REDISTRIBUTION OF CARBON AND NITROGEN IN HETEROGENEOUS WELD JOINTS OF CREEP-RESISTANT STEELS

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23. - 25. 5. 2012, Brno, Czech Republic, EU

REDISTRIBUTION OF CARBON AND NITROGEN IN HETEROGENEOUS WELD JOINTS OF

CREEP-RESISTANT STEELS (P91/P22)

Rudolf FORET1, Zdenk HODIS2and Ji SOPOUEK3

1Faculty of Mechanical Engineering, Brno University of Technology, Technick 2896/2, 616 69 Brno, Czech

Republic,[email protected]

2Faculty of Education, Masaryk University, Po 7, 603 00 Brno, Czech Republic,[email protected]

3Faculty of Science, Masaryk University, Kotlsk 207/2, 611 37 Brno,[email protected], Czech

Republic

Abstract

The experimental work presents the results of carbon and nitrogen redistribution calculations for

heterogeneous weld joints of X10CrMoVNb9-1 (P91)/10CrMo9-10 (P22) steels. The CALPHAD method was

applied for phase equilibrium calculations of both creep-resistant steels. This method can be used for the

solution of both local and global phase equilibrium problems concerning the base material and weld metal.

The creep-resistant steels were thermodynamically considered as Fe Cr V Mo - N - C based closedsystems. The CALPHAD approach complemented with appropriate diffusion model given in DICTRA code

enabled to simulate phase and element profile evolutions inside diffusion-affected zone of weld joint. The

simulation respects an existence of fusion region on the weld interface that is occurred in real weldments.

Keywords: creep-resistant steel; weld joint; CALPHAD; DICTRA

1. INTRODUCTION

Fossil-fired power plants are complex systems, in which it is practically impossible to rule out welding

different kinds of creep-resistant steel. The choice of weld metal is very important for welds used at high

temperatures. In the case of dissimilar weld joints, the redistribution of interstitials (C, N) can be occurred in

the stage of their post weld heat treatment (PWHT) and also in the course of subsequent exploitation. The

carbon redistribution leads to the appearance of carbon depleted zone (CDZ) and carbon enriched zone

(CEZ). The structure of the CDZ forms usually ferrite grains without any apparent carbide precipitate [1], [2],

[3]. The similar redistribution for nitrogen can be also found in dissimilar weld of steels alloyed with nitrogen.

The region of CDZ and CEZ can be the weakest area of the dissimilar weld join under long-time creep but

the other parts of the weld (for example temperature influence zone) have to be considered also.

The aim of the present work is to present the experimental results of redistributions of carbon and nitrogen inthe real weld joint of (P91)/ (P22) steels. These results are compared with theoretical simulation using

DICTRA program [4], which combine diffusion and thermodynamics CALPHAD approach [5]. These methods

can be used with advantage for description of diffusion controlled phase transformations.

According the CALPHAD approach, the investigated steel represents the closed multi-component system in

which each phase have different thermodynamic description defined in the applied thermodynamic database

[6]. The CALPHAD approach permitted a solution of phase equilibrium based on constrained minimization of

the total Gibbs energy in a closed system at a given composition, temperature and pressure [2], [5].

2. EXPERIMENT

Three weld joints were fabricated from creep-resistant steel known as P91 (X10CrMoVNb9-1) and steelmarked as P22 (10CrMo9-10). Straight pipes made from P91 and P22 steels with external diameter 324 mm,
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thickness 25 mm and length 400 mm were jointed together by gas tungsten arc welding (GTAW). The

chemical compositions of the used steels are given in Tab. 1.

Tab. 1 Chemical composition of creep-resistant steels

SteelChemical composition [wt.%]

C Mn Si Cr Ni Mo V W Nb N Fe.

