CORRELATION BETWEEN HARDNESS AND STRENGTH FOR REFINEMENT OF LIFETIME PREDICTION OF HEAVY GT CASING PARTS. Diego Ugel Ansaldo Energia Switzerland

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1 Proceedings of the 1st Globl Power nd Propulsion Forum GPPF 2017 Jn 16-18, 2014, Zurich, Switzerlnd GPPF CORRELATION BETWEEN HARDNESS AND STRENGTH FOR REFINEMENT OF LIFETIME PREDICTION OF HEAVY GT CASING PARTS Aikterini Psrr Diego Ugel Ansldo Energi Switzerlnd Ansldo Energi Switzerlnd Bden, Switzerlnd Bden, Switzerlnd Andres Bumnn Ansldo Energi Switzerlnd Bden, Switzerlnd ABSTRACT Despite current improvements in mnufcturing nd csting processes, csting defects re most of the times inevitble in the production of hevy gs turbine structurl components. After the repir process hs been crried out, inspections re performed to gurntee the qulity of the produced gs turbine components nd strength mesurements re executed t room temperture on smple specimens to control mteril qulity. In ddition, hrdness mesurements cn be performed in criticl res of possible crck inititions nd over the csting surfce of gs turbine structurl prts, to check microstructure homogeneity. In this pper the empiricl proportionlity between tensile stress nd Brinell hrdness hs been obtined to estimte the locl tensile properties due to post-weld het tretment nd re fterwrds used to evlute lifetime t criticl loctions. Hrdness t 175 loctions hs been mesured nd men vlue hs been used to derive n updted globl yield strength vlue of the mteril. The comprison of the updted yield strength ginst the nominl men vlue hs llowed refinement of the lifetime prediction. Finite Element ssessments of two big csing prts re presented to show pplicbility of hrdness-strength mpping on gs turbine components considering opertionl tempertures. INTRODUCTION Csting mnufcturing process Gs turbine csings hve dimensions up to 5 m length nd up to 6 m dimeter. These structures re mde of steel csting or nodulr cst iron mnufctured in snd csting process nd reching weight fter csting of pproximtely 70 tons. Given the complexity of such components, csting nd subsequent mnufcturing steps hve to be considered erly in the design process, demnding brod knowledge from the designer to combine the functionl needs with the requirements on mnufcturbility of the component. Figure 1 Csted Turbine housing Mteril The csings in the hot prt of gs turbines re generlly produced from steels tht cn be used up to the C rnge. Stg10T is one of the most pplicble steels for gs turbine components which operte t high tempertures to stisfy requirements on lifetime nd deformtion. Due to the composition, this mteril hs dvntges like high creep resistnce, yield stress nd LCF limit. In ddition, it hs low therml expnsion coefficient nd good frcture mechnics properties nd resistnce ginst oxidtion. Tble 1 Stg10T Chemicl Composition (%, nominl) G-X12CrMoWVNbN10-11 C Cr Mo W Mn Ni V Nb N This work is licensed under Cretive Commons Attribution 4.0 Interntionl License Cretive Commons Attribution-NonCommercil-NoDerivtives 4.0 Interntionl License

2 with: R p02 = A HB + B. (1) Rp02: Yield strength HB: Brinell Hrdness A,B: numericl coefficients Similr eqution is derived lso for ultimte stress, s shown in Figure 4. Figure 2 Strength properties ginst temperture for Stg10T Defects on big scle components Due to the complex shpe nd size of the component the csting process must be controlled using technology fetures like gting system, risers nd chills to hve defined flow in the mold nd defined solidifiction. After demolding, the prt gets clened, the technology fetures get removed nd qulity het tretment is performed. For steel cstings it is common to find defects during inspection in the form of gs inclusions, hot nd cold crcks or porosities. These defects re excvted nd repired using repir welding. After stress relive tretment (post-weld het tretment) the prt is redy for the finl inspection, dimension control nd hnded over to the mchining workshop. Figure 4 Liner dependency of tensile proof stress nd ultimte stress on hrdness The squre of the correltion between the response vlues nd the predicted response vlues is 92%. Therefore, the correltion is generlly good nd cn be used for the updte of the strength properties bsed on hrdness mesurements. Additionl 120 hrdness mesurements t severl loctions of the two csing prts hve been tken to check the reprtition nd homogeneity of the mteril fter the het tretment t 730 C for 8h relted to repir process. Figure 3 Csting defects t surfce In fct reduction of strength properties is expected fter prolonged permnence t high temperture, s nticipted by metllurgy theory nd confirmed by prctice. In Figure 5 this effect is shown for Stg10T nd Stg9T. HARDNESS AND STRENGTH MEASUREMENTS Hrdness of steels is the resistnce to surfce indenttion under stndrd test conditions. Both hrdness nd tensile properties re indictors of steel resistnce to plstic deformtion nd consequently they re roughly proportionl. Severl empiricl correltions between hrdness nd ultimte strength for steels re normlly vilble in hndbooks nd cn be used for estimtion of tensile strength of given steel from its hrdness. Tensile proof stress, ultimte stress nd hrdness from 9 specimens out of two csing prts in Stg10T hve been mesured t room temperture. The liner correltion between 0.2% tensile proof stress nd hrdness is expressed by the following eqution: Ultimte strength Holding time t 730 C (h) 0.2% tensile proof stress Figure 5 Strength properties reduction fter het tretment for two cst steels 2

