Strength of spot-welded steel sheets in corrosive environment

398 DOI: 10.1002/maco.200905427 Materials and Corrosion 2010, 61, No. 5

Strength of spot-welded steel sheets in corrosiveenvironment

G. Mukhopadhyay, S. Bhattacharya and K. K. Ray*

The load bearing capacity (LBC), often referred to as strength, of spot-welded

joints on as-received and pre-strained sheets of an interstitial free (IF) steel has

been examined under 3.5% sodium chloride solution using tensile-shear (TS)

specimens. These tests have been carried out under three different test

conditions: (i) after immersing the samples in the solution for various duration of

time, (ii) at various slow strain rates by keeping the samples in solution, and

(iii) at slow strain rate under in situ hydrogen charging. Analyses of the results

infer that (i) increased duration of immersion of the samples in aggressive

environment decreases their LBC but increases their extension corresponding to

the maximum load (EML), (ii) slow strain rate tests in the solution indicate

marginal decrease of LBC and EML of the spot-welds, (iii) the LBC and EML of the

spot-welds of pre-strained sheets decrease considerably with cathodic

hydrogen charging while EML of the spot-welded joints on the as-received

sheets is found to improve, and finally, (iv) the detrimental effect of corrosive

environment increases with increased pre-strain of the investigated sheets.

These observations have been discussed together with post-failure

examinations of the broken fractured surfaces, which have assisted in

understanding their failure mechanism.

1 Introduction

Spot-welded sheets are widely used for assembling auto-body

panels which undergo various corrosive environments during

their service period. Especially in the coastal regions, automotives

face the corrosive environment of marine atmosphere and hence

corrosion of its components is a well-recognized problem [1]. It is,

therefore, required to gain knowledge related to the effect of

saline environment on the mechanical behavior of spot-welds in

order to understand their load bearing capacity (LBC) in an

aggressive environment. Somervuori et al. [2] and Alenius et al. [3, 4]have studied corrosion behavior of the spot-welded joints of

austenitic stainless steel, but literature related to the effect of

saline environment on the strength of spot-welds made on

G. Mukhopadhyay, K. K. Ray

Department of Metallurgical and Materials Engineering, Indian

Institute of Technology, Kharagpur-721302 (India)

E-mail: [email protected]

G. Mukhopadhyay

On study leave from R&D and Scientific Services, Tata Steel,

Jamshedpur-831001 (India)

S. Bhattacharya

R&D and Scientific Services, Tata Steel, Jamshedpur-831001 (India)

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

interstitial free (IF) steel which is extensively used in the

fabrication of current automobiles, is almost non-existent.

Hydrogen atoms can get generated during aqueous corrosion

and this is known to degrade mechanical properties of iron-base

alloys like automotive steels. Generally, IF steel is considered

susceptible to hydrogen embrittlement due to the high diffusivity

and permeability of hydrogen in its ferritic microstructure. The

effect of hydrogen on strength and toughness has been widely

studied in pure iron as well as in steels of various compositions;

the reported results are diverse and, at times, contradictory in

nature. The effect of hydrogen absorption in spot-welds of

automotive steel sheets is non-available in the open literature. It

would, therefore, be worthwhile to study themechanical response

of the spot-welded joints made on IF steel sheets to hydrogen

permeation. Further, manufacturing of automotives involve

press-forming of body panels prior to assembling through

spot-welding, which induces varied amount of pre-strain in the

sheet metals. The effect of corrosive environment on the spot-

welds of pre-strained sheets is, thus, of practical importance, on

which emphasis has not been laid so far.

The major objective of this report is to investigate the effect

of 3.5% sodium chloride solution (equivalent to synthetic sea

water) on the strength behavior of spot-weldsmade on as-received

and pre-strained IF steel sheets under three different test

conditions: (i) tests on spot-welded steel specimens after

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Materials and Corrosion 2010, 61, No. 5 Strength of spot-welded steel sheets 399

Table 1. Chemical composition of the investigated EIF steel (in weight percentage)

Steel C Mn S P Si Al Cr Ni Ti Nb N Fe

EIF 0.0025 0.05 0.008 0.007 0.012 0.037 0.023 0.014 0.06 0.001 0.0027 Balance

Figure 1. Spot-welded samples of tensile-shear (TS) configuration with

tabs attached to both ends: (a) schematic and (b) fabricated specimen

immersing these in aqueous solution of 3.5% sodium chloride,

(ii) tests in this solution at different slow extension rates, and

(iii) tests at slow extension rate under in situ hydrogen charging in

the solution. Characterization of the spot-welded joints and post-

failure examinations of spot-welds are some necessary supple-

ments.

