RELATIONSHIP BETWEEN MICROSTRUCTURE OF SOUR GRADESTEELS AND THEIR HIC RESISTANCE

EVA Mazancováa

PETR Wyslychb

KAREL Mazanecc

a) NOVÁ HUŤ, a.s., Research and Testing Institute; 707 02 Ostrava, CZb) VŠB TU – Ostrava, Inst. of Physics, Tř. 17. listopadu, 708 33 Ostrava-Poruba,CZc) VŠB TU – Ostrava, Inst. of Mater. Eng., Tř. 17. listopadu, 708 33 Ostrava-Poruba,

CZ

Abstract Hydrogen induced cracking (HIC) of steel occurs in corrosive environmentcontaining wet hydrogen sulphide what is corresponding to the conditions of steelexposition in oil and wet sour gas. HIC may be found in plain - carbon and low -alloysteels in the absence of external stress if hydrogen content in these steel types ishigher sufficiently. Considering the steady increasing operation requirements on thehigher reliability and durability of pipelines steel, so it is necessary to investigate thephysical metallurgy factors that control the crack formation causing and theparameters leading to the suppressing of this degradation effect. The physical metallurgy parameters of the pipeline C-Mn(Nb,V) steels governingtheir HIC resistance and response to sour service conditions are analysed. Theachieved results show, besides the principal metallurgical characteristics based onthe limitation of carbon, manganese and sulphur contents in steels, that the shapecontrolling of sulphide by Ca-addition and the amount of oxides play a further veryimportant role in the increasing resistance of investigated steels to HIC. The combination of controlled rolling (CR) with accelerated cooling (ACC),modifying the obtained microstructure, represents a perspective processing leadingto the improvement of the HIC resistance. The banded microstructure observed afterCR usually is replaced in case of the ACC application with the uniform ferrite/bainitestructure. The physical metallurgy parameters of the ACC processing assuming anachievement of a higher service reliability and durability of pipeline steel, arediscussed. The model describing the causes of HIC susceptibility of investigated sourservice steel grades and the perspective ways how to assure the high HIC resistanceis presented.

1. INTRODUCTION Hydrogen induced cracking (HIC) of pipeline steels occurs in corrosiveenvironment containing wet hydrogen sulphide what is corresponding to theconditions when steels are exposed in wet sour gas. Plain carbon and low-alloysteels, besides the process of weightless corrosion, may suffer the following types ofcorrosion: a) surface blistering and internal cracking; b) stress corrosion cracking.With respect to cracking, surface blistering and internal cracking (known as hydrogeninduced cracking) also occurs in the absence of external stress, if the hydrogencontent of the steel is high sufficiently. In contrast to HIC, hydrogen induced stresscorrosion, usually referred to as sulphide stress cracking (SSC), occurs in thepresence of external tensile stresses only [1,2]. The hydrogen induced cracks areusually parallel to plate surface and are mostly initiated in the segregated zone of themid-thickness of plate [3]. It is known, that in a low pH environment (at pH=3.0 ÷ 3.5),

however, it is impossible to suppress HIC completely by addition of alloying elementswhich are considered effective for the improvement of hydrogen embrittlement in ahigher pH environment. Considering the steady increasing operation requirements on the higherreliability and durability of pipeline steel, so it is necessary to investigate the physicalmetallurgy factors which control not only the conditions causing HIC, but also theparameters leading to the suppressing of this degradation effect. The mid-thicknesssegregated zone, enriched with alloying elements in steel (in this case with Mn forexample) and characterised with higher hardenability, represents a potential place forHIC. Besides the effect of metallurgical parameters (cleanness and modification ofchemical composition of steel), the process of controlled rolling (CR) in combinationwith accelerated cooling (ACC) enlarging the conventional thermomechanicaltreatment [5, 6] is currently drawing attention as a perspective technology formanufacturing pipeline steel showing a favourable HIC resistance [3]. The pipeline steels that possess adequate HIC resistance are referred to as sourservice grades [7]. HIC tests are performed using test solution of a H2S - saturatedsolution containing 5% NaCl plus 0.5% CH3COOH (known as NACE-solution of pH3.0 to 3.5) [8]. HIC resistance may be evaluated either in terms of HIC crack arearatio (CAR) based on ultrasonic testing or in terms of crack length ratio (CLR) [3,7].

