Metallographic Preparation of Powder Metallurgy Parts Application Notes

Metallographic preparation of powder metallurgy parts Application

Next to casting, mechanical forming lurgy for the high volume production of Notes and machining, powder metallurgy small, intricately shaped, and/or very (P/M) technology is an important precise parts such as gears and links. method of manufacturing metal parts. Undesirable characteristics of ingot- In addition, the process offers the based metals can be greatly reduced, potential to produce a wide variety and desired properties of metals which of alloys with different material would normally not alloy easily can be properties such as high tem- achieved by combining different metal perature toughness and hard- powders or mixtures of metal and non ness. High speed cutting tool metal powders. bits from sintered tungsten carbide powder are an example of The process of making powders, com- the variety of different properties which pacting them into useful shapes and can be achieved with the powder metal- then sintering them is costly, but the lurgical process. finished parts have some specific -ad vantages over wrought or cast parts. As the density of the compacted and sintered part influences its key proper- The main advantages are: ties of strength, ductility and hardness, - the possibility to make fine grained a specific porosity is critical. For pro- homogenous structures cess control, metallography is used to - the ability to form complicated check porosity, non-metallic inclusions shapes with close dimensional and cross-contamination. In research tolerances and failure analysis, metallography is a - and the ability to produce parts with major tool used to develop new prod- a superior surface finish. ucts and improve manufacturing processes. In addition to chemical analysis, Costly machining processes are thus quality control also includes physical reduced or eliminated and consequently methods for checking density, dimen- there is less scrap loss compared to sional changes, flow rate etc. other forming methods. It is therefore most economical to use powder metal-

Experimental P/M stainless Difficulties during metallographic preparation Solution: steel, colour etched Grinding and polishing: Sufficiently long polishing To reveal the structure with the correct and representative porosity.

Fig.1: Porosity of a P/M steel sample after a Fig. 2: Same sample as Fig.1 after 8 min. Different powder metallurgy steel components 4 min. polish with 3 µm diamond polish with 3 µm diamond

28.02.2019 / 62140805 Production of Synchronising hub powder metallurgy parts

Manufacturing process of iron and steel powders

Production produce pure iron powder. It is used for al- A good powder mix not only has the ability to To achieve the desired structure and near net loying and low density applications, such as produce the required properties of a specific shape of a powder metal part, stringent pro- bearings. alloy, but also needs to facilitate handling, cess control of the following production steps compacting and sintering. is required: In the atomizing process molten metal of For instance, the easy flow of powder and its the desired alloy flows through a nozzle and capability to mix evenly with other powders - Making the powder is struck by high pressure water or gas jet. is important for an even powder distribution - Mixing the powder with additions such as Small droplets are formed which solidify into before pressing, and ensures uniform proper- lubricant, carbon and/or alloying elements particles. Atomized powders result in higher ties of the finished part. - Compacting the powder in carbide dies densities than mechanically crushed pow- - Sintering at high temperature (1100- ders, therefore all steel powders are produced For the production of components the mixed 1200°C) in a protective atmosphere by atomizing. powders are first compacted under high pressure in a carbide die. At this stage the part Chemical and atomisation methods are the Copper powder is made by atomizing or elec- has the geometrical features of the finished two most common methods for powder pro- trolytic methods. Tungsten carbide powder component, but not its strength and is called duction. The chemical method converts metal is produced by adding controlled amounts of the “green” part. In order to develop the from ore oxides directly to metal powder at carbon to the tungsten powder and carburis- mechanical and physical properties of the a temperature below the melting point. For ing it at 1400 - 2650°C material, metallurgical bonding has to take example, iron powder is made through direct place through sintering at high temperature reduction from iron ore into sponge iron. The Powder production and mixing is a highly in a sintering furnace. The bonding occurs sponge iron is then mechanically crushed to specialized and complex process which pro- through diffusion between adjacent particles. powder, which is further refined through duces custom made powder mixes designed To avoid oxidation, which would impair the annealing under reducing atmosphere to to satisfy the needs of a specific application. inter-particle bonding, the sintering process is conducted in a protective atmosphere.

