Effect of Nickel on the Precipitation Processes in 12CrMoV Steels During Creep at 550°C

Scripta Materialia, Vol. 38, No. 1, pp. 101–106, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 1 .00

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EFFECT OF NICKEL ON THE PRECIPITATION PROCESSES IN 12CrMoV STEELS DURING CREEP AT 550°C V. Voda´rek and A. Strang* R 38 D Division, Vı´tkovice, Ostrava, Czech Republic *GEC ALSTHOM, Large Steam Turbines, Rugby, UK (Received July 17, 1997) (Accepted September 27, 1997) Introduction In the course of development of 12 CrMoV steels nickel was added in order to improve impact properties and to suppress the presence of d-ferrite in the microstructure. It was found, however, that excessive amounts of nickel, greater than ;0.6 wt. %, caused an accelerated reduction in the creep rupture strength. This resulted in a downward inflexion in time dependence of creep rupture strength which is known as sigmoidal behaviour [1,2]. This phenomenon was particularly evident when these steels were tested at temperatures of 550°C and greater [1]. The effects of nickel in a 12%Cr steel on the tempering resistance and room temperature mechanical properties were attributed to solid solution hardening and a reduced solubility of carbon [3]. The effects of nickel on creep rupture properties, however, are more complex. Microstructural studies conducted on 12CrMoVNb steels after long term creep exposure indicated that in steels with high nickel levels the stability of M2X phase was significantly reduced [4,5]. It was also suggested that nickel had a marked effect on the stability of M23C6 carbides [5]. On the other hand, no change in type of minor phases has been reported. In order to improve understanding of the effect of nickel on the microstructural evolution of 12CrMoV steels the metallographic investigation was undertaken on a series of four casts with different contents of nickel, which had been creep tested at 550°C out to durations of around 100,000 hours. Materials and Experimental Procedures The chemical compositions of the 12CrMoV steels investigated are shown in Table I. As evident all of these casts had a common base composition in which different amounts of nickel were added. All casts were given the same heat treatment. They were solution treated at 1150°C, followed by air cooling and tempered at 675°C for 4 hours with subsequent air cooling. That enabled to study the effect of nickel on the microstructure and the creep properties as a single variable. The results of creep rupture testing programme at 550°C are summarized in Fig. 1. Microstructural investigations were performed on the materials in the as-received condition and on creep rupture testpieces defined in Fig. 1. Detailed microstructural studies were carried out by means of optical and analytical transmission electron microscopy. The electron microscopy was performed on carbon extraction replicas using a Philips CM 20 TEM fitted with an ultra thin window EDAX 9900. 101

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TABLE I Chemical Composition of the 12CrMoV Steels in wt.% Cast

C

Si

Mn

Cr

Mo

V

N

Ni

A B C D

0.13 0.13 0.13 0.12

0.20 0.25 0.32 0.27

0.70 0.60 0.65 0.65

12.32 12.16 12.35 12.32

0.62 0.59 0.58 0.60

0.17 0.18 0.17 0.18

0.04 0.03 0.03 0.03

0.32 0.59 0.97 1.28

SAD and X-ray microanalysis techniques were used for the identification of minor phases. Quantitative microanalyses of the minor phases were performed using PM THIN software, the results in wt.% being normalized to 100%. Vickers hardness HV10 surveys were carried out on materials in the as-received condition as well as on the creep rupture testpieces of individual casts which had been tested for about the same time to rupture. Experimental Results I. Microstructure in As-Received Condition The microstructure of casts A and B consisted of tempered martensite with small amounts of d-ferrite. In the case of casts C and D the microstructure was fully martensitic. TEM revealed extensive M23C6 precipitation at both the prior austenite and martensite lath boundaries. In addition, fine needle-like M2X particles were found dispersed through the matrix, Fig. 2. Electron diffraction studies proved this phase to be isomorphous with Cr2N [6]. The chemical compositions of minor phases in casts A and D are listed in Tables II and III, respectively. The hardness of all materials was the same and reached the value 27964 HV10.

