Recommended Procedures
Class 8

Hardened & Heat Treated Steels

Hardness 28–67 HRC (~290–940 HV) Typical Tool · High-speed · Nitrided Key challenge Carbide retention & edges

Class 8 covers tool steels and surface-hardened steels whose high hardness and carbide content set them apart from the medium-hardness ferrous alloys in Class 5. Cold-work steels (A2, D2, O1, W1) contain massive primary carbides and operate at 58-65 HRC. High-speed steels (M2, M42, T1, T15) add vanadium and tungsten carbides that are harder than the matrix and notoriously prone to pull-out. Hot-work steels (H11, H13, H21) and shock-resisting steels (S1, S5, S7) run at somewhat lower hardness but still demand careful abrasive selection. Nitrided steels add the complication of a graded hardness profile from a very hard nitrogen-diffused surface to a softer core. Across the class, the preparation challenges center on retaining hard carbides, preserving specimen edges for case depth measurement, and revealing martensite morphology without introducing grinding-induced transformation artifacts.

High-alloy tool steel microstructure showing typical Class 8 hardened steel structure
High-alloy tool steel: typical Class 8 microstructure

Overview

Class 8 materials are hard enough that subsurface deformation during grinding is minimal compared to softer classes, but their hardness and carbide content create a different set of challenges: abrasive wear, carbide retention, edge integrity, and the risk of grinding-induced transformation artifacts.

Preparation Challenges

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

Carbide Distribution & Pull-Out Hard primary carbides get ripped from the softer matrix on napped cloths.

Tool steels contain primary carbides (Cr₇C₃ in D2, VC in T15 and M2, M₆C in high-speed steels) that are significantly harder than the martensitic matrix. During polishing on napped cloths, the cloth fibers catch carbide edges and rip them from the matrix, leaving voids that inflate apparent porosity. Napless cloths with moderate diamond loads retain carbides best.

Edge Chipping & Retention Brittle martensite chips at edges where case depth measurements live.

High-hardness martensite is brittle, and specimen edges chip easily during sectioning and grinding. For nitrided and carburized steels, the hard case surface and the case-core transition zone sit at the specimen edge, making edge retention critical for accurate case depth measurement. Edge-retention mounting compounds and careful grinding orientation are essential.

Grinding-Induced Transformation Heat from coarse grinding can temper or re-austenitize a surface white layer.

Excessive heat during sectioning or coarse grinding can temper the martensitic matrix (softening it) or create re-austenitized "white layers" at the surface that misrepresent the actual heat treatment. These artifacts are invisible until etching reveals an unexpected microstructure. Abundant coolant and moderate feed rates prevent thermal damage.

Retained Austenite Untransformed austenite needs tint etchants and is easily transformed by grinding.

Incompletely transformed austenite is present in many hardened steels, particularly high-carbon and high-alloy grades (D2, high-speed steels). Retained austenite can transform to martensite during aggressive grinding, producing a false hardness reading. It also requires specific etchants (Klemm's I or 10% sodium metabisulfite tint etch, which color martensite while leaving retained austenite light) to distinguish from martensite. LePera's reagent is reserved for dual-phase automotive sheet steels, not tool steels.

Rapid Abrasive Wear High-speed steel carbides chew through SiC paper in seconds.

The high hardness and carbide content of these steels wears through SiC grinding papers very quickly, especially for high-speed steels with vanadium and tungsten carbides. Frequent paper changes or switching to diamond grinding discs prevents the loss of grinding efficiency and avoids introducing thermal damage from dull abrasive.

Case-Core Hardness Gradient Soft core polishes faster than hard case, creating relief at the transition.

Nitrided and carburized steels have a continuous hardness gradient from very hard surface (60-70 HRC equivalent) to a softer core (20-30 HRC). The soft core grinds and polishes faster than the hard case, creating relief at the transition zone. Napless cloths and short polishing times minimize this differential removal.

Martensite Morphology & Etching Lath vs. plate martensite respond differently to nital and picral timing.

Different tool steels produce different martensite types: lath martensite in lower-carbon grades (H13, S7), plate martensite in higher-carbon grades (D2, W1). Each responds differently to nital and picral. Over-etching darkens the entire structure, while under-etching fails to resolve grain boundaries. Timing and etchant choice must be matched to the specific steel grade.

Class 8 Materials

Expand any group to view materials grouped by tool-steel family.

