carbide precipitation in 12cr1mov power plant steel

Carbide Precipitation in 12Cr1MoV Power Plant Steel

R.C. THOMSON and H.K.D.H. BHADESHIA

The chemistry and other characteristics of carbide precipitates in 12CrlMoV steel of the type used in the power generation industry were studied using energy dispersive X-ray spectroscopy and electron and X-ray diffraction techniques, and the results have been compared against thermodynamic calculations. As a result of the much larger substitutional solute concentrations present in the alloy, unlike the carbides that occur in steels containing smaller concentrations of chromium and molybdenum, it is found that the equilibrium M23C 6 carbide precipitates very rapidly during heat treatments of the kind used routinely for stress-relief purposes. The chemical compositions of carbides therefore do not change much during subsequent service at elevated temperatures.

I . I N T R O D U C T I O N

T H E vast majority of creep-resisting steels used in power plant or in the petrochemical industry are based on low-carbon, low-alloy steels containing carbide- forming elements, such as chromium, molybdenum, and vanadium, as deliberate additions. I1,2~ In addition to creep resistance, prolonged service at elevated temperatures also requires good oxidation and hot-corrosion resistance, possibly in environments containing hydrogen and sul- fur. In the United Kingdom, the steels are often used within the temperature range 480 ~ to 565 ~ the service stresses being of the order of 15 to 30 MPa over time periods of some 30 years. There is currently con- siderable research in progress to implement higher alloy steels with the aim of improving the creep strength so that the service temperature can be increased. [3.4] Alter- natively, the higher strength can be exploited by reduc- ing section size, which can be beneficial from the viewpoint of welding, thermal fatigue, and the reduced cost of support structures. A lot of the effort is focused on the 12CrlMoV steel, which is the subject of this article.

Given the prolonged and rather severe service conditions, it is of interest to examine the microstructural sta- bility of all power plant steels.J5] A particularly important aspect of the microstructure is the carbide particles, t6] For low-alloy steels, there has been a lot of research to indicate that the initial carbide phase found immediately after the stress-relief heat treatment (typically at 700 ~ for a few hours) is cementite. In these alloyed steels, the cementite is not, of course, the stable carbide. Further- more, it does not at this stage have a chemical composition which would be expected by considering the metastable equilibrium between ferrite and cementite.t7] Instead, the ratio of substitutional solute (X) to Fe atoms is initially the same as that in the ferrite matrix, Is] per- haps because the particles grow rapidly by paraequili- brium ~9,1~ transformation. During service, the cementite therefore enriches in elements such as chromium and molybdenum ~6,11-~3] at the expense of its iron content. This process of enrichment is interesting in several respects.

R.C. THOMSON, Research Student, and H.K.D.H. BHADESHIA, Lecturer, are with the Department of Materials Science and Metallurgy, University of Cambridge/JRDC, Cambridge, United Kingdom CB2 3QZ.

Manuscript submitted July 24, 1991.

For example, it is of importance in determining the point where carbide transitions are expected. Since the enrichment process is thermally activated, it can also be used to estimate the thermal history experienced by the steel, especially where direct data are not available.

As a follow-up to some studies done on low-alloy s tee l s , 16A1-13] the purpose of the present work was to examine the effect of representative heat treatments on the chemistry and some other characteristics of the carbides that are found in 12CrlMoV-type steels.

II. E X P E R I M E N T A L D E T A I L S

A. Materials and Heat Treatment

The material used in this work was a 12CrlMoV steel supplied by National Power Technology and Environment Centre, Leatherhead, U.K. , from heat 60348. The steel was supplied in the form of a rod of diameter 4 cm • 1 m long. Thinner rods of 3 m m diameter and bars 1 • 1 • 4 cm were machined from the original sample. Experimental results were compared with "ex-service" material (i.e., steel which has been in service in a power station), courtesy of Laborelec, Belgium. The service history of this latter pipe was 68,646 hours at 592 ~ followed by 146,000 hours at 587 ~ both at a pressure of 175 bar. The compositions of the steels, which are both within British Standard BS3604 and the German standard X20 for 12CrlMoV steels, are given in Table I. In spite of this, it is worth noting that the chromium concentration of the ex-service steel is significantly higher.

