Evaluation of Mn-V Steel Tendency to Hydrogen Embrittlement Ocjena sklonosti Mn-V čelika prema vodikovoj krtosti M. Gojič, M. Balenovič, Željezara Sisak "IRI" d.o.o., Sisak L. Kosec, FNT, Odsek za metalurgijo in materiale, Ljubljana L. Vehovar, Inštitut za kovinske materiale in tehnologije, Ljubljana L.J. Malina, Metalurški fakultet Sisak, Sisak Recently great attention is given to investigations related to oil - and natural gas exploitation, especially from the aspect of material selection used for production of tubular equipment. In these conditions tubes destructions caused by mechanical damages are less frequent than destructions caused by various corrosion crack forms and especially corrosion cracks in sulphide conditions, with H2S as a dominant component. Results of mechanical properties and evaluation of tendency of seamless tubes in as rolled and heat treated condition, produced from medium-carbon Mn-V steel to hydrogen embrittlement by method of cathode polarization at the constant current density of 4.0 m A/cm2 are presented in this paper besides the above stated possible tube destruction forms in the conditions of oil - and natural gas exploitation. The fractographic analysis of fractured samples surfaces after cathode polarization was also performed by scanning electron microscope. It was found that tubes in the as rolled condition (no heat treatment) shovv great tendency to hydrogen embrittlement with the values of the calculated embrittlement index of 86.4%. By normalization (900" C/30 min. air) of tubes, resistance to hydrogen embrittlement vvas not improved compared with the as rolled condition. The heat treatment (quenching + tempering) resulted in a great resistance of tubes to hydrogen embrittlement with the calculated embrittlement index of 25.1%. The stated results are proved by the fractographic analysis of morphology of fractures that are explicitly brittle for tubes without heat treatment, but tough for quenched and tempered tubes. Therefore it can be concluded that from the aspect of mechanical properties and corrosion resistance the most suitable tubes for use in the oil - and gas wells are those quenched and tempered at high temperatures (70CP C). Danas se u svijetu velika pažnja poklanja istraživanjima vezanim uz problematiku eksploatacije nafte i zemnog plina, posebno s aspekta izbora materijala koji se koriste za izradu cijevne opreme. Propadanja cijevi u tim uvjetima uslijed mehaničkih oštečenja su manje cesto nego propadanja usiijed raz Učiti h oblika korozijskih raspucavanja, a posebno korozijskog raspucavanja u sulfidnim uvjetima gdje je H2S dominantna komponenta. U radu su pored navedenih mogučih oblika oštečenja cijevi u uvjetima eksploatacije nafte i zemnog plina prikazani rezultati mehaničkih svojstava kao i ocjena sklonosti valjanih i toplinski obradenih bešavnih cijevi iz srednjeugljičnog Mn-V čelika prema vodikovoj krtosti metodom katodne polarizacije kod konstantne gustoče struje od 4.0 m A/cm2. Takoder je provedena na scanning elektronskom mikroskopu i fraktografska analiza prelomnih površina uzoraka nakon katodne polarizacije. Utvrdeno je da cijevi u valjanom stanju (bez toplinske obrade) pokazuju veliku sklonost prema vodikovoj krtosti s vrijednostima izračunatog indeksa krtosti od 86.4%. Normalizacijom (900' C/30 zrak) cijevi otpornost na vodikovu krtost nije se poboljšala u usporedbi s valjanim stanjem. Medutim toplinskom obradom (kaljenje + popušatnje) postignuta je velika otpornost cijevi prema vodikovoj krtosti s izračunatim indeksom krtosti od 25.1%. Navedene spoznaje dokazuje i fraktografska analiza morfologije preloma koji su za cijevi bez toplinske obrade izrazito krti cijepajuči dok su žilavi za zakaljene i popuštene cijevi. Prema tome, ovaj rad dokazuje da su s aspekta mehaničkih svojstava i korozijske otpornosti za primjenu u naftnim i plinskim bušotinama najpogodnije cijevi koje su zakaljene i popuštene na visokim temperaturama (70CPC). 1 Introduction Due to ever increasing demand for energy a great attention is given to investigations related to problems connected to oil and natural gas exploitation, especially to the selection of material for tubular equipment. Thirty five years ago medium quality steels having 400 MPa yield strength were used for tubes and pipes in natural gas and oil exploitation from 1000-3000 meters depths. Nowadays the exploitation proceeds not only in higher depths (10000 m) but in more severe conditions also because of the presence of highly corrosive acidic gas (H2S, CO2), chlorides, high pressure and temperature, etc. Therefore, material for tubular ware have to satisfy high requirements for both mechanical and corrosion properties. Cracking of tubular equipment in the presence of sulphide is particularly frequent corrosion phenomenon. The selection of appropriate steel and the heat treatment of fi-nal goods to obtain optimal compromise of mechanical and corrosion properties are used as countermeasure1-6. Different possihle tube damage in oil and natural gas exploitation with special emphasis on SSCC (sulphide stress corrosion cracking) as a form of hydrogen embrittlement is described. Samples made from hot rolled seamless tube manufac-tured from medium carbon low alloyed Mn-V steel vvere in-vestigated Cathodic polarization follovving heat treatment carried out under laboratory conditions vvas used to deter-mine susceptibility to hydrogen embrittlement and evaluate corrosion properties. 2 Forms of oil pipe damage Tubes and pipes used in natural gas and oil exploitation are known as Oil Country Tubular Goods (OCTG). OCTG is divided into tubing, casing and drill tube used in vertical direetion for pumping, extemal proteetion and drilling, re-spectively. In a vvider sense OCTG includes also pipe line used for transport purposes mainly in horizontal direetion. Despite complex stresses pure mechanical failure is less frequent as compared to corrosion failure. Economic op-eration of oil vvells is often dependent on proper selection of tube material. The control of corrosion has become one of main factors in the produetion of energy from geoen-ergetic sources. Corrosion failure of natural gas and oil exploitation equipment is very serious problem from safety as vvell as economic vievvpoint since the costs in oil indus-try amount to several hundreds millions of USS per year. Material failure due to corrosion is also associated vvith produetion breaks. Corrosion problems in oil and gas ex-plotation equipment7~10 can appear in several forms: • vveight loss due to corrosion, • localized corrosion knovvn as pitting, • corrosion fatigue, • galvanic corrosion, • stress corrosion and • sulphide stress corrosion cracking (SSCC) Stress corrosion cracking caused by sulphide is very frequent since typical oil and gas exploitation environment besides chlorides, sulphates, carbonates, C02 and moisture contains considerable amount of H2S also vvhich can reach up to 30%. Hydrogen sulphide attack increases general corrosion, erosion corrosion in turbulent media, stress corrosion cracking, corrosion fatigue of tubes and equipment at bottom of drilled vvells etc. Stress corrosion cracking in the presence of sulphide may appear in tvvo forms: • hydrogen induced cracking (HIC) • sulphide stress corrosion cracking (SSCC) Hydrogen induced cracking is characteristic for OCTG equipment made from lovv alloyed steel vvith ferrite—perlite microstructure and 700 MPa tensile strength. It can occur even in the absence of external stress. This form of corrosion results from atomic hydrogen absorbed on microstruc-tural defecLs (hydrogen traps) vvhich recombines into molec-ular hydrogen. Sulphide stress corrosion cracking occurs in OCTG equipment made from high strength steel as a form of hy-drogen embrittlement. Hydrogen embrittlement has been vvell knovvn and frequently observed in metals for quite a long time. It is caused mostly by corrosion, galvanisation or leaching associated vvith the generation of atomic hydrogen vvhich under certain conditions can diffuse into crystalline lattice resulting in the