analysis of comp action and sintering of stainless steel powders

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Chiang Mai J. Sci. 2006; 33(2) 293

Analysis of Compaction and Sintering of StainlessSteel Powders

Ornmanee Coovattanachai*, Prayoon Lasutta, Nattaya Tosangthum, Rungtip Krataitong,

Monnapas Morakotjinda, Anan Daraphan, Bhanu Vetayanugul and Ruangdaj Tongsri

Powder Metallurgy Research and Development Unit (PM_RDU), National Metal and Materials Technology Center,

Thailand Science Park, 114 Paholyothin Rd., Klong 1, Klong Luang, Pathum Thani 12120, Thailand.*Author for correspondence; e-mail: [email protected]

ABSTRACT

Compaction and sintering are basic processing steps employed for producing powdermetallurgy (P/M) parts. Information of materials property change during the processing stepsis primarily essential for design and manufacturing engineers. In this study, stainless steelpowders (316L, 304L, 409L and 434L) were selected for investigation on the effect of densitieson mechanical property. It was found that measurable parameters such as green density, sintereddensity and mechanical property showed linear relationships with one another for the 304Lstainless steel powder. Equations, derived by using Least Square Method (LSM), may beemployed with high confident for the 304L stainless steel powder. LSM analyses for other

materials (316L, 409L and 434L) failed to get good agreement between the experimental dataand the LSM results. Equations, derived by using LSM, are not recommended for the 316L,409L and 434L stainless steel powders.

Keywords : density, Least Square Method, mechanical property, stainless steel powders.

1. INTRODUCTION

Powder metallurgy process offers greaterflexibility in part design with superior propertiesand dimensional accuracy. The P/M processhas been becoming more attractive among the

various metal forming processes. It offerslower production cost when number of P/Mparts, those are higher than economy-scalequantity, are produced. The attraction of P/Mis mainly resulted from the ability to produce

various parts such as porous, precise (closetolerance), and high performance componentsin an economical manner [1, 2].

Stainless steel P/M parts represent animportant and growing segment of the P/M

industry. The stainless steel powders have beenselected to replace P/M ferrous alloys due totheir superior characters such as corrosionresistance, oxidation resistance, wear resistanceand mechanical properties (ductility and impactstrength) [3]. Stainless steel series 300 powdersare used in several applications such asaerospace, agriculture, appliances, automotive,building and construction, chemical, electricaland electronic, hardware, industrial, jewelry,marine, medical, office equipment, recreationand leisure [4].

Stainless steel series 400 powders havebeen being widely used in P/M applications for

Received: 20 September 2005

Accepted: 14 February 2006.

Chiang Mai J. Sci. 2006; 33(2) : 293 - 300

www.science.cmu.ac.th/journal-science/josci.html

Contributed Paper



294 Chiang Mai J. Sci. 2006; 33(2)

a decade. This is because of their ferromagneticproperties, fairly good corrosion resistance,good strength and low cost. Applications of

400 series alloys can be found as automotiveparts. The examples of them include automotiverearview mirror mounts, antilock brake system(ABS) and sensor rings [5, 6]. Several exhaustcomponents, including coupling flanges andhot exhaust gas oxygen sensor (HEGOS) bosses,are excellent candidate applications for P/Mparts makers to manufacture [7]. Recently, therehas been considerable interest in the potentialfor 400 series P/M parts for automotiveexhaust system components, including flanges[6, 8].

In conventional P/M process, there aremany factors controlling P/M parts characters.Density is one of various important factors.Compaction of loose powder particles intogreen parts results in that green density isdependent on compacting load andpowdered materials property. The greendensity controls sintered density, which in turncontrols mechanical properties [9]. In this study,effect of density on mechanical properties

was analysed using the Least Square Method

(LSM).

2. MATERIALS AND METHODS

Each stainless steel powder grades (304L,316L, 409L and 434L obtained from Coldstreamof Belgium) was compacted into tensile testbars (TTBs) with various green densities. Numbersof green TTBs used for each experimentalcondition were 20 pieces. The green TTBs werethen debinded with batch-type furnace at 600°C for 60 minutes under argon atmosphere.

The debinded TTBs those made from series

300 stainless steel powders were sintered at1300 °C for 45 minutes, and the debinded

TTBs those made from series 400 stainlesssteel powders were sintered at 1350°C for 45minutes. Both series were sintered under purehydrogen atmosphere with a flow rate of 250litres/hr. Densities of green and sintered samples

were determined using the Archimedes method. A universal testing machine (Instron model8801) was employed to measure mechanicalproperties of the sintered TTBs. Hardness of

the sintered TTBs was carried out using ahardness tester (Rockwell scale B).