Heat treatments were performed in order to recreate the microstructures used in the commercial condition when the steels were first implemented for service. The specimens were sealed in silica tubes containing a partial pressure of argon of 150 m m Hg. Austenitization was carried out at 1060 ~ for 15 minutes. It has been shown by Barraclough and Gooch t~4j that the austenitizing temperature for 12CrlMoV steels is crucial in determining the microstructure and mechanical properties. Too low a temperature will cause heavily spheroidized microstructures with dramatically reduced creep resistance, and too high a temperature can result in the formation of 6-ferrite and a large austenite grain size, which is un- desirable. Yet the temperature must be high enough to ensure the complete dissolution of carbides. After

METALLURGICAL TRANSACTIONS A VOLUME 23A, APRIL 1992-- 1171

Table I. Chemical Compositions of the 12CrlMoV Steels in Weight Percent

C Si Mn P S Cr Mo V Ni Cu AI Co Ti Nb + Ta

12CrlMoV 0.21 0.25 0.46 0.009 0.012 10.9 1.03 0.30 0.52 0.02 <0.005 0.02 <0.005 0.06 Ex-service 0.18 0.22 0.58 0.01 0.007 12.4 1.07 0.28 0.64 0.13 0.01 0.03 <0.01 <0.01 X20CrMoV 12 1

austenitization, the specimens were air-cooled and re- sealed in silica tubes and tempered for varying times at 700 ~ and then further tempered at 565 ~ to simulate service conditions.

B. Optical Microscopy

Samples were prepared for microstructural character- ization by hot-mounting in acrylic molding powder, followed by grinding to 120 grit and polishing to 1 /xm cloth. Specimens were etched using Bain-Villela's re- agent (5 ml hydrochloric acid, 1 g picric acid in 100 ml of methanol) for times of up to 3 minutes.

C. Transmission Electron Microscopy

Specimens for transmission electron microscopy (TEM) were prepared by both the carbon extraction replica tech- nique and by twin-jet electropolishing of thin foils. Thin discs were cut from the heat-treated 3 m m rods using a SiC slitting wheel, the discs being mechanically ground to 50 /xm and then electropolished to electron trans- parency in a solution containing 10 pct perchloric acid, 15 pct glycerol, and 75 pct industrial methylated spirits at 50 V, with the solution being cooled to 0 ~ using liquid nitrogen. The thinned samples were examined in a PHILIPS* 400T TEM operated at 120 kV. For the pur-

*PHILIPS is a trademark of Philips Electronics Instruments Corporation, Mahwah, NJ.

poses of electron diffraction analysis, the camera constant was determined using gold film sputtered on a copper grid.

Carbon extraction replicas were prepared using the method described by Smith and Nutting tm from surfaces prepared as for optical microscopy using a lighter etch. The carbon film was removed by electrolytic etching in a solution of 5 pct hydrochloric acid in methanol at + 1.5 V, washing in industrial methylated spirits, and then floating the film off in distilled water and collecting on copper grids for examination in the TEM.

D. Energy Dispersive X-ray Spectroscopy

Detailed microanalyses were carried out on particles using the carbon extraction replicas, with an average of at least 30 isolated particles on each specimen being analyzed, covering an area of several grid squares. A LINK series 860 energy dispersive X-ray spectrometer attached to a PHILIPS 400T TEM was used for all the analyses. X-ray spectra were recorded at a specimen tilt of 35 deg, and live times of at least 200 seconds were used, depending on the count rate from individual particles. The dead time was not allowed to exceed 25 pct. The data were analyzed using the LINK RTS2-FLS software.

Chromium is chosen as an indicator of composition

change because of its large concentration; the iron content simply mirrors the changes in chromium content. The total manganese content is relatively small and therefore any errors will be relatively large. Molybde- num has a much larger atomic mass compared with iron, chromium, or manganese. Since the microanalytic measurements are carried out assuming that all elements ab- sorb the X-rays to a similar degree, the molybdenum data are likely to be flawed, the extent of the error depending on the thickness of the particle along the electron beam direction (a parameter which is very difficult to deter- mine for each individual particle). It should be noted that the energy dispersive X-ray spectroscopy (EDXS) measurements do not take into account the carbon content of the steel, and so for comparison with the thermodynamic calculations, the results should be scaled such that 5 wt pct of C is contained in M23C 6 and 9 wt pct in MTC 3.

