precipitation behavior of type 347h heat-resistant austenitic steel during long … ·...

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Precipitation behavior of type 347H heat-resistant austenitic steelduring long-term high-temperature aging

Yinghui Zhou, Yanmo Li, Yongchang Liu,a) Qianying Guo, Chenxi Liu, Liming Yu, Chong Li, andHuijun LiState Key Laboratory of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering,Tianjin University, Tianjin 300072, People’s Republic of China

(Received 15 July 2015; accepted 19 October 2015)

The microstructural evolution of type 347H heat-resistant austenitic steel during long-termaging at 700–900 °C was investigated by using a transmission microscope, a scanning electronmicroscope, and electron energy spectrum technology. The microstructural examination showedthe typical micrographs of MX carbonitrides and M23C6 carbides after aging. The existence ofthe Z phase (NbCrN) at the grain boundaries during aging was identified. Meanwhile, the possibleprecipitation sequence of these particles was also confirmed. In the beginning of aging, fineNb(C,N) precipitates first, then, M23C6 carbides precipitate along the grain boundaries. Finally,the Z phase is also observed at the grain boundaries. Moreover, the influence of isothermalholding temperature on the precipitation of MX carbonitrides and M23C6 carbides was discussed.The various microstructural characterizations showed that the M23C6 carbides and MXcarbonitrides precipitate more easily with the increase of aging temperature. Furthermore, thenumber and the size of MX particles and M23C6 carbides increase when the isothermal holdingtime is prolonged.

I. INTRODUCTION

Concerns over the protection of the environment and theimprovement of thermal efficiency prompted the develop-ment of ultra-supercritical (USC) power plants that canoperate at much higher steam pressures and temperaturesthan the conventional plants.1 Due to further increases insteam parameters in the USC generator sets, the heat-resistant materials with excellent long-term high temperaturestrength, superior oxidation resistance, good weld-ability aswell as acceptable fabricability are searched. Austeniticheat-resistant steels are potential materials and widely usedfor superheater or reheater tubes in the USC power plants,owing to the lower cost and capability of operations at thedesigned steam pressure (35 MPa or higher) and temper-atures (650–760 °C) conditions.1–3

Austenitic heat-resistant steel such as type 347Haustenitic steel is widely accepted for using in importantconstruction components designed for high temperatureapplications like boilers, nuclear reactors, superheatertubes, and reheater tubes due to its high temperaturemechanical stability.4–7 Type 347H austenite steel isa kind of niobium-stabilized steel and its Nb content isgenerally 8–10 times the total amount of C and N presentin the alloy. Because the Nb atoms are stronger carbide

former elements compared with Cr, the purpose ofadditions of niobium is the formation of Nb(C,N)carbonitrides during aging.6 The Nb(C,N) precipitatesstabilize the austenite steel against the precipitation ofM23C6 carbides, which would lead to intergranularcorrosion. Meanwhile, the dissolved Nb(C,N) carboni-trides during solution heat treatment are capable ofproviding excellent creep resistance in the steel.8

It is well known that the precipitation behavior of thesteel during long-term service at elevated temperatureleads to the change of mechanical properties of thematerials. Therefore, it is necessary to clarify the evolu-tion of secondary phases in the steels. Previous inves-tigations show that MX carbonitrides, M23C6 carbides,Z-phase, laves phase, and sigma phase are the probablephases that existed in type 347H austenitic steel duringlong-time isothermal aging. As discussed above, MXcarbonitrides provide superior thermal strength becauseof pinning of dislocation during aging.9 The M23C6

carbide is commonly found after isothermal treatmentin austenite steel. When type 347H austenitic steel isexposed at the temperature range of 700–900 °C, thelaves phase (Fe2Nb) precipitates at the grain boundariesin the matrix. However, the precipitation of the Lavesphase results in the reduction of ductility, toughness, andcorrosion resistance.10 The Z phase (NbCrN) canstrengthen austenite steel due to its stability and is usuallyobserved in nitrogen and niobium containing heat re-sistant steels during their service at high temperatures.

