Effect of Cu Content on Aging Precipitation Behaviors of Cu-Rich Phase in Fe-Cr-Ni Alloy

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. .;;, ScienceDirect JOURNAL OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2010, 17(5), 63-68

Effect of Cu Content on Aging Precipitation Behaviors of Cu-Rich Phase in Fe-Cr-Ni Alloy TAN Shu-ping' , WANG Zhen-hua/ , LIU Zheng-dong" • HAN Jie-cai",

CHENG Shi-chang" , FU Wan-tangZ

0, School of Astronautics. Harbin Institute of Technology, Harbin 150001, Heilongjiang, China; 2. State Kl1Y Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, Hebei , China; 3. Central Iron and Steel Research Institute, Beijing 100081, China) Abstract, The precipitation characteristics and effect on strengthening mechanism of Cu-rich phases during short-time and long-time aging for Super 304H steels with different Cu content were investigated using X-ray diffraction (XRD) , scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the size of Cu-rich phase particles increases, the interspace of Cu-rich phase particles decreases and the density of Curich phases increases with the increase of Cu content during short-time aging (approximately 800 h) at 650 "C for Super 304H steels. During long-time aging (more than 2000 h) at 650 'C, Cu-rich phase precipitates sufficiently and the strengthening effect of Cu-rich phase is preferable in Super 304 H steel containing Cu of 4 %. The strengthening effect of Cu-rich phase in Super 304H steels containing Cu of 2. 2% or 5% is weaker than that with Cu of 4% during long-time aging (more than 2 000 h). Key words, heat resistant alloy, Cu content, Cu-rich phase, aging, precipitation strengthening

Super 304H steel (ASME S30432 steel), which is a new economical 18-8 type austenitic stainless steel, mostly is used as superheater and reheater for super or ultra super critical boilers. Owing to its elevated temperature long-term creep strength higher than that of the other heat resistant steels such as TP32lH, TP347HFG, ASME SA-2l3, TP347H and TP304H steels, Super 304H steel has been widely used for power plant boilers[I-3]. The main feature of Super 304 H steel is the addition of Cu to 304H austenitic stainless steel, which induces the precipitation of fine Cu-rich phase dispersively distributed in the austenitic matrix during high temperature service. Combined with the particles of Nb(C, N) and MZ3C,; and other carbonnitrides in the steel, the excellent high temperature performance and long service life can be achieved. Tanaka et al[4J investigated the dependence of material properties on M Z3 C6 type carbides and (J phase in 304H (l8Cr-18ND. Marian et al[5 J reported the precipitation

behaviors of MC and MZ3 C,; etc in austenitic stainless steels (lSCr-SNi, l8Cr-lONi, and 2lCr-30ND. The coarsening behavior of Cu-phase in a-Fe matrix was observed by Ryoichi et al[6J. Kozeschnik analyzed the microstructure and component of Cu-rich phase in Fe-Cu alloy based on thermodynamics-", Dieter[8J studied the precipitation hardening behavior of Cuphase in austenitic steels. However, the precipitation behavior and strengthening mechanism of Cu-rich phase in Super 304H steel during long-time aging process up to now are not very clear. The dependence of aging precipitation and its mechanism on Cu content and aging time for Super 304H steel have been studied in this study on theoretic basis and experimental results for further improvement of mechanical properties and expanding application range.

1

Experimental Procedure Investigated experimental steels were melted

Foundation Item: Item Sponsored by National Science and Technology Support Plan of China (2007BAES1B02) Shu-ping
~: TAN

Receiwd Date: March 16, 2009

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twice in vacuum induction furnaces. Mother alloy bar was melted in a 200 kg vacuum induction furnace, and then four type of ingots with different Cu contents were melted in a 25 kg furnace. Their chemical compositions are shown in Table 1. Bars with dimension Table 1

of ~15 mm were forged from ingots at 1100 'C. After solution treatment at 1150 'c and holding for 25 min directly quenched into water, each type of bars were aging at 650°C for 4 h , 300 h , 800 hand 2000 h , respectively.

