Failure Analysis of a LP Turbine Free Standing Blade. Sponsored by. IB Thermal Power Station Orissa Power Generation Corporation Ltd.

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2 FAlLUREANALYSISOFLP TURBINEFREESTANDINGBLADE Report No.: NMLIMST/FA/1.14/2007. October 2007 I. BACKGROUND : MIs IB Thermal Power Station under Orissa Power Generation Corporation Ltd. Approached NML for carrying out a failure analysis of a turbine blade of a 200 MW thermal power plant on September These blades are free standing blade. The incident involved fracturing of a Turbine blade (hereafter, it will be mentioned as LP blade) from the 8thstage of the LP turbine. The blade is under operation from In 2003, three blades were replaced which were at different angles with the present blade. These blades are made of 12%Cr stainless steel. One portion of the fractured portion was recovered by the plant people after opening the turbine. That piece and the remaining portion of the blade were brought to NML on 24/09/07, for detailed investigation to find out the root cause of the failure. The investigation is assigned to NML vide letter No. ITPS/CC-ME dated 5thOctober II. SCOPE OF WORK Based on the discussion with Mr. R.C.Sahu Dy. Manager and Mr. B.Jojwar, Sr. Manager, IB Thermal Power Stationm, OPGCL and the information provided by them, it was decided to do the following Visual examination of the failed blade; Fractographic analysis of fracture surfaces to find out the fracture mode/mechanism; Microstructural and hardness analysis; SEM surface studies of the of the failed blade; Room temperature mechanical property evaluation. III. EXPERIMENTAL RESULTS Visual examination: The photograph of fractured piece of the failed blade is shown in Fig.l a. It is a free standing blade in the LP last stage. The length of each blade in this stage is ~650mm including the root. It is aerodynamically designed with leading (thin) and trailing (thick) edges. The thickness at the leading and trailing edges is 3.2mm and 2.5mm, respectively, where the fracture took place. It has failed at a length of around 400 mm from the root. The 1

3 FAlLURE ANALYSIS OF LP TURBINE FREE STANDING BLADE Report No.: NML/MSTIFA/1.14/2007. October 2007 fractured end is marked with an arrow. The longitudinal surface has smooth shiny finish, except a portion extended from the fractured region to a few centimeter on the leading edge shows different colour contrast (Fig.! b: marked with A)...,..' I ' -r Fig.!: (a) Photograph of the Failed blade; (b) Region A on the leading edge shows different colour contrast b The photograph of the broken piece of the blade which has been recovered from the turbine after its opening has been shown in Fig.2. It is worthy to be noted that there are several parallel cracks emanating from the leading edge of the blade (marked with arrows). 2

4 FAlLUREANALYSISOFLP TURBINEFREESTANDINGBLADE ~ 1:'11,ii" 'liiqlflfll ", \11,"" I ' "i' 'tilljlfl l ' Iiii'I'., IT' if:" "",,; II'.~""" L ~, 7, 5, 91!OJ l',!2 13 l' <i NATARAJ" 621 Fig.2: Photograph of the broken piece of the blade The macrophotographs of the fracture surface (Figs.3a and b) clearly reveals the signature of the fatigue failure, i.e., concentric beach marks covering some of the fracture surface and dull fast fracture zone at the other end of the crack initiation site. The beach marked fatigue zone covers nearly 50% of fracture surface. While rest of the dull region is overload induced fast fracture zone. Therefore, material seems to have sufficient strength and toughness. a Fig.3: (a) Beach marked on the fracture surface; (b) Enlarged portion of the marked area 3

5 FAILUREANALYSISOFLP TURBINEFREE STANDINGBLADE Fractographic analysis of fracture surfaces to find out the fracture model mechanism: ~ Turbine blade fracture surface was further examined by SEM to assess the finer details. At the leading edge, there was evidence of dimples indicating ductile facture mode (FigAa). Apart from that, there are regions showing signatures of rubbing across the fracture surface (FigAb). These rubbed regions might be due to particle impingment (FigAc and d); the particles are found to be silica. The corresponding EDX pattern has been presented in FigAe. Beach marks were also observed on the fracture surface (Fig.5). These marks indicate the propagation of crack under fatigue loading. Secondary cracks were also observed on the fracture surface (Fig.5b). 'b d 4

