Macro and Micromorphology of in Service Cracking and Fracture of Turbine Blades
Makro in mikromorfologija razpok in zlomov nastalih med obratovanjem turbinskih lopatic
F. Vodopivec*, B. Ule, L. Vehovar, J. Žvokelj, Institute of Metals and technologies, Ljubljana, Slovenia
V. Verbič, TNT, Obrenovac, Serbia
After the break down cracked and fractured blades vvere extracted from the turbine and the macro and micromorphology of cracks and fractures surface vvere investigated. Three modes of propagation vvere identified: stable propagation by HISC, stable propagation by HISC and fatigue and instable brittle and ductile propagation. The micromorphological characteristics of the different modes of propagation are explained. Key vvords: Turbine blades, steel, cracking, fracture, corrosion, fatigue, microstructure.
Po zlomu so bile počene in zlomljene lopatice vzete iz turbine in bila je raziskana makro in mikromorfologija razpok in zlomov. Identificirani so trije mehanizmi širjenja: stabilno širjenje zaradi HISC, stabilno širjenje zaradi HISC in utrujenosti ter nestabilno krhko in duktilno širjenje. Opisane so mikromorfološke značilnosti posameznih načinov širjenja. Ključne besede: Jeklo, turbinske lopatice, razpokanje, zlom, korozija, utrujenost, mikrostruktura.
1. Experimental work
The experimental vvork consisted of:
-	examination of microstructure;
-	analvsis of impurities on cracks surfaces, and
-	macro and micro examination on the cracks and fractures surface.
The data on the composition of the steels and mechanical properties will be reported later and will be considered in this paper only vvhen necessarv to explain better the findings relative to the microstructure and the aspect of the cracks and fractures surface.
The composition of ali examined blades corresponded to that required for the martensitic stainless steel X21CrMoV 121 and also the mechanical properties sufficed the requirement of the buver of the turbine. It should be noted that a very lovv notch toughness of 15 J vv as required. Four different cases of cracking and fracturing of the blades vvere identified on the basis of visu-al examination:- one čase of cracking on the rounded trailing edge in the passage betvveen the root and the blade:
-	some cases of cracking in the first root grove mostly at a dis-tance up to 50 mm from this edge (lig. 1, 2 and 3), and
-	fracture of precracked blade in the turbine in the first root grove vv ith an initial crack (fig. 4) or vvithout such crack (fig. 5).
On some in service cracked blades the crack surface vvas opened for examination by bending in laboratorv, generally after cooling in liquid nitrogen.
On the base of the macromorphology of the crack surface three tv pes of in service crack propagation vvere identified:
Prof dr Franc VODOPIV H\
IVI [ Ljubljana. Lepi pot 1 I. f, 100(1 L| .
-	surface shovving near the initial point no fatigue striations but vvith such striations on the remaining area of the crack (fig. 2 and 6),
-	surface of cracks vvithout fatigue striations (fig. 3). and surface vv ith fatigue striations from the starting point of cracks propagation.
Figure 1: Crack on the trailing edge in the first root grove of blade 436. Slika I: Razpoka na izhodnem robu v prvem korenskem žlebu lopatice 436.
2. Micromorphology of cracks and fractures
Several form of propagation vvere observed on specimens cul from different parts of the fracture of blades and on laboratorv
Figure 6: Surface ofthe crack on the blade on fig. 4. Slika 6: Površina razpoke na lopatici na si. 4.
specimens. In order to make the matter easier to follow the fracture micromorphology is deseribed separatelv for different areas: initial point, stable propagation and brutal (instant) rupture on laboratorv specimens and in the turbine.
2.1. Initial point of cracking and stalile propagation Gencrallv. the surface of cracks near the initial point vvas covered vvith corrosion products and also after a verv careful cleaning it vvas rarclv possible to find at SEM observation reli-able details, vvhich vvould characterizc the mechanism of initia-tion. An cxception vvas the specimen in fig. 7. vvhere several crack initials vv ith a perfcctlv clean surface vvere found. Near the tip ofthe pitting vvith a size ofappr. 0.25 mm the fracture surface is brittle trans and intergranular (fig. 8) vv ithout fatigue striations. The micromorphologv ofthe transgranular surface is featherlike and similar to that reported frequently for high strength steels vvith a martensitic microstructure and vvith an inereased content of hvdrogen. This suggests that in presence ofthe pitting the nti-clcation ofthe crack vvas induced bv the overcharging ofthe stccl vvith hydrogen produced bv the corrosion process at the tip ofthe pitting. A similar detail of micromorphologv of fracture surface near the nuclcation point vvas observed also on the blade 430 (fig. 9). It shovvs mixed propagation and small contamination vv ith corrosion products. visible more clcarlv on the intergranular surface. On the clean part of cracks surface vv ithout striations near the border of the brutal fracture the micromorphologv vvas similar as in fig. S and 9 and it shovved mixed trans and intergranular propagation vvith the featherlike surface of transgranular cleavage (fig. 10).
