Review History of Power Plants and Progress in Heat

1. Introduction

Energy conservation and environmental protection areregarded worldwide as highly important issues. Power plantdesign has sought lower fuel costs and CO2 emissionsthrough further improvements in efficiency by elevatingsteam conditions to even higher ranges of pressure and tem-perature. The development of the modern ultra-supercriticalpressure power plant began in the early 1980s, and theworld’s first swing-load ultra-supercritical pressure powerplant with conditions of 31 MPa and 566°C started com-mercial operation in Japan in 1989. Subsequently, powerplants with steam temperatures ranging from 593 to 610°Chave been successively built, and a study has nearly beencompleted for implementation of a 630°C class using ferrit-ic steels. For heat resistant steels used for high temperaturecomponents in power plants, good mechanical properties,corrosion resistance and fabricability are generally re-quired, and creep strength in particular is the most impor-tant property for high pressure and high temperature appli-cations. This has led to ongoing research activities placingemphasis on the improvement of creep strength in alloy de-velopment.

2. History of Power Plants

2.1. Progressive Increases in Steam Pressure andTemperature

The thermal efficiency of fossil-fired thermal powerplants can be raised by reducing exhaust heat and heattransfer losses. The limit has virtually been reached for re-

ducing exhaust heat losses, which are mainly through thecondensers for cooling turbine exhaust and the cooling ofboiler exhaust. Heat transfer losses, however, can be re-duced by raising the pressure and temperature of the steam,but the extent to which this can be done is greatly influ-enced by the materials used.

Figure 11) shows how steam pressures and temperatureshave risen over the years in the US. In the 1920s, the use ofcarbon steel imposed limits on steam pressure and tempera-ture of 4 MPa and 370°C, respectively. After that, the use ofMo steel raised the operating pressure to 10 MPa and thetemperature to 480°C. Further, the development of CrMosteel enabled a plant with operating pressure and tempera-ture of 17 MPa and 566°C to be built in the 1950s. From

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Review

History of Power Plants and Progress in Heat Resistant Steels

Fujimitsu MASUYAMA

Nagasaki Research and Development Center, Mitsubishi Heavy Industries, Ltd., Fukahori-machi, Nagasaki 851-0392 Japan.

(Received on November 14, 2000; accepted in final form on December 22, 2000)

During the last fifty years steam pressure and temperature in fossil-fired power plants have been continu-ously raised to improve thermal efficiency. Recent efforts for raising steam conditions are in response tothe social demand for environmental protection as well as energy conservation concerns. Today the steamtemperature of 600°C for modern power plants equipped with swing load or sliding pressure demand func-tions has already been realized, and a goal for the future is the 630°C to 650°C class with ferritic steels.However the 600°C to 630°C class is possible for current construction, based on already developed materi-als that include ferritic steels for pipework and rotors. Numerous studies on heat resistant steels activelyconducted since the early 1970s have allowed great progress in both 9–12% Cr steels and austenitic steels.This paper presents a historical view of developments in steam pressure and temperature of fossil-firedpower plants and alloy design for heat resistant steels in the 20th century, particularly over the last severaldecades, as well as a survey of the current status of steel development for power plants, mainly with re-gard to creep strengthening and enhancement of corrosion resistance.

KEY WORDS: power plant; boiler; turbine; heat resistant steel; alloy design; ferrite; austenite; creepstrength.

Fig. 1. Historical development of fossil-fired turbine-generators.

the late 1950s to around 1965, larger capacity power plantswere developed in response to increased demand for elec-tricity, and the operating pressures and temperatures bothrose steadily. In 1957, the Philo No. 6 unit (125 MW) start-ed operation at a steam pressure of 31MPa and temperatureof 621°C, and in 1960 the Eddystone No. 1 unit (325 MW),began operating at the highest steam pressure (34 MPa) andtemperature (649°C) thus far achieved. Similarly in Europelarge power plants with advanced steam cycles, the Hülsunit (85 MW, 29 MPa, 600°C) in Germany and theDrakelow No.12 unit (375 MW, 24 MPa, 593°C) in the UKwere ordered in the years through 1960.

However, construction costs increased due to the upgrad-ing of materials needed to raise operating efficiency. Thiscaused a rise in generating costs which resulted in thesteam pressure and temperature being lowered again inmost plants to 24 MPa and 538°C.

In case of Japan, as shown in Fig. 2, steam pressure andtemperature rose sharply after 1950. Meanwhile, thermalpower generation overtook hydroelectric generation to be-come the leading source of energy by 1960, although stillusing domestic coal as the primary fuel. However, contem-porary thermal efficiency was only about 30%, rather lowerthan the figures of approximately 40% that were deliveredby subsequent supercritical pressure (24 MPa, 538°C)plants. Because petroleum, which began to be imported intoJapan in large quantities from around 1965, was found to bemore cost effective, thermal power plants that had beenusing mainly Japanese domestic coal were converted for thefiring of imported heavy crude oil. These plants were againconverted in the wake of the oil crises of 1973 and 1978,this time for the use of imported coals.

From around 1975, frequent starts and stops of fossil-fired power plants have been increasingly required for loadadjustments, accompanied by the expansion of nuclearpower generation and changes in patterns of electricity de-mand. Steam conditions had been fixed at combinations of24 MPa and 538°C or 19 MPa and 566°C, with no addition-al elevation, despite progressive enlargement of plant scaleover the 20 year period from the late 1960s. The oil crisestriggered a call for further elevation of steam conditions,however, leading to the appearance of the first ultra-super-critical sliding pressure plant (31 MPa, 566°C) in 1989. Inthis plant, the steam pressure and temperature were in-creased for the first time in more than 20 years. Conditionsfor subsequent new plants have been successively raised be-yond conventional limitations, and current temperatures are593 or 600°C at pressures of 24 MPa, while the use of630°C is being studied for the next stage of advance. Of the34 units with operation starts scheduled from 1990 through2000,2) 17 units feature a temperature of at least 593°C formain and/or reheat steam, with the majority of these ad-vanced units commencing operations from 1997 onwards.Figure 33) shows the relationship among steam pressure,temperature and thermal efficiency. It is clear from the fig-ure that raising the steam pressure and temperature reducesthe turbine cycle heat consumption, thereby raising turbineefficiency.

2.2. Research and Development for Advanced SteamCycle Power Plants

Research and development work relating to the advancedsteam cycle and/or ultra-supercritical pressure power plantshas been in progress in Japan, the US and Europe.

