1. Introduction. 2. Experimental procedures Material

Transcription

1 Avilble online t ScienceDirect Procedi Structurl Integrity 2 (2016) st Europen Conference on Frcture, ECF21, June 2016, Ctni, Itly Effects of hydrogen pressure, test frequency nd test temperture on ftigue crck growth properties of low-crbon steel in gseous hydrogen Junichiro Ymbe,b,c *, Michio Yoshikw b, Hiso Mtsung b,c,d, Sburo Mtsuok b Interntionl Reserch Center for Hydrogen Energy, Kyushu University, 744 Moto-ok, Nishi-ku, Fukuok-shi, , Jpn b Reserch Center for Hydrogen Industril Use nd Storge, Kyushu University, 744 Moto-ok, Nishi-ku, Fukuok-shi, , Jpn c Interntionl Institute for Crbon-Neutrl Energy Reserch, Kyushu Universit, 744 Moto-ok, Nishi-ku, Fukuok-shi, , Jpn d Deprtment of Mechnicl Engineering, Kyushu University, 744 Moto-ok, Nishi-ku, Fukuok-shi, , Jpn Abstrct Ftigue crck growth (FCG) tests for compct tension (CT) specimens of n nneled, low-crbon steel, JIS-SM490B were performed under vrious combintions of hydrogen pressures rnging from 0.1 to 90 MP, test frequencies from to 10 Hz nd test tempertures of room temperture (RT), 363 K nd 423 K. In the hydrogen pressures of 0.1, 0.7 nd 10 MP t RT, the FCG rte incresed with decrese in the test frequency; then, peked out. In the lower test frequency regime, the FCG rte decresed nd becme nerly equivlent to the FCG rte in ir. Also, in hydrogen pressure of 45 MP t RT, the hydrogen-ssisted FCG ccelertion showed n upper limit round the test frequencies of 0.01 to Hz. On the other hnd, in the hydrogen pressure of 90 MP t RT, the FCG rte monotoniclly incresed with decrese in the test frequency, nd eventully the upper limit of FCG ccelertion ws not confirmed down to the test frequency of Hz. In the hydrogen pressure of 0.7 MP t the test frequency of 1 Hz nd tempertures of 363 K nd 423 K, the stress intensity fctor rnge, ΔK, for the onset of the FCG ccelertion in hydrogen gs ws shifted to higher ΔK with n increse in the test temperture. The lser-microscope observtion t specimen surfce reveled tht the hydrogen-ssisted FCG ccelertion lwys ccompnied locliztion of plstic deformtion ner crck tip. These results infer tht the influencing fctor dominting the hydrogen-ssisted FCG ccelertion is not the presence or bsence of hydrogen in mteril but is how hydrogen loclizes ner the crck tip. Nmely, steep grdient of hydrogen concentrtion cn result in the slip locliztion t crck tip, which enhnces the Hydrogen Enhnced Successive Ftigue Crck Growth (HESFCG) proposed by the uthors. It is proposed tht such peculir dependence of FCG rte on hydrogen pressure, test frequency nd test temperture cn be unified by using novel prmeter representing the grdient of hydrogen concentrtion ner crck tip. Copyright 2016 The Authors. Published by Elsevier B.V. This is n open ccess rticle under the CC BY-NC-ND license ( Peer-review under responsibility of the Scientific Committee of ECF21. * Corresponding uthor. Tel.: ; fx: E-mil ddress: ymbe@mech.kyushu-u.c.jp Copyright 2016 The Authors. Published by Elsevier B.V. This is n open ccess rticle under the CC BY-NC-ND license ( Peer review under responsibility of the Scientific Committee of ECF /j.prostr

