metals

Article Effect of Low-Temperature Sensitization on Embrittlement of 301

Chieh Yu 1,†, Ren-Kae Shiue 1, Chun Chen 1 and Leu-Wen Tsay 2,*

1 Department of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan; [email protected] (C.Y.); [email protected] (R.-K.S.); [email protected] (C.C.) 2 Institute of Materials Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan * Correspondence: [email protected]; Tel.: +886-2-2462-5324 † Current Address: National Chung-Shan Institute of Science and Technology, Materials & Electro-Optics Research Division, Lung-Tan, Tao-Yuan 325, Taiwan.

Academic Editor: Robert Tuttle Received: 7 January 2017; Accepted: 10 February 2017; Published: 15 February 2017

Abstract: The effect of metastable austenite on the hydrogen embrittlement (HE) of cold-rolled (30% reduction in thickness) 301 stainless steel (SS) was investigated. Cold-rolled (CR) specimens were hydrogen-charged in an autoclave at 300 or 450 ◦C under a pressure of 10 MPa for 160 h before tensile tests. Both ordinary and notched tensile tests were performed in air to measure the tensile properties of the non-charged and charged specimens. The results indicated that cold rolling caused the transformation of austenite into α0 and ε-martensite in the 301 SS. Aging at 450 ◦C enhanced the precipitation of M23C6 carbides, G, and σ phases in the cold-rolled specimen. In 0 addition, the formation of α martensite and M23C6 carbides along the grain boundaries increased the HE susceptibility and low-temperature sensitization of the 450 ◦C-aged 301 SS. In contrast, 0 the grain boundary α -martensite and M23C6 carbides were not observed in the as-rolled and 300 ◦C-aged specimens.

Keywords: stainless steel; hydrogen embrittlement; hydrogen charging; G and σ phases; α0 martensite

1. Introduction Austenitic stainless steels (SSs) have been extensively used in industrial applications due to their good combination of resistance and mechanical properties. Metastable austenitic SSs may undergo phase transformation from austenite (γ) into ferromagnetic α0-martensite during plastic deformation [1–5]. The induced martensite during tensile tests at room temperature enhances the elongation to , hardness, and tensile strength [5]. However, the induced martensite increases the susceptibility of austenitic stainless steels to hydrogen embrittlement (HE) [2–8]. In hydrogen-containing environments, the more austenite transforms into martensite, the more ductility loss of the material occurs [6]. Hydrogen suppresses the transformation of austenite to martensite, leading to hydrogen softening and localized brittle fracture [5]. Moreover, the suppressed formation of α0-martensite in the highly strained region of a 304L specimen at 80 ◦C accounts for its lowered HE susceptibility [7]. It has been reported that strain-induced α0-martensite acts as a “hydrogen diffusion highway” in hydrogen-charged 304 SS [8], resulting in an increased hydrogen concentration at the crack tip and accelerated crack growth. Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in chloride-containing solutions [9–15]. In MgCl2 solution, the mechanism of environment-induced cracking of austenitic stainless steels can be either SCC or HE, depending on the test temperature [10,11]. A suitable amount of cold work can lower the SCC susceptibility of austenitic stainless steels, whereas excessive

Metals 2017, 7, 58; doi:10.3390/met7020058 www.mdpi.com/journal/metals Metals 2017, 7, 58 2 of 12 cold working reverses that trend [16,17]. The machined 304L SS showed a marked increase in SCC susceptibility as compared with the solution-annealed, unmachined, and cold worked samples [18,19]. Unlike martensite-free 304 SS, 304 SS with machining-induced martensite is greatly embrittled and undergoes premature failure in 40 MPa hydrogen [20]. As austenitic stainless steels are heated in the temperature range between 500 ◦C and 850 ◦C, carbides form along the grain boundary, leaving an adjacent chromium depletion zone. This phenomenon is called sensitization and it is responsible for intergranular corrosion or intergranular SCC (IGSCC) [21–24]. Cold-working austenitic stainless steels accelerates carbide precipitation, even in a Ti-stabilized (AISI 321) SS [24]. Cathodic hydrogen-charging greatly reduces the ductility of tensile specimens and decreases the time-to-failure of 304L SS and 308L weld metals under constant load tests, especially for the sensitized 304L (650 ◦C/24 h aging) [22]. Furthermore, the nucleation of tiny carbides in many austenitic SSs is enhanced by welding, thermo-mechanical processing, or slow cooling from the solution-annealed temperature. Such thermo or thermo-mechanical processes might not immediately induce IGSCC of the alloys. The number of carbide precipitates remains unchanged, but they grow in size during subsequent long-term service below 500 ◦C. Sensitization of austenitic SSs at temperatures below the classic sensitization range (500 ◦C–800 ◦C) is referred to as low-temperature sensitization (LTS). Cold deformation increases the degree of 304 SS sensitization up to 65 times that of undeformed 304 SS tested at 500 ◦C[25]. In a previous study, 304 SS welds exposed to a temperature of ◦ 450 C for 6600 h suffered from IGSCC due to the presence of (Fe,Cr)23C6 carbides along grain boundaries [26]. Cold working of the steel increases the sensitization kinetics of austenitic SSs by up to 15%, while further cold working shows less effect on sensitization [23]. The induced martensite in cold-worked 304 [27] or 304L [28] SSs causes low-temperature sensitization at 380 ◦C[27] and 500 ◦C[28], thereby increasing the SCC susceptibility in a BWR (boiling water reactor) simulated environment [28]. Furthermore, sensitization causes α0-martensite transformation preferentially along the grain boundaries of 304 and 316 SSs, which provides a high diffusivity path of hydrogen to the crack tip [29] and enhances the intergranular fracture and HE susceptibility of the steel [29]. In this study, cold-rolled 301 SS was hydrogen-charged in an autoclave at 300 ◦C or 450 ◦C at the pressure of 10 MPa for 160 h before straining. The effects of low-temperature sensitization during aging/hydrogen-charging at 300 ◦C or 450 ◦C on the microstructure were investigated. The HE susceptibility of various specimens was correlated with the corresponding microstructures, particularly the induced martensite and fine precipitates in the specimens.