The bonding increases the density, and pressed and sintered powder metal parts generally contain between 5 and 25% residual porosity. Depending on the application some parts may need an additional hot isostatic pressing for better dimensional accuracy, or surface finish or impregnation with oil. Final treatments, such as surface hardening, plat- ing or coating can be applied.

Sponge iron powder, SEM Atomized iron powder, SEM Difficulties in the metallographic preparation of powder metallurgy Fig. 3: Sintered tungsten carbide parts (WC/Co), etched with Murakami’s reagent. 1500 x

The sintering of cemented carbides is carried The main challenge during the preparation is out in a vacuum sintering process. to show the true porosity after grinding and The carbide powder is mixed with 3-25 w% polishing. Depending on the hardness of the cobalt and small amounts of titanium and material this can be more or less successful. tantalum carbides are added to inhibit grain During the grinding of soft metals, abraded growth. This mixture is pressed and sintered. metal is pushed into the pores, which then At 1280 – 1350°C the liquefaction of cobalt has to be removed by polishing. takes place and results in the formation of a Samples from parts in which hard and soft eutectic-like phase of WC/Co. Densification materials are mixed are prone to show pro- begins at lower temperatures and reaches a nounced relief. The preparation of green parts theoretical 100% shortly after the liquefaction needs particular care and patience as they are has occurred. During liquefaction the part very fragile. shrinks up to 40% in volume. Fig. 4: Carbide distribution in conventionally produced steel Recommendations for the Applications preparation of powder Components made by powder metallurgy are mainly used for the following applications: metallurgy parts Cutting - Mechanical and structural parts, mainly iron For sectioning a powder metallurgy com- based, but also from copper, brass, bronze ponent of a specific metal or alloy one can and aluminium. The largest user of P/M parts select an appropriate cut-off wheel using the is the automotive industry. Component sup- recommended charts and guidelines. For pliers make connecting rods, synchronizing mixed materials it is recommended to use a hubs, chain sprockets, cams and gears. cut-off wheel suitable for cutting the material - Refractory metals which, due to their high which constitutes the major part. For sintered melting points, are difficult to produce by carbides a resin bonded diamond cut-off melting and casting. Fig. 5: Carbide distribution in powder wheel is recommended (e.g. B0DXX). Green metallurgically produced steel parts need to be mounted in cold mounting - Porous material in which controlled poro- resin (see section “Mounting”) before cutting, sity serves a specific purpose, for instance so that they are not crushed by clamping. self lubricating bearings. Mounting - Composite materials that do not form al- In order to assure good adhesion of the loys, for instance copper/tungsten for electri- mounting resin to the sample material it is cal contacts, cemented carbide cutting tools essential to degrease the sample thoroughly (Fig. 3), materials for brake linings and with acetone, toluene or Isopar C* before clutch facings, diamond cutting tools, or mounting (Use proper safety precautions metal matrix composites. when handling solvents!). - Special high-duty alloys, such as nickel and cobalt based super alloys for jet engine Sintered parts can be mounted with a resin parts, and high speed tool steels, which have suitables to the mechanical properties of the an even distribution of carbides and have sample material: For hot compression mount- isotropic qualities (Figs. 4 and 5). ed either phenolic resin (MultiFast) or resins containing harder filler material (DuroFast, In addition, different powders and powder LevoFast). For a cold mounting acryllic resins mixtures for thermal spray coatings are pro- with filler (DuroCit-3 or LevoCit) can be used. duced and are also subject to metallographic For more information see www.struers.com quality control. Green parts need to be re-impregnated after sectioning under vacuum with a cold mounting epoxy resin (CaldoFix-2, EpoFix, SpeciFix-40). Powders can be mounted by mixing a small amount (about 1/2 teaspoon) of powder with a slow curing epoxy resin and pouring it into a mounting cup. During the approx.12 hours curing process the particles will form a layer by settling at the bottom of the cup.

