Enhanced Mechanical Properties of Injection Molded 17-4PH … · 2010. 11. 24. · Enhanced Mechanical Properties of Injection Molded 17-4PH Stainless Steel through Reduction of Silica

Enhanced Mechanical Properties of Injection Molded 17-4PH Stainless Steel

through Reduction of Silica Particles by Graphite Additions

Che-Wei Chang*1, Po-Han Chen*2 and Kuen-Shyang Hwang*3

Department of Materials Science and Engineering, National Taiwan University,1, Roosevelt Rd., Sec. 4, Taipei 106, Taiwan, R. O. China

Silica particles are often found in sintered 17-4PH stainless steels when water atomized powder is used. To eliminate the SiO2 particles andto improve the mechanical properties and corrosion resistance, graphite powders were mixed in this study with 17-4PH powders and bindersduring the kneading step of the powder injection molding process. The measurements of carbon and oxygen contents in tensile specimens andmicrostructure observations confirm the reduction of silica particles by graphite powders during vacuum sintering. The optimum content of thegraphite addition is 0.26mass%, with which most SiO2 particles are reduced and the final carbon and oxygen contents are 0.03 and 0.01mass%,respectively. These changes of compositions decrease the amount of �-ferrite in the as-sintered compact to 4 vol%, lower than the 10 vol% of thecompact without any graphite additions. After solutioning and aging treatment, the hardness, tensile strength, and ductility are HRC 41,1310MPa, and 9.0%, respectively. These properties are better than the typical values of 17-4PH reported in the literature and the corrosionresistance remains similar to that without any graphite additions. The relevant mechanisms on the changes of these properties are discussed witha focus on the effects of graphite addition on the residual carbon content, fractions of �-ferrite and martensite, silica amount, and density.[doi:10.2320/matertrans.M2010209]

(Received June 14, 2010; Accepted September 8, 2010; Published October 20, 2010)

Keywords: powder injection molding, 17-4PH stainless steel, water atomized powders, silica, graphite

1. Introduction

During the typical water atomization process of makingstainless steel powders, up to 1mass% silicon is often addedto the melt to act as a de-oxidizer by forming a thin andimpervious oxide layer at the surface of the melt, whichreduces the reaction between the melt and the atmosphereand decreases the total amount of oxygen in atomizedpowders.1,2) During atomization, the remaining silicon in theliquid forms SiO2 films on powder surfaces and also discreteSiO2 particles inside the atomized powder.

3–5) In additionto SiO2, other less reactive elements, mainly chromium,manganese, and iron, also form oxides.5–7) The amounts ofthese oxides are high in fine water atomized powdersbecause of the large surface area of the powder. Ferriss usedelectron spectroscopy for chemical analysis (ESCA) toanalyze the surface of �100 mesh water atomized 316Lstainless steel powder and indicated that the surface oxidewas dominated by SiO2.

3) Furthermore, Tunberg and Nyborgfound that as the particle size decreases, the thickness of theoxide film decreases. In addition, the oxide changes fromiron and chromium-rich oxide to silicon-rich type becauseall silicon near the surface of fine powders forms SiO2,and the fast cooling rate of fine powders does not allowchromium and iron to form oxides at surfaces duringcooling.5) Yasui et al. also showed that higher amounts ofoxygen and more fine silica particles were present in finer17-4PH powders.8)

During sintering, these oxide films and oxide particles canbe reduced, depending on the sintering temperature, time,

and atmosphere. Among these oxides, SiO2 is the mostdifficult to reduce. It remains as SiO2 at low sinteringtemperatures and obstructs the formation of necking betweenstainless steel powders.9) As the sintering temperatureincreases to about 1523K, the SiO2 film transforms intodiscrete particles, as have been observed in the dimples offractured surfaces.10) With the presence of oxide particles,the mechanical properties and corrosion resistance of thesintered stainless steels deteriorate.2,4,11,12)