2. Experimental Procedures The chemical composition of the AISI 301 SS used in this study was 16.71 Cr, 6.89 Ni, 0.08 C, 1.16 Mn, 0.54 Si, 0.02 P, 0.003 S, and the balance Fe in wt %. The 301 SS in the plate form with a thickness of 4.5 mm was solution-annealed at 1050 ◦C for 30 min and had a hardness of Hv 176. Cold rolling of the 301 SS plate with 30 % in thickness reduction was performed at room temperature, and designated as CR (cold-rolled) specimen. For comparison, CR specimens were aged at 300 ◦C or 450 ◦C for 160 h, and these specimens were respectively designated as CR-300 and CR-450. Figure1 shows the dimensions of the double-edge notched tensile and standard tensile specimens, which had a thickness of 3 mm. Cold-rolled specimens were hydrogen-charged in an autoclave at 300 ◦C or 450 ◦C at a pressure of 10 MPa for 160 h before straining. They were designated as CR-300H and CR-450H, respectively. Standard tensile specimens, according to ASTM E8 specification with a gauge length of 25 mm, were wire-cut directly from the cold-rolled plates along the rolling direction. Ordinary tensile tests were carried out at a strain rate of 6 × 10−4 s−1 (crosshead displacement rate of 0.9 mm/min) in laboratory air to determine the tensile properties of the non-hydrogen-charged specimens. Notched tensile tests were performed to evaluate the HE susceptibility of the hydrogen-charged specimens at a crosshead displacement rate of 0.72 mm/min at room temperature. Metals 2017, 7, 58 3 of 12 Metals 2017, 7, 58 3 of 12

Figure 1. SchematicSchematic dimensions dimensions of of ( (aa)) a a double double-edge-edge notched notched specimen specimen and and ( (bb)) a a standard tensile specimen.specimen. (R (RD:D: Rolling Rolling Direction).