Figure 2: Surface of a crack vv ith areas vv itli and vv ithout fatigue striations.
Slika 2: Površina razpoke / deli / in brez. utrujenostnih brazd.
Figure 3: Surface of a crack vv ithout fatigue striations. Slika 3: Površina razpoke brez utrujenostnih brazd.
Figure 4: Fraclure ofthe blade 447 extracted from the disc after the bivak dovvn. The initial crack is on the left side.
Slika 4: Prelom lopatice 447. ki je bila i/ turbine vzela po havariji. Začetna razpoka je na levi strani.
Figure 5: Fracture of the blade 37') extracted from the disc after the bivak dovvn. Slika 5: Prelom lopatice 37l). ki je bila vzeta i/ turbine po havariji.
Figure 7: Fracture initials on blade 450. Slika 7: Začetki preloma na lopatici 450.
Figure X: Surface o!" one of the cracks in l ig. 13 near the bottom of llie pitting.
Slika 8: Površina ene od razpok na si. 13 oh dnu zajede.
Figure 10: Surfacc of the crack in fig. 3 near the border line of the brutal rupture of the blade. Slika 10: Površina razpoke na si. 3 ob meji /. nasilnim /lomom.
Figure 9: Detail of the crack surface without fatiguc striations.
Slika 9: Detajl površine razpoke brez utrujenostnih brazd.
Corrosion pits were the initials of ali the cracks in the first grove of the root, also pitting as small as 0,05 mm (fig. 11).
In ali cases when the cleaning was sufficient to reveal details the surface of cracks without fatiguc striations showed a micromorphologv similar to that in fig. 10. thus brittle trans and inter-granular propagation.
Fatiguc striations were found on crack surface of several blades at various distance from the starting point on the surface. That shows that t\\ ti mechanisms of stablc propagation were ac-tive in the growth of cracks. Consequently, on cracks surface two different micromorphologies of propagation were found. Pure
it* 53

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Figure 11: Pitting and microcrack in the first root grove.
Slika 11: Zajeda in mikrorazpoka v prvem žlebu korena.
fatiguc with striations of different width (fig. 12) was found on-ly in the crack situated in the rounded passage between the root and the leaf of the blade. The propagation is transgranular and the micromorphologv is independent upon the width of the stri-ation. The main feature are striations and small edges oriented in the direetion of crack propagation. It seems safe to conclude that the cause for propagation was the amplitude of fatiguc stress and that large striations represent the operation of the turbine in range of critical number of revolutions. Also the width of the narrovv-est striation is considerable (0.01 mm) and indicates to a relativen high amplitude of dynamic stress. In the second čase the crack surface showed by macroscopic observation an apparent pure fatiguc propagation. By appropriate magnification is SEM a mixed micromorphology was observed (fig. 13). It consisted of groups of steps and microcracks orthogonal to the direetion of propagation alternated vvith vvider bands where the surface indi-cates a specific mechanism of transgranular propagation. Microridges parallel to the direetion of the propagation of cracks trespassed sheafs of steps and microcracks orthogonal to the
direction of propagation. The eonclusion is that the crack propa-gated in conditions when corrosion and fatigue prevailed alternative^. thus a propagation bv fatigue corrosion.
As alreadv shovvn, ali the findings indicate that the initials of cracking in the first grove of the root were corrosion pits. also pils as small as 0.05 mm (fig. 11). The steel at the top of the pits vvas charged vvith hydrogen. that decreased it fracture toughness and craeks vvith mixed trans and intergranular propagation vvere
Slika 12: Površina utrujenostne razpoke na zaobljenem prehodu i/ korena v list lopatice.
Figure 13: Microdetail of the crack surface in l ig. 2 in the area of fatigue striations. Slika 13: Mikrodetajl površine razpoke na sliki 2 na področju utrujenostnih brazd.
Figure 15: Straiaht crack on the vvorking side in the first root grove of blade 41 iT
Slika 15: Ravna razpoka na delovni strani v prvem korenskem žlebu lopatice 41 I
initiated because of statie ordynamic stresses. The initiation took plače either on several points and single mierocracks coalesced in a steplike macrocrack (fig. 141 or in one point and the micro-crack did grovv in a harline slightlv curved macrocrack (fig. 15). If the corrosion process vvas continued, the crack eontinued to propagate bv the same mechanism and a crack surface vvithout striations vvas obtained. If the intensity of corrosion vvas dimin-ished or the corrosion vv as stopped, the propagation continued bv sufficient stress amplitude in conditions of pure fatigue.