Figure 1. Creep Rupture Data on 12 CrMoV Steels at 550°C.

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Figure 2. Microstructure of Cast A in As - received Condition.

II. Microstructural Evolution during Creep Exposure at 550°C Examination of the creep exposed materials indicated that whilst there were no differences in the precipitation sequences between the heads and gauge lengths, the microstructural changes occurred more rapidly and were more pronounced in the creep strained regions of the testpieces. The results of identification of minor phases in creep testpieces investigated are summarized in Table IV. As evident in casts A and B containing 0.32 and 0.59 wt. % Ni respectively, both M23C6 and M2X minor phases remained stable even after creep exposure longer than 100,000 hours. In addition, a small amount of Laves phase appeared in the matrix after long time exposure at 550°C, Table V. On the other hand, in the course of creep exposure of materials C and D containing 0.97 and 1.28 wt. % Ni respectively, M2X particles gradually dissolved. The dissolution of this phase was more pronounced in the cast D where no M2X particles were left after exposure for 14,731 hours at 550°C. The M2X dissolution was accompanied by precipitation of another minor phase. SAD studies demonstrated that this phase had an FCC unit cell with the lattice parameter of a 5 1.08 nm. Furthermore, diffraction patterns along the ^001& directions revealed an absence of the {hk0} reflections for which h 1 k Þ 4n, TABLE II Chemical Composition of the M23C6 Phase in As-received Condition Cast

V

Cr

Fe

Ni

Mo

A D

0.8 6 0.1 1.0 6 0.2

68.3 6 0.4 67.1 6 0.6

24.1 6 1.0 23.7 6 0.7

0.3 6 0.1 1.0 6 0.2

6.5 6 0.6 7.2 6 0.8

TABLE III Chemical Composition of the M2X Phase in As-received Condition Cast

V

Cr

Fe

Ni

Mo

A D

11.6 6 2.0 14.1 6 3.4

78.1 6 2.6 76.3 6 2.8

2.6 6 0.9 2.3 6 0.7

0.9 6 0.5 0.9 6 0.6

6.8 6 1.6 6.5 6 0.7

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TABLE IV Identification of Minor Phases in Creep Rupture Testpieces Tested at 550°C Minor Phases Testpiece Identity

Rupture Life [Hours]

M23C6

M2X

A1 A2 B1 B2 C1 C2 D1 D2

19,109 122,718 13,608 103,943 3,053 15,643 1,253 14,731

y y y y y y y y

y y y y y (y) y

M6X

Laves y y

y y y y

where n is an integer, Fig. 3. The chemical compositions of this phase in testpieces investigated are given in Table VI. The results prove that this phase is of M6X type where X is carbon and/or nitrogen. The results of the hardness studies carried out on creep rupture testpieces A1, B1, C2 and D2 are shown in Table VII. These data clearly indicate that marked changes have occurred in the strength of materials due to the effects of thermal exposure and creep strain accumulation during the course of creep testing. The degree of softening was greater in the creep strained regions of the testpieces. Furthermore, increasing the nickel content in 12CrMoV steels above 0.6 wt. % increased the rate of softening.

TABLE V Chemical Composition of the Laves Phase Testpiece Identity

Si

V

Cr

Fe

Ni

Mo

A2 B2

5.3 6 0.9 4.0 6 0.2

0.2 6 0.1 0.3 6 0.1

10.8 6 0.9 10.9 6 1.1

34.8 6 0.7 35.2 6 0.2

0.6 6 0.1 1.1 6 0.2

48.3 6 0.9 48.5 6 1.5

Figure 3. SADP of the M6X Phase Taken along the [001] Zone Axis.

