Class 3 encompasses cast irons and powder metallurgy (PM) materials. Cast irons contain graphite phases (flakes, nodules, or compacted forms) that must be preserved during preparation for graphite classification per ASTM A247 and nodularity assessment per ASTM E2567. PM materials contain inherent porosity from the sintering process that must be accurately revealed for density and pore distribution analysis. Both material families share the challenges of low ductility, phase pull-out, and pore smearing, requiring techniques that preserve these features rather than mask them.
Preparation Characteristics & Challenges
Nodular Cast Iron, as-polished, 200X. Example of Class 3 material microstructure
Class 3 materials exhibit several distinctive properties that directly impact how they must be prepared:
Graphite Pull-out
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 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
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
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 (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 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
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
The following materials are classified as Class 3 (Lower Ductility Metals). Click on any material to view its detailed preparation procedures.
Preparation Guide
Recommended Preparation Steps
Sectioning
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.
Mounting
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.
Grinding
Start at 240 grit SiC for gray, ductile, malleable, and compacted graphite irons. White cast iron and austempered ductile iron can tolerate 180 grit. PM materials should start at 320 grit to avoid tearing material into open pores. Progress through 320, 400, 600, 800, and 1200 grit with moderate pressure (15-25 N per 30 mm sample) and contra-rotation. Thorough ultrasonic cleaning between every grit change is critical, especially for PM specimens where abrasive particles lodge in pores and produce scratches on subsequent steps. Consider diamond grinding discs as an alternative to SiC for PM materials, as they produce less loose abrasive debris.
Polishing
Use napless cloths throughout to minimize graphite pull-out and pore smearing. Polish with 9 µm diamond on a napless cloth, then 3 µm diamond, followed by 0.05 µm colloidal silica or alumina as a final step. Use moderate pressure (15-20 N) and contra-rotation to reduce directional smearing. Between the 9 µm and 3 µm steps, etch briefly with 2-5% nital to remove any smeared layer covering pores or graphite voids, then re-examine before continuing. Vibratory polishing (2-4 hours with colloidal silica) as a final step is highly effective for removing residual deformation and revealing true porosity in PM specimens.
Etching
Nital (2-5% nitric acid in ethanol) is the standard etchant for most cast irons and PM steels, revealing ferrite grain boundaries and the ferrite/pearlite ratio. Picral (4% picric acid in ethanol) 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. White cast iron may require longer etching or Murakami's reagent to delineate cementite from ledeburite. Always examine specimens as-polished first to assess graphite form and porosity before etching obscures these features.
Quality Verification
Graphite phases retained in their true form without pull-out or distortion
Open porosity in PM specimens preserved, not smeared over by matrix material
No embedded abrasive particles visible in pores under 200x magnification
Carbide phases in white cast iron free of preparation-induced fractures
Clean etching response with distinct ferrite/pearlite boundaries and matrix features