Figure 44) shows an overview of the international re-search and development projects on advanced steam cyclepower plants in these three areas of the world. These pro-jects were initiated in the early 1980s, and in the last twodecades numerous heat resistant steels have been developedto achieve ultra-supercritical pressure steam conditions orsteam temperature of 630°C with ferritic 9–12% Cr steels.

Ultra-supercritical pressure refers to steam pressure ex-ceeding conventional supercritical pressure steam condi-tions, i.e., 24 MPa with a superheater outlet temperature of538°C, but can be more accurately defined as representingturbine inlet steam conditions of at least 24 MPa for mainsteam pressure and a temperature of at least 565°C for mainsteam and reheat steam.5) Such elevated steam conditionsincrease plant efficiency and thereby contribute to improvedresource and energy conservation, as well as environmentalprotection. Power plants incorporating these steam condi-tions had already been constructed in the US and Europe bythe late 1950s, as described above.

While the improvements in ultra-supercritical pressurepower plant are not obvious merely from steam conditionsand plant specifications, there is in fact a very major tech-nological difference between modern plants and those of ageneration ago. Specifically, the older plants were designedand constructed as base load plants without the load-adjust-ing functions considered essential for the plants being usedtoday. Thus, elevated pressure and temperature require-ments were formerly met through the use of large quantitiesof austenitic steels (having potential problems in terms of

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Fig. 2. Trends of steam conditions of power plants in Japan.

Fig. 3. Relation between steam conditions and efficiency.

thermal stresses) for heavy, thick-walled components suchas headers, piping, and turbine equipment in the older gen-eration of ultra-supercritical pressure power plants.

For steam power plants with sliding pressure operationcapable of responding to changes in electricity demand, orfor plants undergoing frequent start and stop cycles, it ispreferable to use ferritic steels having smaller coefficientsof thermal expansion for heavy, thick-walled components inorder to reduce thermal stresses. Accordingly, the heat re-sistance capability of ferritic steels is the major determinantof steam conditions. Recent progress in the development offerritic heat resistant steels has led to gradual elevation ofsteam conditions, up to 630°C presently.

In Japan, research into the use of the ultra-supercriticalpressure for fossil-fired power plants was initiated in orderto obtain data on practical material properties and reliabili-ty of the heat resistant steels through field tests in utilitypower plants. This was because there was a background inJapan of existing developments in 9–12% Cr steels andaustenitic steels, which have excellent creep strength andcorrosion resistance for their costs. In Phase 1 of the devel-opment project for the ultra-supercritical pressure powerplant, initiated by the Ministry of International Trade andIndustry (MITI) and conducted by the Electric PowerDevelopment Co., Ltd. (EPDC) during the period 1981–1993, the boiler and turbine element tests and the fielddemonstration tests were carried out at the temperatures of593°C and 649°C. The Phase 1 program targeted the devel-opment of double reheat steam conditions of 31.4 MPa and593°C/593°C/593°C with the use of ferritic 9–12% Crsteels, and 34.3 MPa and 649°C/649°C/649°C withaustenitic steels.2) In 1994 the Phase 2 program was startedfor the development of single reheat steam conditions of 30MPa and 630°C/630°C with ferritic steels. Both programshave been conducted basically by using existing newly de-veloped heat resistant steels. Concurrently, the NationalResearch Institute for Metals (NRIM) has been conductinga research and development project since 1997 aimed atferritic heat resistant steels to be applicable for 650°C classsteam conditions.

In the US, a large scale and comprehensive developmen-tal project was started by the Electric Power ResearchInstitute (EPRI) in 1986. Boiler and turbine manufacturersin the US, Japan and Europe participated in this project,and the research was conducted worldwide over a period of

8 years. The research results from this project have beenpresented at several international congresses sponsored byEPRI from 1986 onward. Although some of the tasks envi-sioned for the EPRI Project (RP 1403) were dropped or de-layed, extensive tasks covering the aspects of both designand materials studies were conducted. Very recently (start-ing in 2000) the US Department of Energy has providedbacking for the first industry-led design and engineeringprojects, known as the Vision 21 program.6) The overall aimof this effort is to develop the critical building blocks forfossil fuel plants that can produce power, fuels, and chemi-cals at high efficiency, and with virtually no emissions. Thisprogram includes the needs of development of heat resis-tant materials for service in components operated underhigh stress and high temperature conditions.

In Europe, research was started in cooperation amongEuropean countries in 1983. The project in Europe during1983–1997 was named COST 501 (COST: Cooperation inScience and Technology), and covered an extensive rangeof research tasks.7) Due to some differences in design con-cepts among Europe, Japan and the US, the project featureddevelopments somewhat unlike those in Japan and the US.Because both boilers and turbines were directed towards theuse of even stronger steels, for example, development offerritic steels such as high N steels and B-containing steelswere included in the project, and fabrication of the headerand the header nozzle stub as an integral component usingpowder metallurgy was also attempted. COST 522 was ini-tiated in 1998, aiming at steam parameters for ultra-super-critical pressure power plants of 29.4 MPa and 620°C/650°C with ferritic steels, and further efficiency improve-ments are targeted by the European research and develop-ment project known as Thermie Advanced (700°C) PFPower Plant. Steam temperature of up to around 700°C andefficiency of around 55% are the project goals, and this ispartly based on the use of Ni-based super alloys in the mostseverely exposed components. The project aims to demon-strate such a power plant within the next two decades.4)

2.3. Modern Ultra-supercritical Pressure Power Plantsand the Future

Modern ultra-supercritical pressure power plants are al-ready in service and under construction in Japan, Denmarkand Germany. Table 13) lists the ultra-supercritical pressurepower plants in Japan with steam parameters in the range of


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Fig. 4. International research and devel-opment projects on advancedsteam power plants.

24–25 MPa and 31 MPa in pressure, and of 538°C to 600°Cin superheater steam temperature, including in-service,under construction, and designed plants. Table 24) showsplants in service or under construction commissioned dur-ing 1997 to 2002, indicating the materials used for the boil-er superheater, steam line pipe work and turbine rotor, aswell as information about power plant design parameters. Inthese modern ultra-supercritical pressure power plants,newly developed advanced steels (discussed later in thispaper) have extensively been used.

Figure 5 shows steam parameter plots for ultra-supercrit-ical pressure power plants at past, present and future pointson a graph of pressure versus temperature. Time-wise ser-vice temperature ranges are also indicated on this graph forferritic and austenitic steels for heavy section componentssuch as steam line pipe and turbine rotors. The upper tem-perature limit for ferritic steels has thus far risen fromaround 560°C to 630°C, and will increase to 650°C in thefuture.

