International Journal of Minerals, Metallurgy and MaterialsVolume 19, Number 1, Jan 2012, Page 83DOl: 10.1007/s12613-012-0519-0Characterization of 17-4PH stainless steel powders produced bysupersonic gas atomizationXin ming Zhao", Jun Xxu"), Xue xin Zhul:2), Shao-ming Zhang", Wen-dong Zhao", and Guo liang Yuan1) National Engineering and Technological Research Center for Non-ferrous Metals Composites, General Research Insitute for Non frrous Mctals, Beijing 10088, China2) Beijing COMPO Advanced Technology Co. Ltd, Beijing 10088, China(Reccived: 28 December 2010; revised: 20 February 2011; acepted: 26 July 2011)teristics of powder particles were carried out by means of a laser particlc size analyzer, scanning electron microscopy (SEM), and the X-raydifraction (XRD) tchnique. The results show that the mass median particle diameter is about 19.15 um. Three main types of surface micro-structures are observed in the powders: well-developed dendrite, cellular, and cellular dendrite structure. The XRD measurements show that,as the particle size decreases, the amount of fcc phase gradually decreases and that of bee phase increases. The cooling rate is inversely related to the particle size, ie, it decreascs with an increase in particle size.Keywords: gas atomization; metal powder; stainless seel; metal iction molding[This work was financially supported by the National High Tech Research and Development Program of China (No. 2009A4033901) andthe National Natural Science Foundation ofChina (No. 51004019).]the worldwide production of non- ferrous alloy powders by1. Introductiongas atomization reaches 3.5x10 ta [2].MIM is a near-net shaping technology that combines thepreferred method for the production of ultrafine sphericaladvantages of plastic injection molding with conventionalmetal powders, which experience a high cooling ratepowder metallurgy. In addition, it is a cost- effective tech-(0*-100C/s) and deep undercooling [1]. The gas atomiza-nology to manufacture small intricate and precise compo-tion process is described as: the nozzle generates a high ve-nents [4]. Nowadays, over 50% of the injection molded andlocity and high pressure gas flow that disintegrates the liquidsintered components are made from stainless steel composi-metal stream into droplets, which subsequently spheroidize,tions, in which 17-4 PH stainless steel is most widely usedcool, and freeze into metal powder particles ranging from 1s the MIM material. In the MIM process, gas atomizedμm to 1 mm in diameter [2-3]. Powder produced by gas at-powder is generally used because it has a higher packingomization technology exhibits attractive characteristics,density and thus needs less binder for injection moldingsuch as refined microstructure, extended solute solubility,leading to low shrinkage and distortion during sintering.reduced microsegregation, and formed metastable phase.Furthermore, the spherical morphology guarantees a moreCurrently, many commercial metal and alloy powders haveisotropic shrinkage, as the particles cannot take a preferredbeen produced by gas atomization applied in the areas ofdirection during injection molding [5]. In addition, severalthermal spray coating, chemical catalyst, rocket propellant,studies reveal that the sintered MIM components using gasand metal injection molding (MIM), and it is estimated thatatomized powder bave less residual porosity, higher sinteredCoresponding author: Xin .ming Zhao E-mail: xinming. zhao@hotmail.com。University of Science and Technology Beijing and Springer-Verlag Berin Heidelberg 2012包Springer中國煤化工MHCNMHGInt J. Miner. MetalL. Mater, VoL.19, No.1, Jan 2012density, higher mechanical strength, and excellent corrosionthe powders less than 25 um was further analyzed by a laserproperties [6-7].particle size analyzer. The surface morphology of powdersHowever, the microstructure characterization of differentvas examined using a JEOL JSM-6400 scanning electron17-4 PH stainless steel powder particle sizes produced bymicroscope (SEM). The phase present in the powders wasgas atomization is seldom available. During the gas atomi-identified by D8 Advance X-ray