2 Experimental procedure

2.1 Selected material and properties

The material selected for this investigation is an IF steel,

industrially referred to as ‘‘extra interstitial free’’ steel, of

thickness 0.78mm. Extra interstitial free (EIF) steels are ultra-

low carbon (ULC) variety usually stabilized with Ti and are widely

used in the automotive body panels because of their excellent

formability. These steels are industrially manufactured through

sequential stages of cold rolling, batch annealing, and temper

rolling. The chemical analysis of the selected steel has been done

by X-ray fluorescence spectroscopy (model: ARL 8680) and LECO

carbon sulfur determinator, and is given in Table 1.

Small specimens (cross-section� 10mm� 10mm) were cut

from the steel sheet for microstructural studies both in the

transverse and in the longitudinal directions of rolling. The

specimens were suitably mounted with resin and were polished

up to 0.25mm diamond paste using standard metallographic

techniques and were etched with 3% nital for revealing the

microstructures. These were examined using an optical micro-

scope (model: Leica, BMRX) which revealed themicrostructure to

contain predominantly ferrite. The grain size of ferrite was

determined using linear intercept method following ASTM

standard E112-03 [5].

Flat tensile specimens of 12.5mm width and 50mm gauge

length were prepared from the as-received steel sheet following

ASTM standard E8M-03 [6] for determining the tensile proper-

ties. Tensile tests were carried out with the help of a 10 ton

capacity electro-mechanically operated Instron testing machine

(model: 5582) using cross-head velocity of 10mm/min at room

temperature of 298K. The cross-head velocity corresponds to

nominal strain rate of 3.33� 10�3/s.

2.2 Preparation of spot-welded samples

A series of specimen blanks (45mm� 200mm) was uniaxially

pre-strained up to 5, 10, and 15% using the Instron machine. The

gripped portions at both the ends of the sheets were cut off

prior to spot-welding. Spot-welded samples of tensile-shear (TS)

configuration, as shown in Fig. 1, were fabricated from the

as-received and pre-strained steel sheets. Spot-welding was

carried out with the help of a pedestal type inverter-based

90 kVA medium frequency DC machine (model: Lecco-Italia) in

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ambient atmosphere using Cu–Cr electrode having 16mm

diameter shank with a conical cap of 5mm tip diameter at the

end. The electrode was water cooled during welding. Spot-

welding was done by 6.5 kA peak current with constant welding

and holding time of 250 and 200ms, respectively, using an

electrode force of 2.3 kN.

2.3 Characterization of the spot-welds

Representative spot-welded samples were cut approximately at

the center of the weld nugget in the through thickness direction;

these were mounted, ground, polished, and etched as described

in Section 2.1. A series of digital photographs of the cross-

sections of weld nuggets was recorded with the help of a camera

attached to the Leica optical microscope. The diameter of the

fusion zone and the reduced thickness of the conjugate sheets

due to electrode indentation at this zone are referred to as

nugget diameter [7] and nugget thickness [8], respectively. The

nugget diameter and the nugget thickness were measured using

these photographs with the help of an image analyzer coupled to

the microscope. Polished and etched specimens were next

examined using either optical or scanning electron microscope

(Carl Zeiss, Model: Supra 25) to assess the microstructural

variations along the different zones of the prepared spot-welds.

Microhardness profiles across the investigated spot-welds

were determined using a Vickers hardness testing machine

(model: Leitz mini load-II). The hardness values were measured


400 Mukhopadhyay, Bhattacharya and Ray Materials and Corrosion 2010, 61, No. 5

Figure 2. Arrangements of experimental set-up: (a) SSRT at 3.5%

sodium chloride solution using CORTEST machine, (b) schematic

arrangement for hydrogen charging

along the major axis of the weld nugget at regular intervals

of 0.25mm using a load of 100 gmf for indentation duration

of 15 s.

