The Susceptibility to Hydrogen Embrittlement of Lovv Alloy Cr-Mo Steel Tubing Občutljivost cevi iz nizkolegiranega Cr-Mo jekla na vodikovo krhkost M. Gojič, M. Balenovič, Željezara Sisak, Hrvatska L. Kosec, Fakulteta za naravoslovje in tehnologijo, Ljubljana L. Vehovar, Inštitut za kovinske materiale in tehnologije, Ljubljana In the paper the results of research in the susceptibility of as-rolled and heat treated lovv alloy Cr-Mo (microalloy by Nb) tubing to hydrogen embrittlement by method of cathodic polarization are shovvn. The results show that quenching at 870°C (both in vvater and oil) and tem pering at high temperature (720° C) attain excellent resistance to hydrogen embrittlement vvhich is indicated by a small embrittlement index (11.1-23.6%) by tough small-dimple fractures and relatively small content of absorbed hydrogen (3.2-3.4 ppm) at the cathodic polarization. Key vvords: lovv alloy steel, tubing, hydrogen embrittlement, heat treatment, cathodic polarization Raziskovali smo občutljivost valjanih in toplotno obdelanih cevi iz malolegiranega Cr-Mo jekla (mikrolegiranega z niobom) na vodikovo krhkost. Uporabili smo metodo katodne polarizacije. Rezultati kažejo, da s kaljenjem s temperature 870°C (v vodi ali olju) in z visokotemperaturnim popuščanjem (720°C), dosežemo odlično odpornost proti vodikovi krhkosti. Potrjuje jo majhen indeks krhkosti (11.1-23.2%), žilav, jamičasti izgled prelomne površine in relativno majhna vsebnost absorbiranega vodika (3.2-3.4 ppm) pri katodni polarizaciji. Ključne besede: malolegirana jekla, cevi, vodikova krhkost, toplotna obdelava, katodna polarizacija. 1. Introduction Corrosion costs of oil countrv tubular goods (OCTG) in oil industrv are estimated to be some hundred millions dollars every year. At the begin of the fifties a lot of oil tubes in Canada, USA and France failured when API (American Petroleum Institute) pipes grade N-X0 and P-l 10 vvere used in sour vvells, vvith minimum vield strenght of 552 MPa and 758MP, respectively. Novvdavs great efforts are being made to decrease or reduce the process of corrosion to minimum, expecialy the sulfide stress corrosion cracking (SSCC) i.c. hvdrogen embrittlement (HE). Most producers of oil country tublar goods adapt themselvs to difficult conditions of oil and natural gas exploatation by devel-oping and producing pipes from lovv alloy to high allov steels and special allovs. Lovv alloyed steels mostlv used for OCTG are made from medium carbon Mn. Mn-Mo and Cr-Mo type steels. microalloyed vv ith vanadium, niobium, titan and bor1. The sulfide stress corrosion cracking or hydrogen embrittlement of steels is quite a complex phenomenon not being yet completely unambignous determined neither regarding the mechanism2 nor the dominant influence of individual factors (mechanical properties. microstructure etc.). Hovvever it is often used to charac-terize the influences of hvdrogen in steel at room temperatures. These influences are mostly manifested through the loss of duc-tililv (decrease of reduetion area and elongation). Therefore, in this paper is presented the analvse of lovv alloy Cr-Mo steel (mi-croalloved vvith niobium) susceptibility to hvdrogen embrittlement from the point of vievv of mechanical properties, heat treat- ment and microstructure. At the same time, the content of ab-sorpted hydrogen vvas determined and fractographic analvsis of fraetured surfaces' specimens after cathodic polarization vvas carried out. 2. Experimental Material By procedure of laboratory electro slag remelting there vvere received lovv alloy Cr-Mo steel (microalloyed vvith niobium) in-gots. (J) 169 x 380 mm, vvhich vvere after forged into billets Q 135 x 420 mm. The table 1 shovvs chemical composition of the investigated stccl. On the table 1 wc can see that the investigated steel appro-priates to the first type API grade C-90 by addition of niobium as a micro alloy element3. Alloying vvith higher content of molvbdenum and microalloying vvith niobium AISI 4130 steel proved to be useful for inereasing of resistance to hydrogen cracking because of the refined austenite grain and the size of Steel C Mn P S Si V Mo Al Cr Nb Cr-Mo 0.30 0.72 0.022 0.007 0.37 0.01 0.63 11.07 1.12 0.035 Table I: The chemical composition of steel investigated, Tabela I: Kemijska sestava jekla, c/i carbide particles. In Iron Steel Works Sisak, billets □ 135 x 420 mm vvere hot rolled into seamless