*Available from Exxon Preparation method for Grinding P/M bronze

Step PG FG

Foil/Paper MD-Largo

Type SiC Diamond Fig. 6: Abrasive Size #320 9 μm Surface of P/M steel after fine grinding on MD-Allegro Suspension/ Water DiaPro Lubricant Allegro/Largo 9

rpm 300 150

Force [N]/ 35 25 specimen

Time (min) As needed 4

Polishing

Fig. 7: Same sample as in Fig. 6 showing Fig. 9: Larger magnification of surface from Fig. 7, Step DP OP insufficient polish showing metal “lids” covering pores

of diamond polishing to remove the metal Surface MD-Mol MD-Chem pushed into the pores during grinding and Type Diamond Silica reveal the correct porosity. For soft metals the Abrasive final polishing time should not be prolonged Size 3 µm 0.04 µm unnecessarily, as this leads to rounding of Suspension/ DiaPro Mol R3 OP-S NonDry the pore edges. Starting with 500# or 800#, Lubricant green parts are ground manually on silicon carbide Foil/Paper to 4000#. The surface is rpm 150 150 re-impregnated as needed. Polishing can be done on a semi-automatic polishing machine Force [N]/ 25 25 Fig. 8: Same sample as in Fig 7 after longer polish for individual samples. specimen showing correct porosity In order to establish the exact polishing time Time (min) 4 1 for specific alloys and parts, it is recom- Hard metal powders can be hot compression mended to check the structure during polish- mounted by mixing with one measuring spoon ing every two minutes with the microscope, Table 1: Preparation method for 6 samples of P/M bronze, mounted, of the fine grained mounting resin DuroFast. 30 mm dia., using the semi-automatic Tegramin, 300 mm dia. and only proceed to the next polishing step As an alternative to DiaPro polycrystalline diamond suspension P, when all residual metal has been removed The mixture is then poured into the cylinder 9 µm, 3 µm and 1 µm can be used together with red and green or blue from the pores. lubricant. of the mounting press and is topped with phenolic resin.

Grinding and polishing The grinding and polishing procedure for powder metals follows the same rules which are applied to prepare ingot based samples of the same material. Plane grinding high volumes of samples of materials >150 HV can be carried out on an aluminium oxide grinding stone or a diamond grinding disc (MD-Piano). Materials <150 HV can be plane ground on Fig.10: Water stains from cleaning can lead to Fig.11: P/M bronze, unetched, containing 500x silicon carbide Foil/Paper. misinterpretation of structure graphite (grey), and α-δ eutectoid (blue)

For fine grinding with diamond the fine grind- Cleaning and drying ing disc MD-Allegro is suitable for materials After polishing it is essential to clean the sam- >150 HV and MD-Largo for materials <150 ple with a water/detergent mixture to remove HV. This is followed by a thorough diamond remnants of the polishing suspension and polish with 3 µm and a brief final polish with lubricant from the pores. The sample is then 1 µm or with oxide polishing suspensions. rinsed with water, followed by a thorough As one of the major goals of preparing a rinse with isopropanol and dried with a powder metallurgical sample is to show the warm stream of air, holding the sample at an true porosity, it is important that the diamond angle. Do not blow the stream of air directly polishing step is carried out long enough to achieve this goal (see Figs. 6-9). For large or Fig.12: Same as Fig.11, etched with 500x soft samples it can take up to 10-15 minutes iron-III-chloride, showing grain structure of bronze Grinding Preparation method Preparation method Grinding for P/M steel for sintered carbides

Step PG FG Step PG FG

Surface Foil/Paper MD-Allegro Surface MD-Piano MD-Allegro

Type SiC Diamond Type Diamond Diamond Abrasive Abrasive Size #220 9 μm Size #220 9 μm

