Microstructure Evolution of a Novel Super304H Steel Aged

at High Temperatures

Xin-mei Li1, Yong Zou1;*, Zhong-wen Zhang2 and Zeng-da Zou1

1Key Lab of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, P. R. China2Shandong Electric Power Research Institute, Jinan 250002, P. R. China

The microstructure evolution of a novel Super304H stainless steel aged at different temperatures was investigated using various analysismethods. The results reveal that the microstructure of the Super304H steel after aging at 973–1623K consists of the primary � matrix and a smallamount of precipitated phases. Grain size of �-matrix shows a slow increase when the aging temperature is lower than 1373K and it increasesquickly when the aging temperature is beyond 1423K. The lattice parameter of �-matrix varies at different aging temperatures, and this variationcorresponds to the phases precipitated or redissolved. The variation of grain size and precipitated phases can affect the performance of the steelwhen exposing to steam oxidation and be subject to creep at high temperatures. [doi:10.2320/matertrans.MC200916]

(Received August 3, 2009; Accepted December 3, 2009; Published January 25, 2010)

Keywords: Super304H steel, aging temperature, microstructure, lattice parameter, precipitation

1. Introduction

Due to the problems of the earth environment such asthe increase of carbonic acid gas, improvement of thermalefficiency is required in fossil fired power plants. USC (Ultrasuper critical) boilers that are operated under the condition ofa temperature over 873K and steam pressure over 24MPa areexpected as one of the higher heat efficiency plants. To meetthe critical demands for the boilers, a new austenitic stainlesssteel Super304H (0.1C-18Cr-9Ni-3Cu-Nb-N) has been de-veloped.1–3) The Super304H has high strength at elevatedtemperatures are required for superheater tubes in fossil firedboilers. The 105 h creep rupture strength at 973–973K forthis steel is more than 20% higher, compared with ASMESA-213 TP347H which has the highest allowable stressamong the conventional 18-8 steels. This excellent creeprupture strength is based on finely precipitated particles suchas Cu-rich phase, NbCrN, Nb(C,N) and M23C6.

4–7) However,the properties about this new austenitic stainless steel aremainly provided by manufacturer. The study focusing on thehigh temperature properties of Super304H is limited in thepublished literatures.8–11) In order to use them widely in high-temperature environment, the effect of aging at differenttemperatures on the properties is investigated. This studyaims to investigate the microstructure development of theSuper304H steel after being aged at high temperatures. Theanalysis of microstructure, in particular, precipitates forma-tion at different temperatures provides the more fundamen-tals for wide application of the steel at elevated temperatures.

2. Experimental

A 15mm� 12mm� 7mm square test specimen wascut from the commercial tube with the specification of’45mm� 9mm that was available from Sumitomo MetalIndustries. The chemical composition of this Super304H isshown in Table 1. The measured value was obtained by usingquantitative analysis spectrometer.

These specimens were heated respectively in a tubefurnace at temperatures between 973–1623K with soakingtime 3.6 ks and then quickly quenched in water. After aging,the samples were ground to remove surface layer for 0.5mmin order to remove the oxide peels. Then the samples wereanalyzed by X-ray diffractometry (XRD) with CuK� radia-tion at 30 kV and 100mA to identify the phase constitution.The microstructures of the aged samples were observed andanalyzed by using an optical microscope (OM) and scanningelectron microscopy (SEM) equipped with an energy-dispersive spectroscopy (EDS) system. For the OM andSEM observation, the samples were mechanically polishedand etched in a solution of aqua regia. For transmissionelectron microscope (TEM) observation, 3mm diameterdiscs were cut from the aged samples and were mechanicallyground to a thickness of 150 mm. Final foils for TEMobservation were prepared by twin-jet electropolishing inan electrolyte solution. The microstructure of the thin filmswas observed by TEM using an H-800 instrument at 200 kV.

