Executive summery
High-temperature resources are used for many significant components in a broad range of industries; examples include power generation, chemical and petro-chemical processing, as well as gas turbines. As demand rises for effectiveness and throughout in such industries, the tendency has been for elevated services temperatures as well as pressures. Candidate materials for elevated temperature service need to be technically strong and opposed to oxidation or other many corrosion processes. A range of material classes are normally used in high temperature applications. Alloy and stainless steels, and more composite alloys based on nickel or cobalt, are most commonly used.
Various composite materials, for instance oxide -dispersion strengthened mixtures as well as carbon-carbon composites are currently available for particular high-temperature applications, more often than not, strength is the prime goal. Inter-metallic compounds, such as aluminides and silicides, have extremely high yield strengths and significant oxidation resistance at elevated temperatures (Davis1997). This paper goes into detail explaining the materials that are needed in production activities and power generating activities which will not be easily destroyed by the heat ,by corrosion , by oxidation or by any other way in the hot turbines and furnace .1st of all we are going to have an introduction statement which will introduce as in the report enabling us to have a better understanding of the paper ( report).The main body will follow later and it will consist of discussion of material requirement and the whole discussion will be summarized in a conclusion statement .
. Introduction
This report has been written to present the present condition of materials used in applications concerning high temperatures. The center of attention is on materials used in heat-exchangers as well as combustion chambers whose temperatures are as high as 1000 °C or even higher. In the case of a heat exchanger, high-quality heat transmissions Properties are necessary (Annon 1990).
Furthermore, brilliant resistance to decay, oxidization, fouling, and thermal- shocks and fatigue, etc., and sufficient mechanical properties are essential. This confines the potential candidate materials not including, for instance copper (Cu) as well as aluminum (Al) alloys that are ordinary heat- exchanger materials at temperatures which are low. Other significant issues are accessibility, workability, as well as cost of buying the materials. The rationale of this paper is to decide on the very best candidate materials meant for gas-gas as well as gas-water heat- exchangers, and a burning compartment operated in a bio-fuel indirect-fired micro- turbine power- plant unit which are of dimension of 200 kW or 2 MW There exist a great range of types of heat exchangers.
The flow-configurations as well as surfaces for transferring heat depend entirely on the application. Therefore, the workability and the accessibility of materials in the forms which are desired are important (Ashby 2005) for the majority of the materials, perfunctory properties are dependent on temperatures. Generally, the potency of the materials declines with a rise in temperature. . In structures that carry stress, creep becomes important at high temperatures. Moreover, fast sporadic temperatures encourage thermal stresses which may result in a premature breakdown by fragile fracture otherwise thermal-fatigue mechanisms.
Thermal -stresses are characteristically most definite at the surface of materials.
The resistance of materials to failure induced by thermal stress is increased by high fracture strength as well as thermal conductivity, low modulus of flexibility and coefficient expansion induced by temperatures, and low rate of heat transfer.
Regrettably, materials which characteristically have excellent high temperature
firmness as well as resistance to degradation caused by the environment (ceramics) is most vulnerable to failure induced by thermo-shock (Annon 1990). Thermal- fatigue cracks normally set off inside plastically deformed zone. Consequently; high- yield vigor improves thermal- fatigue resistance. At grand temperatures and environments which are corrosive smash up phenomena like hot corrosion, creep, oxidation as well as thermal fatigue might occur. With this short introduction, let us now look at the materials used for high temperature applications.
Main body.
Steels are the mainly used materials at high temperatures in electricity generation and in chemical manufacturing, for example, in boilers, vessels using pressure, pipes, etc. Their greatest service temperatures depends on the type of the steel, alloying, and stability of the surface Chromium is one of the most important element used in alloying in the production of protective oxide scales used on hi-tech steels. The development of layers made of iron oxide is covered up with high sufficient chromium content. Another element of alloying which is equally important is aluminum that improves the performance of steels at high temperatures. Silicon is used to improve the high temperature resistance to corrosion of high alloy steels, particularly in carburizing conditions.
