tempalloy f2 w ang

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DEVELOPMENT OF 2Cr-Mo-W-Ti-V-B FERRITIC STEELFOR ULTRA SUPER CRITICAL BOILERS

(NKK TEMPALOY F-2W)Akira Tohyama , Yusuke MinamiMaterialsand ProcessingResearchCenter, NKKCORPORATION,Kawasaki,JAPAN

AbstractThis paper deals with newly developed 2Cr-Mo-W-Ti-V-B ferritic heat resistant steel rube(TEMPALOY F-2W) which is used for water wall tubes and the economizer tubes of USCboilers, and for superheater tubes and reheater tubes of SC boilers. This steel is made byadding Mo and Was solution strengthening elements, and V and Ti as precipitationstrengthening elements to a base of 2% Cr steel in order to improve its high temperaturestrength, and one whose matrix is a bainite single phase. The developed steel has superiorcreep rupture strength, its ordinary temperature and high temperature strengths are almost1.5 times those of 2.25Cr-1Mo, and its allowable stress is equal to that of 9% Cr highstrength material, SA213-T91 (9Cr-l Mo-Nb-V). Cost advantages can also be expected byreducing the wall thickness of tubing in boiler design due to its high strength. This newlydeveloped steel is a material usable as for water wall tubes up to the 575t range in futurehigh temperature, high pressure plants.Keywords : ferritic steel, bainite phase, solution strengthening, precipitation strengthening

1 Introduction

Recent boilers for the thennal power generation are designed for higher temperature and pressurethan in the past. Especially enonnous efforts were made to establish the technology for the newnitra super critical power plant all over the world. In order to construct these high efficiencyplants, it is essential to obtain materials with improved high temperature strength, superiorresistance to oxidation, and resistance to high temperature corrosion. It is also important thatthese new material s are easy to weld or process, that they can be used for a long time at hightemperatures without their material properties deteriorating, and that their costs are low enoughfor plant economy.As the temperature rises, the final superheater tubes must be made of material with superior hightemperature strength. As the pressure increases, not only the [mal superheater tubes, but all steeltubes inc1uding the economizer tubes and water wall tubes must provide improved hightemperature strength. While it is possible to increase their strength by making their walls thicker,this causes many design problems such as an increase in the overall plant weight. High strengthmaterials which can be used to design plants with present rube dimensions are required from thelow temperature range up to high temperatures.TEMPALOY F-2W(2Cr-Mo-W-Ti-V-B) steel was considered as a prospective material. In orderto improve the creep rupture strength of 2.25Cr-1Mo steel; the new 2Cr steel has been developed.This report describes the development of this new 2Cr-Mo-W-Ti-V-B steel with high temperaturecharacteristics.


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2 Experimental Procedures and Results2.1 MaterialsChemical compositionof steel samplesare given in Table 1.All of the steelswereprepared fromingots cast in 50kg electric fumace and hot rolled to plates with a thickness of 14mm afterheating to 1200"C.Final heat treatmentsare the nonnalizing (1O50"Cx30min,ir cooling) and thetempering (780"Cx1hr,air cooling).The normalizing temperatureis high for low alloy steel.Thepurposeof this is tomakesolutionenoughbecause of the large amountof additions of V andTi which are carbide forming elements. V and Ti are precipitated as fine carbides duringtempering. Steel samplesWA-WDwere investigated for the effectof W content, steel samplesVA-VD for the effect of V content, steel samples TA-TD for the effect of Ti content, steelsamples NA-ND for the effect of Nb content, and steel samples BA-BD for the effect of Bcontent.

2.2 MicrostructureTypical microstructuresof steel samples are shown in Photograph 1. A basie structure of thesesteels is a bainite singlephase, and the grain size of these steelsis ASTMNo.8.5-9.5and a finecomparatively grain in the majorityof materials. The crystal grain grows as Ti decreases, it isASTMNoA in the Ti-freematerial.Moreover,a fine grain of ASTMNo.8.5-9.5and the coarsegrain of ASTMNo.3 levelmightexist togetherby the Nb addition.

_--_n - - -.- ------ ..u --------- -J --.-- ---._- __on_. ,------ -.,Steel C Si Mn Cr Mo W V TI Nb BWA 0.10 0.50 0.50 2.02 0.51


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2.3 TensileTestResultsThe results of tensile test at room temperature and 550"Care shown in Figure 1. These dataindicate that both W and V has the effect to improve tensile strengthby the additionof 0.2%ormore.Moreover, there is a slightincrease in tensile strength for Ti contentsup to 0.15% but witha tendency to decrease with further increasing Ti content. B is also effective in tensile strengthimprovement, and the addition of 0.002% is effective . On the other hand, Nb decreasestensile strengthby rising of the content.