3. Results and Discussion

Figure 1 shows the OM morphology of the as-receivedspecimen and specimens aged at 973K, 1373K, and 1523K,respectively. It can be seen that the microstructure includesthe � phase and precipitated phases, and the grain size of �phase is increased with the increasing of aging temperatures.Figure 2 summarized the variation of grain size with agingtemperatures. As a comparison, the grain size of as-receivedbase metal also is listed in this figure. This figure indicatesthat the austenitic grain size of specimen aged at 973K isalmost kept the same vale as that in as-received condition.After being aged at 1123–1373K, the grain size of austeniticmatrix shows a slow increase. But the austenitic grain sizeshows a rapid growth when the aging temperature isincreased to 1423–1623K.

Figure 3 shows the micro-morphology of precipitates forsamples aged at different temperatures. The shape of pre-cipitated phases include the lath-like and fine spherical, asshown in Fig. 3(a). After being aged at 1123K for 3.6 ks, it*Corresponding author, E-mail: yzou@sdu.edu.cn

Materials Transactions, Vol. 51, No. 2 (2010) pp. 305 to 309Special Issue on Development and Fabrication of Advanced Materials Assisted by Nanotechnology and Microanalysis#2010 The Japan Institute of Metals

can be observed more precipitated phases which distributedin the interior of grain and grain boundary. The precipitatedphases along the grain boundary show the lath-like, and thosein the interior of grain show a spherical shape. With theincreasing of the aging temperature, the size and amount ofprecipitated phases increase continuously. However, theamount of precipitated phases begins to decrease when theaging temperatures exceeds 1273K. When the sample wasaged at 1523K, the size of lath-like precipitated phase showsthe increasing tendency, but the amount of fine sphericalphase decreased obviously. Figure 4 is the statistical results

of the area ration of the precipitated phases and matrix basedon the gray of SEM micrograph, which can be used toestimate the amount changing of precipitated phases duringdifferent aging temperatures.

Figure 5 shows the X-ray diffraction profiles of the as-received Super304H and the samples aged at 973K, 1123K,1273K, 1423K and 1523K for 3.6 ks, respectively. It can besure that the peaks with the highest intensity correspond tothe � phase for all the samples. But for some precipitatedphases, it is difficult to index these phases due to their lowerintensities or overlapping. Therefore, TEM observation andselected area diffraction were used to confirm the precipitatedphases. Figure 6 shows the TEM bright field images andselected area diffraction about the specimen after beingaged at 1123K. Two kinds of precipitate were confirmed.One is M23C6 whose lattice parameter is 1.064, as shownin Fig. 6(a) and (b). The EDS result indicated that thisprecipitate particle included the Cr and Fe elements, and theCr is the major metal element. Therefore, the Cr23C6 isdenoted M23C6 precipitated phase in this study. In Fig. 6(c)and (d), the other Nb(C,N) precipitate is confirmed and itslattice parameter is 0.444. This value is between that of NbNand NbC, which are 0.338 for NbN and 0.447 for NbCrespectively. Combining with the TEM results, the precipi-tated phases are indexed in Fig. 5. Although anotherprecipitated Cu3:8Ni phase is also observed by TEM, it isnot marked in Fig. 5. Because the peaks of Cu3:8Ni areoverlapped with � phase when the diffraction angle is lower

Table 1 Chemical composition of Super304H (mass%).

C Si Mn P S Cr Ni Mo Cu Nb N B

(1)0.07–

�0:30 �1:00 �0:040 �0:01017.00– 7.50–

—2.50– 0.20– 0.05– 0.001–

0.13 19.00 10.50 3.50 0.60 0.12 0.010

(2) 0.10 0.22 0.85 0.033 0.006 18.4 8.56 0.26 2.41 0.48 — —

Note: (1) provided by Sumitomo; (2) measured value

(a) (b)

(d)(c)

Fig. 1 OM morphology of the samples aged at different temperature for 3.6 ks (a) as-received, (b) 973K, (c) 1373K and (d) 1523K.

900 1000 1100 1200 1300 1400 1500 16000

200

400

600

800

1000

Gra

in S

ize

(g/1

0-6m

)

Aging Temperature, T/K

Fig. 2 Variation of austenite grain size of Super304H after being aged at

different temperature for 3.6 ks.

