single step of binder thermal debinding and sintering of injection moulded 316l stainless steel

8/8/2019 Single Step of Binder Thermal Debinding and Sintering of Injection Moulded 316L Stainless Steel

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Single step of binder thermal debinding and sintering of injection moulded

316L stainless steel

Indra Putra Almanar1,a

, Zuhailawati Hussain2,b

and Mohd Afian Omar3,c

1School of Mechanical Engineering,

2School of Materials and Mineral Resources Engineering,

Engineering Campus,

Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

3Advanced Materials Research Centre, Sirim Berhad, Lot 34, Jalan Hi-Tech 2/3, Kulim Hi-Tech Park, 09000

Kedah, Malaysia

[email protected],

[email protected],

[email protected]

Keywords: Metal injection moulding, stainless steel, sintering

Abstract. Metal injection moulding was performed with gas atomized 316L stainless steel powders.

Feedstocks were prepared using a paraffin wax/polyethylene/stearic acid binder system and

subsequently molded into tensile bar specimens. Solvent extraction done at 80○

C has shortened the

debinding process to 30 minute. Single step of thermal binder debinding and sintering was done in a

vacuum furnace. Sintering at 1380○C has reduced the porosity and associated with grain growth that

result in an increase in ultimate tensile strength to 444 MPa while the elongation increased to 29%

before fracture.

Introduction

Injection moulding using stainless steels powder has significant utilisation in the manufacturing

of relatively small, complex shaped parts requiring high strength and good corrosion resistance. The

debinding in PIM is a critical processing step and has been identified as that part of the technique,

which had primarily to be improved. Long debinding times or alternatively high risk of sample

distortion have been a major obstacle for the economic success of PIM. Thermal, solvent, and

vacuum debinding processes have been widely adopted by the powder injection molding (PIM)

industry [1,2].

A more recent developed vacuum debinding process combines debinding and sintering cycles

into a single step and minimizes damage caused by the handling of fragile debound parts [3,4]. In

this process, a high vacuum is employed at low temperatures to enhance the thermal debinding rateof low temperature binder components such as paraffin wax. At high temperatures, low vacuum is

used to remove the remaining major binders. After debinding, the parts are sintered in the same

furnace. This combination of debinding and sintering cycles is made possible by using a special

pumping system which avoids the build up of condensed binder vapours in the pipings. The present

work deals with metal injection moulding of 316L stainless steel using two steps of debinding

process which are solvent debinding followed by thermal debinding. The relationship between the

sintering temperature and mechanical properties of the sintered products are discussed.

Methodology

In this study, gas atomized 316L stainless steel powder supplied by Anval, Sweden with 15 µm

median particle size was used. 55 wt. % of paraffin wax, 35 wt. % of polyethylene and 10 wt. % of

stearic acid were used as binder system. Tensile bar specimen was produced using MCP HEK-

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GMBH Injection Moulding at 180oC and 30 MPa (300 bar). Solvent extraction debinding was

performed to remove paraffin wax component of the binder. The solvent used was heptane which is

heated to 40○C, 60○C or 80 ○C. The specimens were immersed in the heated solvent for 10, 30, 60,

120, 150, 240 and 360 minutes. Then, the specimens were removed from heptane and placed in a

drying oven at 45○C± 5○C for 2 hours to removed any remaining heptane from specimens.

Thermal pyrolysis debinding was done in a vacuum furnace for 440○

C for 1 to remove theremaining binder, polyethylene and stearic acid. Sintering cycle was proceeded in the same furnace

after the thermal pyrolysis process by increasing the temperature 1300○C, 1320○C, 1340○C, 1360○C

and 1380○

C. After sintering, weight loss and dimensions were measured to calculate the sintered

shrinkage and the densities were then examined by using water immersion. Tensile testing was

performed on the specimens after sintering. Nital solution was used to reveal the microstructure

under optical microscope.

