can be represented as a function of chemical composition
percentages of a designated steel as shown in equation 1 [12,
13]. The base metals are from two categories of steels: 1)
Carbon-Manganese (C-Mn) steel; and 2) High Strength Low
Alloy (HSLA) steel. The base metals were selected to be
susceptible to HIC, as the carbon equivalent (CE) of the tested
base metals started at 0.35%. The chemical analysis of the
tested base metals was performed according to ASTM E350
standard test methods for chemical analysis of carbon steel and
low alloy steel. Table 1 shows the chemical composition and
the carbon equivalent (CE) of each tested base metal.
CE=C+(Mn)/6 +(Cr+Mo +V)/5+(Cu+Ni)/15 (1)
Welding Electrodes The welding electrodes were selected with a variety of
diffusible hydrogen content, and most of them have an
undermatch strength; therefore, HIC will be more likely to
occur in HAZ of the base metal [3, 8]. Two different welding
processes were used: 1) Shielded Metal Arc Welding (SMAW);
and 2) Gas Metal Arc Welding (GMAW) with CO2 as an active
shielding gas. Different welding electrodes diameters were used
to provide different welding heat inputs and hence different
cooling rates would be generated. The electrodes types and
diameters, the chemical composition and the mechanical
properties of deposited weld metal are listed in Table 2.
Assessment of Weld Metal Diffusible Hydrogen Content Test piece was manufactured using steel st37-2. The test
piece assembly consists of run-on and run-off pieces and other
three intermediate pieces as individual specimens with a total
length of 135 mm. The weight of each individual specimen was
gravimetrically measured. All surfaces of the test piece parts
were ground and cleaned. The test piece should be kept at
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400oC for about an hour to assure the removal of any source
hydrogen. The test piece was assembled and fixed using a
copper fixture; then it was welded.
Table 2 — Chemical Composition and Mechanical
Properties of Deposited Weld Metal for Welding Electrodes
Ty
pe
of
flu
x
Ele
ctro
de
Typ
e
Cla
ssif
icat
ion
s
(AW
S
Des
ign
atio
n)
Ele
ctro
de
Dia
met
er (
mm
) Chemical Composition,
%
C Si Mn Mo
Cellulose A5.05-81
E7010-G
3.25
4.00
5.00
0.14 0.14 0.60 0.20
Rutile A5.01-81
E6013
3.25
4.00
5.00
0.06 0.40 0.50 -
Basic A5.01-81
E7018
3.25
4.00
5.00
0.05 0.60 0.90 -
Solid wire
CO2-
shielding
gas
A5.18-79
ER70S-6
1.0
1.2 0.08 0.90 1.5 -
The electrode welding current was selected to be about 15
Amperage lower than the highest welding current
recommended by the electrode manufacture. Electrodes with
diameters of 4.0 mm and 1.2 mm were used to measure the
diffusible hydrogen content (H) for both SMAW and GMAW
processes, respectively. The flow rate of CO2 shielding gases in
GMAW process was fixed to be about 25 l/min. SMAW
electrodes were dried according to manufacture’s
recommendations. The welding processes were performed
using Direct Current Electrode Positive (DCEP). All welding
processes were performed at room temperature without
preheating. The weld bead length was nearly 115 mm, and its
length on both run-on and run-off pieces was approximately 35
mm.
The test piece was quenched in ice-water mixture
immediately after 5 seconds of finishing the welding process.
Then, the run-on and run-off parts were removed to prepare the
individual specimens. Each individual specimen was cleaned
and dried. This operation of cleaning and drying should be
finish within 60-90 seconds from finishing the welding process.
After that, the individual specimen was put into a collecting
apparatus of glycerin displacement method. The Hydrogen gas
was collected by immersing the individual specimen in glycerin
at 45 oC for 72 hours. The gas collecting apparatus should be
able to measure the collected gas volume with accuracy of 0.05
ml.
The individual specimens were taken out of glycerin,
washed with water and perfectly dried. Each individual test
specimen was weighted to specify the mass of its deposited
weld metal (WD) by subtracting its original mass (Wr1) from its
mass after the deposition of the weld metal (Wr2) as shown in
equation 2. The accuracy of this gravimetrical method was 0.01
g.