A Fischer MP30 ferritescope (Windsor, CT, USA) can measure the ferrite contents precisely in A Fischer MP30 ferritescope (Windsor, CT, USA) can measure the ferrite contents precisely in austenitic and duplex SSs. The ferritescope was used in this study to determine the amounts of austenitic and duplex SSs. The ferritescope was used in this study to determine the amounts of strain-induced α′-martensite in all specimens [30,31]. The hydrogen-charged samples (CR-300H and strain-induced α0-martensite in all specimens [30,31]. The hydrogen-charged samples (CR-300H and CR-450H) with the dimensions of 1  1  1 cm3 were ground by No. 2000 SiC paper before cleaning. CR-450H) with the dimensions of 1 × 1 × 1 cm3 were ground by No. 2000 SiC paper before cleaning. The hydrogen contents of the charged samples (CR-300H and CR-450H) were measured by using the The hydrogen contents of the charged samples (CR-300H and CR-450H) were measured by using the LECO-TCH 600 (Saint Joseph, MI, USA). After hydrogen-charging, they were melted in a crucible. LECO-TCH 600 (Saint Joseph, MI, USA). After hydrogen-charging, they were melted in a crucible. The amounts of H and O were calculated from the H2O, CO, and CO2 mixture. Fractographs of each The amounts of H and O were calculated from the H2O, CO, and CO2 mixture. Fractographs of each specimen after tensile straining were inspected with a NOVA-450 scanning electron microscope specimen after tensile straining were inspected with a NOVA-450 scanning electron microscope (SEM, (SEM, Hillsboro, OR, USA). Selected specimens were inspected by using a JEOL 2000EX Hillsboro, OR, USA). Selected specimens were inspected by using a JEOL 2000EX transmission electron transmission electron microscope (TEM, Akishima, Japan). A high resolution TEM (HRTEM), FEI microscope (TEM, Akishima, Japan). A high resolution TEM (HRTEM), FEI Tecnai G2 F20 (Hillsboro, Tecnai G2 F20 (Hillsboro, OR, USA), was applied to inspect the nano-sized precipitates in the OR, USA), was applied to inspect the nano-sized precipitates in the specimen. specimen. 3. Results 3. Results 3.1. Microstructural Observation 3.1. Microstructural Observation Figure2 shows optical metallographs of cold-rolled specimens with different aging conditions. FigureFigure2a is a2 compositeshows optical photograph metallographs of the of CR cold specimen-rolled specimens revealing thewith microstructures different agingalong conditions. three Figureperpendicular 2a is a composite directions. photograph No severe textureof the CR was specimen observed revealing in the CR the specimen microstructures after 30% along thickness three perpendicularreduction. The directions. CR specimen No containedsevere texture basket-weaved was observed slip in bands the CR within specimen equiaxial after austenite 30% thickness grains. reduction.In the case ofThe the CR specimens specimen aged contained at 300 (CR-300) basket-weaved and 450 slip◦C (CR-450), bands within slip bands equiaxial could austenite be still observed grains. Inclearly, the case as shown of the in specimens Figure2b,c. aged The ferritescopeat 300 (CR-300) was used and 450 to determine °C (CR-450), the amount slip bands of strain-induced could be still observedα0 martensite clearly, formed as shown in the in tested Figure specimens. 2b,c. The Theferritescope ferrite contents was used of theto determine CR, CR-300, the and amount CR-450 of strainspecimens-induced were α′ 26%,martensite 26%, andformed 29%, in respectively. the tested specimens. It is well The known ferrite that contents heating of strain-induced the CR, CR-300α,0 andmartensite CR-450 to specimensan appropriate were temperature 26%, 26%, will and cause 29%, it to respectively. revert into austenite, It is well leading known to tahat decrease heating in strainferrite-induced content. α′ It wasmartensite deduced to that an appropriate increased amount temperature of α0 martensite will cause in it the to CR-450 revert into specimen austenite, was leadingassociated to a with decrease a specific in ferrite phase content. transformation It was ded inuced the specimen. that increased amount of α′ martensite in the CR-450 specimen was associated with a specific phase transformation in the specimen.

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(a)

(b) (c)

FigureFigure 2.2. OpticalOptical metallographsmetallographs of: of: (a ()a three) three mutually mutually perpendicular perpendicular planes planes of the of CR,the CR, (b) CR-300,(b) CR- and300, (andc) CR-450 (c) CR specimens.-450 specimens.

3.2. Mechanical Properties 3.2. Mechanical Properties Table 1 lists the mechanical properties of the test specimens. The 30% cold rolling obviously Table1 lists the mechanical properties of the test specimens. The 30% cold rolling obviously increased the surface hardness of CR specimen to Hv 455, as compared with the Hv 176 of the increased the surface hardness of CR specimen to Hv 455, as compared with the Hv 176 of the annealed annealed 301 SS. Dislocation recovery during aging at 300 °C was expected to be responsible for the 301 SS. Dislocation recovery during aging at 300 ◦C was expected to be responsible for the slight slight decrease in surface hardness of CR-300 specimen to Hv 445. The surface hardness of CR-450 decrease in surface hardness of CR-300 specimen to Hv 445. The surface hardness of CR-450 specimen specimen further reduced to Hv 414 due to the overage at 450 °C. The results of ordinary tensile tests further reduced to Hv 414 due to the overage at 450 ◦C. The results of ordinary tensile tests revealed revealed that all three tested specimens had the same elongation of 24%. The ordinary tensile that all three tested specimens had the same elongation of 24%. The ordinary tensile strengths of the strengths of the CR and CR-300 specimens were similar. However, the YS and UTS of the CR-450 CR and CR-300 specimens were similar. However, the YS and UTS of the CR-450 specimen were lower specimen were lower than those of the CR and CR-300 ones. than those of the CR and CR-300 ones.

Table 1.1. Mechanical propertiesproperties ofof testedtested specimensspecimens withwith 30%30% reductionreduction inin thickness.thickness. YS a UTS b EL c Hardness NTS d SpecimenSpecimen YS a (MPa) UTS b (MPa) EL c (%) Hardness(Hv) NTS d (MPa) (MPa) (MPa) (%) (Hv) (MPa) CRCR 1350 1350 15061506 24 24455 4551737 1737 CR-300 e 1400 1544 24 445 1743 CR-300 e 1400 1544 24 445 1743 CR-450 f 1200 1296 24 414 1584 f CR-300H g CR-450- 1200 -1296 24 -414 -1584 1339 CR-450H h CR-300H- g ------1339 843 h a YS: offset strength;CR-450Hb UTS: ultimate- tensile strength;- c EL: elongation;- d NTS:- notched tensile843 strength; e CR-300: CRa YS: specimen offset yield aged atstrength 300 ◦C;for b UTS: 160 h ultimate in vacuum; tensilef CR-450: strength; CR specimen c EL: elongation; aged at 450 d◦ NTS:C for 160notched h in vacuum; tensile g ◦ h strength;CR-300H: e CRCR specimen-300: CR hydrogen-chargedspecimen aged at at 300 300 °CC for for 160 160 h;h inCR-450H: vacuum;CR f CR specimen-450: CR hydrogen-charged specimen aged at 450 ◦C for 160 h. 450 °C for 160 h in vacuum; g CR-300H: CR specimen hydrogen-charged at 300 °C for 160 h; h CR-450H: CR specimen hydrogen-charged at 450 °C for 160 h.

