Class 5 is the largest material class and covers the most commonly prepared ferrous alloys: carbon steels, low-alloy steels, stainless steels, maraging steels, HSLA steels, and manganese steels. These materials share moderate hardness and sufficient ductility to resist fracture during preparation, making them the most straightforward class to prepare. The primary challenge is not mechanical but analytical: this class contains the widest variety of microstructures in metallography (ferrite, pearlite, bainite, martensite, austenite, delta ferrite, and their combinations), and each requires specific etching techniques and polishing quality to reveal accurately.
Preparation Characteristics & Challenges
300 Series Cast Stainless Steel, etched. Example of Class 5 austenitic microstructure
Class 5 materials are the most routine to prepare mechanically, but several characteristics still require attention:
Microstructural Variety
No other class spans as many microstructural types. A low-carbon steel shows ferrite and pearlite; a quenched 4340 shows martensite and retained austenite; a duplex stainless shows equal parts ferrite and austenite. Each structure has different hardness, etching behavior, and polishing characteristics. The preparation procedure itself is similar across the class, but etchant selection and polishing quality requirements vary significantly with the analytical objective.
Etch Response Variation
Nital (nitric acid in ethanol) is the standard etchant for carbon and alloy steels, but it attacks ferrite boundaries preferentially and may not adequately resolve pearlite lamellae. Picral resolves pearlite and bainite better but does not reveal ferrite boundaries. Austenitic stainless steels are resistant to nital entirely and require electrolytic or specialized chemical etchants. Duplex stainless steels need color etchants (Beraha, Murakami) to distinguish ferrite from austenite. Matching the etchant to the analytical objective is the most important decision in Class 5 preparation.
Phase Relief
Multi-phase microstructures polish at different rates. Pearlite (mixture of ferrite and cementite) stands slightly higher than surrounding free ferrite. Carbide particles in alloy steels resist polishing and develop positive relief. In duplex stainless steels, the harder austenite phase polishes slower than ferrite, creating measurable height differences. Relief is usually minor in Class 5 (unlike Class 8 tool steels with massive carbides) but can still affect image quality at high magnification. Use napless cloths and moderate pressure to minimize differential removal.
Grinding Deformation
Aggressive grinding creates a subsurface deformation layer that alters the apparent microstructure. In low-carbon steels, grinding can smear ferrite over pearlite colonies, making them appear smaller or absent. In austenitic stainless steels, heavy grinding can induce strain-induced martensite that responds to magnetic etchants and may be misinterpreted as a service condition. Each grinding step must remove the full deformation depth from the previous step. Use moderate pressure and progress through all grits without skipping.
Inclusion Characterization
Non-metallic inclusions (oxides, sulfides, silicates) are critical quality indicators in steels per ASTM E45. Inclusions can be pulled out, fractured, or smeared during polishing, distorting their apparent size, shape, and distribution. Oxide inclusions are hard and brittle; they survive polishing but can be fractured by aggressive grinding. Manganese sulfide inclusions are soft and elongated; they smear easily and can be pulled out by napped cloths. Examine the as-polished surface for inclusion assessment before etching.
Ferrite Smearing
Free ferrite in low-carbon steels is soft enough to smear over adjacent phases and grain boundaries during polishing, especially with excessive force or napped cloths. Smeared ferrite conceals the true ferrite/pearlite boundary, affecting phase fraction measurements. This is most pronounced in hypoeutectoid steels with large ferrite grains. Use napless cloths, light pressure (15-20 N), and contra-rotation. A final vibratory polish with colloidal silica removes the smeared layer without introducing new deformation.
Heat-Affected Zone from Sectioning
Abrasive cutting generates localized heat that can transform the microstructure near the cut surface. In quenched steels, the heat may temper martensite. In cold-worked steels, it may cause recrystallization. The heat-affected zone (HAZ) typically extends 0.5-2 mm from the cut face depending on blade selection, feed rate, and coolant flow. Adequate material removal during grinding (at least 2 mm from the cut face) is required to ensure the analyzed surface represents the true bulk microstructure. Always use coolant during sectioning.
Class 5 Materials
The following materials are classified as Class 5 (Medium Hard, Ductile Metals). Click on any material to view its detailed preparation procedures.
