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Phase transformations and micro cracks induced by hydrogenin cold-rolled and annealed AISI 304 stainless steels

Yongfeng Li a,b, Limin Zhao c, Wenbin Kan a, Hongliang Pan a,*aSchool of Mechanical and Power Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, Chinab School of Mechanical and Electrical Engineering, Henan Institute of Science and Technology, East of HuaLan Road, Xinxiang 453003, ChinacManagement Institute, Xinxiang Medical University, East of JinSui Road, Xinxiang 453003, China

a r t i c l e i n f o

Article history:

Received 1 December 2011

Received in revised form

10 February 2012

Accepted 16 February 2012

Available online 8 March 2012

Keywords:

Phase transformation

Surface cracking

Hydrogen charging

Martensite

* Corresponding author. Tel./fax: þ86 21 6425E-mail address: hirampan@yeah.net (H. P

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.02.083

a b s t r a c t

The effect of hydrogen and stress on the phase transformation and surface cracking in

cold-rolled and annealed AISI 304 stainless steels was investigated. Hydrogen was intro-

duced by cathodic charging in 0.1 M H2SO4 þ 1 g/L Na2HAsO4$7H2O. Scanning electron

microscopy was used to observe structural changes and surface cracks of the specimens.

X-ray diffraction analysis was used to investigate phase transformations in the uncharged

and charged specimens immediately after charging or during aging. Cathodic charging can

cause ε martensite phase transformation and aging after cathodic charging can induce the

ε and a0 martensite transformation. There are a certain amount of surface cracks on the

specimens surface after hydrogen charging or during aging. The surface morphology of the

specimens is studied, and the mechanism of the hydrogen-induced phase transformation

and the surface cracking formation in the charged specimens immediately after charging

or during aging is discussed.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction surface cracking formation are still unknown. Numerous X-

It is well known that the cathodic electrolytic charging of

austenitic stainless steel with the hydrogen that evolves from

acid solutions containing promoters brings about the forma-

tion of numerous surface micro cracks [1e3]. Additionally, ε

and a0 martensites are formed under these conditions [3e6].

Previous studies [7e12] have shown that hydrogen-induced

martensite can play an important role in pitting corrosion,

surface cracking and stress corrosion cracking propagation.

Hydrogen-induced surface cracks are both grain-oriented

transcrystalline and aligned along the grain boundaries [13].

However, the detailed nature and mechanisms of the

hydrogen-induced martensite transformations and the

3622.an).2012, Hydrogen Energy P

ray diffraction studies have confirmed that cathodic hydrogen

charging can lead to a phase transformation [5,7,14,15].

However, no detailed mechanistic or morphological studies

are available for both cold-rolled and annealed AISI 304

stainless steels.

The purpose of this study is to investigate the effect of

cathodic hydrogen charging and aging after cathodic

hydrogen charging on phase transformation in both cold-

rolled and annealed AISI 304 stainless steels, and the effect

of hydrogen charging and aging on surface cracking. We aim

to further expand the hydrogen-induced phase trans-

formation mechanism and the surface crack formation

mechanism.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 7 2 4e8 7 2 8 8725

2. Experiment procedure

2.1. Material

Experiments were performed on commercially available AISI

304 austenitic stainless steel. The specimens for this study

were 15 mm � 30 mm cold-rolled foil with a thickness of

0.2 mm. Specimens were divided into two groups, one was to

maintain cold-rolled state, that is, which was as received; the

other was annealed in a vacuum heat-treatment furnace at

a temperature of 1323K for 2 h. These two types of specimens

were used for the following experiments as the initial state.

The chemical composition of the as received AISI 304 stainless

steels is listed in Table 1.

2.2. Cathodic hydrogen charging

Prior to every cathodic hydrogen charging experiment, the

specimen was rinsed with distilled water and ultrasonically

cleaned in acetone. It was then immersed in an acid pickling

solution (H2SO4: 350 g/L, HF: 110 g/L, H2O: balance) for 3 min

before being immersed in a chemical polishing solution

(H3PO4: 25 wt%, HNO3: 6.5 wt%, H2O: 68.5 wt%, C3H8O3 7.5ml/L,

CH4N2S: 2 g/L, C7H6O6S$2H2O: 1 g/L) at 80e90 �C for a few

minutes to remove the surface oxide layer of Cr2O3, etc. and

activate the surface. The specimen surfaces were rinsed with

distilled water and ultrasonically cleaned in acetone again

and dried quickly by warm air blowing.

Hydrogen was introduced into specimens by the cathodic

charging. Cathodic charging was carried out at a temperature

of 300K with a platinum anode in 0.1 M H2SO4 solution con-

taining 1 g/L Na2HAsO4$7H2O as a hydrogen recombination

poison. The current density was a single charging current

density of 150mA/cm2 and experiments were carried out over

different periods of time ranging from 30 min to 96 h. Some of

the charged specimens were then aged at room temperature

for 20 days.