2. METALLURGICAL CHARACTERISTICS OF SOUR GRADE STEELS The sour grade plain carbon and low alloy steels need special efforts concerningtheir chemical composition. The carbon and/or manganese contents have to bereduced to technologically necessary level in comparison with conventionallymanufactured steel types. Limitation of sulphur content and reduction of centrelinesegregation play a very important role in increasing the resistance of investigatedsteel types to sour gas environment [1,2]. The priority attention is devoted to thedetermination of admissible sulphur content, that according the last investigation hasnot to reach over 0.0015% [3,7] as it has also resulted from analysis of thesusceptibility of OCTG (oil country tubular goods) to SSC performed formerly [1,9].Characteristic chemical composition of pipeline steel for sour gas environment isfollowing (in wt%): 0.05%C; 1.30%Mn; 0.25%Si; 0.008%P; 0.0015%S; 0.04%Ni;0.03%Cu; 0.05%V; 0.03%Nb and Ceq is 0.28÷0.33 approximately. The chemicalcomposition of considered steel type is generally defined by very low carbon and bymanganese content not overreaching the level of 1.35÷1.40%. The high segregationactivity of manganese limits this value. The following average mechanical propertiescan be found in the investigated pipeline steel at room testing temperature:Rp0.2=490÷510MPa, Rm=590÷610MPa, A(2”)=50% and CVN(-20 oC)=200÷250J. Thehardness in the segregation zone has to be lower than 248HV. This hardnesscorresponds (NACE material requirement) to the critical value. The probability of SSCis very high, if this value is overreached. The same limitation can be found in thecase of the resistance to HIC [3]. The sulphur content remaining in steel after its desulphurisation can be treated withan addition of calcium having high affinity to sulphur. The result of this operation isthe shape control of sulphide inclusions as it corresponds to the effective sulphideshape controlling parameter (ESSP) indicating the degree of shape controlling ofsulphide by calcium [3]. Simultaneously, it is necessary to reduce the oxygen contentin steel so that no large amounts of calcium oxides can be found, since oxide stringercan also act as crack starters for HIC [7].

The mid-thickness segregation zone is usually enriched with manganese and alsowith residual elements in relation to the level of steel cleanness [10,11]. Thesegregation of manganese represents a very important problem, since its localconcentration is assumed to be 1.5÷1.7 times higher than that in non - segregatedarea. Therefore, the austenite decomposition in the segregated zone is delayed incomparison with that of the non-segregated area [6]. This effect is amplified due toadditional redistribution of carbon atoms which can diffuse preferentially into zoneenriched with manganese and having a lower carbon activity. An increasedsusceptibility to the HIC is a result of this process. For this reason, it is useful to applythe technology including the combination of CR with final ACC what reduces theredistribution of carbon atoms and assures the formation of uniform microstructurewithout the appearance of unfavourable banding.

3. PHYSICAL METALLURGY OF HIC The recommended application of CR in combination with ACC produces ahomogenous microstructure characterised with the increased HIC resistance. Afterthis heat treatment, the strength and toughness are improved in comparison with themechanical properties obtained after the application of CR [5,6]. The bandedmicrostructure observed after CR, having a low resistance to HIC, is shown in Fig.1.

Figure 1. Microstructure of steel susceptible to HIC.

The schematic illustration of CR with and without subsequent ACC is presented inFig.2. The corresponding microstructures are shown in Fig.3 [5,6]. The ACC startingbelow the Ar3 temperature from the finishing temperature improves the achievedtoughness but the banded microstructure at the mid-thickness of plate does notdisappear what results in a partial susceptibility to HIC. This finding shows that thesecooling conditions are not sufficient to eliminate entirely the investigatedsusceptibility. The start of ACC from higher than the Ar3 temperature (usually morethan 20 °C above the calculated Ar3 temperature, approximately) suppresses theinitiation of HIC completely [3]. This cooling variant produces a homogeneousmicrostructure assuring a high level of the resistance to HIC. In this case, the bandedorientation of microstructure is replaced by uniform distribution of ferrite and bainite[6,10]. The microstructure presented as b) has shown the best HIC resistance (Fig.3).The applied cooling rate at ACC influences also the achieved level of the resistanceto HIC (Fig.4). The banded microstructure in the mid - thickness of plates does not

disappear completely at the cooling rate at 10 °Cs-1. Therefore, it is necessary to cool

Figure 2. Schematic illustration of CR and CR+ACC processing [5,6].

a) b) c) d)

Figure 3. Characteristic microstructures obtained after CR and CR+ACC.