3. RESULTS AND DISCUSSION

3.1 Effect of Green Density on Sintered

Density

LSM analyses of experimental data of 316L, 304L, 409L and 434L materials resultedin equations for green density (

Gρ )-sintered

density ( S

ρ ) relationships (Table 1). The 304Land 409L stainless steel powders (Figure 1(b)and (c)) showed linear relationship betweenexperimental values of

Gρ and

S ρ .The R 2

values, computed from experimental data andcalculated values according to the equations for304L and 409L materials, were higher than0.900 (Table 1). The 316L and 434L stainless

steel powders (Figure 1(a) and (d)), showedthe

Gρ -

S ρ relationships with R 2 values less

than 0.900.Experimental errors occurred with the

316L material. The R 2 value was 0.799.Deviation of experimental values from theLSM ones occurred when green densities werehigher than 6.45 g/cm3. It was observed that

when green densities were higher than 6.45g/cm3 the sintered densities dropped. Thecauses of errors are not understood yet.

In general, the green TTBs with highergreen density have lower porosity, thus containhigher number of points of contact. Whenthe green parts have higher number of pointsof contact, the sintering opportunity wouldbe higher. Therefore densification of the

TTBs with higher green density occurs faster within a certain time.

For the 434L stainless steel, the experimentalgreen-sintered density relationship was not incommon. However, the LSM seemed not tobe fitted with the experimental trend. Thiscauses the R 2 value lowered than 0.900.

The equation general form of the

Gρ -

S ρ relationships for the stainless steel

powders investigated was as follows;C B A

GGS ++−= ρ ρ ρ

2 (1) where A, B and C are materials-dependentconstants. The values of A and B are shown in

Table 1. The constant C for 316L, 304L and409L were too small (<0.001) so they were

omitted.



Chiang Mai J. Sci. 2006; 33(2) 295

Table 1. Equations for green and sintered density.

Sintered Specimens R 2 Equations

316L 0.799 GGS ρ ρ ρ 134.2158.0

2+−=

304L 0.979GGS

ρ ρ ρ 966.1132.02+−=

409L 0.924GGS

ρ ρ ρ 928.1125.0 2+−=

434L 0.896 004.0581.2228.0 2++−=

GGS ρ ρ ρ

(a) 316L (b) 304L

(c) 409L (d) 434L

Figure 1. Plots of sintered density against green density of compacted stainless steels.

3.2 Effect of Sintered Density on

Mechanical Properties

3.2.1 Ultimate Tensile Strength (UTS) The sintered density-UTS relationships of

316L and 304L stainless steels, Figure2(a) and (b), respectively, showed good agreementbetween the experimental data and the LSMresults. The R 2 values for the 316L and 304L

materials were 0.954 and 0.998, respectively.For the 409L material, some TTBs with

different green densities yielded close sintereddensity values (Figure 1(c)). However, thesintered TTBs with close sintered density valuesshowed different UTS values (Figure 2(C)).

This causes the low R 2 value for the sintereddensity-UTS relationship of the 409L material.

The 434L material showed low level of

agreement between the experimental trendand the LSM result (Figure 2(d)).



296 Chiang Mai J. Sci. 2006; 33(2)


316L 0.954 001.0688.982719.143

2−−=

S UTS S ρ ρ σ

304L 0.998 005.0549.905126.1352

−−=S UTS

S

ρ ρ σ

409L 0.731 057.0199.151554.28 2−−=

S UTS S

ρ ρ σ

434L 0.790 170.0453.131306.25 2−−=

S UTS S

ρ ρ σ

Table 2. Equations for UTS and sintered density.

(a) 316L (b) 304L

(c) 409L (d) 434L

Figure 2. Plots of tensile strength against sintered density of sintered stainless steels.

3.2.2 Yield Strength ( Y

σ ) The sintered density (

S ρ ) and yield

strength ( Y

σ ) relationships of some sinteredspecimens (316L, 409L and 434L shown in

Figure 3(a), (c) and (d), respectively) showedlow level of agreement between the experi-

mental trend and the LSM result. The R 2

values for those materials were lower than0.800. The only material showing high R 2 value

was P/M 304L stainless steel.



Chiang Mai J. Sci. 2006; 33(2) 297

Table 3. Equations for yield strength and sintered density.