2.4 Determination of strength of spot-welds

The strength of the spot-welds made on as-received and pre-

strained sheets was determined using TS samples with the help of

the Instron testing machine. These tests were carried out using

cross-head velocity of 10mm/min at the room temperature of

�298K. Displacement values were recorded using a video

extensometer. Tests on the spot-welded TS samples were

continued till the conjugate sheets constituting the weld were

found to separate completely. Tests on each type of specimens

were repeated for at least five times. The peak load in a load–

displacement curve is considered as the failure load which

represents the LBC of a spot-weld. The LBC is popularly referred

to as the strength of a spot-welded joint in several earlier reports

[9–11]; the term ‘‘strength’’ for LBC has been sparingly used for

convenience in this manuscript.

2.5 Determination of strength of spot-welds in corrosive

environment

The strength of the spot-welds was determined in 3.5% sodium

chloride solution under three different test conditions as

described in the following. Tests were also carried out in air

under the same test conditions in order to make comparative

assessment of the effect of corrosive environment on the strength

of spot-welds.

2.5.1 Determination of strength after immersion in 3.5%

sodium chloride solution

Spot-welded TS samples of as-received and pre-strained sheets

were immersed in 3.5% sodium chloride solution which was

continuously agitated with air bubbles. The samples were kept in

the solution for different duration of time, e.g., 3, 10, 20, and

40 days and were then tested in an identical manner as described

in Section 2.4. The load–displacement data generated during

these tests were recorded.

2.5.2 Slow strain rate tensile test in 3.5% sodium chloride

solution

Spot-welded samples of as-received sheets were pulled by a slow

strain rate testing (SSRT) machine in a medium of 3.5% sodium

chloride solution with four different extension rates (1� 10�4,

7.5� 10�5, 5� 10�5, and 3.5� 10�5mm/s) at the room temperature

of�298K. Similar tests were also done in air to study the effect of

corrosion. The apparatus for the slow strain rate test was a

standard CORTESTunit which consisted of a load cell (maximum

limit: 3628 kg), linear variable displacement transducer (LVDT,

maximum limit: 0.5250 cm), a drive mechanism, ball screw

adapter, a motor (1/6 hp, 100–1725 rpm angular speed) and two

speed reducers (connected in series between the SSRT drive

mechanism motor and the ball screw input shaft). Load and

displacement values were manually recorded during these tests

from the digital display attached to the machine to construct the

load–displacement curves.


2.5.3 Slow strain rate tests under cathodic hydrogen charging

Hydrogen susceptibility of the spot-welds has been examined

by tensile testing of hydrogen pre-charged TS samples. For

this purpose, TS samples were cathodically pre-charged with

hydrogen for 3 days in 3.5% sodium chloride solution using a

potentiostat. The arrangement for the cathodic charging is

schematically shown in Fig. 2. The charging was done by making

the specimen as a cathode in 3.5% sodium chloride solution. The

free corrosion potential of the sample in the medium was

�800mV (SCE). A constant potential of �2000mV was applied

for cathodic charging. The pre-charged specimens were quickly

transferred to the grip of the CORTEST machine; suitable

arrangement was made for in situ hydrogen charging under

conditions as described above and SSRT using the CORTEST

machine was started using an extension rate of 1� 10�4mm/s.

The load and displacement values were manually recorded as

mentioned earlier. A few uncharged TS specimens were also

tested in air under identical test conditions.

2.6 Post-failure examinations of the weld nuggets

Macroscopic and microscopic examinations of the failed spot-

welds were done to determine the location of failure and to

understand the physical processes involved during failure. These

investigations were carried out on the failed spot-welds following

the procedure described in Section 2.3. The fractured surfaces of

the spot-welds were examined using a scanning electron

microscope to understand the possible failure mechanism

associated with the different types of tests.

3 Results and discussion

3.1 Microstructure and properties of the selected steel

Typical representative microstructure of the selected EIF steel is

shown in Fig. 3, which predominantly exhibit equiaxed ferrite

grains. The average grain size of the EIF steel is estimated as

18.11� 0.75mm in the transverse orientation to the rolling

direction. Typical engineering stress–strain diagram of the as-

received steel is shown in Fig. 4 and their average tensile

properties are compiled in Table 2.