6 NATIONAL MET ALLURGICAL LABORATORY, JAMSHEDPUR FAlLURE ANAL YSlS OF LP TURBINE FREE STANDING BLADE Full seal. counls: ac initi,'liolu' Ka. I oKa Ii k.v 10 e Fig.4: Fracture surface near the leading edge shows (a) presence of dimples and (b) abraded region; abraded region created by particle impingment (c) and (d); EDX analysis of the particles (e) Fig.5: (a) Beach marked on the fracture surface indicating the propagation of cracks under fatigue loading; (b) Secondary cracks on the fracture surface; (c) and (d) Striations on the fracture surface (arrows indicate direction of crack propagation) 5

7 .: :.."' I J. 'j>.." C=-.-'C-' ~~.7'~""""""', ~" ~.."'...' '" ",. NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR FAlLUREANALYSISOFLP TURBINEFREESTANDINGBLADE.: A close look on the edge of the fracture surface at the leading edge shows that there were some cracks emanating from the edge towards the trailing edge (Fig.6a and b). At one of the edges there were some cylindrical objects (marked as B). The enlarged micrograph of B is shown in Fig.6c resembling to that of blisters developed due to chemical reaction. The EDX pattern obtained from those blisters confirm presence of Chlorine on those places. Presence of chlorine is quite unusual on the fracture surface. There was no signature of intergranular or cleavage type of fracture mode observed under SEM. This indicates that the failure is not due to any embrittlement. b FeLa.! Fe Ka. 50 CIKa. c kev Fig.6: (a) Cracks emanated from the fracture surface (marked by arrow); (b) cracks propagating towards the trailing edge; (c) Enlarged portion ofb; (d) EDX of those B. 6

8 ~~;o-~":fi~r~~. ~..~~~ '". '",-'- -- NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR FAlLURE ANAL YSIS OF LP TURBINE FREE STANDING BLADE Microstructure and hardness of the blade material: Microstructure of the blade shows tempered martensitic structure (Fig.7). It is having Cr 12.35%. This corresponds to martensitic stainless steel of ASTM 410 grade. The hardness of the material is 230BHN which indicates no signature of material degradation. Fig.7: Microstructure of the blade material showing tempered martensitic structure IlIA Analysis of the surface of the blade: In earlier sections, it has been seen that there is no material degradation. Hence, the failure is due to variation in the operational parameters. To elucidate more on that aspect, surface of the blade near the fracture surface has been investigated thoroughly. The macrophotograph of that area is shown in Fig.8a. The same area has been observed under SEM (Fig.8b). The marked area has been enlarged and presented in Fig.8c. It shows nearly similar kind of blisters as observed in Fig.6d. EDX analysis shows presence on chlorine and Si on the surface. Si may be present at Si02. Ca might be present as a constituent of steam. 1t1!!1 III, a 7

9 FAlLUREANALYSISOFLP TURBINEFREESTANDINGBLADE SiKa. AIKa c D kev Fig.8: (a) Macrophotograph ofthe area adjacent to fracture surface on the leading edge; (b) same area observed under SEM; (c) enlarged portion of the marked portion in (b); (d) EDX analysis of area (c) Analysis of the broken and deformed portion of the blade The broken and deformed sample of the blade as shown in Fig.2 indicates several parallel cracks at the edges. Those cracked portions were separated and the fracture surfaces were investigated under SEM. The fracture surface shows several blisters which are shown in Fig.9a and b. The corresponding EDX analysis shows presence of chlorine on the surface. Apart from that, there are Oxygen as well as Ca. The presence of Ca and Chlorine might be from the water. However, Oxygen might be due to iron oxide present on the fracture surface. The blade after the crack initiation has undergone fatigue loading during operation. Due to exposure to steam at this condition, there will be formation of iron-oxide on the surface. However, during operation, due to mating of the cracked surface as well as due to particle impingement, most of the oxide scales might have been peeled off. 8

