Indian Journal of Engineering & Materials Sciences Vol. 10, August 2003, pp. 306-3 13

Powder injection molding and sintering of austenitic stainless steel

A K Mishra", S Paul", A Upadhyaya", T Barriereb & J C Gelin b

"Materi als and Metallurg ical Eng ineering, Indian Institute of Technol ogy, Kanpur, 208 016, Ind ia

bLMA, Universite de Franche Comte, Besant;:on. France

Received 18 October 2002; accepted 8 June 2003

Thi s paper describes powder metallurgy (PIM) process ing approach to co nso lidate ferrous alloys fo r automoti ve applications. The net shape and corrosionloxidation require ments in automotive components, such as exhaust fl anges, necessitate PIM processing route giving the advantage of alloying fl exibi lity and precision manufacturing . One variant o f P/M processing is powder injection mo lding (PIM) where complex-shaped components on a large scale are fabri cated.

In this study, experiments were performed o n 316L stainless steels using compacts shaped by press ing and powder injec tion molding (PIM) . In the conventi onal process, the austenitic stainless steel compacts were solid-state sintered at 12S0°C and supersolidus liquid phase sintered (SLPS) at 1400°C. The injection molded 316L stainless steel feedstock were inj ected. therma ll y debound and sintered . The rheo logica l behavior of the binder was investigated through ill situ imaging. Density o f the stainless steel sa mpl es sintered by supersolidus liquid phase sintering was hi gher than the solid-s tate sintered samples. The effect o f sintering temperature on the mi crostructural evo luti on was characteri zed . The PIM stainless steel compone nt s a ttained nenrl y full density wi th a uniform shrinkage.

Powder Me tallurgy (PIM) is a process ing technique that involves the production of metal powders and conversion of these powders into useful engineered structures . The steps involve shaping of the powder to the desired component by compaction in rigid dies. The as-pressed compacts have pores and the subsequent hea ting of the compacted powder bonds the particles together with the elimination of pores. P/M processing of automotive components has been increasing through the years because of the requirement for greater shape complexity and better mechanical properties. Some of the automotive components manufactured by P/M include connecting rods , valve guides, bearing caps, timing-belt pulleys, anti lock brake systems, shock absorbers, door levers, engine bearings, and signal control systems. t In recent years, the use of ferrous P/M components including austenitic stainless steel in the automobile industry has increased . The principal reason for choosing PIM approach is its cost-effectiveness which results from high material utilization and near net-shape component manufacturing.

[n conventional PIM, powders are shaped by compaction using mechanical or hydraulic pressing. Thi s imposes a limitation on the complexity of shapes that can be achieved. Furthermore, fine powders are not very amenable to pressing and often result in

delamination in the as-pressed component. As the shape complexity increases, PIM becomes viable especially when secondary-fini shing operations can be avoided.

PIM is a process capable of producing complex parts, which can not be made by conventional P/M process by applying plastic JI1J ection molding principle. Thi s process involves several steps; mixing of powders and organic binders, injection molding of the powder-binder mixture, debinding of the injection molded parts and sintering of debound compacts. In PIM, the powders are mixed with a low melting point polymer as a binder. A binder holds the shape of green compacts. Typically, about 40-60 vo\. % polymer is added for injection molding. Powder-polymer mixture is injected at 150-250 0c. At this temperature the polymer melts and therefore the viscosity of feedstock decreases. After injection molding, the polymer has to be removed. This process is called debinding. In order to preserve the structural integrity, debinding is done gradually over a range of temperatures . Typically, a two-stage debinding process is employed which involves solvent extraction followed by thermal debinding, To prevent the parts from cracking due to binder removal , the correct thermal cycle has to be optimized. The

MISHRA et al.: PIM AND SINTERING OF AUSTENITIC STAINLESS STEEL 307

debinding stage plays a very crucial role because failure to remove most of the binders before sintering can result in component distortion, cracking and contamination2.

The P/M components shaped by compaction or injection molding contain porosity, which is eliminated by heating the as-pressed or 'green' part at an elevated temperature. This process called si ntering results in the overall reduction in the system interfacial energy by formation of the interpartic le bonds through mass transport by diffusion at atomistic level. This is accompanied by reduction in the porosity and results in a concomitant increase in the mechanical properties. The main disadvantage of sintered stainless steel over their wrought counterpart is the inherent porosity3. This results in poor mechanical properties and corrosion resistance of P/M stainless steels. There is continued interest in improving the properties of the sintered components by achieving fu ll densification during sintering. In conventional PIM processing, powders less than 10 j..lm in size pose difficulty during compaction. However, in PIM fine powders «5 j..lm) can be employed. The sintered compacts attain over · 95% density due to the fine powder. Another way to enhance densification in a multi-component system is to perform sintering temperature above that of one of the low melting constituent. This is called liquid phase sintering. The properties of sintered products are superior in liquid phase sintering than conventional solid state sintering, as the presence of high-diffusivity liquid allows for rapid densification.