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Figure 3. Microstructure of EIF steel in the transverse direction of

rolling

Figure 4. Typical engineering stress–strain curve of EIF steel having

specimen axis in the direction of rolling

Figure 5. Macro- and microstructures of the spot-welded joints of

EIF steel: (a) macroscopic view of the cross-section of spot-weld,

(b) enlarged view of the spot-weld revealing grain structure in the

various zones, (c) optical microstructure of fusion zone, and (d) laths of

bainitic ferrite, revealed by TEM. A, B, and C represent fusion zone, heat

affected zone, and base metal, respectively

3.2 Characterization of the spot-welds

The cross-section of the spot-welds reveals three distinct zones:

the fusion zone, the heat-affected zone (HAZ), and the base metal

as shown in Fig. 5a and b. The average nugget diameter has been

estimated as 4.59� 0.02mm for the spot-welds of both as-

received and pre-strained sheets. All the three zones of the spot-

weld contain ferrite but of varying morphology. The fusion zone

exhibits a mixture of quasi-polygonal ferrite and bainitic ferrite

(Fig. 5c), the HAZ exhibits elongated ferrite (Fig. 5b) where as

the base metal shows equi-axed ferrite grains (Fig. 3). Figure 5d

depicts the laths of bainitic ferrite in the fusion zone. The

excessive directional grain growth at HAZ is due to higher

amplitude of thermal gradient at this zone as discussed in an

earlier report [12]. The microhardness profile along the spot-weld

on the as-received sheets shows increase in hardness from

base metal to fusion zone through HAZ as shown in Fig. 6a. The

basemetal of the as-received samples exhibits the lowest while the

Table 2. Tensile properties of EIF steel

Steel 0.2% Offset yield strength (MPa) Ultimate tensile stren

EIF 147� 1.3 295� 1.9

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fusion zone for all specimens exhibits the highest hardness

values. The observed variation of hardness along the spot-weld is

due to the morphologic variation of microstructure and variation

in internal stress along the various zones of the spot-weld.

The hardness of the base metal increases with increasing

pre-strain of the sheets due to increased dislocation density. The

hardness profiles along the spot-welds of the pre-strained sheets

are different from that of as-received sheets only at the juncture

of base metal to HAZ. The hardness values decrease gradually

from base metal side towards the HAZ–base metal interfacial

region because of relieving of internal strain energy due to weld

heat input. At the HAZ–base metal interface, a part of the

dislocations gets annihilated during spot-welding, but the remnant

dislocation density at this region increases with increasing pre-

strain. Transmission electron microscopic examinations of the

sub-structures indeed reveal decreased dislocation density at this

interfacial region compared to that at base metal as shown in

Fig. 6b and c. Pre-strain does not alter the microstructures at the

HAZ and the fusion zone because of removal of pre-strain at

these regions during a welding process and hence all the

hardness profiles of the pre-strained specimens almost merge at

these zones.

3.3 Strength and ductility of spot-welds

Typical load–extension plots for the spot-welded TS samples of

the as-received (0% pre-strain) and pre-strained sheets are shown

in Fig. 7a. The recorded maximum load is considered as the LBC

of the spot-weld, and is often referred to as strength of spot-weld.

gth (MPa) Uniform elongation (%) Total elongation (%)

29� 1.14 51� 1.51



Figure 6. Typical hardness distribution and dislocation sub-structures

at different zones of spot-welds made on pre-strained sheets: (a) hardness

profiles along weld nugget, (b) dislocation sub-structure at base metal

of 15% pre-strained sheet, and (c) dislocation sub-structure at HAZ–

base metal interface on 15% pre-strained sheet

Figure 7. Variation of load bearing capacity and extension up to

maximum load (EML) of spot-welds with pre-strain: (a) load–extension

curves of the spot-welds, (b) variation of load carrying capacity and

EML with pre-strain

The extension corresponding to the maximum load (EML) has

been occasionally referred here as the ductility of the spot-weld

following Zhou et al. [13]. In general, the weld quality is described

by the LBC and EML obtained from tension tests of TS samples.