In ref. I it is reported that the enriehment of impurities in the first drops of condensate could reach several orders of magni-tude. The presence of pittings in the first grov e ol the root shovvs that the first drops of eontaminated condensate appeared in this area of the blade. vvhere the statie and dvnamic stress made them particularly harmful. The presence of pittings demonstrates nat-urallv also a poorquality of boiler vvater. at least in some periods of the vvork of the povver station.
2.2. Bruta! fracture
This tvpe of fracture vvas obtained in three different vvavs: - in service.
Figure 14: Step like crack on ihe vvorking side in the first root grove. Blade 435. "
Slika 14: Stopničasta razpoka na delovni površini v prvem korenskem žlebu. Lopatica 435.
ire 12: Surface of the fatigue crack in the rounded area of the transition from the root to leaf of ihe blade.
on laboratorv specimens and
by bending ofcracked blades in laboratorv.
Brutal in service fracture was observed on blades 379. 434. 442 and 447. Fig. 16 shovvs the micromorphology of the fracture in area I on blade 379 fractured vvithout precrack and shovvn in Fig. 5. The micromorphologv shovvs a quasi ductile propagation under shearing stress vvith ven rare intcrcrystallinc details. In area II of the same blade the micromorphology is identical. In area III. vvhere the propagation occurred in conditions of plane strain (1). the micromorphologv is brittle. mixed trans and inter-
Figure 16: Microdetail nI' the rupture surface of the blade on t i«. 5 in area I.
Slika 16: Mikrodetajl površine preloma lopatice na si. 5 v področju I.
Slika 17: Mikrodetajl površine preloma lopatice na si. S v področju III.
granular (fig. 17). Virtually identical vvas the micromorphologv of the fracture of blade 434, vvhich failed in service probablv at the same time and in similar stress conditions. Also the micromorphologv of the brutal fracture of blades 442 (fig. 4) and 447. tvvo blades broken in service or during the break dovvn and pre-cracked in the First grove of the root is similar as that in area III of blade 379.
On noteh toughness specimens the more intercrvstalline brittle propagation vvas found the lovver vvas the value of noteh toughness. By a level of 70 J and more the propagation vvas ductile (fig. IS) vvith mostly small dimples. vvhich indicate that on-ly a thin laver of metal both sides of the crack lips vvas deformed
Figure 18: Fracture surface hv a noteh toughness of 1 10 J. Slika 18: Prelomna površina pri /are/ni žilavosti 110 J.
Figure 19: Fracture surface bv a noteh toughness of 52 J. Slika 19: Prelomna površina pri zarezni žilavosti 52 J.
Figure 17: Microdetail of the rupture surface of the blade on fig. .i in
area lil.
Figure 20: Fracture surface by a notch toughness of 35 J. Slika 20: Prelomna površina pri /are/ni žilavosti 35 J.
Figure 21: Fracture surface bv a notch toughness of 22 J. Slika 21: Prelomna površina pri /are/ni žilavosti 22 J.
during the formation of the voids. Below a toughness of 60 J the surface shows a quasi brittle propagation with frequent areas of propagation through martensite platelets lying in the plane of the fracture and rare duetile details (fig. 19). By a notch toughness of 34 .1 in a similar transcrvstalline matrix intergranular faeets are found (fig. 20) and by a notch toughness of 24 J the intergranular brittle propagation predominated (fig. 21). It seems tlius that the diminution of toughness belovv a level of ap-pr. 35 J is connected to an inereasing part of intergranular brittle crack propagation. The micromorphologv of fracture toughness and of notch toughness specimens of the same steel vvas virtually identical. 576
3. Contamination of crack surface and mechanism of stable crack propagation
On some of the cracked blades broken bv bending in labora-torv small relativelv clean areas of crack surface vvere obtained. On tvvo sueh surfaces. one vvith and the second vvithout fatigue striations the presence of some elements vvas determined vvith surface scanning in a SEM equipped vvith tvvo vvavelength dis-persive speetrometers. Because ofthe uneven surface no quanti-tative analvsis vvas possible. therefore the results given in table 1 have onlv a comparative value. It seems logical to conclude that ali the analvsed elements vvere present on the crack surface as compounds. since ali of them could not reach the crack surface as pure elements. It is assumed also. considering the traces of corrosion on the surface of the blades, that sulphur and chlo-rine are present in form of sulphate rsp. chloride vvhich in vvater solution stronglv inerease the corrosivitv of the droplets in the first area of steam condensation (2. 3. 4). The very great differ-ence in the level of contamination offers a logical support for the follovving explanation of the difference in the process of stable crack propagation and the resulting difference in the morpholo-gy ofthe surface of cracks.
