Effects of cooling rate on mechanical properties and corrosion resistance of vacuum sintered powder injection molded 316L stainless steel

Effects of cooling rate on mechanical properties and corrosion resistance of vacuum sintered powder injection molded 316L stainless steel

Journal of Materials Processing Technology 212 (2012) 164–170 Contents lists available at SciVerse ScienceDirect Journal of Materials Processing Tec...

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Journal of Materials Processing Technology 212 (2012) 164–170

Contents lists available at SciVerse ScienceDirect

Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec

Effects of cooling rate on mechanical properties and corrosion resistance of vacuum sintered powder injection molded 316L stainless steel M. Rafi Raza a , Faiz Ahmad a,∗ , M.A. Omar b , R.M. German c a b c

Mechanical Engineering, Universiti Teknologi PETRONAS, Malaysia Advanced Materials Research Centre (AMREC) SIRIM, Malaysia Mechanical Engineering, San Diego State University, USA

a r t i c l e

i n f o

Article history: Received 27 January 2011 Received in revised form 26 July 2011 Accepted 30 August 2011 Available online 6 September 2011 Keywords: Stainless steel 316L Injection molding Binder system Vacuum sintering Corrosion resistance

a b s t r a c t This study presents the results of corrosion behavior of powder injection molded 316L stainless steel parts sintered in vacuum. The feedstocks of metal powder and plastic binder were prepared and their viscosity was measured. Green samples were injection molded and binder was removed from the green parts. Brown test parts were sintered at 1325 ◦ C with heating rate of 5 ◦ C/min and 10 ◦ C/min for 2 h followed by the same cooling rate. Corrosion response of the sintered test samples was measured by weight loss method in Ringer’s Solution of pH 7.4 for 15 days. The test samples using cooling rate 10 ◦ C/min showed higher mechanical properties and improved corrosion resistance compared to those sintered at low heating and cooling rate. High cooling rate reduced the evaporation of Cr and developed passive chromium oxide layer on the test samples resulting improved corrosion resistance. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Powder injection molding (PIM) is a recently developed technique, for producing complex geometry parts at a relatively low cost. PIM has been tested for production of ceramics and metal composite parts. PIM process consists of mixing of metal powder and binder, injection molding, binder removal followed by sintering near to full density (Zlatkov et al., 2008). The binder provides uniform dispersion of powder particles and produces a flow able mass for injection molding. In addition binder provides sufficient green strength for handling during binder removal and sintering operations (German and Bose, 1997). Normally, low viscosity or molecular weight polymer based binder is preferred to achieve homogeneous feedstock for PIM process and for easy removal of binder (German, 2007; Zaky et al., 2009). The enhanced mechanical properties and corrosion resistance of gas and water atomized stainless steel powder were observed in the parts fabricated by using PIM (Nylund et al., 1995). Reinforcement of carbide particles (TiC) in 316L stainless steel can also improve its mechanical properties (Loh et al., 2001). They also found that powder size, sintering temperature and heating rate have significant effect on the mechanical properties. Omar (1999) reported 96% sintered density of powder injection molded parts of 316L stainless steel

∗ Corresponding author. Tel.: +60 5 3687148; fax: +60 5 365 6461. E-mail address: [email protected] (F. Ahmad). 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.08.019