Since water atomized stainless steel powders for powderinjection molding (PIM) process are typically very fine inorder to attain high sintered densities, the powders containsignificant amounts of SiO2 on the surface. Thus, theelimination of SiO2 inclusions in sintered compacts hasbecome an important focus of research. One solution is tosinter stainless steels in vacuum or low dew point hydrogenor dissociated ammonia at high temperatures.5,7) But whensilica is reduced at high temperatures in dry hydrogen, thereduction product, H2O, may get trapped in closed poresbecause the density at that temperature is high and someinterconnected pores are closed. Suri et al. showed thatthe water atomized 316L powder could be sintered to 97%density only, lower than the 99% of the gas atomized powder,which has little silica. Due to its large molecular size, thetrapped H2O cannot diffuse out and impedes pore annihila-tions.13) This is even more true for fine powders because poreclosure occurs earlier at lower temperatures than it doesin coarse powders. Another solution to eliminate silica wasdisclosed in a patent by Bartone and Das, in which the siliconcontent in the 17-4PH melt was reduced to a quantity ofless than 0.1mass% during the atomization process. Theresulting sintered compact has few SiO2 inclusions.

14) Theother approach to eliminate silica inclusions is to addgraphite powders, for graphite reduction can occur at a lowertemperature than hydrogen reduction. Wu et al. added

*1Graduate Student, National Taiwan University. Present address: Fu

Sheng Industrial Co., No. 9 Hsing-Chung Street, Kwei-shan Industrial

Area, 330, Tao Yuan City, Taiwan, R. O. China*2Graduate Student, National Taiwan University*3Corresponding author, E-mail: [email protected]

Materials Transactions, Vol. 51, No. 12 (2010) pp. 2243 to 2250#2010 The Japan Institute of Metals

http://dx.doi.org/10.2320/matertrans.M2010209

0.1mass% graphite in water atomized 304L powder andobtained improved mechanical properties.15) In anothersimilar study, Tunberg and Nyborg added 0.19mass%graphite in 304L to reduce the SiO2 during vacuum sintering,and the final oxygen level was reduced from 0.31 to0.03mass%. With the elimination of SiO2, the impactstrength was improved by 40%.5) Nylund et al. noticed intheir studies of PIM 316 that parts produced from gasatomized powder had little SiO2 compared to those producedwith water atomized powders. They also observed thatincomplete binder removal leaves carbonaceous binderresidues, which helps reduce the silica and other oxidesduring sintering at high temperatures.10) In a similar study ofPIM 17-4PH, Wu et al. used gas atomized powders andremoved binders at different temperatures to obtain differentresidual carbon contents after debinding, which resultedin carbon contents in as-sintered tensile bars between0.05mass% and 0.13mass%.16) They showed that the speci-men with a 0.13mass% carbon, which exceeds the maximumlimit, had more martensite, less �-ferrite, lower sintereddensity, higher tensile strength, and lower elongation.

Although graphite addition can improve the sinteredproperties of the stainless steels, the control of the residualcarbon could be difficult because the maximum amount ofcarbon allowed is very low; in 304L and 316L, it is only0.03mass%. For 17-4PH stainless steel, the specification formaximum carbon content is more forgiving, at 0.07mass%.Nonetheless, the amount of graphite addition in 17-4PH steelshould still be carefully controlled based on the oxygencontent and the sintering atmosphere used.12) The study ofWu et al.16) has provided a great deal of insight on the effectof carbon content on sintered properties, but the studies onthe use of water atomized powders for 17-4PH compactswith carbon contents within the 0.07mass% limit are stilllimited. Thus, the current study aims to correlate the amountof graphite addition with the reduction of silica, density,mechanical properties, and corrosion resistance of PIM 17-4PH compacts prepared with water atomized powders. Therelevant mechanisms are discussed, with a focus on theeffects of residual carbon content on the fractions of phasesand sintered properties.