2 526 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) Peer-review under responsibility of the Scientific Committee of ECF21. Keywords: hydrogen; ftigue crck growth; low-crbon steel; high-pressure hydrogen gs; elevted temperture; test frequency 1. Introduction The hydrogen sometimes degrdes tensile nd ftigue properties of metllic mterils (Ngumo (2008); Murkmi et l. (2012); Gngloff et l. (2012); Mtsuo et l. (2014)). For the widespred commerciliztion of hydrogen-energy systems, n pproprite design method must be estblished in considertion of the detrimentl effect of hydrogen on mterils (Sn Mrchi et l. (2014); Mtsung et l. (2015); Ymbe et l. (2016); Mtsuok et l. (2016)). To perform sfe nd relible finite-life design, it is importnt to precisely cpture ftigue crck growth (FCG) property of mterils in presence of hydrogen. For low-lloy steel, JIS-SCM435, with tensile strength lower thn 900 MP in high-pressure gseous hydrogen, there exists u upper bound of the FCG ccelertion, lthough reduction in re (RA) during slow strin rte tensile (SSRT) testing is degrded; therefore, this low-lloy steel is considered to be eligible for hydrogen service under the finite-life design (Ymbe et l. (2016)). In contrst, there is no upper bound of FCG ccelertion in the presence of hydrogen for high-strength steel with tensile strength of pproximtely 1900 MP; therefore, this steel is not eligible for hydrogen service under the finite-life design (Ymbe et l. (2012)). These results infer tht the FCG property in presence of hydrogen is strongly dependent on mterils. In order to enble to uthorize vrious low-cost steels for use in high-pressure gseous hydrogen, this study investigted the effects of hydrogen pressure, test frequency nd test temperture on FCG properties of low-crbon steel, JIS-SM490B. 2. Experimentl procedures 2.1. Mteril The mteril used in this study ws n nneled, low-crbon steel, JIS-SM490B, exhibiting ferrite nd perlite structure, composed of 0.16 C, 0.44 Si, 1.43 Mn, P, S in mss %, nd the blnce Fe,. The Vickers hrdness of the mtrix ws HV = 153, mesured (20 points) with the lod of 9.8 N. The lower-yield stress, σ LY, tensile strength, σ B, elongtion, δ, reduction in re, φ, t room temperture in ir t RT were 360 MP, 540 MP, 17 % nd 78 %, respectively. To estimte hydrogen-diffusion properties t crck tip, cold-rolled pltes of JIS-SM490B with rolling rtios of 5, 10, 15, 20, 30 nd 40 % were lso prepred, in ddition to n s-received plte with no cold rolling Determintion of hydrogen-diffusion properties For determining hydrogen-diffusion properties, cylindricl specimen with 2r 0 = z 0 = 19 mm, where 2r 0 is the dimeter nd z 0 is the thickness, ws smpled from the s-received nd cold-rolled pltes. The surfce of the specimens ws finished with #600 emery pper. The specimens were exposed to hydrogen gs t 100 MP nd 358 K for 200 hours to obtin the uniform distribution of hydrogen. After the exposure, the hydrogen contents of the specimens were mesured under constnt tempertures by gs chromtogrphy mss spectroscopy (GC MS). The hydrogen diffusivity ws determined by fitting the solution of diffusion eqution to the experimentl hydrogen contents mesured t vrious constnt tempertures (Ymbe et l. (2015)) Ftigue crck growth test For FCG test, compct tension (CT) specimen with width of, W, 50.8 mm nd thickness, B, of 10 mm ws smpled from the s-received plte. The FCG test ws performed t stress rtio, R, of 0.1 under vrious combintions of hydrogen pressures rnging from 0.1 to 90 MP, test frequencies from to 10 Hz nd test tempertures of room temperture (RT), 363 K nd 423 K, in ccordnce with ASTM E647-08e1 (2010). The purity of hydrogen gs in the cylinder ws % (5N) nd the mesured oxygen contents were lwys less thn 1.0 vol. ppm.

