HIC of HSLA Steels.pdf

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CORROSION SCIENCE SECTION

389CORROSION–Vol. 49, No. 5

Roles of Microalloying Elementsin Hydrogen Induced Cracking ResistantProperty of HSLA Steels

H.-Y. Liou, R.-I. Shieh, F.-I. Wei, and S.-C. Wang*

INTRODUCTION

Aggressive environments such as hydrogen sulfide,carbon dioxide, chloride, and sulfur compounds areencountered often in crude oil survey, oil and naturalgas transportation, and the storage of raw petroleummaterials and products. In these cases, hydrogeninduced cracking (HIC) occurs easily in pipeline andpressure vessel steels. In 1945, blistering occurred inthe oil refining equipment of American PhilipsPetroleum Co.1 In 1984, fracture induced by H

2S

occurred on the monoethanolamine absorption towerof Marshal Fire Co. in Illinois. The fracture was blamedfor 17 deaths and losses above $100 million (U.S.)2 Ithas become important to evaluate the HIC resistanceof steels to prevent environmental destruction, loss of

life, and property damage.External stress is not always necessary for HICoccurrence. A crack usually initiates in the inner part ofsteel and propagates parallel to the rolling direction. Hatoms form on the surface of steel because of thereaction of iron and H

2S. Atomic H can diffuse easily

through the metal lattice because of the poison effectof FeS. Then, H atoms combine preferentially to formmolecular H in H traps such as inclusion/matrixinterfaces, voids, impurities, dislocations, laminations,or microcracks.3 Once molecules of H have formed inany of those structural defects, the internal pressure

Submitted for publication February 1992; in revised form, November1992.

* Steel and Aluminum Research and Development Department, ChinaSteel Corp., Hsiao Kang, Kaohsiung 81233, Taiwan, R.O.C.

0010-9312/93/000093/$5.00+$0.50/0 © 1993, NACE International

ABSTRACT

The effects of alloying elements such as niobium, copper,

chromium, molybdenum, and titanium in high-strength, low-

alloy (HSLA) steels on hydrogen induced cracking (HIC) were investigated. The HIC test was performed to NACE TM-02-84

in BP (saturated H 2 S synthetic sea water) and NACE solutions. The test data were analyzed with an optical

microscope, scanning electron microscope, electron probe

microanalyzer, Auger electron spectrometer, transmission electron microscope, and an image analyzer. Results showed

the addition of Nb increased the aspect ratio of inclusions and decreased HIC resistance. In BP solution, Cu-bearing steel

formed a protective film of -Fe 2 O 3 enriched with Cu on the

outside and FeS on the inside. Cr and Mo showed a synergistic effect on the formation of low-temperature

transformation products and the increase of HIC sensitivity.The suitable parameter for these two elements on HIC

sensitivity was Cr15

+ Mo5 , and the critical value of this parameter

was 0.1%. Additionally, coarse TiN precipitates, as well as elongated MnS inclusions, were the initiation sites of HIC.

Both cooperated to result in HIC.

KEY WORDS: alloying elements, HSLA steel, hydrogen induced cracking, hydrogen sulfide, inclusion




390 CORROSION–MAY 1993

increases. Cracking occurs when pressure is largerthan 1.013 x 109 ~ 1010 N/m2 (104 ~ 105 atm).4

Cracking is divided into straight cracking and stepwisecracking (SWC). The difference is caused by plasticdeformation and shear stress distribution.5 However,blistering is created because of SWC beneath the

material surface.6 In practical experience, HIC initiatesmainly at elongated Type II manganese sulfide (MnS)inclusions and propagates along the pearlite bands orthe bainite and martensite formed in the segregationzone of Mn and P.7