2. Experimental Procedures

The 17-4PH stainless steel powder used in this study wasproduced by water atomization, and thus had an irregularshape and large surface area. The type of oxide on the powdersurface was identified with X-ray Photoelectron Spectros-copy (XPS, Theta Probe, Thermo Scientific, Northumber-land, U.K.). The graphite powder used was a flaky type with apycnometer density of 2:45� 103 kg/m3 and a mean particlesize of 9.6 mm. The characteristics of the stainless steel andgraphite powder are listed in Table 1.

To prepare PIM specimens, the 17-4PH and graphitepowders were kneaded with 7mass% wax-based binders thatconsisted of paraffin wax (PW), stearic acid (SA), andpolyethylene (PE), using a �-blade kneading machine. Theresulting feedstock was molded into tensile test (MPIFStandard 50) and impact energy (3:5� 8:5� 55mm) speci-mens using an injection molding machine (320C/400,

Arburg GmbH, Lossburg, Germany). To remove the binder,compacts were first immersed in 318K heptane for 86.4 ks toextract the paraffin wax and stearic acid, which are soluble inheptane. After solvent debinding, compacts were heated at arate of 0.083K/s to 623K in a tube furnace under hydrogen,followed by a slower heating rate of 0.033K/s between623K and 823K, within which the remaining binder wasthermally removed. The specimens were then heated at a rateof 0.167K/s to 1273K and held at that temperature for 3.6 ksfor pre-sintering. The final sintering was carried out in agraphite furnace under vacuum. Since chromium evaporatesunder high vacuum, the furnace was back-filled with argonand was kept at a pressure of 133–667 Pa. The specimenswere heated at a rate of 0.167K/s to 1593K and sintered for3.6 ks. To attain optimum mechanical properties, the sinteredtensile bars were solution treated at 1323K for 1.8 ks in argonand then water quenched. For aging treatment, the solutiontreated specimens were heated to 755K and held at thattemperature for 3.6 ks.

The sintered density was measured using the Archimedesmethod, following the MPIF Standard 54. The hardness ofthe sintered specimens was measured using a RockwellHardness tester (ARK-600, Mitutoyo Co., Tokyo, Japan). Tocompare the microhardness at different phases, a Vickersmicro-hardness tester (HM-112, Mitutoyo Co., Tokyo,Japan) is used with 100 g loading. The tensile strengthwas measured using a Universal Tensile Tester (AG-10TE,Shimadzu Co., Kyoto, Japan). After tensile testing, thefracture surface was examined with a scanning electronmicroscope (SEM, XL-30, Philips, Eindhoven, Holland).For corrosion test, flat specimens were immersed in a 2%H2SO4 solution at room temperature for 86.4 ks, followingthe MPIF standard 62. The resulting weight loss wasmeasured and then converted into a weight loss per surfacearea per day value. To check whether the organic binder hadbeen completely removed during debinding and sintering,and whether graphite powder had reacted completely withthe silica and other oxides in the powder, the carbon contentwas determined using a carbon analyzer (EMIA-220V,Horiba, Kyoto, Japan). The nitrogen and oxygen contents,

Table 1 Characteristics of the 17-4PH stainless steel and graphite powder

used in this study.

17-4PH Graphite

Grade PF-20F 1651

Pycnometer Density (kg/m3) 7:67� 103 2:45� 103

Size Distribution (mm) D10 ¼ 5:0 D10 ¼ 4:0D50 ¼ 15:2 D50 ¼ 9:6D90 ¼ 33:6 D90 ¼ 19:9

Chemical Composition (mass%)

C 0.048 96

O 0.320 —

N 0.020 —

Si 0.82 —

Mn 0.82 —

Ni 4.27 —

Cr 15.74 —

Cu 3.26 —

Nb 0.30 —

2244 C.-W. Chang, P.-H. Chen and K.-S. Hwang

which are also critical for the mechanical properties andcorrosion resistance, were measured using an Oxygen/Nitrogen Analyzer (TC-136, LECO, St. Joseph, Michigan,U.S.A).

