Powder metals

Metallographic preparation of powder metallurgy parts

The main challenge when preparing powder metallurgy parts for metallographic analysis is revealing the true porosity of the material after grinding and polishing. Learn how to prepare powder metallurgy parts for analysis quickly and with reproducible results.

Download the full application note

The main characteristics of powder metallurgy parts

Powder metallurgy is a relatively common way of producing parts, particularly in the automobile industry, as it enables the high-volume production of small and intricately shaped parts with homogenous structures. In powder metallurgy, mixtures of metal (and sometimes non-metal) powders are compacted and then sintered. The manufacturing process is expensive, but the finished parts have specific advantages over wrought or cast parts.

With powder metallurgy, it is possible to:
  • Alloy metals that do not normally alloy easily
  • Produce a great variety of alloys with different properties
  • Make fine grained homogenous structures
  • Form complicated shapes
  • Create parts with a superior finish

Powder Metals
Fig. 1: Experimental powder metallurgy stainless steel, color etched

Common applications of powder metallurgy include:

  • Mechanical and structural parts, such as connecting rods, chain sprockets and cams
  • Refractory metals which are difficult to produce by melting and casting
  • Porous materials in which controlled porosity serves a specific purpose
  • Composite materials that do not form alloys, such as copper/tungsten
  • Special high-duty alloys, such as nickel and cobalt-based super alloys (used for jet engine parts)
  • High-speed tool steels that have isotropic qualities and an even distribution of carbides

The metallography of powder metallurgy parts

The density of a compacted and sintered component influences its strength, ductility and hardness. Therefore, metallography of powder metallurgy parts usually includes checking for specific porosity.

In process control, metallography of powder metallurgy parts is used to check porosity, non-metallic inclusions and cross-contamination. The metallography of powder metallurgy parts also plays an important role when developing new products or improving manufacturing processes.

Powder Metals
Fig. 2: Powder metallurgy steel with 0.5 % C, diffusion alloyed with Ni, Cu and Mo. Etched in picral, showing areas of fine pearlite surrounded by ferrite, martensite, bainite and Ni-rich austenite

Powder Metals
Fig. 3: Powder metallurgy steel with 0.8 % C, pre-alloyed with 1.5 % Mo. Etched with Nital, showing dense bainite

The production of powder metallurgy parts

Many different metals are used to create powder metallurgy components, including iron, copper and steel powders.

Manufacturing process of iron and steel powders

Powder metals

Powder production
There are two common methods of powder production: chemical and atomization.
  • Chemical: The metal is converted from ore oxides directly to metal powder at a temperature below the melting point.
  • Atomization: The molten metal alloy flows through a nozzle and is struck by high-pressure water or gas jet. Small droplets are formed which solidify into particles.
Once produced, the metal powder is mixed. At this stage, other elements can be added, including lubricant, carbon and/or alloying elements.

Compacting the powder in a carbide die
In order to produce components, the mixed powders are compacted under high pressure in a carbide die. At this stage, the part is shaped like the finished component, but does not have the required strength. These components are known as ‘green’ parts.

Sintering the component
To develop the necessary mechanical and physical properties, the component is sintered at high temperature in a protective atmosphere. Bonding occurs through diffusion between adjacent particles.

Final treatments
Depending on the application, some parts may undergo additional treatments, including hot isostatic pressing, oil impregnation, surface hardening or plating.

Powder Metals
Fig. 4: Sponge iron powder, SEM

Challenges when preparing powder metallurgy parts

The main challenge when preparing powder metallurgy parts for metallographic analysis is to reveal the true porosity of the material after grinding and polishing. This can be particularly challenging with soft materials, or materials in which soft and hard materials are mixed.
  • With soft metals, abraded metal can be pushed into the pores during grinding.
  • Specimens in which hard and soft materials are mixed are prone to show pronounced relief.
  • Green parts: Components that have been compacted but not yet sintered need particular care as they are very fragile.

Overcoming challenges when preparing powder metallurgy parts

The further sections on this page briefly describe how to overcome these challenges. The procedures have been used successfully in practical laboratory applications and have proven to give reproducible results.