D r2 r1W =W -W (2)
The measured volume of the collected gas was converted
to give the volume reading at 0oC and 1.0 atmospheric pressure
(i.e. 101.325 kPa) using the formula in equation 3 for a constant
mass [14].
1 1 2
1 2
P .v .TV=
T .P (3)
where, P1 is the pressure of the collected gas (kPa) at
temperature of T1, v1 is the volume of collected gas (ml), T2 is
equal to 273.15 K (i.e. 0 oC), P2 is equal to 101.325 kPa, and V
is the converted value of the collected gas volume (ml) at 0oC
and 1.0 atmospheric pressure.
The collected gas volume (V) calculated by equation 3
was divided by 100 g of the deposited weld metal (WD)
calculated by equation 2 for each one of the three individual
specimens using equation 4; then, The average collected
hydrogen volume per 100 g of the deposited weld metal was
considered as the diffusible hydrogen content in the deposited
metal (Hg) measured using glycerin method.
g DH = (V×100)/W (4)
where, Hg is the diffusible hydrogen content (ml/100g)
measured using glycerin method, V collected gas volume
calculated by equation 3 (ml), and WD is the mass of the
deposited weld metal (g).
To evaluate the diffusible hydrogen content (H) in the
form of IIW, the following empirical equations was used [3]:
gH=(H +0.8)/0.67 (5)
where, H is the diffusible hydrogen content (ml/100g)
according to IIW standards.
Implant Test Implant test is an external restraint test at which a known
load is applied to simulate the stresses developed from internal
and external restraints.
The geometry of the used Implant test specimen was 6
mm (± 0.02mm) in diameter and 73mm long. A helical groove
with 0.9 mm pitch, 15 mm long and 0.5 mm depth having 40
“V” notch with a root radius of 0.1 mm was machined for each
type of the tested base metals listed in Tables 1. The implant
specimen with 6mm diameter was selected, because larger
specimen diameters were not sufficiently covered by small
weld beads. Figure 1 shows the exact dimensions of the used
implant test specimen.
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Fig. 1 — Implant test specimen dimensions
The implant specimen is fitted into a reamed hole of 6 mm
diameter in a backing steel plate. The test backing plate was
manufactured using St 37-2 with 10 mm and 30 mm
thicknesses to simulate thin and thick weldment thicknesses,
respectively. The tolerances were
0.020.026.0
mm for the Implant
test specimen and
0.000.026.0
mm ream-to-sliding fit for the
backing plat holes [15, 16]. The backing plate is responsible for
controlling the cooling rate of the test weld, and it was used to
support the applied load. Various combinations of arc energy
inputs and plate thicknesses were used to generate different
weld bead cooling times. The surface of the backing plate was
ground before each set of tests. A weld bead with 150 mm
length was deposited on a backing plate assuring that the hole
including the implant test specimen are in the middle of the
deposited weld bead.
The time till HAZ of the implant specimen tip reached
150 C was counted using stop watch, and then the load was
gradually applied. The full targeted implant loading was
reached when the temperature reached 100 C. The time to
fracture under the applied static load was then measured. After
24 hours, if the fracture would not occur, the load was released;
the related stress to this load is nominally known by implant
static fatigue limit stress (σimp) [13, 17].
Microstructure Examination and Hardness Test As it was illustrated previously, the susceptibility to HIC
depends on some factors. The microstructure is one of the main
factors affecting the susceptibility to HIC [3, 9-11]. The
microstructure susceptibility to HIC can be estimated by
measuring its hardness [17]. In the case of the HAZ cracking, it
is usually sufficient to know the maximum HAZ hardness,
which is governed by the base metal chemical composition and
the welding cooling time between 800oC to 500
oC (t800/500) [3,
9-11, 17, 18]. Generally, the harder the microstructure, the
greater is the risk of HIC susceptibility [3-5, 9-11, 17, 19, 20].