For microstructure analysis, the specimens were ground,polished, and then etched with Fry’s solution (HCl: 40 cm3,H2O: 130 cm

3, Ethanol: 25 cm3, CuCl2: 5 g). The lineintercept method was then used to measure the volumefractions of the �-ferrite. The measured values were alsocompared to the phase fractions calculated using the Thermo-Calc software and a TCFE3 database (Thermo-Calc Soft-ware, Stockholm, Sweden). In addition, a Feritscope (MP30,Fisher, Zorneding, Germany) was used to estimate theamount of magnetic phases, including �-ferrite and marten-site. Since these phases are determined by the composition,an Electron Probe Microanalyzer (EPMA, JXA-8600SX,JEOL, Tokyo, Japan) was used to examine the distributionof the major alloying elements, including Fe, Si, Ni, Cr, Nb,and Cu.

3. Results

3.1 Reduction of silica particlesThe ESCA analysis showed that the SiO2, Cr2O3, and

MnO are the dominant oxides covering the surfaces of thefine 17-4PH stainless steel powders, as shown in Fig. 1.The cross-section, as shown in Fig. 2, indicated that a fewsmall silica particles, smaller than 1 mm, were also presentinside the 17-4PH powder. The amount of dissolved siliconmeasured with an EPMA was 0.60mass%, close to the totalsilicon content of 0.82mass%. This indicates that mostsilicon was in the dissolution form. However, large amountsof coarse silica particles, about 4 mm, were observed at thefracture surface of as-sintered tensile specimens, as shownin Fig. 3, and the amount of dissolved silicon decreasedto 0.52mass%. These observations suggest that the silicaparticles originated from the following sources: The firstone was the silica film originally on powder surfaces, whichgrew and formed spherical particles during sintering inorder to reduce the surface energy. The second sourcewas the dissolved silicon, which oxidized during sinteringbecause the partial pressure of oxygen in the furnace wasnot low enough. The last source could be the originalinternal silica particles, which remained oxidized duringsintering.

With graphite addition, the oxygen contents of sinteredspecimens decreased significantly due to the high reductionpower of graphite at 1593K, as shown in Fig. 4. With only0.2mass% graphite addition, the oxygen content decreasedfrom 0.21mass% to 0.02mass%. Assuming that all theoriginal 0.32mass% oxygen had reacted with the graphitepowder and formed carbon monoxide, the amount of graphiterequired was about 0.24mass%. Thus, as the graphiteaddition was over 0.24mass%, extra carbon would be leftin the sintered compacts. For example, as the amount ofgraphite powder increased to 0.3mass%, the amount ofretained carbon reached 0.072mass%, which exceeds themaximum amount allowed for 17-4PH stainless steels. Thissuggests that the graphite addition should be less than0.3mass% for the 17-4PH stainless steel powder selected in

90 100 110 120

Bonding energy, E / eV

50

100

150

200

Cou

nts

(cps

)

SiO2

560 570 580 590 600


150

200

250

300

350

Cou

nts

(cps

)

Cr2O3

630 640 650 660 670


200

250

300

350

400

Cou

nts

(cps

)

MnO

Fig. 1 XPS analysis of the 17-4PH powder surfaces showing the peaks of

SiO2, Cr2O3, and MnO.

10 µ m

Fig. 2 Cross-section of the as-received 17-4PH powder.

Enhanced Mechanical Properties of Injection Molded 17-4PH Stainless Steel through Reduction of Silica Particles 2245

this study. Figure 5 confirms that few silica particles wereobserved in the fracture surface of the 17-4PH + 0.26mass%graphite tensile specimens. The carbon content of thisspecimen was only 0.03mass%, within the specification.

3.2 MicrostructureA comparison of the typical microstructures of the 17-4PH

with and without graphite addition is presented in Fig. 6.