parts sintered in argon atmosphere. Ji et al. (2001) also used Taguchi method to achieve the maximum 96% sintered density in PIM stainless steel parts. Sintering was done in vacuum at 1250 ◦ C for 90 min with heating rate 20 ◦ C/min. Jamaludin et al. (2008) achieved 99% sintered density by sintering in vacuum; the sintering temperature range was 1340 ◦ C–1400 ◦ C, heating rate 10 ◦ C/min and dwell time was 4 h. Yimin et al. (1999) investigated the effects of solid loading on densification and mechanical properties of gas atomized 17-4PH stainless steel. They used solid loading from 60 vol.% to 72 vol.%. Feedstocks were prepared using binder consist of 65% paraffin wax, 30% EVA and 5% stearic acid. The test samples were sintered at 1380 ◦ C for 90 min in 90% Ar and 10% H2 atmosphere. They found that the compact with 68% solid loading showed low viscosity over a wide range of temperatures and was able to achieve densification easily and had better mechanical properties and microstructure as compare to other solid loadings. While many studies have been conducted to study the effects of process parameters on physical and mechanical properties of PIM stainless steel, no study has addressed the corrosion resistance of the PIM 316L stainless steel so far. Corrosion prevention of PIM 316L stainless steel is significantly important for biological application. In the human body, the presence of chloride ions generates localized corrosion such as pitting and crevice corrosion. Corrosion causes allergic and hypersensitivity reaction to 22% of population leaving serious consequences (Aleksandra Kocijan and Marjetka Conradi, 2010). Presently, this problem is minimized by surface treatments, reinforcement of noble metals or carbides and

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Fig. 1. (a) Particle size distribution of water atomized 316L stainless steel powder. (b) SEM micrograph of 316L stainless steel showing spherical particle shape: 3KX.

oxide dispersion with yttria (García et al., 2007). In Duplex stainless steel 2205 and AISI 316L stainless steel Cr plays an important role for corrosion protection. The Cr combines with Mo and N2 to form a passive oxide film, which resists against localized corrosion in artificial saliva and a simulated physiological solution (Aleksandra Kocijan and Marjetka Conradi, 2010). This research presents the results of 316L stainless steel parts produced by PIM. Two formulations F1 and F2 containing 60 vol.% and 65 vol.% respectively, with 316L SS powder were used, and the feedstock was prepared using paraffin wax based binder. The test samples were sintered in vacuum at 1325 ◦ C for 2 h and, were characterized for shrinkage, density, mechanical properties and corrosion.

Table 1 Chemical composition of 316L SS -10PF. Element

Wt.%

C Si Mn P S Ni Cr Mo Cu N

0.024 0.36 0.07 0.029 0.002 10.53 16.57 2.1 0.1 –

Water atomized 316L stainless steel (PF-10R) supplied by PACIFIC SOWA Japan was used in this research. The particle size distribution is d10 = 1.83 ␮m, d50 = 4.42 ␮m and d90 = 7.64 ␮m. Chemical composition of the powder is given in Table 1. Particle size distribution and SEM microstructure of powder is shown in Fig. 1a and b.

polypropylene 25 vol.% and stearic acis 5 vol.%, followed by mixing in Z-blade mixer at temperature 170 ◦ C for 90 min at 60 rpm. The metal mixtures were dried and granulated by using commercially available granulator. The feed stocks were characterized by using TGA and capillary rheometer CFT-500D. The rheology of the feedstocks was done at temperature 160 ◦ C using a capillary die having the dimension 1 mm × 10 mm. A vertical injection molder100 KSA was used to produce tensile specimens according to MPIF-50 standard. The test samples were produced by injection molding at 160 ◦ C; injection time was varied between 20 and 30 s. No physical cracks were observed on the green parts.

2.2. Feedstock preparation and injection molding

2.3. Binder removal

Two formulations F1 (60 vol.%) and F2 (65 vol.%) of stainless steel powders were prepared. The metal powder was dry mixed with a plastic binder containing paraffin wax 70 vol.%,

The binder was removed from the green parts in two steps: solvent extraction and thermal debinding. Solvent extraction process was carried out to remove paraffin wax (PW) from the test samples.

2. Experimental protocol 2.1. Metal powder

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The test samples were immersed in n-heptane at various temperatures from 40 ◦ C to 90 ◦ C upto 10 h by using water circulating bath to optimize the suitable temperature and time followed by thermal debinding at 450 ◦ C, and dwell time 1 h with five different heating rates from 1 ◦ C/min to 10 ◦ C/min to remove the remaining binder. 2.4. Vacuum sintering The debound specimens were sintered in vacuum at 1325 ◦ C for 2 h. Two cooling rates of 5 ◦ C/min and 10 ◦ C/min were used to study the effect of heating and cooling rate on the mechanical properties and corrosion resistance of sintered test samples.