3. Problems with Materials Arising from Ultra-super-critical Pressure Steam Conditions

3.1. High Temperature Strength

Allowable stress is a good representation of the high tem-perature strength characteristics of heat resistant steels, andis often determined by creep rupture strength under actualoperating conditions. In order to improve the reliability ofhigh temperature components, it is therefore necessary toascertain creep rupture strength up to 100 000 h (the basisfor fixing allowable stress), or to make an accurate estimateof this, and to fully appreciate the relationship between thechanges in creep rupture strength and structures over long

periods of time.Both high temperature strength and economy must be

considered in the selection of materials. In general, highercost materials have greater high temperature strength.Figure 68) shows the relationship between the allowabletemperature at an allowable stress of 49 MPa and the rela-tive material cost for various boiler tube steels, includingnewly developed ones. The relative material cost has beendetermined based on a steel tube of normal dimensions(D/T^2.5; T5tube thickness, D5tube diameter), and an al-lowable stress of 49 MPa has been chosen in considerationof the prevailing steam pressures in ultra-supercritical pres-


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Table 1. Ultra supercritical pressure power plants in japan.

Table 2. Ultra supercritical pressure power plants in service or under construction.

Fig. 5. Steam parameter plots for ultra supercritical pressurepower plants.

Fig. 6. Relation between allowable metal temperature at 49 MPaof allowable stress and relative material cost.

sure power plants. Although the standard and newly devel-oped steels cover a wide scatter band, the figure confirmsthat the allowable temperature increases with the materialcost for the same allowable stress conditions. For eachgroup of materials, i.e., those with similar relative costs, itis necessary to raise the high temperature strength so thatthey fall as near as possible to the bottom of the band.

3.2. High Temperature Corrosion

High temperature corrosion is a major factor affectingthe life of superheater tubes, and the corrosion rate increas-es as the temperature goes up. In general, increasing thechromium content makes materials more corrosion resis-tant, and corrosion resistance goes up dramatically whenthe chromium content exceeds 20%.

Coal is one of the main fuels for ultra-supercritical pres-sure power plants. The corrosion caused by coal ash is quitedifferent from that caused by other fuels, as it is highly dependent on the amount of SO2 in flue gas and on the amount of Na2SO4 and K2SO4 in the ash. When largequantities of these substances are present, Na3Fe(SO4)3,K3Fe(SO4)3 and other basic iron sulfates form on the sur-face of the tube, giving rise to severe corrosion. Ash is car-ried upwards with the flue gas, and corrosion occurs whereit accumulates on tube surfaces. It is most severe on sur-faces at an angle of 45° to the upward flow of flue gas.Since basic iron sulfates break up or sublimate at around750°C, the highest point on a bell curve of the corrosionrate against temperature would be 700°C, as shown in Fig.7.9)

As the corrosion resistance of boiler tube materials isgreatly affected by the amount of SO2 in flue gas and theamount of Na2SO4 and K2SO4 in the ash, it is also neces-sary to evaluate different types of coal, particularly in termsof sulfur content. Corrosion has been controlled by using amixture of various types of coal to reduce the sulfur contentto less than 2%. The need to reduce the S content to controlair pollution has also helped to reduce corrosion caused bycoal combustion products, and this latter issue is almost un-heard of in modern coal-fired boilers.

High temperature corrosion due to coal ash is strongly

associated with the chemical composition of the ash, ascoal contains different types of compounds which acceler-ate or inhibit corrosion. A corrosion index taking into ac-count the degree of influence of the respective compoundshas thus been proposed.10) Figure 811) has been preparedbased on this index to obtain the corrosion rate of variousaustenitic alloys. It is important to know the chemical com-position of the ash for evaluation of corrosion resistance ofalloys to be investigated.

3.3. Steam Oxidation

Problems due to steam oxidation include a) creep ruptureresulting from overheating caused by tube plugging, whichis due in turn to exfoliation and buildup of formed scale,and b) solid-particle erosion of turbine components causedby exfoliated scale. Examination of scale from austeniticsteels shows that the outer layer of scale, Fe3O4, is verylikely to exfoliate, whereas the inner layer is a tightlyformed spinal oxide composed primarily of Cr and Niwhich never exfoliates from the tube surface. In scalewhich has grown beyond a certain thickness, the outer layerexfoliates due to the difference in thermal expansion be-tween the tube material and the scale during start and stopof the boiler. Various studies have been conducted with theaim of preventing this, and it is now known that increasingthe Cr content in excess of about 20% is effective in in-hibiting growth of steam oxide scale. Treatment of the innersurface of the tube such as chrome plating and chromizingis also useful. Meanwhile, as a protective measure em-ployed for practical purposes, fine-grained TP347HFGsteels or TP321H with a fine-grained inside surface areused, taking advantage of the fact that the finer the grainsize of stainless steel, the smaller the scale formation. Steeltubes with shot-blasted internal surfaces are also used,given the fact that the cold-worked layer tends to inhibitscale formation. Fig. 912) shows changes in scale thickness-es of austenitic steels in utility boilers for long durations oftens of thousands of hours. Because the linear slope in bothlogarithmic scales is 1/2, it is found that scale growth clear-ly follows the parabolic law. Also, exfoliation of the scaleoccurred after growth to a thickness of between 100 mm


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Fig. 7. Relationship between corrosion rate ofaustenitic alloys and test temperature. Fig. 8. Estimation of corrosion rate in coal ash.

and 200 mm, while the growth is much slower and there isno exfoliation in fine-grained steels or in high-Cr contentsteels even after long periods of service. Also, the growthrate of scale in steels of ASTM No. 8 and finer is equivalentto that in high-Cr steels, with the grain size providing suffi-cient protection against steam oxidation problems.

3.4. Thermal Fatigue

According to the failure experiences13) in the EddystoneNo. 1 unit which started operation in 1960, thermal fatigueand creep fatigue caused substantial damage to the header,main steam pipes and valves, which were mainly made ofaustenitic TP316 steel because of the high steam pressure(34 MPa) and temperature (649°C). The low thermal con-ductivity of this steel was one reason for the damage, be-cause large thermal stresses soon arose when the plantstarted and stopped, even given base load operation. Forthis reason, and because of frequent start and stop operationof recent power plants, ferritic steels must be employedeven in temperature ranges where austenitic steels were for-merly used.