Figure 4. Relationship between the applied cooling rate at ACC and the CAR [7].

after rolling at a cooling rate higher than 15 °Cs-1. The stop temperatures of ACCalso play an important role from point of view the achieved HIC resistance. Theoptimum temperature range is not large and mostly can be found between 400 and520 °C [3]. The presented results demonstrate, that the resistance to HIC isdependent not only upon the cooling start and stop temperatures but also uponapplied cooling rate after rolling [3,5,6]. The formation of pearlite banded microstructure can be associated withheterogeneous redistribution of carbon atoms in steel matrix. In the ACC platescarbon is uniformly distributed irrespective of the non-segregated area or segregatedzone [3], while only after application of CR a carbon concentration region exists atboth sides of the segregated zone. The rejection of carbon to the segregated zonein CR plate is considered to occur for the following reasons: a) the segregatedzone is enriched with manganese (considered 150% enrichment); b) thedecomposition of austenite in ferrite is retarded compared with that of the non-segregated zone; c) the diffusion of carbon atoms is restricted to one direction inregion between the non-segregated area and segregated zone. Besides abovementioned segregation of manganese, it is assumed that carbon is uniformlydistributed in austenite during heating operation before rolling. In the applied model,the different decomposition characteristics in the non-segregated area andsegregated zone in pipeline plates are given by basic segregation of manganeseonly. The difference in transformation time is calculated by supposing the cooling rateof plate by air as 30 oC/min. Schematic illustration of the proposed model ispresented in Fig.5 [10,12]. At air cooling rate and for 1.5 times increased localmanganese concentration in the segregation zone, the calculated difference in thetransformation Ar3 temperatures is 45 oC, approximately [3]. The correspondingdifference in transformation time attains 90s. Assuming austenite-ferritetransformation is governed by the diffusion of carbon in austenite, the thickness offerrite zone (F2, see Fig.5) is given by:

F2 = α.t1/2 1)

where α is the parabolic rate constant for ferrite thickening. This value can beexpressed as:

Dγ1/2 (Cγ

e – C0)α = -------------------------------- 2) (Cγ

e - Cαe)1/2 (C0 - Cα)1/2

where Dγ is diffusion coefficient of carbon in austenite; C0 is the initial bulk carboncontent; Dγ

1/2 and Cαe represent the equilibrium carbon concentration of austenite

and/or ferrite at austenite-ferrite interface [13]. The calculation of the F2-value was carried out under para-equilibrium conditionsince the partitioning of manganese between austenite and ferrite only occurs atrelatively high transformation temperature. The calculated thickness of ferrite zoneunder consideration of Ar3 (the average value of Ar3 temperatures is 715 °C) for thenon-segregated area and Ar3 for the segregated zone is 48µm approximately. Thisprocess is realised at low cooling rate corresponding to air cooling after the finishing

rolling. In the case of ACC technology (at sufficiently high cooling rate) however, thedistribution of carbon is uniform as it follows from the very small amount of carbon

Figure 5. Influence of different Ar3 temperatures in non-segregated area and in segregated zone on the austenite decomposition. Redistribution of carbon atoms in CR plates [10].

atoms diffusing to the segregated zone. The volume fraction of hard phases formedby displaced decomposition of austenite as the products of bainite or martensitephase transformation is decreased. Simultaneously, these hard phases are very finedistributed in the segregated zone [3]. However, when the start temperature of ACC decreases below the Ar3temperature, HIC resistance cannot be improved. In this case, analogously to the CRplates, due to the increased amount of carbon rejected to the segregated zone thedifference in austenite-ferrite transformation time between the non-segregated areaand the segregated zone is enlarged [3,10,12].

4. CONCLUSION The physical metallurgy parameters of plain carbon and low-alloy pipeline steelsinfluencing their HIC resistance are analysed. The obtained results show that besidesthe metallurgical characteristics, based on the limitation of carbon, manganese andsulphur content in steel, the shape of sulphides and the amount of oxides play afurther decisive role in increasing the HIC resistance of sour service steel grades.

The combination of CR with ACC, modifying the steel microstructure, represents aperspective processing improving the resistance to HIC. The banded microstructureusually observed after CR is replaced by the uniform ferrite-bainite microstructureafter the application of ACC. Simultaneously, the model describing the physicalmetallurgy causes of the susceptibility to HIC of steels applied for pipelines undersour service conditions, so as the ways how to assure their high HIC resistance arepresented.

ACKNOVLEDGEMENTS The research has been carried out with a financial support of Grant Agency ofCzech Rep. – project 106/98/0079.

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