View of Effect of precipitation aging temperatures on rejuvenation heat treated microstructures, hardness and Î³' phase stability in Udimet-500

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Effect of Precipitation Aging Temperatures on Rejuvenation Heat Treated microstructures, Hardness and γ' Phase Stability in Udimet-500

Chanon NIEMKONG

¹

, Sureerat POLSILAPA

¹

and Panyawat WANGYAO

^2*

1

Materials Engineering Department, Engineering Faculty, Kasetsart University, Bangkok, Thailand

²

Inovative Metals Research Unit, Metallurgical Engineering Department, Engineering

Faculty, Chulalongkorn University, Bangkok, Thailand

Abstract

This research has studied the effect of precipitation aging temperatures on size and area density ofγ' – particles of the reheat treated microstructures incast nickel basesuperalloy, grade Udimet-500. All specimens were performed the same solutioning treatment at temperature of 1,150°C for 4 hours following with various precipitation aging temperatures of 760, 780, 800 and 820°C for 16 hours. Furthermore, these reheat treated specimens were also performed long-term heating at temperatures of 900°C and 1000°C for 200 hours to evaluate γ'-phase stability. Finally, hardness measurements were also carried out. Form all obtained results, it was found that the standard precipitation aging temperature of 760°C for 16 hours provided the most phase stability after long-term tests as well as the maximum hardness.

Introduction

Udimet-500 is one of cast nickel based superalloysused gas turbine blade material in gas turbine enginesfor electricity generating. After long- term service, themicrostructure of the alloy, which consists of gamma (γ), gamma prime(γ') and/or carbides would be detrimentallydegraded leading to much lower mechanical propertiesat high temperatures.

However, these kinds of γ' precipitated strengthening materials could be microstructallyrestored by rejuvenation heat treatment, which basically consists of solutioning treatment and precipitation aging to provide the microstructural refurbishment as same as the news ones.^(1-4)

The research will study the effect of precipitation aging temperatures in final microstructures in terms of size and area density of γ'-particles as well as the effect of these obtained microstructural characteristics on hardness behavior. Furthermore, the long-term heating at high temperatures were also performed to evaluate the γ'-phase stability at long-term exposures in Udimet-500.

Materials and Experimental Procedures

The investigated material is cast nickel- based superalloy, grade Udimet-500, a turbine blade

material in gas turbine engines, which was used at high temperatures for 50,000 hours by Electricity Generating Authority of Thailand (EGAT). The chemical composition of the alloy shown in Table 1.

Table 1. The chemical composition of cast nickel-base superalloy, grade Udimet-500

Elements Ni Cr Co Ti W Al Ta Mo Fe C B Zr Weight

Percent

(%wt.) Bal. 18 17 3 - 3 - 4 2 0.1 - -

The alloy was performed with various reheat treatment processes as following:

1. Solution treatment at temperature of 1,150°C for 4 hours then following with air cooling.

2. Precipitation aging at temperatures of 760, 780, 800 and 820°C for 16 hours then following with air cooling.

Then these received specimens after various reheat treatment conditions were prepared with cutting, grinding, polishing and etching for metallography analysis by Scanning Electron Microscopy (SEM). Furthermore, to investigate and evaluate the γ'-phase stability then all reheat treated specimen were performed with long-term heating tests at temperatures of 900°C and 100°C for 200 hours. Then these obtained specimens were

*Corresponding anther E-mail: [email protected]

J.Met.Mater.Miner.26 (1) 2016, DOI:10.14456/jmmm.2016.1

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2

also studie treated and carried out

Results an

Figure 1. M 50 Figur Udimet-500 temperature coarse and throughout shows that t coarsening provide low

Figure 2. M

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C/16 hours.

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ostructures of us precipitation 3 a)–d) show er solutioning llowing with emperatures ectively. F s, it could b of precipitati

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820°C/16 hours.

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Figure 6. M v Figur specimens w following w hours. From

a) Aging at 7 and heating hours/ 200

c) Aging at 80 and heating

Hrs.

0 5 10 15 20 25

γ'‐Area Density

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Microstructure various tempe res 6 a) – d) with various with long-ter m these obta

760°C/16 hours g at 900°C/200

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0 5 10 15 20 25 30 35 40 45 50

γ'‐Area Density

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Average Area of γ'Paticles 7

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4 NIEMKONG, C. et al.

