metals

Article Charpy Properties of -Exposed 316L Stainless at Ambient and Cryogenic Temperatures

Le Thanh Hung Nguyen , Jae-Sik Hwang , Myung-Sung Kim, Jeong-Hyeon Kim, Seul-Kee Kim and Jae-Myung Lee *

Department of Naval Architecture and Ocean Engineering, Pusan National University, 30, Jangjeon-Dong, Geumjeong-Gu, Busan 609-735, Korea; [email protected] (L.T.H.N.); [email protected] (J.S.H.); [email protected] (M.S.K.); [email protected] (J.H.K.); [email protected] (S.K.K.) * Correspondence: [email protected]; Tel.: +82-51-510-2342

 Received: 14 May 2019; Accepted: 27 May 2019; Published: 29 May 2019 

Abstract: 316L is a promising material candidate for a hydrogen containment system. However, when in contact with hydrogen, the material could be degraded by (HE). Moreover, the mechanism and the effect of HE on 316L stainless steel have not been clearly studied. This study investigated the effect of hydrogen exposure on the impact of 316L stainless steel to understand the relation between hydrogen charging time and toughness at ambient and cryogenic temperatures. In this study, 316L stainless steel specimens were exposed to hydrogen in different durations. Charpy V-notch (CVN) impact tests were conducted at ambient and low temperatures to study the effect of HE on the impact properties and of 316L stainless steel under the tested temperatures. Hydrogen analysis and scanning electron microscopy (SEM) were conducted to find the effect of charging time on the hydrogen concentration and surface morphology, respectively. The result indicated that exposure to hydrogen decreased the absorbed energy and ductility of 316L stainless steel at all tested temperatures but not much difference was found among the pre-charging times. Another academic insight is that low temperatures diminished the absorbed energy by lowering the ductility of 316L stainless steel.

Keywords: cryogenic temperature; hydrogen embrittlement; impact load;

1. Introduction The Marine Environment Protection Committee (MEPC) of the International Maritime Organization (IMO) regulations met for its 72nd session with the aim to dramatically reduce the greenhouse gas emissions from ships by at least 50% by 2050 compared to 2008 [1]. The new regulations lead to the high demand for new eco-friendly fuels for marine ships and vessels with low greenhouse gas emissions. Among the alternative energies, liquid hydrogen (LH2) has been of great concern because it has zero carbon dioxide emissions in the exhaust gas and a higher energy-to-weight ratio in comparison with conventional fuels, like natural gas or gasoline. Despite these advantages, hydrogen can dissolve into materials and cause hydrogen embrittlement (HE) in hydrogen containers because of its small size [2]. Furthermore, a low temperature of up to 253 C of liquid hydrogen could make the − ◦ materials used for LH2 vessels become brittle. Therefore, the effect of cryogenic temperature and HE on the working capability of materials used for hydrogen containers must be understood. Figure1 illustrates a hydrogen container.

Metals 2019, 9, 625; doi:10.3390/met9060625 www.mdpi.com/journal/metals Metals 2019,, 9,, x 625 FOR PEER REVIEW 22 of of 14

Figure 1. Schematic diagram of a hydrogen container. Figure 1. Schematic diagram of a hydrogen container.

For the transportation and storage of LH2, 316L stainless steel is considered one of the most attractiveFor the material transportation candidates and for storage liquid hydrogen-containingof LH2, 316L stainless vessels steel becauseis considered of its highone resistanceof the most to attraHE [ctive3] and material good mechanical candidates properties for liquid athydrogen low temperatures-containing [4 ].vessels Understanding because of the its ehighffect resistance of cryogenic to HEtemperature [3] and good and mechanical HE on the properties performance at low of 316Ltemperature stainlesss [4] steel. Understanding is very important the effect in selecting of cryogenic 316L temperaturestainless steel and as a HE material on the candidate performance for containing of 316L stainless liquid hydrogen. steel is very However, important the mechanismin selecting of316L HE stainlesson 316L stainlesssteel as a steel material has not candidate yet been for clearly containing understood liquid [5 hydrogen.]. However, the mechanism of HE onFormer 316L stainless studies investigated steel has not the yet influence been clearly of HE understood on the mechanical [5]. properties and microstructure of 316LFormer stainless studies steel. investigated Fukuyama et the al. influence (2004) conducted of HE tensileon the tests mechanical at 10–70 MPa properties of hydrogen and microstructureatmosphere and of at316L ambient stainless temperature steel. Fukuyama for 316L et stainlessal. (2004) steel.conducted They tensile realized tests that at the10–70 impact MPa of hydrogen onatmosphere the tensile and performance at ambient of temperature the material for was 316L negligible. stainless Hydrogen steel. They was realized only distributed that the impactin the thinof hydrogen outer layer on andthe tensile its concentration performance was of notthe uniformmaterial alongwas negligible. with the specimenHydrogen because was only of distributed in the thin outer layer and its concentration was not uniform along with the specimen its low diffusivity [6]. Kanezaki et al. (2008) cathodically charged 316L stainless steel in a H2SO4 because of its low diffusivity 2[6]. Kanezaki et al. (2008) cathodically charged 316L stainless steel in a solution (pH = 3.5) at 27 A/m , 50 ◦C for 672 h. They found that hydrogen was only distributed in the 2 Hthin2SO outer4 solution layer (pH with = 3.5) a thickness at 27 A/m of approximately, 50 °C for 672 h. 100–200 They foundµm after that cathodichydrogen charging was only and distributed the high inhydrogen the thin concentrationouter layer with was a onlythickness distributed of approximately on the 100 µ 100m outer–200 μm layer after [7]. cathodic The high charging content and the of high316L hydrogen stainless steel conce promotesntration awas better only stability distributed of the on austenite the 100 phase, μm outer which layer plays [7] an. The important high nickel role contentin resistance of 316L against stainless hydrogen-assisted steel promotes fracture a better [8 ]. stability In the cubic of the lattice austenite of materials, phase, which the presence plays an of importanthydrogen inrole the in matrix resistance causes against several hydrogen changes,- suchassisted as defects fracture in [8] transformation. In the cubic and lattice phase of materials, formation. theFor presence austenitic of stainlesshydrogen in the like matrix 316L causes stainless several steel, changes, HE leads such to as the defects phase in transformation transformation from and phaseaustenitic formation.γ (face-centered For austenitic cubic) to stainlessε (hexagonal steels close-packed) like 316L stainless and α (body-centered steel, HE leads cubic) to the as proven phase transformationby several studies from [9– 11austenitic]. The di γff usion(face- ratecentered of hydrogen cubic) to in ε the (hexagonalγ-austenite close phase-packed) is lower and than α (body that in- centeredthe martensite cubic) phase;as proven therefore, by several the estudiesffect of [9 HE–11] in. theThe former diffusion phase rate is of lower. hydrogen Furthermore, in the γ-austenite HE also phasechanges is thelower surface than morphologythat in the martensite and microstructure phase; therefore, of hydrogen-exposed the effect of HE specimens in the form [12].er phase is lower.Although Furthermore, some studies HE also have changes reported the the surface effect morphologyof HE on the performance and microstructure of 316L ofstainless hydrogen steel- exposedin the cryogenic specimens environment, [12]. they are different from those reported by this study. Former tensile tests wereAlthough conducted some to study studies the have effect reported of HE onthe the effect mechanical of HE on propertiesthe performance of materials of 316L in stainless the ambient steel inand the cryogenic cryogenic temperatures environment, [13 they–16]. are However, different fracture from those toughness reported is also by anthis important study. Former parameter tensile in testsevaluating were theconducted resistance to of study a pre-cracked the effect material of HE on under the an mechanical applied load. properties The Charpy of materials V-notch (CVN)in the ambientimpact test and [17 cryogenic] is utilized temperatures to estimate the[13– absorbed16]. However, energy fracture during fracturetoughness under is also a high an strain important rate. parameterThis test is in widely evaluating applied the in resistance the industry of a to pre measure-cracked the material material’s under fracture an applied toughness load. because The Charpy of its Vlow-notch cost, (CVN) simplicity impact and test popularity [17] is utilized [18]. Former to estimate studies the concluded absorbed that energy hydrogen during exposure fracture decreased under a highthe impact strain toughness rate. This of test steels is widely [19,20]; applied however, in others the industry found that to measure the effect the of hydrogen material’s exposure fracture toughnesson the impact because toughness of its of low steels cost, was simplicity negligible and [5 ,popularity21]. Nevertheless, [18]. Former the eff ect studies of HE concluded on the impact that hydrogenbehavior ofexposure steels at decreased the cryogenic the impact temperature toughness was of not steels clearly [19,20] investigated.; however, others Thus, found the cryogenic that the effect of hydrogen exposure on the impact toughness of steels was negligible [5,21]. Nevertheless, the