100

75

% Weight Loss

166

Binder

50

F1 F2

25

0

2.5. Characterization of sintered samples

3. Results and discussion 3.1. Thermal curve for feedstock The results of TGA analysis of the binder and feed stock are given in Fig. 2. The results show that the degradation of binder started after 200 ◦ C and no residue was left at the end of the degradation process for binder system. However for F1 and F2 formulations, a large amount of residue was observed. For F1 formulation (60 vol.%) the remaining residue was 91 wt.%, which is steel powder in the feedstock. In case of F2 formulation (65 vol.%) 93.4 wt.% of residue was left, which is equal to the amount of stainless steel powder that exist in the feedstock. 3.2. Rheology of feedstocks Rheology of the feedstocks and binder was studied to understand the flow behavior and homogeneity of the powder with the binder. The viscosity of both feedstocks was tested at 160 ◦ C and the results are shown in Fig. 3. The results show that both feedstocks have typical pseudoplastic behavior, which is suitable for

150

300

450

600

Temperature (ºC) Fig. 2. Thermal degradation of binder and formulations F1 (60 vol.% solid loading), F2 (65 vol.% solid loading).

300 F1

Viscosity (Pa-S)

The sintered test samples were cut into slices using diamond saw cutter. The density of each slice was measured using water immersion technique. The sintered density reported here is the average of five measurements taken from the sintered test samples of each formulation reported. The hardness of sintered test samples was measured according to ASTM E140-02 across the length of bars at five various locations by using BREVETTI® hardness testing machine. The hardness value reported here is the average of five measurements. The tensile strength and elongation of both formulations were measured by using Amsler 100 (Zwick/Roell) according to ASTM E8M-00. The extensometer was used to measure the elongation. The reported tensile strength and elongation is the average of three measurements. The sintered test samples were observed under scanning electron microscope (SEM) to examine the microstructure and porosity. Corrosion test was conducted by using weight loss method to study the corrosion behavior of test samples sintered at 1325 ◦ C for 2 h followed by the cooling rates of 5 ◦ C/min and 10 ◦ C/min, respectively. The test samples for corrosion analysis were taken from the two different positions of the bar with average area 40–45 mm2 . The test specimens were immersed in Ringer’s solution at 37 ◦ C ± 1 ◦ C for 15 days. This temperature was selected as it is the temperature of human body. The pH of the solution was maintained 7.4 by using 1 M solution of HNO3 and NaOH after every 5 days. The area and weight of the test samples were measured according to ASTM G31-72 before and after the immersion in Ringer’s solution. The microstructures of the test samples were examined under SEM and EDX to determine the elemental analysis of corrosion attack.

0

F2

200

100

0 100

1000

10000

Shear Rate(1/S) Fig. 3. Viscosity versus shear rate for formulations F1 and F2 at temperature 160 ◦ C.

PIM. In this type of flow behavior, viscosity decreases with increasing shear rate. The viscosity of both feedstocks is within the range required for PIM (Liu et al., 2005; Faiz Ahmad, 2005) under 100 Pa s at sheer rate of around 1000 s−1 It was observed that an increase in solid loading increased the viscosity as shown in Fig. 3. However, the viscosity remains within the range required for PIM. The results show that both feedstocks are suitable for injection molding. 3.3. Debinding green samples In multi components binder system, major component paraffin wax (PW) is soluble in organic solvent. Therefore, the debinding was carried out in two steps: solvent extraction and thermal debinding. Four temperatures from 40 ◦ C to 90 ◦ C were selected for the solvent extraction process to extract the major binder, which is paraffin wax. The extraction time was varied upto 10 h. The results of removal of PW are shown in Fig. 4a and b. For both formulations at temperature 40 ◦ C a maximum 84 wt.% PW was extracted after 10 h extraction without producing any physical defects. When the extraction temperature was raised from 40 ◦ C to 60 ◦ C, 100 wt.% PW was extracted within 5 h. No physical defects were observed on the surface of the test samples after the extraction of PW. Shrinkage along the length of the test samples were observed and considered as due to residual stresses produced during the flow of feedstock in the mold during injection molding (Omar et al., 2003). The dimensional changes are shown in Fig. 4c. The extraction time was further reduced to 1.5 h when the temperature was raised from 60 ◦ C to 90 ◦ C. However, this rapid removal of PW generated cracks and swelling on the surface of the test samples as shown in Fig. 4d. These defects were considered due to the solubility of major binder in the solvent. Zaky et al. (2009) observed same defects