3.5. Turbine Materials Problems

Until now, most conventional large capacity thermalpower plants have been operated at a pressure of 24 MPaand temperature of 538°C. CrMoV steel has been the majormaterial used for turbine components of such plants, aswell as in plants operating at temperatures of up to 566°C.12% Cr steel is also used for high pressure rotors. However,it is difficult to use these conventional materials for ultra-supercritical pressure turbines operating at temperaturesabove 593°C, and it is accordingly necessary to usestronger steels because of the extra centrifugal stress in therotor, blades, and other rotating parts, and because of thehigher internal pressure stresses in the casing and other sta-tionary parts. And, as mentioned above, ultra-supercriticalpressure power plants operated today are started andstopped frequently to adjust to changes in electricity de-mand. Therefore, resistivity to the thermal fatigue andcreep fatigue associated with the repeated occurrence oflarge thermal stresses is the most important material prop-erty. To reduce thermal stresses it is desirable to use materi-

als with a high conductivity and a low coefficient of ther-mal expansion. A low coefficient of thermal expansion willalso help to reduce the difference in expansion between ro-tating and stationary parts. From this viewpoint, ferritesteels are better than austenitic steels.

A reheating temperature of over 593°C for the low pres-sure rotor may cause some tempering embrittleness in the3.5 NiCrMoV steels currently being used, and attentionshould be paid to high temperature strength as well astoughness in the material selection for low pressure rotors.Tremendous advances over the last 20–25 years in terms ofrefining techniques for making steel clean have contributedgreatly to the manufacture of low pressure rotors and cas-ings for turbines with high strength and toughness.

In addition to the items described above, high tempera-ture bolts, high pressure turbine blades, valves and manyother turbine parts are being considered as sources of prob-lems in the context of higher operating pressures and tem-peratures.

4. Progress in Heat Resistant Steels

4.1. Kinds of Heat Resistant Steels

Heat resistant steels are extensively used for high tem-perature components, and they cover a broad range of ap-plications. The heat resistant steels for fossil-fired powerplants are most suitable as an example to describe the pro-gressive alloy development in recent years.

Various kinds of heat resistant steels are separately usedaccording to their specific purposes. They are generallyclassified into ferritic steels and austenitic steels, but arefurther sub-divided. Ferritic steels include carbon steels (C–Mn, etc.), low alloy steels (0.5%Mo,2.25%Cr–1%Mo), in-termediate alloy steels (5–10% Cr) and high alloy steels(12% Cr martensitic steels and 12–18% Cr ferritic steels ofthe AISI400 series). Austenitic steels include 18%Cr–8%Nisteels and 25%Cr–20%Ni steels of the AISI300 series,21%Cr–32%Ni steels such as Alloy 800H, and Cr–Mnsteels of the AISI200 series.

Figure 10 shows the chemical compositions of typicalheat resistant steels used under stresses in the Fe–Cr–Niternary phase diagram. Ferritic steels generally do not con-tain Ni, and, because Cr compositions of 2%, 9% and 12%are particularly high in strength, they are widely used.


617 © 2001 ISIJ

Fig. 9. Growth of steam oxidation scale at 620–650°C.

Fig. 10. Compositions of heat resistant steels in Fe–Cr–Niternary phase diagram at 800°C.

Among austenitic steels, materials in commercial use arepositioned along the boundary between the full g phase andthe g phase containing a and/or s . The full g phase steelscontain relatively high Ni content and the high cost ofwhich is typcially offset by high creep strength. In contrast,the g phase steels with a and/or s , though less costly, re-quire some improvement to elevate creep strength.

In order to facilitate a better understanding of the differ-ent types of steels, Figure 1114) shows schematically illus-trated microstructures of ferritic and austenitic heat resis-tant materials. In both cases, material upgrades are illustrat-ed from left to right, and the precipitates appearing thereinchange according to type.

4.2. General Concept of Alloy Design for Heat ResistantSteels

Heat resistant steels for practical application must be de-signed by taking their service conditions and environmentsinto consideration, and by examining their various proper-ties. However, when alloy design is performed based onmodification of existing steels, both oxidation and corro-sion resistance as well as their general material propertiesare expected to be nearly equivalent to those of the originalmaterials. Hence, chemical compositions and heat treat-ment conditions are examined in particular consideration ofcreep strength improvement. Figure 12 shows the conceptof alloy design for heat resistant steels to improve creepstrength through the modification of existing steels. For fer-ritic heat resistant steels, research on 9–12% Cr systemsteels is fairly advanced, and approaches for the improve-ment of creep strength through solution strengthening, pre-cipitation strengthening and microstructural stabilization

have been adopted. These techniques are also applicable forthe modification of Cr–Mo low alloy steels as well. On theother hand, chemical compositions of austenitic steels canbe largely classified into the four categories shown in thefigure, and solution strengthening and precipitationstrengthening are designed specifically for each of thesecategories. 18%Cr–8%Ni steels based on Type 304 steelsinclude Type 316 steels solution-strengthened through theaddition of Mo, as well as Type 321 steels and Type 347steels precipitation-strengthened through the addition of Tior Nb. However, these materials were originally developedfor chemical equipment, placing emphasis on corrosion re-sistance, but were not designed from the standpoint ofcreep strengthening. Accordingly, the further enhancementof precipitation strengthening by means of “under-stabiliz-ing” C and/or composition design for improved creepstrength is used. 15%Cr–15%Ni or 21%Cr–30%Ni steelswith full g phase structure are capable of high creepstrength in as-is condition, although they are costly becauseof their high Ni content. Steels containing Cr of 20% andover are likely to have excellent oxidation and corrosion re-sistance, but a costly Ni content of at least 30% is requiredto maintain a full g structure. Nevertheless, low-cost, high-strength, highly corrosion-resistant austenitic steel can bedesigned by adding N of about 0.2% to reduce the Ni con-tent, and by combining the strengthening mechanisms asdescribed above.

4.3. Alloy Design of the 9–12% Cr Steels

Figure 1315) shows an example of the alloy design ofstrong 12% Cr heat resistant steels. First, taking into con-sideration the practical application of this steel in large-di-ameter and thick-walled pipes such as boiler headers andpiping, the principle properties are oxidation resistance,creep strength, weldability and toughness. If the material isassumed to be used for temperatures up to 650°C, 9% Crwould be insufficient in terms of oxidation resistance, and12% Cr must hence be used. Compositional design for im-proved creep strength is as described above, and, particular-ly in this alloy design, Cu, which induces only a minimaldecline in the Ac1 temperature, is added so as to enablehigh temperature tempering for stabilization of the mi-crostructure while inhibiting formation of d-ferrite result-


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Fig. 11. Schematic illustration of microstructures of ferritic andaustenitic steels.