Suspension/ Water DiaPro Suspension/ Water DiaPro Lubricant Allegro/Largo 9 Lubricant Allegro/Largo 9 To reveal the structure, the common chemical rpm 300 150 etching solutions for metals and their alloys, rpm 300 150 that are recommended in the literature, can Force [N]/ 35 30 be used. The following procedure is recom- Force [N]/ 30 30 specimen mended for etching: wet the surface with specimen isopropanol, immerse sample face up into Time (min) As needed 5 etchant and slightly agitate. When the ap- Time (min) As needed 4 propriate etching time has elapsed, take the Polishing sample out of the etchant and rinse either Polishing with isopropanol or water depending on the Step DP 1 DP 2 etchant (see below) and dry with a stream Step DP OP of warm air. Interpretation and photographic documentation should take place immediately Surface MD-Mol* MD-Nap after drying. Surface MD-Dac MD-Chem*

Type Diamond Diamond Etching time depends on the alloy and it needs Type Diamond Silica Abrasive Abrasive Size 3 μm 1 μm some experience for the correct timing. Etch- Size 3 μm 0.04 μm ing too short will not give sufficient contrast Suspension/ DiaPro DiaPro Suspension/ of the different phases. If the sample is over- DiaPro Dac 3 OP-U NonDry Lubricant Mol B 3 Nap B 1 Lubricant etched, it is difficult to distinguish the various phases (see Figs.15-17). When working with rpm 150 150 unfamiliar material it is recommended to etch rpm 150 150 shorter rather then longer and to check the Force [N]/ 30 20 result with the microscope first. More etching Force [N]/ 30 20 specimen specimen can be carried out if necessary, but if the sample is overetched, it needs to be repolished. Time (min) 6-8 0.5 - 1 Following are some of the common etchants. Time (min) 4 1 Follow standard safety precautions when Table 2: Preparation method for 6 P/M steel samples, mixing and working with chemical reagents! Tabel 3: Preparation method for 6 samples of sintered carbides, mounted, 30 mm dia., using the semi-automatic Tegramin, mounted, 30 mm dia., using the semi-automatic Tegramin, 300 mm dia. 300 mm dia. As an alternative to DiaPro polycrystalline diamond suspension P, 9 µm, As an alternative to DiaPro polycrystalline diamond Copper and copper alloys: 3 µm and 1 µm can be used together with green/blue lubricant. suspension P, 9 µm, 3 µm and 1 µm can be used together *Optional step with green or blue lubricant. 1. 100 ml water 20 ml hydrochloric acid *Alternatively MD-Dac/DiaPro Dac 3 can also be used. 5 g Iron-III-chloride from the top onto the sample surface as it 10-20 sec forces the liquids out of the pores, which Rinse with water followed by isopropanol will leave stains on the surface (Fig.10). It is 2. 100 ml water important to use a high quality alcohol for 10 g ammonium persulfate. Use fresh only! rinsing to keep staining to a minimum. Rinse with water followed by isopropanol Steel: Etching It is recommended to examine the unetched 1. 1-3% Nital for iron-carbon alloys, iron- sample first to check the density, shape and carbon-copper alloys and pre-alloyed size of the pores, oxidation and inclusions, iron-molybdenum: 100 ml ethanol sintering necks and free graphite (see Figs.11 Fig.13: P/M steel with 0.5%C, diffusion alloyed with Ni, Cu and and 12). It is important to know the theo- 1-3 ml nitric acid Mo. Etched in picral, showing areas of fine pearlite surrounded retical density in order to compare it to the 10-60 sec depending on carbon content. by ferrite, martensite, bainite and Ni-rich austenite. porosity. Rinse with isopropanol (Fig.14) After this initial inspection it is recommended 2. Picral to develop difference between to etch the sample immediately to avoid dry- martensite and austenite: ing stains, which form when cleaning and 100 ml ethanol drying liquids are gradually released by the 4 g picric acid pores. For very dense sintered carbides this 10-60 sec. depending on carbon content. is not as important as for any powder metal Rinse with water followed by isopropanol with a certain amount of porosity. (Fig.13)