It was observed that more martensite and less ferrite werepresent after graphite addition. The quantitative metallog-raphy measurements showed that the amount of � phase was10 vol% in the graphite-free specimen, as shown in Fig. 7.As graphite addition increased, the amount of �-ferriteincreased first and then decreased. With 0.26mass% graphiteaddition, the �-ferrite amount decreased to 4 vol%. Figure 7

10 µ m

Fig. 3 Fracture surface of the as-sintered 17-4PH tensile specimen.

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Fraction of added graphite (mass%)

0

0.02

0.04

0.06

0.08

Car

bon

cont

ent (

mas

s%)

0.00

0.05

0.10

0.15

0.20

0.25

Oxy

gen

cont

ent (

mas

s%)

Carbon ContentOxygen Content

Fig. 4 The effect of graphite addition on the contents of oxygen and

carbon.

10 µ m

Fig. 5 Fracture surface of as-sintered 17-4PH + 0.26mass% graphite

specimen.

Fig. 6 Microstructure of as-sintered specimen. (a) 17-4PH and (b) 17-

4PH + 0.26mass% graphite.

Fig. 7 Changes of volume fractions of �-phase and magnetic phase with

graphite addition.


also shows that the fraction of the magnetic phase read fromthe Feritscope had a similar trend as the �-ferrite amount. Thereading decreased from 68% of the graphite-free specimen to64% after 0.26mass% graphite additions. It should be notedthat the Feritscope reading is not simply the sum of thefraction of �-ferrite and martensite because the magneticresponses of these phases are different. When solutioned andaged, the amount of �-ferrite in all specimens decreased,following the same trend as in as-sintered specimens, becauseless �-ferrite was present at the solutioning temperatureof 1323K. With less �-ferrite and more martensite, theFeritscope readings decreased because �-ferrite has a strongerferromagnetism than does the martensite.

3.3 Sintered densityThe changes in the fraction of each phase, which is

probably influenced by the changes in the amount ofdissolved carbon and silicon, also influence the sintereddensity. Figure 8 shows that the sintered density increasedslightly from the 7:62� 103 kg/m3 of the graphite-freespecimen to the maximum of 7:69� 103 kg/m3 at about0.20mass% graphite additions, and then decreased as theamount of graphite addition further increased.

3.4 Mechanical propertiesFor as-sintered tensile bars, Fig. 9 shows that the hardness

did not increase greatly until more than 0.2mass% graphite

was added. This hardness change could be a result of thevariations in the fraction and hardness of individual phases.To verify that, the microhardness was measured for theferrite, and was HV 241, lower than that of the martensite.In addition, the hardness of the martensite usually increasesas the amount of carbon increases. In fact, the hardness ofthe martensite in the as-sintered graphite-free specimen wasHV 282, while the hardness of the 17-4PH + 0.26mass%graphite specimen was HV 304.

Figure 10 shows the variations in the tensile strength ofthe as-sintered tensile bars as a function of graphite addition.It follows the same trend as the hardness shown in Fig. 9.It is very likely that the tensile strength is influenced by thesame mechanism as that described earlier for the hardness.

In comparison, the elongation of the as-sintered 17-4PHwas not influenced by the graphite addition, as shown inFig. 11. Impact energy increased as the graphite additionincreased and peaked at 0.2mass%, as shown in Fig. 12.These changes could be a result of complex combinationsof the amount of porosity, silica particles, and �-ferrite in theas-sintered tensile specimen.

For the solutioned-and-aged specimens, however, Fig. 9and Fig. 10 show that the hardness and tensile strength didnot change much with graphite additions. This could be theresult of the small amount of �-ferrite, less than 8mass% inall specimens after heat treatment, as shown in Fig. 7. Inaddition, the strengthening of aged 17-4PH is determined

0.00 0.05 0.10 0.15 0.20 0.25 0.30


7.60

7.65

7.70

7.75

7.80D

ensi

ty,

ρ/1

03kg

m-3

Fig. 8 Effect of graphite addition on the sintered density.

