Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels

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Influence of machining parameters on surface roughness and susceptibility to hydrogen embrittlement of austenitic stainless steels Lucas Renato Queiroga a, Gisele Fernanda Marcolino a, Matheus Santos a, Gabriele Rodrigues a, Carlos Eduardo dos Santos b, Pedro Brito a,* Pontifical Catholic University of Minas Gerais, Department of Mechanical Engineering, Av. Dom Jose Gaspar 500, 30535-901, Belo Horizonte, (MG), Brazil b Federal Center for Technological Education of Minas Gerais, Av. Amazonas 5253, 30421-169, Belo Horizonte, (MG), Brazil a

highlights  Surface roughness and microstructure were varied by machining conditions.  Hydrogen embrittlement of stable/metastable austenitic stainless steels was compared.  Presence of surface martensite and microstrains induced hydrogen embrittlement.

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abstract

Article history:

In the present work, an investigation on the susceptibility to hydrogen embrittlement of

Received 16 August 2019

AISI 304 and 310 austenitic stainless steels was performed. The hydrogen embrittlement

Accepted 16 September 2019

process leads to degradation of mechanical properties and can be accelerated by the

Available online 10 October 2019

presence of surface defects combined with elevated surface hardness. Tensile test specimens of the selected materials were machined by turning with different cutting parame-

Keywords:

ters in order to create variations in surface finish conditions. The samples thus prepared

Hydrogen embrittlement

were submitted to tensile tests before and after hydrogen permeation by cathodic charging.

Strainless steels

Regarding the AISI 304 steel, it was possible to notice that the presence of strain-induced

Mechanical properties

martensite on the material surface led to severe hydrogen embrittlement. In the case of

Surface roughness

the AISI 310 steel, due to its higher nickel amount, no martensite formation could be

Turning

detected, and this steel was found to be less susceptible to embrittlement in the tested conditions. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Austenitic stainless steels are FeeCreNi alloys employed in a variety of industries (including, among others, food

processing, petrochemical, nuclear, thermo-electric and automotive sectors) because of their elevated ductility, mechanical strength and excellent corrosion resistance. Recently, these steels have also been considered for hydrogen

* Corresponding author. E-mail address: [email protected] (P. Brito). https://doi.org/10.1016/j.ijhydene.2019.09.139 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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gas storage pressure vessels for mobile applications. In the presence of high pressure hydrogen gas or liquid environments containing hydrogen, austenitic stainless steels are known to deteriorate [1e4]. Machining processes are commonly applied in the industry for manufacturing of a large array of components. In these processes, the geometry of the final product is obtained by controlled material removal by using an appropriate cutting tool that produces shear stresses on the metal surface. Depending on the cutting parameters, the machined surface may exhibit varying levels of roughness, residual stresses and plastic deformation. In addition, in the case of metastable austenitic stainless steels, the strain induced transformation of austenite to martensite can take place. In this context, a number or studies have been published with the objective of elucidating the influence of machining-induced microstructure modifications on electrochemical processes, with emphasis on corrosion behavior [5e9]. One factor which has received relatively less attention from researchers is the influence of the machining process on the susceptibility to hydrogen embrittlement, which is an essential characteristic to be considered when commissioning stainless steels in hydrogen containing media (gaseous or liquid). The few existing reports [10,11] have focused on metastable austenitic stainless steels, with low Ni-content and did not investigate variations in cutting parameters. The subject is, however, considered relevant given the widespread use of metal cutting operations and the fact that a number of microstructure characteristics altered by machining have an influence on hydrogen embrittlement of austenitic stainless steels, such as: presence of straininduced martensite [12] and pre-existing plastic strain [13,14]. Recently, crystallographic orientation was also found to play a role on hydrogen evolution in a 304 austenitic stainless steel [15]. In the present study, AISI 304 and AISI 310 austenitic stainless steels were submitted to turning operations under different cutting conditions, exposed to hydrogen by cathodic charging and submitted to mechanical testing. The objective was to explore the influence of the machining parameters on the sub-surface microstructure and on the eventual changes in the materials susceptibility to hydrogen embrittlement.

Sample preparation The machining operations were performed using a CNC turning machine. Cutting parameters were determined corresponding to “rough” (1400 rpm, 0.4 mm/rotation) and “fine” (3000 rpm, 0.1 mm/rotation) surface finishes. This resulted in the surface roughness values (average surface roughness, Ra) which are presented in Table 2. Part of the samples were further submitted to cathodic charging in a 0.5M H2SO4 solution (containing naturally dissolved O2) with 0.25 mg/l NaAsO2. The cathodic current was controlled at 100 mA/cm2 for an exposure time of 24 h. The materials were exposed at the center of the tensile test specimen gauge length.