3 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) In ddition to the FCG test under constnt lod rnge, ΔP, the FCG test ws crried out under constnt stress intensity fctor rnge, ΔK, to clrify the effect of f on the FCG rte. These tests re referred to s ΔP-constnt nd K- constnt tests, respectively. The crck size ws obtined by mens of the complince method with crck-opening displcements (CODs) s follows: α = / W = ux ux ux ux ux (1) u x EVg B = ([ ] P 1/ 2 + 1) 1 where is the crck length, W is the specimen width, B is the specimen thickness, E is Young s modulus, V g is the COD. The ΔK ws clculted s follows: K = B P W ( + α ) ( 1+ α ) ( α 13.32α α 5.6α ) 3/ 2 The FCG tests were performed under the following smll-scle yielding (SSY) condition: W 4 ( )( π K σ 4 K mx 2 2 ) = ( )[ ] YS π σ YS(1 R) where σ YS is the yield stress. After the FCG test, slip deformtions were observed by lser microscopy (LM). 3. Results nd discussion 3.1. Hydrogen diffusivity of cold-rolled steels Fig. 1() shows the Arrhenius plot of hydrogen diffusivity, D, of the cold-rolled JIS-SM490B. Literture dt (Kiuchi et l. (1983); Asno et l. (1974)) re lso shown in Fig. 1(). The D of the cold-rolled steel ws lower thn tht of α-iron nd nneled low-crbon steel. This is becuse of lttice defect produced by the cold rolling. The ctivtion energy of D for the steel with the rolling rtio of 40 % ws nerly equl to tht of the cold-rolled low-crbon steel reported by Asno et l. (1974). Fig. 1(b) shows the D of the cold-rolled steel t 303, 363 nd 423 K. Irrespective of the mesured tempertures, the vlues of D decresed with n increse in the rolling rtio; however, becme constnt for rolling rtios higher thn 20 %. Since severe plstic deformtion is produced t crck tip, the hydrogen diffusivity of which is represented by tht of the steel with higher rolling rtios obtined here. Bsed on the Orini s equilibrium theory (Orini (1970)), the experimentl dt were fitted by the following eqution s N X /N L nd E B were unknown prmeters: Hydrogen diffusivity, D [m 2 /s] b d c Rolling rtio 〇 5 % 10 % 40 % Literture dt, b: α-iron (Kiuchi et l.) c: nneled low-crbon steel (Asno et l.) d: low-crbon steel with 40% rolling (Asno et l.) / T [1/K] Rolling rtio [%] Fig. 1. () Temperture dependence of hydrogen diffusivity; (b) Hydrogen diffusivity vs rolling rtio t 303, 363 nd 423 K. b Hydrogen diffusivity, D [m 2 /s] Temperture 〇 303 K 363 K 423 K (2) (3). (4)

4 528 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) D = N 1+ N X L DL = EB N exp( ) 1+ RT N X L D0 ED exp( ) EB exp( ) RT RT where D L is the lttice hydrogen diffusivity in α-iron without ny trps; N L is the number of lttice sites per the unit volume; N X is the number of trp sites per the unit volume; E B is the biding energy; R is the gs constnt; T is the bsolute temperture. According to Kiuchi et l. (1983), the vlues of D 0 nd E D re given s D 0 = m 2 /s nd E D = 5.69 kj/mol. The broken lines for the steels with the rolling rtions of 5, 10 nd 40 % shown in Fig. 1() re fitted ones bsed on Eq. (5). For the steel with the rolling rtio of 40 %, the following fitted prmeters were obtined: N X / N L = nd E B = 28.9 kj/mol. On the other hnd, the sturted hydrogen content, C S, of the cold-rolled steel cn be clculted by using N X / N L nd E B s follows: NX EB C S = { 1+ exp( )} C N RT C LS L 3440 = α F exp( ) T LS where C LS is the sturted hydrogen content in the α-iron; F is the fugcity. The α is mss ppm (Hirth (1980)). According to Sn Mrchi et l. (2007), the fugcity F [MP] is expressed by using the hydrogen pressure s follows: F = p bp RT H2 exp( H2 ) where p H2 is the hydrogen pressure [MP] nd b is the constnt (= m 3 /mol). The vlues of C S nd D for severe plstic deformtion under vrious environmentl conditions were clculted by using Eqs. (5) to (8) Ftigue crck growth behviour nd ftigue crck morphology t room