P91 0.11 0.50 0.29 8.50 0.40 0.93 0.20 - 0.08 0.07 rest

P22 (filler metal) 0.07 0.80 0.50 2.30 - 0.90 - - - 0.01 rest

Welding was carried out using internal protection by inert gas. Inductive heating with thermal insulation

ensured a pre-heating temperature in the range from 200 C to 250 C. Filler metal on the base P22 (2.25Cr-1Mo) was used for welding P91 and P22 steels. We suppose the same composition for filler metal as for

steel P22 [7], [9].Post-weld heat treatment (PWHT) of the P91/P22 dissimilar weld joints was carried out in electric furnace for

2.5 hours at 730 C. The specimens were heat treated at 525C, 550C and 600C. Creep-testing was

carried out using cross-weld specimens also and it is given separately [7].

Fig. 1 Weld joint of P91/P22 600 C/15 875 h (etched by Nital reagent). Creep crack in right hidden part.

Fig. 2 Redistributions of Cr, Mo, Ni, Mn Si, V in P91/P22 real weld joint heat treated at 600 C/15 875 h.

The specimens after creep testing were cut along their longitudinal axis and polished by Villellas reagent or2% Nital reagent. Schematic view is on Fig. 1. The structure of weld joints was observed using scanning

electron microscopy (SEM) Philips XL30. Changes in chemical composition across the weldments were

measured using energy dispersive X-ray (EDAX) spectrometer for metal element and wave dispersive X-rayspectrometer (WDX 400) for light element (carbon and nitrogen).

3. THEORETICAL MODEL

Phase diagrams of the steels and temperature dependent carbon and nitrogen activities of the investigated

steels were calculated using CALPHAD approach [5] and thermodynamic database STEEL16 [8]. The

thermodynamic behaviour of the steels was approximated by Fe- Cr- V- Mo- N- C system. The bulk

composition of the Fe, Cr, V, Mo, N, and C given in Table 1 were used in calculations. The activities of the all

elements in the given steel can be obtained by the same way as the phase equilibrium calculation [10].

The CALPHAD approach complemented with appropriate diffusion model given in DICTRA code enabled us

to simulate element and phase profile evolutions inside diffusion-affected zone of weld joint. The simulationsupposed a coexistence of different phases (carbides, nitrides and carbonitrides) in the weld joint. The

assumption that the local phase equilibrium holds under the examined conditions and that bulk diffusion is

the mayor control process of phase transformations [2] was supposed. The simulations respect an existence


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of fusion region on the weld interface that is occurred in real welds. The linear changes of concentrations of

substitutional elements were assumed within fusion region.

4. EXPERIMENTAL RESULTS

Changes in chemical composition of Fe, Cr, Mo, V, Ni, and Si across the weldment are given on Fig. 2.

Carbon and nitrogen redistributions were measured at the samples heat treated at 525 C/8 690 h,550 C/11 190 h and 600 C/15 875 hours. The example of measured carbon and nitrogen concentrationprofiles are given at Fig. 3 and Fig. 4.

-2 -1 0 1 2

Distance [mm]

0

0.1

0.2

0.3

0.4

0.5

Cconcentra

tion[wt%]

Experiment C

Simulation C

P91 P22

-2 -1 0 1 2

Distance [mm]

0

0.04

0.08

0.12

0.16

0.2

Nconcentra

tion[wt%]

Experiment N

Simulation N

P91 P22

Fig. 3 Experimental and simulated redistribution of carbon in real weld joint P91/P22 600 C/15 875 h

Fig. 4 Experimental and simulated redistribution of nitrogen in real weld joint P91/P22 - 600 C/15 875 h.

Carbon and nitrogen diffused from P22 low alloy Cr-steel into high alloy Cr-steel P91 (again concentration

gradient). The maximum and minimum carbon and nitrogen concentrations in the carbon and nitrogen

enriched zone (CNEZ) and in carbon and nitrogen depleted zone (CNDZ) are given in Table 2.