Fig. 12. General concept of alloy design for heat resistant steels. Fig. 13. Alloy design of 12Cr0.4Mo2WCuVNb steel.

ing from a decrease in the Cr equivalent. Also, this is apreferable compositional design in terms of toughness, suchas reduced Si content, and eventually, the element composi-tion as shown in the figure can be developed.

Much research has been performed since 1960 on the ef-fects of alloying elements on the creep strength of 9–12%Cr steels. Alloying elements for the 9–12% Cr steels areeasy to understand if they are grouped in terms of theirproperties and effects into: 1) Cr; 2) Mo, W, and Re; 3) V,Nb, Ti, and Ta; 4) C and N; 5) B; 6) Si and Mn; and 7) Ni,Cu, and Co.

Cr is the basic alloying element for heat resistant steels,and increased Cr content improves oxidation and corrosionresistance. Although Cr per se does not exhibit a markedeffect on creep strength, high strength is more likely to beobtained near Cr percentages of 2% and 9 through 12% inferritic steels, and strength declines at compositions be-tween the two coverages. The reason for this remains un-known.

Mo, W and Re are all elements useful to solutionstrengthening, and Mo and W have long been used for heatresistant steels. Also, these elements further enhance thecreep strength of heat resistant steels when added in greaterquantities. If their additions exceed a certain limit, however,d-ferrite precipitates and reduces the strength, and precipi-tation of the Laves phase decreases toughness. Furthermore,the effect of W on creep strength is approximately half thatof Mo, and, as described later, the combined addition of Moand W can be effective for strength improvement. Re is re-ported to raise creep strength if added in amount of around0.5%, and this effect is similar to the actions of Mo andW.16)

V, Nb, Ti and Ta all combine with C and/or N to producecarbides, nitrides or carbonitrides, which finely and coher-ently precipitate on the ferritic matrix to exhibit a markedeffect of precipitation strengthening. Among these, V andNb are found to exhibit particularly optimal contents, about0.2% and 0.05% respectively, and, as described later, theeffect of their combined addition can be great. This sug-gests that the formations of precipitates composed by V andNb are associated with each other.

Because C and N are austenite formers, they are useful in

inhibiting d-ferrite. Also, their contents relate to the precip-itation and coarsening of Cr carbides and nitrides. For Cparticularly, if addition exceeds 0.1%, the creep strengthoften declines, and it is believed that there should be an op-timal addition according to the types and contents, etc. ofcarbide-forming elements. N is believed to be an elementessential for raising creep strength in 9% Cr steels.Additions of N are often at about 0.05%, and it is believedthat there should be an optimal content relative to other ni-tride-forming elements such as B.

B improves hardenability and enhances grain boundarystrength, and can greatly improve creep strength. Fur-thermore, a recent publication indicates that it exhibits theeffect of stabilizing carbides by penetrating into M23C6.

17)

With respect to Si and Mn, Si is a ferrite former, whereasMn is an austenite former. These actions are viewed asbeing contradictory to each other, and reduction of the con-tents of both of these elements can improve creep strength.Also, Si works to decrease toughness by promoting theLaves phase, whereas Mn, though useful for toughness im-provement, can impair the high temperature stability of theferrite structure by decreasing the A1 transformation tem-perature in the same manner as Ni.

Ni, Cu and Co are all austenite formers, and if added asalloy elements, they inhibit the formation of d-ferrite by de-creasing the Cr equivalent, but they simultaneously de-crease the A1 transformation temperature. However, level ofthis decrease varies among these elements, and the declineseen with additions of Cu and Co is not greater than thatfound with the addition of Ni. Therefore, if Cu and/or Coare added, the effect of the inhibition of d-ferrite formationcan be expected, making high-temperature tempering possi-ble.

Of the alloy elements in boiler steels as discussed above,the combination effects of Mo versus W and V versus Nbare of interest. As shown in Fig. 14,18) in the combinationof Mo and W, increasing the W ratio while retaining the Moequivalent (Mo10.5W) at 1.5% is most effective for creepstrengthening. Optimal contents of V and Nb may changesomewhat according to temperature, and the combinationof 0.25% and 0.05%, respectively, as already noted, is opti-mized for maximum creep strength.


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Fig. 14. Effect of Mo1W and V1Nb on creep rup-ture strength of 12% Cr steels.

Fig. 16. Effect of W1Mo andCo contents on creeprupture strength of12% Cr turbine steels.

Fig. 15. Effect of Mo and W contents on105 h creep rupture strength at650°C and Charpy absorbed ener-gy at 20°C of 12% Cr turbinesteels (0.13C0.05Si0.5Mn11.2Cr-0.8 Ni0.2V0.05Nb0.05N–Mo–W).

Furthermore, interesting findings on rolls of the alloyingelements W, Mo and Co to 12% Cr steel for turbine rotorshave been reported. Figure 1519) shows the effects of addi-tions of W1Mo and Co on the creep rupture strength of12% Cr steel for turbine rotor. The W1Mo content hasbeen changed by increasing W and decreasing Mo on thebasis of 1%W11%Mo in 12% Cr steel. Time to creep rup-ture is found to be longest with a combination of 1.8% Wand 0.7% Mo, with no decrease in toughness. With regardto Co content, it is found that the maximum time to creeprupture is obtained with a content of 3%, without anymajor effect on toughness.

Figure 1620) shows the effects of Mo and W contents on100 000-h creep rupture strength at 650°C and the Charpyimpact of 12% Cr steels for turbines. It has been empirical-ly shown thus far that creep strength peaks when the Moequivalent (Mo10.5W) is set at 1.5%. It is also known thatincreasing W content induces increased creep strength andreduced ductility and toughness when the chemical compo-sition is determined along the line connecting 3% W and1.5% Mo, i.e., such that the Mo equivalent is 1.5%.

Microstructures of 9–12% Cr steels currently being de-veloped or already commercially available consist of a sin-gle phase of tempered martensite, with some exceptions.High density dislocations exist in this structure, and the dis-location density is principally influenced by the temperingtemperature. It becomes high when the tempering tempera-ture is low, as in the case of turbine rotor steels. Figure 17shows a representative microstructure observed through op-tical microscopy and transmission electron microscopy(TEM) of a typical 9–12% Cr heat resistant steel. The tem-pered martensite is composed of numerous laths, and Crcarbides such as M23C6 precipitate along the lath bound-aries and along the prior-austenite grain boundaries. FineMX carbonitrides of (V, Nb)(C, N) coherently precipitateon the ferrite matrix in laths, and dislocation networks areformed along the lath boundaries or the sub-grain bound-aries. It is considered that the creep strength of 9–12% Crsteels is closely associated with the stabilization of MX car-bonitride and the dislocation structures, and it is inferredthat in W-containing steels, strength rises by suppressingrecovery and recrystallization of martensitic structures dur-ing creep.