0.00 0.05 0.10 0.15 0.20 0.25 0.30

20

30

40

50

Har

dnes

s (H

RC

)

As-sinteredHeat-treated


Fig. 9 Effect of graphite addition on the hardness of as-sintered and

solutioned-and-aged specimens.

σ

Fig. 10 Effect of graphite addition on the tensile strength of as-sintered and


Fig. 11 Effect of graphite addition on the elongation of as-sintered and



mainly by the precipitation of "-Cu, which occurred in allspecimens. Nonetheless, the elongation and impact energyincreased significantly with graphite addition for heat treatedtensile specimens, as shown in Fig. 11 and Fig. 12, respec-tively. This could be due to the elimination of the silicaparticles, which are detrimental for less ductile materials inelongation and impact energy. Without silica particles, therewere far fewer crack initiation sites and fewer weakinterfaces for cracks to propagate through.

3.5 Corrosion resistanceWith graphite addition, it is more likely that the corrosion

resistance will deteriorate due to the formation of chromiumcarbides, particularly when the amount of carbon is too highor when the distribution of carbon is not uniform. This isespecially true for 316L stainless steels, which have amaximum limit of 0.03mass% in the carbon content. But for17-4PH stainless steels, the limit is less restrictive, as themaximum carbon content allowed is 0.07mass%. In addition,the chance of forming chromium carbides and Cr-lean areasis lower because 17-4PH contains 0.15–0.45mass% Nb,which is more prone to forming carbides than is Cr.

Figure 13 shows the weight loss of the specimen that wasimmersed in 2% sulfric acid for 86.4 ks. With a graphiteaddition of 0.26mass% or less, the weight loss was minimal,similar to that of the graphite-free specimen. A close

microstructure examination with EPMA analysis for thespecimen with 0.26mass% graphite addition showed thatno chromium carbides were observed, although some NbCdid form at the grain boundaries, as shown in Fig. 14. Sincethe amount of the niobium carbide was small and nochromium-lean areas were present, the corrosion resistancewas similar to that of the graphite-free specimen. As theamount of graphite further increased to 0.30mass%, thefinal carbon content increased to 0.072mass%, whichexceeds the maximum limit. As a result, the weight lossincreased significantly to 22 gm�2 day�1, which is muchhigher than the 0.5 gm�2 day�1 limit listed in the MPIFstandard 35.

As described above, the SiO2 inclusions originallycontained in water atomized 17-4PH powders can beeffectively reduced by graphite addition during sintering.For the silicon that is originally dissolved in the matrix, theformation of silica during sintering can also be prevented.This beneficial effect of graphite addition should alsoapply to gas atomized powders, in which most silicon isnot present as silica particles or surface films, but in thedissolved form.6,16) The suggested graphite addition shouldbe 0.26mass% or less for the 17-4PH powder examined inthis study, which contains 0.32mass% oxygen. For compar-ison, the optimum amount of graphite addition suggested byTunberg et al.17) for pressed-and-sintered 304L powders withan initial oxygen content of 0.3mass% is 0.19mass%.

4. Discussion

The changes in the mechanical properties as a functionof graphite addition could be due to combined effects ofreduction of silica particles, density, fractions of phases,and residual carbon content. The relevant mechanisms arediscussed in the following sections.

4.1 Effect of graphite addition on �-ferrite formationDuring sintering, 17-4PH stainless steel consists of a

mixture of �-ferrite and �-austenite, which transforms intomartensite after cooling to room temperature. The fraction of

0.00 0.05 0.10 0.15 0.20 0.25 0.30


0

100

200

300Im

pact

ene

rgy,

E /

J As-sinteredHeat-treated

Fig. 12 Effect of graphite addition on the impact energy of as-sintered and


Fig. 13 Effect of graphite addition on the weight loss of as-sintered

specimen immersed in 2% sulfuric acid for 86.4 ks.