diffaction (XRD) using Cuzation process, a range of powder particle sizes is obtained,Ka radiation with a wavelength of λ=0.105406 nm.and this leads to a range of cooling rates, which results inTable 1. Chemical composition of 17 4PH stainless steel wt%diverse structures and phase compositions. In this work, anC_Mn Ni Cr Cu _ NdCoFeinvestigation on the powder size distribution, morphology,microstructure, and phase composition was carried out to0.05 0.80 1.00 4.00 16.00 3.50 0.30 0.10 Bal.study the effects of particle sizes.3. Results and discussion2. Experimental3.1. Powder size distribution of the atomized powderAtomization runs were performed in an industrial-scaleThe powder size distribution of the gas atomized 17 4PHsupersonic gas atomizer with nitrogen as the atomizing gas.Commercial 17-4PH stainless steel, with the compositionstainless steel is shown in Fig. 1. The mass fraction in eachsize range is given in Fig. l(a), and the cumulative distribu-given in Table 1, was used as a base material for this inves-tion is given in Fig. 1(b). It shows that the particle size istigation. In order to minimize the impurity, the melting andatomization chambers were evacuated and nitrogen wasrelatively fine with powders less than 75 pum accounting forbackilled to a pressure of 1.05x105 Pa prior to melting and96.88wt% and the finer particles (D<25 um) approximatelyatomization. The alloy was melted in a magnesia crucible byaccount for 60.42wt%. Based on the cumulative distributionmeans of a medium-frequency induction fumace under a ni-curve (Fig 1()), the mass median powder diameter (dso) istrogen atmosphere. The alloy was heated to 1700°C, and adetermined to be 19.15 um. The specific powder sizes, d16homogenization time of about 10 min was adopted. Theand dgu, which coresponded to the opening of a screenvalue of the atomization gas pressure was 4.0 MPa. Themesh that let through 16wt% and 84wt% of the powders, are17-4PH stainless steel powders were cooled down to ambi-9.15 and 41.74 pum, respectively. The spread of powder sizedistribution is represented by the geometric standard devia-ent temperature in nitrogen atmosphere and then collected.tion 8, defined as (dsv/dso+dsoldhs)/2, assuming that theThe powder size distribution was measured on a standardpowder size obeys a lognormal distribution [8-9]. The geo-sieving machine and the powders were sieved into the fol-metric standard deviation δ of the gas atomized 17-4PHlowing fractions: <25, 25-38, 38- 45, 45-53, 53-62, 62-75,stainless steel powder is 2.14 according to the results men-and >75 pum. After vibration for 15 min, the mass of pow-tioned above, which is consistent with other experiments byders retained on each sieve was measured and convertedthe gas atomization method [1, 8].into a percentage of the total powder sample. The fraction of70廠100s0(日)90- (b)60室800t70十10 F60個50-...0Fi ds>75 62-75 53-62 45-53 38-45 25-38 <251020304050607080Particle diameter/ umParticle diameter 1 umFig1. Powder size dstribution curves for the 17-4PH stainless seel powder: (田) mass fractiono of the atomized powder in each sizerange; () cumulative distribution of the atomized powder.中國煤化工MHCNMHG8Int J. Miner. Metall. Mater, VoL.19, No.1, Jan 2012large particles due to a lowondary dendrite. A local magnification micrograph takenand a higher solidifcation rate for small ones. As a result of from Fig. 4(a) is shown in Fig. 4(b). From this micrograph,various solidification rates, during the atomization process,the secondary dendrite arm spacing is about 2.2 um. As thethe larger droplets might be still in liquid or semisolid state,particle size decreases, the surface microstructure becomeswhile the small ones have already solidified entirely. Underthe cellular dendritic structure with short secondary den-the turbulent conditions formed in the atomization chamber,drites, and the secondary dendrite arm spacing is about 1.3the cllision or impingement of large and small particles letμm, as shown in Figs. 4(c) and 4(d). Fig. 4(e) shows that thesmall solidified particles attach to large liquid or semisolidmixed cellular dendritic and cellular