The alloying elements of niobium (Nb), copper(Cu), chromium (Cr), molybdenum (Mo), and titanium(Ti) often are added in high-strength, low-alloy (HSLA)steels. Among these, the addition of Nb hasprecipitation hardening8 and grain refinementstrengthening effects.9 Ti primarily has the effect ofgrain refinement.10 Cu, Cr, and Mo are added for theircorrosion resistance properties. Although the effect of

these elements on HIC has been reported, conclusionshave not been consistent. There has even beencontroversy. Massive Nb carbonitride precipitates canact as initiation sites,4 but the combined addition of Nband boron increases HIC resistance.11 Addition of Cucan enhance the formation of a tight and adherentblack surface film resulting in a decrease of Hpermeability,12 but its real effect is not clear. Cr additionis beneficial in suppressing HIC.13 Mo addition impairsthe H absorption resistance.14 However, the interactionbetween Cr and Mo complex addition has not beendiscussed in literature. The effect of Ti addition on HICvaries with the size of TiN and Ti(C,N) particles.15

Nevertheless, the relationship between TiN particlesand MnS inclusions also has not been discussed.

This test investigated the effect of alloyingelements such as Nb, Cu, Cr, Mo, and Ti on the HICresistance of HSLA steels.

EXPERIMENTAL PROCEDURES

HSLA steels were melted in a 250 kg vacuuminduction furnace under a controlled protective argon

atmosphere and cast into 160 by 160 mm2 ingots.Chemical compositions of the steels are shown inTables 1 and 2. The cast steel ingots were hot rolledinto 5 to 20 mm thick steel plates by controlled rolling.The HIC immersion tests were performed to NACEStandard TM-02-84 in BP or NACE solution. Three

specimens (100 by 20 mm by full thickness) weresectioned from the steel plates with their longitudinaldirection along the rolling direction. Samples wereground with emery paper number 320, then immersedin synthetic sea water saturated with H

2S (BP solution)

or 5% NaCl + 0.5% CH3COOH (acetic acid) solution

saturated with H2S (NACE solution) for 96 h.

Conditions for the HIC immersion test are listed inTable 3. Steels 0 to 6 were tested in the BP solution toNACE standard. Steels 7 to17 were tested in NACEsolution to prevent the interference of Cu from otherelements to HIC resistance. After the test, the surfaceblistering condition of each sample was examined. In

addition, each sample was cut into four equal pieces tomeasure the crack length ratio (CLR) from thesectioned surface. The CLR, an index of HIC, isdefined below and shown in Figure 1.

CLR = Σa

W x 100% (1)

where a=crack length, W=section width, andsummation over all the cracks in area W x T inFigure 1.

After the CLR value evaluation, the specimenswere welded with foreign extension bars on bothnarrow faces and notched in the middle of two oppositesurfaces. The composite specimens were immersed inliquid nitrogen. When the temperature of eachspecimen dropped to –196°C, the specimen wasremoved and fractured immediately by an impactmachine. The fracture morphology was investigatedwith a scanning electron microscope (SEM). Thecomposite specimen preparation is shown in Figure 2.4

Specimens for the evaluation of inclusion morphologywere cut from the plates in as-rolled condition and

TABLE 1

Chemical Compositions of Experimental Steels with Various Nb and Cu Contents (wt%)

Steel C Si Mn P S Nb Cu Ni Al N Cr Mo Ti

0 0.086 0.24 1.37 0.02 0.010 * * * 0.030 0.0027 * * *1 0.080 0.27 1.39 0.013 0.010 0.011 * * 0.022 0.0034 * * *2 0.088 0.23 1.37 0.02 0.011 0.020 * * 0.037 0.0026 * * *3 0.080 0.25 1.36 0.02 0.011 0.026 * * 0.025 0.0026 * * *4 0.080 0.25 1.37 0.02 0.011 0.037 * * 0.028 0.0024 * * *5 0.068 0.26 1.37 0.02 0.010 0.027 0.29 * 0.033 0.0025 * * *6 0.082 0.26 1.38 0.014 0.011 0.030 0.29 * 0.028 0.0021 * * *

* = Trace.