Microstructure specimens were cut and prepared using a
surface grinding process was carried out with silicon carbide
paper sizes of 220, 320, 400, 600, 1000, 1200 and 2400; then
the surface were polished with alumina paste. The surface
etching process was performed at room temperature with a 2%
nital (2% HNO3 and 98% methylalcohol). The specimens were
then washed with water followed by methylalcohol and dried in
a hot air blast. The specimens were examined and
photographed using electrical optical microscope.
The hardness tests were performed according to the DIN
50133 Sheet 1 and DIN 50163 Part 1 with an interval of 1 mm
between indentations in area of HAZ coarsened grain region.
The hardness was measured in the coarsened grain region,
because the susceptible HAZ microstructure is expected to
appear in this region [21]. The HAZ maximum hardness
(HV10MAX) was measured using Vickers hardness tester with a
10-kg load following the tangential hardness measuring method
procedures according to IIW procedures [22]. The maximum
two values of the measured hardness were averaged to give the
main maximum HAZ hardness for the applied welding
conditions.
Theoretical Approach to Evaluate Heat Input and Cooling Time Heat inputs of 1.2, 1.7 and 2.5 kJ/mm were applied to the
implant test specimens using direct current (DC) for both
SMAW and GMAW welding processes. The amount of heat
input was calculated using the following equation [18, 20] :
h=(E.I/v) (6)
where, h is the arc energy input (J/mm), E is the arc voltage
(Volts), v is the welding speed (i.e. travel velocity of the heat
source) (mm/sec), and I is the arc current (Amperes).
The cooling time from 800C to 500C (t800-500) has a
significant role in the evaluation of HIC susceptibility, since
this is the critical temperature range at which the austenitic
microstructure transformations are characterized [18]. The
cooling time was measured using implanted thermocouple;
however, in order to have a more consistent and precise control
over the implant stress loading process, the measured cooling
time was reevaluated using a heat transfer thermal model.
Rosenthal’s heat transfer models are widely used to
estimate the weldment cooling cycle [10, 23, 24]. For three
dimensional heat flow (thick plate), the simplified form of
Rosenthal’s heat transfer model can be represented as follows
[23, 24]:
2
effo
h rT- T = exp -
2 πkt 4ηt
(7)
where, T is the temperature (C) at time t (s) , To is the initial
plate temperature (C), heff is the effective heat input (J/mm), k
is the thermal conductivity of the metal (-1 -1 o -1J.mm .s . C ), η
is the thermal diffusivity (mm2.s
-1), and r is the radial distance
from the weld (mm).
For two dimensional heat flow (thin plate), the simplified
form of Rosenthal’s heat transfer model can be represented as
follows [23, 24]: 0.5 2
effo
h1 rT- T = exp -
4 πkρCt S 4ηt
(8)
where, is the density of base metal (g/mm3), C is the specific
heat of base metal (-1 o -1J.g . C ), S is the thickness of the plate
(mm), and C is the volumetric specific heat of the base metal
(for steel : C = 0.0044 -3 o -1J.mm . C [15]).
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The effective heat input (heff) can be calculated as follows
[15]:
effh = h.f.m (9)
where, h is the arc heat input (J/mm) from equation 5, f is the
arc efficiency (f= 80% for SMAW and GMAW processes [15]),
and m is a constant depending on the equivalent thickness (1
for bead-on plate, 2/3 for T-joint [15]).
By using the simplified Rosenthal’s models in equations 7
and 8 for a radial distance (r) near the weld (i.e. at r = 0.0 mm),
other two models could be developed to calculate the critical
cooling time from 800 C to 500 C for three and two
dimensional heat flows; the models developed from equations 7
and 8 are known as Adam’s heat transfer models for two and
three dimensional heat flows, respectively. Adam’s heat
transfer models can be used to valuate t800/500 for coarsened
grain region in HAZ near the weld diffusion zone [15, 17, 23].
For three dimensional heat flow (thick plate), Adam’s heat
transfer model can be represented as follows [15, 23, 25]:
1 2
effT /T
2 o 1 o
h 1 1t =
2 πk (T - T ) (T - T )
(10)
where, 1 2T /Tt is the cooling time (sec) from T1 (800 C) to T2
(500 C) at initial plate temperature (To) of 25 C, heff is the
effective heat input (J/mm), and k is the thermal conductivity of
the metal (For steel: k = 0.028 -1 -1 o -1J.mm .s . C [15]).