Test procedures Samples of both stainless steels before hydrogen permeation were also submitted to examination by X-ray Diffraction (XRD) to evaluate whether the machining process led to partial transformation of austenite on the material surface. The level of surface microstrains was also analyzed by employing the Williamson-Hall (WH) method [16]. The XRD analyses were performed in a Shimadzu XRD7000 difractometer operating with Cu Ka radiation at 40 kV and 30 mA. The diffraction angle was varied between 30 and 100 in 0.02 steps. Instrument broadening for the WH analysis was determined by analyzing a Si powder standard. Tensile tests were performed on specimens before and after hydrogen permeation, for all surface finishing conditions. The tensile tests were conducted, in all cases, at a strain rate of 0.003 s1. The main purpose of these examinations was to compare the susceptibility to Hydrogen Embrittlement (HE), expressed quantitively by [17]: HE ¼

l0  lH l0

where l0 and lH are, respectively, the maximum elongation before and after cathodic charging. The stress-strain data were also analyzed in order to obtain information regarding the materials strain-hardening behavior, as modeled by Hollomon’s equation: st ¼ Kεnt

Experimental procedure Materials The materials used in present work were AISI 304 and AISI 310 austenitic stainless steels, received in the form of circular bars which were used for preparing tensile test specimens following the ASTM E8 standard (6 mm gauge diameter and 30 mm gauge length). The chemical composition of the materials is presented in Table 1, along with the Nieq value for each material. Before performing any experimental procedure, the received materials were inspected for surface defects by applying liquid penetrant testing following ASME Section VIII standard.

(1)

(2)

where st and εt are the true stress and strain, respectively, K is the strength coefficient and n is the strain-hardening exponent.

Results Microstructure analysis In order to verify whether the machining process led to straininduced transformation of austenite into martensite on the surface (or sub-surface) regions of the AISI 304 and 310 stainless steel specimens, XRD analyses were performed and the results are presented in Fig. 1(a) and (b), respectively.

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Table 1 e Chemical composition of the AISI 304 and AISI 310 steel used in the present investigation (values provided in weight %). Material 304 310

C

Si

Mn

P

S

Cr

Mo

Ni

Fe

Nieq

0.019 0.072

0.38 0.62

0.95 1.85

0.045 0.025

0.03 0.004

18.1 24.5

0.29 0.49

9.0 19.1

Bal. Bal.

10.05 22.19

Table 2 e Surface roughness (Ra) values obtained on tensile test specimen surfaces. Material 304 310

Fine

Rough

0.178 mm 0.421 mm

15.950 mm 28.864 mm

Concerning the AISI 304 steel, it is possible to notice the presence of martensite for the fine surface finish condition, in agreement with the work by Martin et al. [10]. The reflections corresponding to martensite were found to be compatible with the body-centered cubic form, identified as a’ in Fig. 1(a) e no ε-martensite reflections (hexagonal close-packed crystal structure) could be detected. The presence of martensite could be confirmed by examining the cross section of the AISI 304 steel, as shown in Fig. 2. Vickers microhardness measurements performed close to the surface (approximately 25 mm in depth) revealed an increase in hardness of close to 100 HV in comparison to the interior of the material. The surface hardness of approximately 350 HV is compatible with a dual phase martensite/austenite microstructure. In contrast, for the rough surface finish condition only austenite reflections could be detected, indicating that despite the faster material removal rate in the machining process, no martensite was present on the sample surface (which does not exclude the possibility of martensite formation in the chips removed from the bulk material in the machining process). The microstructure of the AISI 310 steel was in turn found to be more stable regarding the transformation of austenite, since no peaks indicating the presence of other phases could be identified. The absence of martensite in the AISI 310 can be attributed to its higher Ni content (a strong austenite stabilizer), in comparison to the AISI 304 steel (Table 1) [18]. Further XRD analysis is presented in Fig. 3, in the form of WH plots for the tested steels. In these diagrams, the vertical

Fig. 2 e Optical microscopy examination of the surface region in a cross-section sample of the AISI 304 steel machined with fine surface finish (Etchant: Aqua Regia, 10 s).

axis, related to the integral breadth of the diffraction peaks (bexp) minus the term due to instrument broadening (binst), is plotted against 4sinq (q being half the diffraction angle). The variation in integral breadth of the XRD lines is related to the presence of crystalline defects, since localized lattice distortions increase scattering of the diffracted X-rays. The analysis was conducted on the austenite diffraction lines and represent thus only characteristics of that crystalline phase. Ideally, a linear relation is observed and the resulting angular coefficient (m) is proportional to the dislocation density in the materials structure [19]. It is useful to compare the angular coefficient values obtained for each sample analyzed in Fig. 2: 0.18 and 0.45 for the AISI 304 steel with fine and rough surface

Fig. 1 e Qualitative phase analysis by XRD performed on: (a) AISI 304 and (b) AISI 310 austenitic stainless steels.