TABLE 2Chemical Compositions of Experimental Steels with Various Cr and Mo Contents (wt%)

Steel C Si Mn P S Nb Cu Ni Al N Cr Mo Ti

7 0.097 0.37 1.17 0.012 0.0084 * 0.35 0.37 0.028 0.0054 * 0.17 0.0268 0.095 0.38 1.16 0.011 0.0085 * 0.38 0.22 0.036 0.0062 0.77 0.17 0.0309 0.093 0.42 1.17 0.010 0.0084 * 0.40 0.21 0.029 0.0058 1.12 0.15 0.029

10 0.091 0.40 1.18 0.011 0.0084 * 0.38 0.22 0.033 0.0056 * 0.32 0.02811 0.093 0.40 1.16 0.011 0.0090 * 0.38 0.22 0.034 0.0054 0.38 0.32 0.02712 0.093 0.40 1.17 0.012 0.0086 * 0.39 0.21 0.034 0.0052 0.77 0.32 0.02713 0.098 0.41 1.16 0.012 0.0096 * 0.40 0.21 0.035 0.0056 1.12 0.32 0.02914 0.098 0.38 1.16 0.012 0.0088 * 0.37 0.21 0.040 0.0057 * 0.51 0.02815 0.099 0.38 1.16 0.013 0.0090 * 0.39 0.22 0.044 0.0052 0.38 0.51 0.02916 0.095 0.36 1.14 0.011 0.0095 * 0.38 0.21 0.040 0.0060 0.77 0.49 0.02717 0.093 0.38 1.17 0.013 0.0093 * 0.39 0.22 0.039 0.0060 1.14 0.50 0.027

* = Trace.

† Trade name.

addition of Nb can retard the recrystallization ofaustenite and increase the nucleation sites andnucleation rates during ferrite transformation.16,17 Thisresulted in a decrease of ferrite grain size. But, itincreased the deformation resistance of phase at

high temperature.18,19

However, the deformation ofinclusions (especially MnS) during rolling wascontrolled by the relative strength between phaseand inclusions. The higher the deformation resistanceof phase, the more the inclusion was elongated.Figure 4 shows the aspect ratio increased from 4.4 to4.8 with the increase of Nb content. It has beenreported that there is more sensitivity to HIC with lessspheroidization of inclusions.20 On the other hand,when Nb was less than 0.037 wt%, large Nb(C,N)particles were seldom seen. Most of the Nb(C,N)particles were very fine and were observable by TEM.

FIGURE 1. A method of measuring the sensitivity of HIC.

TABLE 3

Test Conditions of HIC Immersion Corrosion Test

H2SBefore After H2S Flow

Solution Test Test Stress Test Time Temperature Concentration Rate

BP 8.2 4.8-5.4 None 96 h 25 ± 3°C Saturated 10 cc/min/1NACE 3.0 4.0-4.5 None 96 h 25 ± 3°C Saturated 10 cc/min/1

pH value

prepared carefully to prevent water contaminationduring grinding and polishing. After preparation, themorphology of inclusions was examined with an opticalmicroscope (OM) and the aspect ratio (length-to-widthratio) was measured quantitatively with a Quantiment

970† image analyzer. Average values of the aspectratio were obtained by measuring more than 200inclusions. The local compositional characteristics ofinclusions were analyzed further by an energydispersive spectrometer (EDS) or an electron probemicroanalyzer (EPMA). The surface protective films ofCu-bearing steels were observed and analyzed byEPMA and an Auger electron spectrometer (AES).Some local precipitates and microphases of steelswere observed by transmission electron microscopy(TEM).

RESULTS AND DISCUSSION

Niobium The relation between Nb content ranging from 0 to

0.037wt% and CLR value after immersion in BPsolution for 96 h is shown in Figure 3. CLR valuesincreased regularly when Nb content increased. Thisdemonstrated that the addition of Nb decreased theHIC resistance of steels. This could be explained since





addition increased HIC resistivity of steels. In the BPsolution, a protective black surface film identified asMackinawite (FeS

1–x), and containing Cu, developed on

steels with more than 0.2% Cu.12,21 Because of theprotection of this film, the corrosion rate and Habsorption rate decreased. It was reported also that Cuaccelerated the recombination of H atoms and thusdecreased the H activity.22 In general, the HICoccurrence can be resisted by adding Cu to steels.