Metals 2019, 9, x FOR PEER REVIEW 3 of 14 Metals 2019, 9, 625 3 of 14 effect of HE on the impact behavior of steels at the cryogenic temperature was not clearly investigated. Thus, the cryogenic impact performance of 316L stainless steel and the effect of HE on thisimpact material performance must be of understood 316L stainless to select steel 316L and the stainless effect ofsteel HE as on a thiscandidate material for must hydrogen be understood-containing to vessels.select 316L stainless steel as a candidate for hydrogen-containing vessels. This study revealed the effect effect of hydrogen exposure on the fracture toughness of 316L stainless steel, includingincluding the the impact impact resistance resistance of 316L of stainless 316L stainless steel under steel low under temperature. low temperatu Variousre. durations Various of durationscathodic hydrogen of cathodic charging hydrogen were charging used to causewere HEused in to the cause 316L HE stainless in the steel316L specimens. stainless steel After specimens. charging, Afterzinc charging, zinc was electroplating performed to was prevent performed hydrogen to prevent from being hydrogen desorbed from out being of the desorbed specimens out and of themaintain specimens HE. The and CVN maintain impact HE. test The at the CVN ambient impact and test low at temperatures the ambient wasand conductedlow temperatures to determine was conductedthe fracture to toughness determine and the the fracture ductility toughness of the specimens and the after ductility being of degraded the specimens by HE. Meanwhile, after being degradedthe hydrogen by HE.amount Meanwhile, in each case the was hydrogen measured amount to study in each the relationship case was measured between the to study hydrogen the relationshipconcentration between and the the absorbed hydroge energyn concentration of the specimens. and the The absorbed microstructure energy ofof thethe fracturespecimens. surfaces The microstructurewas investigated of by the a scanning fracture electron surfaces microscope was investigated to understand by a the scanning characteristic electron of the microscope fracture and to understandascertain the the CVN characteristic impact test of result. the fracture and ascertain the CVN impact test result.

2. Materials and Methods 2. Materials and Methods 2.1. Material 2.1. Material The 316L stainless steel exhibited high resistance to HE, showing excellent mechanical performance The 316L stainless steel exhibited high resistance to HE, showing excellent mechanical at low temperatures. This study used 316L stainless steel made based on the Japanese standard. Table1 performance at low temperatures. This study used 316L stainless steel made based on the Japanese shows the composition of 316L stainless steel. standard. Table 1 shows the composition of 316L stainless steel. Table 1. Composition of 316L stainless steel. Table 1. Composition of 316L stainless steel. Chemical Elements (wt. %) Specimen Chemical Elements (wt. %) Specimen C Si S P Mn Mo Ni Cr Fe C Si S P Mn Mo Ni Cr Fe 316L 316L0.020 0.0200.56 0.560.003 0.003 0.028 0.028 1.33 1.33 2.12.1 10.1910.19 16.416.4 Bal. Bal.

2.2. Specimens Specimens The specimens were made based on ASTM E23 E23—16b:—16b: Standard Standard Test Test Methods for Notched Bar Impact Testing Testing of Metallic Materials [[17]17].. V V-notched-notched specimens are widely used to test the absorbed energy of metals because they are easy to prepare and the CVN impact test results can be achieved quickly and cheaply. Each specimen had a specificspecific dimension of 55 mmmm × 10 mm × 1010 mm mm in in length, length, × × width andand thickness,thickness, respectively. respectively. The The V-notch V-notch was was 2 mm 2 mm in depth in depth and and 45 degree 45 degree in V-angle. in V-angle. The notch The notchradius radius was 0.25 was mm. 0.25 Figuremm. Fig2 illustratesure 2 illustrates the dimension the dimension of each of V-notched each V-notched specimen. specimen.

Figure 2. ((aa)) Photograph Photograph and and ( (bb)) dimension dimension of of the the V V-notched-notched specimen (unit: mm) mm)..