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Fig. 4. (a) Weight percentage of paraffin wax removed during solvent extraction for 60 vol.% (F1) solid loading. (b) Weight percentage of paraffin wax removed during solvent extraction for 65 vol.% (F2) solid loading. (c) Comparison of dimensions of test sample after solvent extraction with green test samples at 60 ◦ C, without physical defects. (d) Appearance of swelling and cracks on the surface of test sample at temperatures 80 ◦ C and 90 ◦ C for solid loading 60 vol.% (F1).

in 17-4PH stainless steel. When the temperature was increased to 90 ◦ C and above, the solvent started boiling which produced vapors that slowed down the solvent extraction process (Lin and German, 1989). Based on these results, 60 ◦ C was considered a suitable temperature for PW extraction for both formulations. The test samples were thermally debond successfully at temperature 450 ◦ C and dwell time 1 h. The reason of selecting this temperature was complete decomposition of binder and is shown in Fig. 2. Five different heating rates were used from 1 ◦ C/min to 10 ◦ C/min to select for debinding process. It was observed that at heating rates 1 ◦ C/min, 3 ◦ C/min, 5 ◦ C/min and 7 ◦ C/min, the test samples were successfully debond as shown in Fig. 5a. However, when the heating rate was increased to 10 ◦ C/min the swelling and cracks appeared on the surface of the test samples of both formulations as shown in Fig. 5b. Based on these results, it was considered that the most suitable heating rate for debinding rate is 7 ◦ C/min. 3.4. Shrinkage The dimensions of the green and sintered test samples were measured to calculate shrinkage. The average shrinkage measured for sintered test samples ranges from 15.7 to 16.14% and is illustrated in Fig. 6a. In the present study, a minimum shrinkage was observed in F2 sintered test samples with cooling rate 10 ◦ C/min. The results showed that shrinkage was reduced at higher solid loading and cooling rate. Fig. 6b shows the densification results of feedstocks F1 and F2 sintered at 1325 ◦ C and Cool down using different cooling rates. Approximately, 96% of theoretical density was achieved for F2 (65 vol.% solid loading) during cooling rate 10 ◦ C/min, at same conditions formulation F1 (60 vol.% solid loading) attained 94% of theoretical density. The difference in sintered density was considered due to the lower volume loading of powder in F1. The results show that the sintered density of F1 test samples using cooling rate 10 ◦ C/min was lower than the sintered density attained using cooling rate 5 ◦ C/min. This was considered due to the presence of porosity observed clearly in the SEM micrograph, in Fig. 9a. Bose

Fig. 5. (a) Comparison of dimensions of test samples after thermal debinding. Test samples subjected to heating rate 7 ◦ C/min at temperature 450 ◦ C for 1 h show no defects. (b) Thermaly debond test sample subjected to heating rate 10 ◦ C/min at temperature 450 ◦ C for 1 h shows swelling and cracks.

et al. (2008) reported 97% densification at 1100 ◦ C by using 5 ␮m particle size powder. Their work has shown that reducing the particle size lower the sintering temperature and the resultant part has increased tensile strength and improved finishing compared to the