Typically, in order to maintain good dimensional precision, the P/M stainless steels are sintered in solid-state, which results in residual porosity. In conventional PIM processing, powders less than 10 j..lm in size pose difficulty during compaction . However, in PIM fine powders «5 j..lm) can be employed. The sintered compacts attain over 95% density due to the fine powder. The properties of sintered products are superior in liquid phase sintering than conventional solid state sintering, as the presence of high-diffusivity liquid allows for rapid densi-fication.

One of the novel liquid-phase sintering routes for prealloyed powders is supersolidus liquid-phase sintering (SLPS). In SLPS, temperature is selected between the solidus and liquidus. During SLPS, upon melt formation, the liquid penetrates the solid-solid grain boundaries and fragment the same. Typically,

SLPS is associated with prealloyed powder; therefore the formation of melt preferentially occurs at the grain boundaries and at triple points. The capillary stresses that occur due to the melt, cause compact densifi-cation . There is concomitant grain coarsening that occurs due to solution-reprecipitation and coale-scence. The steps of SLPS are liquid formation, particle fragmentation, fragment rearrangement, grain packing and sliding, coarsening, and eventual pore elimination by solution-reprecipitation. A typical schematic diagram of SLPS is shown in Fig . 1. In thi s, homogenous distribution of liquid occurs within the particles4 . Subsequent repacking of the fragments under the capillary force of the liquid results in rapid densification . Compared to solid-state sintering, densification mechanism in SLPS shows an extreme sensitivity to temperature. Densification occurs within minutes after passing the critical temperature. Furthermore, SLPS can only be done for preaUoyed powders.

The first work on SLPS was reported by Westerman5 on prealloyed nickel-base super-alloy. Extensive work on densification mechanisms and microstructural evolution during SLPS of prealloyed powders was carried out by Tandon and German6. Lal et al. 7 examined the shape loss during SLPS on various prealloyed powders, including bronze, 316L stainless steel and T 15 tool steel. They in vestigated the shape loss during SLPS and rationalized the processing and material factors with regard to distortion . Shankar and Upadhyaya8 did extensive research on the SLPS of 3 16L and 434L stainless steel powders with rare-earth oxide additions.

The present investigations compare the effect of sintering temperature on the densification and microstructural evolution in 316L ferritic stainless steel shaped by conventional pressing and through PIM.

supersolldus slntering

Fig. I- Different stages of SLPS4

308 INDIAN J. ENG. MATER. SC I. AUGUST 2003

Experimental Procedure Gas ato mi zed 3 16L austenitIc s tainless stee l

powder (0.025C- 12.9Ni-1 6.5Cr-2.4S Mo-0 .93Si -0.2 1 Mn-O.OOSS-

MISHRA et al.: PIM AND SINTERING OF AUSTENITIC STAINLESS STEEL 309

temperature to 130°C in 2 h, to eliminate the residual moisture in the green parts . Afterwards, the parts were heated from 130°C to 250°C at a constant heating rate of 5°CIh. It takes 24 h to complete the cycle. Debound compacts were sintered in an argon atmosphere at temperature ranging from 800°C to 1360°C. To measure the variation in shrinkage and shrinkage rate with temperature, the injection molded stainless steel samples were investigated using a OIL 402 C dilatometer, supplied by NETZSCH, Germany. The thermal cycle used for dilatometric studies is shown in Fig. 4. Temperature was kept constant at 350°C for 90 min in order to remove the binders left after debinding. Holding at 550°C for 60 min was done to eliminate organic additives. Again, temperature is kept constant at lOOO°C for 60 min to remove the contaminants. Finally, heating was done at 1360°C for 60 min, to get a highly densified sample. All the densities were measured using helium pycnomter, AccuPyc 1330, supplied by Micrometrics Instrument Corp., USA.

Results and Discussion Conventional P/M processing

Fig. 5 shows the variation of sintered densities for the 316L stainless steel samples with compaction pressure. Fig. 5 also shows the effect of sintering temperature on the samples. From the figure, it is observed that the sintered density of the compacts increases with increase in pressure and the maximum sintered density was obtained for the sample compacted at 600 MPa. This can be attributed to the higher green density of the sample. Table 1 presents the density of 316L compacts in solid-state and supersolidus sintering temperature. The samples

Heating Cycle of316L

1400

1200

1000 !-' e 800 .a f l 600 E !

