Need to evolve materials for steam turbine components

Friction as we know is the resistance to motion of one object moving relative to another. It may not be a fundamental force, like gravity or electromagnetism. Instead, scientists believe it to be the result of the electromagnetic attraction between charged particles in two touching surfaces. For instance, while two rough surfaces (such as sandpaper) rubbing against each other sometimes have more friction, very smoothly polished materials (such as plates of glass) that have been carefully cleaned of all surface particles may actually stick to each other very strongly. Some advantages and disadvantages of friction as observed are;

ADVANTAGES OF FRICTION

Friction plays a vital role in our daily life. Without friction we are handicap.

1. It is becomes difficult to walk on a slippery road due to low friction. When we move on ice, it becomes difficult to walk due to low friction of ice.

2. We cannot fix nail in the wood or wall if there is no friction. It is friction which holds the nail.

3. A horse cannot pull a cart unless friction furnishes him a secure Foothold.

DISADVANTAGES OF FRICTION

Despite the fact that the friction is very important in our daily life, it also has some disadvantages like:

1. The main disadvantage of friction is that it produces heat in various parts of machines. In this way some useful energy is wasted as heat energy.

2. Due to friction we have to exert more power in machines.

3. It opposes the motion.

4. Due to friction, noise is also produced in machines.

5. Due to friction, engines of automobiles consume more fuel which is a money loss.   

Here, in this post I intend to share a unique advantage of friction shared by one of our professors at engineering college way long back in 1994 that was an eye opener for me on understanding its duality when applied in fluid systems viz. hydraulic, pneumatic and steam. In hydraulic and pneumatic systems, you would expect energy losses when the fluid (incompressible/compressible) would rub against each other or against the piping/container in which they would be contained during their transition from one point/elevation to the other point/elevation. However, we find in a steam operated system, the same friction could be understood as one that helps reheat the steam to recover its any energy lost during such a transition.

The post ordinarily is based on the thought I had developed then after our professor had revealed to us on such a duality of friction in mechanical systems. So, any monolithic material that would justify as having a sufficiently low thermal conductivity and also help achieve frictional heat recovery for the steam in the cycle were gradually unlearnt when I happened to be introduced in the latter years via. internet sources to the upcoming metal matrix composites that seemed better to tackle firstly the very high temperature and pressure required in today’s steam operated power plants in addition to achieving a low thermal conductivity and maybe such friction heat recovery. The fact that such knowledge on composites had existed much before but the urgent need to apply was felt to offset the dual problem faced by the world i.e. growing environmental pollution caused by burning coal and dwindling coal reserves has gone a long way to help relearn engineering today.  

ABSTRACT

In a steam power plant the maximum temperature of steam that could be used is fixed from metallurgical considerations of conventional materials. When the maximum temperature Tmax is fixed, as the operating steam pressure at which heat is added in the boiler increases, the mean temperature of heat addition increases.

But when the turbine inlet pressure increases, the ideal expansion line shifts to the left and the T-S diagram shows increased moisture content at the turbine exhaust. If the moisture content of steam in the later stages of the turbine were to be high, the entrained water particles along with the vapour coming out from the nozzles with high velocity would strike the blades and erode their surfaces, as a result of which the longevity of the conventional blade material would decrease. From a consideration of the erosion of the blades in the later stages of the turbine, it would therefore be desirable that most of the turbine expansion should take place in the single phase or vapour region.

Therefore, alternative materials in the form of composites was studied that could not only improve the cycle efficiency but as well ensure long life replacing the most known conventional materials due to their excellent microstructure and properties.

INTRODUCTION

Ref 1. suggests that climate change concerns and the rising price of coal are driving the power generation market towards more efficient cycles than the conventional subcritical steam plant. Steam turbines need to operate at substantially higher pressures and temperatures in the supercritical (SC) and ultra-supercritical (USC) domain. The SC steam plant design is rapidly becoming the preferred option for many owners, given its cost - effective use of coal, an abundant domestic fossil fuel. The three types of plants are defined below:

·        SC is a thermal cycle with a main steam temperature of less than 1,112 °F (600 °C) operating at pressures between 3,208 and 4,000 psia.