For two dimensional heat flow (thin plate), Adam’s heat
transfer model can be represented as follows [15, 23, 25]:
1 2
2
effT /T 2 2
2 o 1 o
h1 1 1t = -
4 πkρC S (T - T ) (T - T )
(11)
where, is the density of base metal (g/mm3), C is the specific
heat of base metal (-1 o -1J.g . C ), S is the thickness of the plate
(mm), and C is the volumetric specific heat of the base metal
(for steel : C = 0.0044 -3 o -1J.mm . C [15]).
The critical thickness (Scr) defining the border between
two and three dimensional heat flows can be developed using
equations 9 and 11 as follows [15, 23]: 1/2
effcr
2 o 1 o
h 1 1S = +
2ρC (T - T ) (T - T )
(12)
where, Scr is the critical thickness of the plat at which the thick
plate equation is applied when S > Scr, and the thin plate
equation is applied when S < Scr.
Welding current and traveling speed were adjusted during
testing to obtain the desired heat input. The voltage and the
amperage were checked occasionally by using a digital
ACA/DCA clampmeter. The implant test loading times were
measured using stopwatch with accuracy 0.05 sec. The
calculated and the measured cooling time between 800 oC and
500 oC (t800/500) are shown in Table 3. There is a slight
difference between the measured and the calculated values of
the 2-D heat flow as a result of neglecting the surface heat
transfer in the Adam’s model; however, in general, there is a
very close match between the calculated and the measured
values of cooling time from 800C to 500C (t800-500).
Therefore, Adam’s model was highly suited to be used to
control the loading time of the applied implant load.
Results and Interactive Integrated Analysis Approach By applying the developed integrated experimental
procedures, interactive integrated results were produced to
illustrate: 1) how the welding factors individually affect the
HIC susceptibility; and 2) how these welding factors
interactively integrated to affect the HIC susceptibility.
Individual Effect of Each Welding Factor on HIC Susceptibility Diffusible hydrogen content (H) is the main source for
HIC. Through the cooling cycle of the weld, hydrogen escapes
from the solidified weld bead by diffusion [26, 27]. The
escaped hydrogen is mainly divided into two categories [3, 9,
28]: 1) residual hydrogen, which loses its ability to diffuse
during the cooling temperature range between 300 C and 200
C; and 2) diffusible hydrogen, which is the main cause of HIC
at relatively low cooling temperature range between 150 C and
100 C. The atomic diffusible hydrogen content will build up in
voids and rifts at relatively low temperature between 150 C
and 100 C forming molecular hydrogen accompanied by a
very high pressure [1]; as an example, it has been estimated that
5 ppm of molecular hydrogen in a steel void (1.0 ppm= 0.9 ml
per 100 gm) would cause over 17000 atmospheric pressure in
this voids at 20 C [17, 29]. Table 4 shows the different values
of the diffusible hydrogen content (H) measured using glycerin
method for the testing welding electrodes. SMAW with
cellulose electrodes gave the highest diffusible hydrogen level,
which highly increases the susceptibility of HAZ to HIC. The
rutile electrode came after with HIIW of 30 ml/100g, then the
basic electrode with 5 ml/g. The lowest H value was observed
for GMAW process with CO2 as a shielding gas; that can be
attributed to two factors: 1) the lower moisture content in the
shielding gas; and 2) the spherical shape of the molten metal
droplets within CO2, as the spherical shape has the minimum
contact surface with the surrounding environment; therefore,
the amount of the diffused hydrogen in the weld pool was
reduced [11].
For steel B: 42% CE at t800/500 of 4.2 sec, Figures 2 and 3
illustrate the effect of H on implant test and σimp, respectively.
Figures 2 and 3 show a noticeable drop in the values of σimp by
increasing the amount of H; therefore, for certain CE and
t800/500, the amount of stresses supported in implant test (i.e.
σimp) is decreased by increasing H. As a result, a higher H value
means a higher susceptibility to HIC. That can be explained as
a consequence of increasing the amount of formed molecular
hydrogen resulting from increasing the amount of diffusible
hydrogen content (H).