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2.4 CharpyImpact TestResultsThe results of charpy impact test at Ot are shown in Figure 2. The specimen used is ns NoAtype (lOxlOx55mm), and the V notch depthof the direction ofthiclcnessof the plate is2mm.W,V,and Ti indicate in any case an excellent value hardly exerting the effect on the absorbedenergy in the range of the component examined at this time. On the other hand, Nb and Bdecrease the absorbedenergy respectively by the addition of 0.1% or more and the addition of0.004%ormore.

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lai O'CI

0.8.2 0.4 0.6W content (wt%)a) Effect of W

400.'J"-"gM


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2.5 CreepRuptureTestResulteThe results of creep rupture test at 650'C-147MPaand 600'C-196MPaare shown in Figure 3. Theaddition of TI, W of 0.2% or more, V of 0.05% or more and B of 0.004% are effective to theimprovementof the creep rupture strength .The additionof Nb of 0.1%or less is effective to theimprovement of the creep rupture strength,but the creep rupture time is shortened and the creeprupture strength is decreasedby increasingtheNb contentbeyond0.1%.

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10.0 0.6.4 0.8.2. W content (wt%)a) Effect ofW

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e) Effect ofBB content (wt%)

Figure 3 Effectof alloyelementon the creep rupturestrength

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3 Manufacture of Steel Boiler Tubes and High Temperature Characteristics3.1 Manujactureoj SteelBaUerTubesUsing a five ton vacuummelting fumace, one heat of this new developed steel was melted. Thesteel ingot obtained from the five ton fumace was forged and machined into round billets, hotroIledto manufacturethe boilertubeswith an OD of 108mmand wall thickness of 13mm. Thetubes were heat treated by normalizing at the temperature of 1O50'Cand tempering at thetemperatureof 780'C. The chemicalcompositionof the tubematerialis given in Table 2. W andMo are contained respectively by about 0.3 and 0.6% as solid solution strengthening and aprecipitation strengthening element based on Cr of 2%, and V and Ti are added as carbideformingelements 0.37 and 0.14%.For the investigation of properties,tests were carried out onsamples taken from rube materials. Tensile tests were carried out at room temperature andelevatedtemperaturesup to 650'C.Creep rupture testswere carriedout at 500,550,600and 650'C.

3.2 Microstructure and High Temperature CharacteristicsMicrostructure of rube material is shown in Photograph 2. This photograph shows the structure ofthe rolling direction etched with nital. The structure is a bainite single phase, and the grain size iscomparatively fine with ASTM No.8.5-9.5.1.\~ "-?d~, tO,"~>:' ."r- ",;'00' -,It;!, 'ooo:..~j;!-;;o'\ ~I"'~o~-" ~~ -~ ,':;:"'~\ '.Y:! '"o;~eo /co~ "O,' .

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Figure5 Charpyimpactproperties

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1OO 200 300 400 500 600 700 800(212) (392) (572) (752) (932) (1112) (1292)( 1472)

Terr,perature "C ("F)

The creep rupture test results are shown in Figure 6. The results of tests conducted on creeprupture strength arranged as Larson-Miller parameters (constant C= 25) are given in Figure 7.The creep rupture strength of this steel is high from 500'C to 600'C, and it decreases rapidly for along time at 650'C.

---as~::g 300-OJ~ 100~.....UJ 60

Tensile characteristicsat room and high temperatures

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104(hr)

Figure 6 Creep rupture strength

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Figure 7 Creep rupture strength(Larson-Miller Plot)

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Allowablestresses of this steel provisionally calculated are shown in Figure 8 as comparedwithSA213-T22(2.25Cr-lMo) and SA213-T91(9Cr-lMo-Nb-V). The allowable stress of thisdevelopedsteel is higherthan that of T91 at temperaturesup to 5751: and is significantlyhigherthan thatof T22. 200-.!:$p-!~-"

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SA213-T22

200 400 600 800Temperature CC)

Figure8 Allowablestressesof eachgrade steel

4 DiscussionThe effect of the small amount of the alloying elements on the creep rupture strength of thelow Cr ferritic heat resistant steel was investigated. Using the creep ruptured specimen of the rubematerial (550'C-245MPa-12338hr ruptured specimen), the precipitates were extracted and the

residue was analyzed. Identification results by X-ray diffraction of extracted residue are shown inFigure 9. ~C, M23C6and V4C3are clearly identified and the precipitation of (Ti,V)C is alsonoted. The state of precipitates was observed by using TEM(Transmission Electron Microscopy).The TEM observation results are shown in Photographs 3 and 4. The grain boundary is mainlycomposed of ~C and M23C6precipitation. On the other hand, precipitation of [me V4C3and(Ti,V)C are present in the grains. Based on the state of precipitation and the effect of the alloyelements on the creep rupture strength which was mentioned above, it is considered that fineprecipitation of these carbides (V4C3and (Ti,V)C) in the newly developed steel contribute to highcreeprupturestrength. ,.:? .;:.~1 o

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28 (degree)Figure 9 Identification results of extracted residue


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tempalloy f2 w ang

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