Hydrogenenvironmentembrittlementofaustenitic stainlesssteelsatlowtemperatures
Thorsten Michlera*，Jorg Naumannb
°Adam Opel GmbH 65423 Risselsheim GermonybBMW AG 80788 Munchen Germany
ＡRTICLE N Ｆ O
ABSTRACT
Hydrogen environment embrittement (HEE) Is a well-known phenomenon in materials science. For the automotive industry this topic is of special importance due to thestainless steels are presented which show that the Nickel content is one of the main uping hydrogen technology. Results on a wide variety of mercially austeniticparameters to meet HEEresistance.In slow strain rate tensiletesting no loss in reduction ofinfluence of minor alloying elements and impurities like Mo Mn Ti and S is negligible area at 50°C was found for Ni contents above 12.5 wt%. It was also found that thewithin the range investigated here. Another important parameter is local metallurgy. t is shown that the HEE resistance strongly depends on the type of the semi-fnished producthomogeneous structure show a better resistance pared to rolled plate materials with e s q ueoan esegregations despite identical nominal chemical positions.
Article history: Received 1 October 200712 February 2008 Received in revised formAccepted 12 February 2008
Keywords:Hydrogen embrittlementLow temperature Stainless steel
2008 International Association for Hydrogen Energy Published by Elsevier Ltd. Allrights reserved.
between 8 and 12.5wt%. Unfortunately the price for thesecontent e.g. by the end of 2006 AISt 316 steel was twice as (o pue) In Suseaus m Aueguis sostau seuaesBusArs so Btq aq pnom a as p0E ISIV se aAsuadxa effect if it were possible to stabilize the low Ni materials atlow HEE levels since the price for these steels is primarily a function of their Ni content. Unfortunately the interaction ofpositional and metallurgical variables influencing HEEare still not understood well enough although there are a lot of very detailed investigations published. Reviewing thedisciplines e.g physicists chemists metallurgists engineers literature we got the impression that several scientificexpertise but cross-functional work seems to be necessaryto link those single results to get a broad understanding Theof hydrogen technology to selet appropriate materials. current situation makes it very diffcult for mercial users
1. Introduction
Hydrogen environment embrittlement (HEE) is a well-knownphenomenon in materials science. A cost and time effective method to assess the susceptibility of materials to HEEis slowstrain rate tensile testing under real hydrogen gas tempera- tures and pressures (e.g. ASTM G 142). In most cases thequantify HEE because yield strength is not affeted at all and reduction of area (RA) of the tensile specimens is used toultimate tensile strength and elongation are usually lessaffected by HEE. A prehensive study of Fe-CrNi alloys being tensile tested in hydrogen at 700bar and 20°C wasthe degree of HEE of austenitic stainless stees (SS) strongly published by Caskey [1]. He was the first who reported thatdepends on their Ni content. He found a strong increase inHEE-resistance from 8% to 12% Ni and a large scatter within this Ni range. Most of the mercial SS have Ni contents
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Especially for automotive OEM's it is very important to meet the cost requirements which means that affordable materialsare to be identified without promising safety. Unfortu-reason for initiating this investigation. Furthermore much nately there is far too few data available which was themore experience is needed to create suitable standards for HEE testing for automotive applications. The results pre-sented here might be a contribution to this effort.
scope readings were multiplied by a factor of 1.7 [10]. the Feritscope readings into martensite contents the Ferit-
3. Results and discussion
Table 2 shows the detailed results of the tensile testsperformed in this investigation. It can be seen that yield strength was not influenced by hydrogen whereas the ultimatethose alloys that were extremely affected by HEE The most tensile strength was decreased by a certain extent but only foraffected parameter was the RA which is taken in the followingin mechanical properties were observed between testing as the indicator to assess HEE. In general no distinct deviationscylindrical and flat specimens with the dimensions given in Section 2 although the RA values of steels 22 and 26 tend to beatmospheres H and He the effect on RRA is minor. a little lower for the fat specimens. Since this is true for both
2. Experimental details
Tensile tests with cylindrical specimens were performed inhydrogen (99.999%) atmosphere at 10bar and 50°C with a cross-head speed of 0.1 mm/min. Reference testing was per-formed in helium or nitrogen also at 50 °C and 0.1 mm/min.Aq paguan sem i asneaq os le pauuopad sem Susa sereral authors [13] that embrittlement reaches a maximumat around 50°C for austenitic Ss. The tests were done at the Materials Testing Institute University of Stuttgart (Germany)finished product different smooth cylindrical specimens with as au uo upuada [ upaqusap sneredde sa e umwere used which results in strain rates of 5.5 × 10- s-1 and the dimensions ②3 × 30 3 × 50 2 5 × 30 and ( 5 × 50 mm3.3 × 10- s-1 respetively In addition plate materials werelength 6mm width 2mm thickness) which results in a also tested by using a smooth flat specimen geometry (20 mmand then tapered by 0.02mm in the middle of the gauge strain rate of 8.3 × 10- s-1. The specimens were machinedlength with 800 grid paper. The specimens were placed in theconditioning or precharging. Several mercially available pressure chamber just prior to testing without any hydrogenSS were tested with the details given in Table 1. All materials were in solution treated condition. It shall be noted herethat specimens made out of bar material were machined parallel to the rolling direction and specimens made out ofplate material were machined parallel and perpendicular tothe rolling direction. The degree of HEE was assessed by the relative reduction of area (RRA) which was defined as thereduetion of area in hydrogen (RAsu ) divided by the reduction of area in helium (RAae) namely RRA = RAa /RAHe × 100 [%]Fracture modes were assessed by conventional SEM techni-with Lichtenegger-Bloech etchant. The latest systematic que. Metallographic cross-sections were prepared and etchedinvestigations regarding HEE of SS were published by Fukuyama et al. [2 3 58] from the Japanese National Institute of Advanced Industrial Science and Technology (AIST) who performed tensile testing of different merciallyavailable SS at various temperatures and pressures inof 4.2 × 10- s~. Because their testing conditions are very hydrogen (99.9999%) and argon (99.999%) with a strain ratesimilar to those in this study their results are implemented in the following chapters. They even patented a Ni-equivalentequation for the assessment of austenitic SS for HEE [9] which will be discussed later.