The cooling time between 800 oC and 500
oC (t800/500) is
another factor affecting HIC susceptibility in steel weldment, as
it is the temperature range at which austenitic transformation
takes place [2, 3, 9-11, 17, 18, 21]. Figures 4-a and 4-b show
the microstructure of base metal A: 0.42% CE and the
microstructure of the coarsened grain region of its HAZ at
t800/500 of 4.5 sec, respectively.
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Table 3 — Measured and Calculated Values of t800/500 at Different Welding Conditions
Welding Conditions
Electrode Type
Cellulose (E7010-
G) – Rutile
(E6013) –
Basic (E7018)
Solid Wire CO2
Shielding Gases
ER70S-6
Electrode Diameter (mm.) 3.25 4.0 5.0 1.0 1.2 1.2
Current (Amp.) 125 170 225 200 250 250
Voltage (Volt.) 24 25 28 27 30 30
Heat input(KJ/mm.) 1.2 1.7 2.5 1.2 1.7 2.5
Welding Speed (mm/sec.) 2.5 2.5 2.5 4.5 4.5 3
Welding Time for 150mm
Bead on Plate (sec.) 60 60 60 33.3 34 50
Critical thickness (mm.) 19.2 22.9 28.0 19.2 22.9 28.0
t800/500
Thick
Plate,
30
mm.
(sec.)
Using the
thermocouple 4.7 5.8 10.1 4.9 6.5 9.8
Using
Adam’s
model
4.5 6.3 9.3 4.5 6.3 9.3
Thin
Plate,
10
mm.
(sec.)
Using the
thermocouple 15.7 29.7 67.3 14.8 30.9 65.1
Using
Adam’s
model
16.5 33.1 71.5 16.5 33.1 71.5
Fig. 2 — Effect of diffusible hydrogen content (H) on
implant test results for steel B: 0.42CE at t800/500 of 4.5 Sec.
Figures 4-a shows the microstructure of the base metal A:
0.42% CE, which is constituted by a ferrite structure (lighter
color areas) along with a pearlite (darker color areas) structure
appearing as ghost bands due to the effect of the hot rolling.
Fig. 3 — Effect of diffusible hydrogen content (H) on σimp
for steel B: 0.42CE at t800/500 of 16.5 Sec.
This can be attributed to the high melting point of manganese
sulphide (MnS), which is a slightly soluble in iron, and it is
collected in irregular distributed large globules through the
steel; these large globules are plastic at high temperature, and
they are elongated into threads by hot rolling without seriously
impairing the properties of the material [21, 30, 31]. During
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cooling, the manganese sulphide particles have a nucleated
pearlite forming these ghost bands of pearlite dark grains
shown in Figure 4-a [30]. Figure 4-b shows the transformations
in HAZ coarsened grain microstructure of steel A: 0.42% CE;
ferrite was nucleated and grown from the grain boundary
toward the inner of the grain forming the Widmanstatten ferrite
structure shown in Figure 4-b, which is considered relatively as
a brittle microstructure [32].
Table 4 — Weld Metals’ Diffusible Hydrogen Contents
Consumable Electrode
Type
Electrode
Conditions
Heat
Input,
KJ/mm.
H (IIW),
ml/100gm. Process
Electrode
Type
Classifications
(AWS
Designation)
SMAW A5.05-81
E7010-G N/A 1.7 40
SMAW A5.01-81
E6013
1 hour at
temperature
150 oC
1.7 30
SMAW A5.01-81
E7018
2 hours at
temperature
260 oC
1.7 5
GMAW
CO2
Shield
Gas
A5.18-79
ER70S-6 N/A 1.7 2
(a) (b)
Fig. 4 — Microstructure for metal B: 0.42 (CE) showing (a)
Base metal, 200 x, and (b) HAZ grain coarsened region with
t800/500: 4.5 Sec, 200 x.