stnuctures are present.ones firmly.The finer particle (D<25 um), as shown in Fig. 4(), dis-Three kinds of surface microstructures of the 17-4PHplays a complete cellular structure without the formation ofstainless steel powder have been observed, namely, den-secondary dendrite arms. In addition, the intercellular spac-dritic structure, cellular dendritic structure, and cellularing (or the width of the cells) is about 0.5 pum. The result in-structure, as shown in Fig. 4. Fig. 4(a) is the microstructuredicates that, as the particle size varies from coarse to fine,corresponding to the powder of Dr -75 pum and exhibiting ahe corresponding surface microstructure changes fromwell- defined dendritic structure with a well- developed sec-dendritic to cellular dendritic/cellular structure. The varia-a)(t020gmd)5ume)5 yuim1 umFig. 4. Surface morphologies of 17 4PH stainless steel powders with dilerent sizes: (a) D> 74 pum; () local magnification of (@); (<),(d) D-53-62 pm; (e) D :38-45 pm,用D<25 pm.XC中國煤化工MHCNMH GXM. Zhao et al, Characterization of 17- 4PH stainless stee powders produced by supersonic gas atomization87tion of the surface microstructure of powder particles is re-particle size dependence. The amount of fce phase decreaseslated to the solidification undercooling, and the undercool-and that of bcc pbase increases with decreasing the particleing depends strongly on the initial undercooling and thesize.cooling rate [13].0o25Although most particles have the surface microstructure-98--- bec phasedescribed above, it should be noticed that in each case a告20very small fraction of the particles also has a microstructure+94.that presented in the other batch. For example, in powder-92+90particles of D> 75 um, a small fraction of the particles pos-sesses a cellular dendritic structure similar to that found in5 10+8powder particles of D<75 um and vice versa.3.4. XRD of the atomized powder-82-8XRD patterns of 17-4PH stainless steel powders in dif->7562-7553-6245-5338-4525-38<25ferent particle sizes at room temperature shown in Fig. 5 in-Particle diameter 1 umdicate that the particles contain a mixture of bcc () and fccig. 6. Volume fraction of fcc and becc phases as a function of(y) phases and bcc phase is the primary solidification phaseparticle size.in all the powder particles. The volume fraction of fcc phaseand bcc phase as a function of powder particle size is shownKelly et al. concluded that the probability of nucleationin Fig. 6. The volume fraction of the phases was calculatedor nucleation site for a given droplet decreases rapidly withusing the ratio of the integrated intensities ofbcc {10} anddecreasing the particle size, namely, a small droplet have thefcc {1]} diffraction peaks [14]. Since 17-4PH stainlesslow probability of nucleation, while the large one has thesteel powders only contain two phases, bcc and fcc phases,high probability of nucleation [15]. For this reason, in thethe expression for the volume fraction of each phase can besmall droplets of 17-4PH stainless steel, the droplet nucle-written as Vj/V~=(sR)(CIRs) and Vg+V=1, where Vg is theates at a relative high undercooling due to the low probabil-volume fraction of bcc phase, Vy the volume fraction of fccity of nucleation, leading δ (bcc) phase to form, as sbown inphase, Is the measured integrated intensity of the {1 l0}bcehe Fe-Cr-Ni equilibrium phase diagram (Fig. 7). Subse-diffaction peak, Iy the measured integrated intensity of thequently, during the recalescence period after nucleation, a{11}ec diffraction peak, and factor R depends only on thesmall fraction of the undercooled particles was heated to thediffracting 2θ angle and the diffracting plane {hkl} [14].δ+L zone before final cooling. Then, the high cooling rateHence, the volume fractions, Vs and Vp are calculated by themakes the small droplets (Dr<25 pum) to retain the bcc (6)above equations.pbase (97.8vol%) and fcc (Y) phase (2.2vol%) to room tem-perature. As the particle size increases, the droplet has a500F110%high probability of nucleation and achieves a low degree ofundercooling prior to nucleation. As a result, the larger par-2000-bcc phase- fcc phase .