3.1. Role of nickel
Fig 1 shows RRA measured at 50°C and 10bar as a function of the Ni content of the mercial steels listed in Table 1.Each data point represents one single test in hydrogen as wellat identical test conditions. It can be seen that no HEE was as in helium performed with identically oriented specimensfound for Ni contents higher than 12.5% and that RRA decreases almost in a linear way down to 20% at 8% Ni. The700 bar and 20°C. It shall be mentioned as a side note that this same general relationship was found by Caskey [1] for tests atrelationship is only true for Ni-austenitic SS (FeCr-Ni alloys) e.g Ni maraging steels do not follow this curve. A sophisti- pauasaid se (ah) uaaqa ualopau iog ioa paesby Lee [11] and recently Gavriljuk et al. [12 13] acted on this suggestion to improve explanations for hydrogen enhancedthe electron density of states at the Fermi energy level N(E) localized plasticity (HELP). Lee proposed that HE is related tosasdaN(Er) values because they have significantly more electronic states available for H electrons to interact with the metalvalence electrons in the d-band than metals with lower values of N(Er). This effect leads to metal-bond decohesion.Although both Lee and Gavrljuk interpret the role ofhydrogen in a different way (decohesion on the one hand and HELP on the other hand) this theory is the only one thattries to correlate atomistic with macroscopic properties. Lee was able to correlate atomistic data (N(Er) values) of puremetals with their macroscopic embritlerment data (e.g RA or notched tensile strength) with a surprising accuracy. Heextended his theory to termary (Fe-Cr-Ni) alloys and hefrorm 8% to 12%. His theory implicates that HE is primarily a a useau q a u aseaap dreqs e paadfunction of the chemicalposition and furthermore mainly of the main elements Fe Cr and Ni which clearlystates the importance of Ni in these alloys. Based on this idea some engineering metrics were developed to assess thesusceptibility of SS to HE namely by (a) Ni content (b) Nickelbe discussed in detail in Section 3.4. In contrast HELP tries equivalent and (c) austenite stability. These topics willto explain HE by metallurgical variables especially slip.
The martensite contents of all specimens were measureds rs s ao e Germany. The measurements were performed in the neckingarea of the specimens to get the maximum values. To convert
(%) 00198 2 2 2 5 2 217.138 10.18 1 12 185 13 1517 13 419d. (%m） 3 0.023 0.029 188o0.024 9100 EO000 0.0040.019Strain rate 1: 5.5 × 10-5 4: 4.2× 10-5(s-)1212121144equivalent NTATE1 11.4301 14306 1.4401 1.4404 1.4435 1.4571 tsns T9tEsnS 9TESns 9TESnso'ou 42012 244429 2012612327
uqp psspsu
Chemical mpositisere taken fm the 3.1B cerictes f the stel msnfctue Als given sre the chical cpoitios of SUS6 materil iestigted by Hn t al [2] and Fukyma
Please cite this article as: Michler T Naumann J. Hydrogen environment embrittlerment of austenitic stainless steels at low temperatures. Int J Hydrogen Energy (2008) doi:10.1016/j.jhydene.2008.02.021
(ou uu q paos saas) aaq pees sas a jo snsa sa z aq
Internal Steel Gas Specimen Orientation YS S RAno. (MPa) (MPa) (%)4 1.4301 He/N H Cylindrical BarL 322 312 537 949 25 7422 1.4301 H Cylindrical PlateT PlateL 387 400 596 635 15 18He/No Cylindrical Flat PlateT Plate-T 365 371 SEOL 536 15 75Flat PlateT Plate-1L 337 347 1028 990 79 611.4301 H Cylindrical Bar-L 465 649 201.4305 He/N; Cylindrical BarL 464 994 7712 He/N; H 504 511 1095 746 17 6124 1.4306 H Cylindrical PlateT PlateL 316 280 688 722 29 25He/N Cylindrical Flat Plate7 PlateT 325 315 668 851 31 75Flat PlateT PlateL 280 312 827 822 82 6941 1.4305 H Cylindrical BarL 330 692 4042 1.4401 He/N H Cylindrical BarL 333 294 764 776 83 52He/N; 297 800 812 1.4404 He/N; H Cylindrical BarL 280 278 699 058 26 789 1.4404 H Cylindrical BarL 241 244 642 636 6820 1.4404 He/N H Cylindrical PlateT 337 638 84 22Flat PlateT Plate1. 331 395 742 683 25 25He/N Cylindrical PlateT PlateL 339 331 881 841 74 82Flat PlateT 392 831 7043 1.4404 He/N H Cylindrical BarL 297 627 820 33 821 1.4435 He/N H Cylindrical 7188 327 322 760 761 80 8323 1.4435 H Cylindrical Plate-T 349 770 72He/N Plate-T PlateL 336 352 759 771 72 751.4429 H Cylindrical Plate-L Plate-T 331 428 755 826 80 7526 Flat Plate-T PlateL 424 425 812 766 82 62He/N Cylindrical PlateT Plate1 422 446 827 817 85 841.4571 H Cylindrical Flat Plate-T Bar-L 423 242 771 734 72 433 He/N; 238 760 7621 1.4571 H Cylindrical PlateT Plate1L 310 278 697 740 33 31He/N Cylindrical Flat PlateT PlateT 327 355 726 850 31 76Flat PlateT PlateL 273 337 791 763 77 72