Chemical composition of experimental steels

(mass percent,

%)

Steel

C

Si

Mn

Ni

Cr

Nb

N

Cu

Fe

2Cu

0.093

O. 19

0.82

9.95

18.25

0.41

O. 10

2.21

Balance

3Cu

O. 094

O. 18

0.81

9.58

18.23

0.40

0.11

3.12

Balance

4Cu

0.097

0.18

0.86

10.36

18.08

0.40

0.12

3. 97

Balance

5Cu

0.092

0.16

0.84

9. 98

18.06

O. 39

0.11

5.04

Balance

The specimens with different Cu contents and solution treatments were polished mechanically. After etching with a solution containing FeCl 3 of 10 % and distilled water of 90 %, the microstructures and copper distribution were observed. Specimens were cut by using wire-cutting method; after mechanical thinning to 50 flm, thin foils specimens were finally thinned by double-jet electrolytic thinning technique using HCl0 4 -C z H, 0 6 solution (volume ratio 1 : 19) at room temperature with the etching voltage of 30 V, then observed by Hitachi H-800 transmission electron microscope at 200 k V. Phase analysis of the specimens was carried out on Rigaku 3014 X-ray diffract meter. The tube voltage and current were 40 kV and 40 rnA, respectively. The tube anode was COKUl (). = O. 178 90 nrn) and the receiving slit is 2 mm.

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] ournal of Iron and Steel Research. International

Experimental Results and Discussion After different solution treatments, the extracTable 2

ted residue of each type of specimens was analyzed by X-ray diffraction (XRD). The results were compared with ASTM cards. Table 2 shows the XRD data from extracted residue of Super 304H steel containing Cu of 4 % after aging for 800 h. The main precipitations are M Z3 C 6 , Nb (C, N), M 6 C and Curich phase, which is in consistent with the results in other reports[9-1O]. 2. 1 Impact of aging time on precipitation behavior of Cu-rich phase The microstructures of Super 304H steel containing Cu of 3 % after solution treatment at 1150 'C and aging at 650 'C for short-term (4 h) and long-term (2000 h) are shown in Figv l , respectively. Obviously, austenitic grain sizes are similar under both aging condition. The TEM morphologies of Cu-rich phase in Super 304 H steel containing Cu of 4 % after solution treatment at 1150 'C and aging at 650°C for

X-ray diffraction data of investigated steels Standard data from ASTM cards (cl/Crystalographic plane index)/nm

Number of peaks

1/ ld

d/nm Nb(C,N)

M 2JC 6

M,C

Cu-rich phase

O. 2546 (11])

1

100

0.25541

2

47

O. 23673

3

5

O. 23422

4

100

O. 22078

5

56

0.21652

6

8

O. 20776

7

8

0.20701

8

100

0.20368

9

50

O. 18715

10

4

0.17916

11

30

0.15673

O. 1559 (220)

12

28

O. 13354

0.1330 <31])

13

10

O. 1280

0.1278 (220)

14

13

0.1083

0.1090 <311 )

0.2361 (420) O. 2348 (] 1]) 0.2205 (200) 0.2165 (422) O. 2088 (111) O. 2062 ( 53]) 0.2033 (442) O. 1867 (440) 0.1808 (200)

Issue 5

Effect of Cu Content on Aging Precipitation Behaviors of Cu-Rich Phase in Fe-Cr-Ni Alloy

(a)

Fig. 1

4 hI

(b) 2 000 h.

Microstructure of Super 3048 steel containing Cu of 3% at 650 't for different agiog time (OM)

4. 300. 800 and 2000 h are shown in Fig. 2. respectively. Evidently the size of granular Cu-rich phase dispersed in the matrix gradually increases with aging time, however. the growing speed is relatively slow. After aging for 800 and 2000 h. the average sizes of Cu-rich phase are 13 and 17 nm , respectively.

2. 2 Impact of Co content on precipitation behaviors of Co-rich phase Cu distribution in Super 304H steels with 4 different Cu contents was analyzed for further investigation of the relationship between Cu content and precipitation behaviors of Cu-rich phase. Fig. 3 and Fig. 4 show the Cu distribution results of Super 304H steel after aging for 800 and 2 000 h. Under both aging conditions. Cu distribution density increases with Cu content from 2. 2 % to 4 %. however. decreases when the Cu content reaches 5 % (see Fig. 3 and Fig. 4). Therefore. the effect of dispersion of Cu-rich phase in Super 304H steel with Cu content of 5% on strengthening is the best when the aging term is

(a) 4 hi

Fig.2

• 65 •

(b) 300 hI

short (approximately 800 h). whereas when the aging time reaches 2000 h. the effect decreased due to the aggregation growth of Cu-rich phase. In Super 304 H steel containing Cu of 4 %. this effect is better after aging for 2000 h. Fig. 5 shows the effect of Cu content on average size and distribution of Cu-rich phase particles in Super 304H type steels after solution treatment at 115-0 'C and aging at 650 'C for 800 hand 2000 h. It can be found that the average size of Cu-rich phase, which is about 16 nm in the specimen containing Cu of 5 % and increases linearly with Cu content after aging for 800 h in Fig. 5 (a). The same phenomenon was found in the specimens after aging for 2000 h; however. the average size of Cu-rich phase particles is obviously coarser (30 nm) in the specimen containing Cu of 5 %. The interspace between Cu-rich phase particles also increases with increasing Cu content after aging for 800 h. After aging for 2000 h. the interspace is the smallest in the specimen containing Cu of 4 %. which should be the

(c) 800 h,

(d) 2000 h.