E. Bulk Extraction of Carbides and X-ray Diffraction

The carbides were isolated by electrochemical dissolution of the matrix using the method described by Andrews and Hughes. t~61 The specimen was anodically dissolved in a cell with a platinum cathode and a solution of 10 pct hydrochloric acid in methanol as the electrolyte at a voltage of 1.5 V. Typical extraction times were 6 to 8 hours. The electrolyte was then decanted off and the precipitate thoroughly dried in an oven at approximately 60 ~ Careful weighing at all stages of the extraction to an accuracy of 10/xg was performed in order to find the exact amount of precipitate extracted. An internal standard, CeO2, was added to the precipitates. This was accurately weighed to approximately 25 pct of the total mass of the precipitate. A Siemens D500 diffractometer (CuK~ radiation) was used to scan between 30 and 65 deg 20 at a step size of 1 second duration, corresponding to 0.004 deg 2 0, where 0 is the Bragg angle. The diffraction peak positions were located using Siemens DIFFRAC 500 software, which was also used to cal- culate the associated integrated intensities of the peaks.

F. Particle Size Measurements

The size of each carbide particle was determined by measuring a number of random intercepts across it on the photographic negative. Hence, particle size is ex- pressed in terms of a mean linear intercept. This measure was chosen as it is closely related to the diffusion dis- tance across the particle and can be used for irregular geometries. The particles are in the form of thin plates, so that most of the enrichment should be from diffusion in a direction normal to the plate plane. The linear intercept as measured above should give a good description of the particle size along this diffusion direction.

1172--VOLUME 23A, APRIL 1992 METALLURGICAL TRANSACTIONS A

G. Thermodynamic Calculations

Thermodynamic calculations were performed using a computer package (called MTDATA) developed by the National Physical Laboratory, Middlesex, U.K., in which the equilibria in multicomponent, multiphase systems are determined using critically assessed data for simpler systems. [171 The amounts of species within each phase for a given temperature and pressure or volume can be calculated by minimizing the Gibbs free energy of the sys- tem for specified component amounts. The main advantage is that calculations can be performed for the specific composition of individual materials used. The equilibrium phases in the 12CrlMoV steel used in this work were calculated. The sequence of carbide precipitation was also determined by first finding the equilibrium carbide and then suppressing its formation and repeating the calculation. Thus, the previous carbide to form in the sequence was found.

I I I . R E S U L T S

A. Microstructural Changes

A typical optical micrograph is shown in Figure 1. It can be seen that the microstructure is 100 pct martensite and does not contain any 6 ferrite. Transmission electron micrographs of the as-quenched microstructure are presented in Figure 2. Figure 2(a) shows martensite plate- lets containing some internal twins, confirmed by selected area electron diffraction. Figure 2(b) demonstrates the fact that there are no carbides in the as-quenched microstructure, i.e., that no autotempering has occurred. Gooch tlSJ reported that the microstructure obtained by cooling from 1100 ~ contained a fine dispersion of cementite particles. This difference is attributed to the relatively slower speed of the quench.

Figures 3(a) and (b) illustrate the carbides beginning to form at prior austenite grain and lath boundaries, and also intralath, in a specimen which has been tempered at 700 ~ for 15 minutes. The carbides were identified as both M7C 3 and M23C 6.

The carbide M7C 3 w a s also found in specimens aged for up to 30 minutes at 700 ~ but these had all dissolved at the end of the stress relief heat treatment. Figure 4 shows that M7C 3 w a s mainly found within the

Fig. l - -Opt ica l micrograph for specimen austenitized at 1060 ~ 15 min, air-cooled, and tempered for 2 h at 700 ~

Fig. 2 - - ( a ) and (b) Transmission electron micrographs from the as- quenched microstructure. Fig. 2(a) illustrates the twinned martensite microstructure; the inset is a selected area diffraction pattern showing the twin (t) and matrix (m) reflections. The twin plane is 211. Fig. 2(b) is a higher magnification image showing that no autotempering has occurred.

martensite laths and distant from the clustered M23C 6 precipitates. This is in agreement with Beech and Warrington t191 who found that M23C 6 and M7C 3 w e r e both present from an early stage of tempering and that on spheroidization the particles within the martensite laths disappeared. The diffraction pattern in Figure 4 illustrates the characteristic streaks of M7C 3 resulting from its faulted structure compared with that of pure CrTC3.t2~ No difference in morphology was found between M7C 3 and M23C6, apart from a tendency for the former carbides to be much finer.

