Effects of milling time and temperature on phase evolution of AISI 316 stainless steel powder and subsequent sintering

Accepted Manuscript Effects of milling time and temperature on phase evolution of AISI 316 stainless steel powder and subsequent sintering R. Esmaeilzadeh, M. Salimi, C. Zamani, A. M. Hadian, A. Hadian PII:

S0925-8388(18)32446-0

DOI:

10.1016/j.jallcom.2018.06.325

Reference:

JALCOM 46661

To appear in:

Journal of Alloys and Compounds

Received Date: 17 May 2018 Revised Date:

25 June 2018

Accepted Date: 26 June 2018

Please cite this article as: R. Esmaeilzadeh, M. Salimi, C. Zamani, A. M. Hadian, A. Hadian, Effects of milling time and temperature on phase evolution of AISI 316 stainless steel powder and subsequent sintering, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.06.325. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effects of Milling Time and Temperature on Phase Evolution of AISI 316 Stainless Steel Powder and Subsequent Sintering R. Esmaeilzadeha , M. Salimia , C. Zamani*a , A. M. Hadiana, A. Hadiana a

School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran.

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* Corresponding author. [email protected]

Abstract

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In this study, effects of milling conditions on mechanical and microstructural properties of AISI 316 stainless steel powder were examined. Milling process was performed at ambient and cryogenic (-100 to -90℃) temperatures for 1 to 12 hours. The cryogenic tests were performed using liquid nitrogen without direct contact to the main powder. In order to investigate the effect of milling parameters on final properties of sintered samples, hot pressing was performed on selected as-milled powders at 1150 ℃ for 1 hour under 65 MPa pressure. XRD analysis on as-milled powders confirmed the phase transformation of austenite () to   martensite in all samples. It was also evident from quantitative measurements carried out by Rietveld refinement method that the   weight fraction was a function of time and temperature. Based on this method, for all phases, crystallite size reduced to nanometer regime. Microstructural analysis for all samples was carried out by Scanning Electron Microscopy (SEM) equipped with Energy Dispersive Spectroscopy (EDS) that confirmed Rietveld analysis results. For as-sintered samples, relative density and Vickers hardness measurement were also performed to assess the physical and mechanical properties.

Key words: AISI 316 stainless steel, nanostructure, cryogenic milling, microstructure, sintering, Rietveld refinement

1- Introduction

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Austenitic stainless steels have been the center of attention in various industrial applications such as petrochemistry, implants, and automotive, due to their desired properties especially high ductility, good weld-ability [1–3]. Many attempts have been made in order to improve the strength and wear resistance without sacrificing the toughness [2]. Most of the approaches focus on Severe Plastic Deformation (SPD) techniques such as Equal Channel Angular Pressing (ECAP), High Pressure Torsion (HPT), Accumulative Roll Bonding (ARB), Surface Mechanical Attrition Treatment (SMAT), and Mechanical Milling (MM) [4– 10]. In these techniques, grain refinement to sub-micron and nanometer regimes results in improvement of strength and hardness [11,12]. Mechanical milling (MM) has been widely used in Powder Metallurgy (PM). As a typical process, it has become the focal center of interest to achieve desired both equilibrium and non-equilibrium crystalline, quasi-crystalline, and amorphous phases. This process is mainly operated via attrition ball milling, planetary ball milling, multidirectional milling, etc. Among these, attrition ball milling is one of the simplest and perhaps the most cost effective one to synthesize nanostructured materials, with the possibility of large-scale production [13]. Morphology and microstructural evolution after MM significantly depend on milling parameters such as rotating speed (rpm), Ball to Powder Weight Ratio (BPR), Process

ACCEPTED MANUSCRIPT Control Agent (PCA), temperature, time, and inner atmosphere. During mechanical milling, there are some mechanisms controlling the final properties of the milled powders. Among them, powder cold welding and fracture are the two dominant phenomena along with phase transformation to less extent [13].

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Milling process at cryogenic temperatures, named as cryo-milling, has received tremendous interest as a novel method to synthesize nanostructured materials. Reduced milling time to attain nano-powders is the main advantage of cryo-milling over conventional room temperature milling. This phenomenon is generally attributed delaying recovery process and overcoming fracture to cold welding. At cryogenic temperatures, diffusional processes which are mainly responsible for recovery processes are postponed due to lower mobility of atoms. During cryo-milling, particles mostly absorb less energy in collision; therefore, their toughness decreases. Eventually, the reduction in toughness prevents particles from cold welding. Another advantage is the limitation of both oxidation reactions and powders agglomeration at low temperatures, which is vital for materials having a high tendency to be oxidized [14].

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Generally, in cryo-milling, liquid nitrogen is directly injected into the milling chamber. Thus, powder is in direct contact with liquid nitrogen entitled to deep cryogenic milling.

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Many researches have been conducted using cryo-milling in order to reach nanostructure materials and enhance mechanical properties after consolidation especially about Cu, Zn, Al, Mg and their alloys. During these experiments, nano-particles like oxides and nitrides formed that acted like a pin hindering excessive grain growth during heat treatment or consolidation [15–18]. According to a study done by Lau et al. deep cryo-milling of M50 tool steel led to the formation of nitride phases like Cr2N [19]. Formation of these phases is not always beneficial. For example, in case of stainless steel, formation of chromium carbide will reduce corrosion resistance and mechanical properties of the final product could later deteriorate due to the brittle nature of this phase [20,21].

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Unlike the conventional cryo-milling operations, and in order to prevent any changes in the final composition, a special attrition apparatus is designed for this work in which liquid nitrogen is not directly in contact with the main powder. Although this method of cryomilling has been carried out elsewhere[18,22], there is no attempt about indirect cryo-milling especially about stainless steels until now. In this paper, the effect of milling time and temperature on the final phases, morphology, and microstructure of the AISI 316 stainless steel powder is assessed. To further explore the effect of milling temperature, as-milled powders were hot pressed (HP furnace) to achieve bulk samples. Finally, the mechanical and microstructural analysis was performed to evaluate these samples. 2- Experimental:

Commercially available gas atomized AISI 316 stainless steel powder (100 microns) was used as the raw material. The chemical composition of the powder is shown in Table.1. About 14 wt% oxygen was also detected in the initial powder due to the production process.

Table.1. chemical composition of AISI 316 stainless steel Element Weight percent

Cr 20

Ni 7

Mo 2.5

Si 0.5

Fe Balance

ACCEPTED MANUSCRIPT 2.1- Attrition milling:

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Fig. 1. shows a schematic view of the designed chamber of the cryogenic attrition apparatus. The chamber consists of three concentric shells. The milling process takes place in the inner-zone (Fig. 1 (a)) of the chamber which is cooled down by the intermediate shell containing liquid nitrogen (Fig. 1 (b)). The desired milling temperature is controlled by the liquid nitrogen flow in this area. In order to minimize the heat transfer and to optimize nitrogen consumption, vacuum was applied to the outer zone of the chamber (Fig 1. (c)).

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Fig. 1. Schematic view of the cryogenic attrition milling (a) main milling chamber (b) cooling chamber containing liquid nitrogen (c) vacuum chamber.

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Milling experiments were performed at cryogenic (-100 to -90oC) and ambient temperatures for different periods of time as summarized in table 2. Other fixed parameters were as follows: rpm = 400, BPR = 25, PCA = ethanol, and Balls (d=10mm) (total mass of the balls=625g). As-milled powders were dried in an oven at 40 oC for 24 hours followed by sieving technique to determine particle size distribution. Chemical etching was performed using Kalling solution prior to microstructural examination of each class of the particle size.

Table .2. Milling conditions Milled Sample

Po(raw powder)

P1C

P3C

P6C

P12C

P1A

P3A

P6A

P12A

Time (h)

-

1

3

6

12

1

3

6

12

Temperature

-

C

C

C

C

A

A

A

A

C= cryogenic A= ambient

Consolidation of Po, P6C, and P6A samples was carried out via hot pressing (KOVACO KHP-200) furnace at 1150 ℃ for 1 hour under constant pressure of 65 MPa in a vacuum and then cooled in furnace. The heating cycle of hot-pressing experiments is shown in Fig. 2. Final codes for the sintered samples of Po, P6C, and P6A are So, S6C, and S6A respectively.

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1 hour 1150

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800

450

100 50

100

150 Time (min)

200

250

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o Temperature ( C)

10 oC/min

Fig.2. Heating cycle of the HP furnace for sintering of So, S6C, and S6A. Vickers hardness with 10 kgf load was carried out according to ASTM E92 standard [23] (Nexus 4000 series, INNOVATEST). Archimedes method was performed to measure the

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density according to ASTM B962 [24]. Microstructures of the as-milled and as-sintered samples were examined by a scanning electron microscope (FEI Nova NanoSEM). Phase analysis was performed using X-ray diffraction (XRD, Philips X'Pert Pro diffractometer using Cukα (=1.5406oA) radiation at 40 kV and 30 mA) for all samples. Additionally, quantitative analyses were carried out using Rietveld refinement method via MAUD software.

3- Results and discussion:

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Fig. 3. shows X-ray diffraction of AISI 316 stainless steel powder before and after milling at different times and temperatures. Two main phases, austenite and chromite (FeCr2O4), can be identified in the initial powder (Po). According to Fig. 3.as the milling process takes place, the transformation of austenite to BCC-martensite (α ) occurs in both milling conditions. This phenomenon, named as strain-induced martensitic transformation, has been observed in metastable austenitic stainless steels (i.e. 304L, 316L, 301) [25–30]. Additionally, a weak HCP-martensite (ε) peak is only visible for P1C and P3C. Due to the fact that there is no need of high deformation for ε martensite formation, it can be concluded that milling for 1 and 3 hours induced adequate strain for ε martensite formation. It is known that prolonged milling exercise is associated with higher number of collisions leading to more strain. As presented in Fig. 4. phase evolution with milling time can be traced from low ε content at low milling times to nearly complete α with trace austenite at extended milling times. The presence of α martensite in the microstructure leads to enhancement in strain hardening rate, consequent ductility, and postponing necking which is industrially attracted in many applications [9,31].

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ε/ α

/

α

* Chromite (Fe Cr2O4) γ Austenite

α

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12h

f e d

ε

b

ε

γ

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/

h

30

HCP-martensite BCC-martensite

α

i

a

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Similar to these results, Seetharaman et al. [32] reported that during mechanical deformation of 316 stainless steel at -196oC, two kinds of martensite have been observed (i.e. α , ε). The HCP martensite (ε) formed at first stages of deformation up to 10 vol%. Higher strain levels reduced the volume fraction of this phase along with an increase in the amount of α phase. However, α can also be formed directly from austenite which results in a gradual increase of its volume percent to nearly 100 [32].

12h 6h

Cryo 3h 1h

γ

50

1h

γ

* 60

70

γ 80

90

γ

Unmilled

100

2 Theta (deg)

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Fig. 3. X-ray diffraction of the AISI 316 stainless steel powder before and after milling at different times and temperatures. a) Po b) P1C c) P3C d) P6C e) P12C f) P1A g) P3A h) P6A i) P12A.

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α

ε

HCP-martensite /

BCC-martensite

γ

Austenite

γ

γ

ε

c

6h at Cryo

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d

3h at Cryo

γ

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ε

b

γ

a

43

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α

44

45

1h at Cryo

Unmilled

46

2Theta (deg)

Fig. 4. X-ray diffraction of the milled powder representing martensite formation,

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annihilation, and   formation. a) Po b) P1C c) P3C d) P6C.

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In metastable austenitic stainless steels, α martensite is the thermodynamically favorable phase at low temperatures. However, pronounced driving force is required for the transformation of γ → α due to the presence of nickel as an austenite stabilizer. Therefore, mechanical work is required to provide sufficient energy to accelerate the phase transformation [33]. In steels having low SFE amount (less than 20 mJ/m2), this transformation has an intermediate step. At first,  transforms to , which then transforms to   (  → →   ). However, the formation of α martensite in AISI 316 stainless steel relies exceedingly on strain amount and SFE [9,34,35]. As stated earlier, the presence of this phase can be noted in both milling conditions (Fig. 4). Using Rietveld refinement calculations for α weight fraction (i.e. ignoring Chromite phase) higher values of this parameter were obtained for cryo-milling condition (Fig. 5). It is assumed that this behavior is related to lower SFE amount at cryogenic temperatures. This causes more intersections of irregular overlapping of stacking faults generating extra nucleation sites for α martensite that controls α growth [9]. Moreover, this phase is more thermodynamically stable at cryogenic temperatures which decreases the amount of sufficient plastic work for transformation [33].

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Cryogenic Temperature

0.8

Ambient Temperature

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0.4

0.2

0.0 0

2

4

6

10

12

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Milling time (h)

8

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' f α

0.6

Fig. 5.  martensite weight fraction after milling at different times and temperatures using MAUD software based on Rietveld refinement calculations considering no oxide phase.

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As presented in Fig. 5. the weight fraction of α martensite increased gradually up to 0.78 after 6 hours of cryo-milling followed by a moderate drop to 0.69 when milling time extended to 12 hours. Similarly, for milling at ambient temperature, its weight fraction increased to 0.68 and then decreased to 0.62 for 6 and 12 hours, respectively. The decreasing trend has also been observed in some other austenitic stainless steels known as reversion of martensite to austenite [27]. This behavior is probably related to dominating other mechanisms like Grain Boundary Sliding (GBS) and localized increases in temperature arising from collision inside the attritor [13,27].

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Peak broadening in x-ray diffraction patterns was observed in all milling conditions (Fig. 4). This behavior is related to crystallite refinement to nanometer regime and residual strain from mechanical work [36]. In order to analyze crystallite size, different methods are suggested in literature. The Scherrer equation, although used by majority of researchers, does not take the effect of strain into account. In this aspect, the Williamson-Hall method is expected to provide more precise results [15,37]. Since the y-intercept was negative for our data, unfortunately, this method did not reach to accurate conclusions as it was the case for other researchers too [29]. However, Rietveld refinement method does not suffer from this limitation [38]. The MAUD software based on Rietveld refinement calculations was used to perform analysis on crystallite size and phase weight fraction. Fig. 6. represents crystallite size of different phases versus milling time in various milling conditions. For all phases, crystallite size reduced sharply after 1-hour milling, and then gradually dropped down to almost constant values. The refinement is due to high dislocations density arising from severe mechanical work in the high energy milling [10,13,14]. Moreover, further reduction in crystallite size is observed in cryogenic milling conditions due to the somehow poor ductility of steel in this condition [39]. The cooling-induced reduction in ductility in different phases depends on copious material properties like chemical composition, lattice structure, microstructure, etc. [40,41]. At the early stages of milling, austenite phase deformed

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plastically. Further time caused to increase in strain rate hardening, strain induced martensite transformation, and dislocations density leading to a reduction in crystallite size. After 6 hours, as it was pointed out above, GBS may dictate deformation mechanism hindering further grain refinement [13,14,25,26,41,42]. SEM image of crystallite size (P6C) also confirms the calculations of Rietveld refinement method (Fig. 7). Fig. 8 also shows the correlation between Rietveld refinement results and experimental x-ray analysis revealing the quality of simulation.

220

Cryogenic Temperature

Ambiant Temperature 180

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60

20 0

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Crystallite Size (nm)

a)

4

6

8

10

12

100

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Milling Time (h)

Cryogenic Temperature

Ambiant Temperature

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b)

60

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Crystallite Size (nm)

80

40

20

0

2

4

6

8

10

12

Milling Time (h)

Fig. 6. a) Austenite, and b) Martensite crystallite size using Maud software based on Rietveld refinement method.

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Fig. 7. SEM image of nano-sized crystallites of P6C after mechanically polished and chemically etched. The arrows show some crystallite size.

Experimental

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Calculated

Martensite (Im-3m) Chromite (Fd-3m) Austenite (Fm-3m)

30

40

50

60

70

80

90

2 Theta(deg)

Fig. 8. The difference profile is shown based on the experimental and calculated diffraction pattern for P6C (Rwp= 10.6% , R= 4.5%).

Fig. 9. illustrates SEM images of powder particles before and after milling. Milling process led to morphology alteration from spherical to flattened particles with more intense effect at

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longer milling times. As shown in Fig. 9 (b), (c), and (f), samples milled for shorter periods (P1A, P3A, P1C) contain a small fraction of spherical and large particles. However, after 6 hours of milling at both temperatures, approximately all powders became flat. Due to the presence of the both ductile (austenite) and brittle (chromite) phases, all types of collision may have happened [13]. Severe deformation induced by milling caused austenite spherical particles to plastically deform into flattened particles which mainly transformed to martensite (strain-induced martensitic transformation) [8,43].

Fig. 9. SEM images of un-milled and as-milled powders. a) Po b) P1A c) P3A d) P6A e) P12A f) P1C g) P3C h) P6C i) P12C.

ACCEPTED MANUSCRIPT It can be noted that milling at cryogenic temperature resulted in finer particles than the room temperature for the same milling period (Fig. 9). It is known that most steels exhibit lower toughness as the temperature decreases. Thus, this morphology difference is probably related to the reduced toughness of the powder at cryogenic temperature [39].

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According to the results obtained from milling experiments, nano-size crystallites, fine particles and the highest amount of   martensite can be achieved after 6 hours of milling in both conditions. Therefore, P6C and P6A samples were selected to be sintered together with Po in order to study the effect of milling condition on the final product.

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Fig. 10. represents x-ray diffraction of the selected samples before and after sintering. It can be noted that sintering has led to the conversion of martensite (c and e) to austenite (d and f) in S6A and S6C. Similar results were also reported during annealing or sintering of asmilled austenitic stainless steels [8,43,44]. Reports on stainless steels show two reversing mechanisms (diffusion and diffusion-less reversion) for this phenomenon [45,46]. Depending on heating rate, chemical composition, prior mechanical work, temperature, and holding time, one of these mechanisms may control the transformation. In this investigation, different milling conditions led to stored strain energy [13]. Due to the relatively low heating rate through sintering (Fig. 2.) and high amount of Cr/Ni ratio, the reversion transformation has probably taken place by the diffusional mechanism. Another fact confirming this speculation is the equiaxed austenite grains (Fig. 11.) which is a characteristic of a diffusional process [45].

As-sintered f

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γ

γ / α

Austenite

*

BCC-martensite

γ

Chromite (Fe Cr2O4)

O Chromium Carbide

(Cr23C6)

γ

γ

O

γ

/

α

γ

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O O

6h Cryo

/

α

/

α

γ

γ

γ

γ

γ

/

α

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Intensity (arb.unit)

As-milled e

As-milled c

As-sintered b

raw powder a *

30

/

α

γ γ O

γ

6h Ambient

/

α

γ γ

γ

γ

γ

γ

γ

γ

* 40

50

60

70

80

Unmilled

90

2 Theta(deg)

Fig. 10. X-ray diffraction of the samples before and after sintering. a) Po b) So c) P6A d) S6A e) P6C f) S6C.

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It can be stated from Fig. 10 and 11 that chromium carbide (Cr23C6) has been formed in S6A after sintering. Its first peak (the weak peak at 43.8o) can also be traced in So and S6C. In the sintering condition, formation of chromium carbide is thermodynamically favorable [33], though the corresponding peak is weak in both So and S6C samples. There is a direct relationship between the nature of the main phases and the final hardness of the samples [11]. Due to the higher intensity of chromium carbide x-ray diffraciton peaks in S6A(Fig. 10.) the eventual hardness is higher than other samples as Fig. 12. represents. Additionally, ultrafine microstructure also led to these high hardness numbers, which means that although the crystallites grew during sintering, they are still in nanometer regime.

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In this investigation, pre-cold work and sintering conditions determine chromium carbide formation as two major factors. Different milling conditions stored various energy levels in the powders [13]. Therefore, releasing of this energy during sintering may then have changed the kinetics of its formation. It is noteworthy to mention that although S6C has the lowest hardness value (213 ±4 Hv(10kgf) ), it is still harder than the one reported using hot-press by Bartolomeu et al. (176 Hv(3kgf) [47].

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Chromium carbide has negative impacts on the toughness and corrosion resistance of austenitic stainless steels. Hence, the presence of this detrimental phase in the final microstructure should be absolutely avoided by performing some treatments like cooling rapidly above critical temperatures in which chromium carbide forms (i.e. 425 to 870 ℃). Another alternative is adding some elements like Nb, Ti etc. which could form carbides preferably instead of chromium carbide. Neither of which were performed due to the fact that fast cooling was not practical because of hot pressing system limitations and our approach was not to change chemical composition [33].

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Table 3. represents the relative densities measured by the Archimedes method. S6C has the highest density (96%) among all samples. However, S6A also reached relative density close to S6C which means that different milling temperatures did not have such significant impact on the final density but performing milling caused to reach higher densities than the un-milled sample. This observation is related to the wide distribution of flattened particles formed by the attrition milling. Microstructure of the as-sintered samples is depicted in Fig. 13. The microstructure after sintering is related to the milled particles’ morphology. S6C and S6A have more flattened particles connected to each other and the black dots correspond to pores formed during sintering (Fig. 13 (b) and (c)). Un-milled powder (Po) has spherical morphology while after milling, particles become flattened. S6A has also thinner particles compared to S6C, due to the fact that ductility of the powder at ambient temperature is higher than the cryogenic ones[13,39]. Due to similarity in densities for both S6A and S6C, the main reason for the difference in hardness measurement is the carbide formation.

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Fig. 11. SEM image of S6A microstructure (etchant: kalling). Carbide and austenite phases distinguished using both X-ray and EDS analysis.

500

449

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300

369

213

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Hardness (Hv)

400

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100

0

So

S6A

S6C

Fig. 12. Vickers hardness of as-sintered samples.

Table 3: Theoretical density of as-sintered samples Sample Code % of theoretical density

S6C 96

S6A 95.5

So 93.5

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Fig. 13. SEM-backscatter images of as-sintered samples via HP furnace at 1150 ℃ for 1 hour. All images were taken from interior sections after mechanical polishing. a) So b) S6A c) S6C.

4- Conclusion: AISI 316 stainless steel powder was milled at both cryogenic and ambient temperature for 1 to 12 hours. Attrition milling resulted in the refinement of powder crystallites into the nanometer regime and no nitride phase formation as an impurity. Additionally, not only did attrition milling lead to more amount of strain-induced   martensite, but it also refined the

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powder into finer particle with flattened shapes. However, microstructural analysis showed that ambient-milling led to morphological change into more flattened particles with less thickness in comparison to cryo-milled ones. Highest amount of   martensite (0.78 weight fraction) was achieved after 6 hours of cryo-milling. As-sintered 6 hour cryo-milled sample had the lowest amount of hardness (213±4 Hv) while the as-sintered ambient one showed the highest hardness (449±9 Hv) among all samples. Surprisingly, sintering of cryo-milled sample has no appreciable detrimental phase, typically Cr23C6, comparing to other samples. However, milling at ambient temperature led to formation of chromium carbide after sintering. Another point is that milling at both temperatures resulted in higher relative densities. Moreover, sintering of the cryo-milled sample reached the highest density (96%). Considering all the results, 6 hours of cryo-milling resulted in higher density and acceptable hardness without considerable formation of chromium carbide.

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ACCEPTED MANUSCRIPT Highlights

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Milling of AISI 316 stainless steel powder at cryogenic and ambient temperature for 1 to 12 hours No compositional change in samples thanks to the novel design of the mill. Sintering of selected as-milled and un-milled powder at 1150 oC via hot pressing. Improved mechanical properties as result of cryomilling. Detailed qualitative and quantitative analysis based on X-ray diffraction pattern and SEM observations.

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