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NotchedNotched tensile tests were performed in air to evaluate the notch brittleness of the specimens. TheThe NTSsNTSs ofof thethe CR,CR, CR-300,CR-300, and CR-450CR-450 specimens were 1737, 1743 1743,, and 1584 MPa, respectively. respectively. TheThe NTSNTS ofof thethe CR-450CR-450 specimenspecimen waswas lowerlower thanthan thosethose ofof thethe CRCR andand CR-300CR-300 ones.ones. This is consistent withwith the YS and and UTS UTS results results for for the the CR, CR, CR CR-300,-300, and and CR CR-450-450 specimens. specimens. It is It noted is noted that thatthe NTS the NTS was wassignificantly significantly higher higher than than the the corresponding corresponding UTS UTS for for all all specimens, specimens, indicating indicating that that the notched notched specimensspecimens possessedpossessed enoughenough toughnesstoughness toto resistresist localizedlocalized brittlebrittle fracturefracture beforebefore rupturerupture in in air. air. NotchedNotched tensile strengths tested in airair versusversus displacementdisplacement curves of thethe non-chargednon-charged (solid(solid lines)lines) andand hydrogen-chargedhydrogen-charged (dotted (dotted lines) lines) specimens specimens are are shown shown in Figurein Figure3. The 3. The NTSs NTSs of the of CR the and CR CR-300and CR specimens-300 specimens were similar, were similar, but the CR-300 but the specimen CR-300 had specimen a slightly had higher a slightly notch displacement higher notch (notchdisplacement ductility) (not thanch theductility) CR one. than In contrast,the CR one. the NTSIn contrast, and notch the displacement NTS and notch of the displacement CR-450 specimen of the wereCR-450 obviously specimen lower were than obviously those of lowerthe CR than and CR-300 those of ones the tested CR and in air. CR- Subsequently,300 ones tested aging in theair. ◦ cold-rolledSubsequently 301, aging SS at the 450 coldC was-rolled found 301 toSS deteriorate at 450 °C was its notched found to tensile deteriorate properties. its notched As shown tensile in Figureproperties.3, hydrogen-charging As shown in Figure caused 3, hydrogen a significant-charging drop caused in NTSs a significant and notch drop displacements in NTSs and of notch both thedisplacements CR-300H and of both CR-450H the CR specimens.-300H and HE CR was-450H responsible specimens. for HE such was a responsible severe degradation for such ofa severe notch tensiledegradation properties of notch of the tens hydrogen-chargedile properties of specimens. the hydrogen The results-charged also specimens. indicated that The the results CR-450H also specimenindicated wasthat morethe CR damaged-450H specimen by hydrogen-charging was more damaged than theby CR-300Hhydrogen one.-charging Aging than the the cold CR worked-300H specimenone. Aging at the higher cold temperature worked specimen was expected at higher to improve temperature its ductility was expected and reduce to improve brittleness, its butductility these improvementsand reduce brittleness, did not occurbut these in the improvements CR-450 specimen. did not This occur result in the would CR-450 be addressedspecimen. inThis the result later discussionwould be addressed of the mechanism in the later of deteriorationdiscussion of inthe CR-450. mechanism of deterioration in CR-450.

FigureFigure 3.3. NotchedNotched tensiletensile teststests ofof non-hydrogen-chargednon-hydrogen-charged (solid(solid lines)lines) andand hydrogen-chargedhydrogen-charged (dash(dash lines)lines) specimens.specimens.