A typical carbon extraction replica from a sample which had been given the commercial stress relief heat treatment, i.e., tempering at 700 ~ for 2 hours, is shown in Figure 5 and clearly illustrates the distribution of the carbides in relation to the martensite boundaries. The carbides at the end of the stress-relief heat treatment were found to consist chiefly of a dispersion of M23C 6 particles concentrated on the austenite grain and lath boundaries.

A comparison between the distribution of coarse M23C 6 carbides in the ex-service material and in a specimen isothermally heat-treated at 700 ~ for 1173 hours is presented in Figure 6 . The empirical Larson-Miller t2q


Fig. 5 - - C a r b o n extraction replica from a specimen tempered for 2 h at 700 ~ illustrating the distribution of M23C6 carbides with respect to martensite lath boundaries.

Macrohardness measurements were made on all the specimens using a 30 kg load. These results are presented in Table II. Each data point is the average of three measurements on each sample, with the total scatter being no more than 10 HV. The macrohardness data confirm that the microstructures of the ex-service material and

Fig. 3 - - ( a ) and (b) Transmission electron micrographs illustrating carbide formation at both the grain and lath boundaries in a specimen tempered for 15 min at 700 ~

parameter, defined as T(20 + log t), where T is the temperature in Kelvin and t is the time in hours, indicates that these two different heat treatment conditions are comparable. The carbide size and distribution are similar in the two materials, although for reasons which are not clear, there appears to be a tendency for increased clus- tering of the carbides in the ex-service material.

Fig. 4 - - C a r b o n extraction replica showing small M7C3 particles dis- tributed within the martensite laths and distant from the larger M23C6 particles clustered on the lath boundaries. The inset is a selected area diffraction pattern of M7C3, the zone axis being [2i5] and showing characteristic streaks, t19]

Fig. 6 - - ( a ) and (b) A comparison between the distribution of M23C6 carbides in the ex-service material (a) and in the specimen isothermally heat-treated for 1173 hours at 700 ~ which represent comparable heat treatments according to the empirical Larson-Miller 121j parameter. The inset in (b) is a selected area electron diffraction pattern of M23C 6 in the [ i l 1] orientation.


Table II. Macrohardness Measurements

Specimen Macrohardness/HV

Pretempering 622 700 ~ 15 min 315 700 ~ 30 min 309 700 ~ 60 min 296 700 ~ 120 min 315 700 ~ 1173 h 293

Ex-service material 302

the 1 2 C r l M o V steel are s imilar and that li t t le change in carbide prec ip i ta t ion occurs on temper ing.

The appearance of addi t ional phases , such as Laves phase, on t emper ing a 1 2 C r l M o V steel depends cri t i- cally upon the base compos i t ion o f the steel. In the steels used in this work , no addi t ional phases were found during tempering, which is consistent with the work of Briggs and Parker . t221

B. Thermodynamic Calculat ions

The carb ide M23C 6 w a s found to be the stable carbide , coexist ing with ferri te, at all tempera tures in the range of interest , 400 ~ to 800 ~ The equi l ib r ium solut ion temperature o f the carbides was found to be 950 ~ It should be noted that whether or not carbides will dis- solve at the solut ion temperature wil l depend on the holding t ime; for example , Bar rac lough and Gooch I14] found that 30 minutes at 950 ~ was not an adequate solution heat t reatment , a l though 30 minutes at 1000 ~ was sat isfactory. The tempera ture at which delta ferrite became stable on heat ing was ca lcula ted at 1220 ~ in the 1 2 C r l M o V steel and as 1110 ~ in the ex-serv ice material , which conta ined an addi t ional 1.5 wt pct chromium. This difference is consistent with the work of Irvine et al. t23~ who found that an increase in ch romium content of 1 wt pct led to an increase in 6-ferri te content o f 14 pct. This conf i rms that the commerc ia l austenizat ion temperature range of 1020 ~ to 1070 ~ is adequa t e to comple te ly d isso lve carbides and wil l not p roduce large amounts o f ~-ferrite. M23C 6 w a s found to be the most stable carbide , fo l lowed by M7C3, and then by cementite. A precip i ta t ion sequence o f M3C ~ M7C 3 ~ M23C 6 is therefore poss ib le .