Figure 1818) shows the effect of the tempering tempera-ture on long term creep rupture strength of 12% Cr boilersteels as an example to show that creep strength of 9–12%Cr steels is greatly affected by the stability of the mi-crostructure. If the tempering temperature is low, the creeprupture strength in the short term region is typically high,whereas it rapidly decreases in the long term region, andthe strength time to rupture curve is crossed over by that ofhigh temperature tempered steels. This is caused probablybecause, in low temperature tempered steels, recrystalliz-tion from martensite to equi-axed ferrite occurs duringcreep, thereby rapidly dropping the strength, whereas hightemperature tempered steels have microstructures where thedislocation density of martensite is too low in terms of tem-pered conditions to derive recovery and recrystallization.From those microstructural observations, weakening due tothe change in microstructure is less likely to occur duringcreep in the case of unstable structures. The same is known

to be true for 12% Cr turbine steels, and tempering has thusbeen conducted in recent years at a temperature of approxi-mately 700°C, although the temperature for turbine steelswas formerly about 650°C.

4.4. Alloy Design of Austenitic Steels

The features of alloy design of austenitic heat resistantsteels are discussed below. As shown in Fig. 12, “under-sta-bilizing” is one of the techniques for improving the creepstrength of 18%Cr–8%Ni steels. This method enhancescreep strength through improvement of precipitation mor-phology by fixing C in alloys and decreasing carbide form-ing elements such as Ti and Nb, which hinder Cr carbideformation, to the point where their contents are insufficientfor the C fixation. Figure 1921) shows this, and the peakpoint of the creep rupture strength against the ratio of (Ti10.5Nb)/C is at a position far apart from the peak point ofthe conventional Type 321 or Type 347 steels, showing thatreducing additions of Ti and Nb relative to the C contentcan be useful. Figure 2022) shows the effect of the Cu addi-tions on the creep rupture strengths of 18Cr9NiNbN steels.Although the Cu addition does not show a major change upto about 2%, a substantial enhancement in creep strengthby means of Cu addition of about 3% or more can be ob-served. However, because the strength tends to be saturated,and decline in creep rupture ductility can occur when theCu addition exceeds 3%, the addition of Cu at 3% shouldbe suitable.

4.5. Historical Development of Heat Resistant Steels

Figure 2111) shows elevation of the creep rupturestrength of heat resistant steels for boilers, viewed in termsof change in the 105 h creep rupture strength at 600°C, formaterials developed during the 20th century (the steelnames shown in the figure and their compositions are de-scribed later). After World War II, 18%Cr–8%Ni steels al-ready developed in Germany before the war came to beused worldwide, thereby increasing the steam pressure andtemperature of fossil-fired power plants. Also, ultra-super-critical pressure power plants constructed in the latter halfof the 1950s were realized by further applying theseaustenitic steels to thick walled components. For example,TP316H was used for boiler headers and steam piping, and17Cr14NiCuMoNbTi23) and TP321H for superheater and re-heater tubes at Eddystone unit No. 1. Regarding ferritic


© 2001 ISIJ 620

Fig. 17. Typical microstructure of tempered martenstitc 9–12%Cr steel.

steels, low alloy steels or 9–12% Cr steels at about 40 MPaof 105 h creep rupture strength had been used over a longperiod of years, and the problem of cost increases existedbecause, especially in the case of the superheater and thereheater, there was an alloy “gap” between the low alloysteels and the 18%Cr–8%Ni steels, arising as a result oftemperature elevation. Accordingly, development of thehigh strength 9–12% Cr steels was initiated in order to fillthis gap, and the materials for 60 MPa class (first genera-tion) were developed over the period from 1960 to 1970.Further developments were advanced, and creep rupturestrength reached the 100 MPa class (second generation) inthe 1980s, with the 140 MPa class (third generation)achieved in the 1990s. Materials for the 180 MPa class, asthe next generation, are expected to emerge. The outside di-ameter and wall thickness of pipes and tubes can be greatlyreduced through elevation of the creep rupture strength.The thermal stresses can accordingly be reduced, and con-struction will be possible for fossil-fired power plants capa-ble of load sliding operation under steam conditions of fur-ther elevated pressure and temperature.

4.6. Ferritic Boiler Steels

Table 3 shows nominal chemical compositions of ferriticheat resistant steels for boilers, and Fig. 22 shows develop-

ment progress for ferritic steels representing 2% Cr, 9% Crand 12% Cr steels with 105 h creep rupture strength at600°C. The high strength 9–12% Cr steels exhibit relativelygood corrosion resistance and can be used as low-cost alter-natives to 18%Cr–8%Ni steels. Furthermore, in comparisonwith the conventional 2.25%Cr–1%Mo steels, pipe wallthickness can be reduced and oxidation and corrosion resis-tances can also be enhanced. The 9–12% Cr steels devel-oped most recently have strengths between those of2.25%Cr–1%Mo steels and 18%Cr–8%Ni steels, orstrength equal to or higher than strength of the 18%Cr–8%Ni steels. 9Cr2Mo24) is a low carbon 9%Cr–2%Mo steelhas 28 years of service experience in utility plants andabout 2 000 tons of service experience as superheater andreheater tubes and piping since development. The creeprupture strength is between those of 2.25Cr1Mo steels andTP304H, and the material is used especially for reheatertubes as a substitute for 18%Cr–8%Ni steels. LowC9Cr1MoVNb,25) 9Cr2MoVNb26) and 9Cr1MoVNb (ASMET91)27) are modified 9% Cr steels with high temperaturestrength being enhanced by adding carbonitride-forming el-ements such as V and Nb. Of these modified 9% Cr steels,T91 developed in the US has a high allowable stress andhas already been used extensively worldwide not only forsuperheater tubes but also for thick walled components


621 © 2001 ISIJ

Fig. 18. Effect of tempering temperatureon long-term creep rupturestrength of 12Cr1Mo1WVNbsteel.

Fig. 19. Effect of (Ti1Nb)/C ratio on creep rup-ture strength of 18Cr10NiNbTi steel.

Fig. 20. Effect of Cu content oncreep rupture strength of18Cr9NiCuNbN steel.

Table 3. Nominal chemical compositions of ferritic steels for boiler.