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TABLE VI Chemical Composition of the M6X Phase Testpiece Identity

Si

V

Cr

Fe

Ni

Mo

C1 C2 D1 D2

7.0 6 0.9 7.5 6 1.2 6.7 6 0.4 7.0 6 0.9

6.0 6 0.5 6.8 6 0.5 5.4 6 0.4 6.6 6 0.4

34.5 6 1.3 33.4 6 3.2 35.1 6 1.0 33.1 6 1.9

9.2 6 1.1 7.8 6 1.2 7.6 6 0.4 6.6 6 0.7

24.7 6 1.5 26.2 6 1.8 26.5 6 0.8 28.3 6 0.9

18.6 6 1.2 18.3 6 2.0 18.7 6 0.5 18.4 6 1.4

TABLE VII Evolution of Hardness during Creep Exposure at 550°C Hardness HV10

Testpiece Identity

Head

Gauge Length

A1 B1 C2 D2

269 6 3 269 6 6 254 6 4 237 6 5

213 6 4 212 6 3 202 6 6 178 6 2

Discussion Fig. 1 demonstrates that increasing the nickel content in 12CrMoV steels has a deleterious effect on creep rupture strength at 550°C. It has been confirmed that increasing the nickel content increased the rate of dissolution of M2X phase during creep exposure. The reduced stability of M2X phase was a

Figure 4. Microstructure of 12CrMoV Steels after Creep Exposure at 550°C (a). Cast B, tr 5 13,608 hours (b). Cast D, tr 5 14,731 hours.

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TABLE VIII Effect of Thermal Exposure at 550°C on the Chemical Composition of the M23C6 Phase Testpiece Identity

V

Cr

Fe

Ni

Mo

A1 A2 B1 B2 C1 C2 D1 D2

0.7 6 0.1 0.6 6 0.1 0.7 6 0.1 0.8 6 0.1 1.1 6 0.2 0.7 6 0.1 1.2 6 0.2 0.6 6 0.1

69.3 6 1.5 72.3 6 1.2 69.8 6 0.9 70.9 6 1.7 69.0 6 0.5 70.5 6 0.8 68.6 6 0.7 69.3 6 1.2

21.8 6 1.4 18.8 6 1.5 21.3 6 0.7 19.4 6 0.7 21.6 6 0.6 20.2 6 1.1 22.1 6 0.4 21.3 6 1.3

0.4 6 0.1 0.6 6 0.1 0.8 6 0.1 0.8 6 0.1 1.0 6 0.1 1.1 6 0.1 1.1 6 0.1 1.2 6 0.2

7.8 6 1.1 7.7 6 0.4 7.4 6 0.7 8.1 6 1.5 7.4 6 0.4 7.5 6 0.5 7.0 6 0.5 7.1 6 0.7

consequence of precipitation of M6X phase. The driving force for precipitation of nickel rich M6X phase increased with increasing the nickel content in 12 CrMoV steels. It has been very well known that coarsening of M6X phase in ferritic steels is much faster than coarsening of M23C6 carbides [7]. This phenomenon is clearly demonstrated in Figs. 4a, b where typical microstructures of the testpieces B1 and D2 are shown. M6X particles can partly substitute M23C6 carbides which form most precipitated particles in 12CrMoV steels. M23C6 carbides dissolve only a very small amount of nickel. The nickel content in M23C6 particles did not change during long term exposure at 550°C. The chemical compositions of M23C6 phase in heads of creep rupture testpieces investigated are shown in Table VIII. The observed changes of chromium and iron contents in this phase were found to be in good agreement with the THERMOCALC predictions [8]. In spite of the fact that only very small amounts of nickel are dissolved in M23C6 phase the results of Marrison and Hogg [5] who studied the low strain creep curves of 12CrMoVNb steels suggest that nickel can affect the stability of M23C6 carbides. Voda´rek and Strang [9] recently confirmed that the rate of coarsening of M23C6 phase increased with increasing the nickel content in 12CrMoVNb steels. Conclusions The results of microstructural investigations on 12CrMoV steel after quality heat treatment and subsequent long term creep exposure at 550°C prove that the drop of creep rupture properties of casts containing 0.97 and 1.28 wt. % of nickel can be at least partly attributed to both precipitation of M6X particles and reduced stability of M2X phase. References 1. 2. 3. 4. 5. 6. 7. 8. 9.

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