Please cite this article as: Michler T Naumann J. Hydrogen environment embrittlement of austenitic stainless steels at lowtemperatures. Int J Hydrogen Energy (2008) doi:10.1016/j jhydene.2008.02.021
Table 2 cntinund)
Intemal ou Steel Gas Specimen Orientation YS (ew) UTS (MPa) RA7 SUS 316L H Cylindrical BarL 323 744 63He/N; 321 752 83SUS 3161 H Cylindrical PlateT PlateL 329 313 714 742 43 57He/Ns PlateT PlateL 352 317 773 727 08 80
Fig. 1 Relative reduction of area (RRA versus Ni content of the steels investigated here including data from Han et al. [2 and cl (xna) ge sa eeAnxn
Dissolved hydrogen enhances dislocation mobility and slipplanarity [14 15] which leads to highly localized plastic strain suffcient to enable subcritical crack growth. Due to thisaadde a edose a ds reed paueais brittle whereas the microscopical appearance shows the familiar ductile features like microvoid coalecence [16]. Nickelincreases the stacking fault energy in austenitic SS which promotes cross-slip (see [15] and references therein) and thus macroscopic ductile fracture behavior. It can be assumed thathigh Ni contents (>12.5 wt% in this study) pensate the negative effects of hydrogen and that the bined effect ofthe high HE-resistance characterized by macroscopic ductile austenite stabilization and promotion of cross-slip leads tofracture of high Ni containing SS. A detailed review can be found in [15].
It is quite surprising that both theories decohesion (Fermi)and HELP predict the beneficial role of nickel because they seerm to be contradictory (decohesion versus enhancedplasticity). A very detailed review of environmentally assisted fracture by Lymch [16] shows that most likely both decohe-sion and enhanced plasticity are parts of the fracture -op jo) uondaospe e s uopeardaaqu st wsqueqougen) weakens the interatomic bonds at crack tips therebyfacilitating the nucleation of dislocations at crack tips so that crack-tip sources are now activated before extensive disloca-tion activity occurs ahead of cracks" [16]. with this explana-
tion the fracture mechanism seems to be understood but thefractographic properties are still unknown. If these properties unique atomic properties of Ni which lead to this enhancedSuedas fq padojanap aq u@ru shoe iadeaup umoux aiamNi by a bination of alternate alloying elements.
Referring back to Fig. 1 it can also be seen that the scatter ofcontents is low whereas the scatter in the range of 1012 wt% the data at low (12.5 wt%) Nifirst variables to look after are deviations in test equipment Ni (316 grades) is significantly higher. To assess scatter theand testing parameters. All of the tests in this study weremachine and testing parameters (50C 10bar H-purity performed at the same test laboratory with the same test99.999%). Since the results of Han et al. [2] and Fukuyama et al. [3] (50°C 10 bar H-purity 99.9999%) and those of thisstudy fit very well the inffuence of the test machine andH-purity seems to be negligible in the range investigated here. A very important parameter is the strain rate. Strain ratesmust be sufciently low to allow the hydrogen to interact with the material. It was found by several authors [1719| thatbelow 10~ 10~ s1 deviations bee negligible. Alltests presented here were performed at lower strain rates. Ifdeviations in testing and testing procedure can be excludedmaterial properties and chemical positions. Comparing the scatter can. only be explained by local deviations in thesteel 21 (RRA = 43%) with steel 42 (RRA = 64%) this large