Table. 2 Extreme carbon and nitrogen concentrations in the CNEZ and in CNDZ (simulated values in

brackets will be explained in capture Discussion)

Annealing NExpC max

[wt.%]

NExpC min

[wt.%]

NExpN max

[wt.%]

NExpN min

[wt.%]

525 C/8 690 h 0.33 (0.2) 0.03 (0.017) 0.095 (0.07) n.a (0)

550 C/11 190 h 0.375 (0.31) 0.015 (0.013) 0.12 (0.071) n.a (0)

600 C/15 875 h 0.45 (0.39) 0.01 (0.009) 0.13 (0.096) n.a (0)

The experimental observation confirmed that the matrixes of both steels are ferritic at annealing

temperatures 525 C, 550C and 600C. The CNDZ of the P22 steel is formed by rough ferritic grains andhigh precipitate density is occurred in the CNEZ of the P91 steel (see Fig. 5 and Fig. 6). The chromium rich

M23C6 type carbide and vanadium rich MX carbonitrides in P91 steel were detected and analysed. The

molybdenum rich phase (M6C type carbide or Laves phase) was observed in P22 steel on weld interface.

5. PHASE DIAGRAM AND ACTIVITY CALCULATIONS

The ThermoCalc program code was used for the phase diagram calculations of the steels P91 (Fig. 7) and

P22 (Fig. 8). The phase diagram isothermal cross section at 550 C for the steel P91 is given on Fig. 9

(MX=Vanadium Carbonitride, M23= M23C6, M7= M7C3, M6=M6C, M3= M3C, BCC=Ferrite and

FCC=Austenite).


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Fig. 5 Weld joint of P91/P22 after 600 C/15 875 h (over all view, Villella Bain etched, light microscopy).

Fig. 6 Precipitates on P91/P22 weld interface after 600 C/15 875 h (detail, Villella-Bain, SEM).

6. PHASE DIAGRAM AND ACTIVITY CALCULATIONS

The ThermoCalc program code was used for the phase diagram calculations of the steels P91 (Fig. 7) and

P22 (Fig. 8). The phase diagram isothermal cross section at 550 C for the steel P91 is given on Fig. 9

(MX=Vanadium Carbonitride, M23= M23C6, M7= M7C3, M6=M6C, M3= M3C, BCC=Ferrite and

FCC=Austenite).

Fig. 7 Phase diagram of the P91 steel (1...BCC+MX+M23+M6, 2..BCC+FCC+MX+M23; dash line represent

carbon content of the steel).

Fig. 8 Phase diagram of the P22 steel (1..BCC+M6+MX, 2..BCC+M7+M6+MX, 3..BCC+M7+MX,

4..FCC+BCC+MX; dash line represent carbon content of the steel).

The thermodynamic activities of the substitutional and interstitial elements represent important informationfrom point of diffusion [1], [11]. The temperature dependences of the calculated activities of carbon and

nitrogen for P91 and P22 steels are given on Fig. 10 and Fig. 11.

7. DIFFUSION SIMULATIONS

An important result of phase equilibrium calculations is the evaluation of the activities of interstitial elements

in the steels with respect to standard element reference (101 325 Pa, 25 C). The activity difference of the


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Fig. 10 Calculated carbon and nitrogen activity of steels P91 and P22.

8. DIFFUSION SIMULATIONS

An important result of phase equilibrium calculations is the evaluation of the activities of interstitial elements

in the steels with respect to standard element reference (101 325 Pa, 25 C). The activity difference of thegiven element in different materials (see Fig. 10 and Fig. 11) can be used as a first approximation for weld

joint stability judgement because each element diffuses to a place with lower activity and this diffusion flux is

roughly proportional to the product of the activity difference and element mobility [1]. Here both carbon and

nitrogen diffuse from P22 steel to P91 steel. It is in agreement with our experimental finding (see Fig. 3 and

Fig. 4). The temperature dependences of the carbon and nitrogen activity for the examined steels predict

that the carbon will diffuse from P22 to P91 at temperatures 500-840 C. The same direction of diffusion ispredicted for temperatures higher than 840 C, when the matrices of steels are completely austenitic.