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a

a

20

100

F1

F1 F2

16 14 12

50 25

10 0

5

10

0

15

Cooling Rate (ºC/min) 100

b

F1

98 96 94 92 90 5

10

5

10

15

500 F1

F2

0

0

Cooling Rate (ºC/min)

Tensile Strength (MPa)

% Sintered Density

b

F2

75

Hardness (HRB)

% Shrinkage

18

F2

450

400

15

0

Cooling Rate (ºC/min)

conventional powders. In the present research reported results of sintered density are comparable with the published data (Ji et al., 2001). 3.5. Tensile strength of sintered samples The maximum tensile strength measured for F2 (65 vol.% solid loading) is 481 MPa and elongation is 21.7% for test samples sintered using cooling rate 10 ◦ C/min. For the test samples of F1 (60 vol.% solid loading) the values of hardness and tensile strength are less than the test samples of F2 as shown in Fig. 7a and b, respectively. The F2 test sample also showed higher sintered density. Higher tensile strength and hardness for F2 test samples are attributed to the increased volume contents of powder. The results achieved in this study are comparable to those for wrought 316L (according to ASTM standard) (Becker et al., 2000). 3.6. Corrosion behavior of sintered samples The results of vitro test for corrosion of sintered F1 and F2 test samples are shown in Fig. 8. A higher corrosion attack was observed at the test samples of F1, which is considered due to the presence of porosity as shown in SEM micrograph Fig. 9a. Pitting type corrosion was observed under SEM as shown in Figs. 10a, 11a and 12a. Pitting corrosion occurred due to the existence of chloride ions in the solution used for the immersion of test samples. Relatively less corrosion was observed in both F1 and F2 sintered test samples cooled with cooling rate 10 ◦ C/min as compared to the sintered test samples with cooling rate 5 ◦ C/min. This was considered due to the formation of passive oxide layer enriched with Cr on the surface of test samples, which provided the resistance against corrosion (Trepanier et al., 2000). The higher corrosion attacked at F1 sample prepared using cooling rate 10 ◦ C/min could be either due to the presence of porosity in the test sample or the amount of Cr is less than 12% (Krug and Zachmann, 2009). Both facts have been proved by SEM and EDX analyses shown in Fig. 9a and Table 2, respectively.

20

30

40

50

% Elongation Fig. 7. (a) Effect of cooling rate on hardness of sintered 316L SS. (b) Relationship between tensile strength and elongation of PIM 316L SS at temperature 1325 ◦ C with cooling rates 5 ◦ C/min and 10 ◦ C/min.

Corosion Rate (Miles per year)

Fig. 6. (a) Effect of cooling rate on the shrinkage of sintered 316L SS test samples. (b) Effect of cooling rate on the sintered density.

10

10 F1

8

F2

6 4 2 0 0

5

10

15

Cooling Rate (ºC/min) Fig. 8. Corrosion on 316L SS in the presence of Ringer solution after 15 days. Table 2 EDX analysis of F1 and F2 test samples at different cooling rates. Formulation

Ni (%)

Cr (%)

Fe (%)

O (%)

F1 at 10 ◦ C/min F2 at 5 ◦ C/min F2 at 10 ◦ C/min

14.86 10.87 5.31

7.16 16.62 36.51

70.16 50.97 39.74

7.29 21.53 18.43

As the result of increasing the cooling rate, the time to evaporate the Cr is reduced and formation of chromium carbide is minimized. The evaporation of Cr and formation of carbide have been identified is responsible for the initiation of corrosion (Krug and Zachmann, 2009). The results of EDX analysis shown in Table 2 indicated that no evaporation of Ni and Cr was observed for samples with cooling 10 ◦ C/min. In F2 test samples, it is noted that the percentage of chromium at surface test sample with cooling 10 ◦ C/min is twice than test sample with cooling 5 ◦ C/min, and this amount is greater than the amount of Cr present in the bulk. The presence of high percentage of Cr has facilitated the formation of passive layer of CrO2 , which provided protection against chloride ions.