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2.3. Cathodic Hydrogen Charging and Zn Electroplating 2.3. Cathodic Hydrogen Charging and Zn Electroplating The two predominant methods used to generate HE in specimens are electrochemical charging The two predominant methods used to generate HE in specimens are electrochemical charging and hydrogen gas charging. Hydrogen gas charging involves exposing the specimen to hydrogen and hydrogen gas charging. Hydrogen gas charging involves exposing the specimen to hydrogen atmosphere, while electrochemical hydrogen charging, or cathodic hydrogen charging, is a method by atmosphere, while electrochemical hydrogen charging, or cathodic hydrogen charging, is a method which a specimen is immersed in an electrolytic solution. Electricity is then used to generate water by which a specimen is immersed in an electrolytic solution. Electricity is then used to generate water electrolysis. Cathodic hydrogen charging was used herein to generate HE in the 316L stainless steel electrolysis. Cathodic hydrogen charging was used herein to generate HE in the 316L stainless steel specimens because this method was simple to perform. Cathodic hydrogen charging was conducted specimens because this method was simple to perform. Cathodic hydrogen charging was conducted based on the ISO 16573-2015: Steel—Measurement method for the evaluation of HE resistance of based on the ISO 16573-2015: Steel—Measurement method for the evaluation of HE resistance of highhigh-strength-strength steels steels [22] [22].. The The generated generated hydrogen hydrogen diffused diffused into into the the specimen specimen surface, surface, which which played played a role as a cathode. A platinum mesh was used as an anode. The notched specimens were totally a role as a cathode. A platinum mesh was used as an anode. The notched specimens were totally immersed in the electrolyte solution composed of 3% NaCl + 0.3% NH4SCN. The NaCl molecules immersed in the electrolyte solution composed of 3% NaCl + 0.3% NH4SCN. The NaCl molecules in + in the H2O solvent were dissolved to Na and Cl− ions, which acted as electrolytes in the solution, the H2O solvent were dissolved to Na+ and Cl− ions, which acted as electrolytes in the solution, while + while NH4SCN prevented the H ions from recombination to H2 during hydrogen charging [23]. The NH4SCN prevented the H+ ions from recombination to H2 during hydrogen charging [23]. The charging’s current density was kept constant at 20 A/m2 using the potentiostat mode of a WBCS3000S charging’s current density was kept constant at 20 A/m2 using the potentiostat mode of a WBCS3000S StandardStandard Type Type Battery Battery Cycler Cycler ( (WonATech,WonATech, Seoul,Seoul, Korea).Korea). The charging charging durations durations were were 12 12 h, h, 24 24 h h and and 4848 h. h. Hydrogen Hydrogen was was liberated liberated at at the the cathode cathode surface surface (specimen) (specimen) and and might might diffuse diffuse into into the the specimen. specimen. TheThe amount of of liberated liberated hydrogen hydrogen depended depended on the on current the current density. density. A larger A current larger densitycurrent liberated density liberatedmore hydrogen more hydrogen on the specimen on the specimen surface. Figure surface.3 illustratesFigure 3 illustrates the schematic the schematic diagram of diagram hydrogen of hydrogencharging forcharging the V-notched for the V specimens.-notched specimens.

Figure 3. Schematic diagram of the cathodic hydrogen charging. Figure 3. Schematic diagram of the cathodic hydrogen charging. After the hydrogen exposure, each specimen was slightly polished by a grit sanding sponge after beingAfter charged the hydrogen to remove exposure, the little dirt each on specimen the specimen was surface.slightly polished Next, zinc by electroplating a grit sanding was sponge conducted after beingto prevent charged hydrogen to remove from being the little desorbed. dirt on The the thin specimen Zn layer actedsurface. as a Next, barrier zinc that electroplating prevents hydrogen was conductedfrom diffusing to prevent out during hydrogen the loading from test. being The desorbed. schematic The diagram thin ofZn the layer electroplating acted as a was barrier the same that preventsas the hydrogen hydrogen charging, from except diffusing for the out current during density the loading and the test. The solution. schematic In this diagram procedure, of the the 2 2+ electroplatingapplied current was density the same was 300as the A/ mhydrogen. The plating charging, solution except included for the Zn currentions, density which and were the referenced plating solution.from the In ISO this 16573-2015 procedure, standard the app [lied22]. current Each specimen density waswas initially300 A/m electroplated2. The plating for solution approximately included 8 Znmin.2+ ions, The which specimen were was referenced then rotated from and the continuedISO 16573- to2015 be electroplatedstandard [22] again. Each inspecimen approximately was initially 8 min electroplatedto ensure that for the approximatelyZn layer homogeneously 8 min. The covered specimen the specimen. was then The rotated electroplating and continued times could to be be elslightlyectroplated adjusted again to makein approximately sure that the 8 Zn min layers to ensure sufficiently that the covered Zn layer the specimenshomogeneously with homogeneous covered the specimen.thicknesses. The Figure electroplating4 illustrates times the specimens could be before slightly (a) adjusted and after to (b) make electroplating. sure that the Zn layers sufficiently covered the specimens with homogeneous thicknesses. Figure 4 illustrates the specimens before (a) and after (b) electroplating.

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Figure 4. CCharpyharpy V V-notch-notch (C (CVN)VN) impact specimens before ( a) and after (b) electroplating.electroplating.

2.4. Hydrogen Concentration Measurement After being hydrogen pre pre-charged,-charged, the hydrogen concentration on the surface of each specimen was measured byby aa ONH-2000ONH-2000 hydrogenhydrogen analyzeranalyzer (ELTRA (ELTRA GmbH, GmbH Haan,, Haan Germany), Germany for) for the the 12 12 h, h, 24 24 h hand and 48 48 h chargedh charged and and uncharged uncharged specimens. specimens For. For this this measurement, measurement, each each sample sample has 1has g in1 g weight in weight was wascut from cut from the specimenthe specimen surface. surface. The The samples samples were were then then investigated investigated for hydrogenfor hydrogen concentration concentration for forapproximately approximately 2.5 2.5 min min inside inside the the dual dual range range thermal thermal conductivity conductivity cell cell of theof the hydrogen hydrogen analyzer. analyzer. At Atleast least five samplesfive samples were analyzed were analyzed for each for duration each duration of hydrogen of hydrogen exposure to exposure ascertain to the ascertain repeatability the repeatabilityof the hydrogen of the concentration hydrogen concentration results. The hydrogen results. concentrationThe hydrogen was concentration used to unveil was the used relationship to unveil thebetween relationship the absorbed between energy the and absorbed the hydrogen energy concentration and the hydrogen for each concentration CVN specimen. for each CVN specimen. 2.5. CVN Impact Test 2.5. CVNAfter Impact being Test electroplated, the specimens were prepared by controlling the desired temperature conditions.After being Then, electroplated, the Charpy impactthe specimens tests were were conducted. prepared Theby controlling CVN impact the test desired demonstrated temperature the conditions.absorbed energy Then, during the Charpy the material impact fracture, tests were which conducted. also indicated The CVN work impact needed test to de makemonstrated the material the absorbedfracture at energy the experimental during the temperaturematerial fracture, of the which CVN also impact indicated test. The work absorbed needed energy to make was the calculated material fractureas follows: at the experimental temperature of the CVN impact test. The absorbed energy was calculated E = MR(cosβ cosα)g, (1) as follows: CVN − where E is the absorbed energy; M is the mass of the ; R is the specimen length; β is the CVN ECVN = MR(cosβ − cosα)g, (1) angle after the impact; α is the falling angle before the impact; and g is the gravitational . Thewhere absorbed ECVN is the energy absorbed of the energy; specimens M is was the recordedmass of the to estimatehammer; the R is toughness the specimen and ductilitylength; β ofis the angleimpact after tests. the Ductile impact; fracture α is the has falling high angle absorbed before energy the impact; in the CVN and impactg is the test gravitational as well as betteracceleration. impact Thetoughness, absorbed while energy brittle of the fracture specimens has low was absorbed recorded energy to estimate in the the CVN toughness impact test.and ductility Therefore, of the impactductile fracturetests. Ductile was preferred fracture has in most high applications. absorbed energy Aside in fromthe CVN the specimens impact test used as well for the as better CVN impact testtoughness, at the ambient while brittle temperature, fracture thehas others low absorbed were pre-cooled energy in in the the CVN desired impact low temperaturetest. Therefore, for the ductile fracture was preferred in most applications. Aside from the specimens used for the CVN 40–50 min to ensure their thermal equilibrium. To lower the temperature to 50 ◦C and 125 ◦C for impact test at the ambient temperature, the others were pre-cooled in the desired− low temperature− the low-temperature impact test, liquefied nitrogen (LN2) was injected to an insulated chamber and forthe 40 inlet–50 flow min rateto ensure of nitrogen their wasthermal automatically equilibrium. adjusted To lower to maintain the temperature the desired to − temperature50 °C and −125 by the°C fortwo the thermocouples low-temperature connected impact to test, a computer. liquefied For nitrogen196 (LNC, the2) was specimens injected were to an directly insulated immersed chamber to − ◦ andliquid the nitrogen inlet flow at atmospheric rate of nitrogen pressure was inautomatically another insulated adjusted chamber. to maintain After pre-cooling the desired for temperature 40–50 min, bythe the CVN two impact thermocouples test was conducted connected within to a 5 computer. s for each specimen For −196 since°C, the the specimens time that each were specimen directly wasimmersed brought to liquid out of nitrogen the insulated at atmospheric chamber [ 17press]. Eachure in case another was repeated insulated at chamber. least three After times pre to-cooling ensure forthe 40 reliability–50 min, of the the CVN testing impact results. test Figurewas conducted5 presents within the experimental 5 s for each procedure specimen ofsince the CVNthe time impact that each specimen was brought out of the insulated chamber [17]. Each case was repeated at least three times to ensure the reliability of the testing results. Figure 5 presents the experimental procedure of

