Chiang Mai J . Sci. 2006; 33(2) 293 Analysis of Compaction and Sintering of Stainless Steel 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 T echnology Center, Thailand Science Park, 1 14 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 powder metallurgy (P/M) parts. Information of materials property change during the processing steps is primarily essential for design and manufacturing engineers. In this study, stainless steel powders (316L, 304L, 409L and 434L) were selected for investigation on the effect of densities on mechanical property. It was found that measurable parameters su ch as green density , sintered density and mechanical property showed linear relationships with one another for the 304L stainless steel powder. Equations, derived by using Least Square Method (LSM), may be employed with high confident for the 304L stainless steel powder. LSM analyses for other materials (316L, 409L and 434 L) failed to get good agreement between the experimental data and 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. I NTRODUCTION Powder metallurgy process offers greater flexibility in part design with superior properties and dimensional accuracy. The P/M process has been becoming more attractive among the various metal forming processes. It offers lower production cost when number of P/M parts, those are higher than economy-scale quantity , are produced. The attraction of P/M is mainly resulted from the ability to produce various parts such as porous, precise (close tolerance), and high performance components in an economical manner [1, 2]. Stainless steel P/M parts represent an important and growing segment of the P/M industry. The stainless steel powders have been selected to replace P/M ferrous alloys due to their superior characters such as corrosion resistance, oxidation resistance, wear resistance and mechanical propert ies (ductilit y and i mpact strength) [3]. Stainless steel series 300 powders are used in several applications such as aerospace, agriculture, appliances, automotive, building and construction, chemical, electrical and electronic, hardware, industrial, jewelry, marine, medical, office equipment, recreation and leisure [4]. Stainless steel series 400 powders have been 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.cm u.ac.th/journal-science/josci.html Contributed Paper
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Analysis of Comp Action and Sintering of Stainless Steel Powders
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8/3/2019 Analysis of Comp Action and Sintering of Stainless Steel Powders
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.
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
8/3/2019 Analysis of Comp Action and Sintering of Stainless Steel Powders
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.
8/3/2019 Analysis of Comp Action and Sintering of Stainless Steel Powders
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)).
8/3/2019 Analysis of Comp Action and Sintering of Stainless Steel Powders
Table 3. Equations for yield strength and sintered density.
Sintered Specimens R 2 Equations
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)
8/3/2019 Analysis of Comp Action and Sintering of Stainless Steel Powders
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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[4] Reinshagen, J. H. and Neupaver, A. J.,Fundamentals of P/M Stainless Steels, Advances in Powder Metallurgy , 1989, 283-295.
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