hydrogenization of metal. In the beginning the effect of hydrogen vvas attributed to stress corrosion. Recently the similarity betvveen hydrogen embrittlement and certain types of stress corrosion, partic-ularly SSCC8-10 has been pointed out. There is no unique theory capable of explaining ali phe-nomena associated vvith hydrogen attack because it depends on a number of factors e.g. type of steel, its microstructure, electrolyte, etc. At present, the proposed mechanisms of hydrogen attack are based on inerease in the inner pres-sure, surface adsorption, decohesion, inerease or decrease in plasticity and the formation of hydrideu. Acidic enviro-ment in natural gas and oil exploitation enhances hydrogen embrittlement and SSCC as its particular form because of • the presence of H2S at lovv pH value of media, • sulphides vvhich inerease the amount of hydrogen dif-fusing into the crystalline lattice of metal and • tendency for the localization of anodic part of corrosion reaction vvhich promotes initial cracks. There are tvvo sources of atomic hydrogen; the inner gener-ated by manufacture and heat treatment of steel and the ex-terior resulting from the effect of definite environment. During application of material hydrogen may be adsorbed from gaseous phase in molecular form vvith subsequent dissoci-ation into atoms or by electrochemical dissolving of liquid phase i.e. surrounding corrosive media vvhich takes plače during the exploitation. In this čase hydrogen is formed in molecular or atomic form. The overall reaction of hydrogen formation is in acidic solutions: 2H30++2e~ — H2 + 2H20 (1) in caustic solutions: 2H20+2e~ — H2 + 20H~ (2) The transport of HjO+ ion (front now on H+ ) or H20 moleculc to electrode surface and subsequent formation of adsorbed hydrogen atoms can be described by (3) and (4): H++e" - Hads (3) H20 + e- - Hads + OH- (4) Irrespective on the nature of solution, hydrogen is adsorbed on metal electrode surface. To be continuous the electrochemical process requires permanent presence of Hais on electrode surface from vvhere it can be removed in one among three ways12,13: • by catalytic recombination (Volmer-TafeTs mecha-nism) vvhere both the adsorption and desorption take plače simultaneously: H++e" - Haiis (5) Hads + Hads — H2 (6) • by Volmer-Heyrowski's mechanism of electrochemical desorption vvhere desorption results from the reduetion Table 1. Chemical composition of the Heat (T) and billet (A ) Steel Sample Composition in wt. % C Mn P S Si V Mo Al Cr Mn-V T 0.33 1.10 0.017 0.004 0.23 0.21 - 0.027 _ K 0.33 1.14 0.025 0.005 0.29 0.23 0.02 0.04 0.08 of H+ ion or H2O molecule according to (7), (8) and (9): H+ + e" H+ + Hads + e" H20 + Hads + e- Hads H2 H2 + OH" (7) (8) (9) • by emission mechanism where adsorbed hydrogen atoms vaporize from electrode surface: H Uds H (10) Consequently, hydrogen atoms adsorbed on the surface (Hads) can recombine into either harmless gaseous hy-drogen through (6) vvhich is bubbled out of solution or diffuse into metal and enhance embrittlement: Hads —* Hahs It can be concluded that sulphide stress corrosion crack-ing is caused by absorbed hydrogen. The dissolving of iron and the presence of H2S in vvater solution create conditions for increase in hydrogen content of steel14. Usually, only a small part of hydrogen generated on cathode diffuses into metal. Diffusion rate depends on numerous faetors, e.g. the type of steel or alloy, its composition and previous thermo-mechanical treatment, nature of electrode surface, type of electrolyte, its composition, cathode current density, etc. Hydrogen embrittlement is often used in evaluation of the effect of hydrogen on steel at room temperature vvhich results in a loss of ductility (reduetion in elongation and contraction), decrease in tensile strength and enhanced brit-tleness. 