Carbon Steels
- A36 Structural Steel
- AISI 1018 Carbon Steel
- AISI 1020 Carbon Steel
- AISI 1035 Carbon Steel
- AISI 1045 Carbon Steel
- AISI 1095 High Carbon Steel
Low-Alloy Steels
- AISI 4130 Chromium-Molybdenum Steel
- AISI 4140 Chromium-Molybdenum Steel
- AISI 4142 Chromium-Molybdenum Steel
- AISI 4340 Nickel-Chromium-Molybdenum Steel
- 5160 Spring Steel
- 52100 Bearing Steel
- AISI 6150 Chromium-Vanadium Steel
- AISI 8620 Case-Hardening Steel
- AISI 9310 Case-Hardening Steel
Specialty Steels
Austenitic Stainless Steels
- 304 Stainless Steel
- 309 Stainless Steel
- 310 Stainless Steel
- 316 Stainless Steel
- 321 Stainless Steel
- 330 Stainless Steel
- 347 Stainless Steel
- 904L Super Austenitic Stainless Steel
- AM 316L Stainless Steel (SLM)
Ferritic & Martensitic Stainless Steels
- 410 Stainless Steel
- 420 Stainless Steel
- 430 Stainless Steel
- 431 Stainless Steel
- 440C Stainless Steel
Duplex & PH Stainless Steels
Preparation Guide
Recommended Preparation Steps
Sectioning
Standard abrasive cut-off with an aluminum oxide blade and continuous coolant is suitable for most Class 5 materials. Use moderate feed rates to minimize the heat-affected zone. For harder alloys (52100, 440C, maraging steels in aged condition), a silicon carbide or CBN blade may cut more efficiently. Precision diamond wafering saws are unnecessary for this class but can be used for small specimens. Always remove at least 2 mm of material during subsequent grinding to clear the sectioning HAZ before analysis.
Mounting
Compression mounting with phenolic or diallyl phthalate compounds is acceptable for most Class 5 steels, which tolerate the mounting temperature (150-180°C) without microstructural change. Use edge-retaining, mineral-filled compounds when case depth, decarburization, or coating thickness must be measured. Castable (cold) epoxy is preferred for heat-treated steels where the mounting temperature could temper martensite (particularly quenched and low-tempered conditions). For inclusion analysis per ASTM E45, ensure the mounting compound provides good contrast with the steel surface.
Grinding
Start at 180 or 240 grit SiC and progress through 320, 400, 600, 800, and 1200 grit. Use moderate pressure (20-30 N per 30 mm sample) and contra-rotation. These steels grind predictably and tolerate standard procedures. Ensure each step removes the full damage depth from the previous grit. For softer steels (A36, 1018, 1020), lighter pressure reduces subsurface deformation. For harder steels and stainless steels, standard pressure is appropriate. Clean thoroughly between grit changes to prevent carryover scratching.
Polishing
Use napless cloths for the diamond polishing steps to minimize phase relief and ferrite smearing. Polish with 9 µm diamond, then 3 µm diamond, followed by 0.05 µm colloidal silica or alumina as a final step. Use moderate pressure (15-25 N) and contra-rotation. For inclusion assessment (ASTM E45), stop after the 1 µm diamond step and examine as-polished; colloidal silica can etch inclusions and alter their appearance. Vibratory polishing (1-4 hours with colloidal silica) produces excellent results for austenitic stainless steels and maraging steels where residual deformation is difficult to remove by hand.
Etching
For carbon and low-alloy steels: 2-5% nital reveals ferrite grain boundaries, pearlite, bainite, and martensite. Picral (4% picric acid in ethanol) is preferred when pearlite lamellae resolution or bainite identification is the objective, as it does not attack ferrite boundaries. For austenitic stainless steels: electrolytic oxalic acid (10% at 6V for 15-60 seconds) reveals grain boundaries and sensitization; glyceregia (glycerol, HCl, HNO3) provides general-purpose results. For duplex stainless steels: Beraha II or modified Murakami's reagent distinguishes ferrite (colored) from austenite (uncolored). For martensitic and PH stainless: Vilella's reagent (picric acid, HCl, ethanol) reveals prior austenite grain boundaries and lath structure. Maraging steels respond to modified Fry's reagent.
Quality Verification
Grain boundaries and phase boundaries etch cleanly without residual deformation artifacts
No ferrite smearing over pearlite colonies or grain boundaries
Non-metallic inclusions retained in their true shape without pull-out or smearing
Minimal relief between phases (ferrite/pearlite, austenite/ferrite in duplex)
Sectioning HAZ fully removed; analyzed surface represents true bulk microstructure