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A Study of Parameters and Properties Influencing the Size, Morphology and Oxygen Content of Water Atomized Metal Powders
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Process.ORCID iD: 0000-0001-7376-9062
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The production of metal powders by water atomization is a well-established process, which can be used to produce a wide range of particle sizes for different applications. In general, there is a lack of detailed knowledge about what process parameters that affect the powder properties for water atomized metal powders. More specifically, this thesis focuses on the particle size, morphology and oxygen content of water atomized iron powders. A careful control of the particle size distribution is necessary to atomize powders with a high quality and at a low production cost. Demands on the particle morphologies vary depending on the application for the final product. It is important to control both the melt properties and atomizing parameters, to produce powders with an even particle shape and sintered steel components with tight tolerances. The oxidation of the liquid metal should also be as low as possible during the water atomization, to avoid a large amount of harmful oxide forming in the final powder. Pores are generally considered as defects in metal powders. Therefore, the powder porosity should be as low as possible.

The main objective of this thesis is to obtain a more in-depth knowledge of water atomization of metal powders, by investigating some fundamental parts of the process. The study investigates how the median particle size (d50 value) for iron powders is influenced by the water pressure, the melt stream diameter, the jet angle, the water level in the atomizing tank, changed configurations of the water jets, superheat of the melt, and the carbon and sulfur content in the liquid steel. Similarly, the thesis also investigates factors that influence the particle shape, porosity and oxidation of water atomized iron powders.

Laboratory and pilot experiments show that the effect on the d50 value was large for the water pressure, medium for the viscosity, surface tension and water to metal ratio, and small for the melt stream diameter. Calculations indicate that the water jet angle has a large effect on the d50 value. In practice, this effect cannot be exploited beyond certain limits caused by instabilities in the atomizing system, which occur if the jet angle is too large.

The particle size decreases when the carbon and sulfur contents in the liquid iron are increased. This is attributed to decreased viscosities and surface tensions, respectively. An alternative explanation could be that the superheats at increased carbon contents result in a longer time spent in the molten state before the atomization is completed. This may also lead to a decrease in the particle size. Calculations using a developed d50 model estimate that a decreased viscosity from 6.8 mPa s to 4.3 mPa s leads to a reduction in the d50 value by 33%. Similarly, a decreased surface tension from 1840 mN/m to 900 mN m-1 reduces the d50 value by 27%.

The distribution of oxides in pilot water atomized Fe-Mn-C powders was determined by using optical and scanning electron microscopy, combined with energy dispersive X-ray microanalysis. The oxygen in the atomized powders was mainly present as thin surface oxide layers, which increase in thickness from 10 nm to 50 nm as the particle sizes increase from 10 microns to 750 microns. Manganese oxides were observed to be unevenly distributed at the surface of several particles, when the alloy contained 0.3 wt.% manganese. Experimental data indicate that between 10 - 20% of the manganese was present as oxides in the powders. However, equilibrium calculations at 1550 °C estimate that only 4% of the initial manganese content remained in the steel after a completed atomization.

The sphericity of the atomized powders decreases as the particle size increases. One feasible explanation is that some larger particles are irregular, since they are formed by collisions of smaller particles. Conversely, smaller particles are formed directly from breakups of the melt and are not the product of collisions between droplets. The sphericity of the size fraction 20-45 microns increases as the carbon content in the iron increases from 0.2 wt.% to 4.2 wt.%. The atomized droplets with larger carbon contents spend a longer time in the molten state, which allows them more time to form a spherical shape during the atomization process. The porosity of iron-carbon powders increases with increasing carbon contents in the melt. Dissociation of steam to hydrogen at the melt surface and precipitation of hydrogen pores in the melt were the most likely mechanisms to cause a pore formation in the powders.

Keywords:    water atomization; metal powder: particle size; oxygen content; particle shape; porosity; steelmaking

 

Abstract [sv]

Vattenatomisering är en väletablerad produktionsprocess, som kan användas för att tillverka metallpulver för flera olika produktapplikationer och partikelstorlekar. Generellt råder det en brist på detaljerad kunskap över hur processparametrar påverkar pulveregenskaperna för vattenatomiserade metallpulver. Följande avhandling studerar hur parametrar och egenskaper påverkar partikelstorleken, morfologin och syrehalten för vattenatomiserade järnpulver. En noggrann styrning av partikelstorleken är väsentlig för att kunna atomisera metallpulver med hög produktkvalitet och låg tillverkningskostnad. Krav på partikelform varierar beroende på den slutliga applikationen för produkten. Styrning av både atomiseringsparametrar och smältans fysikaliska egenskaper är här väsentliga, för att kunna producera pulver med en jämn partikelform och sintrade ståldetaljer med snäva toleranser. Oxidationen av metallsmältan bör också minimeras i processen, för att undvika att en större mängd skadliga oxider bildas i det atomiserade pulvret. Porer betraktas generellt som defekter i metallpulver, varför partikelporositeten bör vara så låg som möjligt.