Figure 5 represents the relationship of t800/500-σimp at
different values of carbon equivalent (CE) and diffusible
Hydrogen content (H). Figure 5 demonstrates that the value of
σimp increases, minimizing the susceptibility to HIC, by
increasing the values of t800/500. The relationship of t800/500-
HV10MAX is represented in Figure 6 for base metals E: 0.58 CE
and F: 0.69 CE. As shown in Figure 6, for each value of CE,
the value of HV10MAX is decreased, being less susceptible to
HIC, by increasing the values of t800/500.
As shown in Figures 5 and 6, the base metal carbon
equivalent (CE) is interactive with other welding factors
affecting the susceptibility to HIC. For HIC in HAZ, it is
adequate to know the hardness, determined mainly by the base
metal chemical composition and the cooling time of the
austenitic transformation temperature range between 800 oC
and 500 oC, to be able to asses the susceptibility of developed
HAZ microstructure to HIC [3-5, 19, 20]. Therefore, carbon
equivalent (CE) is considered to be one of the main factor
affecting HIC susceptibility in steel elements [3, 9-11]. Figure
7 shows the effect of CE on implant test results of steels D:
0.52% CE, E: 0.58% CE and F: 0.69% CE. Figure 7 shows that
by increasing CE the implant static fatigue limit (σimp)
decreased at constant values of t800/500 and H. Furthermore, the
increase of CE causes the maximum HAZ hardness (HV10MAX)
to be increased as shown in Figure 8 for different steels at
t800/500 of 16.5 sec. Therefore, it can be concluded that the
increase of CE causes HIC susceptibility to increase in steel
weldments, which can be attributed to the effect of CE on
creating a microstructure that is susceptible to HIC.
Fig. 5 — The relationship between t800/500 and σimp at
different values of CE and H.
Fig. 6 — The relationship between t800/500 and HV10MAX for
E: 0.58 CE and F: 0.69 CE base metals.
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Fig. 7 — Effect of carbon equivalent of base metal on the
results of the implant test for steels D: 0.52% CE, E:
0.58% CE and F: 0.69% CE at t800/500 of 6.3 sec and H of 40
ml/100g.
Fig. 8 — Effect of carbon equivalent (CE) on maximum
HAZ hardness (HVMAX) at different values of CE and t800/500
of 16.5 Sec.
Interactive Integrated Mapping for Relationships between Welding Factors and HIC Susceptibility
As demonstrated previously, there are main controlling
factors that can be expected to affect σimp and HV10MAX as
follows [3, 9-11]: 1) the cooling time between 800 oC and 500
oC (t800/500); 2) the base metal carbon equivalent (CE); and 3)
the diffusible hydrogen content (H). On the other hand,
HV10MAX can be considered as another controlling factor that
can affect σimp. The proposed integrated experimental
procedures were developed to demonstrate the interactive
integrated effect of these controlling welding factors on σimp
and HV10MAX, as they form the measures for HAZ susceptibility
to HIC. To illustrate these interactive relationships between the
main welding factors and HIC suitability, three-dimensional
mapping representation of the interactive relationships among
the diffusible hydrogen content (H), the implant static fatigue
limit stress (σimp), and the cooling time between 800 oC and 500
oC (t800/500) at CE: 0.42 is depicted in Figure 9, respectively.
Figure 9 illustrates the interactive effect of t800-500 and H on the
values of σimp. It could be perceived from Figure 9 that the
variation in t800/500 has a stronger effect on the values of σimp
than the effect generated by the variation of H.
Fig. 9 — 3D meshing showing the effect of cooling time
between 800C and 500C (t800/500) and diffusible hydrogen
content (H) on implant static fatigue limit stress (σimp) for
steel B : 0.42 CE.
Fig. 10 — 3D map demonstrating the effect of cooling time
between 800C and 500C (t800/500) and carbon equivalent
(CE) on maximum HAZ hardness (HV10MAX) measured for
base metals with CE of 0.52 up to 0.69.
The interactive integrated relationship in Figure 10
ascertains that the susceptibility to HIC, represented by
HV10MAX, is increased by increasing CE and decreasing t800/500.
From Figure 10, it can be assumed that the variation in t800/500
has a stronger effect on the maximum HAZ hardness
(HV10MAX) than the variation in CE.