306 X. Li, Y. Zou, Z. Zhang and Z. Zou

than 96�, it can be distinguished with � phase only when thediffraction angle is higher than 96�. Therefore, the peaks ofCu3:8Ni phase are not marked in this figure. In Fig. 5, it canbe seen the trendy about the type of precipitated phases.When the specimen is heated at 973K, the Cr23C6 begins toprecipitate although its peaks are very weak. With theincreasing of aging temperature, it almost disappears whenthe aging temperature is up to 1423K. This indicates theCr23C6 phase is precipitated at a lower aging temperatureand re-dissolved at a higher temperature. When the agingtemperature is as 1523K, a new NbCrN phase is detectedas shown in Fig. 5. At this temperature, other precipitatedphases almost disappear.

The variation of lattice parameter of � phase at differentaging condition is shown in Fig. 7, which is calculated frommain peak of � phase based on the X-ray diffraction data. Itcan be seen the lattice parameter of as-received base metalhas a bigger value, it begins to decrease with the increasing ofaging temperature and it reaches the minimum at 1123K.Although many conditions can affect the lattice parameter,

the variation of lattice parameter of � phase is thought to becaused mainly by the solid solution of some elements,because all the samples were heat treated at similar conditiononly the temperature is different. The sample at as-receivedcondition is solution treated and some elements (such as C, Nand Nb, Mo) solid dissolved in the � matrix, the latticeparameter of as-received base metal has a bigger latticedistortion. After being aged at 973K, some C, N and Nbprecipitated from � matrix and formed Cr23C6 and Nb(C,N)phases, this will result in the decreasing of lattice parameterfor � matrix. When the aging temperature is up to 1123K, theamount of precipitated phases has a maximum value andcorresponded to the minimum value of lattice parameter for �matrix. Continue to increase the aging temperature, someprecipitated Cr23C6 phase begins to re-dissolve and some Catoms are solid dissolved in the � matrix, then the latticeparameter of � matrix increase again. According to the X-rayand TEM results, the precipitated Cr23C6 phases almost

(a) (b) (c)

(f)(e)(d)

Fig. 3 SEM images of samples aged at different temperature (a) 973K, (b) 1123K, (c) 1173K, (d) 1273K, (e) 1423K, (f) 1523K.

1000 1100 1200 1300 1400 15000

0.5

1

1.5

2

Aging Temperature, T/K

Rel

ativ

e A

rea

(%)

Fig. 4 Variation of relative content for precipitated phase of Super304H

aged at different temperature.

25 30 35 40 45 50 55 60 65 70 75 80

γ phase

Inte

nsity

(a.

u.)

2θ (degree)

1523K

Nb(C,N)Cr23C6

NbCrN

1423K

1273K

1123K

973K

as-received

Fig. 5 X-ray diffraction patterns of Super304H aged at different temper-

ature.

Microstructure Evolution of a Novel Super304H Steel Aged at High Temperatures 307

disappeared when the aging temperature is higher than1323K. The lattice parameter of matrix almost returnsthe initial stage of as-received condition. When the agingtemperature is above 1323K, although the amount ofprecipitated Nb(C,N) is increasing, the total amount ofprecipitated phases is limited, so the lattice parameter ofmatrix at this aging temperature only has a slight changing.When the aging temperature is above 1423K, the latticeparameter of matrix increase again due to the Ostwaldripening. When the specimen is aged at 1623K, the alloyelements in steel almost re-dissolve in matrix, so the latticeparameter reaches maximum value.