Metals 2019, 9, 625 6 of 14 Metals 2019, 9, x FOR PEER REVIEW 6 of 14 thetest. CVN Table impact2 illustrates test. Table the experimental 2 illustrates scenariothe experimental of the hydrogen scenario pre-charging, of the hydrogen zinc electroplatingpre-charging, andzinc electroplatingCVN impact tests. and CVN impact tests.

Figure 5. ExperimeExperimentalntal procedure of the CVN impact test.test.

Table 2. Experimental scenario. Table 2. Experimental scenario. Electroplating Temperature of Charging ChargingCurrent CurrentChargingCharging Electroplating Current Temperature of CVN No No 2 Current Density CVN Impact Test Density (A/mDensity2) (A/mDuration) Duration(h) (h)Density (A/m2) 2 Impact Test (°C) (A/m ) (◦C) 1 10 00 00 0 2 2 12 12 2525 3 320 20 24 24 300 300 4 448 48 5 50 00 00 0 6 12 6 12 50 720 24 300 −−50 7 820 24 48 300 8 48 9 0 0 0 9 10 0 0 12 0 125 10 1120 12 24 300 − −125 11 1220 24 48 300 12 13 048 0 0 14 12 13 0 0 0 196 1520 24 300 14 12 − 16 48 −196 15 20 24 300 16 48 2.6. Scanning Electron Microscopy (SEM)

2.6. ScanningFor each Electron case of theMicroscopy CVN impact (SEM) test, a Supra 25 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) was used to analyze the microstructure in the middle of the fracture surfaces For each case of the CVN impact test, a Supra 25 scanning electron microscope (Carl Zeiss AG, after the impact test. This step focuses on the effect of the hydrogen charging time on the surface Oberkochen, Germany) was used to analyze the microstructure in the middle of the fracture surfaces morphology (e.g., tortuousness, depth and size of dimples) on the specimen’s fracture surface after the after the impact test. This step focuses on the effect of the hydrogen charging time on the surface CVN impact test. ImageJ software was employed to measure the dimple size. The absorbed energy and morphology (e.g., tortuousness, depth and size of dimples) on the specimen’s fracture surface after microstructure combination could unveil the impact properties of the specimens. The microstructure the CVN impact test. ImageJ software was employed to measure the dimple size. The absorbed energy and microstructure combination could unveil the impact properties of the specimens. The microstructure of the ductile fracture mainly has a tortuous appearance with large and deep dimples.

MetalsMetals 20192019,, 99,, 625x FOR PEER REVIEW 77 of 1414 Metals 2019, 9, x FOR PEER REVIEW 7 of 14 In contrast, the brittle fracture microstructure mainly contains small spherical dimples and cleavage ofInfacets. thecontrast, ductile the fracture brittle mainlyfracture has microstructure a tortuous appearance mainly contains with large small and spherical deep dimples. dimples In and contrast, cleavage the brittlefacets. fracture microstructure mainly contains small spherical dimples and cleavage facets. 3. Results and Discussion 3. Results and Discussion 3.1. Hydrogen Concentration in the Specimens 3.1. Hydrogen Concentration in the Specimens Figure 6 illustrates the hydrogen concentration at the surface of the uncharged and 12 h, 24 h and Figure48 h pre6 6 illustrates- chargedillustrates specimens. thethe hydrogenhydrogen This concentration concentrationchart shows that atat thethe the surfacesurface hydrogen ofof thethe concentration unchargeduncharged andandin the 1212 charged h,h, 2424 hh andspecimens 48 hh pre-chargedpre was-charged higher specimens.specimens.than that in Thisthe uncharged chart shows specimens that the . hydrogenThe average concentration hydrogen concentration in thethe chargedcharged in specimensthe uncharged waswas highersamples than was thatthat 3.52 in wt. the ppm, uncharged which specimens.specimens was approximate. The average to that hydrogen in the previous concentration research in thethe[24] unchargeduncharged. In comparison samplessamples with was was the 3.52 3.52 non wt. wt.-exposed ppm, ppm, which whichsamples, was was approximatethe approximate results for to thattheto that 12 in theh, in 24 previousthe h previousand 48 research h researchcharged [24]. In[24]samples comparison. In comparison are 5.79, with 10.1 thewith and non-exposed the 8.74 non wt.-exposed ppm, samples, which samples, the correspond results the results for theto 164%,for 12 h,the 24 287% 12 h andh, and 24 48 h 248%, h and charged 48 respectively h samplescharged. aresamplesThe 5.79, hydrogen 10.1are and5.79, concentration 8.74 10.1 wt. and ppm, 8.74 dramatically which wt. ppm, correspond increasedwhich tocorrespond 164%,and reached 287% to and164%, a saturation 248%, 287% respectively. andpoint 248%, at 24 The hrespectively of hydrogen charging.. concentrationTheAfter hydrogen 24 h of dramaticallyconcentration charging, the increased dramatically hydrogen and concentration increased reached aand saturation remainedreached pointa saturation relatively at 24 h closedpoint of charging. at to 24 theh of After saturatedcharging. 24 h ofAfterconcentration charging, 24 h of the of charging, hydrogen hydrogen. the concentration Besides, hydrogen the concentrationerrorsremained of hydrogen relatively remained concentration closed relatively to the saturatedwere closed relatively toconcentration the large. saturated This of hydrogen.concentrationresult confirmed Besides, of hydrogen.the the high errors resistance Besides, of hydrogen theof 316L errors concentration stainless of hydrogen steel were toconcentration HE relatively because large. werehydrogen This relatively result hardly large. confirmed diffused This theresultinto high this confirmed resistance material the andof high 316L was resistance stainless homogeneously steelof 316L to HE stainless distributed because steel hydrogen on to theHE thinbecause hardly outer dihydrogen ff layerused ofinto hardly the this hydrogen materialdiffused- andintoexposed was this homogeneouslyspecimens. material and The was distributed next homogeneously section on explains the thin distributed outer the effect layer on of the the the thinhydrogen-exposed hydrogen outer layer charging of specimens. t he time hydrogen and The the- nextexposedtemperature section specimens. explains of the CVN the The e impactff nextect of section test the hydrogenon explainsthe absorbed charging the effectenergy time of of and the 316L the hydrogen stainless temperature charging steel. of the time CVN and impact the testtemperature on the absorbed of the CVN energy impact of 316L test stainless on the absorbed steel. energy of 316L stainless steel. 12 12 10 10 8 8 6 6 4 4 2 2 Hydrogen(ppm) concentration 0 Hydrogen(ppm) concentration 0 12 24 36 48 0 0 12 Charging24 time (h) 36 48 Charging time (h) Figure 6. Hydrogen concentration at the uncharged and 12 h, 24 h and 48 h charged specimens. FigureFigure 6. Hydrogen 6. Hydrogen concentration concentration at the at the uncharged uncharged and and 12 12h, 24 h, 24h and h and 48 48h charged h charged specimens specimens.. 3.2.3.2. CVN Impact Absorbed Energy 3.2. CVN Impact Absorbed Energy InIn a a CVN CVN impact impact test, test, the the ductile ductile fracture fracture has has higher higher absorbed absorbed energy energy andand higher higher plasticplastic In a CVN impact test, the ductile fracture has higher absorbed energy and higher plastic deformationdeformation thanthan thethe brittlebrittle fracture.fracture. Figure7 7 shows shows thethe temperature-dependenttemperature-dependent absorbed absorbed energy energy of of deformation than the brittle fracture. Figure 7 shows the temperature-dependent absorbed energy of thethe CVNCVN impactimpact testtest forfor thethe 1212 h,h, 2424 hh andand 48 h hydrogen charged andand unchargeduncharged 316L316L stainlessstainless steelsteel the CVN impact test for the 12 h, 24 h and 48 h hydrogen charged and uncharged 316L stainless steel atat 2525 °CC (ambient(ambient temperature),temperature), −50 °C,C, −125125 °CC and and −196196 °C,C, respectively. respectively. at 25 ◦°C (ambient temperature), −−50 °C,◦ −125 °C◦ and −−196 °C,◦ respectively.

