Scholarly article on topic 'Damage Mechanism of Low Cycle Fatigue in an Advanced Heat Resistant Austenitic Stainless Steel at High Temperature'

Damage Mechanism of Low Cycle Fatigue in an Advanced Heat Resistant Austenitic Stainless Steel at High Temperature Academic research paper on "Materials engineering"

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{"Heat resistant austenitic stainless steel" / Superheater / A-USC / "low cycle Fatigue" / damage.}

Abstract of research paper on Materials engineering, author of scientific article — Guocai Chai

Abstract Sandvik Sanicro 25 is a newly developed heat resistant austenitic stainless steel grade for the next generation of coal fired advanced ultra-super critical (AUSC) power plants. In this paper, low cycle fatigue behavior and damage mechanisms of the material were studied. The low cycle fatigue test was performed in air at room temperature, 600°C to 700°C. The microstructures were studied using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a conventional hardening and softening behavior as most metal materials. At high temperatures, however, it shows only a cyclic hardening behavior. Dynamic strain ageing is found to be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the fatigue damage in grains.

Academic research paper on topic "Damage Mechanism of Low Cycle Fatigue in an Advanced Heat Resistant Austenitic Stainless Steel at High Temperature"

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Procedia Materials Science 3 (2014) 1754 - 1759

20th European Conference on Fracture (ECF20)

Damage mechanism of low cycle fatigue in an advanced heat resistant austenitic stainless steel at high temperature

Guocai Chai1, 2*

1Sandvik Materials Technology, SE-811 81 Sandviken, Sweden 2Linkdping University, Engineering Materials, SE-58183 Linkoping, Sweden

Abstract

Sandvik Sanicro 25 is a newly developed heat resistant austenitic stainless steel grade for the next generation of coal fired advanced ultra-super critical (AUSC) power plants. In this paper, low cycle fatigue behavior and damage mechanisms of the material were studied. The low cycle fatigue test was performed in air at room temperature, 600 °C to 700°C. The microstructures were studied using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a conventional hardening and softening behavior as most metal materials. At high temperatures, however, it shows only a cyclic hardening behavior. Dynamic strain ageing is found to be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the fatigue damage in grains.

© 2014ElsevierLtd.Thisisanopenaccessarticle under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selectionandpeer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering

Keywords: Heat resistant austenitic stainless steel, Superheater, A-USC, low cycle Fatigue, damage.

* Corresponding author. Tel.: +0-000-000-0000 ; fax: +0-000-000-0000 . E-mail address: author@institute.xxx

2211-8128 © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of the Norwegian University of Science and Technology (NTNU), Department of Structural Engineering doi: 10.1016/j.mspro.2014.06.283

1. Introduction

The demand for electric power is continuously increasing around the world. Meanwhile the consciousness of the environmental impact from human action is growing. Although combustion processes generate carbon dioxide, coal-fired thermal power generation is still one of the most important methods in the medium to long-term future to satisfy this demand, as coal is available at a competitive price and often is the single domestic energy source, by IEA (2009). However, the biggest challenge facing coal-fired power plants is to improve their energy efficiency. This can be accomplished by increasing the maximum steam temperature and the steam pressure. Conventionally, the heat efficiency of coal-fired power plants has stayed at around 41% in the super critical (SC) condition with a temperature of 550°C and pressure of 24.1MPa. In order to attain a power generating efficiency of about 43%, ultra super critical (USC) conditions with a steam temperature at about 600°C should be reached. By increasing the temperature up to 700°C (A-USC condition) and pressure of above 300 bars, a power plant efficiency of higher than 50% can be reached and CO2 emission can be reduced by about 45% comparing with that of SC condition, by Blum et al. (2004). However, the steam data in practice will be limited by the material properties of the boiler tubes, especially tensile strength at elevated temperatures and creep strength combined with corrosion resistance.