Contributing Editor: Eric A. Stacha)Address all correspondence to this author.e-mail: [email protected]

DOI: 10.1557/jmr.2015.343

J. Mater. Res., Vol. 30, No. 23, Dec 14, 2015 �Materials Research Society 20153642

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The sigma phase is an intermetallic phase with a Fe–Crtype, which is known as sigma phase embrittlement.11

This work is focused on the research of microstructuralevolution that may occur in type 347H heat-resistantaustenite steel after isothermal treatment at elevatedtemperature. The type of carbides and phases duringaging at 700 °C were identified. The existence of the Zphase (NbCrN) at the grain boundaries was observed.Meanwhile, the typical micrographs of MX carbonitrides,M23C6 carbides, and Z phases were presented. Moreover,the possible precipitation sequence of phases duringaging at 700 °C was confirmed. The effects of isothermalholding temperature and annealing time after hot de-formation on the precipitate of MX carbonitrides andM23C6 carbides were investigated.

II. EXPERIMENTAL DETAILS

The material analyzed in this investigation was a niobiumbearing austenitic steel, type 347H heat-resistant austeniticsteel, whose chemical composition is listed in Table I. Theexperimental samples with a size of 10mm� 10mm� 10mmwere machined from as-received steels and homogenized at1150 °C for 30 min. The specimens for microstructuralcharacterization were aged from 700 to 900 °C in the rangeof 1–100 h then quenched in water. To analyze theprecipitation behavior during long-term aging, the sampleswere isothermally aged at 700 °C for 1–2200 h thenquenched in water. Compression experiments were con-ducted by using a Gleeble-3500 thermal mechanical simu-lator (DSI Company, St. Paul, Minnesota) at 1100 °C and0.01 s�1. All of the samples were prepared by mechanicalgrinding and polishing and then etched with a mixedsolution (CuCl2:HCl:CH3CH2OH 5 1:20:20, in volume).

The carbon replicas method was used to observe MXcarbonitrides. After a thin carbon layer was jetted on thesurface of the specimens, the carbon foils consisting ofprecipitates were extracted by etching and supported ona 600-mesh Cu grid. The phase identification analysis by x-ray diffraction (XRD) was done by an extraction method.The samples were cut into slice-like specimens with a sizeof 10 mm � 10 mm � 2 mm and etched in a solution ofhydrochloric acid for several days. Then, the pulverousspecimens are extracted by using a centrifugal machine.

Phases in microstructures were identified using a scan-ning electron microscope (SEM) equipped with an x-rayenergy dispersive spectroscope (EDS), a transmissionelectron microscope (TEM) operating with a voltage of200 kV, and an x-ray diffractometer.

III. RESULTS AND DISCUSSIONS

A. SEM observations

Figure 1 shows the microstructure observed by SEM oftype 347H austenitic steel after aging at 700 °C fordifferent holding periods. The typical microstructure ofaustenitic with some twins is shown in Fig. 1(a) and highdensity of twins was always randomly presented in thematrix during aging as evident in Fig. 1. A few ofparticles were observed to be undissolved Nb(C,N)carbonitrides survived from the as-cast microstructurein the specimen which was solution-treated at 1150 °Cfor 30 min. The microstructure exhibits rather a homoge-neous grain structure with an average grain size of about45 lm. As is shown in Fig. 1, the grain size of theaustenite matrix increased inconspicuously with the agingtime prolonged. Because the aging temperature is ratherlow of 700 °C, the grain growth may be neglected. Forthe specimens of the steel annealed for longer times,a large number of the particles in the form of a chain werefound and the coarsening of precipitates was apparent.

B. The identification of carbides and phases

The identification of different types of phases thatprecipitate in type 347H austenitic steel during aging at700 °C was analyzed by TEM examination. In the in-vestigation, three types of secondary phases (MX, M23C6,Z-phase) were identified.

1. MX carbonitrides

Figure 2(a) exhibits the original morphology of MX-type nanoprecipitates with M 5 Nb and X 5 C/N (it willbe confirmed later that these are Nb(C,N) carbonitrides).Actually, a large amount of very fine Nb(C,N) carboni-trides nucleated densely along dislocations formed byquenching, and their size increased with the increase ofisothermal holding time.12 The typical microstructures ofMX particles are shown in Fig. 2(c). During aging at700 °C for 2200 h, the size of MX precipitates is about20–150 nm. The highly fine Nb(C,N) carbonitrides wererandomly distributed at crystal defects such as subgrainboundaries and dislocations, and even most of them weresurrounded by dislocations. The formation of these leadsto the retardation of dislocation movements and improvesthe creep resistance of the steels at high temperature.13

The types of carbonitrides were determined by TEM,EDS, and XRD analysis. The content of powerfulcarbide-former component Nb in type 347H austeniticsteel is about 0.54%. The stabilizing component has asa function higher affinity to carbon than chromium, andthey form MX carbides. These carbides combine carbonwithin the grains and reduce the possibility of intergran-ular corrosion.14 MX carbonitrides in type 347H austen-itic steel were classified as intragranular carbide though

TABLE I. Chemical composition of the explored type 347H austeniticsteel (in wt%).