tubing <1> 60,3 x 4.83 mm for oil industrv. Heat treatment The temperatures of phase transformations needed for heat treatment vvere tested by dilatometer Lk.02 "Adamel Lhomargy". The specimens <|> 2 x 12 mm vvere heated and cooled by heating and cooling rate of 0,05"C/s. On registered diagram dilatation/temperature, the temperature values of particular phase transformation vvere read off. On Ihe basis of there results. the heat treatment of pipes vvhich is consisling of normalization and tempering as vvell as quenching (vvith cooling in vvater and oil) and tempering vvere carried out in the laboratorv electric resistance chamber furnace. Before and after heat treatment me-chanical properties of ASTM standardized specimens vvere tested. The hardness lest vvas performed by BrinelFs method. In a vievv of obtaining phase composition a phase analysis by X-ray diffraction device and Philips numerical couting technique by tise CoKa radiation. Corrosion tests Since the hvdrogen embrittlement of the material presents in faet the loss of its ductil ity (due to absorbed hvdrogen) a decrease of ductility parameters is obvious, i.e. the reduetion area and elongation of specimens are always reduced2'4'6. Among many electrochemical methods the cathodic polarization is one of the most appropriate methods for the determination of relative material susceptibilitv to hydrogen embrittlement. The specimens <|> 3.5 x 1 1(1 mm made from steel investigated in as-rolled and heat treated state vvere put into electrochemical celi (fullfilled vvith 0.5 M II,SO, + 10 mg As,(),/1 solutin) vvhich vvas put in Zvvick 50 kN tensile machine and subjeeted lo static load of 60 and 8()'/f ils of yield strenght7. The cathodic polarization vvas carried out by Wenging's potentiostat at current density of 1,6: 4,0: 8,0 and 12.0 mA/cnr. After cathodic polarization (duration of tvvo hours) of stressed specimens testing to ihe fracture vvith deformation rate of 2.4 x 10'V vvas immidiately carried out. On the base of change of specimens' reduetion area embrittlement index vv as calculated according lo the follovving equation: vvhere are: RAji, - reduetion area prior cathodic to polarization (uneharged by hydrogen) RAh - reduetion area after cathodic polarization (charged b\ hvdrogen ) After corrosion tests the content of absorbed hvdrogen in cathodic polarized specimens is determined on the exalograph EA-1 by the method of hot extraction. Metallographic and fractographie testing Microstructure of polished and etehed (in nital) specimens before and after heat treatment vvere carried out by the scanning electronic microscope (SEM) tvpe JOF.L JXA-50 A. voltage to 50 kV. For determination series and manner fracture the analy-sis fraetured surfaces of specimens vvere carried out. 3. Results of investigation Investigation of mechanical properties vvere carried out b\ Instron 1196 tensile machine on tvvo samples in as-rolled and heat treated state. Energv impact testing vvas carried out bv Charpv clapper on three IŠO specimens vvith V-notch at temperature 20"C. Average testing values of mechanical properties are shovved in table 2. On table 2 can be seen that the tubing vvithout heat treatment according to mechanical properties correspond to P-l 10 API grade vvhich does not belong to the corrosion resistant oil coun-try tubular goods. By normalizing at 900"C and tempering at 700°C vvas obtained corrosion resistance L-SO API grade. By the tubing heat treatment consisting of quenching at 870"C and tempering at high temperature of 720"C (specimens 36 and 39) there vvere obtained OCTG vvith mechanical properties C-90 grade. The index embrittlement as per equation (1) taking into account the specimens reduetion area prior and after cathodic polarization. The average values of embrittlement index for as-rolled and heat treated tubes are shovvn in table 3. 