X-ray diffraction (Debye-Scherrer camera method),EPMA, and AES were used to analyze the structureand composition of the black protective film. X-ray

a

In addition, HIC cracks initiated mainly from the tips ofelongated MnS inclusions in this series of steels.Therefore, the detrimental effect of Nb on HICresistance was attributed to its effect on the increase inaspect ratio of inclusions.

Copper Figure 3 also shows the difference in HIC

resistance between Nb-bearing steels with 0.29 wt%Cu and without Cu. The CLR values of Cu-bearingsteel dropped below 1% and the blistering of thesurface was decreased or absent. This indicated Cu

FIGURE 2. Schematic diagrams showing composite specimen preparation: (a) a small section cut from HIC test specimen and (b) weld extensions made to (a).

FIGURE 3. Effect of Nb content on crack length ratio obtained by HIC immersion test in BP solution.

FIGURE 4. The relation between Nb content and the average

aspect ratio of inclusions.

(b)(a)





HIC. The film’s outside was -Fe2O

3 enriched with Cu,

and the inside was FeS. This differed from resultsobserved by Inagaki, et al.,12 and Smith, et al.,21 whoreported that Cu enriched the FeS film.

Chromium and Molybdenum Cr and Mo usually are added to HSLA steelsbecause they impart excellent corrosion resistance, butthis addition has tremendous effects on microstructure.These elements — like aluminum, silicon, vanadiumand tungsten — can increase the A

3 temperature,

decrease the Bs and M

s temperatures and delay the

transformation of ferrite and pearlite.23 Thus, adding Crand Mo will increase the hardenability and promote theformation of low-temperature transformation products.24

The effect of Cr on the steel microstructure is shown inFigure 7. Increasing Cr content changed the

FIGURE 5. The black film morphology of Cu-bearing steel observed by EPMA: (a) protective film morphology and (b) line scanning analysis.

(a) (b)

FIGURE 6. The AES depth profile of the Cu-bearing steel.

spectra indicated the corrosion products of Cu-bearingsteel were composed primarily of -Fe

2O

3 and FeS.

Most of EPMA line-scanning analyses showed theouter part of this film was enriched with Cu, and theinner part contained a high level of sulfur (Figure 5).

However, some showed sulfur only, without theexistence of a Cu peak at the surface. Figure 6 showsthe AES depth profile of a Cu-bearing steel. The totalsputtering time was 220 min. Cu was not detected atthe surface. A possible reason for the absence of Cu ina few data of the films on Cu-bearing steels was therandom enrichment of Cu in the films or thecontamination of the specimen preparation. From theconcentration depth profile of O, Fe, and S, AESindicated the outer film was -Fe

2O

3, and the inner was

FeS. In summary, adding more than 0.2% Cu washelpful to form a protective black film which resisted





FIGURE 7. Optical micrographs of HSLA steels with 0.32% Mo

and (a) 0.02% Cr, (b) 0.38% Cr, and (c) 0.77% Cr. ( =ferrite,P=pearlite, B=bainite, and GB=granular bainite)

FIGURE 8. Optical micrographs of HSLA steels with 0.02% Cr

and (a) 0.17% Mo, (b) 0.32% Mo, and (c) 0.51% Mo. ( =ferrite,P=pearlite, GB=granular bainite, and RP=regressive pearlite)

(c)(c)

(a)

(b)

(a)

(b)





microstructure from pearlite, ferrite, and small amountsof bainite to a ferrite and bainite structure. When Crcontent reached 0.77%, the microstructure belongs togranular bainite. Figure 8 shows the effect of Mo on thesteel microstructure. Increasing Mo content caused asimilar variation of the structure. When Mo content

reached 0.5%, the structure contained black massiveregressive pearlite in addition to the original granularbainite. Because of the high hardenability effect of Crand Mo, excessive additions of these elements easilygenerated low-temperature transformation products, asshown in Figures 7 and 8. From TEM observation, asshown in Figure 9, the microphases between theadjacent bainitic ferrite laths were mainly martensite/ austenites-constituents (M/A-C). This high carbonmicrophase, belonging to hard and brittle structure,increased with Cr and Mo contents. The effect ofM/A-C on HIC is discussed later in detail.