For a more detailed description of the procedures outlined here, download the full application note.

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Fig. 5: Same specimen as Fig.1 after 8 minutes of diamond polishing (3 μm)


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Fig. 6: Porosity of a powder metallurgy steel specimen after 4 minutes of diamond polishing (3 μm)

Recommendations for cutting powder metallurgy parts

Powder metallurgy parts can be made with a range of materials, including iron, copper and steel powders. The appropriate choice of cut-off wheel is dependent on the material type.
  • When sectioning a single-material powder metallurgy component, select a cut-off wheel appropriate for that material.
  • If you are sectioning a mixed-material component, choose a cut-off wheel suitable for the major material.
  • For sintered carbides, use a resin-bonded diamond cut-off wheel.

Powder Metals
Fig. 7: Sintered tungsten carbide (WC/Co), etched with Murakami’s reagent 1500x

Recommendations for mounting powder metallurgy parts

To ensure good adhesion between the mounting resin and specimen material, degrease the specimen thoroughly with acetone or toluene before mounting.

As with cutting, the best mounting method depends on the material you are working with.
  • For sintered parts (mounted specimens):
    -When hot compression mounting, use either phenolic resin (MultiFast) or resins containing a harder filler material (DuroFast or LevoFast).
    -When cold mounting, acrylic resins with filler (DuroCit-3 or LevoCit) can be used.
  • Green parts must be re-impregnated after sectioning under vacuum with a cold mounting epoxy resin (CaldoFix-2, EpoFix or SpeciFix-40).
  • Powders can be mounted by mixing a small amount of powder with a slow-curing epoxy resin. The mixture can be poured directly into the mounting cup.
  • Hard metal powders can be hot compression mounted by mixing them with a fine-grained mounting resin (DuroFast). Pour the mixture into the mounting press cylinder and top it with phenolic resin.

Powder Metals
Fig. 8: Carbide distribution in conventionally produced steel

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Fig. 9: Carbide distribution in powder metallurgically produced steel

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Recommendations for grinding & polishing powder metallurgy parts

As a basic principle, when grinding, fine grinding and polishing powder metals, you should use the same procedures as for ingot-based specimens of the same material.

Plane grinding
  • Plane grinding large volumes of materials (>150 HV) can be done on a diamond grinding disc (MD-Piano). Stainless steel materials can be plane ground on an aluminum oxide grinding surface (MD-Alto).
  • Materials <150 HV can be plane ground on silicon carbide foil or paper.
Fine grinding
  • For fine grinding of materials >150 HV use diamonds on MD-Allegro.
  • MD-Largo with diamonds is suitable for fine grinding of materials <150 HV. 
Diamond polishing
During metallographic grinding, metal is pushed into the pores. If the next polishing steps are not carried out properly, residual metal ‘lids’ will be left over the pores (particularly in soft materials). If not removed, these lids will obstruct evaluation.

Therefore, fine grinding should be followed by a thorough diamond polish. It is important that the diamond polishing step is carried out long enough to reveal the true porosity of the material (see below Figs. 10-13).

Preparation method for powder metallurgy bronze

Powder Metals
Table 1: Preparation method for 6 specimens of P/M bronze, mounted, 30 mm dia., using the semi-automatic Tegramin, 300 mm dia.

As an alternative to DiaPro polycrystalline diamond suspension P, 9 μm, 3 μm and 1 μm can be used together with red, green or blue lubricant.

Preparation method for powder metallurgy steel

Powder Metals
Table 2: Preparation method for 6 P/M steel specimens, mounted, 30 mm dia., using the semi-automatic Tegramin, 300 mm dia.

As an alternative to DiaPro polycrystalline diamond suspension P, 9 μm, 3 μm and 1 μm can be used together with green or blue lubricant.
*Alternatively, MD-Dac/DiaPro Dac 3 can also be used.

Preparation method for sintered carbides

Powder Metals
Table 3: Preparation method for 6 specimens of sintered carbides, mounted, 30 mm dia., using the semi-automatic Tegramin, 300 mm dia.