temperture Fig. 2() shows the FCG rte, d/dn, s function of ΔK in ir nd in the vrious pressures of hydrogen gs. In low ΔK regime, i.e., ΔK < 20 MP m 1/2, the FCG ccelertion incresed with n increse in ΔK. Conversely, in higher ΔK regime, i.e., ΔK > 20 MP m 1/2, the d/dn ΔK curves in hydrogen gs were prllel to the curves in ir. Fig. 2(b) presents the reltive FCG rte (RFCGR), (d/dn) H2 /(d/dn) ir, s function of p H2 obtined t ΔK = 30 MP m 1/2 under f = 1 Hz nd R = 0.1, where (d/dn) H2 nd (d/dn) ir re the FCG rtes in hydrogen nd ir, respectively. The RFCGR ws nerly constnt t p H2 = 0.7 ~ 90 MP. Fig. 2(c) exhibits the reltionship between the RFCGR nd f. Under p H2 10 MP, the RFCGR grdully incresed with reduction in f; then, suddenly decresed close to 1.0. Similr behviour hs been reported for Cr-Mo nd pipeline steels (Mtsuok et l. (2011); Somerdy et l. (2013)) nd ustenitic stinless steels (Itog et l. (2014)). It is to be noted tht the pek of ccelertion shifted towrds the lower f with n increse in p H2. Under p H2 45 MP, the reduction of the RFCGR did not occur in the low-frequency regime down to the f of Hz. It is lso noteworthy tht t the p H2 of 45 MP, the RFCGR sturted t bout 30. On the other hnd, t the p H2 of 90 MP, the upper bound of the FCG ccelertion did not exist down to the f of Hz. Figs. 3 nd 4 show LM imges of the crck morphology t the surfce of the CT specimen, fter the ΔK-constnt test performed t ΔK = 30 MP m 1/2. In Fig. 3, the test ws initited in ir, fter the test tmosphere ws switched to 0.7-MP hydrogen gs. In ir, extensive slip bnds were observed long the ftigue crck (Fig. 3(b)). The sme proved to be the cse for the crck grown in 0.7-MP hydrogen gs t the frequency of Hz (Fig. 3(d)), where the crck growth rte ws nerly equivlent to tht observed in ir. In contrst, in the test in 0.7-MP hydrogen gs t the frequency of 1 Hz, where the FCG ws ccelerted by bout fctor of 10, very few slip bnds were observed long the crck (Fig. 3(c)). This ws ssumed to be due to the locliztion of plstic deformtion t the crck tip under the influence of hydrogen. Conversely, in the test in 90 MP hydrogen gs (Fig. 4), where growth ws ccelerted t the test frequencies of 1 Hz nd Hz, very few slip bnds were observed long the crck in both cses. (5) (6) (7) (8)

5 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) b c Fig.2. () Reltionship between d/dn nd ΔK in 0.1 ~ 90-MP hydrogen gs t RT; (b) RFCGR vs p H2 ; (c) RFCGR vs f. b c d Fig.3. Crck growth morphologies t the specimen surfce fter the FCG test in 0.7-MP hydrogen gs t RT: () Low mgnifiction; (b) Mgnifiction of (A); (c) Mgnifiction of (B); (d) Mgnifiction of (C). Fig.4. Crck growth morphologies t the specimen surfce fter the FCG test in 90-MP hydrogen gs t RT Ftigue crck growth behviour nd ftigue crck morphology t elevted tempertures Fig. 5() shows the reltionship between d/dn nd ΔK for the ΔP-constnt tests t R = 0.1 nd f = 1 Hz in ir or 0.7-MP nitrogen gs nd in 0.7-MP hydrogen gs t RT, 363 K nd 423 K. Irrespective of the test tempertures, the FCG rte ws ccelerted in hydrogen gs; however, the RFCGR ws lower with higher tempertures. The ΔK for the onset of the FCG ccelertion, ΔK onset, ws shifted to higher ΔK with n increse in the test temperture, showing ΔK onset < 13 MP m 1/2 for RT, ΔK onset = 16 MP m 1/2 for 363 K nd ΔK onset = 19 MP m 1/2 for 423 K. Fig. 5(b) shows LM imges of the crck morphologies t the surfce of the CT specimen fter the ΔP-constnt test. The LM observtion ws performed t ΔK 30 MP m 1/2. In 0.7-MP hydrogen gses t 423 K, extensive slip bnds were observed long the ftigue crck (Fig. 5(b-3)), where the FCG ws slightly fster thn tht observed in ir t RT. In the FCG tests in 0.7-MP hydrogen gs t 363 K, where the FCG ws significntly ccelerted s well s tht in 0.7-MP hydrogen gs t RT, very few slip bnds were observed long the crck (Fig. 5(b-2)).