Det övergripande målet med detta arbete är att öka kunskapen om vattenatomisering av metallpulver, genom att i detalj studera några fundamentala delar av processen. Avhandlingen undersöker hur pulvrets mediandiameter (d50 värdet) påverkas av vattentrycket, stålstrålens diameter, dysvinkeln, vattennivån i atomiseringstanken, ändrade dysmunstycken, smältans övertemperatur samt kol- och svavelhalten i stålsmältan. Studien behandlar även faktorer som påverkar pulvrets partikelform och porositet samt oxidationen av det vattenatomiserade järnpulvret.

Försök i laboratorie- och pilotskala visar att inverkan på d50 värdet var stor för vattentrycket, medelstor för vatten/metall kvoten, smältans viskositet och ytspänning, samt låg för stålstrålens diameter. Modellberäkningar indikerar även att dysvinkeln har stor inverkan på d50 värdet. I praktiken begränsas detta dock av att atomiseringen blir instabil vid för höga dysvinklar.

Pilotskaleförsök visade även att partikelstorleken minskar med ökande kol- och svavelhalter i stålsmältan, vilket kan bero en sänkt viskositet respektive ytspänning. En annan möjlig förklaring kan vara att de högre övertemperaturerna vid högre kolhalter har ökat tiden som de atomiserade partiklarna är flytande under atomiseringen. Detta kan också ha minskat d50 värdet. Beräkningar med en utvecklad d50 modell visar även att en sänkt viskositet från 6.8 mPa s till 4.3 mPa s minskar d50 värdet med 33%. En sänkt ytspänning från 1840 mN/m till 900 mN/m uppskattas på samma sätt minska d50 värdet med 27%.

Fördelningen av oxider i Fe-Mn-C pulver atomiserade i pilotskala undersöktes med optiskt ljus- och svepelektronmikroskop, med energidispersiv röntgendetektor. Syret var huvudsakligen fördelat som tunna ytoxidskikt i de atomiserade pulvren. Oxidskikten uppskattas öka i tjocklek från 10 nm till 50 nm, vid en ökad partikelstorlek från 10 mikron till 750 mikron. Vidare var manganoxider ojämnt fördelade på ytan av ett flertal pulverpartiklar för järnpulver legerade med 0.3 vikts% mangan. Experimentella data indikerar även att 10 - 20% av manganet förelåg som oxider i materialet. Dessutom visar termodynamiska beräkningar vid 1550 °C en stark drivande kraft att oxidera mangan, där endast 4% av den ursprungliga manganhalten uppskattades vara kvar i stålet efter atomiseringen.

Rundheten för de atomiserade pulvren minskade med en ökad partikelstorlek. En möjlig förklaring till att några större partiklar är oregelbundna kan vara att de är bildade genom kollisioner mellan mindre partiklar. Vidare kan små partiklar ha bildats direkt vid sönderdelningen av stålsmältan och inte genom kollisioner av metalldroppar. Rundheten ökar för storleksfraktionen 20 - 45 mikron med en ökad kolhalt, från 0.2 till 4.2 vikts%. De atomiserade metalldropparna med högre kolhalter tillbringar en längre tid i flytande tillstånd under atomiseringen, vilket kan medföra en längre tid att nå en rund partikelform jämfört med partiklar med lägre kolhalter. Porositeten för järnpulver ökar med stigande kolhalt i smältan. Spjälkning av vattenånga till vätgas vid smältans yta samt vätgasutskiljning i metalldropparna bedöms vara den mest troliga mekanismen för att bilda porer i pulvren.

Nyckelord:  vattenatomisering; metallpulver; partikelstorlek; syrehalt; partikelform; porositet; stålframställning

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2021. , p. 199
Series
TRITA-ITM-AVL ; 2021:42
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
URN: urn:nbn:se:kth:diva-305316ISBN: 978-91-8040-085-5 (electronic)OAI: oai:DiVA.org:kth-305316DiVA, id: diva2:1614544
Public defence
2021-12-20, https://kth-se.zoom.us/s/68638736525?pwd=dHpsVnB4WE1rUXpISit3QnhMUHZHUT09, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2021-11-26 Created: 2021-11-25 Last updated: 2022-06-25Bibliographically approved
List of papers
1. Prediction of particle size for water atomised metal powders: parameter study
Open this publication in new window or tab >>Prediction of particle size for water atomised metal powders: parameter study
2012 (English)In: Powder Metallurgy, ISSN 0032-5899, E-ISSN 1743-2901, Vol. 55, no 1, p. 45-53Article in journal (Refereed) Published
Abstract [en]

This work aims to investigate how some significant atomising parameters influence the mass median particle size d50 of water atomised metal powders. More specifically, these were water pressure, melt flowrate, water jet angle, liquid metal viscosity and surface tension. Existing models for the prediction of d50 during water atomisation were reviewed. The selected models were fitted and compared with atomising experiments of liquid iron containing 0.5–4.4%C. Experimental results and model calculations were used in a parameter study to investigate how the different parameters influenced d50. The effect on d50 was large for the water pressure, medium for the viscosity and low for the melt flowrate and surface tension. Model calculations indicate that the jet angle has a large effect on d50, which should be verified by additional studies. The model proposed by Bergquist (B. Bergquist: Powder Metall., 1999, 42, 331–343) showed the best agreement with the current experimental data.