Element C Cr Ni Nb N Mn Mo Fe

wt% 0.06 17.60 10.71 0.54 0.01 1.59 0.12 Balance

Y. Zhou et al.: Precipitation behavior of type 347H heat-resistant austenitic steel during long-term high-temperature aging

J. Mater. Res., Vol. 30, No. 23, Dec 14, 2015 3643



a few of precipitates of MX were also observed at thegrain boundaries. Figure 2(e) illustrates the presence ofNb(C,N) particles at the grain boundaries. It is probablynot a detrimental tendency for steels due to the thermalstability of MX carbonitrides. Above all, MX carboni-trides are the predominant strengthening precipitates intype 347H austenitic steel during isothermal annealing at700 °C.

2. M23C6 carbides

As is known to all, a critical aspect of microstructuralstability in the precipitation strengthened steels for heat-resistant applications is the problem of formation ofdetrimental particles in the matrix during long-term hightemperatures exposure.15–17 The extremely coarse detri-mental precipitates can result in the embrittlement anddisappearance of strengthening particles. M23C6 carbidesare present at the grain boundaries and they can result in

susceptibility to intergranular corrosion. However, thefine discontinuous grain boundary M23C6 precipitateslikely improved resistance against grain boundary slidingof type 347H austenitic steel and as well as contributed toimproving the creep strength. Rojas et al. found that theformation of nanosized Cr-rich M23C6 carbides canprovide good creep resistant due to the pinning effectsof grain boundaries.9 While, the coarsening rate of M23C6

carbides is much faster than that of MX carbonitrides.The presence of M23C6 precipitates and undissolvedprimary Nb(C,N) particles plays the key role in de-creasing the creep strength of the explored steels.Therefore, the evolution of M23C6 particles has to bewell known and understood.

The precipitation behavior of M23C6 carbides has beenobserved upon long-time holding at 700 °C (see Fig. 3).The M23C6 carbides were identified through energydispersive spectra and x-ray analysis results [see Figs. 3(e)and 3(f)]. Figure 4 shows the distribution of elements in

FIG. 1. SEM micrographs of type 347H austenitic steel after isothermal annealing at 700 °C for different periods: (a) 0 h, (b) 1 h, (c) 100 h,(d) 500 h, (e) 1000 h, and (f) 1500 h.


J. Mater. Res., Vol. 30, No. 23, Dec 14, 20153644



the form of elemental energy-dispersive X-ray detector(EDX) maps. The results described that the M23C6

particles consist of much more chromium and a littlemore carbon, and less nickel and iron as compared to thatdistributed in the matrix. In the steel specimens annealedat 700 °C for 500 h, the existence of discontinuous shortrod-like M23C6 particles distributed at the grain bound-aries can be seen. By increasing the annealing time, thenumber of M23C6 carbides increases. It is also apparentthat the size of M23C6 increased with the increase of theisothermal holding time. The growth of the M23C6

carbide is faster than that of MX particles. Actually, inthe isothermal aged steels, M23C6 carbides precipitatesuccessively on grain boundaries, incoherent twin bound-aries, coherent twin boundaries, and finally intragranu-lar.18 When the isothermal holding time is increased up to2200 h, the M23C6 carbides only precipitated at the grain

boundaries [see Fig. 3(d)]. There are no M23C6 particlesthat can be found within the grains because of the shortholding period. As shown in Fig. 4, M23C6 carbides firstformed with the shape of short rod-like particles thengrew into long bar-like carbides and finally became chain-like precipitates along the grain boundaries. Furthermore,bulk M23C6 particles are easily formed at triple points ofgrains. The coarsening of M23C6 particles is very harmfulto the properties of materials.15–17,19 To decrease thenumber of continuous chain-like M23C6 carbides at thegrain boundaries, increasing the content of niobium andboron in the austenitic steels is an effective method toreduce the detrimental to properties at high temperature.20