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Fig. 9. (a) SEM micrograph of F1 test sample showing 94% sintered density at cooling rate 10 ◦ C/min. (b) SEM micrograph of F2 test sample showing 96% sintered density at cooling rate 10 ◦ C/min. (c): SEM micrograph of F2 test sample showing 94% sintered density at cooling rate 5 ◦ C/min.

3.7. Microstructural analysis The typical microstructures of austenitic steel is shown in Fig. 9a–c. Fig. 9a shows the presence of small pore in the sintered samples of F1. The pores have round shape and their size is relatively small for F2 test samples produced with cooling 10 ◦ C/min

compared to the F1 test samples. The SEM results show that in both formulations F1 and F2 with cooling rate 10 ◦ C/min have less porosity as compared to those of low cooling rate 5 ◦ C/min. The grains are well grown during the cooling rate 10 ◦ C/min, which reduced the porosity as can be seen in Fig. 9b; this observation is in accordance with Ji et al. (2001). The grain growth is responsible for the

Fig. 10. (a) SEM micrograph showing formation of Cr oxide layer for sample sintered at cooling rate 10 ◦ C/min for 60 vol.% solid loading. (b) EDX analysis of F1 test sample after corrosion test showing the existence of Oxygen with Cr.

Fig. 11. (a) SEM micrograph showing formation of Cr enriched oxide layer for sample sintered at cooling rate 10 ◦ C/min for 65 vol.% solid loading. (b) EDX analysis of F2 test sample after corrosion test showed the existence of Oxygen with Cr.

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• For thermal debinding temperature, heating rate and time is 450 ◦ C, 7 ◦ C/min and 1 h for both formulations, respectively. • The sintered 316L SS test samples developed from 65 vol.% solid loading, sintered at 1325 ◦ C with cooling 10 ◦ C/min showed tensile strength 481 MPa, elongation 22% and hardness 76HRB, while the sintered density was 96%. • Reducing the cooling rate reduced the density, mechanical properties and corrosion resistance of the test samples. • During sintering cycle, a higher cooling rate increased the corrosion resistance. Acknowledgements The authors would like to express their appreciation to Universiti Teknologi PETRONAS for providing the fund and laboratory facilities. The authors are also acknowledging Advanced Materials Research Centre (AMREC) SIRIM, Kulim, Malaysia for providing the experimental support. References

Fig. 12. (a) SEM micrograph showing the formation of Cr enriched oxide layer for sample sintered at cooling rate 5 ◦ C/min for 65 vol.% solid loading. (b) EDX analysis of F2 test sample after corrosion test showing the existence of Oxygen with Cr.

mechanical properties and the difference in grain growth is clear in Fig. 9a–c. Fig. 10a shows the SEM micrograph of F1 sintered test sample with cooling rate 10 ◦ C/min. The SEM micrograph shows that the test sample has corroded due to the presence of porosity verified in Fig. 10a. The results were also confirmed by EDX analysis that indicates the presence of Ni (Fig. 10b), which causes the corrosion. Figs. 11a and 12a show the SEM micrographs of F2 test samples at different cooling rates. It is observed that the test samples with higher density and mechanical properties have less corrosion than the samples with lower density. In the micrographs, the lighter colored layer is the combination of Fe and O2 while the layer with dark color is the combination of the Cr, O2 and small amount of Ni. These results were confirmed by EDX analysis. Figs. 11b and 12b show the comparison in existence of Cr with O2 in F2 test samples. Fig. 11b shows that the amount of Cr in existence with Oxygen is higher than that shown in Fig. 12b. The presence of more Cr with Oxygen forms the enriched Cr oxide layer, which protects the test sample from the chloride environments. 4. Conclusion This study concluded the following; • The viscosity of both formulations is within the range required for PIM. • The solvent extraction temperature and time to extract major binder from green parts without causing any defects to green parts is identified 60 ◦ C and 5 h, respectively.

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