In the case of real welds both the weld preparation and annealing history is important. In our experiment, all

dissimilar P91/P22 welds under view reveal the fusion regions. It was found that the thickness of the fusion

regions is fluctuating within 80-120 m range and the concentrations of substitutional elements changebetween steels linearly. This arrangement of the fusion zone was therefore supposed at all diffusion

simulations. The results of diffusion simulations (at 600 C/15 875 h, fusion zone thickness 100 m) give tous carbon and nitrogen calculated profiles presented on Fig. 3 and Fig. 4. The relevant phase profiles are

given on Fig. 12. The results of the simulations at 525 C and 550C were qualitatively the same. The

calculated maximum and minimum carbon and nitrogen concentrations in the CNEZ and in CNDZ at all

temperatures are given in Table 3.

Fig. 12 Simulated phase profiles of P91/P22 weld joint 600 C/15 875 h

Fig. 13 Micro hardness HV 0.05 measurement; P91/P22, 600 C/15 875 h).


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9. DISCUSSION

The presented experiment and theoretical calculations focussed on the real P91/P22 welds after PWHT

extent former results dealing with model joints having neglect able thickness of fusion zone [10], [12], [13].The chemical composition profiles of the substitutional elements (Fig. 2) show that the fusion zone P91/P22

real weld is formed. The thickness of the fusion zone in the investigated samples prepared by GTAW

process can be put to 100m. The changes of chemical compositions of the substitution elements can berepresented with linear function crossing the fusion zone. This linear substitution element redistribution is

created during GTAW welding process and the shape of the redistribution is highly influenced by

convectional fluxes in the melt. The mass balance is fulfilled in any case. The melted portion of P91 material

is conventionally spread in melted filling material and the substitution element profiles close to linear tend are

formed in place where convectional fluxes are not active (in the fusion zone). The linear composition change

is the result of steady state diffusion [3], [14] in convection unaffected liquid melt. The profiles are very stable

after solidifying due to their neglect able diffusion of the substitution elements in solid state. PWHT neither

following weld service do not change the substitution element profiles. The approximation of stability ofsubstitutional element profiles can be supposed for consequent carbon and nitrogen diffusion simulations.

The carbon and nitrogen are the interstitial elements in P91 and P22 steels. Their element profiles are

changing also in solidified welded steels. The formation the CNR and CND zones start just at PWHT. The

propagation of the carbon and nitrogen redistributions continues at next heat treatment (compare Fig. 4).

The directions of diffusion fluxes are in accordance with carbon and nitrogen activity differences (see Fig. 10

and Fig. 11). The formation and solving of the carbonitride phases were [15], [16] occurred and predicted in

P91/P22 joint (see Fig. 5 and Fig. 12). The Laves phase was predicted at temperatures 525 C and 550 C.

Moreover the M6C phase was predicted at all temperatures. Fortunately, because of kinetic reasons, the

microstructure having molybdenum rich phases was occurred inside fusion zone close weld interface ( Fig. 6)

only. The microstructure has influence on mechanical properties of the P91/P22 weld interface as given on

Fig. 13.

The maximum of the micro hardness (Fig. 13) respect the prediction of CNEZ (Fig. 3 and Fig.4). The good

agreement between experimental and predicted carbon a nitrogen maxima (see Table 2) was occurred at

600 C/15 875 h because the higher temperatures the system is closer to phase equilibrium. The nitrogenmaxima are less reliable because the accuracy of weaker theoretical carbonitride description. The presented

approach applied for the investigated creep samples gives results in accordance with experimental findings

on P91/P22 real weld interface.