400

200

0 0 300 600 900

time, min

Fig. 4--Heating cycle of 316L stainless steel

sintered at 1400°C (SLPS) were having higher sintered densities than the sample sintered at 1250°C (solid state sintering). The higher sintered density due to SLPS can be attributed to the homogeneous melt formation. The diffusion rate is much faster through the liquid phase, which consequently enhances densification.

Figs 6a and 6b show the optical photomicrograph of 316L stainless steel sintered at 1250°C and 1400°C, respecti vely. Clearly, the supersolidus sintered stainless steel has less porosity. Figs 7a and 7b reveal the microstructure of 316L PIM stainless steel samples sintered at 1250°C and 1400°C. It can be noticed that the sample sintered at 1400°C has higher grain growth and lower porosity.

The effect of the sintering temperature on the mechanical properties of stainless steel samples were studied by measuring the hardness. Table 2 presents the Vickers hardness values (0.3 kg load) for 316L samples sintered in solid-state and supersolidus conditions . The hardness of the pure stainless steel sample sintered at 1250°C was lower than the sample sintered at 1400°C. This can be attributed to the higher sintered density of the 1400°C sample.

Table l- Sintered densities of pressed and sintered 3l6L stainless steel samples

Sintered density, g/cm3 (% theoretical)

Solid-state sintering Supersolidus sintering

(1250°C)

6.30 (78.7)

(1400°C)

6.81 (85.1)

90 ,....------r---.,...-----,----; 7 .1 -.- 1250·C ~ 85 -e--

! 80

310 INDIAN 1. ENG. MATER. SCI, AUGUST 2003

.. .~- '.

t b .... . ' ~ ' . . \. ;Por.~S :. ' ; .- ~ ') .

. ." - .-~

. -" "- . .,

(a)

" ' . ... . ~ ~ ..

Pores

.--- ,. .. (b)

Fi g. 6--Optical micrograph showing pore di stribution in the austenitic stainless steel samples sintered at (a) 1250°C and (b) 1400°C

. ' k ,'. . .'\' J ', : . . .. --{ ~ .. .,... '. . r •

~~h, r '~'-- .. ' . ).~! .. ' ': : j" . . .' . '

~ " (.\, ... ' . . 't"';~ . , ;i .'

: ., .~ .. .:.,; ~ . : . .'~ , ': . :.l. .I ' .. r .. ... ~ ... "

:-::.t,' ''1'. .. .. , ' ' . \.

(a) (b)

Fig, 7--Optical micrograph of 3 16L P/M stainless steels sintered at (a) 1250°C and (b) 1400°C. The samples were compacted at 600 MPa

Table 2- Vickers hardness values (0.3 kg load) for solid-state and supersolidus sintered austenitic stainl ess stee ls

Temperature A verage Vickers Standard hardness HVO.3 deviation

1250°C 8 1 0,7

(solid-state sintering)

1400°C 124 1.2

(supersolidus sintering)

Powder injection molding

In order to study the preci sion of injection, weights and dimensions of the tensile and transverse rupture bars were measured. Table 3 reports ' the standard deviation (in %) of the measured weight, length, width and thickness of the injection-molded components. For all the parameters, the variation was

MISHRA et al.: PIM AND SINTERING OF AUSTENITIC STAINLESS STEEL 311

Fig. 8--Flow behaviour inside the mold

8f. Final injected sample is shown in Fig. 8h. Fig. 9 shows the effect of solids loading on the mold-cavity filling by the feedstock . From the figure, it is evident that a low solids loading results in jetting whereas the mold gets more uniformly filled at an increasing the solids loading. However, beyond a critical solids loading the viscosity of feedstock will rapidly increase the viscosity and restrict mold filling.

The thermal debinding behaviour of the injection molded feedstock was examined. It was observed that heating up to 130°C from ambient temperature results minimal binder removal. Up to 130°C, only the

moisture gets removed. Increasing the temperature from l300e to 220°C causes binder removal progressively at nearly constant rate. The behavior in the density of the component as a function of debinding temperature is shown in Fig. 10. The density of 316L increases as debinding temperature increases because of binder removal. At 220°C, the debound parts attained nearly 87% of theoretical density (7 g/cm3) and had adequate handling strength . Thus, 220°C is chosen as a preferred debinding temperature.