The results of the the rmodynamic calculat ions for 700 ~ (the stress re l ie f heat t reatment) and for 565 ~ (the service temperature) are presented in Table III. The a l loying e lement content o f the two carbides o f interest , M7C 3 and M23C6, is presented as a funct ion o f temperature in F igures 7(a) and (b).

M,C

:1 ~ ' ~

~ 6 o -

*N

4o- ~

"~ 3o-

- C

0 I I I ! I 600 T ~ I I ~ 900 1000 1100

T e m p e r a t u r e / K (a)

80-

70-

d l0 -

M23C6

Mo

0 - - I I I ! I 6O0 TOO O ~ 900 1ooo no0

T e m p e r a t u r e / K (b)

Fig. 7 - - (a ) and (b) Alloying content of M7C3 and M23C6 carbides as a function of temperature in the 12CrlMoV steel.

Table IIl. Chemical Compositions of the Carbides in Both the 12CrlMoV Steels Used in Weight Percent*

12CrlMoV Steel Ex-Service 12CrlMoV Steel

Fe Cr Mo Mn C Fe Cr Mo Mn C

700 ~ MTC 3 5.2 82.2 3.1 0.6 8.9 4.6 83.2 2.7 0.7 8.8 M23C6 10.7 65.1 19.1 - - 5.1 9.0 66.4 19.5 - - 5.1

565 ~ M7C 3 1.6 84.4 4.5 0.7 8.8 1.4 85.1 4.0 0.7 8.8 M23C 6 4.2 70.3 20.4 - - 5.1 3.6 70.9 20.4 - - 5.1

*The calculations were performed using MTDATA at 700 ~ and 565 ~

METALLURGICAL TRANSACTIONS A VOLUME 23A, APRIL 1992- - 1175

The carbides found in the microstructural investiga- tion w e r e M7C 3 and M23C6, in good agreement with the sequence predicted by thermodynamic calculations. The EDXS results show that the equilibrium chromium content in the carbide M7C 3 is higher than that in M23C 6. The absolute levels of Cr and Mo predicted to be in the carbides are slightly higher than those observed experi- mentally ( e . g . , 65 wt pct Cr and 20 wt pct Mo are predicted in M23C6, whereas 60 wt pct Cr and 10 wt pct Mo, allowing for 5 wt pct C, are measured experimen- tally); however, there is good general agreement. The chromium content of both carbides in the ex-service material is larger due to the increased bulk chromium content of the alloy. The carbides can support a greater substitutional alloying content as the temperature is low- ered. It is possible, therefore, that when the steel is in service at the lower temperature of 565 ~ after the stress relief heat treatment, the chromium content of the M23C 6 may increase by approximately 4 to 5 wt pct. It is likely, however, that this approach to equilibrium will be extremely slow and difficult to detect within the experimental error of EDXS.

C. C e m e n t i t e P r e c i p i t a t i o n

It is interesting to note that no cementite was found in any of the specimens, even during the earliest stages of tempering, because cementite is usually expected to be the first carbide to form on tempering martensite. Therefore, a computer model developed by Bhadeshia t13~ was used to investigate the time for cementite, with an initial composition determined by assuming a paraequi- librium mechanism, to reach its predicted equilibrium composition. The results of these calculations are presented in Figure 8. The chromium concentration in the cementite is plotted against the time allowed for diffusion at 700 ~ for a range of particle sizes between 10 and 30 nm. It can be seen that cementite in fact saturates in an extremely short time, of the order of a few minutes. It is concluded that with the large amount of chromium in the base composition of the steel, the driving force for alloy carbide precipitation is large and cementite will only be seen in this steel if tempering takes place at a lower temperature, or possibly immediately after tempering has begun.