2.3. The examination of surface cracks and phasetransformations

In order to observe the structural changes and surface cracks

of the specimens, scanning electron microscope was used. X-

ray diffraction analyses were used to investigate the phase

transformation in the uncharged and charged specimens

immediately after charging or during aging. X-ray diffraction

studies were performed using a Rigaku D/max2550VB/PC X-

ray diffractometer with Cu Ka radiation at 40 kV and 100 mA.

Measurements were made from 40� � 2q � 100� in steps of

Table 1 e Chemical composition of AISI 304 stainlesssteel, wt%.

Element C Mn Si Cr Ni

Percentage (%) 0.072 1.43 0.57 18.24 8.06

Element Mo Cu S P Fe

Percentage (%) 0.16 0.07 0.0088 0.048 Balance

0.02� at a scan rate of 0.12 s/step. On the basis of Bragg’s law

and the crystallographic relationships between lattice

parameters and interplanar spacing, d, the lattice parameters

of the phases were determined.

3. Results and discussion

3.1. Effect of cathodic hydrogen charging on phasetransformation

After cathodic hydrogen charging, the annealed specimens

were immediately analyzed by X-ray diffraction. Fig. 1 shows

the effect of cathodic hydrogen charging on the phase trans-

formation. The specimens were charged at 150 mA/m2 for

various lengths of time. From the X-ray diffraction analyses,

only the austenite phase, g, (face-centered cubic (fcc) struc-

ture) was present in the uncharged specimen and the average

lattice constant, a, was 3.5851A, which was close to the

reported parameter (a ¼ 3.593 A) [3]. For the charged speci-

mens a new reflection was found. It was indexed as belonging

to an εmartensite phase, which has a close-packed hexagonal

structure. For the specimens charged ranging from 0.5 h to

96 h, the lattice parameter of g austenite phase and ε

martensite phase varied with the different hydrogen charging

time.When the specimenswere charged for 0.5, 1, 24 and 96 h,

the average lattice constant (a) of g austenite phase which

measured in this work was 3.5884, 3.5910, 3.5958 and 3.5972 A,

respectively, and the lattice parameters of ε martensite phase

were a ¼ 2.5655 A and c ¼ 4.0743 A, a ¼ 2.5696 A and

c ¼ 4.0691 A, a ¼ 2.5720 A and c ¼ 4.0615 A, a ¼ 2.5832 A and

c ¼ 4.0813 A, respectively. Peak broadening was different

between the original peaks associated with the uncharged

specimen and the subsequent peak of the charged specimen

while the peak position changed slightly. This phenomenon

may be related to the entry of hydrogen into the specimens by

cathodic charging, which severely strains the metal lattice

resulting in compressive stress at the surface and specimen

Fig. 1 e X-ray diffraction spectra of the annealed AISI 304

stainless steel taken immediately after cathodic charging

at 150 mA/cm2 at different charging times.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 7 2 4e8 7 2 88726

warping. The twist and deformation of the specimen increase

with the amount of hydrogen charging and the amount of

hydrogen charging increases with an increase in the charging

current density or charging time. All the peaks, therefore, shift

to slightly lower angles and broaden considerably. Further-

more, the amount of ε phase increased with an increase in

charging time. However, no reflections that correspond to the

a0 phase for the sample was observed immediately after the

cathodic charging of the specimens. As thematrix of the cold-

rolled 304 stainless steels already contained a certain amount

of a0 martensite phase it is difficult to determine if the

a0 martensite phasewas induced by cold-rolling or by cathodic

hydrogen charging. Therefore, the cold-rolled specimenswere

not analyzed by X-ray diffraction immediately after cathodic

hydrogen charging.

3.2. Effect of aging after cathodic hydrogen charging onphase transformation

X-ray diffraction patterns of the uncharged and the charged

specimens that were aged at room temperature for 20 days are

shown in Fig. 2. During aging after the cathodic hydrogen

Fig. 2 e X-ray diffraction patterns of the uncharged and the

charged specimens aged at room temperature for 20 days.

(a) cold-rolled specimens; (b) annealed specimens.

charging, a large amount of hydrogen escaped from the

specimens and this resulted in significant structural changes.

The X-ray diffraction patterns reveal that new reflections are

present and these reflections can be indexed as belonging to

a0 and ε martensite phases whereas no a0 phase was observed

immediately after charging. From the X-ray diffraction

patterns of cold-rolled specimens in Fig. 2a, the g and a0 phaseexist in all of the specimens, and when the specimens were