3.3.3.3. Fractographic Examinations TheThe macroscopic macroscopic fracture fracture appearances appearances of selected of selected notched notched tensile tensile specimens specimens are shown are in shown Figure in4. TheFigure fractographs 4. The fractographs of the CR of and the CR-300 CR and specimens CR-300 specimens were alike, were comprising alike, comprising extensive shearextensive fractured shear (SF)fractured regions (SF) (shear regions lips) (shear on the lips) lateral on surfacesthe lateral and surfaces triangular and flat triangular fracture flat (FF) fracture zones ahead (FF) zones of the ahead notch fronts,of the notch as shown fronts, in as Figure shown4a. in A Fig noticeableure 4a. A reduction noticeable of reduction the SF regions of the and SF regions an increase and an in theincrease size of in thethe FFsize zone of the were FF observed zone were in theobserved CR-450 in specimen the CR-450 after specimen the notched after tensile the notched test (Figure tensile4b). test Moreover, (Figure the4b). fractographMoreover, the shown fractograph in Figure shown4b exhibited in Figure lamellar 4b exhibited tears andlamellar many tears fine and secondary many fine cracks. secondary Even withoutcracks. Even hydrogen-charging, without hydrogen the-charging, CR-450 specimen the CR-450 showed specimen inherent showed notch inherent brittleness notch in brittleness comparison in withcomparison the CR andwith CR-300 the CR specimens.and CR-300 specimens. It is known that hydrogen tends to accumulate and embrittle the elastic-plastic boundaries of high-strength steels [32], and this embrittlement becomes more severe at the locations of stress concentration, such as a notch [33,34]. Moreover, a notch can produce a triaxial stress state, and limits the plastic deformation ahead of the notch tip. The marked decreases in NTSs and notch displacements of the hydrogen-charged specimens (CR-300H and CR-450H in Figure 3) were

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associated with significant changes in their fractographs as illustrated in Figure 4c,d. In the CR-300H specimen, most SF zones were replaced by FF regions with parallel secondary cracks (Figure 4c). For Metals 2017, 7, 58 6 of 12 the CR-450H specimen, the fractured surface was dominated by FF regions (Figure 4d) due to the high HE susceptibility.

FigureFigure 4 4.. MacroMacro-fracture-fracture appearances appearances of (a) of CR- (300,a) CR-300, (b) CR-450, (b) (c CR-450,) CR-300H (c,) and CR-300H, (d) CR-450H and (dspecimens.) CR-450H specimens.

ItFailure is known analyses that hydrogen of all fractured tends tosurfaces accumulate after tensile and embrittle tests were the performed elastic-plastic using boundaries an SEM. ofDuctile high-strength dimple fracturesteels [32 was], and observed this embrittlement in all ordinary becomes tensile more specimens severe at (not the shown locations here). of stress SEM concentration,fractographs of such selected as a specimens notch [33, 34after]. Moreover,notched tensile a notch tests can are produce presented a triaxialin Figure stress 5. The state, fracture and limitsmodes the of the plastic CR- deformation300 and CR-450 ahead specimens of the notchdiffered tip. slightly. The marked In the CR decreases-300 specimen, in NTSs predominant and notch displacementsfine shallow dimples of the hydrogen-charged mixed with sparsely specimens flat cleavage (CR-300H-like and fracture CR-450H were in Figure observed3) were within associated the FF withzones significant ahead of changesnotch front in their (Figure fractographs 5a). In contrast, as illustrated mainly in cleavage Figure4 c,d.fracture In the with CR-300H secondary specimen, cracks mostalong SF austenite zones weregrain replacedboundaries by was FF regionsfound ahead with of parallel the notch secondary front of the cracks CR- (Figure450 specimen4c). For (Fig theure CR-450H5b), indicating specimen, the the brittle fractured nature surface of was the dominated sample. by The FF CRregions-450 (Figure specimen,4d) due even to the without high HEhydr susceptibility.ogen-charging, was likely to suffer intergranular fracture under strain. Regarding the hydrogenFailure-charged analyses specimens, of all fractured extensive surfaces cleavage after tensile-like fracture tests were together performed with using numerous an SEM. secondary Ductile dimplecracks fracture was observed was observed in the CRin all-300H ordinary specimen tensile (Fig specimensure 5c), (not while shown the CR here).-450 SEM specimen fractographs mainly ofexhibited selected intergranular specimens after fracture notched (Fig tensileure 5d). tests are presented in Figure5. The fracture modes of the CR-300 and CR-450 specimens differed slightly. In the CR-300 specimen, predominant fine shallow dimples mixed with sparsely flat cleavage-like fracture were observed within the FF zones ahead of notch front (Figure5a). In contrast, mainly cleavage fracture with secondary cracks along austenite grain boundaries was found ahead of the notch front of the CR-450 specimen (Figure5b), indicating the brittle nature of the sample. The CR-450 specimen, even without hydrogen-charging, was likely to suffer intergranular fracture under strain. Regarding the hydrogen-charged specimens, extensive cleavage-like fracture together with numerous secondary cracks was observed in the CR-300H specimen (Figure5c), while the CR-450 specimen mainly exhibited intergranular fracture (Figure5d).

Figure 5. Cont.

Metals 2017, 7, 58 6 of 12 associated with significant changes in their fractographs as illustrated in Figure 4c,d. In the CR-300H specimen, most SF zones were replaced by FF regions with parallel secondary cracks (Figure 4c). For the CR-450H specimen, the fractured surface was dominated by FF regions (Figure 4d) due to the high HE susceptibility.

Figure 4. Macro-fracture appearances of (a) CR-300, (b) CR-450, (c) CR-300H, and (d) CR-450H specimens.