Austenitic stainless steel grade UNS S31035 (Sandvik Sanicro® 25) has been developed for the next generation of 700°C A-USC power plant, by Rautio et al. (2004) or Chai et al. (2011). This new grade shows very good resistances to steam oxidation and hot corrosion, and higher creep rupture strength than other austenitic stainless steels available today. Actually, Sanicro 25 has a highest creep strength among the commercial heat resistant austenitic stainless steels, by Chai et al. (2013). This makes it an interesting alternative for super-heaters and reheaters in future high-efficient coal fired boilers. They have been test-installed in different boilers in Europe and have run for more than five years, and are still in very good conditions. This paper will discuss the low cycle fatigue, LCF, properties of Sandvik Sanicro 25 material, mainly the LCF damage and crack initiation behavior and mechanisms.

2. Materials and experimental

The material used was Sandvik Sanicro 25 bar material with a diameter of 150mm in an annealing condition. Table 1 shows the nominal chemical composition. This alloy contains large amounts of Cr, W, Cu and N to achieve a stable microstructure, high creep strength and high corrosion resistance. Fig. 1a shows the grain microstructures of the material, which contains plenty of annealing twins.

Table 1. Nominal composition of Sandvik Sanicro 25 (wt%).

Cmax Si Mn Cr Ni W Co Cu Nb N Fe

0.1 0.2 0.5 22.5 25 3.6 1.5 3.0 0.5 0.23 Bal.

For low cycle fatigue test, cylinder-shaped samples with a diameter of 10 mm and a measure length of 20 mm were prepared from the bar material in the longitudinal direction. The low cycle fatigue was carried out in a computer-assisted Instron 1342 servo-hydraulic machine. The tests were performed at RT (20°C), 600°C, 650°C and 700°C using a heating resistance chamber with a temperature accuracy of 1°C. The total strain amplitude ranged from 0.3 to 0.8% with a push-pull mode and a frequency of 0.05Hz.

The fatigue pre-initiation damage (dislocation slip bands) was investigated using a JEOL 840 scanning electron microscope (SEM). In order to investigate the fatigue damage mechanism and process, two SEM techniques, electron back scatter diffraction (EBSD) and electron channeling contrast image (ECCI), were used. The EBSD technique was used to analyze the strain or stress localization or the influence of LCF on the strain localization. Misorientation and strain mapping were performed in a 6500 F JEOL field emission gun-scanning electron microscope (FEG-SEM) equipped with a TSL OIM EBSD system at 20 kV acceleration voltage and a working distance of about 13.6 mm. The ECCI technique has been recently proven as a powerful technique to image deformation damage and even dislocation structures steels. ECCI observations were carried out in a Zeiss Crossbeam instrument (XB 1540, Carl Zeiss SMT AG, Germany) consisting of a Gemini type field emission gun (FEG) electron column and an focused ion beam (FIB) device (Orsay Physics). ECCI was performed at 10 kV acceleration voltage and a working distance of 5 mm, using a solid state 4-quadrant BSD detector. The microscope was run in the "high current" mode and an objective lens of an aperture of 120 |im was used.

3. Results and discussion

3.1. Cyclic deformation behavior

Fig. 2 shows the cyclic stress-strain responses of Sanicro 25 with different total strain amplitudes at RT, 600°C, 650°C and 700°C. At RT, the alloy shows an initial hardening followed by a continuous softening behavior (Fig. 2a). Increase of applied strain increases this hardening and softening process and the number of cycles for a maximum stress also increases. The cyclic plastic strain amplitude slightly decreases first and then increases again. However, the amount of plastic strain amplitude changes little during the cyclic loading (Fig. 2b). At temperatures 600°C, 650°C and 700°C, only monotonic cyclic hardening can be observed (Fig. 2c), and the cyclic plastic strain continuously decreases during cyclic loading (Fig. 2d), especially at low strain amplitude. The cyclic plastic strain amplitude decreases rapidly after 100 cycles. To understand the monotonic cyclic hardening, the stress versus strain hysteresis curve was evaluated Chai et al. (2013). Serrated curves can be observed at temperatures from 600°C to 700°C. The amplitude of the serrated points (peak and valley) increases with increasing temperature and strain amplitude. These phenomena indicate that dynamic strain ageing could have occurred during the cyclic loading at these temperatures. The monotonic increase of stress can be attributed to dynamic strain ageing. Dynamic strain ageing can greatly reduce the dislocation annihilation rate and consequently increases the cyclic hardening rate. When the dynamic strain ageing is so effective that dislocation annihilation rate can be greatly reduced, only cyclic hardening can occur. Increase of applied strain increases amount of dislocations that increases the probability for dynamic strain ageing.