Carbon and Low Alloy Steels whose use is constrained at high temperatures (above 300-400 °C) because of oxidation, graphitization, as well as creep. Carbon -steels structures three dissimilar layers of oxidation in air, i.e. magnetite (Fe3O4), hematite (Fe2O3), as well as wustite (FeO) which is formed at temperatures above 570 °C, and has a very elevated mobility of ions as well as electrons and, consequently, its protection against oxidation is poor. Alloying by means of chromium, manganese, nickel, and silicon makes oxidation as well as creep resistance of steels better. Particularly, some of the high- temperature Cr-Mo steels that have been made in a way that they can be used at temperatures up to 700 °C.Alloying of Cr prevents the formation of the the layer of wustite and, thus, decreases the rate of oxidation. When the content of Cr goes beyond 12 %, defensive chromium oxide (Cr2O3) coating is created on the exterior and steels are referred to as stainless steels (Gethro 1987).
Austenitic Stainless Steels which basically are Ni alloying decreases the rate of oxidation of Cr steels and therefore changes the BCC (ferrite) crystal structure to the FCC (austenite) structure. Cr-Ni steels contain relatively little cost, fine mechanical properties, as well as moderate resistance to corrosion .Consequently; they are recurrently used in applications of high-temperature with low performance. These stainless steels display advanced strength and creep resistance at higher temperatures than that of non-strengthened stainless steels made up of iron, given that the crystal like structure of austenite is more stable at high temperatures (Anon1990). In addition, the resistance of lattice to the motion of displacement is not that susceptible to temperature in austenite thus diffusion rate is lesser than in the ferrite. The AISI-300 series of stainless steel which is common- AISI 304 in accumulation AISI 316- can be used at temperatures which are up to 900- 950 °C above which the protective Cr2O3 layer starts to decompose (Gethro 1987). The resistance of oxidation of stenitic stainless steels can be made better by mixing them with small quantities of Silicon as well as rare earth metals For instance, AISI 310 and MA252 steels
Are able to be used up to 1150 °C also MA353 steel up to temperatures as high as 1175 °C. The AISI 300 series austenitic stainless -steels are vulnerable to stress- corrosion cracking (SCC) in surroundings containing chloride. Resistance to corrosion and resistance to SCC are improved in the so-called super austenitic classes by mixing with adequate quantity of molybdenum, nitrogen, or Ni.
Martensitic and Ferritic Stainless Steels .Oxidation -resistance of ferrous stainless steels is of better-quality as compared to Cr-Mo steels. AISI 446 steel (27 wt. % Cr) contains equal highest service heat to austenitic MA 353 steel. Nevertheless; ferritic stainless- steels drop their strength when exposed to temperatures above 650 °C (Annon 1990). The strength decrease is connected to the decline in the Peierls stress at high temperatures. The Peierls-stress, which resembles the magnitude of stress necessary for the displacement movement all the way through the crystic lattice, is highly sensitive to temperatures in BCC metals, resembling ferritic stainless- steels.Martensitic as well as ferritic stainless steels encompass enhanced thermal conductivity and lesser coefficients of thermal-expansion than that of austenitic grades. As a result, they are more technically used in thermal-cycling conditions. Nonetheless, ferritic steel grades containing chromium concentration of more than 13 % suffers from 475 °C embritterment those constraints their use at temperatures ranging from 370 to 540 °C. The sternness of the 475 °C embrittlement rises with the increasing chromium content. Martensitic stainless- steels contain higher force than ferritic steel grades and therefore their mechanical characteristics remain moderate to the extend of 540 °C. Resistance to corrosion of martensitic steel grades is lesser as compared to ferritic steel grades.