350 350 300 300 250 250 200 200 150 150 0h 100 0h12h

Absorbed energy (J) 100 12h24h 50 48h

Absorbed energy (J) 24h 50 48h 0 -200 -150 -100 -50 0 50 0 -200 -150 Temperature-100 (o-50C) 0 50 Temperature (oC) Figure 7. Temperature-dependent absorbed energy behavior of 316L stainless steel for uncharged, 12 hFigure charged, 7. Temperature 24 h charged-dependent and 48 h chargedabsorbed specimens energy behavior (the error of 316L bars stainless indicate steel the incertitudefor uncharged, of the 12 Figure 7. Temperature-dependent absorbed energy behavior of 316L stainless steel for uncharged, 12 absorbedh charged, energy). 24 h charged and 48 h charged specimens (the error bars indicate the incertitude of the habso charged,rbed energy) 24 h charged. and 48 h charged specimens (the error bars indicate the incertitude of the absorbed energy).

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Overall,Overall, the the tested tested 316L 316L stainless stainless steel’s steel’s absorbed absorbed energy energy gradually gradually decreased decreased when when decreasing decreasing thethe temperature temperature of of the the CVN CVN impact impact test. test. Specifically,Specifically, in in comparison comparison with with 25 25◦ °C,C, the the average average absorbed absorbed energyenergy of of the the uncharged uncharged specimens specimens at at −5050 C, °C, −125125 °CC andand −196196 °CC are are 76.0%, 76.0%, 69.4% 69.4% andand 61.0%61.0% − ◦ − ◦ − ◦ respectively.respectively. These These ratios ratios for for the the 12 12 h, h, 24 24 h h and and 48 48 h h chargedcharged specimens specimens were were approximate approximate to to those those of of thethe uncharged uncharged specimens. specimens. Therefore,Therefore, the the ductility ductility of of impact impact tests tests decreased decreased gradually gradually from from ambient ambient temperaturetemperature toto −196196 °C.C. TheThe decreasedecrease inin thethe absorbedabsorbed energyenergy asas wellwell asas thethe ductilityductility waswas causedcaused byby − ◦ loweringlowering the the temperature temperature of of impact impact tests, tests, which which resulted resulted in in the the more more brittle brittle fracture fracture of of 316L 316L stainless stainless steelsteel [ 25[25,26],26]. . FigureFigure8 8shows shows the the charging charging time-dependent time-dependent absorbed absorbed energy energy behavior behavior of of 316L 316L stainless stainless steel steel at at 2525 C°C (ambient (ambient temperature), temperature),50 −50C, °C,125 −125C °C and and196 −196C, °C, respectively. respectively. At theAt the same same temperature temperature of the of ◦ − ◦ − ◦ − ◦ CVNthe CVN impact impact test, the test, absorbed the absorbed energy of energy the pre-charged of the pre specimens-charged specimensrelatively dropped relatively in comparison dropped in tocompariso that of then unchargedto that of the specimens. uncharged For specimens. the CVN impactFor the testCVN conducted impact test at theconducted ambient at temperature, the ambient thetemperature, drop in the the absorbed drop energies in the absorbed at the 12 h, energies 24 h and at 48 the h charged 12 h, 24 specimens h and 48 corresponded h charged specimens to 16.6%, 14.2%corresponded and 12.6%, to respectively,16.6%, 14.2% when and compared12.6%, respectively, to those in when the uncharged compared specimens. to those in This the resultuncharged also indicatesspecimens. that This hydrogen result also pre-charging indicates slightlythat hydrogen decreased pre the-charging ductility slightly of the notcheddecreased specimens the ductility at the of ambientthe notched temperature. specimens at the ambient temperature.

400 25 oC 350 - 50 oC - 125 oC 300 - 196 oC 250

200

150

100 Absorbed Energy (J) 50

0 0 12 24 36 48 Charging Time (h)

FigureFigure 8. 8.Charging Charging time-dependent time-dependent absorbed absorbed energy energy behavior behavior of of 316L 316L stainless stainless steel steel at at 25 25◦ C°C (ambient (ambient temperature), 50 C, 125 C and 196 C (the error bars indicate the incertitude of the absorbed temperature),− −50 ◦°C, −−125 ◦°C and −−196 ◦°C (the error bars indicate the incertitude of the absorbed energy).energy).