Fig.14: P/M steel with 0.8% C, pre alloyed with 1.5% Mo. Etched with nital, showing dense bainite Step PG FG

Struers ApS Pederstrupvej 84 DK-2750 Ballerup, Denmark Phone +45 44 600 800 Fax +45 44 600 801 [email protected] www.struers.com Fig.15: Etched too short 3. For contrast between pearlite, bainite and martensite: AUSTRALIA & NEW ZEALAND NETHERLANDS Struers Australia Struers GmbH Nederland 200 ml ethanol 27 Mayneview Street Zomerdijk 34 A Milton QLD 4064 3143 CT Maassluis 4 g picric acid Australia Telefoon +31 (10) 599 7209 Phone +61 7 3512 9600 Fax +31 (10) 5997201 1-2 ml nitric acid Fax +61 7 3369 8200 [email protected] [email protected] 20-100 sec, depending on carbon content NORWAY and alloying elements. BELGIUM (Wallonie) Struers ApS, Norge Struers S.A.S. Sjøskogenveien 44C Rinse with water followed by isopropanol 370, rue du Marché Rollay 1407 Vinterbro F- 94507 Champigny Telefon +47 970 94 285 sur Marne Cedex [email protected] Stainless steels: Téléphone +33 1 5509 1430 Vilella’s reagent: Télécopie +33 1 5509 1449 AUSTRIA [email protected] Struers GmbH Step DP OP 45 ml glycerol Zweigniederlassung Österreich Fig.16: Etched too long, over etched BELGIUM (Flanders) Betriebsgebiet Puch Nord 8 15 ml nitric acid Struers GmbH Nederland 5412 Puch Zomerdijk 34 A Telefon +43 6245 70567 30 ml hydrochloric acid 3143 CT Maassluis Fax +43 6245 70567-78 Telefoon +31 (10) 599 7209 [email protected] 30 sec to 5 min Fax +31 (10) 5997201 Rinse thoroughly with water followed by [email protected] POLAND Struers Sp. z o.o. isopropanol CANADA Oddział w Polsce Struers Ltd. ul. Jasnogórska 44 7275 West Credit Avenue 31-358 Kraków Sintered tungsten carbides: Mississauga, Ontario L5N 5M9 Phone +48 12 661 20 60 Phone +1 905-814-8855 Fax +48 12 626 01 46 Murakami‘s reagent Fax +1 905-814-1440 [email protected] 100 ml water [email protected] ROMANIA 10 g potassium or sodium hydroxide CHINA Struers GmbH, Sucursala Struers Ltd. Bucuresti 10 g potassium ferricyanide No. 1696 Zhang Heng Road Str. Preciziei nr. 6R Zhang Jiang Hi-Tech Park 062203 sector 6, Bucuresti Etching by immersion or swab etch Shanghai 201203, P.R. China Phone +40 (31) 101 9548 Rinse thoroughly with water followed Fig.17: Correct etching Phone +86 (21) 6035 3900 Fax +40 (31) 101 9549 Fax +86 (21) 6035 3999 [email protected] by isopropanol [email protected] Author SWITZERLAND CZECH REPUBLIC & SLOVAKIA Struers GmbH Elisabeth Weidmann, Birgitte Nielsen, Struers GmbH Organizační složka Zweigniederlassung Schweiz Summary Struers A/S, Copenhagen, Denmark vědeckotechnický park Weissenbrunnenstraße 41 Powder metallurgy is a method of produc- Judy Arner, Struers, Inc., Westlake, OH, USA Přílepská 1920, CH-8903 Birmensdorf CZ-252 63 Roztoky u Prahy Telefon +41 44 777 63 07 ing parts from metals which normally would Phone +420 233 312 625 Fax +41 44 777 63 09 Acknowledgements Fax +420 233 312 640 [email protected] not alloy easily, or from mixtures of metals We wish to thank Höganäs AB, SE 26383 Höganäs, [email protected] [email protected] SINGAPORE and non-metals, taking advantage of their Sweden, for the co-operation and supplying information Struers Singapore combined properties. Although the powder and material for reproduction: all photos of P/M