Failure analyses of all fractured surfaces after tensile tests were performed using an SEM. Ductile dimple fracture was observed in all ordinary tensile specimens (not shown here). SEM fractographs of selected specimens after notched tensile tests are presented in Figure 5. The fracture modes of the CR-300 and CR-450 specimens differed slightly. In the CR-300 specimen, predominant fine shallow dimples mixed with sparsely flat cleavage-like fracture were observed within the FF zones ahead of notch front (Figure 5a). In contrast, mainly cleavage fracture with secondary cracks along austenite grain boundaries was found ahead of the notch front of the CR-450 specimen (Figure 5b), indicating the brittle nature of the sample. The CR-450 specimen, even without hydrogen-charging, was likely to suffer intergranular fracture under strain. Regarding the hydrogen-charged specimens, extensive cleavage-like fracture together with numerous secondary cracks was observed in the CR-300H specimen (Figure 5c), while the CR-450 specimen mainly Metals 2017, 7, 58 7 of 12 exhibited intergranular fracture (Figure 5d).

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Figure 5. Cont.

FigureFigure 5.5.. SEM fractographs of (a)) CR-300,CRCR--300, (b)) CR-450,CRCR--450, (c)) CR-300H,CRCR--300H,, and (d)) CR-450HCRCR--450H specimens after notchednotched tensiletensile test.test.

3.4.3..4.. TEM TEM Examinations FigureFigure6 6 displays displays detailed detailed TEM TEM micrographs micrographsicrographs of of of the the the CR CR CR specimen. specimen. specimen. ThereThere were were three three distinct distinct structures—structures—εε-martensite,-martensite,α α′0-martensite,-martensite,, and and austenite—in austenite—inin the thethe CR CRCR specimen. specimen.specimen. The TheThe microstructure microstructuremicrostructure of the ofof CRthe specimenCR specimen was was primarily primarily comprised comprised of parallel of parallel strips strips of ε -martensiteof ε-martensite (hcp) (hcp) in the in austenitethe austenite (γ, fcc) (γ,, matrix,fcc) matrix, as illustrated as illustrated in Figure in Fig6a.ure The 6a..ε The-martensiteThe εε-martensite formed formed in slip in bands slip bands due to due the 30%to the cold 30% rolling cold ofrolling the 301 of SS. theThe 301α SS.0-martensite The α′-martensite (bcc) had also(bcc) been had found also been mainly found at the mainly intersections at the intersections of ε-martensite of strips.ε-martensite Similar strips. results Similar were obtained results were in prior obtained study ofin theprior SCC study of cold-rolled of the SCC 304L of cold SS [4-,rolled12], consistent 304L SS with[4,12], the consistent location ofwithα0 -martensite,the location asof shownα′-martensite, in Figure as6 shownb. in Figure 6b..

FigureFigure 6.6.. TEM micrographs of the CRCR specimenspecimen illustratingillustrating (aa)) εε-martensite--martensite strips inin thethe austeniteaustenite matrix,matrix, ((bb)) αα′α′0--martensite-martensite at the int intersectionsersections of ε-martensite--martensite strips.

In the CR-450 specimen, further aging at 450 °C for 160 h enhanced the transformation of In the CR-450 specimen, further aging at 450 ◦C for 160 h enhanced the transformation of metastable austenite into α′0 -martensite and the precipitation of M2323C66 carbides along the grain metastable austenite into α -martensite and the precipitation of M23C6 carbides along the grain boundaries, as displayed in Figure 7. In the CR and CR-300 specimens, the grain boundary boundaries, as displayed in Figure7. In the CR and CR-300 specimens, the grain boundary α′0-martensite and M2323C66 carbides were not observed. Due to the presence of fresh grain boundary α -martensite and M23C6 carbides were not observed. Due to the presence of fresh grain boundary α′-martensite, the ferrite contents of the CR-450 specimen were higher than those of the CR and α0-martensite, the ferrite contents of the CR-450 specimen were higher than those of the CR and CR-300 specimens. Both the precipitation of M2323C66 carbides and increased α′-martensite along the grain boundaries resulted in increasing HE susceptibility of the CR-450H specimen, as compared with the CR-300H specimens. It was obvious that cold rolling the 301 SS induced partial austenite transformation into α′- and ε-martensite. The subsequent aging at 450 °C promoted the formation of grain boundary α′-martensite and M2323C66 carbides, which were responsible for the low-temperature sensitization of the SS.

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0 CR-300 specimens. Both the precipitation of M23C6 carbides and increased α -martensite along the grain boundaries resulted in increasing HE susceptibility of the CR-450H specimen, as compared with the CR-300H specimens. It was obvious that cold rolling the 301 SS induced partial austenite transformation into α0- and ε-martensite. The subsequent aging at 450 ◦C promoted the formation of 0 grain boundary α -martensite and M23C6 carbides, which were responsible for the low-temperature Metals 2017, 7, 58 8 of 12 sensitization of the SS.

Figure 7.7. TEM images of the CR CR-450-450 specimen: ( a)) bright bright field field image, ( b) dark field field image of carbides 0 using (11 1) diffraction spot, (c) dark fieldfield imageimage of αα′ martensite using ( 101) spot, (d) selected area diffraction pattern with two identifiedidentified structures.