3 Experimental Mn-V steel billet of cfl 135 • 420 mm dimensions vvas pro- duced under laboratory conditions on Metallurgical Institute Hasan Brkič, Zenica. The billet vvas hot rolled into 060.3 -4.83 mm pumping oil tube (tubing) under industrial conditions in Seamless Tube Mili of Željezara (Iron and Steelvvorks) Sisak. Table 1 presents Heat (T) and a control (K) analysis of the steel. Samples cut from rolled tube vvere subjeeted to heat treatment (annealing, annealing + tempering, quenching + tempering) in electric resistance chamber furnace. Inves-tigation of mechanical properties vvere canied out on IN-STRON type 1196 machine using samples prepared according to ASTM standard. Brinell or Rockvvell C hardness vvas determined depending on the sample hardness. Corrosion resistance vvas measured by the method of cathodic polarization vvhich is knovvn as one among the most appropriate for determination of relative susceptibility to hydrogen embrittlement. After cleansing vvith acetone the samples for cathodic polarization vvere put in the elec-trochemical celi of ZWIC'K 50 kN tensile machine (fig. 1) and subjeeted to static load of 80% of yicld stress. 1 - POSTOE .[ I M! MANI/ A n KIDAL 2- tajus 11 KioAuti -i ■ H I K I R (?K E 111 1 S K A r F L I JA - -UZORAK 0T0PINA 0 M n T LJ EI I KTR0DE / 1 IJGGINOVA KAPI| ARA REFLRENTNOM E LEKIR000M ISPIIŠIANJE OTOPINE P0KAZIVAČ ',111 10 -1' I '-.Ar 11 -REGULATOR BRZINI 12 ZAUSTAVI JANJE RA,7 VLAČ t N JA H IKLJUL IVAN JE RAZVLAEENJA ] i. -POVRAT Ni H0Q 1 S-U TE Z NA KI A I NU lb-PREKIU A POGONA , DA! IC E ref elektioda proTuelektroda rodna elektroda Figure 1. Schematic illustration of equipment for hydrogen emhriltlement evaluation by cathodic polarization method. Slika 1. Shematski prikaz aparature za ocjenu vodikove krtosti metodom katodne polarizacije. Sample of Mn-V steel vvas used for vvorking electrode and saturated calomel electrode (SCE) situated in Luggin's capillary tube as a reference. Tvvo graphite Johnson Matthey electrode of 6.5 nun diameter vvere used as counter electrode. The solution used vvas IN H2S04 vvith addition of 10 mg/l As20? used to aetivate the generation of Hads- The solution vvas deaerated by nitrogen blovving for 30 min. For cathodic polarization WENKING potentiostat modeli 68 FR 0.5 vvas used and constant 4 niA/cm2 current den-sity vvas applied. After 2 hours of polarization samples vvere taken out of the celi and immediately tested on INSTRON machine at very lovv deformation rate of 2.4- 10~4s_1. The overall tensile test lasted for 3-4 rnins. Aftervvard the cross seetion of fraetured surface vvas measured to determine the contraction. Due to the loss of ductility (contraction) embrittlement index F vvas calculated from: F = Z o Zn — • 100 vvhere: Z0 Z1 contraction before polarization contraction after polarization Table 2. Results of mechanical testing of Mn-V steel tube samples in as rolled and heat treated state. Sample Heat treatment Re Rm A2" Hardness, HB: API MPa MPa % I II III IV grade 2 - 605.5 759.5 25.5 216 216 211 213 L-80 900°C/30' air 20 + 670° C/60' air 535.0 657.0 24.6 222 215 229 235 J-55 870°C/30' vvater 21 + 640° C/60' air 794.0 836.0 21.2 276 278 276 278 P-105 870730' vvater 23 + 700760' air 692.0 723.0 23.4 232 234 255 231 C-90 2 ?0/Jm I----1 — trakasta feritno-perlitna mikrostruktura — a) valjano stanje — popušteni martenzit — b) K: 870"C/30 voda + P: 700°C/60 zrak Figure 2. Microstructure of tubing Mn-V steel in rolled (a) and heat treated (b) condition. Slika 2. Mikrostruktura cijevi iz Mn-V čelika u valjanoni (a) i toplinsko obradenoni (b) stanju. 4 Results Results of mechanical and metallographic investigation Investigation of mechanical properties (tensile strength, yield stress and strain) vvere carried out on two tube sam- ples in as rolled state and another tvvo in heat treated state. Ring-like 060.3 '1.83-30 mm samples vvere used for Brinell or Rockvvell C (for quenched samples) hardness measure-ment using three impressions per quadrant (in the middle of sample wall). Average values of mechanical properties for as rolled and different heat treated state are seen in table 2. Mechanical properties of Mn-V steel tubes in as rolled state i.e. vvithout heat treatment correspond to L-80 API grade of corrosion