Fig. 21. Historical improvement of creeprupture strength of boiler steels.

such as headers and main steam pipes. The emergence ofthis material made it possible to use ferritic steels for fabri-cation of major pressure parts for ultra-supercritical pres-sure power plants using temperatures up to 593°C.Furthermore, 9% Cr steels [9Cr0.5Mo1.8WVNb (ASMET92)28) in the early 1990s, and 9Cr1Mo1WVNb (ASMET911)29) in the late 1990s] having a higher allowable stressthan that of the T91 have been developed. These were ob-tained based on steels with Mo content replaced by additionof W. Mo was decreased to 0.5% and 1.8% of W added toT91 in the case of T92, while 1% W was added to T91 inthe case of T911.

Of 12% Cr steels, 12Cr1MoV (DIN X20CrMoV121)30)

is extensively used for superheater tubes, steam pipes, etc.in Europe, and has extensive service experience. However,because this steel has a carbon content as high as 0.2%,weldability is found to be somewhat poor, and because hightemperature strength is not satisfactorily high, this materialis hardly used in Japan or in the US. Meanwhile, improved12% Cr steels for boilers, 12Cr1Mo1WVNb31) and12Cr0.4Mo2WCuVNb (ASME T122)15) were developed byeliminating the drawbacks of conventional 12% Cr steels.Namely, 12Cr1Mo1WVNb is a 12% Cr steel of dual phasestructures consisting of tempered martensite and d-ferrite,with weldability and creep rupture strength being markedlyimproved. The creep rupture strength of this steel has beenstabilized by using fine VN precipitation strengthening andhigh temperature tempering at above 800°C, and the allow-able stress is somewhat superior to T91. This steel already

has service experience in boilers over a duration of morethan 15 years, and, taking advantage of its excellent corro-sion resistance, large quantities of this steel have been usedfor the superheater tubes of soda recovery boilers exposedto severe high temperature corrosion attack. T122 is a mod-ified type of 12Cr1Mo1WVNb, and can be used for thickwalled components such as large diameter pipes withtoughness being enhanced by eliminating d-ferrite forma-tion.

In ferritic steels, various approaches for further elevationof creep rupture strength are being sought, and as shown inthe development progress of Fig. 22, some steels having 105

h creep rupture strength of 180 MPa at 600°C or 130 MPaat 650°C have already been obtained on a laboratory scale.All of them have chemical compositions with Co added andW content increased. 11Cr2.6W2.5CoVNbBN32) containsCo of 2.5% and W of 2.6%, with B addition slightly in-creased in comparison with conventional steels. Meanwhile,11Cr3W3CoVNbTaNdN33) has 3% of both Co and W, char-acterized by the addition of Ta and Nd. It is reported thatboth Ta and Nd form fine, stabilized nitrides, which can en-hance creep strength in the temperature range of 600°C to650°C.33)

4.7. Ferritic Turbine Steels

Table 4 shows chemical compositions and normalizingand tempering temperature of 9–12% Cr turbine steels in-cluding those developed and reported most recently.Because emphasis is placed on strength at ordinary and in-


© 2001 ISIJ 622

Fig. 22. Development progress of ferritic steels for boiler.

Table 4. Chemical compositions and heat treatment conditions of 9–12% Cr turbine steels.

termediate temperatures for turbine steels, their temperingtemperatures are generally low as compared with boilersteels. Accordingly, the heat treatment conditions as well aschemical compositions can greatly influence the creepstrength of turbine steels. The steels in the table were devel-oped for small components, rotors and casings, and the de-velopment progress is shown in Fig. 23, although theircomparison may not be especially useful. 12Cr0.5MoVNbN34) is an original high strength 12% Cr steel andcontains V, Nb and N. The C contents in turbine steels haverisen to higher values because these steels are not used in components for welded structures except cast steels, and because their strengths at ordinary and intermediatetemperatures must be enhanced. 12Cr0.5MoVNbN is noexception. However, 12Cr0.5MoVNbN contains a relative-ly large amount of Nb, and the composition cannot nec-esarily be said to have been optimized in terms of creepstrength if the normalizing temperature is required to belowered. Developments based on 12Cr0.5MoVNbN are10.5Cr1MoVNbN35) and 10.5Cr1.5MoVNbB,36) with theformer used for turbine rotors and the latter for gas turbinedisks. 10.5Cr1MoVNbN is a steel for rotors with large di-ameters, and because toughness must be secured, the nor-malizing temperature is lowered, thereby decreasing the Nbcontent to match the solubility level of Nb carbide inaustenite. Also, formation of d-ferrite which can adverselyaffect toughness and creep strength in the short-term andhigh stress region is suppressed by decreasing the Cr equiv-alent. 10.5Cr1.5MoVNbB is also a steel for small compo-nents like 12Cr0.5MoVNbN, and because the normalizingtemperature can be increased, Nb content has been set rela-tively high and the strength is substantially increased by ad-dition of B. Chemical compositions employed for these twotypes of steels had a great influence on the development ofultra-supercritical pressure turbine rotor and casing materi-als, initiated early in 1980s. 10.3Cr1.5MoVNbN37) and9.5Cr1MoVNbN38) are typical examples, with creepstrengths increased by slightly decreasing the C content andusing optimized contents of V, Nb and N. Because9.5Cr1MoVNbN is a cast steel which must take weldabilityinto account, C content is lowered, and it can be used as acasing material at 593°C and above, which exceeds the al-lowable service temperature limit for low alloy steels.Although known as 12% Cr steels, the most recent heat re-sistant turbine steels contain Cr of about 10%, as the Crcontent must be decreased in order to reduce the Cr equiva-lent. Furthermore, in the context of application at even

higher temperatures or for further elevation of the strength,CrMo steels appear to have already reached a limit, and de-velopment of steels using W as an alloy element will beneeded. 10.3Cr1.5MoVNbN, 10.3Cr1.2Mo0.3WVNbN39)

and 10Cr1Mo1WVNbN39) were proposed as rotor steels for593°C in the early 1980s, and W was added at 0.3 and1.0%, respectively, for the latter two steels. 10.2Cr0.2-Mo1.8WVNbN40) was developed for potential use as a rotorsteel for 621°C, with Mo decreased to 0.5% and W in-creased to 1.8%, while 11Cr0.2Mo2.5WVNbN41) uses0.2% of Mo with W increased to 2.5%. For all of these ma-terials, the 105 h creep rupture strength at 600°C exceededthe 98 MPa objective.