Duplex Stainless- Steel .This type of stainless steels contains an austenitic ferritic micro-structure. Even though the duplex-stainless steels exhibits a good resistance to oxidation because of their high Cr contents, they mostly suffer from embrittlement of the ferrite- phase if put to temperatures which are above about 350°C, and therefore their use is limited to lower temperatures.
ODS Steels. Perfunctory characteristics of martensitic and ferritic steels at high temperatures are able to be enhanced by dispersion hardening with particles of oxides, like Y2O3 or TiO2. For instance, with yttrium mixed ODS (oxide dispersion strengthened) steels, gives rise to stress of 210 MP a that can be reached by means of 9 wt. % Cr at 700 °C .The service temperature which is maximum depends entirely on the instability of the so called dispersoids at temperatures which are above 1100 °C . Due to the fact that the dispersoids are alloyed mechanically, it is essential to practice a good mixing of the support material and oxide particles to attain material properties which are uniform The resistance to creep of ferritic -ODS steels is fine up to 1100 °C. Habitually the conditions for loading in heat exchangers are organized in a way that the life of the service is rather dogged by oxidation than creep. Oxide- dispersoids also advance the resistance to oxidation by steels because of the early formation of
the protective Cr2O3 coating. The said oxidation resistance of ODS steels could be enhanced with addition of aluminum that enables the configuration of a shielding Al2O3 layer on the surface. Incoloy alloy MA956 is able to be used up to 1300 °C, and as broad section up to 1370 °C. Kanthal APTM steel can be used up to 1250 °C (Gethro 1987). ODS steels are more superior than super alloys with precipitates, since the fine scattering of motionless oxide particles in ODS steels doesn’t roughen with time, as a result keeping the high strength for a longer period of time at temperatures temperatures which are relatively high . Compared to traditional super alloys, ODS steels exhibits a higher resistance to recurring oxidation as well as scale spallation.
Super alloys are used in high -temperature applications that require exceptional creep resistance as well as high temperature force in addition to good resistance to oxidation and surface stability, as in gas turbine blades. Resistance to oxidation is based on Cr or Al mixing which enables the formation of Cr2O3 or Al2O3 layers on to the surface (Fontana 1986). Super alloys are conventionally classified as Fe-, Ni-, as well as Co-based super alloys. In recent times, refractory super alloys have been prepared. These Super alloys have a close-packed FCC crystal structure, which is able to maintain relatively reliable and high tensile, rupture, creep, as well as thermo-mechanical fatigue characteristics to homologous temperatures which are much higher than the ones of the equivalent BCC systems. This is because of the high modulus of the FCC lattice, its numerous slip systems and its low diffusivity for less important elements. Super alloys are mainly strengthened by rainfall of compounds between the metals, assisted by the wide solubility of less important elements. Other intensification mechanisms include solid-solution hardening, carbide rain and grain edge control, directional solidification, as well as generation of single crystal (Fontana 1986).
Nickel-Based Super alloys generally have a better metallurgical stability, are more resistant to creep, oxidation, carburization, as well as halogen attack than austenitic stainless steels (Annon 1990), however, Ni increases susceptibility to sulfidation. Ni-based super alloys can be made stronger by mixing (alloying) them with Al along with Ti which bring to mind precipitation hardening due to logical intermetallic precipitates Ni3(Al,Ti) in the FCC Ni matrix. Other intensification mechanisms are rock-hard solution hardening, carbide rain, and grain boundary control. Oxide dispersion strengthening in Ni-based super alloys is obtained with Y2O3 or ThO2 oxides. These oxide particles even out the protective oxide layer and reduce creep at high temperatures by pinning grain boundaries.
Chromium-Based Super alloys Interest in this type of super alloys as materials of high temperature has recently increased, particularly at temperatures above which Ni-based super alloys will fail (Getro 1987). Benefits of Cr include its high melting point, good resistance to oxidation, low density, as well as high temperature conductivity. Nevertheless, the use of Cr-alloys is constrained due to their high ductile-to-brittle conversion temperature (DBTT) which is normally above room temperature. Due to the contamination by nitrogen, Cr-alloys need surface shield in air at temperatures beyond 1100 °C.