According to the analyses above, the Super304H showsthe good thermal stability when the specimens are agedtreatment below 973K, the austenite matrix still keep thefine microstructure. This could be attributed to the dis-

persively distributed Nb(C,N) and Cu-rich phase, which caneffectively pin dislocations, subgrain boundaries and impedethe movement of boundaries. So the recrystallization of thealloys is retarded in the process of ageing. However, theaustenite grains of Super304H also begin to grow when theaging temperature is above 973K, especially higher than1423K. In the previous reports,3) the excellent high temper-ature steam oxidation resistance of Super304H is benefitfrom the fine austenite grain. In order to keep the hightemperature steam oxidation resistance of Super304H, theservice temperature should be limited under 973K to avoidthe grain coarsening. At the same time, it also is noticedthat there are lots of Cr23C6 precipitates along the grainboundary when the specimen is aged at 1123K, this willweaken the creep rupture strength of Super30H. Therefore,it should avoid this temperature range during manufacturingprocess of Super304H parts, such as welding and heattreatment.

4. Conclusion

The microstructure of Super304H is consisted by � phaseand small amount of precipitated phases when it is aged at973–1623K for 3.6 ks and the Super304H shows excellentthermal stability at high temperature. Grain size of �-matrixis grown slowly when aging temperature is lower than1373K and it increases quickly when the aging temperatureis beyond 1423K. The Cr23C6, Cu3:8Ni, Nb(C,N) and NbCrNprecipitated phases appears at different aging temperatures.The lattice parameter of �-matrix shows some changing atdifferent aging temperatures, and this variation correspondsto the phases precipitated or redissolved.

(b)

(111)

(220)

(311)

(133)

(331)

(402)

(a)

(c) (220)

(311)

(111)(002)

(310)(312)

(d)

γ

γ

γ

γ

γ

γ

Fig. 6 TEM images and selected area electron diffraction patterns of Super304H aged at 1123K (a) and (c) bright field image and

corresponding SAD pattern; (b) SAD calibration of �-phase and Cr23C6; (d) SAD calibration of �-phase and Nb(C,N).

1000 1200 1400 16003.590

3.595

3.600

3.605

3.610

Aging Temperature, T/K RT

Lat

tice

Para

met

er o

f γ

Phas

e

Fig. 7 The variation of lattice parameter of � phase at different aging

condition.

308 X. Li, Y. Zou, Z. Zhang and Z. Zou

REFERENCES

1) Y. Sawaragi, K. Ogawa, S. Kato and A. Natori: The Sumitomo Search

48 (1992) 50–58.

2) Y. Sawaragi, N. Otsuka, K. Ogawa, S. Kato and S. Hirano: Sumitomo

Metals 43 (1991) 24–31.

3) Y. Sawaragi, N. Otsuka, H. Senba and S. Yamamoto: Sumitomo Search

56 (1994) 34–43.

4) M. Igarashi: 18Cr-9Ni-3Cu-Nb-N steel, (Springer Berlin Heidelberg,

2004) pp. 260–264.

5) T. Kan, K. Sawaragi, Y. Yamadera and H. Okada: Materials for

advanced power engineering, (Julich, Germany; Forschungszentrum

Julich; 1998, Conference; 6th—1998: Liege; Belgium, 1998) pp. 441–

450.

6) K. Ogawa, Y. Sawaragi, N. Otsuka, H. Hirata, A. Natori and S.

Matsumoto: ISIJ Int. 35 (1995) 1258–1264.

7) Y. Yang, S. Cheng and G. Yang: Teshugang/Special Steel 23 (2002)

27–31.

8) A. Y. Kina, V. M. Souza, S. S. M. Tavares, J. M. Pardal and J. A.

Souza: Mater. Charact. 59 (2008) 651–655.

9) H. Matsuo, Y. Nishiyama and Y. Yamadera: 4th Int. Conf. on Advances

in Materials Technology for Fossil Power Plants, (ASM International,

9639 Kinsman Road, OH 44073-0002, United States, Hilton Head

Island, SC, United States, 2005) pp. 441–450.

10) K. Natesan and J. H. Park: Int. J. Hydrog. Energy 32 (2007) 3689–3697.

11) Y. Noguchi, M. Miyahara, H. Okada and M. Igarashi: Zairyo/J. Soc.

Mater. Sci. Japan 56 (2007) 136–141.

Microstructure Evolution of a Novel Super304H Steel Aged at High Temperatures 309