For the CVN impact test conducted at 50 C, the absorbed energy for the pre-charged specimens For the CVN impact test conducted at− −50◦ °C, the absorbed energy for the pre-charged specimens was almost alike to the uncharged specimens. Specifically, the dropped ratio for the CVN impact test was almost alike to the uncharged specimens. Specifically, the dropped ratio for the CVN impact test for the 12 h and 48 h pre-charged specimens is approximately 0.23% and 1.7% respectively, compared for the 12 h and 48 h pre-charged specimens is approximately 0.23% and 1.7% respectively, compared to the uncharged specimens. The absorbed energy for the 24 h pre-charged specimen increased by to the uncharged specimens. The absorbed energy for the 24 h pre-charged specimen increased by 0.45%. Therefore, the effect of hydrogen pre-charging at 50 C was negligible. 0.45%. Therefore, the effect of hydrogen pre-charging at− −50◦ °C was negligible. At 125 C, the absorbed energy also exhibited a slight decrease with the pre-charging time At− −125 ◦°C, the absorbed energy also exhibited a slight decrease with the pre-charging time of of hydrogen. The drop in the absorbed energy is 7.9%, 11.3% and 9.7% for the 12 h, 24 h and 48 h hydrogen. The drop in the absorbed energy is 7.9%, 11.3% and 9.7% for the 12 h, 24 h and 48 h pre- pre-charged specimens, respectively. Finally, at 196 C, the drop in the absorbed energy for the 12 h, charged specimens, respectively. Finally, at −196− °C,◦ the drop in the absorbed energy for the 12 h, 24 24 h and 48 h pre-charged specimens is 12.9%, 9.2% and 10.0%, respectively. Overall, most of the h and 48 h pre-charged specimens is 12.9%, 9.2% and 10.0%, respectively. Overall, most of the hydrogen pre-charged specimens exhibited a drop in the absorbed energy, indicating that hydrogen hydrogen pre-charged specimens exhibited a drop in the absorbed energy, indicating that hydrogen pre-charging decreased the ductility of 316L stainless steel in the CVN impact tests at the ambient pre-charging decreased the ductility of 316L stainless steel in the CVN impact tests at the ambient temperature, 50 C, 125 C and 196 C. Former studies also revealed that hydrogen exposure temperature, −−50 ◦ °C, −−125 ◦ °C and −−196 ◦ °C. Former studies also revealed that hydrogen exposure changes the fracture mode of materials from ductile to brittle [2,27] and therefore decreased the changes the fracture mode of materials from ductile to brittle [2,27] and therefore decreased the absorbed energy in the CVN impact test of metallic materials. In Figures7 and8, the exposure to absorbed energy in the CVN impact test of metallic materials. In Figure 7 and Figure 8, the exposure hydrogen decreased the absorbed energy in the CVN impact tests but the drop in absorbed energy to hydrogen decreased the absorbed energy in the CVN impact tests but the drop in absorbed energy remainedremained stablestable fromfrom 2424 hh toto 48 48 h h of of charging, charging, which which reflected reflected a a similar similar behavior behavior with with hydrogen hydrogen concentration in 24 h charged and 48 h charged specimens. A former study also revealed that when concentration in 24 h charged and 48 h charged specimens. A former study also revealed that when increasing the hydrogen outgassing time to low carbon stainless steel, not much difference in hardness of the specimens was observed [28].

Metals 2019, 9, 625 9 of 14 increasing the hydrogen outgassing time to low carbon stainless steel, not much difference in hardness Metalsof the 2019 specimens, 9, x FOR wasPEER observedREVIEW [28]. 9 of 14

3.3. Fracture Fracture Surface Morphology Figure 99 illustrates the specimens specimens under under CVN CVN impact impact tests. tests. In In Figure Figure 9,9 c,racks cracks were were found found on on the Vthe-notched V-notched specimen specimen after after the theCVN CVN impact impact test. test. The The appearance appearance of the of thecracks cracks mostly mostly depended depended on theon thetemperature temperature of ofthe the impact impact tests. tests. The The presence presence of of plastic plastic deformation deformation increased increased in in the the order order of −196196 °C,C, −125125 °C,C, −5050 °C Cand and ambient ambient temperature. temperature. For For CVN CVN impact impact tests tests conducted conducted at at−196196 °C, allC, allV- − ◦ − ◦ − ◦ − ◦ V-notchednotched specimens specimens were were separated separated into into 2 2 pieces pieces and and showed showed very very little little plastic deformation. The specimens in CVN impact teststests conductedconducted atat −125125 °CC and −5050 °CC were were partly partly separated separated by the the impact − ◦ − ◦ testeststs and showed relatively little plastic deformation. Meanwhile, impact tests conducted at ambient temperature showedshowed extensiveextensive plastic plastic deformation deformation and and all all specimens specimens were were not not separated. separated. Further, Further, the thediff erencesdifferences in the in appearancethe appearance of impacted of impacted specimens specimens according accord toing charging to charging times times were were not clear.not clear.

Figure 9. Photograph of tested specimens specimens..

Scanning electron electron microscopy microscopy was was conducted conducted after after the CVN the CVN impact impact test to test investigate to investigate the fracture the fracturesurface of surface the specimens. of the specimens. The brittle The fracture brittle has fracture little plastic has deformation little plastic and deformation mainly contains and mainly small containsspherical small dimples, spherical flat facets dimples, and flat cleavage facets facetsand cleavage on the fracturefacets on surfaces. the fracture For surfaces. ductile fracture, For ductile the fracture,fracture surfacethe fracture mainly surface includes mainly a tortuous includes appearance a tortuous withappearance deep and with large deep dimples. and large The dimples. ductile Thefracture ductile has fracture more plastic has more deformation plastic defo thanrmation the brittle than fracture. the brittle Therefore, fracture. the Therefore, absorbed energythe absorbed in the energyCVN impact in the testCVN of impact ductile test fracture of ductile was higher fracture than was that higher of brittle than fracture.that of brittle fracture. Figure 1010 showsshows thethe fracturefracture surfaces surfaces for for the the impact impact test test conducted conducted at ambientat ambient temperature temperature for thefor (a)the uncharged,(a) uncharged, (b) 12 (b) h 12 charged, h charged, (c) 24 (c) h charged24 h charged and (d) and 48 ( hd) charged 48 h charged specimens specimens after the after CVN the impact CVN impacttest. As test. shown As inshown this figure, in this the figure, fracture thesurfaces fracture havesurfaces a tortuous have a appearance tortuous appearance and mainly and covered mainly by coveredlarge dimples. by large The dimples. cleavage The fracture cleavage was fracture not observed was not in observed the SEM imagesin the SEM at ambient images temperature at ambient temperatureand the large and dimples the large were dimples relatively were deep. relatively Those weredeep. the Those signs were of plastic the signs deformation. of plastic deformation. Therefore, at Therefore,ambient temperature, at ambient the temperature, type of impact the type fracture of impact was mainly fracture ductile. was mainly For diff ductile.erent charging For different times, chargingthe morphology times, of the the morphology uncharged specimen of the uncharge was the roughestd specimen appearance was the by roughest included appearance many tortuous by includedand large many dimples, tortuous the depth and oflarge those dimples, dimples the was depth bigger of those thanthe dimples other chargedwas bigger specimens, than the whichother chargedconfirms specimens, the negligible which drop confirms in absorbed the energynegligible of charged drop in specimens absorbed atenergy ambient of charged temperature. specimens Besides, at ambientthe dimples temperature. on the fracture Besides, surface the dimples of 12 h charged on the fracture specimen surface were relativelyof 12 h charged smaller specimen and shallower were relativelythan the other smaller specimens and shallower at ambient than temperature, the other specimens confirms at that ambient the absorbed temperature, energy confirms of 12 h chargedthat the absorbedspecimen energy was the of smallest 12 h charged among specimen the charging was the time. smallest Therefore, among the the eff chargingect charging time. times Therefore, at 12 h the on effectthe surface charging morphology times at 12 as h well on the as deformation surface morphology of CVN impactas well testsas deformation at ambient temperatureof CVN impact was tests the atlargest, ambient followed temperature by 24 h was and the 48 h largest, charging followed time. Besides, by 24 h thereand 48 were h charging negligible time. diff erencesBesides, between there were the negligibledeformation differences as well as between absorbed the energy defor betweenmation 24as hwell charged as absorbed specimen energy and 48 between h charged 24 specimen h charged at specimenambient temperature. and 48 h charged specimen at ambient temperature.