5 µ m

Fig. 14 Microstructure of the 17-4PH + 0.26mass% graphite specimen.


the �-ferrite in the mixture depends on the composition.Using the Thermo-Calc software, the calculated amount of�-ferrite at 1593K is 54mass% for the 17-4PH with thecomposition given in Table 1. This value will vary withgraphite additions due to the influence on the amounts ofcarbon and silicon dissolved in the matrix, which areaustenite and ferrite stabilizers, respectively. When theamount of graphite addition was small, most graphite wasconsumed to reduce the silica particles, and thus the amountof dissolved silicon increased, resulting in an increase inthe �-ferrite amount, as shown in Fig. 7. However, as thegraphite content continued to increase, a small amount ofcarbon was also dissolved, as shown in Fig. 4. With anincrease in the amount of both �-ferrite and dissolvedcarbon, the amount of �-ferrite reached a maximum at thegraphite addition of about 0.1mass%. As the amount ofgraphite addition further increased, little more silica couldbe reduced, while the amount of dissolved carbon increasedsignificantly. As a result, more austenite formed at theexpense of �-ferrite.

These changes in the amount of �-ferrite are consistentwith those reported by Wu et al. for the 17-4PH sintered at1603K using gas atomized powders.16) Their results show acontinuous decrease in the �-ferrite amount from 2.7 to0mass%, as the carbon content increases from 0.06 to0.12mass%. This trend is similar to that observed in thisstudy for carbon content in a similar range, except that theiramount of �-ferrite is lower due to the lower silicon contentof 0.45mass% in their gas atomized powders.

4.2 Effect of �-ferrite on sintered densityA comparison of Fig. 7 and Fig. 8 indicates the beneficial

effect of �-ferrite on sintering densification. The first effectis that the volume diffusion rate in ferrite is about two ordersof magnitude higher than that in austenite. The second effectis that, with dual phase sintering in the �þ � region, thegrain growth is less serious due to the constraint exerted bythe neighboring unlike phases. With finer grains and more�=� interfaces, the grain boundary diffusion mechanism,which is a faster mass-transport mechanism than volumediffusion, could contribute more to densification.18,19)

4.3 Effect of �-ferrite, density, and carbon content onmechanical properties

The mechanical properties of the PIM 17-4PH aredetermined by the density, phase fractions, and the hardnessof the individual phases. As described in previous sections,low amounts of graphite addition enhanced densification dueto the increase in the amount of �-ferrite. This means adecrease in the fraction of martensite, which negated theimprovement in hardness and tensile strength given by theincreased density. Thus, the strength and hardness did notchange much with small amounts of graphite additions. Butas the amount of graphite addition further increased, themartensite fraction increased, and the hardness of theindividual martensite grain also increased, due to the increasein the amount of dissolved carbon. Thus, the overall hardnessand tensile strength of as-sintered specimens increasedsignificantly when the amount of graphite increased to0.26mass% and higher.

However, the hardness and tensile strength of agedspecimens seemed to be irrelevant to the graphite addition.The most likely explanation is that the strengtheningmechanism was no longer the low carbon martensite. Inaddition, the difference of the amount of �-phase among thespecimens also decreased after solutioning. The strength andhardness of aged 17-4PH are determined mainly by theprecipitation of "-Cu, which occurred in all specimens. Asa result, the aged tensile specimens consisted of similaramounts of martensite and similar properties.

5. Conclusions

(1) This study confirms that water atomized 17-4PHpowders are covered with silica films. During sintering,the silica film evolves into spherical particles. Withthe addition of graphite powders, the silica film andparticles can be effectively reduced during vacuumsintering.

(2) The presence of �-ferrite enhances densification butdeteriorates mechanical properties. With differentamounts of silicon and carbon dissolved in the matrix,the fractions of �-ferrite and martensite phases varyand influence the sintered density and mechanicalproperties.

