Recommended Procedures
Class 3

Lower Ductility Metals

Hardness 60 HRB – 450 HB Typical Cast irons · PM steels Key challenge Graphite pull-out & porosity

Cast irons (gray, ductile, white, malleable, CGI) and powder metallurgy steels. Graphite form and porosity are the analytical targets; both are easily destroyed by aggressive preparation. ASTM A247 / E2567 demand true shape preservation.

Nodular cast iron microstructure showing typical Class 3 material structure
Nodular cast iron: typical Class 3 microstructure

Overview

Cast irons carry graphite (flakes, nodules, vermicular) that must survive preparation intact. PM materials carry inherent sintered porosity that must read accurately. Both demand techniques that preserve, not mask, the features being measured.

Preparation Challenges

Seven properties drive the prep procedure. Tap a card for full detail.

Graphite Pull-out The most common cast-iron artifact: graphite torn out, leaving false voids.

Graphite flakes, nodules, and compacted graphite particles can be torn from the matrix during grinding and polishing, leaving voids that may be mistaken for casting defects or porosity. This is the single most common artifact in cast iron metallography. Use napless cloths, moderate pressure, and short polishing times to retain graphite. Verify retention by comparing as-polished appearance against expected graphite form for the iron type.

Pore Smearing Matrix flows over pores and graphite voids, faking density.

Matrix material smears over open pores and graphite voids during polishing, concealing true porosity and distorting graphite shape measurements. This is critical for PM materials where porosity percentage determines sinter quality, and for cast irons where graphite form classification (ASTM A247) depends on accurate shape preservation. Intermediate etching with 2-5% nital between polishing steps removes the smeared layer.

Abrasive Trapping in Pores PM pores and shrinkage cavities trap loose abrasive, scratching later steps.

Open and interconnected porosity in PM materials and shrinkage cavities in castings trap abrasive particles from grinding and polishing steps. These trapped particles scratch subsequent polishing surfaces and can be mistaken for inclusions. Thorough ultrasonic cleaning in a solvent bath between every step is essential. Vacuum impregnation before grinding fills pores and prevents trapping entirely.

Wide Hardness Variation 187 HB gray iron and 450 HB white iron can't share one procedure.

White cast iron (450 HB with massive carbides) and gray cast iron (187 HB with soft graphite flakes) require very different grinding and polishing pressures. Austempered ductile iron (310 HB) sits between. PM materials are often softer (60-90 HRB) and more porous. Match preparation parameters to the specific material rather than using a single procedure for the entire class.

Brittle Phase Fracture Massive cementite cracks under aggressive grinding, mimicking service damage.

Massive cementite (Fe3C) in white cast iron, ledeburite, and other carbide phases are extremely brittle and can crack or fracture during aggressive grinding. These fractures propagate into subsurface regions and can be mistaken for service-related cracking. Use moderate feed rates during sectioning and avoid excessive grinding pressure. Start with a finer grit (240-320) rather than coarse grit to limit damage propagation.

Edge Fragility Low ductility plus PM porosity makes edges chip easily.

Low ductility makes edges and corners prone to chipping during sectioning and grinding. For PM materials, the porous structure weakens edges further. Use edge-retaining mounting compounds and vacuum impregnation to mechanically support fragile edges. Orient the specimen so edges trail the grinding direction to reduce chipping forces.

Matrix Composition Varies Ferrite/pearlite ratio drives etchant choice; tailor per material.

The ferrite/pearlite ratio in gray, ductile, and malleable irons varies with cooling rate and heat treatment, affecting both hardness and etching response. Austempered ductile iron has an ausferrite matrix. White cast iron has a cementite-austenite matrix. PM materials may have mixed ferrite-pearlite or fully martensitic matrices depending on alloying and sintering. Etchant selection and etching time must be tailored to the specific matrix.

Class 3 Materials

Cast irons + sintered powder metallurgy steels.

Cast Iron 6
  • Austempered Ductile Iron
  • Compacted Graphite Iron
  • Ductile Cast Iron
  • Gray Cast Iron
  • Malleable Cast Iron
  • White Cast Iron
Powder Metallurgy 2
  • Sintered 316L Stainless Steel (PM)
  • Sintered Iron (Fe-Cu PM)

Recommended Procedure

Five-stage workflow. Always examine as-polished before etching to assess graphite form and porosity.

  1. 1

    Sectioning

    Al₂O₃ abrasive cut-off for gray/ductile/malleable; SiC blade for white & ADI; diamond wafering for PM. Moderate feed throughout.

    More detail

    Use abrasive cut-off wheels with continuous coolant for most cast irons. Select an aluminum oxide blade for gray, ductile, and malleable irons, and a silicon carbide blade for the harder white and austempered ductile irons. Low-speed diamond wafering is preferred for PM materials, where the porous structure can trap abrasive and fragment under aggressive cutting. Keep feed rates moderate to avoid fracturing brittle carbide phases in white cast iron. For PM specimens requiring porosity analysis, minimize heat input to prevent sinter neck distortion.

  2. 2

    Mounting

    Castable epoxy with vacuum impregnation for PM (always) and for porous cast irons. Avoid compression mounting on PM; pressure closes pores.