·        USC is a thermal cycle with a maximum steam temperature greater than 1,112 °F (600 °C) operating at pressures higher than 4,000 psia.

·        Advanced USC is a thermal cycle with a steam temperature of 1,300 °F (705 °C) or greater.

The Figure 1 from Ref 2. shows the properties of steam on a T-S diagram wherein the principles of thermodynamics states that the cycle efficiency is maximum when the net work done area is maximum.

Now consider the area A-B-C-D-E-F-G-H-I-A where we have highest cycle efficiency due to a larger net work area thereby exhibiting a more efficient cycle compared to A-B-N-M-J-K-H-I-A and A-B-C-D-E-F-M-I-A. But the temperature due to superheating would be higher than Tmax set for the conventional blade materials. Hence the need for new materials that could resist effects of high pressure and temperature of steam viz. corrosion and erosion as in an ultra supercritical case. In the ongoing struggle to balance cost and performance, one ought to consider more affordable materials.

Figure 1 T-S diagram

The post is comprised of two parts:

Literature survey on improving the longevity of blade materials and

Study section with relevant results and discussions on improving the cycle efficiency. 

LITERATURE SURVEY

Conventional Materials

Ref 3. suggests factors for blade failures as Unknown 26%, Stress - Corrosion Cracking 22%, High-Cycle Fatigue 20%, Corrosion-Fatigue Cracking 7%, Temperature Creep Rupture 6%, Low-Cycle Fatigue 5%, Corrosion 4%, Other causes 10%, TOTAL 100%. Besides, many damage mechanisms have been found to operate in combination of poor steam/water chemistry, certain blade design factors that varied from one turbine manufacture to other and system operating parameters.

Ref 4. suggests that the high thermal efficiency of the SC and USC steam power plants cannot be achieved without the use of new alloys with higher creep strength and improved oxidation resistance. Operation above 1,000 °F was possible due to the continuous development effort to improve the 9 – 12 percent ferritic steels, as well as some advanced austenitic alloys. It need be noted that the high temperature strength of ferritic steels was equal to that of the low - end austenitic alloys, but their resistance to oxidation was lower. Attaining USC main steam conditions of about 1,300 ºF and 5,000 psia (345 bar) was possible only by using nickel and chrome-nickel superalloys (e.g., Inconel 740), which exhibit the mechanical properties needed for these high temperatures and pressures but are expensive.

Comparison with Metal Matrix Composites (MMC)

Compared to monolithic metals, MMCs have:

·Higher strength-to-density ratios

·Higher stiffness-to-density ratios

·Better fatigue resistance

·Better elevated temperature properties

*Higher strength

*Lower creep rate

·Lower coefficients of thermal expansion

·Better wear resistance

The advantages of MMCs over polymer matrix composites are:

·Higher temperature capability

·Fire resistance

·Higher transverse stiffness and strength

·No moisture absorption

·Higher electrical and thermal conductivities

·Better radiation resistance

·No outgassing

·Fabricability of whisker and particulate-reinforced MMCs with conventional metalworking equipment.

Some of the disadvantages of MMCs compared to monolithic metals and polymer matrix composites are:

·Higher cost of some material systems

·Relatively immature technology

·Complex fabrication methods for fiber-reinforced systems (except for casting)

·Limited service experience

Ref 5. suggests that the strength of all engineering materials reduces as their temperature increases. Steel was no exception. However, a major advantage of steel was that it could fully recover its strength following a heat, most of the times. During the heat, normally steel absorbs a significant amount of thermal energy. After this exposure to heat, steel returns to a stable condition after cooling to ambient temperature. During this cycle of heating and cooling, the individual steel members may become slightly bent or damaged, generally without affecting the stability of the whole structure. This however would be the main inability to use conventional steels where high temperature and pressure would be involved. On the other hand use of expensive materials for the turbine blades would not be feasible. Therefore the composite use of high temperature alloy steel as the base material and a coating of the requisite alloying elements on the steel were considered suitable.