FigureFigure 88 shows shows TEM TEM images images of of the the G G and and σ σ phases phases in in the the CR-450CR-450 specimen. specimen. Nano-sizedNano-sized precipitates, G an andd σσ phases,phases, were were found found in in the the α′α martensite0 martensite of of the the CR CR-450-450 specimen, specimen, as asshown shown in inFig Figureure 8a8–a–c.c. The The average average size size of of the the G G phase phase precipitates precipitates was was greater greater than than that ofof the the σ phase precipitates. The lattice image was transformed in intoto a diffraction pattern by fast Fourier transform (FFT) for identificationidentification of the crystal structure of nano nano-sized-sized precipitates by by high high resolution resolution TEM. TEM. FigureFigure8 b 8b is is a lattice a lattice image image of the of G the phase G ph formedase formed in the inα' matrix the α' with matrix a [100] with zone a [100] axis. zone A σ precipitate axis. A σ inprecipitate the α0 matrix in the with α′ matrix a [013] with zone a axis[013] is zone shown axis in is Figure shown8d. in Figure 8d.

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Figure 8.8. TEM images and high resolution lattice images with fast Fourier Fourier transform transform diffraction diffraction patterns ofof thethe CR-450CR-450 specimen:specimen: (a,,b)) GG phase;phase; andand ((cc,,dd)) σσ phase.phase.

4.4. Discussion In general, general, an an increase increase in annealing in annealing temperature temperature reduces reduces the strain-hardening the strain-hardening effect of cold-rolled effect of steels.cold-rolled Annealing steels. treatmentAnnealing can treatment improve can the improve resistance the ofresistance work-hardened of work- steelhardened to HE. steel As to shown HE. As in Figureshown6 , in cold Figure work 6, enhanced cold work the enhanced formation the of formationε- and α’-martensite of ε- and α’ in-martensite the metastable in the austenitic metastable SS. Moreover,austenitic metastableSS. Moreover, austenitic metastable SSs undergo austenitic traditional SSs undergo sensitization traditional when sensitization heated above when 500 ◦ heatedC, and low-temperatureabove 500 °C, and sensitization low-temperature occurs sensitization below 500 ◦occursC, if the below cold 500 work °C, is if applied. the cold Thework formation is applied. of inducedThe formationα0-martensite of induced is responsible α′-martensite for is its responsible easy sensitization. for its easy In sensitization. this work, aging In this the work, cold-rolled aging ◦ 0 301the cold SS at-rolled 450 C 301 (CR-450) SS at 450 assisted °C (CR the-450) formation assisted of the grain formation boundary of grainα -martensite boundary and α′- Mmartensite23C6 carbides. and InM23 contrast,C6 carbides. grain In boundarycontrast, grainα’-martensite boundary andα’-martensite M23C6 carbides and M were23C6 carbides not observed were not in theobserved CR and in CR-300the CR and specimens. CR-300 specimens. As illustratedillustrated inin Table Table1, both1, both UTS UTS and and YS ofYS the of CR-300the CR- specimen300 specimen were were slightly slightly higher higher than those than ofthose the of CR the one. CR In one. contrast, In contrast, UTS and UTS YS and of the YS CR-450 of the specimenCR-450 specimen were obviously were obviously lower than lower those than of CRthose and of CR-300CR and ones. CR-300 Moreover, ones. Moreover, all specimens all specimens showed equivalent showed equivalent tensile elongation tensile elongation tested in air. tested The resultsin air. The indicated results increasing indicated the increasing annealing the temperature annealing temperature to 450 ◦C did to not 450 improve °C did notthe ductility improve but the causedductility a declinebut caused in strength a decline of in the strength cold worked of the 301 cold SS. worked The results 301 SS. of notchedThe results tensile of notched tests in airtensile also revealedtests in air that also the revealed NTS of CR-450that the specimen NTS of CR was-450 lower specimen than those was oflower the others.than those As shown of the inothers. Figure As4, theshown CR-450 in Fig specimenure 4, the comprised CR-450 specimen of extensive comprised flat fracture of extensive region (Figure flat fracture4b), in contrast, region (Fig shearure fracture 4b), in dominatedcontrast, shear the fractographs fracture dominated of CR and the CR-300 fractographs specimens of CR (Figure and CR4a).-300 It demonstrated specimens (Fig theure fact 4a). of It highdemonstrated notch brittleness the fact ofof thehigh CR-450 notch specimen,brittleness andof the could CR- be450 related specimen, to the and brittle could microstructures be related to the in brittle microstructures in the specimen. SEM fractographs of the CR-450 specimen after the notched tensile test in air showed cleavage-like fracture along with intergranular fracture (Figure 5b), which