    More detail

    Castable (cold) epoxy is preferred for most Class 3 materials. Vacuum impregnation is essential for all PM specimens and recommended for cast irons when porosity, shrinkage cavities, or graphite retention at edges must be preserved. Impregnation fills open porosity, prevents abrasive trapping during subsequent grinding, and mechanically supports fragile edge features. Compression mounting is acceptable for solid cast irons when edge preservation is not required, but avoid it for PM materials where the pressure can close pores. Use edge-retaining, mineral-filled epoxy when surface features or coatings need to be examined in cross section.

  3. 3

    Grinding

    For graphitic cast iron, use 500 grit ALO paper plane-grind then 1200 grit ALO. For PM materials, 320 grit SiC start, step through 400 / 600 / 800 / 1200.

    More detail

    Graphitic cast iron (gray, ductile, malleable, CGI) per Don §11.3.2: Use alumina (Al2O3) paper, not silicon carbide. Two paper steps are enough: 500 grit (P500) ALO paper until plane, then 1200 grit (P1200) ALO paper for 1 minute. Water lubricant, 5-10 lb, 200/200 rpm complementary rotation. Alumina paper cuts cast iron cleanly with less matrix smearing over the graphite than SiC; smearing is the dominant cast iron preparation failure mode. Longer SiC progressions are not needed and each additional step is another chance to smear matrix metal into graphite cavities.

    White cast iron and ADI: Same ALO paper sequence works but is slow due to hardness. An alternative is a rigid diamond grinding disc (~75 µm grit) for plane grinding followed by diamond suspension on a rigid disc, eliminating paper entirely.

    PM materials: Start at 320 grit SiC (avoid 240 grit which tears material into open pores) and progress through 400, 600, 800, and 1200 grit. Thorough ultrasonic cleaning between every grit is critical; abrasive lodges in pores and scratches subsequent steps. Diamond grinding discs are an alternative that produces less loose abrasive debris.

  4. 4

    Polishing

    Graphitic cast iron: 1 µm DIAMAT on GOLDPAD, then 0.05 µm Nanometer alumina on TRICOTE. PM materials: 9 → 3 → 1 µm DIAMAT then colloidal silica.

    More detail

    Graphitic cast iron (Don §11.3.2): Go directly from the 1200 grit ALO paper to 1 µm DIAMAT diamond on GOLDPAD with DIALUBE Purple Extender (5-10 lb, 200/200 rpm, 2 minutes). Final polish with 0.05 µm Nanometer alumina on TRICOTE (5-10 lb, 100/100 rpm, 1 minute). Adding intermediate 6 or 3 µm diamond steps does not improve graphite retention; it adds more opportunities to smear matrix metal over graphite. Cast iron uses alumina rather than colloidal silica for final polish because silica's chemical action can attack interfacial regions and cause graphite pullout in soft-matrix grades. Inspect at 500× after polishing: properly polished graphite is grey at this magnification, not black. Black at 500× means matrix metal is smeared over the cavity; return to the 1 µm DIAMAT step.

    PM materials: Use napless cloths throughout to minimize pore smearing. 9 µm diamond on a napless cloth, then 3 µm diamond, then 0.05 µm colloidal silica as a final step. Moderate pressure (15-20 N) and contra-rotation. Between the 9 µm and 3 µm steps, etch briefly with 2-5% nital to remove smeared layer covering pores, then re-examine. Vibratory polishing (2-4 hours with colloidal silica) is highly effective for revealing true porosity.

  5. 5

    Etching

    Examine as-polished first to assess graphite form and porosity. Then nital or picral depending on what you're characterizing.

    More detail

    Nital (2-5%) is the standard etchant for most cast irons and PM steels, revealing ferrite grain boundaries and the ferrite/pearlite ratio. 4% Picral is preferred when pearlite characterization is the primary objective, as it does not attack ferrite boundaries and provides better lamellar resolution. For austempered ductile iron, use 2% nital with short immersion times (5-10 seconds) to reveal the ausferrite matrix. For white cast iron, 4% Picral or Vilella's reagent delineates cementite from ledeburite cleanly; Murakami's reagent is reserved for carbide-type identification (M₃C, M₇C₃, etc.) in highly alloyed high-chromium white irons. Always examine specimens as-polished first to assess graphite form and porosity before etching obscures these features.

    Common etchants by family

    Graphite form (A247)
    None; examine as-polished. Etching distorts measured shape and obscures pull-out.
    Gray / Ductile / Malleable iron
    2–5% nital (ferrite/pearlite ratio); 4% picral (pearlite resolution); Stead's reagent (phosphide ID)
    Austempered Ductile Iron
    2% nital, 5–10 s immersion (ausferrite); Beraha's tint for retained austenite
    White cast iron
    4% picral; Vilella's (cementite vs. ledeburite); Murakami's (carbide-type ID in high-Cr)
    Sintered iron / steel PM
    2–3% nital; 4% picral for pearlitic PM grades
    Sintered stainless (316L PM)
    Vilella's reagent; oxalic acid electrolytic; glyceregia

    Cast iron etchant guide → Stainless steel etchant guide → Learn about etchants → Shop etchants →

Quality Checks

  • Graphite retained in true form, with no pull-out or distortion
  • PM porosity preserved, not smeared by matrix
  • No embedded abrasive in pores at 200×
  • White iron carbides free of preparation-induced fracture
  • Clean etch; ferrite/pearlite boundaries distinct