MCrAlY materials

Ref 6. suggests MCrAlY’s as a family of materials, that have a base metal (M) of superalloy plus coating of chromium, aluminium, yttrium and sometimes other alloying elements and only around 300 micrometers thick.

The M of MCrAlY stands for either Ni or Co or a combination of both (when applied to steels it could also be Fe) (refer Figure 3), depending on the type of superalloy. Co - based appear to have superior resistance to corrosion.

Cr provides hot - corrosion resistance, but the amount that could be added would be limited by the effect it is expected to have on the substrate and the formation of Cr - rich phases in the coating.

Al content would typically be around 10-12 wt%. Since oxidation life would essentially be controlled by the availability of Al, it would be tempting to increase the aluminium content. However, this could result in significant reduction of ductility.

MCrAlY also typically contain 1 wt% yttrium (Y) that enhances adherence of the oxide layer. It was initially thought that Yttrium helped the formation of oxide pegs which helped anchor the oxide layer to the coating. However, it has been shown that there was little if any correlation and it is now believed that the main role of Y was to combine with sulfur and prevent its segregation to the oxide layer, which usually was otherwise detrimental to its adhesion.

Additions of hafnium (Hf) played a similar role

It was found that silicon (Si) significantly improved cyclic oxidation resistance; however it also decreased the melting point of the coating. 5 wt% are enough to lower the melting temperature to about 1140^oC. There was also evidence that it affects phase stability. For cyclic oxidation at 1000^oC, 2.5 wt% was found to be the optimum content. Further additions were detrimental.

Ref 7. tells about MCrAlY’s use as overlay coating as shown in Figure 2 especially for turbine components improving their resistance and provide a longer lifetime for turbine even under hard environmental conditions. High temperature oxidation resistance was achieved by one or more alloy components, which tend to form a dense, stable, slow growing, external oxide layer such as Al2O3.

Figure. 2 Typical MCrAlY coating

 

Figure. 3 Comparative corrosion and oxidation resistance of different bond coats

  

Ref 7. emphasis on Al content in the coating alloy as important, because selective oxidation of the Al occurs only on the surface of alloys with adequate Al contents. However, a high Al content in coatings leads to coating brittleness and a strong tendency to crack. Such cracks could propagate into the substrate material and lead to premature failure of the coated component. Al was found to have proved that a fine grain size had a positive effect on the oxidation behavior of alumina forming alloys.

Ref 7. also emphasized on the addition of rhenium, a rare metal characterized by a very high melting point and high density upto 2 % to a mixture of cobalt, nickel, chromium, aluminium and yttrium that imbues the complex mixture with extraordinary properties. At high temperatures, the mixture forms a barrier of aluminium oxide on the MCrAlY surface that protects turbine blades from superheated steam. The rhenium improves the mechanical properties of the protective coating and simultaneously prevents the aluminium from diffusing into the base material. The coating stops the base material from oxidizing. Additions of rhenium (Re) have been shown to improve isothermal or cyclic oxidation resistance and thermal cycle fatigue. Without Rhenium, the nickel base alloy in the blade would only survive 4,000 hours of operation at maximum operating temperatures. With the coating however, the alloy could hold out against the oxidation for more than 25,000 hours, longer than power plant operators’ demand as a minimum.

STUDY SECTION

1) Study of a ceramic coating on MCrAlY and its use to contain the steam heat within the system:

Ref. 7. also suggests MCrAlY coating could have another function i.e. to serve as an adhesive agent for ceramic thermal insulation layers or thermal barrier coating (TBC). The Figure 4 shows the reduction in temperature due to the thermal insulation layer. The newest thermal insulation coating systems could even accommodate ceramic surface temperatures of up to 1,350 °C.