312 INDIAN J. ENG. MATER. SCI, AUGUST 2003

Fig . 9--Effect of solids loading on the flow behavior of feedstock in the mold cavity during injection

7 . 5

7 . 0

"E 6 . 5 ~ at

~ to c: ~ 6 .0

5 . 5

5 . 0

t hermal deb ln d l ng of316L

--_._----_._ ... _._----_ .

2 5 7 5 1 2 5 . 1 7 5 225 temper et ure , ·C

Fig. 10-Thermal debinding of injected 316L stain less steel

The SEM images of as-molded and debound part are shown in Figs lIa and lIb. On debinding at 220°C for I h, the binder gets removed and grain coarsening occurs. The parts thermally debound at 220°C were sintered in argon for I h at various at

Table 4--Average density of samples after sintering for I hat different temperatures

Temperature A verage density Sintered density,

°C g/cm3 % theoretical

800 7.54 95 .8

1000 7.60 96.6

1200 7.76 98 .6

1360 7.83 99 .5

Table 5--Vickers hardness values for solid state sintering and supersolidus sintered PIM austenitic stainless steels

Temperature Average, Standard

HV03 deviation

1200°C III 1.5 (solid state sintering)

I 360°C 172 2.1 (supersolidus sintering)

various temperatures ranging between 800°C to 1400°C. Table 4 presents the density of PIM 316L stainless steel sintered at various temperatures. Density increases as temperature increases as shown in Fig. 12. At 1360°C, a maximum density of 7.83 g/cm3 was achieved which is 99.5% of the theoretical density.

MISHRA et af. : PIM AND SINTERING OF AUSTENITIC STAINLESS STEEL 3 13

( "0 5.5

5

__ _ 0 _ _ ____ • __ •• _____ • _____ -- --_ ._ - - • •• • ,

+---i Dcbinding j

Sinler ing

o 200 400 600 800 1000 1200 1400 temperature, °c

Fig. 12--Effect of temperatu re on the sintered density of 3 16L stainless steel

Table 5 summari zes the Vickers hardness values (0.3 kg load) for PIM 3 16L sintered in solid-state and super-solidus conditions. The hardness of the pure stainless steel sample sintered at 1200 °C was lower than the sample sintered at 1360°C. As compared to Table 2, the hardness of PIM component is higher than pressed and sintered 316L. Thi s can be attributed the higher sintered density of injection molded stainless steel.

Conclusions The density of 3 16L austenitic stainless steel

increases with increase in compaction pressure and sinlering temperature by conventional PIM route. The PIM stainless steel parts had higher hardness values compared to conventional (pressed and sintered) P/M parts. As compared to solid-state sintering, super-solidus sintered injection molded components have higher sintered density and also higher hardness . While the injection mo lded 316L attains nearl y full density when supersolidus sintered at 1360°C, the as-pressed stainless steel compacts attains only 85% of the theoretical density even when sintered at 1400°C.

The fl ow of feedstock during Inj ection molding was visuali zed using a tran sparent mold . The PIM stainless steel parts show less than 0.5% deviation in weight and dimensions, which confirms the accuracy of mold design and the 0Plimum injection parameters chosen.

Acknowledgement

Thi s work was financially supported by the All India Council fo r Technical Education (AICTE); the Mini stry of Human Resource and Development (MHRD), Indi a; and through the research grant fo r the Department of Science and Technology (DST). The authors wish to thank J Shankar fo r hi s ass istance with the experiment set up . The authors express their gratitude to Dr Kishore Kulkarni fo r supplying the 3 16L feedstock for the present investigation.

References I German R M. Powder Metallurgy of iroll and steel (John

Wiley, New York) , 1998.

2 German R M & Bose A, Inj ection molding of metals alld ceramics (John Wiley, New York), 1997.

3 Upadhyaya G S, Sintered metallic and ceramic materials (John Wiley, New York), 1999.

4 German R M, Sintering theory alld praclices (John Wiley, New York) , 1996.

5 Westerman E J, Trans AlME, 224 ( 1962) 159- 164.

6 Tandon R & German R M, 1111 J Powder Metall , 30 ( 1994) 435-443.

7 Lal A, Iacocca R & German R M, Mater Melall TrailS A. 30 ( 1999) 220 1-2208.

8 Shankar J & Upadhyaya A, Trans PMAI, 28 (2002) 168- 184.

9 Rodriguez D & Tschiptschin A P, Proc. Firsl European Symp Powder Illjeclion Molding (PIM-97) , EPMA, Shrewsbury, UK, (1 997) 162- 169.