The typical TEM morphologies of M23C6 precipitated atdifferent nucleation sites are shown in Fig. 5. It was observedthat M23C6 precipitates at the grain boundaries and agglom-erates with a diameter or length of about 0.2–0.8 lm. Since

FIG. 2. TEM micrographs of MX carbonitrides in type 347H austenitic steel: (a) the original Nb(C,N) carbonitrides nucleated at dislocations,(b) the fine Nb(C,N) carbonitrides distributed in the matrix, (c) the Nb(C,N) carbonitrides surrounded by dislocations, (d) the EDS analysis of MXcarbonitrides, (e) the Nb(C,N) particles within the grain, (f) the TEM analysis of MX precipitates within the grain, (g) the Nb(C,N) particles alongthe grain boundaries, and (h) the TEM analysis of MX precipitates along the grain boundaries.





the carbon atom diffuses fast at grain boundaries viadislocation and vacancies, M23C6 particles nucleate and growfirstly along grain boundaries.11 Figure 5(a) illustrates the

micrograph of the steel samples after aging at 700 °C for500 h, the misty granular particle is the initial microstructureof M23C6 carbide. The M23C6 carbide with the size of 1 lm

FIG. 3. SEM micrographs of M23C6 carbides aged at 700 °C for different holding periods: (a) 500 h, (b) 1000 h, (c) 1500 h, (d) 2200 h, (e) EDSanalysis of M23C6 carbides, and (f) XRD profiles.





is present at triple junctions of grain boundaries [seeFig. 5(b)]. Figures 5(e) and 5(g) exhibit the discontinuousrod-like M23C6 carbides. After aging for 2200 h, lots oftriangular M23C6 carbides are observed along the grainboundaries [see Fig. 5(k)]. The rod-like M23C6 carbides aremore beneficial to the effect of creep-fatigue of the steelcompared with triangular M23C6 carbides.21 There are notenough credible results to verify that the sigma phaseprecipitates after aging at 700 °C for 2200 h in this work,though M23C6 carbides can facilitate the formation of thesigma phase at the grain boundaries during aging.22

3. Z phases

The Z phase (NbCrN) is a kind of very fine nano-precipitate, which is of tetragonal or cubic structure. The

Z phase is capable of strengthening S31042 austeniticsteel due to its stability. After long-term exposure underhigh temperature, a number of cirriform Z phases mayoccur inside the grains of S31042 austenitic steel. Withthe increase of holding time, Z phases may also pre-cipitate along the grain boundaries.23 A large amount ofZ phases were observed in type 347H austenitic steelafter aging at 700 °C for 2200 h (see Fig. 6), the size ofthem is about 100–200 nm. However, it was found thatall Z phases were formed at the grain boundaries or twinboundaries, as shown in Figs. 6(a) and 6(c), respectively.Furthermore, as can be seen from Figs. 6(b)–6(e), theresults of TEM and EDS analyses indicate that thegranular and triangular Z phases are NbCrN particles.The coherent relationships between the Z phase with a sizeof 200 nm and the matrix cannot be found in Fig. 6(b).

FIG. 4. The SEM micrographs and EDS maps showing distribution of alloying components in M23C6 carbides forming intergranular in type 347Haustenitic steel: (a) the SEM image, (b) C, (c) Cr, (d) Fe, (e) Ni, and (f) Nb.





As shown in Fig. 6(f), these XRD results indicate theformation of Nb(C,N) carbonitrides and NbCrN particlesin the type 347H austenitic steel at 700 °C for 1000 h.Unfortunately, the size and nucleation sites of the Z phasedo not conform to the previous observations. The possiblereason is the transformation of Nb(C,N) carbonitrides to Zphases with the increase of aging time. The presence of Nb(C,N) at the grain boundaries provides the possibility oftransformation. The diffusion rate of atoms at grain

boundaries is extremely fast, making it possible to trans-formation. Danielsen et al. found that MX precipitateparticles transform directly into the Z phase by interdiffu-sion of Cr from the matrix in 12% CrTaN steel.24 Thoughthe Z phase is formed at the grain boundaries, the Z phaseis more stable than that of Nb(C,N) carbonitrides: thegrowth of the former is slower than that of the latter.12

Z phases with the size of 200 nm may strengthen the grainboundaries and increase the creep resistant of the steels.