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Fig. 3 e Williamson-Hall plots for the: (a) AISI 304 and (b) 310 austenitic stainless steels. finish, respectively, and 0.24 and 0.44 for the AISI 310 steel with fine and rough surface finish, respectively. For both steels, it was possible to notice a positive (but non-linear) relation between surface roughness and microstrains (related to the level sub-surface plastic deformation).

Mechanical behavior A summary of the results obtained is presented in Table 3. The average standard deviation registered for Yield Strength (YS), Ultimate Tensile Strength (UTS) and maximum elongation were, respectively, 42.5 MPa, 22.5 MPa, 3.25%. The mechanical behavior of AISI 304 stainless steel before and after hydrogen permeation is presented in Fig. 4(aed). The engineering stressstrain diagrams are registered in Fig. 4(a) and (c) for fine and rough surface finishes, respectively. The linearized true stressstrain data used for obtaining the strain-hardening coefficient are analyzed in Fig. 4(b) and (d) for the fine and rough surface finish conditions, respectively. The mechanical behavior of the AISI 310 stainless steel specimens, presented in a similar structure, are analyzed in Fig. 5(aed). The results registered in Table were used for determining the susceptibility to hydrogen embrittlement (Equation (1)), as shown in Fig. 6. For the AISI 304 steel, the hydrogen embrittlement was determined to be 25.77 and 18.39% for the fine and rough surface finish conditions, while for the AISI 310 steel, hydrogen embrittlement was of 5.88 and 18.75% for the fine and rough surface finish conditions.

Table 3 e Summary of tensile properties obtained for the AISI 304 and 310 steels before and after hydrogen charging (þH). Steel

Condition

YS (MPa)

UTS (MPa)

Max. Elongation (%)

n

AISI 304

Fine Rough Fine þ H Rough þ H Fine Rough Fine þ H Rough þ H

490 520 420 480 550 600 660 620

700 690 620 640 620 640 710 660

92.0 87.0 72.0 71.0 34.0 33.0 32.0 26.0

0.36 0.36 0.39 0.34 0.13 0.14 0.14 0.14

AISI 310

By analyzing Fig. 4(a) and (c), it is possible to notice a decrease in mechanical strength and ductility for the AISI 304 steel, for both surface roughness conditions (Table 3). On the other hand, the results presented in Fig. 4(b) and (d) did not reveal significant differences in the strain-hardening behavior with the presence of hydrogen, for either surface roughness. This observation is consistent with the results obtained by Ji et al. [20] who noticed that, for pre-strained specimens, the presence of hydrogen in austenitic stainless steel led to small changes in tensile strength and strain-hardening rate (ds/dε) but did cause a noticeable decrease elongation. Regarding the influence of hydrogen on the mechanical behavior of the AISI 310 steel, analyzed in Fig. 5, it is possible to notice that, for low surface roughness values and the corresponding reduced level of lattice strain (Fig. 3) practically no hydrogen embrittlement could be noticed for the hydrogen charging conditions employed: the 5.88% reduction in maximum elongation (Fig. 6) is close to the standard deviation for this parameter (3.25%). However, for increasing levels of surface roughness and lattice strain, significant hydrogen embrittlement could be noticed (18.75%). No significant influence of the presence of hydrogen on the strain-hardening behavior was registered, as observed for the AISI 304 steel.

Discussion The AISI 304 and 310 steels examined in the present work revealed different behaviors concerning the surface modification by machining. Even though the XRD data suggested similar amounts of lattice strain in the austenite phase present in both steels after the turning operations for the same sets of cutting conditions, the AISI 304 steel exhibited systematically lower surface roughness values relative to the AISI 310 steel, an indication of superior machinability. Considering the results presented in Fig. 3, it is possible to notice that for the fine surface roughness conditions relatively low levels of defects were to be found on the surface of either steel, and that the defect density increased for the rougher surface finish. The main difference between both steels was that for the AISI 304 steel, strain-induced transformation of austenite to martensite was noted on the material surface after turning.

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Fig. 4 e Results of tensile tests performed on AISI 304 stainless steel samples before and after hydrogen charging: (a) stressstrain diagram (fine finish), (b) linearized true stress-strain diagram (fine finish), (c) stress-strain diagram (rough finish) and (d) linearized true stress-strain diagram (rough finish).