resistant OCTG wares. Annealing treatment (900°C/30' in air) follovved by tempering at 670°C produces lovver quality OCTG vvares vvith mechanical properties correponding to J-55 API grade. Quenching in vvater coupled vvith subsequcnt tempering at 640° C yields OCTG vvares of higher mechanical properties (P-105 grade) vvhich are not desired because of poor resistance to SSCC, i.e. a high susceptibility to hydrogen embrittlement. Hovvever, tempering at 700°C results in API C-90 grade correspond-ing to corrosion resistant OCTG vvares. Mechanical properties measured are in accordance vvith corresponding microstructures as can be seen on fig. 2. Results of cathodic polarization tests Since the determination of susceptibility to hydrogen embrittlement by catodhic polarization is based on the loss of ductility caused by absorbed hydrogen, samples vvere sub-jected to tensile test vvith 2.4 • 10~4s~' deformation rate immediately after the polarization. Embrittlement index F was calculated from equation (11) taking into account con-traction measured before (Zu) and after (Z\) polarization. The results are given in table 3. Histograms given on figs. 3 and 4 present the change in contraction and embrittlement index, respectively for as rolled and heat treated Mn-V steel sample caused by cathodic polarization. Samples in as rolled state and that in annealed state shovv high susceptibility to hydrogen embrittlement i.e. a great decrease in contraction (fig. 3) and high index of embrittlement (fig. 4). Samples annealed and subsequently tempered at 670° C have significantly lovver embrittlement (F = 28.1% only). The best resistance to hydrogen embrittlement (F = 25.1%) have samples vvhich vvere quenched and tempered at 700° C because of highly tempered martensite microstructure as seen in fig. 2b. Fracture surface of samples after cathodic polarization vvas analysed by electron scanning microscope as seen in fig. 5. Fracture surface of Mn-V steel samples in as rolled state after cathodic polarization 100 80 60 - 40 20 0 [Že K + P VS N N+P t o p L i n s k a obrada Figure 3. Concentralion change resulting from cathodic polarization of Mn-V steel in different heat treatment conditions VS-rolled. Heat treatment . N: 900°/30' air . N + P 900°/30' air + 670°/60' air . K + P: 870°/30' water + 700°/60' air Slika 3. Promjena kontrakcije uslijed katedne polarizacije za različita stanja toplinske ohrade Mn-V čelika. VS—valjano stanje. Toplinska obrada • N: 900°/30' zrak . N + P 900°/30' zrak + 670°/60' zrak • K + P: 870°/30' voda + 700°/60' zrak 100 80 - 60 - 40 - 20 - VS N N + P K + P toplinska obrada Figure 4. Embrittlement index change resulting from cathodic polarization of Mn-V steel in different heal treatment conditions VS—rolled. Heat treatment • N: 900°/30' air • N + P 900°/30' air + 670°/60' air . K + P: 870°/30' vvater + 700°/60' air Slika 4. Promjena indeksa krtosti uslijed katodne polarizacije za različita stanja toplinske obrade Mn-V čelika. VS—valjano stanje. Toplinska obrada . N: 900°/30" zrak • N + P 900°/30' zrak + 670°/60' zrak • K + P: 870°/30' voda + 700°/60' zrak shovvs typical brittle fraeture (fig. 5a) vvhereas quenched and highly (700°C) tempered samples shovv duetile fraeture (fig. 5b) coiresponding to a lovv index of embrittlement (fig. 4). 5 Discussion Despite the fact that mechanical properties of Mn-V steel seamless tube samples in as rolled state correspond to API L-80 grade of corrosion resistant OCTG vvares, the cathodic polarization at 4.0 mA/cnr current density displayed great susceptibility to hydrogen embrittlement. As a result of cathodic polarization the contraction dropped from initial (as rolled state) 56.9% to 7.7% corresponding to embrittlement index F = 86.4% (figs. 3 and 4). The susceptibility to hydrogen embrittlement of Mn-V steel tubes in as rolled state is also seen (fig. 5a) from brittle fraeture surface. It results from strip like ferrite-perlite microstructure vvith elongated inclusions (fig. 2a) vvhich is favorable for accu-mulation of the critical antount of hydrogen required for the initiation of cracking. From the vievvpoint of resistance to SSCC viz. hydrogen embrittlement, elongated sulphide inclusions (especially MnS vvhich act as hydrogen trap) and high tendency for segregation of ntanganese (martensite and bainite islands observed in microstructure) are unfavorable and have a dontinant influence on corrosion resistance of Mn-V steel in as rolled state. The annealing treatment carried out (900° C/30' air) on tested tubes did not intprove the resistance to hydrogen embrittlement since strip like ferrite-perlite strueture vvith prevailing influence of MnS inclusions aeting as hydrogen traps vvas preserved. On the contrary, tempering (670°C/60' air) of annealed tubes resulted in considerable inerease of the resistance to hydrogen embrittlement since martensite and bainite islands vvere removed. In respect to mechanical properties the heat treatment combined of quenching and tempering at temperatures vvithin 640-700° C range yielded J-55, P-105 and C-90 (table 2) API grades. Cathodic polarization test of tubes corresponding to API C-90 grade shovved high resistance to hydrogen embrittlement vvith only 25.1%' reduetion in contraction. The obtained high resistance to hydrogen embrittlement is illustrated also by fig. 5b shovving duetile nature of fraeture surface. As comparcd to samples in as rolled state the microstructure of heat treated sample tubes of Mn-V steel instead of bands of ferrite-perlite vvas composed of honto-geneous highly tempered martensite ntarked by high duc-tility and capacity for accumulation of higher amounts of energy generated e.g. by Zappfe's mechanism11 of hydro-gen embrittlement. Since MnS vvas observed in heat treated samples also, it is evident that highly tempered martensite reduces harmful influence of MnS on corrosion properties of OCTG vvares. Table 3. Loss of ductility of Mn-V steel as detemiined by the method of cathodic polanzation. Samplc Heat treatment Re Applied ^0 Zi i F MPa stress % % mA/cnr % 2-3 - 599.1 0.8 Re 56.9 7.7 4.0 86.4 20N-4 900° C/30'air 594.5 0.8 Re 61.1 10.9 4.0 82.0 900°C/30' air 20-5 + 670°C/60' air 519.4 0.8 Re 67.2 48.3 4.0 28.1 870°C/30' vvater 23-4 + 700°C/60' air 681.8 0.8 R, 70.1 52.5 4.0 25.1 — krti cijepajuči prelom — Uzorak 2—3 (valjano stanje) — žilavi prelom — Uzorak 23-4 )K: 870°C/30 voda + P: 700"C/60 zrak) Figure S. Fracture moiphology of specimens form Mn-V steel in rolled (a) and heat trealed (b) condition after cathodic polarizatton. Slika S. Morfologija preloma uzoraka iz Mn-V čelika u valjanom (a) i toplinsko obradenom (b) stanju nakon katodne polarizacije. Summarv Based on the investigation of susceptibility to hydrogen embrittlement of seamless Mn-V steel tubes utilized in natural gas and oil industry the following conclusions can be de-rived. • Mechanical properties of Mn-V steel tubes in as rolled state with higly oriented ferrite-perlite microstructure correspond to L-80 API grade. • Beside J-55 and P-105, C-90 API grade conforming to corrosion resistant OCTG vvares was also attained by heat treatment (annealing + tempering, quenchung + tempering) of tubes. • Cathodic polarization of tubes in as rolled state shovved small resistance to hydrogen embrittlement since embrittlement index F vvas 86.4%. • The resistance to hydrogen embrittlement vvas not im-proved by annealing (900°C/30' air) (F = 82.0%) as compared to as rolled state. • Based on brittle fracture surface revealed by fractographic analysis of samples subjected to cathodic polarization it vvas established that Mn-V steel tubes in annealed or as rolled state are not suitable for the use in oil industry. • High resistance to hydrogen embrittlement proven by comparatively small embrittlement index (F = 25.1%0 and ductile nature of fracture surface vvas ac-quired by quenching and tempering at a high temperature (700° C). • In respect to both mechanical and corrosion properties Mn-V steel tubes quenched and temepered at a high temperature are suitable for the use in natural gas and oil vvells.