Figure 9. Microstructures of specimens after aging at various temperatures then heating at higher temperature

at 1,000°C for 200 hours.

Figures 9 a) – d) show the microstructures of specimen with various reheat treatment conditions following with long-term heating at 1,000°C for 200 hours. The received results clearly show the much higher effect of higher heating temperature on γ'-particle coarsening than those of figures 6a) – 6d). There is also the same trend asit was found in long-term heating 900°C for 200 hours that the higher aging temperatures (800 and 820°C/16 hours)

resulted in rapidly increasing of both size and area density of γ'-particle, see also figures 10 and 11.

Figure 10. The relationship between γ’ particles size and various agingtemperatures and heating at 1000°C/200 hours.

Figure 11. The relationship between γ’ area density and various agingtemperatures and heating at 1000 °C/200 hours.

a) Aging at 760°C/16 hours and heating

b) at 1,000°C /200 Hrs.

b) Aging at 780°C/16 hours and heating at 1,000°C/200 Hrs.

c) Aging at 800°C/16 hours and heating at 1,000°C/200 Hrs.

d) Aging at 820° /16 hours and heating at 1,000°C/200 Hrs.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

760 °C 780 °C 800 °C 820 °C

Aging Temperatures ( °C )

Aging Long term testing at 900 °C Long term testing at 1000 °C 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

760 °C 780 °C 800 °C 820 °C

0 10 20 30 40 50 60 70 80

760 °C 780 °C 800 °C 820 °C

γ'‐Area Density

Figure 12.The relationship between γ’-particles size and various agingtemperatures after heating at high temperatures

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Hardness and γ' Phase Stability in Udimet-500

Form overall results re-expressed in Figures 12 and 13, which show the effects of aging temperatures and long-term heating temperatures on size (average area of γ’-particle) and γ’-phase area density, respectively. It could be summarized that long-term heating at high temperatures provided

much more effect of both very rapid γ’-particle coarsening and increasing in γ’-phase area density.

This result was much more pronounced when the specimens were performed with the higher precipitation aging temperatures of 800 and 820°C for 16 hours.

0 10 20 30 40 50 60 70

760 °C 780 °C 800 °C 820 °C

γ'‐Area Density

Aging Long term testing at 900 °C Long term testing at 1000 °C

Figure 13. The relationship between γ’-area density and various agingtemperatures after heating at high temperatures

350360 370380 390400 410420 430

Hardness ( HV )

Heating conditions

Figure 14.The relationship between hardness and reheat treatment conditions

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Figures 14 and 15 show the obtained hardness results from specimens with various reheat treatment conditions and long-term heating tests. From these obtained results, it was found that the higher of precipitation aging temperatures resulted in lower hardness values. Furthermore, it was also found that

the higher heating temperature resulted in much lower hardness values.^(6-10) From these, it could be concluded that the hardness ismore dependanton the size of γ’-particle much more than the γ’-phase area density, which usually provides higher hardness, see details in Figures 16 and 17.

350 360 370 380 390 400 410 420 430

760 °C 780 °C 800 °C 820 °C

Hardness ( HV )

Heating conditions

Aging Heating at 900 °C Heating at 1000 °C

Figure 15.The relationship between hardness and various agingtemperatures after heating at high temperatures

370 380 390 400 410 420 430

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Hardness ( HV )

Figure 16. The relationship between hardness and γ' particles size

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Hardness and γ' Phase Stability in Udimet-500

Conclusions

1. Higher precipitation aging temperature provided higher results in size and area density of γ'-particles but lower in hardness.

2. Higher heating temperature provided also both increasing in size and area density of γ'- particles but decreasing in hardness.

3. The effect of size of γ'-particles on hardness was much more pronounced than that of γ'-phase area density.

4. The lowest precipitation aging (standard aging temperature) form temperature even provided the lowest area density and size of γ'- particles but

resulted in the highest hardness and the most γ'- phase stability after long-term heating tests.

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360 370 380 390 400 410 420 430

0 10 20 30 40 50 60 70

Hardness ( HV )

Figure 17.The relationship between hardness andγ' area density