As an alternative to DiaPro polycrystalline diamond suspension P, 9 μm and 3 μm can be used together with green/blue lubricant.
*Optional step.

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Fig. 10: Surface of powder metallurgy steel after fine grinding on MD-Allegro

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Fig. 11: Same sample as in Fig. 10 showing insufficient polish, showing a surface of powder metallurgy steel


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Fig. 12: Same specimen as in Fig 11 after a longer polish showing correct porosity


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Fig. 13: Larger magnification of surface from Fig. 11, showing metal ‘lids’ covering pores

Cleaning and drying powder metallurgy parts

After polishing, powder metallurgy specimens should be cleaned with a water/detergent mixture to remove remnants of the polishing suspension and lubricant. The specimen should then be rinsed with water, before being thoroughly rinsed with isopropanol.

Powder Metals
Fig. 14: Water stains from cleaning can lead to misinterpretation of the structure

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Recommendations for etching powder metallurgy parts

When analyzing powder metallurgy specimens, it is important to know the theoretical density in order to compare it to the porosity.

We recommend examining the unetched specimen first to check the density, shape and size of the pores, oxidation and inclusions, sintering necks and free graphite (see Figs. 15 and 16). The specimen should then be etched immediately to avoid drying stains.

The recommended etching procedure for powder metallurgy specimens:
  • Wet the surface with isopropanol, immerse the specimen face up into the etchant and slightly agitate.
  • When the appropriate etching time has elapsed, take the specimen out of the etchant and rinse with isopropanol or water depending on the etchant.
  • Dry with a stream of warm air.

Powder Metals
Fig. 15: Powder metallurgy bronze, unetched, containing graphite (grey), and α-δ eutectoid (blue) 500x

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Fig. 16: Same as Fig.15, etched with iron-III-chloride, showing grain structure of bronze 500x

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Fig. 17: A sample that has not been etched long enough – it is difficult to distinguish the various phases

Etching solutions

Common chemical etching solutions for the relevant metal or alloys can be used. When working with chemicals the standard safety precautions must be observed.

Etchants for copper powder metals and copper powder alloys
100 ml water
20 ml hydrochloric acid
5 g Iron-III-chloride
Etch for 10-20 seconds
Rinse with water followed by isopropanol
100 ml water
10 g ammonium persulfate (use fresh only)
Rinse with water followed by isopropanol
Etchants for steel powder metals
1-3% Nital for iron-carbon alloys, iron-carbon-copper alloys and pre-alloyed iron-molybdenum:
100 ml ethanol
1-3 ml nitric acid
Etch for 10-60 seconds depending on carbon content Rinse with isopropanol
Etchants for stainless steel powder metals
Vilella’s reagent:
45 ml glycerol
15 ml nitric acid
30 ml hydrochloric acid
Etch for 30 seconds to 5 minutes
Rinse thoroughly with water followed by isopropanol
Etchants for tungsten carbide powder metals
Murakami‘s reagent:
100 ml water
10 g potassium or sodium hydroxide
10 g potassium ferricyanide
Etch by immersion or swab etch
Rinse thoroughly with water followed by isopropanol

Powder Metals
Fig. 18: Etched too long

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Fig.19: Correct etching

DOWNLOAD THE FULL APPLICATION NOTE INCLUDING PREPARATION METHODS

Summary

Powder metallurgy is used to produce components from metals which normally would not alloy easily. Common materials include iron, copper and steel powders.

The density of a powder metallurgy component affects its strength, ductility and hardness. Therefore, metallographic control of porosity is integral to quality control.

During metallographic grinding and fine grinding, metal can be pushed into the pores, leaving residual metal ‘lids’ that obstruct evaluation. Therefore, careful grinding and polishing with diamonds is essential to ensure a true representation of the material’s structure.

Powder Metals
Fig. 20: Powder metallurgy steel with copper infiltration

Get insight into other materials

Learn more about the materialography of other metals and materials. Check out our materials page 

Holger Schnarr

All images by Birgitte Nielsen, Applications specialist, Denmark
For specific information about the metallographic preparation of powder metallurgy parts, contact our application specialists.