Thus, if the surface microstructure features alone are considered, it is possible to separate the specimens examined in the present work into the following categories: austenite with low lattice strain (AISI 310 steel, fine finish), austenite with the presence of lattice strains (AISI 304 and 310 steels, rough finish) and martensite (AISI 304, fine finish). The presence of a strain-hardened layer and martensite bear significant influence on the role of hydrogen in the microstructure of austenitic stainless steel. The increased dislocation density associated with lattice strains constitutes trapping sites for hydrogen atoms, increasing their equilibrium concentration in the metal. Alternatively, even though hydrogen solubility is larger in austenite than in martensite, hydrogen diffusivity in martensite is approximately five times larger than in austenite, effectively creating fast diffusion paths for hydrogen which assist its dispersion in the material [2]. Thus, for both steels with the rough surface finish (elevated lattice strain) and for the AISI 304 steel with fine finish (presence of martensite), stronger effects of hydrogen are expected in comparison to the AISI 310 steel with fine surface finish. These observations are consistent with the results summarized in Fig. 6. For a rough surface finish, both AISI 304 and 310 steels exhibited similar lattice strains and similar susceptibility to hydrogen embrittlement

(approximately 18%), while for the low surface roughness condition the AISI 310 steel was practically unaffected by hydrogen charging. Of the three dominant microstructure features identified in the investigated specimens, the presence of martensite was found to be more liable to hydrogen embrittlement (25.77%). It is interesting to notice that, in case of the AISI 304 steel, the substantial increase in surface roughness, from 0.178 to 15.950 mm (Table 2), did not overcome the effect of martensite, even though deeper or sharper surface asperities act as stress-raisers and allow the accumulation of aggressive species in the environment which may enhance other electrochemical processes, such as stress corrosion cracking [6]. The results obtained demonstrate therefore that susceptibility to hydrogen embrittlement is strongly dependent on surface microstructure, and less on the microstructure of the bulk of the material, in agreement with the observations by Zhang et al. [21], who showed that hydrogen induced cracks originated at the metal surface, indicating the primary role of hydrogen surface concentration on the embrittlement process. In addition, it has been shown that hydrogen induced cracks originate between martensite-rich and austenite-rich zones [22], which would explain why for the AISI 304 steel (fine finish) the hydrogen embrittlement effect was stronger in

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Fig. 5 e Results of tensile tests performed on AISI 310 stainless steel samples before and after hydrogen charging: (a) stressstrain diagram (fine finish), (b) linearized true stress-strain diagram (fine finish), (c) stress-strain diagram (rough finish) and (d) linearized true stress-strain diagram (rough finish).

comparison to the other analyzed situations. That hydrogen embrittlement is predominantly influenced by the surface hydrogen concentration is further supported by the fact that

no significant differences in strain-hardening behavior (related, in contrast, to the material bulk) could be noticed in the tested steels when comparing the tensile test results before and after hydrogen charging (Figs. 4 and 5). In this regard, it is important to consider that the presence of hydrogen in austenitic stainless steel can lead to changes in the overall tensile strength behavior, provided the strain rate employed during testing is slow enough to allow hydrogen diffusion (~104 s1 [1]) e which was not the case in the present investigation (~103 s1).

Conclusions

Fig. 6 e Comparison of hydrogen embrittlement for AISI 304 and AISI 310 austenitic stainless steels after cathodic charging for fine and rough surface finish conditions.

In the present work, an investigation of the susceptibility to hydrogen embrittlement of austenitic stainless steels (AISI 304 and AISI 310) with different levels of surface roughness obtained by machining was carried out. Examination by XRD revealed that cutting parameters associated with “rough” surface finishes (Ra > 15 mm) were associated with higher levels of lattice strain in comparison to the conditions that led to “fine” surface finish values (Ra < 1 mm). The experimental results obtained indicated that the turning process leads to different microstructure alterations on the metal surface.

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Qualitative phase analysis performed by XRD revealed the presence of strain-induced martensite on the surface of the AISI 304 stainless steel in the fine surface finish condition, which was not observed for the AISI 310 steel. It was possible to observe that cathodic charging in (0.5M H2SO4 þ 0.25 mg/l NaAsO2) with 100 mA/cm2 for 24 h was sufficient to induce hydrogen embrittlement on the tested steels, except in case of the 310 steel with a fine surface finish. It was found that surface microstructure was the predominant factor associated with hydrogen embrittlement, with the presence of martensite being the most liable feature, followed by increased lattice strain.

Acknowledgements ~ o de This study was financed in part by the Coordenac¸a Aperfeic¸oamento de Pessoal de Nı´vel Superior - Brasil (CAPES) - Finance Code 001. The authors also acknowledge funding from PIBIC/CNPq (1044/2017).

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