Figure 4 Thermal conductivity of different layers

Ref 8. suggests Zirconia or zirconium oxide (ZrO₂) an extremely refractory compound of zirconium and oxygen having low thermal conductivity and therefore could be used for outer ceramic zone, for which low thermal conduction was required. In most cases, ZrO2 oxide could be stabilized with Y2O3 (YSZ – yttria stabilized zirconia), a material of one of the lowest values of thermal conductivity that enabled reduced thermal stresses. Usually, thickness of the outer ceramic layer was within a range 250 -375μm.

Ref 9, 10 suggests Zirconia (ZrO2) that usually was partially or fully stabilised by Yttria, magnesia or any other alkaline earth metal oxide, ceria or any other rare earth metal oxide or mixtures of these oxides that could be employed as thermal barrier coating materials. Zirconia could be stabilised with the above noted oxides to inhibit a tetragonal to monoclinic phase transformation at about 1000ºC, which resulted in a volume expansion that could cause spallation. At room temperature, the more stable tetragonal phase is obtained and the undesirable monoclinic phase is minimised if zirconia is stabilised by at least about 6 weight percent yttria. An yttria content of seventeen weight percent or more ensures a fully stable cubic phase. Though thermal conductivity of YSZ decreases with increasing in yttria content, the conventional practice has been to stabilise zirconia with at least six weight percent and more typically to only partially stabilise zirconia with six to eight percent weight yttria, with the understanding that 6 - 8% YSZ thermal barrier coating was more adherent and spall resistant to high temperature thermal cycling than YSZ thermal barrier coating containing greater and lesser amounts of yttria. The Figure 5 shows a typical thermal barrier coating with materials and functions.  

Figure 5 Thermal barrier coating

Contrary to the conventional practice of stabilizing zirconia with at least six weight percent yttria it could be shown that zirconia partially stabilized by less than six weight percent yttria exhibits superior erosion and impact resistance as compared to conventional YSZ. The basis for this improvement has not been well understood, though it is believed that YSZ thermal barrier coating containing less than six weight percent yttria, particularly about four percent yttria, exhibits increased fracture toughness that was responsible for improved erosion and impact resistance.

2) Study of characteristics for friction and wear of the ceramic material to contain the steam heat within the system:

As suggested by Ref 11. the characteristics could be determined by a combination of its bulk microstructure parameters, surface conditions and environmental factors (temperature, atmosphere pressure, etc.) which may be summarised as below:
      .    Effect of microstructure on tribological properties of ceramic:

1) Parameters of microstructure and their influence on friction and wear of ceramics based on

Ø Grain size

Ø Critical flaw size (the size of a flaw that results in rapid fracture)

Ø Homogeneity

2) Manufacturing processes forming microstructure of ceramics viz.

Ø Powder preparation

Ø Compaction (shape forming)

Ø Sintering

·      Effect of surface characteristics on tribological properties of ceramics

     1) Surface characteristics such as

Ø Surface topography

Ø Surface defects

Ø Surface composition and tribochemical reactions

2) Methods of modification of ceramic surfaces by

Ø Plasma oxidizing

Ø Ion nitriding and carburizing

Ø Ion implantation

Ø Laser densification

Ø Electron beam densification

Ø Chemical etching

Ø Sputter etching 

RESULTS AND DISCUSSIONS

1) Binary yttria stabilized zirconia (YSZ) was particularly found to be of wide use because of its low thermal conductivity, high temperature capability including desirable thermal cycle fatigue properties and relative ease of deposition by plasma spraying, flame spraying and physical vapour deposition(PVD) such as electron beam physical vapour deposition(EBPVD) which was found to yield a strain tolerant columnar grain structure as shown in Figure 6 that was able to expand and contract without causing damaging stresses that lead to spallation. Also zirconia partially stabilized with not more than three to four percent yttria (YSZ) and to which one or more additional metal oxides are alloyed increases crystallographic defects and lattice strain energy in the coating grains and optionally form second phases of zirconia and/or compound(s) of zirconia and/or yttria and the additional metal oxide(s).