FIG. 5. The various TEM micrographs of M23C6 carbides: (a) the original micrograph and its diffraction pattern (b); (c) bulk M23C6 carbides attriple grain boundaries and its diffraction pattern (d); (e) short rod-like M23C6 carbides and its diffraction pattern (f); (g) chain-like M23C6 carbidesand its diffraction pattern (h); (i) triangular M23C6 carbides and its diffraction pattern (j).





FIG. 6. The TEM analysis of Z phases (NbCrN): (a) Z phases at the grain boundaries and its diffraction pattern (b); (c) Z phases at the twinboundaries and its diffraction pattern (d); (e) EDS analysis of Z phases and its XRD profiles (f).





Based on the above investigation, the possible pre-cipitation sequence of type 347H austenitic steel duringaging at 700 °C could be described as: a few ofundissolved Nb(C,N) particles exist after solution treat-ment. A large number of fine Nb(C,N) precipitates areformed at the beginning of aging. With the increase ofisothermal holding time, lots of M23C6 carbides pre-cipitate along the grain boundaries. Finally, Nb(C,N)nanoprecipitates transform to the Z phase (NbCrN).

C. Effect of aging temperature on precipitation

Figure 7 apparently indicates that the aging tempera-ture has a strong effect on the number of precipitates intype 347H austenitic steel. During aging from 700 to900 °C for 100 h, the size of grain swiftly increases.However, when aging temperature increases up to900 °C, the austenite grain size decreases because theM23C6 carbides occupy the grain boundaries and hinderthe increase in grain size. Obviously, the increasing

temperature (below the melting point) results in thenumber density and size of the M23C6 carbides increasingat the grain boundaries. Moreover, hot deformationaccelerates the formation of M23C6 precipitates aroundgrain boundaries and deformation bands [see Fig. 7(f)].The experimental results suggest that the strain inducesthe precipitation of M23C6 carbides. After hot deforma-tion at 1100 °C and 0.01 s�1, the occurrence of re-crystallization and the formation of deformation twinninglead to the earlier precipitation of M23C6 carbides.25,26

Compared with the sample at 900 °C for 100 h, type 347Haustenitic steel precipitated more M23C6 particles afterthermal deformation. It suggests that the precipitation ofM23C6 carbides in steels is more greatly affected by hotdeformation at temperatures above aging treatment.

Microstructural examples of the aged Nb(C,N) nano-precipitates at 700, 800, 850, and 900 °C for 100 h areshown in Fig. 8. Both the amount and the size of MXcarbonitrides increase with the aging temperaturesascended. Furthermore, when the temperature rose up

FIG. 7. The SEM images of samples being aged for 100 h at different temperatures: (a) 700 °C, (b) 750 °C, (c) 800 °C, (d) 850 °C, and (e) 900 °C,(f) samples deformed at 1100 °C and 0.01 s�1.





to 900 °C, the secondary MX carbonitrides precipitatedmore easily as compared to the cases annealed in thetemperature range of 700–800 °C. Therefore, the increaseof isothermal holding temperature facilitates the pre-cipitation of secondary MX carbonitrides.

IV. CONCLUSIONS

The microstructural evolution of type 347H heat-resistant austenitic steel during aging at 700–900 °Cwas investigated. It can be summarized as:

(1) Various precipitates, MX, M23C6, and Z phase, wereidentified during the long-time high-temperature aging.The size and the amount of these phases increase with theincrease of the isothermal temperatures and holding time.

(2) MX carbonitrides are the most predominantstrengthening phase in 347H austenitic steel. The par-ticles uniformly distributed within the grain as well as atthe grain boundaries and tangled with dislocations.

(3) M23C6 carbide is a primary harmful precipitateduring aging at 700 °C. The size of M23C6 carbides isabout 0.2–1 lm, some of them even grow up to 2 lmwith the increase of the holding time.

(4) Z phases (NbCrN) are observed with the size of200 nm at the grain boundaries during aging at 700 °C. Intype 347H austenitic steel, NbCrN transformed from Nb(C,N) at the grain boundaries.

(5) The possible precipitation sequence during aging:fine Nb(C,N) precipitates first, then, M23C6 carbidesprecipitate along the grain boundaries. Finally, nano Nb(C,N) transforms to the Z phase at the grain boundaries.

ACKNOWLEDGMENTS

The authors are grateful to the China National Funds forDistinguished Young Scientists (Grant No. 51325401), theNational High Technology Research and DevelopmentProgram of China (Grant No. 2015AA042504), and theNational Natural Science Foundation of China (Grant No.51474156) for grant and financial support.

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