D. M i c r o a n a l y t i c a l D a t a

It has been demonstrated for the early stages of carbide enrichment, in the absence of soft impingement* in

*Soft impingement is the overlap of the diffusion or temperature fields of adjacent particles, or from active regions of the same particle.

the matrix, that the kinetics of composition change are given by tl31

~r[x~ cO)] 2

tc = 16D~(c~O _ c_)2

where tc is the time required for the carbide to reach a concentration c e, x ~ is the thickness of the carbide plate, D~ is the diffusion coefficient for the substitutional alloying element in ferrite (assumed to be equal to that in

Calculated cement i t e e n r i c h m e n t - 700"C

80-

�9 i.... 70- ....'"

i ~ - I: I ! , ," - - - - 4 0 - ! . : 1 / _ , " m s ~ v n

J / i / / . . . . s ~ , ,

I l l o

'eld 30 ::s:oo - - - S 2 0 n m

tO ~ . . . . S30nm 2 0

10 I I i I 0.0 0.5 1.0 1.5 2.0

Time / H o u r s

Fig. 8--Calculated rate of cementite enrichment with respect to chromium concentration for particles of sizes 10 to 30 nm using a finite difference model, t~3j

carbide), c ~~ is the concentration of the substitutional solute element of interest, and ~ is the corresponding bulk concentration in the alloy. This equation shows clearly that the degree of enrichment to be found after any specified heat treatment will depend on the particle size. Consequently, measurements of carbide compositions should be presented alongside the corresponding particle sizes, the latter measured in a way related to the prob- lem. Thus, the results of the EDXS analysis on the carbides contained in specimens tempered at 700 ~ for 15, 30, 60, and 120 minutes, are presented in Figures 9(a) through (d), respectively, as plots of the chromium concentration against particle size measured in terms of a mean linear intercept.

It can be seen in the 15 minute specimen that M7C 3 is found, with a composition of approximately 75 wt pct Cr, 20 wt pct Fe, and small amounts of molybdenum and manganese, to coexist with M23C6, with a composition of approximately 60 wt pct Cr, 30 wt pct Fe, and 10 wt pct Mo. These compositions are in general agreement with those of Beech and Warfington, t~9J the absolute values being dependent on the base composition of the steel. The average chromium concentration in M23C 6 in all the specimens ranged from 60 to 63 wt pct. The transition from M7C 3 to M23C 6 is picked up in the 30 minute specimen. After 1 hour, there is very little evidence of any MTC 3 being present in the microstructure, and after the completion of the stress-relief heat treatment, all the MTC 3 has redissolved. Figure 9(e) compares data from the ex-service material and the specimen tempered for 1173 hours at 700 ~ The chromium content in the carbides in the ex-service material is larger than that in the isothermally tempered specimen, but this difference can be attributed to the higher chromium content in the base composition of the steel and the longer tempering time. In both cases, however, the chromium content is.constant.


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M E T A L L U R G I C A L T R A N S A C T I O N S A V O L U M E 2 3 A , A P R I L 1 9 9 2 - - 1 1 7 7

E. X-ray Diffraction Analyses

X-ray diffraction analyses for specimens tempered at 700 ~ for 10 minutes and 2 hours are presented in Figures 10(a) and (b), respectively. In the spectra for the specimen tempered for 10 minutes, the 420 and 202 peaks from MTC 3 are clearly visible, whereas these have disappeared after tempering for 2 hours. The 420 and 202 peaks are the strongest visible peaks for M7C3, because the strong 421 peak overlaps with the strong 511 peak of M23C6. There was no further change in the diffraction pattern for specimens tempered up to 1200 hours at 700 ~ The lattice parameters for M23C 6 extracted from all the heat-treated specimens of the 12CrlMoV steel were calculated using the measured values of d-spacings. Er- rors can arise in the measured values of the d-spacings due to the geometry of the diffractometer, absorption in

g * SIEMENS * DIFFRAC 500 Ra~,RAI t 2 t~ - 71o aEo c - 11 MZN+ StRZU+ i 0~+ , +,0+

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4001 ( ) 12221

TWO - THETA (DEGREESI

(b)

Fig. lO--(a) and (b) X-ray diffraction patterns for specimens tempered at 700 ~ for 10 rain and 1 h, respectively, illustrating that M7C3 initially present along with M23Ca has dissolved after tempering for lh.

the specimen, etc. These were corrected for by fitting a polynomial function to the d-spacings of the internal standard, which are known to a high degree of accuracy. M23C 6 is cubic and therefore the d-spacings and plane indices are related by the equation

a 2

h 2 + k 2 + 12

The adjusted d-spacings were fitted to this equation using a nonlinear least-squares procedure. The calculated lattice parameters are presented in Table IV.