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FigureFigure 10. 10.Scanning Scanning electron electron microscopemicroscope (SEM)(SEM) images obtained obtained from from the the impact impact test test at atthe the ambient ambient temperature:temperature:Figure 10. S (canninga ()a uncharged,) uncharged, electron ( b( bmicroscope)) 1212 hh charged,charged, (SEM (c)) images 24 h h charged charged obtained and and from (d (d) )48 the 48 h h impactcharged charged. test. at the ambient temperature: (a) uncharged, (b) 12 h charged, (c) 24 h charged and (d) 48 h charged. FigureFigure 11 11 shows shows the the microstructure microstructure analysis of of the the fracture fracture surface surface in in the the CVN CVN impact impact test test at at 50−50C. °CFigure The. The fracture fracture11 shows surfaces surfaces the microstructure atat −50 °CC a arere analysis primarily primarily of coveredthe covered fracture by by relatively surface relatively in large the large CVNdimples. dimples. impact However, However, test at − ◦ − ◦ thethe−50 fracture fracture °C. The surface surfacefracture at at surfaces this this temperature temperature at −50 °C waswasare primarily less circuitous circuitous covered than than by that thatrelatively in in the the ambient large ambient dimples. temperature, temperature, However, the the dimplesdimplesthe fracture in Figurein Figuresurface 11 are11 at relativelyarethis relativelytemperature smaller smaller was and less shallowerand circuitous shallower than than than the that dimples the in dimples the in ambient Figure in Figure 10 temperature, and 10 an and increased thean presenceincreaseddimples of in presence the Figure small of11 spherical the are small relatively dimplesspherical smaller anddimples flat and and facets shallower flat was facets observed.than was the observed. dimples Thus, Thus, this in Figure appearancethis appearance 10 andof an the ofincreased the fracture presence surfaces of the indicated small spherical that the dimples deformation and flat was facets less wasductile observed. in the impactThus, this tests appearance at −50 °C fracture surfaces indicated that the deformation was less ductile in the impact tests at 50 ◦C than that thanof the that fracture at the surfaces ambient indicated temperature that theand deformation led to a drop was in lessthe absorbedductile in energythe impact from −tests the atambient −50 °C at the ambient temperature and led to a drop in the absorbed energy from the ambient temperature temperaturethan that at tothe − 50ambient °C in Figure temperature 7. Moreover, and le thed to appearance a drop in theof the absorbed fracture energy surfaces from in Figure the ambient 11 are to 50 C in Figure7. Moreover, the appearance of the fracture surfaces in Figure 11 are made quite made−temperature◦ quite analogous, to −50 °C confirmingin Figure 7. thatMoreover, the impact the appearanceproperties of of specimens the fracture at −surfaces50 °C are in roughly Figure 11alike. are analogous, confirming that the impact properties of specimens at 50 C are roughly alike. Overall, Overall,made quite the analogous,effect of hydrogen confirming pre -thatcharging the impact on the properties CVN impact of specimens properties− ◦ at of−50 316L °C are stainless roughly steel alike. at the effect of hydrogen pre-charging on the CVN impact properties of 316L stainless steel at 50 C was −Overall,50 °C was the negligible. effect of hydrogen The drop pre in the-charging absorbed on energythe CVN from impact the ambientproperties temperature of 316L stainless to −50− °Csteel◦ was at negligible. The drop in the absorbed energy from the ambient temperature to 50 C was caused by caused−50 °C wasby the negligible. decrease Thein the drop temperature in the absorbed of the energyCVN impa fromct thetest. ambient temperature− ◦ to −50 °C was thecaused decrease by the in the decrease temperature in the temperature of the CVN of impact the CVN test. impact test.

Figure 11. Scanning electron microscope (SEM) images obtained from the impact test at −50 °C: (a) Figureuncharged,Figure 11. 11.Scanning S(canningb) 12 h electroncharged, electron microscope (microscopec) 24 h charged (SEM)(SEM and) images (d) 48 hobtained obtained charged .from from the the impact impact test test at at−5050 °C: C:(a) (a) − ◦ uncharged,uncharged, (b )(b 12) 12 h h charged, charged, (c ()c) 24 24 h h charged chargedand and ((dd)) 4848 hh charged.charged.

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Figure 12 illustratesillustrates the the microstructure microstructure images images of of the the fracture fracture surface surface in in the the CVN CVN impact impact test test at at 125 C of 316L stainless steel according to the hydrogen charging time. As shown in this figure, −125− °C ◦of 316L stainless steel according to the hydrogen charging time. As shown in this figure, the thefracture fracture surfaces surfaces were were mainly mainly covered covered by by small small spherical spherical dimpl dimples.es. The The presence presence of of the the small and spherical dimple fracture was more frequent here than in the ambient temperature and 50 C, making spherical dimple fracture was more frequent here than in the ambient temperature− and◦ −50 °C, the average size of dimples at 125 C smaller than at the ambient temperature and 50 C and the making the average size of dimples− ◦at −125 °C smaller than at the ambient temperature− and◦ −50 °C sphericaland the spherical dimples are dimples relatively are shallow. relatively Thus, shallow. thisappearance Thus, this of appearance the fracture of surfaces the fracture indicated surfaces that the CVN impact behavior at 125 C was more brittle than at the ambient temperature and 50 C, indicated that the CVN impact− behavior◦ at −125 °C was more brittle than at the ambient temperature− ◦ confirming that the average absorbed energy at 125 C was smaller than at 50 C. Besides, there and −50 °C, confirming that the average absorbed− energy◦ at −125 °C was smaller− ◦ than at −50 °C. wereBesides, diff thereerences were in thedifferences fracture surfacein the fracture according surface to the according charging to times. the charging The surface times. morphology The surface of themorphology uncharged of specimenthe uncharged was rougher specimen than was the rougher charged than specimens. the charged Specifically, specimens. the Specifically, size of dimples the ofsize the of unchargeddimples of the specimen uncharged was specimen relatively was larger relativel and deeper,y larger which and deeper, confirms which the confirms drop in absorbed the drop energy of charged specimens at 125 C. In addition, the fracture surfaces of the charged specimens in in absorbed energy of charged specimens− ◦ at −125 °C. In addition, the fracture surfaces of the charged Figurespecimens 12 are in roughly Figure alike,12 are which roughly mainly alike, covered which by mainly small spherical covered dimples. by small Therefore, spherical the dimples. surface morphologyTherefore, the and sur theface average morphology size of and dimples the average confirmed size thatof dimples hydrogen confirmed pre-charging that hydrogen degraded pre the- impact toughness and surface morphology of the CVN impact test conducted at 125 C but the charging degraded the impact toughness and surface morphology of the CVN impact− test ◦conducted diat ff−erences125 °C but among the differences the charged among specimens the charged were negligible. specimens were negligible.

