Integrated Experimental Procedures Assessing Hydrogen ... · PDF fileSING HYDROGEN INDUCE. D...

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

1 Copyright © 2015 by ASME

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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



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.



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]).



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.



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



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.



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



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).



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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