316L 0.744 006.0733.440730.64

2−−=

S Y S ρ ρ σ

304L 0.976 003.0947.323807.49 2−−=

S Y S

ρ ρ σ

409L 0.509 077.0458.133898.22 2−−=

S Y S

ρ ρ σ

434L 0.738 129.0204.48481.11 2−−=

S Y S

ρ ρ σ

(a) 316L b) 304L

(c) 409L (d) 434L

Figure 3. Plots of yield strength against sintered density of sintered stainless steels.

3.2.3 ElongationFor sintered metals, the ductility change

with fractional density ( ρ ), which is the density of the porous material (equivalent to sintereddensity (

S ρ ) divided by the density of

equivalently processed wrought material), wasapproximated as follows [10]:

2/12

2/3

)1( ψ

ρ ε

c+=

where c is an empirical constant that relatesto the sensitivity to pores, and y is porosity.

The relative ductility ( ε ) is the ductility,indicated by elongation ( ε ) value, of theporous material divided by the ductility of equivalently processed wrought material.

According to Equation (2), it was notuncommon that elongation of most sinteredstainless steels (316L, 304L and 434L) increased

with increasing sintered density (Fig. 4).Increased sintered density indicates increased

volume fraction of metallic bonds (sintered necks)

and decreased volume fraction of pores,during sintering process.

(2)



298 Chiang Mai J. Sci. 2006; 33(2)

Table 4. Equations for elongation and sintered density.


316L 0.902001.0613.129452.18 2

−−=S

S

ρ ρ ε

304L 0.959 001.0812.79669.11 2−−=

S S

ρ ρ ε

409L 0.033 009.0659.19443.2 2++−=

S S

ρ ρ ε

434L 0.993 001.0504.27057.4 2−−=

S S

ρ ρ ε

(a) 316L (b) 304L

(c) 409L (d) 434L

Figure 4. Plots of elongation against sintered density of sintered stainless steels.

3.2.4 HardnessRelationships between sintered density

and hardness as shown in Fig. 5 were analyzed

by using the LSM. Results of fitting equationsof hardness versus sintered density are shown

in Table 5. The R 2 values indicates that theequations for 316L, 304L and 409L stainless

steels are acceptable, particularly the equationfor the 304L material.



Chiang Mai J. Sci. 2006; 33(2) 299

Table 5. Equations for hardness and sintered density.


316L 0.9200 001.0776.361068.51

2−−=

S S

H ρ ρ

304L 0.9870 001.0453.166896.23 2−−=

S S

H ρ ρ

409L 0.9575 002.0689.47685.7 2−−=

S S

H ρ ρ

434L 0.853 044.0581.89508.13 2−−=

S S

H ρ ρ

3.3 Degree of Confidence

It can be concluded that all the equationsfor the 304L stainless steel, shown in Table 6,can be used with high confidence. For other

materials (316L, 409L and 434L), equationsfor relationships between physical properties

(a) 316L (b) 304L

(c) 409L (d) 434L

Figure 5. Relationship between sintered density and hardness.

(i.e. green and sintered densities) and mechanicalproperties (strength, hardness and elongation)are not recommended to apply without

laboratory test results as references.



300 Chiang Mai J. Sci. 2006; 33(2)

RelationshipMaterials

316L 304L 409L 434LGreen-sintered density 0.799 0.979 0.924 0.896

Sintered density-UTS 0.954 0.998 0.731 0.790

Sintered density-YS 0.744 0.976 0.509 0.738

Sintered density-E 0.902 0.959 0.033 0.993

Sintered density-H 0.920 0.987 0.958 0.853

Note: = R 2 value > 0.900

Table 6. The R 2 values for stainless steel powders.

4. CONCLUSIONS

During P/M processes, the 304L stainless

steel powder exhibited linear relationshipsbetween parameters such as green density,sintered density and mechanical properties.

The R 2 values computed from the experimentaldata and the LSM results were higher than0.900. Equations, derived by using LSM, may be employed with high confident for the 304Lstainless steel powder. LSM analyses for othermaterials (316L, 409L and 434L) failed to get

good agreement between the experimental

data and the LSM results. Equations, derivedby using LSM, are not recommended for the316L, 409L and 434L stainless steel powders.

ACKNOWLEDGEMENTS

The authors express their gratitude toNational Metal and Materials Technology Center (MTEC), Pathum Thani, Thailand, forfinancial support.

REFERENCES

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