Metals 2017, 7, 58 10 of 12 the specimen. SEM fractographs of the CR-450 specimen after the notched tensile test in air showed cleavage-like fracture along with intergranular fracture (Figure5b), which could be associated with 0 the formation of grain boundary α -martensite and M23C6 carbides, as shown in Figure7. Without the presence of brittle microstructures along grain boundaries, the notched fracture appearance of the CR and CR-300 specimens showed predominantly shallow dimple fracture (Figure5a). It was reported that the HE susceptibility of the austenitic SSs is affected by the compositions [35,36] and prestrained level of the SS [37,38]. Dynamic interactions between hydrogen and the induced martensite play important roles in the HE of the hydrogen-charged 304 SS [37]. The hydrogen enhances the formation of α0-martensite, which facilitates HE and dominates the fracture process of the metastable austenitic SS [7]. Moreover, the hydrogen source influences the location of crack initiation and propagation in austenitic SSs. An external hydrogen source promotes crack initiation and propagation at the surface. Similar to surface cracks, the propagation of internal cracks is accelerated by the presence of internal hydrogen in the 304 and 316 SSs [35]. In Table1, both CR-300H and CR-450H specimens were highly susceptible to internal HE, especially for the CR-450H one. It was deduced that high sensitivity to HE of the cold worked 301 SS could be attributed to its low austenite stability under straining. The CR-450H specimen suffered from a greater loss of ductility than the CR-300H specimen, as displayed in Figure3. Both macro- and micro-fractographs, as shown in Figures4 and5, also confirmed that the CR-450H specimen was more susceptible to internal HE than the CR-300H specimen. High extent of intergranular fracture in the CR-450H specimen was associated with the brittle characteristics of grain boundaries. Nano-sized G and σ particles were observed in α0-martensite of CR-450 specimen (Figure8). The precipitation of G phase in the ferrite matrix has also been reported in the duplex SS after prolonged aging above 350 ◦C[39]. The precipitation of nano-sized G and σ phases further strengthened the interiors of the grains of the CR-450 sample. The strengthening of the grain interior by nano-sized precipitates highlighted the weakness of the embrittled grain boundaries and favored crack propagation therein. Actually, hydrogen-charging caused more hydrogen to diffuse into the specimen at 450 ◦C than that at 300 ◦C. The hydrogen concentrations of the CR-300H and CR-450H were 12 and 20 ppm, respectively. A high hydrogen concentration is usually accompanied by high HE susceptibility. Both the deteriorated microstructure and the high hydrogen content accounted for the high HE susceptibility and extensive intergranular fracture of the CR-450H specimen.

5. Conclusions The effects of cold rolling and subsequent hydrogen-charging at 300 ◦C or 450 ◦C for 160 h on the microstructure, tensile properties, and HE susceptibility of 301 SS were investigated. Cold rolling caused ε-martensite to form in parallel strips of the slip bands, and α0-martensite was found mainly at the intersections of ε-martensite. Hydrogen-charging at 300 ◦C and 450 ◦C led to severe HE of the cold-rolled 301 SS, particularly in the 450 ◦C charged specimens. In specimens that were not hydrogen-charged, the notched tensile fracture of the 450 ◦C-aged one showed brittle fracture appearance, which comprised of cleavage-like fracture together with intergranular separations. In the 0 CR-450 specimen, α -martensite and M23C6 carbides were formed along the grain boundary. Because of this fresh grain boundary of α0-martensite, the ferrite contents of the CR-450 specimen were higher than those of the CR and CR-300 specimens. Moreover, very fine precipitates including the G and σ 0 0 phases were found in the α -martensite. The formation of grain boundary α -martensite and M23C6 carbides together with nano-sized precipitates in the α0-martensite were responsible for the high HE susceptibility and low-temperature sensitization of the CR-450 specimen. In contrast, those of grain boundary precipitates and nano-sized phases were not observed in the CR and CR-300 specimens.

Acknowledgments: The authors gratefully acknowledge the partial financial support of this research by the National Science Council, Republic of China, Taiwan. Metals 2017, 7, 58 11 of 12

Author Contributions: L.W. Tsay and R.K. Shiue designed and planned the experiment. C. Chen contributed to the discussion of the results. C. Yu performed the tests and TEM examinations. All co-authors contributed to the MetalsMetalsmanuscript 2017 2017, 7,, 758 proof, 58 and submissions. 1111 of of 12 12 Conflicts of Interest: The authors declare no conflict of interest. ConflictsConflicts of of Interest: Interest: The The authors authors declare declare no no conflict conflict of of interest. interest. References ReferencesReferences 1. Böhner, A.; Niendorf, T.; Amberger, D.; Höppel, H.W.; Göken, M.; Maier, H.J. Martensitic transformation 1. 1. Böhner,Böhner,in ultrafine-grained A.; A.; Niendorf, Niendorf, T.; stainless T.; Amberger, Amberger, steel D.; AISI D.; Höppel, Höppel, 304L under H.W.; H.W.; monotonic Göken, Göken, M.; M.; and Maier, Maier, cyclic H.J. H.J. loading. 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