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Vol. 8. Issue 6.
Pages 6413-6419 (November - December 2019)
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Vol. 8. Issue 6.
Pages 6413-6419 (November - December 2019)
Short Communication
DOI: 10.1016/j.jmrt.2019.10.048
Open Access
Study of mechanical and metallurgical properties of Hastelloy X at cryogenic condition
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Pavithra Ekambaram
Department of Mechanical Engineering, Vel Tech Rangarajan Dr Sagunthala R & D Institute of Science and Technology, Avadi, Chennai – 62, Tamilnadu, India
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Tables (2)
Table 1. Chemical composition of Hastelloy X (courtesy: https://www.azom.com/article.aspx?ArticleID=4460).
Table 2. The results of tensile testing.
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Abstract

Hastelloy X is widely used in aero space applications. In this research work, the investigation of Hastelloy x has been carried out at low temperature, that is, cryogenic temperature (−196°C). The effects have been studied when the material was treated under 12 and 24h of cryogenic temperature respectively. The mechanical behavior of the material relating to hardness, tensile strength and metallurgical property were studied using Optical Microscope (OM), Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDAX) and X-ray diffraction (XRD) after it was cryo treated and a comparison was made with the raw material. The results indicated the cryo treated Hastelloy X had retained the mechanical and metallurgical properties at sub zero temperature. Hence, the results of this study can be used in aerospace industries where the components are subjected to cryogenic temperatures.

Keywords:
Hastelloy X
Cryogenic treatment
SEM
XRD
Full Text
1Introduction

The Ni-Cr-Fe-Mo alloy, Hastelloy X, finds extensive use in high temperature applications [1,2]. Nickel based alloys are widely used in high heat resistant jobs extensive use of Hastelloy X at high temperatures is seen compared to Inconel 718 [3]. Thermal exposure leads to the formation of molybdenum and chromium rich carbides [4]. The physical properties of Hastelloy C-276 and the mechanical properties of fully coated buffer layer, as substrate for semiconductors at low temperatures were studied [5,6]. The material used for high temperature is also used for low temperature applications. The treatment of materials carried at -196°C is referred to as cryogenic treatment or deep cryo treated samples. Cryogenics improves wear resistance and mechanical properties, impact toughness and dimensional stability [7]. In aerospace industries, cryo fluids are used in the pipelines. The components used are nickel based alloys. The author has made a thorough study of the relevant literature for findings to support the research work related to cryogenic conditions. But, no research attempt appears to have been made towards the study of Hastelloy X at cryogenic condition. The purpose of this paper is to analyse the effect of cryogenic medium with respect to the change in the property of the material (Hastelloy X). Some research studies have addressed machining of the hard material using cryogenics as coolant which results in a positive performance. Most of such research articles are based on the cryogenic medium for machining. Deep cryogenic treatment (DCT) appears as the subject seen only in a few studies. The entire treatment of the sample was in a cryogenic bath. Franco et al have referred to the DCT improving the wear resistance in Aluminium alloy [8]. Cryogenic chill results in finer grains, improved mechanical property and good bonding have been reported in a research article by Anil Kumar et al [9]. In addition, better dimensional stability and refined grain structure were obtained in cryogenic-treated materials [10]. The surface characterisation techniques play a vital role in materials including cryo treated materials. The characterisation techniques namely optical microscope, XRD, SEM, TEM (Transmission Electron Microscope) and EDAX are widely used for analyzing micro and nano particles [11–15]. These techniques examine the process mechanism, phase transformation and formation, residual stress, elemental composition and size and shape of the grains.

To the best of the author’s knowledge, no research has been made on the mechanical and metallurgical study of deep cryo treated alloys. Hence, this paper has taken up the mechanical and metallurgical properties of cryogenic treated samples Hastelloy X at 12h and 24h as the subject of investigation. The results are thus compared with the untreated sample.

2Materials and methods

In this research work Hastelloy X, material of sheet thickness 1.6mm was used. The material was prepared using wire cut EDM for the required dimensions for conducting mechanical and metallurgical tests. Details of the composition of the Hastelloy x are shown in Table 1. The sample chosen was as 10mm×10mm. It was treated with cryogenic medium (liquid nitrogen, −196°C) for 12 and 24h. Mechanical tests namely, Vickers hardness and tensile test were conducted in three conditions, namely, raw sample, 12h and 24h cryo treated samples. Metallographic study of microstructure, SEM with EDAX and XRD was carried out.