Place, publisher, year, edition, pages
Informa UK Limited, 2012
Keywords
Powder metallurgy, Modelling, Water atomisation, Metal powder, Particle size
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-53622 (URN)10.1179/1743290111Y.0000000016 (DOI)000302546300008 ()2-s2.0-84859701828 (Scopus ID)
Note

QC 20220609

Available from: 2011-12-29 Created: 2011-12-29 Last updated: 2022-06-24Bibliographically approved
2. Influence of liquid metal properties on water atomised iron powders
Open this publication in new window or tab >>Influence of liquid metal properties on water atomised iron powders
2012 (English)In: ISIJ International, ISSN 0915-1559, E-ISSN 1347-5460, Vol. 52, no 12, p. 2130-2138Article in journal (Refereed) Published
Abstract [en]

The main focus of the present study was the influence of liquid metal properties on the particle size during water atomisation. Experiments for liquid iron showed that alloy additions of carbon and sulphur decreased the particle size. Moreover, it was indicated that the reduced d50 value at increased %C and %S contents may be related to a decreased viscosity and surface tension respectively. An alternative mechanism could be that raised superheats at increased carbon contents increased the total available time for atomisation. This may also have decreased the particle size. The influence of surface tension and viscosity on the d50 value was further analysed with a theoretical d50 model proposed in a previous work. A reduced viscosity from 6.8 to 4.3 mPa s decreased the d50 value with 33%. In addition, the particle size was estimated to decrease with 27% by decreasing the surface tension from 1 850 to 900 mN m-1.

Keywords
Metal powders, Particle size, Superheat, Surface tension, Viscosity, Water atomisation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-116149 (URN)10.2355/isijinternational.52.2130 (DOI)000312968000003 ()2-s2.0-84871778211 (Scopus ID)
Note

QC 20130118

Available from: 2013-01-18 Created: 2013-01-16 Last updated: 2024-03-15Bibliographically approved
3. Oxidation of Water Atomized Metal Powders
Open this publication in new window or tab >>Oxidation of Water Atomized Metal Powders
2014 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 85, no 12, p. 1629-1638Article in journal (Refereed) Published
Abstract [en]

This study focuses on the oxidation of water atomized metal powders. Pilot plant experiments were performed using liquid iron alloyed with manganese and carbon. The powder particle shape and the oxides were determined using optical and scanning electron microscopy with energy dispersive X-ray microanalysis. The oxygen in the atomized powders was mainly present as thin surface oxide layers, which were determined to increase from 10 to 40-60 nm, at increased particle sizes from 10 to 750 mu m. In addition, manganese oxides were observed to be unevenly distributed at the surface of several particles for iron powders alloyed with 0.3mass% Mn. Experimental data indicated that between 10 and 20% of the manganese was present as oxides in the powders. However, equilibrium calculations predicted a strong driving force for oxidation of manganese. More specifically, it was estimated that only 4% of the initial manganese content remained in the final atomized powders.

Keywords
metal powders, atomization, oxidation
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:kth:diva-158283 (URN)10.1002/srin.201300466 (DOI)000345832000006 ()2-s2.0-84913617856 (Scopus ID)
Note

QC 20150109

Available from: 2015-01-09 Created: 2015-01-07 Last updated: 2024-03-15Bibliographically approved
4. Particle morphology of water atomised iron-carbon powders
Open this publication in new window or tab >>Particle morphology of water atomised iron-carbon powders
2022 (English)In: Powder Technology, ISSN 0032-5910, E-ISSN 1873-328X, Vol. 397, p. 116993-, article id 116993Article in journal (Other academic) Published
Abstract [en]

Water atomisation can produce metal powders faster and at lower cost than gas atomisation, but it is well known that the powder particles are irregular and may contain a large number of pores. The current study analyses three iron-carbon alloys with different superheats, produced as powder by water atomisation and compares the particle shapes and porosity in each. The alloy with the most carbon (4.2 wt%) showed the highest circularity (0.72) for 20-40 µm particles, but the lowest (0.59) for 180-210µm particles. This is consistent with collisions between droplets affecting particle shape. The lowest-carbon melt (0.22 wt%) solidified fastest, so underwent fewest collisions and showed similar circularity for all particle sizes. The breakdown of water to form hydrogen and the formation of hydrogen bubbles was the most likely cause of porosity. The findings of this study may be used to inform future water atomisation process design to control particle shape and minimise porosity.

Keywords
Water atomisation; turbulent collisions; porosity; water breakdown; hydrogen evolution
National Category
Metallurgy and Metallic Materials
Research subject
Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-305315 (URN)10.1016/j.powtec.2021.11.037 (DOI)000820281000003 ()2-s2.0-85120423992 (Scopus ID)
Note

QC 20211130

Available from: 2021-11-25 Created: 2021-11-25 Last updated: 2023-06-07Bibliographically approved

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