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Research ArticleReceived: 24 July 2007 Revised: 13 December 2007 Accepted: 18 December 2007 Published online in Wiley Interscience: 13 February 2008

(www.interscience.com) DOI 10.1002/sia.2759

Characterization of the carbides in the steelX20CrMoV12.1 used in thermal power plantsDanijela Anica Skobir,a Matjaz Godec,a Monika Jenkoa andBostjan Markolib

Microstructure of the steel X20CrMoV12.1, used for thermal power plants, after tempering at approximately 500 C consists oftemperedmartensite with carbide precipitates.The evolution of the chemical and phase composition of carbide precipitates in X20CrMoV12.1 steel was studied in the

as-received state after 56 000 h at 470530 C under load and in heat-treated state (11344 h at 800 C) using transmissionelectronmicroscopy (TEM) and electron backscatter diffraction (EBSD) technique.The precipitates found in service-loaded state as well as in heat-treated state were of M23C6 type. In all samples two

morphologically different types of carbides were established. Copyright c 2008 JohnWiley & Sons, Ltd.

Keywords: chromium steel; X20CrMoV12.1 steel; carbide morphology; tempering

Introduction

The 912% Cr martensitic steels have been widely used aspipework in the power-generating industry. The first 912%chromium steel, X20CrMoV12.1, was developed in Germanyand it was standardized for use in steam pipes under theGerman standard DIN17175 designation. Since 1960s it has beenwidely used for steam pipes in power plants in Europe andelsewhere.[1 3]

The heat-treatment of 912% chromium steels includesaustenitizing, quenching and high-temperature tempering (at730780 C) in order to achieve a good combination of high-temperature strength, toughness and creep strength. The mi-crostructure of such a steel is composed of highly temperedmartensite with finely dispersed carbide precipitates along theboundaries of ex-austenitic grains and ferritic sub-boundaries.An important aspect of the microstructural stability is the dis-tribution of carbide precipitates. Carbides change their chemicaland phase composition as well as their size with time and tem-perature until equilibrium is reached. The carbides which canbe expected in steel depend strongly on the service temper-ature, initial composition, amount and kind of carbide-formingelements and thermodynamic stability of carbides. Steels des-tined for power plant applications might contain any of thefollowing precipitates M23C6, M7C3, M2X, and MX (M standsfor the metal elements and X for nonmetallic elements of Cand N).[4]

The aim of this work was to investigate the morphologicaland compositional changes of carbides in X20CrMoV12.1 steelafter long-term service of 56 000 h within a temperature rangeof 470530 C at pressures up to 18 MPa. In order to study theevolution of the carbide precipitates, some samples were heat-treated at 800 C for different periods of time (11344 h) andinvestigated.

Transmission electron microscopy (TEM) and electron backscat-ter diffraction (EBSD) techniques were used to determine the typeof carbides.

Experimental

Steel pipes manufactured from X20CrMoV12.1 were cut from a 325MW steam boiler after long-term service for 56 000 h (470530 Cat up to 18 MPa) and investigated. The chemical compositionof the investigated steel was performed using optical emissionspectroscopy (OES) and is presented in Table 1. Specimens wereprepared from the wall of an industrial 42 4.5 mm pipe in theas-received state after long-term service. In order to simulate theevolution sequence of carbides, some specimens of the samematerial were prepared, but they were first quenched in oil at atemperature of 1040 C. This temperature ensured the solution ofall carbide particles in austenite.[3] Specimens of steel were thentempered from 1 to 1344 h at 800 C.

Microstructures of the sample in as-received state and samplestempered at 800 C at different times were investigated byfield emission scanning electron microscope JEOL JSM 6500Fsupplemented by energy dispersive spectroscopy (EDS) and EBSDanalytical techniques. EDS analyses were performed at 13 kVaccelerating voltage and a probe current of 0.35 nA, while EBSDanalyses were done at 15 and 20 kV and a probe current of 1.2 and2 nA, respectively.

Samples for EBSD measurements were finally polished for 3 minby colloidal silica oxide (Struers OPS). Extraction carbon replicaswere prepared from the material, tempered 1344 h at 800 C forthe investigation on TEM. The crystallographic characterisation ofprecipitates was performed on a JEOL AEM 2000 FX.

Correspondence to: Danijela Anica Skobir, Institute of Metals and Technology,Lepi pot 11, SI-1000 Ljubljana, Slovenia.E-mail: danijela.skobir@imt.si

a Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Slovenia

b University of Ljubljana, Faculty of Natural Sciences and Engineering, Ljubljana,Slovenia

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Table 1. Chemical composition of the steel of the investigated pipe(mass%)

Steel C Si Mn P S Cr Mo Ni V

X20CrMoV12.1 0.18 0.24 0.51 0.009 0.014 11.7 0.96 0.66 0.27

Results and Discussion

Some of the steam pipes have been in service more than 20 years.Owing to high-operating temperatures, long-exposing time andhigh pressure, the mechanical properties and the microstructurechanged significantly. In order to prolong the service time and topredict the life time it is of great importance to understand theevolution of the carbide precipitates in microstructure.

Figure 1(a) shows the microstructure of the as-received materialas well as microstructures of material, tempered at temperature800 C for 1,336 and 1344 h (Fig. 1(b)(d)). In the as-received statethere is still a typical tempered martensite microstructure withcarbide precipitates along martensite laths and previous austenitegrain boundaries. After tempering for 1 h at 800 C the microstruc-ture is similar to that in as-received state. Morphologically, twotypes of carbides were distinguished, in the shape of laths andoctahedral (Fig. 1(e)). The size of the first ones was about 200 nmand the size of the octahedral was about 500 nm. After 336 h the

coarsening of the precipitates was observed. The precipitates ofthe lath shape disappeared or coarsened into octahedral shape.In the microstructure after 1344 h of tempering at 800 C themartensite habitus is not present any more. There are still twosizes of carbides, the larger one up to 4 m and smaller onearound 500 nm. The octahedral shape is preserved.