4. Diseussion of results The mechanical properties of Cr-Mo steel tubing in as-rolled state are high (API grade P-105, table 2) due to the chemical composition (modification vvith molybdenum and microallov ing vvith niobium) and the presence of bainite microstructure (figure Table 2: The mechanical properties of tubings Cr-Mo steel in as-rolled and heat treated state Tabela 2: Mehanske lastnosti cevi iz Cr-Mo jekla v valjanem in toplotno obdelanem stanju Specimen Heat treatment Yield strenght MPa Tensile strenght MPa Elongation ck Hardness HB Energv impact at+20°C Fracture toughnes MPa \ M ? - 972 1145 8.9 400 4 50 30 Normalized 900 C/min, air + Tempered 700°C/60 min, air 605 725 25.5 230 IS 84 36 Quenched 870°C/30 min. vvater + Tempered 720°C/60 min, vvater 721 765 25.5 252 19 94 39 Ouenched 870 C/30 min, oil + Tempered 720°C/60 min, air 703 759 22.1 250 22 100 Table 3: The values of embrittlement index and content absorbed hydrogen Cr-Mo steel by cathodic polarization Tabela 3: Vrednost indeksa krhkosti in vsebina absorbiranega vodika Cr-Mo jekla pri katodni polarizaciji Yield lndex Current Content Specimen Heat treatment strenght embrittlement dcnsity hydrogen MPa F(%) M A/cm2 ppm 3-6 - 972 87.6 1.6 2.7 30 - 4 Normalized 900°C/30 min, air 27.2 4.0 30 - 7 + 605 55.2 8.0 - 30 - 6 Tempered 700°C/60 min, air 88.5 12.8 36 - 3 Quenched 870°C/30 min, vvater 11.1 4.0 3.2 36 - 5 + 721 30.7 8.0 4.0 36-4 Tempered 720°C/60 min, air 86.2 12.0 4.8 39 - 3 Quenched 870°C/30 min. oil 23.6 4.0 3.4 39 - 5 + 703 31.1 8.0 4.4 39-4 Tempered 720°C/60 min, air 88.6 12.0 7.0 vvhere are: KIC = ReH(0,646 CVN/ReH-0,00635)"2 ( 2 ) ReH - Uppcr yield strenght (MPa) CVN - Charpy energy impact (J) On the base of calculated K[c - values (table 2) it is clear that the fracture toughness of quenched and tempered pipes has high values vvhich may be up to 100 MPam"2. The figure 1, in the manner of histogram, shovvs the change of embrittlement index at the cathodic polarization both for different states of material and different current densities and it can be seen that pipes resistance to hydrogen embrittlement increases through appliance of the heat treatment, specially quenching and tempering. Although pipe specimens in as-rolled state vvere at test stressed on the level of 60% yield strenght and polarized at current density of 1,6 mA/cnr a small resistance to hydrogen embrittlement vvith embrittlement index of 87,6% vvas obtained. The microfractography (figure 2a) of a fractured specimen made after cathodic polarization shovvs the presence of mixed fracture in vvhich predominantes brittle cleavage type of fracture. The reason for such a small resistance to hydrogen embrittlement is in the presence of untempered bainite microstructure (figure 3) vvhich is by many investigators9'10 considered, after martenzite structure. to be the most unfavourable microstructure vvith regard to the resistance in corrosion environmetals, expecially in sulfide environmetals. By normalization and tempering at 700"C" the resistance to hydrogcn embrittlement vvas increased can be seen from the in-dex value of embrittlement 27.2% and from the ductile fracture vvith a small energy fracture (figure 2a). The fracture began at the large inclusion particle being hovvever mighty traps fro hy-drogen because their great intersurface encures the accummula-tion of sufficient hydrogen quantites for the initation of cracking"'12. By quenching and tempering at 720"C the tube resistance to hydrogen embrittlement vvas increased vvhich vvas expressed by a smaller embrittlement index, particulary specimens quenched in vvater vvhere the embrittlement index is only 11.1 c/<. The fracture are ductile, vvith fine dimple appearence (figure 2c). The increase of embrittlement index vvas induced mosttly by presence of high tempered martenzite microstructure (figure 3b) in vvhich by means of X-ray diffraction the distribution of fine sphero carbides FeC, Fe,C, Cr,C2 and aMo,C vvas determined. Fine carbides microstructured is a main microstructural parameter for improving of hydrogen embrittlement because in this čase a longertime is needed for the accummulation of critical amount 3a) appearing at usual air cooling of tubes vvith finish rolling temperature. The hardness is homogenous through the vvhole cross seetion and amounts to 400 HB and 230-250 HB for the pipes in as-rolled and heat treated state as vvell. As OCTG are also used in arctic fields they are supposed to be as tough as possible. expecially at lovv temperatures. Energy impact of heat treated pipes is high and amounts to 18-22 J at 20"C retaining to same values also at -40"C. The fracture toughness (KIC-value) is knovvn to be an important characteristic of material, hovvever, because of vv all-thinnes (4,83 mm) KIC vvas not defined by the way of Charpy's energv at 2()"C as per Rolf-Novak's equation8. 