After immersion in NACE solution, the HIC resis-

tant property of HSLA steels had a greater tendency todeteriorate when Cr and Mo contents were increased.However, a consistent relation was not observed. Theleast CLR value was 3.3%, but the highest valuereached 42%. Parrini and DeVito13 found 0.3% Cr wasthe most beneficial to HIC resistance in BP solution.Iino, et al.,25 indicated 0.6% Cr had a pronounced effecton lowering H absorption on steel surface in the HICtest, but the addition of Mo to 0.4% had a contraryeffect. However, those researchers only considered thecase of individual additions of Cr or Mo. In this study,Cr and Mo were added together. As a result, thecomplicated synergistic effect between Cr and Mo hadto be considered. Because cold cracking at welds andHIC occurrence of steels are both induced by H, therelative coefficient of these two elements on HIC couldbe referred to the International Institute of Welding(IIW) carbon equivalent formula (CEQ) and theIto-Bessyo formula (Pcm, weldment crackingparameter of material).26 These equations resulted:

Ceq = C + Mn

6 + Cr+Mo+V

5 + Cu+Ni

15 (2)

Pcm = C + V10

+ Mo15

+ Mn+Cr+Cu20

+ Si30

+ Ni60

+ 5B (3)

The possible parameters of these two elements

were Cr5

+ Mo5

and Cr20

+ Mo15

. Figure 10 shows the variation

of CLR value, and Cr

5+ Mo

5 were too large to derive any

relationship. Similar variation occurred in CLR value

and Cr

20+ Mo

15. Because these parameters didn’t match

the HIC data, other parameters were chosen by

mathematical calculation. The suitable parameter was

derived as Cr

15+ Mo

5. The relationship between CLR value

(a)

(b)

FIGURE 9. Microphases of granular bainite in HSLA steel: (a)bright field and (b) dark field. (M/A-C=martensite/austenites-

constituent and RA=retained austenite)

FIGURE 10. Variation of CLR value of HSLA steels with

Cr5

+ Mo5

in NACE solution.

and Cr

15+ Mo

5 is shown in Figure 11. The variation of CLR

values could be divided into two regions. When Cr15

+ Mo5

was lower than 0.1%, the CLR values of HSLA steels

could be controlled to below 7%. When this parameter





was larger than 0.1%, the CLR values were higher

than 9%. This result indicated the parameter ( Cr15

+ Mo5

)

was suitable to evaluate the combination effects of Crand Mo on HIC sensitivity. The critical value of thisparameter was 0.1%. This phenomenon occurredbecause Cr and Mo had a synergistic effect on theformation of low-temperature transformation products.

FIGURE 11. Variation of CLR value of HSLA steels with

Cr

15 + Mo

5 in NACE solution.

(a)

Figure 12 shows the H-induced cracks initiated at theinterface between M/A-C and matrix and the interfaceamong M/A-C, MnS, and matrix. The more the M/A-Cwas, the higher the HIC sensitivity was. From OMobservation, adding Mo generated M/A-C more easilythan adding Cr. This explained why alloying the steelswith Mo was more detrimental than alloying with Cr.

FIGURE 13. HIC (a) initiating at the interface between a TiN inclusion and matrix and (b) penetrating through the TiN

inclusion.

FIGURE 12. SEM micrographs showing the cracks of HIC initiating at the interfaces (a) between M/A-C and matrix and

(b) among M/A-C, MnS, and matrix.