It is necessary to note that the creep break of the real P91/P22 weld was located inside P22 temperature

influenced zone for all experimental samples. The break times were given by creep conditions (stress and

temperature) applied for the P91/P22 weld samples. The break times of the samples used in our

investigations do not over pass 1.8 yrs. It implies that the P91/P22 weld combinations cannot be used at

these creep conditions in industry where much longer live time is necessary. At the least, the lower applied

creep stress is needed. At these conditions, degradation processes inside diffusion affected zones can over

come the degradation processes inside temperature-affected zone. In this case, the described theoretical

method can be used with advantage for diffusion simulations at long annealing times close to industrial

operating times (10 years or more) because the simulation is not time limited as creep experiment.

10. CONCLUSIONS

The presented simulations of the investigated weld joints (P91/P22) enable better understand of interstitial

element diffusion and phase transformations. The simulations performed provided information that can be

used for failure risk predictions for long annealed weld joints. The simulations respect an existence of fusionregion on the weld interface that is occurred in real welds (GTAW).


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These calculated carbon and nitrogen redistribution temperature dependences are very important for a

design of dissimilar weld joints under long-time high temperature creep. The presented method is of great

importance for power industry.

ACKNOWLEDGEMENTS

This w ork has b een sup port ed by project MSM/00216/22410 and FSI-J-11-37.

REFERENCES

[1] FORET, R., ZLMAL, B., SOPOUEK, J. and BURK, J. In J. Lecomte-Beckers, M. Carton, F. Schubert, P.J.Ennis (Eds.), Materials for Advanced Power Engineering 2006, Vol.53, Part III, Forschungszentrum Jlich, Liege

(2006), p. 1449.

[2] SOPOUEK, J., FORET, R. and JAN, V. Science and Technology of Welding and Joining. (2004) p.59.

[3] PILOUS, V. and STRNSK, K. Structural stability of deposits and welded joints in power engineering,Cambridge Int. Science Publishers, UK (1998[4] BORGENSTAM, A. ENGSTRM, A., HGLUND, L. and GREN, J.Journal of Phase Equilibria, 21/3 (2000), p.

269-280.

[5] SAUNDERS, N. and MIODOWNIK, A. P. CALPHAD, Pergamon Materials Series, Amsterdam (1998).

[6] GREN, J. J. Phys. Chem. Solids, 42 (1982), p. 385-391.

[7] JANDOV, D., KASL, J., FOLKOV, E. and KANTA, V. In J. Lecomte-Beckers, M. Carton, F. Schubert, P.J.Ennis (Eds.), Materials for Advanced Power Engineering 2006, Vol.53, Part III, Forschungszentrum Jlich, Liege

(2006), p.1401.

[8] KROUPA, A., HAVRNKOV, J., COUFALOV, M., SVOBODA, M. and VEL, J.Journal of PhaseEquilibria, Vol. 22, (2001) p. 312.

[9] MESSLER, R.W. and SAVAGE, W.F. Principles of Welding: Processes, Physics, Chemistry and Metallurgy,

Wiley-VCH, Berlin (1999).

[10] SOPOUSEK, J. and FORET, R. Science and Technology of Welding and Joining, 13/1, p. 17-24.

[11] HODIS, Z. and SOPOUEK, J. Defect and Diffusion Forum, 263 (2007) p. 225-230.

[12] FORET, R., ZLMAL, B. and SOPOUEK, J. Welding Journal, 85/10, OCT 2006, p. 211-217.

[13] SOPOUEK, J. and FORET, R. Journal of Phase Equilibria and Diffusion, 27/4 (2006) p. 363-369.

[14] FORET, R. et al. Science and Technology of Welding and Joining, 6/6 (2001) p. 405-411.

[15] KUNZE, J. Nitrogen and Carbon in Iron and Steel, Academia Verlag, Berlin, (1990).

[16] CARPENE, E., SCHAAF, P., ARIELY, S. et al. Hyperfine Interactions 139/140 (2002) p. 495-499.

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REDISTRIBUTION OF CARBON AND NITROGEN IN HETEROGENEOUS WELD JOINTS OF CREEP-RESISTANT STEELS