The load starts dropping as crack initiation is followed by its

propagation around the weld nugget. After the maximum load,

the change in displacement does not follow any regular pattern

because of the uncertainty in tearing of the base metal. The spot-

welds exhibit partial pullout of nuggets followed by tearing of the

base metals in the loading direction. The LBC of the spot-welds

increases while EML decreases with increasing pre-strain of the


base metal as shown in Fig. 7b. The increase in LBC due to

increased pre-strain can be explained simply by the increase in

dislocation density as has been explained in an earlier report [14].

3.4 Failure of spot-welds after immersion in 3.5%

sodium chloride solution

Typical load–extension plots for the spot-welds subjected to

immersion in 3.5% sodium chloride aqueous solution for

different duration are shown in Fig. 8a. The LBC of the spot-

welds is found to decrease with increasing duration of immersion

in the solution as shown in Fig. 8b, but interestingly the amount

of EML is found to increase. The LBC of the spot-welds made on

0% (as-received), 5, 10, and 15% pre-strained sheets decreases by

approximately 21, 23, 24.5, and 25.4% after immersion in 3.5%

sodium chloride solution for 40 days. Thus, the severity of

deterioration in LBC due to immersion in 3.5% sodium chloride

solution gets marginally enhanced with increasing pre-strain of

the sheets. Contrary to this observation, for dissimilar stainless

steel–non-stainless steel joints (EN1.4318 2H—ZStE260BH),

Alenius et al. [3] have reported that the LBC of specimens exposed

to 3.5% sodium chloride solution does not change significantly

even after 90 days of immersion. The EML of the spot-welds are

found to increase with increased duration of immersion; this can

be explained by increased extension of the base metal region

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Figure 8. Variation of failure load with immersion duration and pre-

strain of base metals: (a) load–displacements curves of the spot-welds

of as-received sheets for different duration of immersion, (b) variation

of failure load with immersion duration for the spot-welds of as-

received (0%) and pre-strained (5, 10, and 15%) sheets

Figure 9. (i) Cross-section of the failed spot-weld (a) nugget without

immersion, (b) nugget shows reduction in thickness after 40 days of

immersion. (ii) Failed spot-weld at the location of failure (c) nugget

does not exhibit any secondary cracking when tested without

immersion in 3.5% sodium chloride aqueous solution, (d) nugget tested

after immersion shows corrosion and secondary cracks indicated by

arrows near fracture surface. (iii) Fracture surface exhibits (e) dimples

when tested without immersion, (f) stretched surface embedded with

small voids and corrosion product when tested after immersion

which gets reduced in thickness due to corrosion with increased

duration of immersion. The spot-welds in the present study are

usually found to fail from the interface of HAZ–base metal

regime, except in samples immersed for 40 days in which failure

is observed to initiate at the base metal.

Representative cross-sectional views of the failed spot-welds

without and with immersion in 3.5% sodium chloride solution

are shown in Fig. 9a and b, respectively. Spot-welds subjected to

immersion in the aggressive solution exhibit reduction in

thickness of the weld joint and the conjugate sheets due to

corrosion as illustrated in Fig. 9a and b. The reduction in

thickness is found to increase with increased duration of

immersion. The spot-welds exhibit 11% reduction in thickness

after 40 days of immersion. Representative macrofractographs of

the failed nuggets are shown in Fig. 9c and d. Numerous

secondary cracks are observed at the corroded interface around

the weld nugget tested after immersion in the solution (Fig. 9d)

unlike that in specimens tested without immersion in 3.5%

sodium chloride solution (Fig. 9c). Thus, the decrease in LBC of

the immersed spot-welds can be explained by the reduction in

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thickness of the weld joints as well as relatively easy growth of

the secondary cracks formed due to corrosion. A comparative

examination of the broken fractured surfaces has been made

using representative microfractographs as shown in Fig. 9e and f.

This examination reveals large dimples (Fig. 9e) on the fractured

surfaces of the spot-weld tested without immersion unlike that for

the immersed specimens (Fig. 9f). Spot-weld broken after

immersion in the aggressive solution exhibits small dimples

associated with nearly smooth surface with ripples formed due to

stretching (Fig. 9f). Similar type of fracture mechanism has been

described by Brooks and Choudhury [15] in stainless steel.