(a)

(a)

(a)





Titanium Elongated MnS inclusions were the most

important initiation sites of HIC in HSLA steels.However, the interaction between the Ti componentand MnS on HIC merited study. Figure 13(a) shows theHIC crack initiated at the interface between rhombicTiN inclusion and the matrix. In addition to initiation atthe interface, the crack penetrated the TiN inclusion(Figure 13[b]). Some researchers have reported that Tihas a faint tendency to cohere with MnS and to form

globular TiS inclusions.27

But, the current study foundthat TiN and MnS inclusions existed together. FromTEM microscopy (Figure 14), the rectangular TiNparticle could be found in the interior of an MnSinclusion. This indicated TiN particles acted asnucleation sites for MnS formation, and thesecombined inclusions cooperated to result in HIC. Thiscould be seen in fracture morphology in the through-thickness direction of specimens (Figure 15). Theeffect of Ti addition on HIC also has been reported.Wilde, et al.,4 reported that the major initiation site ofHIC was elongated Type II MnS inclusion. In addition,

glassy silicate inclusions or massive Nb(C,N)precipitates acted as initiation sites. However, they didnot mention Ti and other inclusions. Shimogori, et al.,15

found that fine and dispersed Ti(C,N) with a diameterless than 0.1 µm decreased the H diffusion constantand increased the resistance of HIC. Shimizu, et al.,28

found that fine and dispersed Ti(C,N) suppressed Hsegregation at the inclusion-matrix interfaces andthereby decrease HIC. On the other hand, coarseTi(C,N) and TiN were reported to be HIC initiation sites

and increase the HIC occurrence.15,29

In this study,coarse TiN precipitates (about 1 µm or more) such aselongated MnS inclusions acted as HIC initiation sites.The bigger the TiN particle was, the more theopportunity for crack initiation. At the same time, TiNcombined with MnS to accelerate HIC occurrence.

CONCLUSIONS

Aspect ratios of inclusions increased with theincrease of Nb content, decreasing HIC resistance ofsteels.

(a) (b)

(c)

FIGURE 14. TEM micrographs showing the morphology and orientation relationship of a compound TiN and

MnS: (a) bright field; (b) dark field; and (c) and (d) electron diffraction pattern.

(d)





transformation products (M/A-C) which increased HICsensitivity. The suitable parameter of these two

elements for HIC sensitivity was Cr

15+ Mo

5, and the critical

value of the parameter was 0.1%. Coarse Ti precipitates ( >1 µm) such as elongatedMnS inclusions were HIC initiation sites. Additionally,

TiN and MnS combined to induce HIC occurrence.

ACKNOWLEDGMENTS

The authors acknowledge the assistance of C.-F.Wu and J.-M. Hsu in the experiments and the guidanceof L.-H. Hsieh in the image analysis.

REFERENCES

1. M.H. Bartz, C.E. Rawlins, Corrosion 4, 5 (1948): p. 187.2. L.T. Guo, Petroleum Newsletter—Technology 2, 4 (1990): p. 10.3. D. Warrew, MP 26 (1987): p. 38.4. B.E. Wilde, C.D. Kim, E.H. Phelps, Corrosion 11, 36 (1980): p. 625.

5. A. Ikeda, Y. Morita, F. Terasaki, M. Tekeyama, 2nd Int. Cong. Hydrogenin Metal, Vol. 2, paper no. 4A7 (Coral Gables, FL: Int. Assoc. forHydrogen Energy, 1977).

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21. J.S. Smith, J.D. Millar, Brit. Corros. J. 10 (1975): p. 136.22. R. Blondeau, Ironmaking and Steelmaking 18, 3 (1991): p. 201.23. T. Kasugai, M. Inagaki, J. of Japan Welding Society 3, 8 (1975): p. 220.24. R.V. Fostin, M. Semchyshen, Transaction of the ASM 57 (1964): p. 632.25. M. Iino, N. Nomura, H. Takezawa, H. Gondoh, Revue de Metallurgie,

Vol. 76 (1979): p. 591.26. Y. Ito, K. Bessyo, Int. Ins. of Welding Document No. IX-631-69 (1969).27. K. Narita, A. Tomita, M. Boukuya, Y. Mazhon, J. Hajiki, Tetsu-to-

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25 (1985): p. 982.

The outer part of the protective film formed on theCu-bearing steel was -Fe

2O

3 enriched with Cu, while

the inner part was mainly FeS. The addition of both Cr and Mo had a synergisticeffect on the microstructure, forming low-temperature

FIGURE 15. SEM micrographs showing the fracture morphol-

ogy in the through-thickness direction.

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