3 Figure 6 presents ll mesured hrdness vlues for both csings prts, using set of stndrd nd comprble loctions. Hrdness vlues rnge between 200HB nd 250HB nd this spred is relted lso to the fct tht one prt underwent more repir effort nd consequent het tretments thn the other. Frequency Histogrm of Hrdness Norml Men StDev N 120 with: n( T) 100 s s e = (2) E ( T ) r( T ) R ( T ) stt r: hrdening fctor [-] n: stress exponent [-] ε : strin mplitude [%] σ : stress mplitude [MP] E stt : sttic E-Modulus [MP] R p02 : yield strength [MP] Ł p02 Bsed on eqution (2), the stress-strin curves t mbient temperture for minimum nd men dt re shown in Figure 8, together with other curves derived using n interpolted vlue for yield stress. ł Hrdness men mteril properties Figure 6 Hrdness mesurements results on two csing prts The grph shown in Figure 7 divides the 120 hrdness vlues into ctegories ccording to mesured prt. The two hlves of the prt more het treted exhibit n verged hrdness vlue which is lower compred to the other two hlves. minimum mteril properties Density Histogrm of Hrdness Norml Lbel Prt1_LH Prt1_UH Prt2_LH Prt2_UH Men StDev N Figure 8 Stress-strin curves t mbient temperture Considering the hrdness-strength correltion shown in Figure 4, men nd minimum vlues of yield strength correspond to defined hrdness vlues. By chnging R p02 (T) in eqution (2) degrdtion of the mteril due to PWHT cn be trnsformed into different stress-strin curves which re between those derived from men nd minimum mteril vlues Hrdness EFFECT OF HARDNESS ON LIFETIME Figure 7 Norml distribution of the hrdness vlues derived from on Minitb softwre EFFECT OF HARDNESS ON STRESS-STRAIN CURVE Rmberg-Osgood eqution is normlly used to model the elstic-plstic behviour of ductile steels. The eqution cn be written for mono-xil pplictions in the form of (2): Lrge mount of LCF dt for Stg10T is vilble, both from in house test results nd from externl public funded progrms. Since most of gs turbine components operte t high temperture, dmge temperture hs to be defined bsed on therml cycle experienced by the structurl prt t different loctions, rnging typiclly between 400 C nd 500 C. Such dmge temperture is used to ccess corresponding Mnson-Coffin curves nd quntify the number of therml cycles to crck initition for given level of elstic-plstic strin. 3

4 For the purpose of the tsk described in this pper, bckwrd ppliction of Neuber hyperbol (3) to elsticplstic strins from the Mnson-Coffin curves llows generting σ-n curves for Stg10T in cse of 400 nd 500 o C (see Figure 9): E with: n ( T ) * n ( ) ( T ) s eq ( ), = T 3 E( T ) 2 ( ) ( T ) s + eq, E ( ) = ( ) s eq, ( T ) s ( T ) Ł 0 ł n ( T ) -1 ( s ) 2 eq, (3) EFFECT OF HARDNESS ON LIFETIME OF CASING PART 1 (INNER CARRIER) FEA clcultions Vne Crriers re criticl components in gs turbine rchitecture, considering their cyclic opertion under high temperture. Despite their design is optimized in such wy to stisfy the LCF requirements, the risk of ftigue filure t some loctions is inevitble t highly cycling opertionl regimes, s shown in Figure 10. nd: ( ) = ( ) ( ) r: cyclic hrdening fctor [-] n: stress exponent [-], : equivlent elstic stress mplitude [MP] σ eq, : equivlent elstic-plstic stress mplitude [MP] υ: Poisson rtio [-] E: sttic E-Modulus [MP] R p02 : Yield strength [MP] In Figure 9, the continuous lines correspond to the men nd minimum LCF dt. Assuming Coffin-Mnson coefficients re not significntly ffected by hrdness vritions, the dshed lines represent the resulting lifetime curve for n rbitrry mesured vlue of hrdness. It hs to be mentioned tht reduction of hrdness leds to decrese in LCF lifetime only becuse of bigger plstic deformtion. Figure 10 Simulted LCF lifetime, Csing Prt 1 The corresponding dmge temperture is globlly pproximtely 450 C. Hrdness Mesurements Out of 60 stndrd hrdness mesurements, men vlue corresponds to yield vlue, which is slightly below the men yield vlue vilble from mteril dtbse. It mtches the fct tht this prt underwent only one PWHT. No loctions hve hrdness vlues lower the vlue corresponding to the minimum yield vilble from mteril dtbse. Summry for Hrdness Prt = Prt1 A nderson-drling N ormlity Test A -Squred 0.59 P-V lue Men StDev 5.66 V rince Skewness Kurtosis N 60 Minimum st Qurtile Figure 9 Effect of dmge temperture nd hrdness on the lifetime Medin rd Qurtile Mximum % C onfidence Intervl for Men % Confidence Intervl for Medin % Confidence Intervls 95% Confidence Intervl for StDev Men Medin Figure 11 Norml distribution considering 60 mesurements, Csing prt