Table 1.

Chemical composition of Hastelloy X (courtesy: https://www.azom.com/article.aspx?ArticleID=4460).

Mg  Si  Cr  Ni  Mo  Fe  Ti  Al  Co  Cu 
0.15  0.040  0.030  23  Bal  10  20  0.15  .50  2.5  0.01  0.50 

The hardness of the samples was measured using Vickers hardness tester (Matsuzawa Co Ltd with model MMT-X7) with a load of 10kg and a dwell time of 3s.

The tensile samples were prepared using a wire cut EDM as per the dimensions (ASTM E8) shown in Figs. 1 and 2. The samples were then cryo treated for 12h and 24h. Following cryo treatment, the samples were loaded in the tensile testing machine - Jinan Testing Equipment IE Corporation (Model : WDW 200).

Fig. 1.

Dimensions of tensile specimen.

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Fig. 2.

Tensile test samples (raw sample).

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Hastelloy X samples such as raw material, 12h and 24h cryo treated were finely polished and etched using 10ml HNO3, 20–50ml HCl and 30ml glycerol, known as glycergia reagent. The microstructures of the samples were captured using an optical microscope (LEICA – DM4000M) with four different magnifications.

Surface morphology and fractography of the prepared samples were examined using a Scanning Electron Microscope (COXEM–CX200) under the secondary electron mode. Also, the elemental composition of the treated samples was obtained through Elemental Dispersive X-ray Spectroscopy (Bruker –X#6I1).

In addition, analysis of the metallurgical properties of the treated samples was done using Rigaku XRD miniflex II – C with a scanning rate of 0.04°.

3Results and discussion3.1Hardness

Details relating to hardness of the samples observed are shown in Fig. 3. Averages of three iterations are shown in Fig. 4. A 10% decrease in the hardness level compared to raw samples was seen following the cryogenic treatment. This led to increase in the toughness and ductility of the material. The hardness of 12h cryo treated sample was low compared to 24h cryo treated sample.

Fig. 3.

Samples for hardness testing.

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Fig. 4.

Hardness.

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3.2Tensile test

The fractured samples are shown in Fig. 5 and the results obtained are tabulated and shown in Table 2. The tensile strength of the raw material is seen as comparatively low compared to cryo treated samples. This was due to the decrement in the hardness of the cryo treated samples which was the result of an increase in the coarsening of grains caused by cryogenic cooling. The grain enlargement was the result of the occurrence of recrystallisation at low temperature; causing an increase in the size of the grains with simultaneously decreases in yield strength. A higher elongation was also observed in 12h cryo treated samples compared to the other conditions as they increase the ductility of the material.

Fig. 5.

After tensile testing of samples cryo treated at 24h, 12h and raw material.

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Table 2.

The results of tensile testing.

Sample conditions  Yield strength (MPa)  Average (MPa)  Tensile Strength (MPa)  Average (MPa)  Elongation (%)  Average (%) 
Raw material  348  352  747  773  47.00  47.8 
  352    786    47.50   
  355    787    49.00   
After 12hrs cryo treated  348  345  787  784  49.00  48.3 
  342    789    49.50   
  346    776    46.50   
After 24hrs cryo treated  351  352  783  780  45.00  44.6 
  347    777    43.50   
  359    780    45.50   
3.3Microstructure analysis

The microstructure of Hastelloy X samples is shown in Fig. 6. Uniform appearance of carbides is seen on the grain boundaries and on the grains. The average grain sizes measured were found to be 5.5, 6.5 and 6 microns for raw sample, 12 and 24h respectively. The grain growth in the cryo treated samples confirmed the occurrence of the recrystallisation process. Variation in this level of action were made through variations in the duration of cryo treatment in the Hastelloy X. The results showed the 12h cryo treated sample including a higher level of recrystallisation which resulted in, grain coarsening effect followed by lower hardness and higher ductility found. The continuous cooing of Hastelloy X by 24h, led to the retention of the grain structure close to the raw material. However, there were no significant variations seen in the three different samples. The twinning (rectangular bands) of the grains along with the grain boundaries was predominantly seen in the higher magnifications.

Fig. 6.

Microstructure of raw material, 12h and 24h cryo treated samples at four different magnifications.

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3.4Surface morphology

The four different magnifications of the samples were observed. Details are shown in Fig. 7. Twinning was seen on all the three different samples. The presence of nucleation and cluster of nucleation growth was seen on the grains and grain boundaries. Compared to other two samples, increased growth of nucleation was seen on the 24h cryo treated sample. This nucleation was continuously present on the grain boundaries. This happened due to the deep cryo treatment of the materials which caused the occurrence of dynamic recrystallisation. The effect of dynamic recrystallisation was confirmed by grain coarsening in the cryo treated materials. Despite this, dynamic recrystallisation was limited at 12h and 24h cryogenic treatments. Further extension of cryogenic treatment, could have led to the formation of a new grain structure. A few elements diffused between grains and their boundaries at lower temperature were also seen. EDAX analysis had shown in Table 3 confirms this. The location of the sample was seen both on the grains and on the grain boundaries. The capture of location of the EDAX is shown in Fig. 8. The presence of M6C, molybdenum rich carbide precipitates were seen [16]. Variations in the elemental composition seen were with minimum levels. The cryo treatment of the samples did not have much impact on the changes from the raw samples. However, no other changes were observed in the treated materials.

Fig. 7.

SEM analysis of Hast X samples at three different samples with four different magnifications.

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Table 3.

Elementary composition of Hast X.

 
Fig. 8.

EDAX locations away from the grain boundary and on the grain boundary.

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3.5XRD analysis

Fig. 9 shows the XRD peaks of different material conditions. Peaks such as (111), (200) formed in the various conditions confirm the FCC phase of the material [17]. No shifting of peaks was observed despite the employment of deep cryogenic treatment. There was no significant occurrence of grain refinement or coarsening as result of this. However, there was a slight increase in the peak width in the deep cryogenic treatment conditions compared to the raw material. The occurrence was the result of the enlargement of grains in the cryo treated material. This was confirmed by the higher grain size that of the raw material. A higher peak width was found in the 12h cryogenic treated condition which induced the coarsening of the grains (Fig. 6); as a result, hardness was decreased (Fig. 2). Also, residual stress was examined using “d” spacing vs. sin2Ψ method. Compressive residual stress was found. However, the residual stress of the different conditions remained the same (−155MPa). Insignificance in the shifting of peaks also restricted the accumulation of residual stress in the crystal structure leading to the maintenance of the lattice spacing between the atoms centre at the same level. Hence the ability of the Hastelloy X to retain the material properties under sub zero temperature environment was confirmed.

Fig. 9.

(a) XRD of three samples. (b) Phase shift of the peak (111).

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3.6Fractography analysis

SEM micrograph in Fig. 10 shows the fractography investigations of the tensile samples at four different magnifications and three different conditions. The mode of failure observed predominantly was ductile fracture. But the mixed mode of failure was also seen in few patches. The dimples of smaller size were observed. The striations were seen in all the three conditions. The cryo treated samples fractography proved that there were no much variations observed when compared to the raw material. The SEM mages revealed that the presence of molybdenum rich carbides (M6C) on the grain boundaries.

Fig. 10.

Fractography study of Hastelloy X at three different samples with four different magnifications with scale bar 20 μm.

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4Conclusion

Hastelloy x was deep cryo treated at 12h and 24h respectively. The mechanical and microstructural properties were investigated. There was a 10% decrease in the hardness of the cryo treated samples compared with raw sample. The decrement of the hardness caused an increment in the tensile strength of the cryo treated sample. The cryogenic treatment enlarged the grain size due to the occurrence of recrystallisation.

Further prolonging at the same temperature led to the refinement of the grains. This resulted in the continuous nucleation on the grain boundaries. The effect of dynamic recrystallisation was observed by the grain coarsening of the cryo treated samples. However there were no major changes in the grain structure observed. This was confirmed by XRD peak analysis. Thus the results of the above material characterisation proved the absence of any significant variation on the cryo treated Hastelloy X. Hence Hastelloy X can be used at low temperature for aerospace applications.

Acknowledgement

This experimental work was funded from SEED fund by Vel Tech Rangarajan Dr Sagunthala R & D Institute of Science and Technology. I thank my colleague, Dr. N. Yuvaraj, Assistant Professor for his valuable comments that greatly improved the manuscript.

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Copyright © 2019. The Author
Journal of Materials Research and Technology

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