3.5 Slow strain rate tensile test in 3.5% sodium chloride

solution

Load–extension plots for the spot-welds of the as-received sheets

tested in air and in 3.5% sodium chloride solution at different

extension rates are shown in Fig. 10a. The LBC of spot-welds in

air as well as in 3.5% sodium chloride solution decreases with

decreasing extension rate (Fig. 10b). The trend of the results in

Fig. 10b is in agreement with the conventional deformation

behavior of metallic materials. In addition, these results also

illustrate the difference between LBC of the welded joints in air

and in 3.5% sodium chloride solution. The tensile properties like



Figure 10. Variation of load carrying capacity in air and in 3.5% sodium

chloride solution at different extension rate: (a) load–extension plots at

various extension rates, (b) variation of load carrying capacity in air and

in 3.5% solution

LBC and EML of the spot-welds are found to degrade marginally

in 3.5% sodium chloride solution compared to that tested in air.

Similar observations have beenmade byAlenius et al. [3] who havereported 9% decrease in LBC for the spot-welds of EN 1.4318

steels in 3.5% sodium chloride solution at an extension rate of

6� 10�6mm/s. The degradation of tensile properties in 3.5%

sodium chloride solution can be explained by the phenomenon of

easy crack initiation at the interface around the weld nugget due

to exposure in the corrosive media under SSRT. The degradation

of the tensile properties of spot-welds gets pronounced with

decreasing extension rate due to increasing exposure time in the

corrosive media. For example, the reduction in LBC increases

from 4 to 9%when the extension rate is changed from 1� 10�4 to

3.5� 10�5mm/s. In contrast to the expected variation of LBC

with extension rate, EML is also found to decrease with

decreasing extension rate; for standard tensile test of metallic

materials, ductility usually increases with decreasing extension

rate. The EML has been referred earlier [13] as ductility of spot-

welds. But this is an apparent observation. The extension of the

spot-weld is dictated by the rotation and deflection of the welded

joint as well as by the stretching of sheet metal around the nugget.

The extension of the sheet metal is expected to increase with

decreasing extension rate while the extension due to rotation and

deflection is considered to have decreased with decreasing


extension rate. The overall balance has resulted in a decrease in

EML of weld-joint. Post-failure examinations reveal that spot-

welds fail from the interface of HAZ–basemetal in both air and in

3.5% sodium chloride solution under SSRT, similar to the

illustration shown in Fig. 9a.

3.6 Slow strain rate tests under cathodic hydrogen

charging

Typical load–extension plots obtained during SSRT under in situ

hydrogen charging of the pre-charged spot-welds are shown in

Fig. 11a. The LBC of the spot-welds increases with increasing

amount of pre-strain of the steel sheets, as discussed in Section

3.3. The deformation behavior of the spot-welds considerably

alters with hydrogen charging; the nature and the extent of

alterations due to hydrogen charging are different for spot-welds

on as-received and pre-strained sheets. The LBC of the spot-welds

on as-received sheets remains almost unchanged and interest-

ingly, the EML improves with hydrogen charging where as both of

these properties for pre-strained sheets degrade with hydrogen

charging (Fig. 11b and c). For example, the LBC and EML

decrease by 4 and 25%, respectively, due to hydrogen charging for

the spot-welds on 15% pre-strained sheets. The deterioration in

LBC and EML for pre-strained sheets is found to get enhanced

with increased amount of pre-strain. Liu and Wu [16] have

observed similar behavior during tensile tests of annealed and

cold-worked IF steels where hydrogen pre-charged annealed

specimens showed improvement in ductility without any change

in strength, but cold-worked specimens exhibited deterioration of

these properties with hydrogen charging.

The effect of increasing pre-strain on the hydrogen

susceptibility of spot-welds has shown an expected behavior.

Cathodic hydrogen charging produces high input fugacity of

hydrogen [17]. Hydrogen entrapment in a deformed material is

usually significantly higher than that in an un-deformed material

[18], and as a consequence critical concentration of hydrogen

required for hydrogen damage is easily attainable in pre-strained

sheets. This can be simply explained by increased hydrogen

trapping sites resulting from increased dislocation density and

vacancies [18–20] in deformed specimens. Numerous theories

have been proposed for explaining hydrogen damage phenom-

enon, e.g., pressure theory, decohesion theory, dislocation theory,

etc. [21–23]. Local accumulation of hydrogen at microstructural

heterogeneities is considered to trigger crack nucleation [24–26].