5 Scling the empiricl yield vlue vilble t room temperture for clculted dmge temperture, Rmberg- Osgood nd Neuber equtions llow updting the mx expected lifetime reduction, s described bove. Mximum reduction of lifetime prediction is less thn 20% for ctul elstic stress rnge nd dmge temperture. Figure 14 Simulted dmge temperture, Csing Prt 2 Figure 12 Ftigue curves σ-n for different hrdness vlues EFFECT OF HARDNESS ON LIFETIME OF CASING PART 2 (OUTER CASING) Hrdness Mesurements Out of 60 stndrd hrdness mesurements, men vlue is 211HB, corresponding to yield vlue which is very close to the minimum yield vlue vilble from mteril dtbse. It cn be concluded tht the repeted repir cycles globlly decresed mteril strength cross the prt, resulting in some single loctions below strength specifiction/limits. FEA clcultions Hot outer csings re criticl components in gs turbine rchitecture, considering their cyclic opertion under high temperture, their complex shpe, big size nd the severe therml grdients. Despite their design is optimized in such wy to stisfy the LCF requirements, the risk of ftigue filure t some loctions is inevitble t highly cycling opertionl regimes, s shown in Figure 13. Summry for Hrdness Prt = Prt2 A nderson-drling N ormlity Test A -Squred 1.84 P-V lue < Men StDev 8.02 V rince Skewness Kurtosis N 60 Minimum st Qurtile Medin rd Qurtile Mximum % C onfidence Intervl for Men % Confidence Intervl for Medin 95% Confidence Intervls % Confidence Intervl for StDev Men Medin Figure 15 Norml distribution considering 60 mesurements, csing prt In order to check tht such loctions do not coincide with criticl ones, life limiting res hve been hrdness-tested in detil, see Figure 16. Loctions with hrdness vlues between 200HB nd 210HB re highlighted nd most of these low hrdness loctions re round the cn holes. Figure 13 Simulted LCF lifetime, Csing Prt 2 The corresponding dmge temperture is visulized in Figure 14 nd is lmost equl to 500 o C t the criticl res. Figure 16 Hrdness mesurements for lifetime updte 5

6 The hrdness mesurements hve been used to estimte the tensile strength of the mteril nd provide n updte of the LCF lifetime burners holes re for this prticulr csting prt, tking into ccount ny irregulrity of the mteril. The dmge temperture of the outer csing t cn-hole re hs been ssumed equl to 480 o C s shown in Figure 14. Using the procedure previously described, updted elsticplstic strin mplitudes hve been quntified nd lifetime figures reclculted, to mke sure tht, despite globl mteril degrdtion for this prt, lifetime requirements re still met. Engineering, Volume 528, Issue 9, 15 April 2011, Pges [4] Muecke, R., Bernhrdi, O., A constitutive model for nisotropic mterils bsed on Neuber s rule, Comput. Methods Appl. Mech. Engrg. 192 (2003) [5] Muecke, R., Kiewel, H., Nonlocl Cyclic Life Prediction for Gs Turbine Components With Shrply Notched Geometries, ASME Turboexpo Conference 2007 Figure 17 σ-n curves t 480 C CONCLUSIONS An empiricl liner reltionship between Rp02/Rm nd hrdness hs been derived using dedicted specimens from specific csing prts in the form of Rp02/Rm=A*HB+B Hrdness-strength correltion hs been used to estimte from set of stndrd hrdness mesurements whether the PWHTs hve cused significnt globl reduction of strength properties nd lifetime figures. For the cse of csing prt which underwent severl PWHTs, n dditionl set of hrdness mesurements hs been performed nd used to loclly estimte the lifetime reduction. NOMENCLATURE HB Hrdness Brinell Rp02 Proof yield stress Rm Ultimte stress PWHT Post-weld Het Tretment LCF Low Cycle Ftigue REFERENCES [1] Stephens RI, Ftemi A, Stephens RR, Fuchs HO, Metl ftigue in engineering, ISBN-13: , John Wiley & Sons, Nov 3, [2] Bordmn, B.; Ftigue resistnce of steels, ASM Hndbook Committee, p , 1990 [3] Csgrnde A., Cmmrot G.P., Micele L., Reltionship between ftigue limit nd Vickers hrdness in steels, Mterils Science nd 6