GERMANY 627A Aljunied Road, components, diagram and SEM photos on page 2, Figs. Struers GmbH #07-08 BizTech Centre metallurgical process is costly, it has the Carl-Friedrich-Benz-Straße 5 Singapore 389842 1, 2, 6, 7, 8, 9, 10, and 13 - 17. Special thanks go to D- 47877 Willich Phone +65 6299 2268 advantage of economically producing large M. Carlsson who generously shared her professional Telefon +49 (0) 2154 486-0 Fax +65 6299 2661 experience with us. Fax +49 (0) 2154 486-222 [email protected] amounts of small and intricately shaped parts [email protected] Figs. 4 and 5 courtesy Böhler Edelstahl GmbH, SPAIN with homogenous structures. The density of Kapfenberg, Austria FRANCE Struers España compacted and sintered parts affects their Struers S.A.S. Camino Cerro de los Gamos 1 Bibliography 370, rue du Marché Rollay Building 1 - Pozuelo de Alarcón strength, ductility and hardness. Therefore F-94507 Champigny CP 28224 Madrid Höganäs Handbook for sintered components, 6. sur Marne Cedex Teléfono +34 917 901 204 metallographic control of porosity is an inte- Metallography, Höganäs AB, 1999 Téléphone +33 1 5509 1430 Fax +34 917 901 112 Télécopie +33 1 5509 1449 [email protected] gral part of quality control. Höganäs iron and steel powders for sintered [email protected] During metallographic grinding, metal is components, Höganäs AB, 1998 FINLAND Structure 24, Metallographic examination of P/M HUNGARY Struers ApS, Suomi pushed into the pores and if the subsequent Struers GmbH Hietalahdenranta 13 stainless steel matrix composites, N. Frees, Institute for Magyarországi Fióktelepe 00180 Helsinki polishing steps are not carried out properly, Product Development, Technical University, Lyngby, 2040 Budaörs Puhelin +358 (0)207 919 430 Szabadság utca 117 Faksi +358 (0)207 919 431 residual metal “lids” covering the pores will Denmark, 1991 Phone +36 2380 6090 [email protected] obstruct the evaluation of the correct poros- Metallographic etching, G. Petzow, ASM, Metals Park, Fax +36 2380 6091 Ohio, 1978 Email: [email protected] SWEDEN ity. Thorough grinding and polishing with Struers Sverige IRELAND Box 20038 diamond, and microscopic checks between Struers Ltd. 161 02 Bromma Unit 11 Evolution@ AMP Telefon +46 (0)8 447 53 90 polishing steps, assure a true representation Whittle Way, Catcliffe Telefax +46 (0)8 447 53 99 Rotherham S60 5BL [email protected] of the structure. The given procedures for Tel. +44 0845 604 6664 Fax +44 0845 604 6651 UNITED KINGDOM automatic preparation and the chemical etch- [email protected] Struers Ltd. ants have been used successfully in day to Unit 11 Evolution @ AMP ITALY Whittle Way, Catcliffe day practical laboratory application and give Struers Italia Rotherham S60 5BL Via Monte Grappa 80/4 Tel. +44 0845 604 6664 reproducible results. 20020 Arese (MI) Fax +44 0845 604 6651 Tel. +39-02/38236281 [email protected] Fax +39-02/38236274 [email protected] USA Struers Inc. JAPAN 24766 Detroit Road Marumoto Struers K.K Westlake, OH 44145-1598 P/M steel with copper infiltration Takanawa Muse Bldg. 1F Phone +1 440 871 0071 3-14-13 Higashi-Gotanda, Fax +1 440 871 8188 Shinagawa [email protected] Tokyo 141-0022 Japan Phone +81 3 5488 6207 Fax +81 3 5488 6237 [email protected] 28.02.2019 / 62140805