As illustrated previously, the cooling time of austenitic
transformation temperature range between. 800 C to 500 C
(t800/500) and the base metal carbon equivalent (CE) affect the
developed HAZ microstructure, which can be assessed using
maximum HAZ hardness (HV10MAX) [3, 9-11, 17, 18];
therefore, the implant static fatigue limit stress (σimp) could be
assumed to be interactively affected by maximum HAZ
hardness (HV10MAX) and diffusible hydrogen content (H).
Figure 11-a represents the projection in σimp- t800/500 axis
of the developed 3-D mapping shown in Figure 9. In this
projection, a bandwidth of HIC susceptibility values (i.e σimp
values) is formed as a result of increasing the diffusible
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hydrogen content (H) in the range between 2ml/100g to
40ml/100g. For C-Mn steels, by increasing the carbon
equivalent (CE) from 0.38 to 0.48, the developed bandwidth
size is reduced significantly, as shown in Figure 11-b. On the
other hand, increasing the value of CE from 0.52 to 0.69 for
HSLA steel almost did not have any significant effect over the
bandwidth size generated by increasing the value of H from
2ml/100g to 40ml/100g, as shown in Figures 12-a and 12-b
respectively. This could be attributed to the increasing effect of
the diffusible hydrogen content (H) as a result of the relatively
high percentages of some alloying elements found in the
chemical compositions of HSLA steels. These elements are: the
vanadium (V) in steel D (CE:0.52) with relatively high
percentage of 0.13%; and the chromium (Cr) and the
molybdenum (Mo) in steel F (CE:0.69) with relatively high
percentages of 1.250% and 0.215%, respectively. The relatively
high percentage of the vanadium (V) in steel D (CE:0.52)
increases the amount of absorbed hydrogen [33]. On the other
hand, the relatively high percentages of chromium (Cr) and the
molybdenum (Mo) in steel F (CE:0.69) affect interactively to
increase the diffused hydrogen [34].
(a) (b)
Fig. 11 — 2-D projection of a developed 3-D map for implant static fatigue limit stress (σimp) versus cooling time between
800C and 500C (t800/500) in a range of diffusible hydrogen content (H) between 2 to 40 ml/100g for C-Mn steels: (a) A: 0.38
(CE) and (b) C: 0.48 (CE).
(a) (b)
Fig. 12 — 2-D projection of a developed 3-D map for implant static fatigue limit stress (σimp) versus cooling time between
800C and 500C (t800/500) in a range of diffusible hydrogen content (H) between 2 to 40 ml/100g for for HSLA steels: (a) D:
0.52 (CE) and (b) F: 0.69 (CE).
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Conclusions Integrated experimental procedures were proposed to
asses HIC susceptibility in steel weldments. In the proposed
experimental procedures: HIC susceptibility was investigated
and assessed using implant static fatigue limit stress (σimp) and
maximum hardness of HAZ coarsened grain region (HV10MAX).
It was found that the susceptibility to HIC increased with
increasing the carbon equivalent of the base metal (CE) as well
as increasing the diffusible hydrogen content (H). On the other
hand, the HIC susceptibility reduced with increasing the
cooling time between 800 oC and 500
oC (t800/500).
Using the data generated from the developed integrated
experimental procedures, the interactive integrated
relationships between welding factors (i.e. the diffusible
hydrogen content (H), the base metal carbon equivalent (CE)
and the cooling time between 800 oC and 500
oC (t800/500)) and
HIC susceptibility (i.e. the implant static fatigue limit stress
(σimp) and the maximum hardness of HAZ coarsened region
(HV10MAX)) were successfully developed and mapped using 3D
mapping techniques. Using the 2-D projection of the developed
3-D maps, it was proven that the effect of diffusible hydrogen
content (H) on the susceptibility of HSLA steels to HIC was
more than the effect of H on the HIC susceptibility of C-Mn
steels. This was attributed to the relatively high percentages of
some alloying elements in HSLA steel compositions, such as
vanadium (V) and chromium (Cr)/molybdenum (Mo)
combination. These alloying elements increase the amount of
diffused hydrogen, and this leads to increase the sensitivity of
HIC susceptibility to the amount of diffusible hydrogen content
(H).
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