Cyclic hardening rate has been evaluated using the cyclic hardening curves as shown in Fig. 3a. At RT, the cyclic hardening rate is high at some first cycles, and then the cyclic hardening rate becomes slightly lower. Since the number of cycles for the maximum stress increases with increase of applied strain, the average cyclic hardening rate decreases slightly with increase of strain amplitude. On the other hand, cyclic hardening rate increases with increasing strain and temperature at high temperatures (Fig. 2c and 3a). Another interesting phenomenon is that the stress increases more near the failure period at a comparatively low temperature such as 600°C than at a higher temperature. This is probably related to a long fatigue time that can cause the precipitation that increase cyclic deformation hardening rate. It should be mentioned that the cyclic deformation hardening rate at RT is much higher than that at high temperaturea. At 650°C, a plateau of stress can be reached after 100 cycles with a strain amplitude of 0.8%.

♦ 0.6%

■ 1.6%

A 1.2%

A AAAààttk A 1 1 1IMM A A H HIM A X

x xxxxm x xxx«» x x>X«s< x x>s* Js

RT (b)

10 100 1000 10000 100000 Number of cycles

10 100 1000 10000 100000 Number of cycles

CO 300

.i 200 X re

A A AAA

0,87% 0,80% a 1,60% 0,60% 1,20% ♦ 600°C-0.6% 700°C-0.6%

O .VV-i

a ' -o* o$C t

650°C

(/> re a E

0,87% 0,80% A 1,60% 0,60% 1,20% ♦ 700°C-0.6%

A A AAA** A A t^AMàt^k A AAAI 600°C-0-6%

650°C

1 10 100 1000 10000 1 10 100 1000 10000 Number of cycles Number of cycles

Fig. 2. Stress response curves in Sanicro 25 bar material during LCF test , (a). At RT, (b). Influence of temperature with strain amplitude of 0.3%,

(c). Influence of strain amplitude at 650°C.

e 1 0) re

"E 0,1

0,01 0,

650°C

A A à ♦

Total strain range

0,01 -:

♦ 650°C »700°£

• 600°C ART

(b) u\-

1E+2 1E+3 1E+4 1E+5 1E+6 Number of cycles to failure 2Nf

Fig. 3. (a) Cyclic deformation hardening rate, (b) Total strain rate versus number of cycles to failure.

Fig. 3b shows the total strain range versus number of cycles to failure for Sanicro 25 at different temperatures. Generally, the fatigue life decreases with increase of temperature. At temperature between 600°C and 700°C, the fatigue life is similar at a given strain from this investigation.

3.2. Fatigue damage mechanism

Fig. 4 shows the EBSD images from the LCF tested specimen of Sanicro 25 with a strain range of 0.6% at 600 °C, which shows the damage in the specimen during cyclic loading at the temperature. Fig. 4a shows the grain and deformation structures. The misorientations with low angle (<10°) (white lines) or dislocation structures are mainly concentrated at grain boundaries (black lines) and twin boundaries (red lines). Some of twin boundaries are surrounded by amounts of low angle misorientations. This result also shows that the damage in the low fatigue tested specimen are very selective. Fig. 4b shows the strain contour mapping that indicates the comparative strain levels in the material. The red color indicates higher strain than the green color. This analysis further shows that the damage in this LCF tested sample is localized. The area where has high density of low angle misorientations shows high strain concentration that can cause the damage during cyclic loading. As known, the occurrence of plastic deformation in multi-grained material is usually related to the Schmid factor of the grain. The grain that has a higher Schmid factor may undertake a plastic deformation first. Fig. 4a shows the Schmid factor in the grains after the LCF test. However, it can be found in this study that Schmid factor and plastic deformation is not well correlated during cyclic loading. The grain that has a high strain concentration has a small Schmid factor. This is different from that of uniaxial deformation. Cyclic plastic loading that can cause repeated reversed crystal rotations may the change of Schmid factor after each cycle.