(3) The amount of graphite addition should be less than0.26mass% for the 17-4PH powder with an oxygencontent of 0.32mass%. When the carbon content is keptbelow the 0.07mass% limit, the corrosion resistance ofthe graphite-added specimens is similar to that of thegraphite-free specimen.

(4) With 0.26mass% graphite powder addition, the hard-ness, tensile strength, and elongation in the as-sinteredspecimen improved to HRC 30, 1050MPa, and 9.8%,respectively. After solutioning and aging, these prop-erties increased to HRC 41, 1310MPa, and 9.0%,respectively. The ductility was 100% higher than thatof the graphite-free specimen, and the impact energyof the aged specimen also improved by 107%, mainlydue to the elimination of silica particles.

Acknowledgement

The authors are grateful for the support of this work by theNational Science Council of the Republic of China underContract No. NSC-98-2221-E-002-54-MY2.

REFERENCES

1) J. J. Dunkley: Prog. Powder Metall. 37 (1981) 39–45.

2) N. Dautzenberg and H. Gesell: Powder Metall. Int. 9 (1976) 14–17.

3) D. P. Ferriss: Int. J. Powder Metall. Powder Technol. 19 (1983) 16–19.

4) D. H. Ro and E. Klar: Modern Developments in Powder Metallurgy,

ed. by H. H. Hausner, H. W. Antes and G. D. Smith, (Metal Powder

Industries Federation, 1981) pp. 247–287.

5) T. Tunberg and L. Nyborg: Powder Metall. 38 (1995) 120–130.

6) L. Nyborg and I. Olefjord: Powder Metall. 31 (1988) 33–39.

7) R. M. Larsen and K. A. Thorsen: Powder Metall. 37 (1994) 61–66.

8) N. Yasui, H. Satomi, H. Fujiwara, K. Ameyama and Y. Kankawa:

Extended Abstracts of 2006 Powder Metall. World Congress, ed. by

K. Y. Eun and Y. S. Kim, (Korean Powder Metallurgy Institute, Seoul

Korea, 2006) pp. 39–40.


9) L. Fedrizzi, A. Molinari, F. Deflorian, A. Tiziani and P. L. Bonor: Br.

Corros. J. 26 (1991) 46–50.

10) A. Nylund, T. Tunberg, H. Bertilsson, E. Carlstrom and I. Olefjord: Int.

J. Powder Metall. 31 (1995) 365–373.

11) I. Costa, C. V. Franco, C. T. Kunioshi and J. L. Rossi: Corrosion 62

(2006) 357–365.

12) E. Klar, M. Svilar, C. Lall and H. Tews: Adv. Powder Metall. and

Particulate Mater., ed. by J. M. Capus and R. M. German, (Metal

Powder Industries Federation, 1992) pp. 411–426.

13) P. Suri, R. P. Koseski and R. M. German: Mater. Sci. Eng. A 402 (2005)

341–348.

14) K. J. Bartone and S. K. Das: U. S. Patent 6,770,114 B2, (2004).

15) Y. Wu, H. Lai, Y. Dong and C. Liu: Adv. Powder Metall. and


Powder Industries Federation, Princeton, NJ, 1992) pp. 353–362.

16) Y. Wu, R. M. German, D. Blaine, B. Marx and C. Schlaefer: J. Mater.

Sci. 37 (2002) 3573–3583.

17) T. Tunberg, L. Nyborg and C. X. Liu: Adv. Powder Metall. and


Powder Industries Federation, Princeton, NJ, 1992) pp. 383–396.

18) T. M. Puscas, A. Molinari, J. Kazior, T. Pieczonka and M. Nykiel:

Powder Metall. 44 (2001) 48–52.

19) G. J. Shu, K. S. Hwang and Y. T. Pan: Acta Mater. 54 (2006) 1335–

1342.


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