6 530 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) Crck growth rte, d/dn [m/cycle] 10 5 ΔP-constnt ΔK-incresing f = 1 Hz, R = MP hydrogen gs d/dn = ΔK 3.79 RT in H K in H K in H K in N 2 RT in ir b-1 b-2 H 2 t RT FCG direction 100 μm K= 31 MP m 1/2, d/dn = m/cycle, RFCGR = 14 b-3 b-4 H 2 t 423 K FCG direction H 2 t 363 K K= 31 MP m 1/2, d/dn = m/cycle, RFCGR = 9 N 2 t 423 K FCG direction 100 μm FCG direction Stress intensity fctor rnge, ΔK [MP m 1/2 ] K= 31 MP m 1/2, d/dn = m/cycle, RFCGR = 3 Fig. 5. () Reltionship between d/dn nd ΔK t elevted tempertures; (b) Crck growth morphologies: (b-1) in 0.7-MP hydrogen gs t RT; (b-2) in 0.7-MP hydrogen gs t 363 K; (b-3) in 0.7-MP hydrogen gs t 423 K; (b-4) in 0.1-MP nitrogen gs t 423 K 3.4. Understnding ccelertion of ftigue crck growth in presence of hydrogen K= 30 MP m 1/2, d/dn = m/cycle The fore-mentioned results indicte tht FCG ccelertion due to hydrogen is lwys ccompnied by slip locliztion t the crck tip. Once gin, it is importnt to note tht, even in hydrogen gs, FCG ccelertion does not occur when the slip deformtion is not loclized. Similr phenomen with respect to slip locliztion t the crck tip were lso observed in JIS-SCM435 (Mtsuo et l. (2010)). In order to understnd this peculir frequency dependence of the FCG rte, Mtsuo et l. (2010) performed FCG testing in 0.7-MP hydrogen gs t vrious test frequencies, thereby detecting the peculir frequency dependence of hydrogen-induced ccelertion, using JIS-SCM435. They explined the ccelertion mechnism bsed on the hydrogen-enhnced successive ftigue crck growth (HESFCG) model (Murkmi et l. (2008); Mtsuok et l. (2011); Mtsuok et l. (2016)), representing tht the ccelertion is not determined by either the presence or bsence of hydrogen t the crck tip, but is determined by the distribution of hydrogen ner the tip of the ftigue crck. They suggested tht steep grdient of hydrogen t the crck tip cuses locliztion of plsticity which prevents crck tip blunting nd shrpens the crck tip. As result, the crck growth per cycle is incresed. In contrst, Somerdy et l. (2013) performed FCG testing on the pipeline steel, X52, t vrious test frequencies in 21-MP hydrogen gs contining 10, 100 nd 1000 vol. ppm oxygen, discovering tht the frequency dependence of FCG ccelertion in hydrogen ws ltered by the oxygen content. As mentioned erlier, the hydrogen gs in the cylinder used in this study lwys ws less thn 1 vol. ppm, which is considerbly lower thn tht reported by Somerdy et l. (2013); therefore, this study investigted the peculir frequency dependence of the FCG rte in terms of hydrogen distribution ner the crck tip, i.e., the HESFCG model. A series of experimentl evidences infer tht steep grdient of hydrogen concentrtion cuses slip locliztion t crck tip; hence, we propose new prmeter quntifying the onset of the FCG ccelertion due to hydrogen, in considertion of the following two fctors: (1) hydrogen concentrtion t the surfce, (2) the rtio of the penetrtion depth of hydrogen per cycle to the ordinry plstic zone produced in ir. As shown in Fig. 1(b), the C S nd D of the present steel round crck tip, hving severe plstic deformtion, is considered to be nerly equivlent, respectively. Thus, s illustrted in Fig. 6, when the initil hydrogen content is zero, the hydrogen distribution, C H, ner the crck tip bsed on the normlized distnce from crck tip, x', my be pproximtely expressed s follows: 100 μm 100 μm x ωp CH ( x ) = CS{1 erf ( )}, 2 Dt x x = (9) ω p where x is the distnce from the crck tip; t is the loding time per cycle, defined s t = 1/(2f); erf is the error function. The vlues of C S nd D of the steel with higher rolling rtio should be used to reproduce the hydrogen-diffusion properties round the crck tip, following Fig. 1. ω p is the ordinry plstic zone for plne strin in ir:

7 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) Hydrogen concentrtion, C H C S Slop bsed on the normlized distnce from crck tip: CSω p f Condition for the FCG ccelertion: G = = CSωp 2 Dt 2D CSωp f G = = C ω p : ordinry plstic zone Sωp > GC 2 Dt 2D G C : criticl slope for the FCG ccelertion Smller G Dt ω Hydrogen distribution: p x ωp CH ( x ) = CS{1 erf ( )} Lrger G 2 Dt ω Crck Normlized distnce from crck tip, x' Fig.6. Schemtic illustrtion of pproximte grdient of hydrogen concentrtion round crck tip. 1 K mx 2 2 p = ( ) = { } 3π σ YS 3π (1 R) σ YS 1 K For quntifying the grdient of hydrogen concentrtion ner the crck tip in considertion of the rtion of the penetrtion depth of hydrogen per cycle to the ordinry plstic zone in ir, the following prmeter, G, ws defined: CSω p G = = CSωp 2 Dt f 2D When the vlue of G in Eq. (11) exceeds the criticl vlue, G C, n onset of the FCG ccelertion occurs. f f G = CSω p = G0ωp GC, G0 C 2D = S 2D (12). Under the ΔK-constnt tests t R = 0.1 nd RT, insted of the G in Eq. (12), we cn use (p H2 f) 1/2 for quntifying the onset of the FCG ccelertion, since C S is pproximtely proportionl to p H2 1/2. Fig. 7() shows the reltionship between the RFCGR nd (p H2 f) 1/2 for the ΔK-constnt tests t R = 0.1 nd RT (cf. Fig. 3). The onset of the FCG ccelertion ws quntified by the prmeter, (p H2 f) 1/2, reveling tht the FCG ccelertion occurred t (p H2 f) 1/ From (p H2 f) 1/2 = 0.1, the verge G 0 vlue for ΔK onset t pressures rnging from 0.1 to 90 MP t RT ws 1.86 mss ppm/mm; therefore, the vlues of G 0 nd ω p for the ΔK onset in the ΔK-constnt tests t RT cn be obtined. The vlues of G 0 nd ω p for the ΔK onset in the ΔP-constnt tests t 363 K nd 423 K cn be lso obtined from Fig. 5(). These three reltionships between G 0 nd ω p were fitted by G 0 ω p = G C s the G C ws n unknown prmeter. Fig. 7(b) shows the reltionship G 0 nd ω p for the ΔK onset in the ΔK-constnt tests t RT nd the ΔP-constnt tests t 363 K nd 423 K. As reference, the experimentl result of the ΔP-constnt test t RT, where ΔK onset is not observed in Fig. 5(), is shown here. The reltionship between G 0 nd ω p for ΔK onset could be fitted by G 0 ω p = G C. These results demonstrte tht the onset of the FCG ccelertion in presence of hydrogen ws stisfctorily quntified (10). (11). (d/dn) H2 / (d/dn) ir ΔK-constnt ΔK = 30 MP m/ 1/2 R = 0.1 Hydrogen pressure, p H2 [MP] FCG ccelertion (p H2 f) 1/2 b G 0 [mss ppm/mm] MP H 2, RT, f = 1 Hz, ΔP-constnt G 0 ω p = G C FCG ccelertion (G 0 ω p > G C ) 0.7-MP H 2, 363K f = 1 Hz, ΔP-constnt 0.7 ~ 90-MP H 2, RT, f = ~ 10 Hz, ΔK-constnt MP H 2, 423K, f = 1 Hz, ΔP-constnt ω p [mm] Fig. 7. () Reltionship between the RFCGR nd (P H2 f) 1/2 for the ΔK-constnt tests t RT; (b) Reltionship between the G 0 nd ω p for the ΔK onset in the ΔK-constnt tests t RT nd the ΔP-constnt tests t RT, 363 K nd 423 K.

8 532 Junichiro Ymbe et l. / Procedi Structurl Integrity 2 (2016) by Eq. (12) nd the peculir dependence of FCG rte on hydrogen pressure, test frequency nd test temperture could be unified by using novel prmeter representing the grdient of hydrogen concentrtion ner crck tip. 4. Conclusions This pper presented the effects of hydrogen pressure, test frequency nd test temperture on the ftigue crck growth (FCG) properties of low-crbon steel, JIS-SM490B. To estimte hydrogen-diffusion properties t crck tip, hving severe plstic deformtion, hydrogen diffusivity ws lso determined with cold-rolled JIS-SM490B. The hydrogen diffusivity ws lower with n increse in the rolling rtio; however, becme constnt t rolling rtios higher thn 20 %. The hydrogen-ssisted FCG ccelertion lwys ccompnied locliztion of plstic deformtion ner crck tip nd it ws inferred tht steep grdient of hydrogen concentrtion cused the slip locliztion t crck tip. The peculir dependence of FCG rte on hydrogen pressure, test frequency nd test temperture could be