管1508 10001450L+yL+8211;|500-200,200% 220,。1400Ltry+δ1300γ+δ2030405060708090~201(9)1250Expcrimental alyFig. 5. XRD patterns of the 17-4PH stainless steel powder.0.51.0152.0253.03.54.0455.0As seen in Fig. 6, the volume fraction of the fcc and bccphases in the 17-4PH stainless steel powder shows a strongFig. 7. Equilibrium phase diagram of the FeCr-Ni alloy.中國煤化工YHCNMHG88Int. J. Miner. Metall. Mater, Vol.19, No.1, Jan 2012ticles undercool to the δ+L zone of the phase diagram. Be[4] A. Nylund, T. Tunberg, H Bertilsson, et al, Injection mold-sides, the cooling rate also decreases as the particle size in-ing of gas and water. atomized stainless steel powders, Int. J.creases, as shown in Fig. 2. The above-mentioned two rea-Powder Metall, 31(1995), p.365.ons promote the increment of the amount of fcc phase in[5] P. Suri, R.P. Koseski, and R.M. German, Microstructuralevolution of injection molded gas- and water atomized 316Llarge droplets. Compared to the small particles (D 25 μm),stainless steel powder during sinteing, Mater. Sci. Eng. A,the large particles (D>75 um) solidify as bcc phase402(2005), p.341.(81.2vol%)} and fcc phase (18.8vol%).[6] Z.W. Xu, C.C. Jia, CJ. Kuang, et al, Fabrication and sinter-ing behavior of high-nitrogen nickel-ftee stainless steels by4. Conclusionsmetal injection molding, Int. J. Miner. Metall. Mater,17(2010), p.423.(1) The mass median powder diameter (dso) for 17-4PH[7] L.X. Cai and R.M. German, Powder injection molding usingstainless steel powders is about 19.15 um. The specificwater atomized 316L stainless steel, Int. J Powder Metall,powder sizes, d1o and dga, are about 9.15 and 41.74 um, re-31(1995), p.257.spectively. The standard deviation is 2.14.[8] J. Juarcz-lslas, Y. Zhou, and EJ. Lavemia, Spray atomizatioof two Al-Fe binary alloys: solidification and microstructure(2) The cooling rate is inversely related to the dropletcharacterization, J. Mater. Sci, 341999), p.1211.diameter, i.e, it decreases as the droplet diameter increases.The cooling rates for the droplets of 10 and 90 um in di-[9]、C. Srivastava and S.N. Ojha, Effect of aspiration andgas- melt configuration in close coupled nozzle on powderameter are 1.46x 10' and 3. 10x 105 K/s, respectively.productivity, Powder Metall, 49(2006), p.213.(3) Three main types of surface microstructures have[10] F. Duflos and JF. Stoht, Comparison of the quench rates at-tained in gas- atomized powders and melt- spum ribbons of Co-been observed in the powders: dendritic structure, cellularand Ni-base superalloys: influence on resulting microstruc-dendritic structure, and cellular structure.tures, J. Mater. Sci, 17(1982), p.3641.(4) The particles contain a mixture of bcc and fcc phases[11] A. Inoue, T. Masumoto, T. Ekimoto, et al, Preparation ofFe,and the bcc phase is the primary phase in all particles. WithCo-，and Ni-based amorphous alloy powders by high-pre-a decrease in particle size, the amount of fcc phase graduallyssure gas atomization and their structural relaxation behavior,Metall. Trans. A, 19(1988), p.235.decreases and that of bcc phase increases.[12] P. Dong, W.L. Hou, X.C. Chang, et al, Amorphous andnanostructured AlgsNisY6Co2Fer powder prepared by nitro-Referencesgen gas-atomization, J. Alloys Compd., 436(2007), p.118.[1] 1.E. Anderson and R.L. Terpstra, Progress toward gas atomi- [13] R. Xu, Y.Y. Cui, D. Li, et al, Solidification microstructure ofzation processing with increased uniformity and control, Ma-super-a2 alloy prepared by gas atomization, J. Mater. Sci,ter. Sci. Eng. A, 326(2002), p.101.32(1997), p.3821.[2] N. Zeoli and s. Gu, Computational validation of an isentopic[14] N.H. Pryds and A.S. Pedersen, Rapid solidification of mart-plug nozzle design for gas atomisation, Comput. Mater. Sci,ensitic stainless steel atomized droplets, Metall. Mater. Trans.42(2008), p.245.A, 33(2002), p.3755.3] J. Ting, J. Connor, and S. Ridder, High-speed cinematogra-[15] TF. Kelly, M. Cohen, and J.B.V. Sande, Rapid solidifcationphy of gas-metal atomization, Mater. Sci. Eng. A, 390(2005),of a droplet processed stainless steel, Metall Trans. A,p.452.15(1984), p.819.中國煤化工MHCNMHG