The lattice parameter of (Fe, Cr)23C 6 containing 60 wt pet Cr extracted from a commercial steel containing 14 wt pct Cr has been measured as 10.595 ]k. [241 In order to estimate the change in lattice parameter due to the molybdenum content in the M23C 6 in this work, the rel- ative sizes of the atoms are considered. Molybdenum atoms are 10 pct larger than chromium and may replace up to eight out of 92 of the metal atoms in the unit cell. [2~1 The 10 wt pct Mo measured in the carbide corresponds tO Crl6Fe6Mo]C6; therefore, an increase in lattice parameter to 10.59(1 + 0.1 • 1 / 2 3 ) = 10.64 ,~ is predicted, This is in good agreement with the calculated value. The calculated lattice parameters differed by no more than 0.02 ,~, indicating again no significant differences in the composition of the M23C 6 after tempering.

IV. D I S C U S S I O N

The carbides precipitating during the stress-relief heat treatment in 12CrlMoV steel were identified by X-ray diffraction and TEM as M7C3 and M23C6. Therefore, before entering service at approximately 565 ~ the steel contains a distribution of M23C6 particles. The initial composition of the carbides is close to that predicted using equilibrium thermodynamics, and it has been established by EDXS that there is no further change in the composition of the carbides with tempering. No significant dependence of chromium concentration on particle size was found. Lattice parameter measurements and comparison with ex-service material confirm that there is no change in composition of the M23C6 on tempering, especially with respect to molybdenum, which might have been expected from the thermodynanaic calculations. The fact that there is no enrichment occurring on tempering in 12CrlMoV steel is in contrast to the low-alloy steels. Du t]2] found that the chromium content in M23C6 precipitating in a 0.5Cr0.5Mo0.25V steel increased with time.

Table IV. Lattice Parameters of Mz3C6 Determined by X-ray Diffraction

Time at 700 ~ Lattice Parameter//k

10 min 10.64 - 0.01 15 min 10.65 -L-- 0.01 30 min 10.66 --- 0.01 1 h 10.65 -+ 0.01 2 h 10.65 +- 0.01 1173 h 10.66 -4- 0.01 2 h + 16 h at 565 ~ 10.65 +- 0.01

1 1 7 8 - - V O L U M E 23A, APRIL 1992 M E T A L L U R G I C A L T R A N S A C T I O N S A

Whether or not alloy carbides precipitate at their equilibrium composit ion is therefore dependent on the concentration of alloying elements available in the base composition of a steel. It has been established that there is no significant change in the carbide identity or composition during service after the stress-relief heat treatment. Therefore, the question arises as to which other microstructural changes could be fruitfully investigated. It seems from a comparison of Figures 9(d) and (e), which show an increase in particle size from about 125 to 300 nm, that carbide coarsening could potentially be used as the microstructural parameter, and future work will focus on this aspect.

V. C O N C L U S I O N S

The kinetics of carbide precipitation in 12CrlMoV steels are rapid when compared with other low-alloy steels of the type commonly used in power plant. This is attributed to the fact that the steel studied has relatively large concentrations of carbide-forming substitutional solutes. Thus, unlike the low-alloy steels, relatively stable alloy carbides have been found to dominate in the microstructure immediately after the stress-relief heat treatment. Since this heat treatment is always necessary before implementing the alloy in service, there seems little prospect of estimating the thermal history of a component from the chemical composition of its carbides. In fact, both the thermodynamic analysis and the experimental data show that the chromium concentration of the M23C 6 carbide is very sensitive to the average chromium concentration of the steel. It is found that variations in the chromium concentration within the accepted industrial specifications can lead to larger corresponding variations in carbide compositions than would be caused during service.

A C K N O W L E D G M E N T S

The authors are grateful to Professor C. Humphreys for the provision of laboratory facilities at the University of Cambridge. One of the authors (RCT) also wishes to thank the Science and Engineering Research Council and National Power for funding this work. The assistance of Mrs. P. Dolbear, Dr. R.C. Reed, and the MTDATA group of the National Physical Laboratory is greatly appreci- ated. HKDHB's contribution was made under the aus- pices of the Atomic Arrangement: Design and Control

project, which is a collaborative venture between the University of Cambridge and the Japan Research and Development Corporation.

R E F E R E N C E S

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carbide precipitation in 12cr1mov power plant steel

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