charged for 0, 0.5, 1, 24 and 96 h, the average lattice constant

(a) of g austenite phase which measured in this work was

3.5786, 3.5877, 3.5900, 3.5981 and 3.6013 A, respectively, and

the average lattice constant (a) of a0 phase was 2.8621, 2.8633,

2.8641, 2.8658 and 2.8662 A, respectively. When the charging

time is 24 and 96 h, a new reflection was found. It was indexed

as belonging to an ε martensite phase, and the lattice

parameters of ε martensite phase were a ¼ 2.5800 A and

c ¼ 4.0864 A, a ¼ 2.5825 A and c ¼ 4.0918 A, respectively. In

Fig. 2b, when the specimens were charged for 0, 0.5, 1, 24 and

96 h, the average lattice constant (a) of g austenite phase

whichmeasured in this workwas 3.5767, 3.5779, 3.5786, 3.5796

and 3.5825 A, respectively, and the average lattice constant (a)

of a0 phase was 2.8872, 2.9028, 2.9331, 2.9382 and 2.9513 A,

respectively. When the charging time is 1, 24 and 96 h, an ε

martensite phase was found, and the lattice parameters of ε

martensite phase were a ¼ 2.5236A and c ¼ 4.1845 A,

a ¼ 2.5249A and c ¼ 4.200 A, a ¼ 2.5262 A and c ¼ 4.2113 A,

respectively.The uptake of hydrogen caused the expansion of

the lattice in the specimen surface layer during cathodic

hydrogen charging and this resulted in a compressive stress

on the surface [14]. During aging, hydrogen escaped from the

external surface, the hydrogen concentration at the surface is

quickly reduced to low values, redistribution of hydrogen

caused a high tensile stress at the surface and compressive

stress beneath the surface, and promoted the formation of

a0 martensite phase [14,16,17]. The intensity and amount of

a0 and ε phase increased with charging time upon aging. A

comparison between Fig. 2a and b shows that much less ε

phase is formed in the charged cold-rolled specimens than in

the charged annealed specimens. Holzworth and Louthan [6]

suggested that the cold working of 304 stainless steel before

cathodic hydrogen charging reduces the extent of the g -to- ε

transformation, which occurs upon charging.

3.3. Effects of hydrogen charging and aging on surfacecracking

The surface morphologies of the 150 mA/m2 cathodically

charged specimens were observed by scanning electron

microscopy. The charging times of the specimens were 1 h

and 96 h while the aging time was 20 days. An examination of

the specimens surface by scanning electron microscopy

showed that as hydrogen escaped from the surface during

room temperature aging, cracking occurred on the specimens

surface. The cracks are shown in Fig. 3. No cracking was

observed in the annealed specimen uncharged, as shown in

Fig. 3a, b and c show that there is a certain amount of cracks

on the annealed specimens surface charged for 1 h and 96 h.

And when the cold-rolled specimens were charged for 96 h,

some very broad cracks appeared on the specimens surface,

as shown in Fig. 3d. Both specimens behaved in a similar

Fig. 3 e Surface cracking observed during aging after the specimen was charged at 150 mA/cm2 for different charging times.

(a) annealed specimen uncharged; (b) annealed specimen charged for 1 h; (c) annealed specimen charged for 96 h; (d) cold-

rolled specimen charged for 96 h.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 8 7 2 4e8 7 2 8 8727

manner in that the existing cracks propagated in the length

and depth directions with charging time and, additionally,

more cracks appeared on the specimens surface. Therefore,

the crack density, the average crack depth and the crack

length increased with charging time. When the specimens

were charged for a certain time, there was high hydrogen

concentration at the surface. This will lead to the volume

expansion, and consequently will cause high compressive

stresses at the surface. The high stresses can cause the

surface deformation [17]. During outgassing stage, hydrogen

mainly escaped from the external surface, the hydrogen

concentration quickly became lower at the surface and then

decreased at greater depthswhich caused a high tensile stress

at the surface. While there was a high compressive stress

beneath the surface. As the formation of a0 martensite is

accompanied with volume expansion [3], the appearance of

the tensile stress and lack of constraint at the surface will

promote the a0 martensite and consequently surface cracking

during outgassing. Q Yang et al. [2] carried out an in situ

observation of surface cracking process during aging, they

suggested that the crack formation and growth might be

related to the hydrogen outgassing process.

4. Conclusions

Cathodic hydrogen charging can lead to ε martensite trans-

formation and aging after cathodic hydrogen charging can

induce the ε and a0 martensite phase transformation, and the

amount of both the a’ and ε martensite phases produced

increases with an increase in charging time, whilemuch less ε

phase is formed in the charged cold-rolled specimens than in

the charged annealed specimens. Additionally, surface

cracking appears on the specimens surface after hydrogen

charging or during aging, and the cracks propagate in the

length and depth directions with charging time. Permeated

hydrogen strains the metal lattice, leading to a change (a shift

to slightly lower angles) in peak position and considerable

increase in broadening of all peaks. Hydrogen charging leads

to the volume expansion and consequently high compressive

stresses beneath the surface. Hydrogen outgassing causes

a high tensile stresses at the surface. High compressive

stresses during charging and tensile surface stresses during

aging promote the formation of surface cracking.

Acknowledgments

The authors gratefully acknowledge the National Science

Council of China for financial support of this research under

Contract No. 50875084.

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