Fig. 4. EBSD image from the LCF tested specimen of Sanicro 25 with a strain range of 0.6% at 600°C, 7617 cycles. (a) Grain structure, black line: grain boundary, red lines: S3 twin boundary, white lines: low angle misorientations (<10°), the deeper color (blue): high Schmid factor, (b).

Strain contour mapping, red color shows high strain.

During the electron channel contrast image (ECCI) analysis, three types of interactions that may lead to fatigue damages have been observed. The first is the interaction between slip bands and grain boundaries, the second is the interaction between multi-slip bands at grain boundary, and the third is the interaction between multi-slip bands and twin boundary. These phenomena can be observed in the specimens fatigue-tested at RT and high temperature with high strain amplitude. These observations are consistency with the EBSD results as shown in Fig. 4a and b. Grain and twin boundaries are the main damage locations during LCF test.

Fig. 5 shows the fatigue crack initiation and damage observed in the specimens after the LCF tests. Here the definition of crack initiation is that some small crack has formed. Fig. 5a shows the crack initiation at the grain boundary due to the interaction between multi-slipping bands. The interaction between multi-slip bands causes high stress concentration (light contrast) that leads to the formation of fatigue crack initiation at the grain boundary. A small crack in the slip band can be observed. It can be typical localized deformation as usually observed in LCF

specimens. For the specimen tested at high temperature with small strain amplitude, strain localization in grain can be an important factor for fatigue damage or crack initiation. With a high strain amplitude at high temperature, crack initiation at grain or twin boundaries caused by the interaction between multi-slapping at grain boundary will be dominant again (Fig. 5b).

Fig. 5. Electron channel contrast images; (a). At RT, 0.8% and 5948 cycles, (b). At 650°C, 1.2% and 1571 cycles.

4. Conclusion

A new austenitic stainless steel grade, Sanicro 25 (UNS S31035), has been developed intended for super-heater and reheaters for A-USC. The alloy has good low cycle fatigue properties

The alloy shows monotonic cyclic hardening and serrated stress versus strain hysteresis curves during LCF at high temperatures. Dynamic stain ageing is one of the cyclic hardening mechanisms.

LCF damage in the alloy occurs mainly at grain and twin boundaries by the interaction between multi-slipping and in grains by dislocation slipping.

LCF crack initiation can start at the grain or twin boundary by stress concentration due to the interaction of slip bands, and in the grain by stress localization

5. Acknowledgements

This paper is published by permission of Sandvik Materials Technology. The assistance of ECCI study by Mr Jerry Lindqvist, and supports from Mr Peter Stenvall and Dr Jesper Ederth are gratefully acknowledged.

References

Blum, R., Vanstone, R.W., and Messelier-Gouze, C., 2004, Materials Development for Boilers and Steam Turbines Operating at 700 °C, Proc. 4th

Int. Conf. on Adv. in Mater. Technol. for Fossil Power Plant, 116-124. Chai, G., Bostrom, M., Kjellstrom, P., Forsberg, U., Creep and LCF Behaviors of Newly Developed Advanced Heat Resistant Austenitic

Stainless Steel for A-USC, Procedia Engineering, 55 (2013), 232-239. Chai, G., Nilsson, J.O., Bostrom, M., Hogberg, J., Forsberg U., 2010, Advanced Heat Resistant Austenitic Stainless Steels, Proc. of ICAS 2011, 56-66.

IEA, 2010, 2009 energy statistics, http://www.iea.org/stats, (2010-02-25).

Rautio, R., Bruce, S., 2004, Sandvik Sanicro 25, a new material for ultra supercritical coal fired boilers, Proc. 4th Inter Conf. on Adv. in Mater. Technol. for fossil power plants, 274-284.