10Cr1MoWVNbN(501E)42) and 10Cr1MoVNbN(501F)42)

are rotor steels from the aforementioned European COST501 research and development project; W is contained in501E but not in 501F. These rotor steels have been usedEuropean ultra-supercritical pressure power plants recentlyconstructed.

As shown in Fig. 23, Mo or Mo1W added steels withthe Mo equivalent set at about 1.5% are applied for steamtemperature of about 600°C. For the temperatures above600°C, materials with even higher creep strengths are re-quired. Accordingly, research and development efforts havebeen actively pursued in recent years with respect to 12%Cr steels. These are represented by 11Cr2.6W3CoNiVNbB43) and 10Cr0.7Mo1.8W3CoVNbB44) which con-tain further increased W and additions of Co and B, asshown in Table 4 together with their chemical composi-tions. These development efforts are being undertaken inview of accommodation of up to 650°C, and the goal is for105 h creep rupture strength at 650°C in excess of 98 MPa.Such efforts are also common to the development of smallturbine components such as blades and disks. For these ap-plications, 12% Cr steels with compositions similar to thoseof turbine rotor steels such as 11Cr2.6W3CoVNbB43) or10.5Cr2.5W1CoVNbBRe45) have been proposed.

4.8. Austenitic Boiler Steels

Chemical compositions of austenitic heat resistant steelsare given in Table 5, with development progress presentedin Fig. 24. Because 18%Cr–8%Ni steels are used for thehighest temperature boiler components, various improve-ments have been made to enhance corrosion resistancewhile maintaining high creep strength. Furthermore, newsteels with Cr content of 20% or more have been developedfor the purpose of improving creep strength and corrusion


623 © 2001 ISIJ

Fig. 23. Development progress of 9–12% Cr turbine steels.

resistance.18%Cr–8%Ni steels such as TP304H, TP321H, TP316H

and TP347H are still used for fossil-fired power plants op-erating under conventional steam conditions. TP347H,which has the highest allowable stress among these fourtypes of steels, was improved to have a fine-grained struc-ture with grain size No. 8 and finer for steam oxidation re-sistance and creep strengthening, designated asTP347HFG46) in ASME. This steel is very useful for the re-liability improvement of superheater tubes, being applicableto ultra-supercritical pressure power plants up to the 593°Cclass. It is already fully employed for the superheater tubesof a substantial number of ultra-supercritical pressurepower plants in Japan.

Because 17Cr14NiCuMoNbTi and 15Cr10Ni6MnVNbTi47)

are stable austenitic 15%Cr–15%Ni steels, high strengthsare likely to be obtained, and their allowable stresses arevery high. However, they have the disadvantage of inferiorcorrosion resistance due to small amounts of Cr contents.Furthermore, among 18%Cr–8%Ni steels, the allowablestress of 18Cr9NiCuNbN22) is much higher than that of17Cr14NiCuMoNbTi, which is conventionally believed tohave the highest strength at temperatures up to about670°C. Also, 18Cr10NiNbTi48) has allowable stress higherthan those of existing conventional steels. Because both

18Cr9NiCuNbN and 18Cr10NiNbTi have been developedon the basis of Type 304H, their cost effectiveness is excel-lent. They are also advantageous from the standpoint of re-sistance to steam oxidation because they are fine-grainedsteels.

Although 20–25% Cr steels and high Cr-high Ni steelssuch as 30Cr50NiMoTiZr49) and 23Cr43NiWNbTi50) haveexcellent resistances to high temperature corrosion andsteam oxidation as compared with other austenitic steels,their drawback lies in the fact that they are too costly fortheir allowable stresses. However, as shown in Table 5, thematerials developed most recently, particularly 20–25% Crsteels, have excellent high temperature strength, as well asbeing relatively inexpensive. They are practically applied ashigh strength steels taking high temperature corrosion resis-tances into account. Allowable stresses of 25Cr20NiNbN(ASME TP310CbN),51) 20Cr25NiMoNbTi52) and 22Cr15NiNbN53) are far higher than that of Alloy800H, and theycan be used in higher steam conditions and in corrosive en-vironments. Alloy800H has a stable austenite structure, sta-bilized by using a large addition of Ni, but high temperaturestrength was insufficient in relation to cost. Although thereis currently no choice but to use austenitic steels for super-heater and reheater tubes for ultra-supercritical pressureboilers, certain materials have already been developed thatare sufficient to meet the steam conditions of 650°C classboiler superheater and reheater tubes, as indicated previous-ly. Recently, materials taking cost-effectiveness into consid-eration have also been developed. 22.5Cr18.5NiWCuNbN54)

is an example, which uses 0.2% addition of N to stabilizethe austenitic structure based on TP310CbN, in addition toa small amount of Nb addition aimed at precipitationstrengthening by means of “under-stabilizing”. Furthermore,comprehensive strengthening techniques covering a widerange of temperature have been employed by introducingthe concept of Cu addition in 18Cr9NiCuNbN and W addi-tion in 23Cr43NiWNbTi. In the case of this steel, cost-ef-fectiveness has been secured by stabilizing the austeniticstructure through additions of N and Cu, and decreasing theNi addition to 18% by reducing Cr content to a level slight-ly below that of TP310CbN.


© 2001 ISIJ 624

Fig. 24. Development progress of austenitic steels for boiler.

Table 5. Nominal chemical compositions of austenitic steels for boiler.

5. Summary

Steam conditions for power plants have recently beenraised in order to respond to environmental protection andenergy conservation concerns. In Japan and Europe steamconditions with a temperature of 600°C have already beenadopted for newly constructed plants, and, pending orders,a 630°C class is expected to be realized soon. The next goalwill be a 650°C class with ferritic steels. Since 1960 nu-merous studies on heat resistant steels for boiler and turbineapplications have been actively conducted. Among the vari-ous steels developed for advanced steam cycles, majorprogress has been seen in 9–12% Cr steels for boiler pipework and turbine components, and in austenitic steels forsuperheater and reheater tubing. In particular, recently de-veloped ferritic 9–12% Cr steels are stronger than conven-tional austenitic stainless steels. The most recent 9–12% Crsteels have a creep rupture strength of 140 MPa at 600°Cfor 100 000 h. Such enhancement in creep strength isachieved by alloying with tungsten to reduce some of themolybdenum. Strengths of 180 MPa at 600°C or 130 MPaat 650°C are anticipated in the near future for ferritic steels.Significant improvements are also being achieved inaustenitic steels for boiler superheater and reheater tubing,and development is now being oriented towards high-per-formance and low-cost steels capable of superior resistanceto oxidation/corrosion in high temperature environments.

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