The higher the pre-strain, the higher is the microstructural

heterogeneity for hydrogen accumulation, and thus higher is the

damaging effect.

The improvement of EML with hydrogen charging of the

spot-welds of the as-received sheets can be explained following

Beachem [27], Birnbaum [28], Liu and Wu [16]. Beachem [27] and

Birnbaum [28] have suggested that presence of hydrogen in an

annealed steel considerably enhances dislocation mobility, while

Liu and Wu [16] have proposed that increased ductility of

hydrogen-charged annealed steels is due to increased mobility of

dislocations caused by the formation of tiny titanium hydride in

the matrix. Post-failure examinations (Fig. 12) of the hydrogen-

charged spot-welds of pre-strained sheets exhibit negligible

necking (Fig. 12a) at the location of failure, and fractography

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Figure 11. Variation of load carrying capacity and extension up to

maximum load (EML) of the spot-welds of as-received and pre-strained

sheets under in situ hydrogen charging: (a) load–extension curves,

(b) comparison of failure loads with and without hydrogen charging,

and (c) variation of extension up to maximum load (EML) with and

without hydrogen charging

Figure 12. SEM micrographs of fracture surface of the spot-welds

tested (i) with hydrogen charging: (a) macroview of the failed weld-

nugget, (b) overview of the fracture surface, (c) magnified view of the

fracture surface, and (ii) without hydrogen charging (d) fractography of

the spot-weld tested in air showing elongated dimples associated with

serpentine glide

reveals predominantly planar quasi-cleavage type fracture

(Fig. 12b and c) indicating relative brittleness unlike specimens

fractured without hydrogen charging; in the latter, pronounced

necking is observed at the location of failure and the fracture

surface exhibits elongated dimples associated with serpentine

glide indicating ductile fracture. The observed features on

the fractured specimens of spot-welds with hydrogen charging,

on the other hand, appear to support the de-cohesionmechanism.

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4 Conclusions

Spot-welded joints of TS configuration have been made on as-

received and pre-strained sheets of an IF steel to determine their

deformation behavior in 3.5% sodium chloride solution. The

effect of 3.5% sodium chloride solution on the strength of spot-

welded joints has been determined under three different test

conditions: (i) after immersing the samples in the solution for

various duration of time, e.g., 10, 20, and 40 days followed by

tensile tests, (ii) tests at various extension rates by keeping the

samples in solution, and (iii) tests at slow strain rate under in situ

hydrogen charging. Post-failure examinations have been carried

out to understand the failure mechanism. Analyses of the results

lead to the following major conclusions:

(i) T
he LBC decreases while EML, increases with increased
duration of immersion of the spot-welded joints in 3.5%

sodium chloride solution. The decrease in LBC of the joints

has been explained by the reduction in their thickness and

easy crack initiation due to the attack of corrosive

environment. The increase in EML of the spot-welds has

been explained in terms of increased extension of the base

metal part, which is found to be reduced in thickness by

corrosion with increased duration of immersion.

(ii) T
he LBC and EML of the spot-welds in pre-strained sheets
considerably deteriorate with hydrogen absorption while

LBC remains almost unchanged and EML is found to

improve for as-received sheets. This phenomenon can be

explained by higher entrapment of hydrogen in the spot-

welded joints of pre-strained sheets due to their increased

dislocation density and vacancy concentration.

(iii) T
he influence of corrosive environment in deteriorating LBC
and EML of the spot-welds of pre-strained sheets increases

with increasing pre-strain.

(iv) T
he LBC and EML of the spot-welds under SSRT in 3.5%
sodium chloride solution decrease marginally. The influence



� 20

of the corrosive media gets pronounced with decreasing

strain rate; this is due to higher duration of exposure in the

solution at slower strain rate.

Acknowledgements: The authors would like to thank Tata Steel

Ltd., Jamshedpur, India for their financial support to this work via

grant no. R&D-SS/PROD-SM/IN-02/45/09. One of the authors

(G.M.) would also like to thank Tata Steel Ltd. for sponsoring his

graduate studies at the Indian Institute of Technology, Kharagpur.

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(Received: June 14, 2009)

(Accepted: July 6, 2009)

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Strength of spot-welded steel sheets in corrosive environment

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