unified by using novel prmeter representing the grdient of hydrogen concentrtion ner crck tip, in considertion of the rtio of the penetrtion depth of hydrogen per cycle to the ordinry plstic zone in ir. This work ws supported by the New Energy nd Industril Technology Development Orgniztion (NEDO), Fundmentl Reserch Project on Advnced Hydrogen Science (2006 to 2012) nd Hydrogen Utiliztion Technology (2013 to 2018). The uthors grtefully cknowledge the support of the Interntionl Institute for Crbon-Neutrl Energy Reserch (I2CNER), estblished by the World Premier Interntionl (WPI) Reserch Center Inititive funded by the Ministry of Eduction, Culture, Sports, Science, nd Technology (MEXT), Jpn. References Asno, S., Hr, K., Nki Y., Ohtni, N., The trpping effect of disloctions on hydrogen diffusion in mild steel. Journl of the Jpn Institute of Metls 38, Itog, H., Mtsuo, T., Orit, A., Mtsung, H., Mtsuok, S., Hirotni, R., SSRT nd ftigue crck growth properties of high-strength ustenitic stinless steels in high-pressure hydrogen gs. ASME PVP Gngloff, R.P., Somerdy, B.P., Gseous hydrogen embrittlement of mterils in energy technologies. Woodhed Publishing, Cmbridge. Hirth, J.P., Effects of hydrogen on the properties of iron nd steel. Metllurgicl nd Mterils Trnsctions A 11, Kiuchi, K., McLelln, R.B., The solubility nd diffusivity of hydrogen in well-nneled nd deformed iron. Act Metllurgic 31, Mtsung, H., Yoshikw, M., Kondo, R., Ymbe, J., Mtsuok, S., Slow strin rte tensile nd ftigue properties of Cr Mo nd crbon steels in 115 MP hydrogen gs tmosphere. Interntionl Journl of Hydrogen Energy 40, Mtsuo, T., Mtsuok, S., Murkmi, Y., Ftigue crck growth properties of quenched nd tempered Cr Mo steel in 0.7 MP hydrogen gs. Proceedings of the 18th Europen conference on frcture. Mtsuo, T., Ymbe, J., Mtsuok, S., Effects of hydrogen on tensile properties nd frcture surfce morphologies of Type 316L stinless steel. Interntionl Journl of Hydrogen Energy 39, Mtsuok, S., Tnk, H., Homm, N., Murkmi, Y., Influence of hydrogen nd frequency on ftigue crck growth behvior of Cr Mo steel. Interntionl Journl of Frcture 168, Mtsuok, S., Ymbe, J., Mtsung, H., Criteri for determining hydrogen comptibility nd the mechnisms for hydrogen-ssisted, surfce crck growth in ustenitic stinless steels. Engineering Frcture Mechnics 153, Murkmi, Y., Knezki, T., Mine, Y., Mtsuok, S., Hydrogen embrittlement mechnism in ftigue of ustenitic stinless steels. Metllurgicl nd Mterils Trnsctions A 39, Murkmi, Y., Mtsuok, S., Kondo, Y., Nishimur, S., Mechnism of hydrogen embrittlement nd guide for ftigue design. Yokendo, Tokyo. Ngumo, M., Fundmentls of hydrogen embrittlement. Uchid Rokkuho, Tokyo. Orini, R.A., The diffusion nd trpping of hydrogen in steel. Act Metllurgic 18, Sn Mrchi, C., Somerdy, B.P., Robinson, S.L., Permebility, solubility nd diffusivity of hydrogen isotopes in stinless steels t high gs pressures. Interntionl Journl of Hydrogen Energy 32, Sn Mrchi, C., Somerdy, B.P., Nibur, K.A., Development of methods for evluting hydrogen comptibility nd suitbility. Interntionl Journl of Hydrogen Energy 39, Somerdy, B.P., Sofronis, P., Nibur, K.A., Sn Mrchi, C., Kirchheim, R., Elucidting the vribles ffecting ccelerted ftigue crck growth of steels in hydrogen gs with low oxygen concentrtions. Act Mterili 61, Ymbe, J., Mtsumoto, T., Mtsuok, S., Murkmi, Y., A new mechnism in hydrogen-enhnced ftigue crck growth behvior of MP-clss high-strength steel. Interntionl Journl of Frcture 177, Ymbe, J., Awne, T., Mtsuok, S., Investigtion of hydrogen trnsport behvior of vrious low-lloy steels with high-pressure hydrogen gs. Interntionl Journl of Hydrogen Energy 40, Ymbe, J., Itog, H., Awne, T., Mtsuo, T., Mtsung, H., Mtsuok, S., Pressure cycle testing of Cr-Mo steel pressure vessels subjected to gseous hydrogen. ASME Journl of Pressure Vessel Technology , 1 13.