These second phase precipitates were believed to provide scattering sites for lattice vibrations (phonons) which contribute to the thermal conductivity of the coating. Increasing the crystallographic defects and lattice strain energy within the YSZ lattice significantly reduced the thermal conductivity of the YSZ compared to that obtained with conventional 6 - 8 percent YSZ. This improvement was evident when compared with YSZ coatings having the columnar grain structure whose thermal conductivity was known to increase overtime.

As a result of exhibiting greater resistance to heat transfer YSZ thermal barrier coatings could be heated to higher temperatures and though grain and pore coarsening may occur the thermal barrier coating would maintain a thermal conductivity at a level equal to or lower than that possible with conventional 6 - 8 percent YSZ thermal barrier coatings subjected to identical conditions and also thinner coatings could suffice reducing processing and material costs and promoting component life and thermal cycle efficiency.

Figure 6 Columnar grain structure of TBC

2) The losses in the steam turbine are those associated with frictional effects and heat loss to the surroundings. However, if there was heat loss to the surroundings, enthalpy would decrease, accompanied by a decrease in entropy. It may so happen that the entropy increase due to frictional effects just balances the entropy decrease due to heat loss, with the result that the initial and final entropies of steam in the expansion process would be equal. Thus characteristics for friction and wear could be optimally worked out with the parameters given in the previous section so as to arrive at an entropy increase due to such frictional effects.  

CONCLUSIONS

1) The steam turbine blade component need therefore comprise of a superalloy substrate, a metallic bond coat on a surface of the substrate and a thermal barrier coating having a columnar microstructure, the thermal barrier coating essentially consisting of zirconia partially stabilized by 1 to 3 weight percent yttria and to which is alloyed, in weight percent, one of the following additional metal oxides: 1.0 to 5.0 % calcia, 1.9 to 8.8 % strontia, 2.7 to 12.3 % barium oxide, 5.8 to 22.5 % lanthana and 1.0 to 20 % ceria, the thermal barrier coating having a microstructure containing crystallographic defects formed by the additional metal oxide and which reduces the thermal conductivity of the thermal barrier coating resulting in improving the availability of steam energy. Further study to achieve lower thermal conductivity need to be looked into to prevent degradation in the qualities of the TBC layer.

2) Assuming the heat loss to the surrounding to be negligible (due to vacuum conditions of a condensing steam turbine) the friction here between the coating and the steam could result in reheating the moisture in the steam and convert it to dry steam. Therefore the effect of friction of zirconia ceramic coating for dry steam recovery would need to be studied with a view to improving the cycle efficiency.

REFERENCES

1.  Steam turbine design considerations for supercritical cycles by Justin Zachary, Paul Kochis and Ram Narula presented at the COAL GEN 2007, Betchel Power Corporation at www.betchel.com.

2.  P.K Nag (1990) Engineering Thermodynamics, Tata McGraw-Hill Publishing Company Limited, 295-304.

3.  http://www.massengineers.com/Documents/QASteamturbines.htm

4.  V.D Kodgire & S.V Kodgire (2003) Material Science and Metallurgy, Everest Publishing House, 427-433.

5.  http://nptel.iitm.ac.in/courses/IIT- MADRAS/Design Steel Structures I/ 1_introduction/6_fire_resistance.pdf

6.  http://www.thomas-sourmail.org/coatings/mcraly.html

7.  http://www.siemens.com/innovation/en/index.php

8.  G.Moskal, Thermal barrier coatings: characteristics of microstructure and properties,   generation and directions of development of bond, Journal of Achievements in Materials and Manufacturing Engineering 37/2 (2009), 323-331.

9.  http://news.google.com/patents/about?id=WLwNAAAAEBAJ

10. Yttria-stabilized zirconia with reduced thermal conductivity by Joseph David Rigney et al. at http://news.google.com/patents/about?id=WLwNAAAAEBAJ

11. http://www.substech.com/dokuwiki/doku.php?id=tribology_of_ceramics