10 FAlLURE ANALYSIS OF LP TURBINE FREE STANDING BLADE 0 K.. Fe L j CJL... GOO 400 Fe K C.,K.. 5 ~ev 10 c Fig.9: Blisters on the cracked surface as shown in Fig.2 (a and b); EDX analysis of the blisters (c). It has been seen from the above results that the microstructure of the material didn't show any degradation as observed by the tempered martensitic structure and the corresponding hardness. Therefore; the blade didn't fail due to materials problem. Presence of beachmarks as well as striations on the fracture surface corresponds to crack propagation under fatigue loading. Turbine blade rotating at 3000 rpm experiences fatigue loading. Hence, we need to identify the cause for the crack initiation. The small cracks shown in Fig.6 show that initiation of the cracks is at the leading edges. Fig.8a and b show roughness on the surface of the samples adjacent to the fracture surface. The corresponding EDX shows presence of Chlorine and silica on that surface. It is well known that chlorine is very much detrimental for corrosive attack on the blade material. Within the rough surfaces, there might be small cracks which initiated first. The cracks grew under stress as well as corrosive environment. The formation of blisters along with presence of chlorine on the fracture surface itself indicate corrosion assisted crack growth. The cracked surface of the deformed sample (Fig.9) also supports the above process of corrosion assisted crack propagation. 9

11 FAlLURE ANALYSIS OF LP TURBINE FREE STANDING BLADE Evaluation of Mechanical properties Standard tensile samples having a gauge length of 30mm were prepared from the failed blade away from the fracture surface. The room temperature tensile tests were carried out on an Instron machine at a strain rate of 6x 10-4Is. The room temperature mechanical properties were calculated from the stress-strain curves and are given in Table 1. Each data is an average of two test results. Table I: Room temperature tensile properties of failed and virgin blades Yield Stress (MPa) Ultimate Tensile % Elongation Stress (MPa) As per the standard steel of this composition i.e. AISI 400 grade, having hardened and tempered martensitic stainless steel has the tensile properties: Yield Stress: minimum 495 MPa, Tensile Stress: MPa and % elongation: The present material has higher yield strength compared to the standard; however UTS is nearly same. This confirms the observations made earlier that there is no embrittlement effect on the failure of the blade. IV. CONCLUSIONS Failure of the blades may be due to two reasons: a. Degradation of the blade material; b. Failure due to the operations conditions. Based on the microstructural investigation, it is concluded that there is no microstructural degradation. There is no signature of intergranular feature resembling to that of embrittlement. The mechanical properties of the material are also within limit; there is no deterioration of the properties. Therefore, the possibility of the failure due to material degradation is ruled out. On the fracture surface there are presence of SiOz particles. Also, on the blade surface adjacent to the fracture surface at the leading edge shows presence of grooves. Presence of 10

12 FAlLUREANALYSlSOFLP TURBINEFREE STANDINGBLADE Report No.: NML/MST/FA/ Octob~r Chlorine at the fracture surface as well as on the cracked surface of the broken piece of the blade indicate occurrence of corrosion. Corrosion of the blade surface at the leading edge leads to initiation of cracks. These cracks then propagate under fatigue loading due to vibration. Initiation of these kinds of cracks on the leading edge has been observed on the broken sample and presence of chlorine was also observed on the fracture surfaces. The fracture surface of the remaining blade as well as the broken blade sample show evidence for blisters as well as globules. These were caused as a result of chlorine attack. Henceforth, the failure of the blade is due to the chlorine assisted corrosion with silica particle impingement. V. RECOMMENDATION: It is recommended that the source of the chlorides be eliminated and that the remaining blades be inspected at regular maintenance intervals for evidence of pitting and cracking. 11