The smaller carbides were examined by TEM and it was foundthat they are of the type M23C6 (Fig. 2). Identical results wereobtained also by Zheng-Fei and Zhen-Guo.[5] Thermo-Calc (version1.4.4. Software AB 2000) simulation was also performed andshowed that the composition given in Table 1 corresponds tothe microstructure of -ferrite and M23C6 carbides. The chemicalcomposition given by Thermo-Calc was very close to that, obtainedby EDS measurement on larger carbide precipitates. Table 2presents the comparison of chemical compositions in at.% for theM23C6 carbide phase, calculated with Thermo-Calc and obtained

Table 2. Chemical composition of M23C6 obtained by EDS measure-ment and calculated with Thermo-Calc (at.%)

C V Cr Fe Mo

Thermo-Calc 20.8 1.0 52.8 20.2 5.2

EDS 29.4 0.7 47.2 19.4 3.3

Figure 1. SEM images of microstructure (a) in as-received state; and after tempering at 800 C for (b) 1 h; (c) 336 h; (d) 1344 h; (e) higher magnificationimage of (b) of two morphologically different carbides, laths, and octahedral shape.

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Figure 2. (a) Transmission electron image of an extraction carbon replica with (b) corresponding diffraction pattern and (c) indexing of the diffractionpattern of M23C6 carbide. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

Figure 3. SEM image and corresponding EBSD patterns of the material in the as-received state. The point of the EBSD analysis is marked by cross on theSEM image. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

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by EDS measurement. The results of EDS measurements givenin Table 2 are an average of three carbides shown in Figs 3, 4and 5. The amount of carbide-forming elements, measured byEDS coincides very well with the calculated values. Carbon valueobtained by EDS measurement was larger, due to carbon built-upunder electron beam.

In the literature there is not much found about the investigationsof the carbides by EBSD technique, which is a rather new technique.This technique was primarily developed for texture measurements,

but also phase analysis can be successfully provided.[6] The carbideprecipitates are usually studied by TEM technique, which requiresvery difficult and time consuming preparation of thin foils as wellas carbon replicas. The main advantage of EBSD is because thistechnique works by positioning a stationary beam of electrons onselected sampling points on the specimen surface.

Figure 3 shows 70.5 tilted and tilt-corrected SEM image ofas-received material. Final polishing of microstructure leads tosurface topography. The harder carbide phase is slightly above

Figure 4. SEM image and corresponding EBSD patterns of the material tempered for 1 h at 800 C. The point of the EBSD analysis is marked by cross onthe SEM image. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

Figure 5. SEM image and corresponding EBSD patterns of the material tempered for 1344 h at 800 C. The point of the EBSD analysis is marked by crosson the SEM image. This figure is available in colour online at www.interscience.wiley.com/journal/sia.

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the surface. EBSD measurements were performed on a largercarbide phase and also on the small one. Both Kikuchi patternsconfirm Cr23C6 carbide types. It has been quite an effort todistinguish between Cr7C3 and Cr23C6, because patterns arevery similar. At higher accelerating voltage the diffraction linesare narrower and can be easily distinguished between similarpatterns. To avoid overlapping of patterns it is better to uselower accelerating voltage for the determination of small carbideprecipitates. On the Fig. 3 there is also a pattern of -Fe. The sametype of carbide was also found on the samples, tempered for 1,336 and 1344 h at 800 C. Figures 4 and 5 show EBSD carbidesanalyses of sample tempered for 1 and 1344 h, respectively. Inthe cases when carbides are smaller than 400 nm some patternsoverlapping appeared, as shown in Fig. 5. On the SEM image(Fig. 5) the coarsening of carbides is clearly seen. There aretwo carbides 3 and 4 m diameter with orientation differingby a few degrees. From this image, we might suppose that twocarbides grow together and when they coalesce one orientationpredominates. The other explanation might be in connection withthe orientation of martensite matrix. Therefore, the orientationsof two close carbides which mostly nucleate from the samemartensite habitus have similar orientation. The phenomenonof carbides coalescence was quite frequently observed. Suchcoalescent carbides have mostly the same orientation within5 differences. The exact mechanism of the coarsening is notcompletely known.

Conclusions

The samples, taken from the steam pipes manufactured fromthe X20CrMoV12.1 steel after 56 000 h in service within thetemperature range of 470530 C and pressures up to 18 MPa,have carbides in various morphologies and coarsened duringexposure. The martensite structure changed from the primarilyneedle-like martensite to ferritecarbide microstructure with stillnoticeable martensite habitus.

The microstructure obtained after tempering for 336 h at 800 Cis very similar to that in as-received state. In all samples twomorphologically different types of carbides were found but bothwere proven by EBSD and TEM to be of the type M23C6.

References

[1] Hald J. Steel Res. 1996; 67: 369.[2] Blum R, Hald J, Bendick W, Rosselet A, Vaillant JC.VGBKraftwerkstech.

1994; 74: 641.[3] Foldyna V, Kubon Z, Filip M, Maier KH, Berger C. Steel Res. 1996; 67:

375.[4] Fujita N, Bhadeshia HKDH. ISIJ Int. 2002; 42: 760.[5] Zheng-Fei H, Zhen-Guo Y. Journal of Materials Engineering and

Performance JMEP 2003; 12: 106.[6] Michael JR. In Electron Backscatter Diffraction in Materials Science

Schwartz AJ, Kumar M, Adams BL (eds). Kluwer Academic Press: NewYork, 2000; 75.

Surf. Interface Anal. 2008; 40: 513517 Copyright c 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia