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Metallurgical investigation of defective superalloy rings
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ELSEVIER
C~)DIRECT°
Engineering Failure Analysis 11 (2004) 449~462
ENGINEERING
FAILURE ANALYSIS www.elsevier.com/locate/engfailanal
Metallurgical investigation of defective superalloy rings S.K. Bhaumik*, T.A. Bhaskaran, R. Rangaraju, M.A. Venkataswamy, M.A. Parameswara, R.V. Krishnan Failure Analysis Group, Materials Science Division, National Aerospace Laboratories, Post Bag 1779, Bangalore 560 017, India
Received 8 May 2003; accepted 17 May 2003
Abstract
During the final machining of nickel base superalloy rings, some cavity-like defects were observed in a few components. Some of them were exposed to the surface, while the others were detected by X-radiography. The defective rings were examined and a study was carried out to identify the processing step during which the defects would have been introduced in the components. Under the scanning electron microscope, the cavity surface was found to be smooth with typical appearance of a molten and solidified structure. Detailed microstructural study at these defective regions showed an acicular microstructure with faceted/globular phases. EDX analysis confirmed these areas to be rich in Nb. It was also found that the cavities observed were always associated with such unacceptable microstructures. A detailed analysis of the results is presented in this article. © 2003 Elsevier Ltd. All rights reserved. Keywords: Superalloys; Microstructures; Solidification; Defects; X-ray analysis
1. Introduction
A m o n g the commercially available superalloys, nickel-base superalloys such as I N 718, I N 706 and I N 625 are most widely used for aeroengine applications. They account for the majority of the total production of nickel-base superalloys. Niobium is present in these alloys in quantities varying from 3 to 5.5 wt.%. Inconel 718 (Ni-19Cr-5.1Nb-3Mo-0.9Ti-0.5AI-18.5Fe) was introduced by Inco Alloys in 1959. Niobium's primary role in this alloy is solid solution strengthening and precipitation hardening. Addition of N b results in the formation of MC-type and M6C-type carbides in addition to the 7" phase, Ni3Nb. The strengthening mechanism involves precipitation of an intermetallic compound (Ni3Nb) in the nickel matrix during heat treatment. Although there are other alternatives to niobium as a strengthener, niobium is found to be unique in its ability to avoid strain-age cracking during fabrication of the final components, especially during welding. Since niobium provides a slower aging response, parts can be thermally stress relieved without cracking. I N 718 shows much better performance than the other precipitation-strengthened superalloys available, such as Astroloy (Ni-15Cr-17Co-5.25Mo~4A1-3.5Ti-0.06C-0.03B) and Ren6 * Corresponding author. Tel.: + 91-80-527-3351; fax: + 91-80-5270098. E-mail address: [email protected](S.K. Bhaumik). 1350-6307/$ - see front matter © 2003 Elsevier Ltd. All rights reserved. doi: 10.i 016/j.engfailanal.2003.05.019
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41 (Ni-19Cr-11Co 10Mo-I.5A1-3.1Ti-0.09C-0.005B). I N 718 also exhibits better creep properties in the range 1000-1200 °F than Rene 41 and Astroloy. I N 718's residual strength and ductility properties are not impaired by exposure in this temperature range. Even after more than 40 years since first being introduced by INCO, I N 718 is still the most important all-purpose nickel-base superalloy [1]. The normal practice for the production of nickel base superalloy ingots is by vacuum arc remelting (VAR) or electroslag remelting (ESR). The consumable electrode required for the above is produced by vacuum induction melting (VIM). The progressive directional solidification in V A R facilitates obtaining ingots nearly free from shrinkage cavities. In addition, a homogeneous structure can be obtained with minimum amount of macrosegregation. The initial consumable V A R is followed by a second VAR, and sometimes even third V A R step, to assure better chemical and structural homogeneity in the ingot. The structure in these ingots is further refined by extrusion, pressing, rolling etc, to form billets or bars which are then forged to the final shape. However, producing a segregation-free structure is very difficult in nickel base superalloys containing nickel and iron even after VAR. A very stringent process control is required to obtain ingots free from defects or with minimum defects which can be easily tackled in the subsequent metal working operations [2]. This paper describes in detail a metallurgical investigation that has been carried out on defective nickel base superalloy (composition equivalent to I N 718) rings. The nominal composition of the alloy is given in Table 1. The defects were observed at the final machining stage and the objective of the investigation was to identify the stage in the processing steps during which the defect could have been introduced.
2. Background 2.1.
The process
The raw material was obtained from the supplier in round bar form. These round bars were manufactured by basic melting processes such as V I M and VAR, and the cast ingot had a diameter of 450 mm. The cast ingot was forged/rolled to bars and then turned to the required diameter of 160 mm. F r o m the supplied raw material, unit stock of 160 m m qbx 310 m m length was cut and was subjected to upset forging and piercing to obtain an upset cheese of size 350 m m O D x 150 m m I D x 6 0 m m HT. The cheese was then ring rolled to size 471 m m OD x 370 m m I D x 40 m m HT. The ring thus obtained was then machined to get the required profile. The processing flow chart for the production of these rings is shown in Fig. 1.
Table 1 Nominal composition of the alloy Element
Wt. %
C Mn Si Cr Mo Nb Ti A1 Fe Cu Ni
0.04 0.2 0.2 18.5 3.0 5.1 0.9 0.5 17.0 0.2 Balance
S.K. Bhaumik et al./ Eng#wer# N Failure Analysis 11 (2004) 449-462
Starting material
451
Unit stock 160 ~ x 310 L
350 OD x 150 ID x 60 HT
Upset forging & piercing I
I
I
i 1 1
481 OD x 360 ID x 55 HT
Ring rolling I
Solution heat treatment followed by machining (471 OD x 370 ID x 40 HT) All dimensions in mm.
Fig. 1. Schematicshowing the processingflow chart.
2.2. The defect
While final machining of one of the rings out of a batch of five, cavity-like defects readily visible to the unaided eye were observed at the fillet region of the change of section. X-radiography was conducted on all the machined rings and one more ring was found to have similar defects (not exposed to the surface) at the central web region. The photograph of one of the defective rings is shown in Fig. 2. These two defective rings were analysed in the laboratory to determine whether (a) these defects were created during upset cheese forging and/or ring rolling or (b) these defects were created during machining due to presence of some very hard phase which might have fallen during machining leaving behind a cavity on the surface. These questions were addressed during the investigation and a detailed analysis is presented.
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Fig. 2. Photograph showing one of the defectiverings (ring-l).
3. Experimental details The defects exposed to the surface were examined visually and under a stereobinocular microscope. The defective area was cut from the ring, cleaned ultrasonically in acetone and then observed in a scanning electron microscope (SEM, LEO 440I). Microstructural study was carried out by using both optical microscope (Nikon, Epiphot-200) and SEM. Samples for microstructural study were prepared by standard metallographic techniques. Energy Dispersive X-ray analysis was carried out by EDX attached to the SEM. Hardness measurements were carried out using a S H I M A D Z U HSV-20 Vickers microhardness tester at a load of 500 g. 3.1. Visual and stereo-binocular 3.1.1. Ring-1
Visual observation revealed a cavity like defect in ring-1 at the fillet at the change of section as shown in Fig. 3. Under the stereo-binocular microscope, this defect looked like a volume defect, which was present in the material and became exposed to the surface during machining. The inside surface of the cavity had a smooth appearance. 3.1.2. Ring-2
!
The defect in ring-2 did not show any such cavity, but looked more like a dent on the surface under the stereo-binocular microscope. The location of this defect was in the central web region close to the fillet as in ring-1. Careful examination revealed a number of cracks around the dent.
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Fig. 3. Photograph showing the cavity-likedefect in ring-1.
Fig. 4. SEM photograph showing the close-up view of the defect shown in Fig. 3. 3.2. Scanning electron microscopy ( S E M ) 3.2.1. Ring-1 The sample containing the defect was ultrasonically cleaned in acetone and observed in the SEM. The appearance of the defect under SEM is shown in Fig. 4. The internal surface of the cavity i~s shown in Fig. 5. It had a smooth surface similar to a solidified morphology resembling a dendritic structure. Fractographically it was featureless. However, in some places, parallel band like features were seen (Fig. 6). At higher magnification, this region also showed the surface morphology similar to that shown in Fig. 5.
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Fig. 5. SEM photograph showing the internal surface of the cavity in ring-1. The surface has a smooth appearance similar to a solidified morphology.
Fig. 6. SEM photograph of the internal surface of the cavity in ring-l. The banded structure with solidified morphology indicates limited deformation in this region.
3.2.2. Ring-2 The S E M p h o t o g r a p h o f the dent in ring-2 is shown in Fig. 7. A thin flaky m a t e r i a l was observed at the dent region. E D X analysis confirmed this m a t e r i a l to be the same as t h a t o f the p a r e n t material. W h e n the flaky m a t e r i a l was r e m o v e d by a sharp needle/edge, a h i d d e n cavity was observed (Fig. 8). A r o u n d the dent, extensive c r a c k i n g was also seen. D e t a i l e d e x a m i n a t i o n revealed t h a t the n a t u r e a n d m o r p h o l o g y o f this
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Fig. 7. SEM photograph showing a dent-like defect in ring-2. The dent mark was surrounded by a number of cracks.
Fig. 8. SEM photograph showing the cavity after removing the thin flaky layer of material at the dent region shown in Fig. 7.
cavity are similar to that mentioned earlier for ring-1. The inside surface of the cavity had the same smooth appearance with no fractographic features (Fig. 9).
3.3. Metallography Sample containing the defect in ring-1 was mounted along the cross section, while that in ring-2 was mounted along the central web region for microstructural study.
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Fig. 9. SEM photograph showing the internal surface of the cavity shown in Fig. 8. The surface has a smooth appearance similar to a solidified morphology.
Fig. 10. Optical micrograph showing the microstructure near the cavity in ring-1, 50 x.
3.3.1. Ring-1 The optical microstructure in the region of cavity is shown in Fig. 10. At low magnification (50x), the microstructure showed dark etched regions in the vicinity of the cavity and lightly etched regions away from it. At higher magnification (500x), the dark etched region was resolved and found to consist of an acicular type of structure (Fig. 11). The cavity was completely surrounded by this acicular phase. Away from the cavity, the microstructure consisted of deformed nickel rich gamma grains Wlth twins, typical of hot worked nickel base superalloy structure (Fig. 12). However, there was significant grain size variation from region to region. Microstructural observations also suggest variation in the extent of deformation from region to region.
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457
Fig. 11. Optical microstructure of the gray area shown in Fig. 10. It consists of acicular phase, 500x.
Fig. 12. Optical micrograph at a region away from the defective area showing deformed grains with twins, typical of hot-worked nickel base superalloy structure, 500 x.
The acicular structure was resolved better under the scanning electron microscope (Fig. 13). EDX analysis ' the alloy). In addition, "g " " has showed this region to be rich in Nb (12 OYo compared to 5 0V0 in some faceted/ globular particles rich in Nb (about 88%) were also observed in this region. It can be noted that such type of structure was seen only in the vicinity of the cavity. Away from the cavity no such phases were present and the microstructure was found to be normal.
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Fig. 13. SEM micrograph showing acicular and faceted/globularphases in gray region shown in Fig. 10.
Fig. 14. Optical micrograph showing the microstructurenear the cavity in ring-2, 50x. 3.3.2. Ring-2
The microstructure around the cavity was found to be exactly similar to that described in ring-1 (Fig. 14). However, in the present ring, this kind of microstructure was found spread over a distance of about 20 mm along the central web region. At higher magnification, this region consisted of acicular phase. As in ring-l, in this case also, the microstructure consists of acicular and faceted/globular phases, and carbides at the grain boundaries. The population of the faceted/globular phase and the aspect ratio of the acicular phase were found to be much more in ring-2 than those observed in ring-1 (Fig. 15). The X-ray mappings showing the distribution of elements in different phases are shown in Fig. 16.
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Fig. 15. SEM micrograph showing acicular and faceted/globular phases near the cavity in Fig. 14.
3.4. Hardness measurements 3.4.1. Ring-1 Considerable variation in hardness values was observed near the cavity and away from the cavity. Near the cavity, where the microstructure is acicular, the hardness value was measured to be HV 320 compared to HV 220 measured elsewhere in the defect free region. 3.4.2. Ring-2 Similar variation in hardness values was also observed in ring-2. In the acicular phase region, the hardness was about HV 407 compared to HV 265 measured elsewhere in the defect free region.
4. Analysis of results Visual and stereobinocular observations clearly indicated the existence of volume defects in both the rings. It appears that in ring-l, the defect was present in the material and got exposed to the surface during subsequent machining. The size of the defect on the plane of observation was about 2.6 mm. No estimation of the depth could be made in the SEM. In ring-2, the defect was not exposed to the surface. However, it was just covered with a thin layer of material and when this layer of material was removed, the hidden volume defect (cavity) could be seen. Therefore, it can be inferred that the dent observed was actually due to collapsing of the thin layer of material on top of the cavity under the tool pressure during machining. The inside surface of both the defects was extensively studied in the SEM. The nature and morphology of the exposed (in ring-1) and unexposed (in ring-2) defects were found to be exactly similar. Ti~is observation establishes the fact that the volume defects were nothing but cavities and also eliminates the possibility of any particle being embedded in the defective area, and falling away during machining to leave a cavity behind.
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Fig. 16. X-ray mapping showing distribution of elements in different phases. Note that the faceted/globular phases are rich in Nb and Ti.
The inside surfaces of both the cavities had a smooth appearance and resembled closely a solidified morphology, and possibly remained unaffected during hot forming. Fractographically, these surfaces were featureless. This clearly indicates that these defects would not have been created during hot forming. Otherwise, one would have expected a surface with some fractographic features."Moreover, the two mutually opposite surfaces inside the defect should also match with each other. Microstructural studies indicated inhomogeneous composition and microstructure near the cavity. Such abnormality was seen only near the cavity. The defective area consists of acicular phase as well as faceted
S.K. Bhaumik et al./ Engineering Failure Analysis 11 (2004) 449 462
46l
and globular phases. EDX analysis confirmed these areas to be rich in Nb. It was not possible to find out the exact composition of the acicular phase by semiquantitative EDX analysis in the SEM, but the result does indicate a high concentration of Nb (12-15 wt.% as compared to about 5 wt.% in the alloy) in these regions. The globular/faceted phases contain about 88 wt.% Nb and 6.5 wt.% Ti. These results suggest that there was chemical inhomogeneity in the ingot which might have resulted in the formation of an inhomogeneous microstructure during subsequent hot working. The microstructure elsewhere, away from the cavity, was found to be normal. However, the grain size variation and the microstructure clearly indicate the variation in the extent of deformation from region to region in the same ring. The co-existence of volume defects and chemical/microstructural inhomogeneity in localized regions suggest that the present problem might have resulted during the solidification stage itself. It is possible that during vacuum arc remelting, some solid particles, rich in Nb, would have broken from the electrode and fallen in the melt pool. It is also possible that the solidification rate was such that these particles did not get enough residence time in the liquid to melt and homogenize with the remaining ingot. In such a situation, not only did these Nb rich particles remain solid but also they would have obstructed the fluid movement during solidification resulting in a cavity in the vicinity. It is well known that unless stringent process control is exercised such problems do occur in vacuum arc remelting of superalloys containing Ni and Fe [2]. These defects are known as "Freckles". Subsequent hot working of the freckeled ingot gives rise to defective microstructure consisting of acicular delta phase (orthorhombic Ni3Nb) globular Nb3Ti phase and numerous primary carbides (Nb rich-MC) similar to that observed in the present study. It is also reported that when such defects are present, it is often difficult to break down the ingot microstructure because of the poor deformability of the Nb rich regions. This can be confirmed in terms of the hardness values measured at different regions i.e., defective region and defect free region. When a material containing two regions with varying hardness is plastically deformed, it is expected that the softer region surrounding the harder region will deform preferentially. If the hardness difference is more as in the present case, it is probable that during hot working the plastic deformation is confined to the softer region only and the harder region remains relatively intact except for diffusion related phase changes. In such a case, it is possible for the volume defects present in the ingot to survive the subsequent hot deformation without any noticeable change in the appearance and morphology. However, there could be a very limited deformation in the harder region surrounding the cavity, which might have produced steps on the free surface (inside surface of the cavity). These steps looked like parallel bands under the SEM. Even in these regions, the surface structure resembling the solidified morphology is still preserved.
5. Conclusions
1. From the nature of defects and other microstructural evidence, it appears that the defects observed in the superalloy rings are nothing but solidification cavities. 2. The volume defects (cavities) could survive the subsequent hot deformation due to the wide difference in hardness values in the cavity region and away from the cavity.
Acknowledgements The authors thank Mr. C.R. Kannan and Ms. Kalavathi for NDT and Scanning Electron Microscope support. The help rendered by Mr. Keshab Barai for metallographic sample preparation is acknowledged.
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The a u t h o r s also t h a n k Dr. A.C. R a g h u r a m a n d Dr. R. K r i s h n a n for fruitful discussions d u r i n g the course o f this analysis. The a u t h o r s are t h a n k f u l to D i r e c t o r , N A L , for p e r m i s s i o n to p u b l i s h this work.
References [1] Carneiro T, Tither G. Special supplement on superalloys of American metals market. 24 August 2000. [2] Pridgeon JW, Darmara FN, Huntington JS, Sutton WH. Superalloys source book. Metals Park (OH): American Society for Metals; 1984 pp. 201~16..
ELSEVIER
C~)DIRECT°
Engineering Failure Analysis 11 (2004) 449~462
ENGINEERING
FAILURE ANALYSIS www.elsevier.com/locate/engfailanal
Metallurgical investigation of defective superalloy rings S.K. Bhaumik*, T.A. Bhaskaran, R. Rangaraju, M.A. Venkataswamy, M.A. Parameswara, R.V. Krishnan Failure Analysis Group, Materials Science Division, National Aerospace Laboratories, Post Bag 1779, Bangalore 560 017, India
Received 8 May 2003; accepted 17 May 2003
Abstract
During the final machining of nickel base superalloy rings, some cavity-like defects were observed in a few components. Some of them were exposed to the surface, while the others were detected by X-radiography. The defective rings were examined and a study was carried out to identify the processing step during which the defects would have been introduced in the components. Under the scanning electron microscope, the cavity surface was found to be smooth with typical appearance of a molten and solidified structure. Detailed microstructural study at these defective regions showed an acicular microstructure with faceted/globular phases. EDX analysis confirmed these areas to be rich in Nb. It was also found that the cavities observed were always associated with such unacceptable microstructures. A detailed analysis of the results is presented in this article. © 2003 Elsevier Ltd. All rights reserved. Keywords: Superalloys; Microstructures; Solidification; Defects; X-ray analysis
1. Introduction
A m o n g the commercially available superalloys, nickel-base superalloys such as I N 718, I N 706 and I N 625 are most widely used for aeroengine applications. They account for the majority of the total production of nickel-base superalloys. Niobium is present in these alloys in quantities varying from 3 to 5.5 wt.%. Inconel 718 (Ni-19Cr-5.1Nb-3Mo-0.9Ti-0.5AI-18.5Fe) was introduced by Inco Alloys in 1959. Niobium's primary role in this alloy is solid solution strengthening and precipitation hardening. Addition of N b results in the formation of MC-type and M6C-type carbides in addition to the 7" phase, Ni3Nb. The strengthening mechanism involves precipitation of an intermetallic compound (Ni3Nb) in the nickel matrix during heat treatment. Although there are other alternatives to niobium as a strengthener, niobium is found to be unique in its ability to avoid strain-age cracking during fabrication of the final components, especially during welding. Since niobium provides a slower aging response, parts can be thermally stress relieved without cracking. I N 718 shows much better performance than the other precipitation-strengthened superalloys available, such as Astroloy (Ni-15Cr-17Co-5.25Mo~4A1-3.5Ti-0.06C-0.03B) and Ren6 * Corresponding author. Tel.: + 91-80-527-3351; fax: + 91-80-5270098. E-mail address: [email protected](S.K. Bhaumik). 1350-6307/$ - see front matter © 2003 Elsevier Ltd. All rights reserved. doi: 10.i 016/j.engfailanal.2003.05.019
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S.K. Bhaumik et al./ Engineering Failure Analysis, 11 {2004) 449~62
41 (Ni-19Cr-11Co 10Mo-I.5A1-3.1Ti-0.09C-0.005B). I N 718 also exhibits better creep properties in the range 1000-1200 °F than Rene 41 and Astroloy. I N 718's residual strength and ductility properties are not impaired by exposure in this temperature range. Even after more than 40 years since first being introduced by INCO, I N 718 is still the most important all-purpose nickel-base superalloy [1]. The normal practice for the production of nickel base superalloy ingots is by vacuum arc remelting (VAR) or electroslag remelting (ESR). The consumable electrode required for the above is produced by vacuum induction melting (VIM). The progressive directional solidification in V A R facilitates obtaining ingots nearly free from shrinkage cavities. In addition, a homogeneous structure can be obtained with minimum amount of macrosegregation. The initial consumable V A R is followed by a second VAR, and sometimes even third V A R step, to assure better chemical and structural homogeneity in the ingot. The structure in these ingots is further refined by extrusion, pressing, rolling etc, to form billets or bars which are then forged to the final shape. However, producing a segregation-free structure is very difficult in nickel base superalloys containing nickel and iron even after VAR. A very stringent process control is required to obtain ingots free from defects or with minimum defects which can be easily tackled in the subsequent metal working operations [2]. This paper describes in detail a metallurgical investigation that has been carried out on defective nickel base superalloy (composition equivalent to I N 718) rings. The nominal composition of the alloy is given in Table 1. The defects were observed at the final machining stage and the objective of the investigation was to identify the stage in the processing steps during which the defect could have been introduced.
2. Background 2.1.
The process
The raw material was obtained from the supplier in round bar form. These round bars were manufactured by basic melting processes such as V I M and VAR, and the cast ingot had a diameter of 450 mm. The cast ingot was forged/rolled to bars and then turned to the required diameter of 160 mm. F r o m the supplied raw material, unit stock of 160 m m qbx 310 m m length was cut and was subjected to upset forging and piercing to obtain an upset cheese of size 350 m m O D x 150 m m I D x 6 0 m m HT. The cheese was then ring rolled to size 471 m m OD x 370 m m I D x 40 m m HT. The ring thus obtained was then machined to get the required profile. The processing flow chart for the production of these rings is shown in Fig. 1.
Table 1 Nominal composition of the alloy Element
Wt. %
C Mn Si Cr Mo Nb Ti A1 Fe Cu Ni
0.04 0.2 0.2 18.5 3.0 5.1 0.9 0.5 17.0 0.2 Balance
S.K. Bhaumik et al./ Eng#wer# N Failure Analysis 11 (2004) 449-462
Starting material
451
Unit stock 160 ~ x 310 L
350 OD x 150 ID x 60 HT
Upset forging & piercing I
I
I
i 1 1
481 OD x 360 ID x 55 HT
Ring rolling I
Solution heat treatment followed by machining (471 OD x 370 ID x 40 HT) All dimensions in mm.
Fig. 1. Schematicshowing the processingflow chart.
2.2. The defect
While final machining of one of the rings out of a batch of five, cavity-like defects readily visible to the unaided eye were observed at the fillet region of the change of section. X-radiography was conducted on all the machined rings and one more ring was found to have similar defects (not exposed to the surface) at the central web region. The photograph of one of the defective rings is shown in Fig. 2. These two defective rings were analysed in the laboratory to determine whether (a) these defects were created during upset cheese forging and/or ring rolling or (b) these defects were created during machining due to presence of some very hard phase which might have fallen during machining leaving behind a cavity on the surface. These questions were addressed during the investigation and a detailed analysis is presented.
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Fig. 2. Photograph showing one of the defectiverings (ring-l).
3. Experimental details The defects exposed to the surface were examined visually and under a stereobinocular microscope. The defective area was cut from the ring, cleaned ultrasonically in acetone and then observed in a scanning electron microscope (SEM, LEO 440I). Microstructural study was carried out by using both optical microscope (Nikon, Epiphot-200) and SEM. Samples for microstructural study were prepared by standard metallographic techniques. Energy Dispersive X-ray analysis was carried out by EDX attached to the SEM. Hardness measurements were carried out using a S H I M A D Z U HSV-20 Vickers microhardness tester at a load of 500 g. 3.1. Visual and stereo-binocular 3.1.1. Ring-1
Visual observation revealed a cavity like defect in ring-1 at the fillet at the change of section as shown in Fig. 3. Under the stereo-binocular microscope, this defect looked like a volume defect, which was present in the material and became exposed to the surface during machining. The inside surface of the cavity had a smooth appearance. 3.1.2. Ring-2
!
The defect in ring-2 did not show any such cavity, but looked more like a dent on the surface under the stereo-binocular microscope. The location of this defect was in the central web region close to the fillet as in ring-1. Careful examination revealed a number of cracks around the dent.
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Fig. 3. Photograph showing the cavity-likedefect in ring-1.
Fig. 4. SEM photograph showing the close-up view of the defect shown in Fig. 3. 3.2. Scanning electron microscopy ( S E M ) 3.2.1. Ring-1 The sample containing the defect was ultrasonically cleaned in acetone and observed in the SEM. The appearance of the defect under SEM is shown in Fig. 4. The internal surface of the cavity i~s shown in Fig. 5. It had a smooth surface similar to a solidified morphology resembling a dendritic structure. Fractographically it was featureless. However, in some places, parallel band like features were seen (Fig. 6). At higher magnification, this region also showed the surface morphology similar to that shown in Fig. 5.
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Fig. 5. SEM photograph showing the internal surface of the cavity in ring-1. The surface has a smooth appearance similar to a solidified morphology.
Fig. 6. SEM photograph of the internal surface of the cavity in ring-l. The banded structure with solidified morphology indicates limited deformation in this region.
3.2.2. Ring-2 The S E M p h o t o g r a p h o f the dent in ring-2 is shown in Fig. 7. A thin flaky m a t e r i a l was observed at the dent region. E D X analysis confirmed this m a t e r i a l to be the same as t h a t o f the p a r e n t material. W h e n the flaky m a t e r i a l was r e m o v e d by a sharp needle/edge, a h i d d e n cavity was observed (Fig. 8). A r o u n d the dent, extensive c r a c k i n g was also seen. D e t a i l e d e x a m i n a t i o n revealed t h a t the n a t u r e a n d m o r p h o l o g y o f this
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Fig. 7. SEM photograph showing a dent-like defect in ring-2. The dent mark was surrounded by a number of cracks.
Fig. 8. SEM photograph showing the cavity after removing the thin flaky layer of material at the dent region shown in Fig. 7.
cavity are similar to that mentioned earlier for ring-1. The inside surface of the cavity had the same smooth appearance with no fractographic features (Fig. 9).
3.3. Metallography Sample containing the defect in ring-1 was mounted along the cross section, while that in ring-2 was mounted along the central web region for microstructural study.
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Fig. 9. SEM photograph showing the internal surface of the cavity shown in Fig. 8. The surface has a smooth appearance similar to a solidified morphology.
Fig. 10. Optical micrograph showing the microstructure near the cavity in ring-1, 50 x.
3.3.1. Ring-1 The optical microstructure in the region of cavity is shown in Fig. 10. At low magnification (50x), the microstructure showed dark etched regions in the vicinity of the cavity and lightly etched regions away from it. At higher magnification (500x), the dark etched region was resolved and found to consist of an acicular type of structure (Fig. 11). The cavity was completely surrounded by this acicular phase. Away from the cavity, the microstructure consisted of deformed nickel rich gamma grains Wlth twins, typical of hot worked nickel base superalloy structure (Fig. 12). However, there was significant grain size variation from region to region. Microstructural observations also suggest variation in the extent of deformation from region to region.
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Fig. 11. Optical microstructure of the gray area shown in Fig. 10. It consists of acicular phase, 500x.
Fig. 12. Optical micrograph at a region away from the defective area showing deformed grains with twins, typical of hot-worked nickel base superalloy structure, 500 x.
The acicular structure was resolved better under the scanning electron microscope (Fig. 13). EDX analysis ' the alloy). In addition, "g " " has showed this region to be rich in Nb (12 OYo compared to 5 0V0 in some faceted/ globular particles rich in Nb (about 88%) were also observed in this region. It can be noted that such type of structure was seen only in the vicinity of the cavity. Away from the cavity no such phases were present and the microstructure was found to be normal.
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Fig. 13. SEM micrograph showing acicular and faceted/globularphases in gray region shown in Fig. 10.
Fig. 14. Optical micrograph showing the microstructurenear the cavity in ring-2, 50x. 3.3.2. Ring-2
The microstructure around the cavity was found to be exactly similar to that described in ring-1 (Fig. 14). However, in the present ring, this kind of microstructure was found spread over a distance of about 20 mm along the central web region. At higher magnification, this region consisted of acicular phase. As in ring-l, in this case also, the microstructure consists of acicular and faceted/globular phases, and carbides at the grain boundaries. The population of the faceted/globular phase and the aspect ratio of the acicular phase were found to be much more in ring-2 than those observed in ring-1 (Fig. 15). The X-ray mappings showing the distribution of elements in different phases are shown in Fig. 16.
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Fig. 15. SEM micrograph showing acicular and faceted/globular phases near the cavity in Fig. 14.
3.4. Hardness measurements 3.4.1. Ring-1 Considerable variation in hardness values was observed near the cavity and away from the cavity. Near the cavity, where the microstructure is acicular, the hardness value was measured to be HV 320 compared to HV 220 measured elsewhere in the defect free region. 3.4.2. Ring-2 Similar variation in hardness values was also observed in ring-2. In the acicular phase region, the hardness was about HV 407 compared to HV 265 measured elsewhere in the defect free region.
4. Analysis of results Visual and stereobinocular observations clearly indicated the existence of volume defects in both the rings. It appears that in ring-l, the defect was present in the material and got exposed to the surface during subsequent machining. The size of the defect on the plane of observation was about 2.6 mm. No estimation of the depth could be made in the SEM. In ring-2, the defect was not exposed to the surface. However, it was just covered with a thin layer of material and when this layer of material was removed, the hidden volume defect (cavity) could be seen. Therefore, it can be inferred that the dent observed was actually due to collapsing of the thin layer of material on top of the cavity under the tool pressure during machining. The inside surface of both the defects was extensively studied in the SEM. The nature and morphology of the exposed (in ring-1) and unexposed (in ring-2) defects were found to be exactly similar. Ti~is observation establishes the fact that the volume defects were nothing but cavities and also eliminates the possibility of any particle being embedded in the defective area, and falling away during machining to leave a cavity behind.
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Fig. 16. X-ray mapping showing distribution of elements in different phases. Note that the faceted/globular phases are rich in Nb and Ti.
The inside surfaces of both the cavities had a smooth appearance and resembled closely a solidified morphology, and possibly remained unaffected during hot forming. Fractographically, these surfaces were featureless. This clearly indicates that these defects would not have been created during hot forming. Otherwise, one would have expected a surface with some fractographic features."Moreover, the two mutually opposite surfaces inside the defect should also match with each other. Microstructural studies indicated inhomogeneous composition and microstructure near the cavity. Such abnormality was seen only near the cavity. The defective area consists of acicular phase as well as faceted
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and globular phases. EDX analysis confirmed these areas to be rich in Nb. It was not possible to find out the exact composition of the acicular phase by semiquantitative EDX analysis in the SEM, but the result does indicate a high concentration of Nb (12-15 wt.% as compared to about 5 wt.% in the alloy) in these regions. The globular/faceted phases contain about 88 wt.% Nb and 6.5 wt.% Ti. These results suggest that there was chemical inhomogeneity in the ingot which might have resulted in the formation of an inhomogeneous microstructure during subsequent hot working. The microstructure elsewhere, away from the cavity, was found to be normal. However, the grain size variation and the microstructure clearly indicate the variation in the extent of deformation from region to region in the same ring. The co-existence of volume defects and chemical/microstructural inhomogeneity in localized regions suggest that the present problem might have resulted during the solidification stage itself. It is possible that during vacuum arc remelting, some solid particles, rich in Nb, would have broken from the electrode and fallen in the melt pool. It is also possible that the solidification rate was such that these particles did not get enough residence time in the liquid to melt and homogenize with the remaining ingot. In such a situation, not only did these Nb rich particles remain solid but also they would have obstructed the fluid movement during solidification resulting in a cavity in the vicinity. It is well known that unless stringent process control is exercised such problems do occur in vacuum arc remelting of superalloys containing Ni and Fe [2]. These defects are known as "Freckles". Subsequent hot working of the freckeled ingot gives rise to defective microstructure consisting of acicular delta phase (orthorhombic Ni3Nb) globular Nb3Ti phase and numerous primary carbides (Nb rich-MC) similar to that observed in the present study. It is also reported that when such defects are present, it is often difficult to break down the ingot microstructure because of the poor deformability of the Nb rich regions. This can be confirmed in terms of the hardness values measured at different regions i.e., defective region and defect free region. When a material containing two regions with varying hardness is plastically deformed, it is expected that the softer region surrounding the harder region will deform preferentially. If the hardness difference is more as in the present case, it is probable that during hot working the plastic deformation is confined to the softer region only and the harder region remains relatively intact except for diffusion related phase changes. In such a case, it is possible for the volume defects present in the ingot to survive the subsequent hot deformation without any noticeable change in the appearance and morphology. However, there could be a very limited deformation in the harder region surrounding the cavity, which might have produced steps on the free surface (inside surface of the cavity). These steps looked like parallel bands under the SEM. Even in these regions, the surface structure resembling the solidified morphology is still preserved.
5. Conclusions
1. From the nature of defects and other microstructural evidence, it appears that the defects observed in the superalloy rings are nothing but solidification cavities. 2. The volume defects (cavities) could survive the subsequent hot deformation due to the wide difference in hardness values in the cavity region and away from the cavity.
Acknowledgements The authors thank Mr. C.R. Kannan and Ms. Kalavathi for NDT and Scanning Electron Microscope support. The help rendered by Mr. Keshab Barai for metallographic sample preparation is acknowledged.
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The a u t h o r s also t h a n k Dr. A.C. R a g h u r a m a n d Dr. R. K r i s h n a n for fruitful discussions d u r i n g the course o f this analysis. The a u t h o r s are t h a n k f u l to D i r e c t o r , N A L , for p e r m i s s i o n to p u b l i s h this work.
References [1] Carneiro T, Tither G. Special supplement on superalloys of American metals market. 24 August 2000. [2] Pridgeon JW, Darmara FN, Huntington JS, Sutton WH. Superalloys source book. Metals Park (OH): American Society for Metals; 1984 pp. 201~16..