Results & Discussion

Fig. 1 shows the percentage of paraffin wax removed at various times for three different

temperatures during solvent extraction process. It is proposed that the solvent reaction should be

done at 80oC for 30 minute to shorten the solvent extraction process. During immersion in heptane

in the water bath, the paraffin wax begin to dissolve in the heptane and fine pore channels begin to

form. As the time increased, the weight loss of the specimen due to the dissolution of paraffin wax

is increased and the pore channels enlarged. Fig. 2 presents a scanning electron micrograph of the

moulded specimens after immersed in the heptane for 30 minute at 80○

C which give evidence that

the open pore channels were formed by the removal of paraffin wax. The formation of open pore

channels allows the rapid removal of the remaining binder without cracking, blistering or swelling

during subsequent thermal pyrolysis. The whole debinding cycle is about 7 hours in duration. This

is short enough compared with the 10 hours taken by conventional thermal degradation process.

Fig. 1 The effect of debinding time and temperature on the percentage of paraffin wax removed.

Fig. 2 SEM show the development of interconnected pores

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0 100 200 300 400 P a r a f f i n w a x r e m o v e d ( % )

Time ( Minutes )

40 oC

60 oC

80 oC



Fig. 3 shows the density of the sintered sample and shrinkage. Higher shrinkage contributed to

higher density. A relative density of sintered specimens was obtained as 87.8% at 1300○

C and as

99.8%, very close to theoretical density, at 1380○C.

Fig. 3 Shrinkage and density of the sample after sintering at different temperature

Fig. 4 presents the optical micrographs of the specimens for different sintering temperature. The

porosity of the sintered specimens is decreased as the sintering temperatures are increased. Pore of

sintered specimens become more rounded and appeared to be isolated in then distribution as the

sintering temperature was increased. The pore present in the specimen generally very fine, closed

and uniformly distributed (at 1360○

C and 1380○

C). The pores after sintering process occupy both

location that is the grain boundaries and in the grain interiors. Substantial grain growth has been

observed at higher temperature.

Fig. 4 Optical micrographs of etched 316L stainless steel for different sintering temperature (a)

1300○

C, (b) 1320○

C, (c) 1340○

C, (d) 1360○

C and (e) 1380○

C

The variation of the ultimate tensile strength and percentage of elongation for the specimens with

the sintering temperature are shown in Fig. 5. Low porosity gives good mechanical properties of

20 µm

20 µm20 µm

20 µm20 µm

(e)(d)

(a) (b) (c)

0

2

4

6

8

10

12

1416

1300 1320 1340 1360 1380Temperature (C)

S h r i n k a g e ( % )

6.6

6.8

7

7.2

7.4

7.6

7.88

D e n s i t y ( g / c m 3 )

Shrinkage

Density



the specimens. The ultimate tensile strength increased from 327 to 444 MPa when the sintering

temperature was increased from 1300○

C to 1380○

C while the elongation increased from 19% to 29 %

before fracture.

Conclusions

In this study, solvent extraction debinding process coupled with the thermal removal of theremaining binder phase during sintering has been applied on injection molding part of 316L

stainless steel. The rate of extraction of paraffin wax from the green body increased with increasing

solvent extraction temperature. As the time increased, the weight loss increased and the pores

channels enlarged. The pores present in the sintered part generally very fine, closed and uniformly

distributed (at 1360○

C and 1380○

C). Low porosity in the sample sintered at higher temperature

exhibited good ultimate tensile strength and elongation.

Acknowledgement

Financial support provided by Universiti Sains Malaysia under the schemes of TPLN and Research

University Grant are acknowledged. Acknowledgement is also extended to Ms. Yew Sin Lin for her

assistance in experimental work.

References

[1] R. Seidel, F. Petzoldt, G. Veltl, and H.-D. Kunze: Met. Powder Rep. Vol. 47 (1992), p. 56

[2] M.Y. Cao, J.W. O’Connor and C.I. Chung: A New Water Soluble Solid Polymer Solution

Binder For Powder Injection Moulding, PIM Symposium Proceedings, San Francisco (1992).

[3] K.S. Hwang, K.H. Lin and S.C. Lee: Mater. Manuf. Processes Vol. 12 (1997), p. 593

[4] M.A. Omar, R. Ibrahim, M.I. Sidik, M. Mustapha and M. Mohamad, J. Mater. Process.

Technol. Vol. 140 (2003), p. 397.

single step of binder thermal debinding and sintering of injection moulded 316l stainless steel

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