100 80 60- 40 20 1.6 mA/cm 4.0 mA/cm 8.0 mA/cmz 12.0 mA/cm2 / M / without heat 900°C/air 870"C/water 870°C/oil treatment + » ♦ (as-rolled) 700'C/air 720°C/air 720°C/air Heat treatment Figure 1: The influence heat treatment on embrittlement index of lovv allov Cr-Mo steel Slika 1: Vpliv toplotne obdelave na indeks krhkosti malolegiranega Cr-Mo jekla Figure 2: The microfractography of fractured surfaces of low alloy Cr-Mo steel after cathodic polarization a) as-rolled. current density 1.6 mA/cnv b) quenched and tempered. current density 4,0 mA/cm c) normalized and tempered, current density 4.0 mA/cm' d) quenched and tempered. current density 12.0 mA/cnr Slika 2: Mikrofraktografije prelomnih površin malolegiranega Cr-Mo jekla po katodni polarizaciji a) valjano, gostota toka 1,6 mA/cnr b) kaljeno in popuščeno, gostota toka 4,0 mA/cnr c) normalizirani in popuščeno, gostota toka 4.0 mA/cnr d) kaljeno in popuščeno, gostota toka 12.0 mA/cnr' of hydrogen inducing the brittlc material decay. Since the microstructure influence is manifested mainly through the absorption and trapping of hydrogen on the interfaces carbide/matrix. the defined fine chrome carbides Cr,C, and «Mo,C inerease resistance to hvdrogen embrittlement. 5. Conclusion The tubing of investigated Cr-Mo steel in as-rolled state (without heat treatment) in regard to mechanical properties cor-respond to API grade P-110 vvith bainite microstructure appeared in usual way by air cooling of pipes at finished rolling temperature. Their resistance to hydrogen embrittlement is small vvith high values of embrittlement index of 87,6 %. It proves also the presence of mixed fracture vvith mainlv brittle cleavage frac-tures. By normalizing of tubing at 900"C and tempering at 700"C is obtained API grade L-80 vvith a great resistance to hvdrogen embrittlement (F = 27,8%) and ductile fracture vvith a small fracture energv. I lovvever, by quenching and tempering at 720"C API grade vvas obtained C-95 vvith significant resistance to hydrogen vvilh ductile mainly fine dimple fraetures. The reason of there are carbides Cr,C2 and aMo.C in tempered martenzite microstructure. Hovvever, by inereasing of current densitv from 4,0 mA/cm2 to 12.0 mA/cm2 at cathodic polarization some quintity of hydro-gen (5-7 ppm) vvas absorbed vvhich remarkably decreased resistance to hydrogen embrittlement (F = 86-89% ) in the presence of the brittle cleavage transgranular fracture. The results of the test shovv that for the obtaing of API grade C-95 vvith high resistance to hvdrogen embrittlement the heat treatment of tubing from investigated Cr-Mo steel needs to bc carried out by quenching in vvater after having reached the temperature at 870"C and air tempering from 720"C. Figure 3: The microstructures tubings from lovv alloy Cr-Mo steel in as-rolled (a) and quenched and tempered state (h) Slika 3: Mikrostruktura cevi iz malolegiranega Cr-Mo jekla v valjanem (a) ter kaljenem in popuščanem stanju (b) Literature 1 H.A. Uljanin: Struktura i korrozija metallov i splavov. Metalurgija, Moskva, 1989, s 139 2 G.M. Pressouvre: Current solutions to hydrogen problems in steel, Proceeding of the first International Conference on Current Solutions to Hvdrogen Problems in Steels, Washington, 1.-5. 11.1982. s 18 3 API Specification 5CT, American Petroleum Institute. Washington, 1990, s IS 4 F. Mansfeld: Corrosion mechanisms, Marcel Dekker Inc. Nevv York, 1987, s 344 5 P. Mclntyre: Hydrogen effects in high strenght steels, Edited by R.A. Oriani, J.P. Hirth: Hydrogen degradation of ferrous alloys, Noyes Publication. Nevv Jcrsev, 1985. s 763 6 L. Vehovar: International Conference on Materials Development in Rail. Tire, Wing. Hull Transportation, Euromat 92, Genoa, s 1367 7 M. Gojič et al.: Evaluation of Mn-V steel tendency to hvdrogen embrittlement, Kovine Zlitine Tehnologije, 26, 1992, s 349 s S.T. Rolfe, S.R. Novak: Slovv-bend KIC testing of medium-strenght high-toughness steels - STP 463, Philadelphia, American Society for Testing and Materials, 1970. s 124 ' E. Snape: Rolles of composition and microstructure in sulfide cracking of steels, Corrosion, 24, 1968. s 261 111 J.B. Greer: Fractors affecting of sulfide stress cracking per-formance of high strenght steels. Materials Performance, 3, 11, 1975 " G.M. Pressouyre, I.M. Bernstein: A Quantitative Analvsis of Hydrogen Trapping, Metallurgical Transactions A. 9A. 1978. s 1571 12 J.P. Hirth: Effects of Hydrogen on the Properties of Iron and Steel, Metallurgical Transactions A, 11A. 1980, s 861.