Table 1: Results ofthe analvsis of crack surfaces. Tabela 1: Rezultati analize površine prelomov.
Blade Mode of crack			Element, mg/enr	
No. propagation	C1	Na	Ca Si S	Cl+S
435 vvithout lat. str.	47.7	4,S.4	55.6 112 50.2	D7.')
436 vv ith lat. str.	1.0	0.17	5.2 12.4 0.19	1.74
Chloride ions break the passive laver on the surface on the blade. cause a rapid local process of corrosion and pittings are formed because the cathodic area is much greater than the anodic area. On the bottom of the pittings the condition for the initi-ation of cracks are present: an aggressive solution, small active tip. great passive lateral surface as vv ell as brittle steel charged in hvdrogen produced at the tip bv the corrosion process through the follovving electrochemical reactions: M -> M' = e. and
Figure 22: Microstructure bv a notch toughness of 1 10 J. Slika 22: Mikrostruktura pri /are/ni žilavosti 110 J.
Figure 23: Microstructure bv a notch toughness of 35 J. Slika 23: Mikrostruktura pri /are/ni žilavosti 35 J.
vrči +H,0 = MOH + Cl + H . Metal chloride produces through hvdrolvsis metal hydroxide as deposit on the crack surface and ions of hydrogen and chloride. The formation of acid in the pitts lovvers the pH value. produces hydrogen ions vvhich promote the hvdrogen induced stress cracking (HISC). In references 2. 3 and 4 the brittle cracking of martensitic stainless steel in the presence of a corrosion process vvhich generates hvdrogen ions in cathod-ic areas is confirmed. Tvpical features of this type of cracking are non branchcd cracks. vvhich vvere found in ali the blades cracked in the first grove of the root. vvhile in čase of stress corrosion cracking the cracks are branchcd. Hvdrogen in interstitial solution segregates to areas of tensile stress concentration. lovvers the ductilitv and the fracture toughness ol' the steel and causes a mi.ved trans and intergranular brittle fracture.
4. Microstructure and notch toughness
The examination in optical microscope did not shovv significant differences in microstructure of the steel, vvhile the obser-vation in SEM vv as more instructive. In ali cases the microstructure consisted of tempered mostlv acicular martensite. By observ ation in SEM it vvas possibly to connect partlv the microstructure. especiallv the size and distribution of tempered carbide particles. to the notch toughness. Bv high notch toughness the carbide particles are coarse and the habitus of martensite
poorlv marked (fig. 22). By intermediate toughness level the particles of carbide are smaller, frequently aligned along grain boundaries and along martensite platetels, and the habitus of martensite is vvell marked (fig. 23). Bv a very lovv notch toughness of 20 J the microstructure is similar. A careful evaluation in-dicates that the differencc in notch toughness and the increasing part of intergranular fracture can not be explained onlv in terms of microstructure. The tempering temperature required for a high limit o f c I a s t i c i t y for this type of steel is in the range of reversible intergranular segregation of some elements, especiallv phospho-rus (5). It seents thus that the intergranular fracture bv lovv toughness is partlv due also to the britlleness produced bv intergranular segregation. This conclusion is confirmed bv the fact that frequenlly intergranular facets are perlectlv smooth (fig. 21). thus tvpical for intergranular britlleness produced bv reversible intergranular segregation (5).
Conclusions
In the paper the results of the investigation of the cracks and fractures surface of turbine blades are presented.
On the base of the cracks macro and micromorphology three meehanisms of stable crack propagation vvere established: - mixed intcr and transgranular propagation bv HISC and transgranular propagation bv corrosion fatigue. and transgranular propagation bv fatigue.
In the first tvvo cases cracks started on corrosion pits as small as 0,05 111111. Brutal fracture in turbine and in laboratorv occurred bv mixed brittle trans and intergranular propagation. On the initial part of the in turbine rupture of the blades vvithout crack the fracture vvas ductile, in the second area the propagation vvas brittle trans- and intergranular vvhile the fracture of precracked blades vvas eompletely brittle. The lovvering of the notch toughness of the steel belovv appr. 35 J is characterised bv an increasing part of intergranular fracture vvith a smooth surface suggest-ing that the steel britlleness vvas connected to the microstructure as vvell as to an intergranular segregation of phosphorus.
References
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J. Bohnstedt. I'. H. Effertz. P. Forchanimer, L. Hagn: Dur Mas chinenschaden 51. 19X7, 73-79.
F. Gelhaus: "SCC in Stcam Turbine Materials" in Metals I lami hook. Volume 13, Corrosion, p. 952-959.
1 O. Jonas: Corrosion of Stcam Turbines in Metals Handhook, Volume 13. Corrosion. p. 993-1001.
s H. Erhart, H. J. Grabke: Železarski zbornik 15. 1981, 149.