Cobalt-Based Super alloy. In general, the characteristics of nickel alloys exceed those of cobalt alloys because of the lack of an analogous precipitation stiffening mechanism. Nonetheless, cobalt-based super alloys continue
to be used due to definite advantages such as higher melting temperature as well as better resistance to hot corrosion and thermal fatigue (Gethro 1987). They exhibit also better Weld ability than those super alloys based on nickel.
Iron-Based Super alloys Fe-based super alloys are typically modifications of austenitic stainless steels. Improved Mechanical properties at high temperatures are obtained by precipitation hardening because of molybdenum, tungsten, titanium, and niobium alloying.
Conclusions
Requirements for high temperature combustion- chambers as well as heat exchangers include good resistance to oxidation, carburization, hot corrosion, nitridation, thermal and Thermo-mechanical fatigue and finally resistance to creep. Firstly, the mechanical necessities must be met. Secondly, the candidate materials must have adequate corrosion characteristics or the use of coatings is essential.
Suitable structural materials are found among different material groups e.g. stainless steels e. t. c. Austenitic stainless steels in the (353 MA) and (AISI 310) in addition to ferritic stainless steel (AISI 446) have almost equal maximum service temperatures as compared to super alloys. Nicrofer 6025 and Hastelloy X, but the super alloys exhibit a greater creep resistance. Finally, the recommended materials to be purchased are found in the recommendations below.
Recommendations
The four materials that are recommended as being suitable construction materials are described below in a summery form. 353MA refers to austenitic chromium-nickel steel. It is mixed with nitrogen and rare earth metals. The manufacturer of this material is Sandvik AB under license from Avesta Sheffield AB.It has high creep strength, resistance to rust, resistance to combustion gases and good weld ability.
Incoloy alloy MA956 UNS S67956 is an ODS Fe-Cr-Al alloy. MA856 is readily machined. It requires to be handled with diligence in high strain rate forming operations. It is produced by Special Metals Corporation.
Nicrofer 6025 HT refers to a nickel-based super alloy. It has high carbon content. The alloy has additions titanium and zirconium as well as aluminum and yttrium. It can be readily hot- and cold-worked as well as machined.
Haynes 214 is a nickel-based super alloy. It is available in the form of plate, sheet, as well as strip. It is manufactured by Haynes International. More information on these materials can be established in the appendices at the end of this paper (Davis1997).
References
Anon (1990) Elevated-Temperature characters of Ferritic- Steels, In: Metals Handbook, Vol 1, Properties and Selection: Irons, Steels, and High-Performance Alloys, ed : Davis, J.R.10th ed. ASM International, USA. 1063 p. ISBN 0-87170-007-7.
Ashby, M.F. (2005) Materials collection in Mechanical plan. 2nd ed. Butterworth-
Heinemann, Great Britain. 603 p. ISBN 0-7506-6168-2.
Gethro, A.M. (1987) Cobalt-Base metals. In: Super alloys II, eds.: Sims, C.T., Stoloff, N.S., and Hagel, W.C. John Wiley & Sons, New York, USA. 615 p. ISBN 0-471-01147
Davis, J.R., Hampshire, S., and Quinn, G.D. (1997) Structural Ceramics, In: Heat-Resistant Materials, ed., Davis, J.R. 1st ed., ASM International, USA. 591 p. ISBN 0-87170-596-6.
Fontana, M.G. (1986) Corrosion Engineering. 3rd ed. McGraw-Hill, Inc., Mexico. 556 p.ISBN 0-07-021463-8.Garret-Price, B.A., Smith, S.A., Watts, R.L., Knudsen, J.G., 211 p. ISBN 0-412-75010-4.