Figure 12. ScanningScanning electron microscope (SEM)(SEM) images obtained from the impact test at −125125 °C:◦C: ( a) − uncharged, (b)) 1212 hh charged,charged, ((cc)) 2424 hh chargedcharged andand ((dd)) 4848 hh charged.charged.

FigureFigure 13 illustrates the microstructure imagesimages on the fracture surface in the CVN impact test at 196 C. The appearance of the fracture surfaces in this figure is dominated by small and spherical −196 °C◦ . The appearance of the fracture surfaces in this figure is dominated by small and spherical dimples, which are smaller and more circular than those in the fracturefracture surface of the impact tests conducted at the ambient temperature, 50 C and 125 C. Besides, the deep and large dimples are conducted at the ambient temperature, −−50 ◦°C and −125 °C◦ . Besides, the deep and large dimples are rarely observed herein, confirming that the type of fracture at 196 C was the least ductile fracture rarely observed herein, confirming that the type of fracture at −−196 ◦°C was the least ductile fracture among the tested temperatures.temperatures. That That is, is, this this was the reason why the absorbed energy in the CVN impact test at 196 C was the smallest among the tested temperatures. impact test at −196 °C◦ was the smallest among the tested temperatures.

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Figure 13. SScanningcanning electron microscope (S (SEM)EM) images obtained from the impact test at −196196 °C:C: ( (aa)) − ◦ uncharged, ( b) 12 h charged, (c) 24 h charged and (d)) 48 hh charged.charged.

Among the the SEM images in Figure 13,13, the uncharged specimen’s size size and depth were largest among the fracture surfaces obtained from the impact test at −196196 °CC,, confirming confirming that that the the type of − ◦ fracturefracture of the uncharged specimen at −196196 °CC was was the the most most ductile ductile fracture. fracture. Besides, Besides, the the differences differences − ◦ among the fracture surfaces the charged specimens were not noticeable. That That is, is, they were covered by s similarimilar shallow and small spherical dimples, confirming confirming that hydrogen exposure decreased the ductility and the absorbed energy of the charged specimens in the CVN impact test at −196 °C.C. − ◦ 4. Conclusions Conclusions The CVNCVN impact impact behavior behavior of the ofhydrogen-exposed the hydrogen-exposed 316L stainless 316L stainless steel at the steel ambient at the temperature, ambient 50 C, 125 C and 196 C was studied herein. The following conclusions can be drawn from this temperature,− ◦ − ◦ −50 °C, − −125 ◦°C and −196 °C was studied herein. The following conclusions can be drawnstudy: from this study:  Exposure to hydrogen increased the hydrogen concentration of tthehe samples collected at the • specimen surface. After After 24 24 h h of of charging, charging, the the hydrogen concentration in the charged specimens reached a saturation point.point.  Hydrogen charging resulted in a slight reduction in the absorbed energy and ductility of 316L Hydrogen charging resulted in a slight reduction in the absorbed energy and ductility of 316L • stainless steel at most of the tested specimens. The drop in absorbed energy varied from 0.23% stainless steel at most of the tested specimens. The drop in absorbed energy varied from 0.23% to to 16.6%. 16.6%.  The surface morphology of the uncharged specimens was more ductile than that of the pre- The surface morphology of the uncharged specimens was more ductile than that of the pre-charged • charged specimens impacted at ambient temperature, −125 °C and −196 °C. While the differences specimens impacted at ambient temperature, 125 C and 196 C. While the differences for for specimens impacted at −50 °C were negligible.− ◦ − ◦ specimens impacted at 50 C were negligible.  Another academic insight− obtained◦ herein is that low temperature decreased the ductility of the Another academic insight obtained herein is that low temperature decreased the ductility of the • V-notched specimens in the CVN impact test. The loss of ductility caused by the ductile to brittle V-notched specimens in the CVN impact test. The loss of ductility caused by the ductile to brittle transformation was attributed to the lowering of the temperature in the CVN impact tests. transformation was attributed to the lowering of the temperature in the CVN impact tests. Therefore, the impact properties of 316L stainless steel have a high resistance against HE and this materialTherefore, can the be impacta possible properties candidate of316L as material stainless for steel the havehydrogen a high containment resistance against system. HE However, and this usingmaterial 316L can stainless be a possible steel candidateat low temperature as material should for the be hydrogen carefully containment considered system.because However,of the losses using in ductility316L stainless and fracturesteel at low resistance temperature caused should by this be carefully low temperature. considered Former because studies of the losses investigated in ductility the effectand fracture of HE resistanceon the mechanical caused by propertiesthis low temperature. of materials. Former A standard studies forinvestigated the absorbed the e ff energyect of HE of hydrogenon the mechanical-exposed properties materials of in materials. specific temperature A standard for ranges the absorbed must be energy surveyed of hydrogen-exposed and created. The resultsmaterials of this in specific study coul temperatured contribute ranges to the must research be surveyed database and for created. hydrogen The tanks, results hydrogen of this study pipelines could and fuel cell vehicles containing 316L stainless steel.

Metals 2019, 9, 625 13 of 14 contribute to the research database for hydrogen tanks, hydrogen pipelines and fuel cell vehicles containing 316L stainless steel.

Author Contributions: L.T.H.N. wrote the paper, designed the testing program and performed CVN impact tests. J.S.H. coordinated the sample acquisition, specimen machining, measured hydrogen concentration. M.S.K. prepared the experimental testing facility and analyzed impact test results. J.H.K. optimized experimental setup for microstructure analysis and contributed reviewing the original manuscript. S.K.K. optimized experimental setup for CVN impact tests and contributed reviewing the original manuscript. J.M.L. supervised the experiments and contributed reviewing the original manuscript. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT (MSIT) (No. 2018R1A2B6007403). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) through GCRC-SOP (No. 2011-0030013). Conflicts of Interest: The authors declare no conflicts of interest.

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