Micro-morphology of Super 3048 steel containiog Cu of 4% at 650 't for different aging time

Journal of Iron and Steel Research, International

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best effect of dispersion strengthening. When the Cu content reaches 5 %, this strengthening effect significantly decreases. It also can be seen in Fig. 3

( a) 2.2%;

Fig. 3

that the Cu-rich phase particle size is finer (no more than 20 nm) when the Cu content is 2. 2%, however, which has weaker effect on strengthening due to lower

(c)

4%;

Cd) 5%.

Surface distribution of Cu in Super 304H steels with different Cu content after aging at 650 "c for 800 h

Ca) 2.2%;

Fig. 4

(b) 3%;

Vol. 17

(b) 3%;

(c) 4%;

Cd) 5%.

Surface distribution of Cu in Super 304H type steels with different Cu content after aging at 650 "c for 2 000 h

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Effect of Cu Content on Aging Precipitation Behaviors of Cu-Rich Phase in Fe-Cr-Ni Alloy

30 (a)

~
<:l 20

i'a ~

"15.

"i

25

~300

E60

I..,

15


10

B is

5 2

3

5

4

(a) Size of particles I

Fig. 5

(c)

400

180 <:l

i'a

200

b

40

~ 100 2

4 3 Content of Cw'l6

(b) Interspace between particles I

5

2

3

4

5

(c) Density of particles.

Effect of Cu on size and distribution of Cn-rich phase particles

Cu-rich phase particle density.

2. 3

Strengthening effect of Co Cu in austenitic matrix can be precipitated as fine dispersed Cu-rich phase particles from solutiontreated Super 304H steel (1150 'C for 25 min) during aging process at 650 'C, which characterizes good strengthening effect (Fig. 2). Since the issues of precipitation of Cu from FeCu system were firstly proposed by Wasserman and Wincierz, the Cu-rich phase in steels had been focused on all along. Many researchers have found that fcc Cu-rich phase precipitated from bee Fe-based alloy belongs to coherent precipitation'P' P", In addition, microstructure changes of bee Cu-rich phase will take place during aging process Cbcc copper9Rcopper-fcc copper )[13]. TEM non-contrast band morphology on Cu-rich phases in the Super 304H steel containing Cu of 3 % after aging for 2000 h is shown in Fig. 6. It can be concluded that the appearance of non-contrast band indicates the coherent relation

between Cu-rich phase and the fcc matrix (see Fig. 6) , which induced apparent strengthening effect. Fig. 7 shows the TEM morphology of Cu-rich phase near stacking fault in the Super 304H steel containing Cu of 4 % after aging for 2000 h. The interaction between stacking fault and solute-atom Cu can lead to solute-atom segregation to stacking fault forming Suzuki atmosphere, which pins dislocations and induces strengthening effect.

FIg. 7 Distribution of Cn-rich phase near stacking faults io Super 304H steel with Co of 4 % aged at 650 't for 2000 b

3

FIg. 6

NOD-contrast band 00 Cn-rich phase in Super 304H steel with Cu of 3 % aged at 650 't for 2 000 h

Conclusions

1) Cu in austenitic matrix can be precipitated as fine dispersed Cu-rich phase particles from Super 304H steel during aging process at 650 'C, which has excellent dispersion strengthening effect. 2) The precipitation fraction of Cu-rich particles as well as its average size increases with increasing aging time. The average interspace between Cu-rich phase particles decreases' with the increase of Cu content, but its density increases. The interspace is the smallest and the distributing density is the high-

• 68 •

est for Super 304H type steel contammg Cu of 4 % after aging for 2000 h , which has the best dispersion effect among all investigated Super 304 H type steels. However, when the Cu content reaches 5 %, the dispersion strengthening effect will decrease significantly. 3) After short-time aging (approximately 800 h) at 650 'C, the dispersion effect is the best in Super 304 H steel containing Cu of 4 %. The growth of Curich phase weakens the dispersion strengthening effect after long-time aging (2000 h) at 650 'C in Super 304 H steel containing Cu of 5 %. 1) The appearance of non-contrast band on Curich phase particles in Super 304 H steel indicates the coherent relation between the precipitates and matrix, which leads to coherent strengthening effect. Cu atoms segregated to stacking fault can also pin dislocations and induce matrix strengthening effect.

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