Cold-Work Tool Steels 6
  • A2 Air-Hardening Tool Steel
  • A4 Air-Hardening Tool Steel
  • A6 Air-Hardening Tool Steel
  • D2 Tool Steel
  • O1 Oil-Hardening Tool Steel
  • W1 Water-Hardening Tool Steel
Hot-Work & Mold Steels 4
  • H11 Hot-Work Tool Steel
  • H13 Hot-Work Tool Steel
  • H21 Hot-Work Tool Steel
  • P20 Plastic Mold Steel
High-Speed Steels 4
  • M2 High-Speed Steel
  • M42 High-Speed Steel
  • T1 High-Speed Steel
  • T15 High-Speed Steel
Shock-Resisting Tool Steels 3
  • S1 Shock-Resisting Tool Steel
  • S5 Shock-Resisting Tool Steel
  • S7 Shock-Resisting Tool Steel
Surface-Hardened Steels 1
  • Nitrided Steel Cross-Section

Recommended Procedure

Five-stage workflow tuned to retain carbides, preserve edges, and avoid grinding-induced transformation.

  1. 1

    Sectioning

    Aluminum-oxide ferrous blades with generous coolant; moderate feed rates so the cut face does not temper.

    More detail

    Use ferrous-metal abrasive blades (aluminum oxide) with generous coolant flow. These steels are brittle at high hardness and chip easily, so clamp firmly and use moderate feed rates. For nitrided or carburized specimens, cut perpendicular to the case surface to preserve the case-core transition. Precision wafering with diamond blades produces less subsurface damage for critical case depth measurements. Avoid excessive cutting speed, which generates heat and can temper the martensitic structure at the cut face. Note on hardened stainless: fully hardened martensitic (440C aged, heat-treated 410/420) and precipitation-hardening (17-4 PH H900, 15-5 PH H1025) stainless grades follow the Class 5 stainless preparation procedure for grinding, polishing, and final-polish (Nanometer alumina on TRICOTE), but step up the sectioning blade to MAXCUT MAX-VHS (HRC > 45). See also the Stainless Steel Preparation Guide.

  2. 2

    Mounting

    Per Don §11.8.1: epoxy or DAP compression mounts (phenolic NOT recommended for hardened steels because shrinkage compromises edge retention). Mineral-filled epoxy for case-depth work.

    More detail

    Don §11.8.1 specifies epoxy or diallyl phthalate (DAP) compression resin for tool steels and nitrided steel; phenolic is not on Don's list. Phenolic shrinks more than epoxy or DAP, producing gaps at the mount-sample boundary that compromise edge retention exactly where case-depth, decarburization, or carbide-rich edge measurements are made. For case-hardened cross-sections (nitrided, carburized), use mineral-filled (edge-retention grade) epoxy; the hard filler particles polish at a rate close to the steel, eliminating the matrix-mount step that causes interface rounding. The thermal cycle (150-180 °C) is well below tempering temperature for most tool steels and will not affect hardness.

  3. 3

    Grinding

    Per Don §11.8.1: single 120 µm diamond grinding disc plane grind (replaces SiC paper progression). Water, 5-10 lb, 200/200 rpm, until plane.

    More detail

    Don §11.8.1 replaces multi-step SiC paper progressions with a single 120 µm diamond grinding disc plane-grind step (water, 5-10 lb, 200/200 rpm, until plane). The rigid diamond disc cuts hardened steels (HRC 60+) cleanly and uniformly; SiC papers wear quickly against these materials, glaze partway through, and produce inconsistent removal. The rigid disc also maintains flatness across the carbide-rich microstructure, which is what preserves carbides through the subsequent polishing steps. This single disc replaces the entire SiC paper progression and the multi-step diamond grinding disc sequence (75/40/15 µm) used in some Buehler/Struers procedures. Maintain consistent 5-10 lb force; excessive pressure can fracture brittle carbides at the plane-grind stage and propagate damage into all subsequent steps.

  4. 4

    Polishing

    9 µm DIAMAT on SIRIUS → 3 µm DIAMAT on ORION → 1 µm DIAMAT on GOLDPAD → 0.05 µm Nanometer alumina on TRICOTE. Rigid composite discs preserve carbides.

    More detail

    Don §11.8.1 polishing sequence: 9 µm DIAMAT on SIRIUS composite disc (DIALUBE Purple Extender, 5-10 lb, 200/200 rpm, 3 min) → 3 µm DIAMAT on ORION composite disc (DIALUBE Purple Extender, 5-10 lb, 200/200 rpm, 3 min) → 1 µm DIAMAT on GOLDPAD polishing pad (DIALUBE Purple Extender, 5-10 lb, 200/200 rpm, 2 min) → 0.05 µm Nanometer alumina on TRICOTE polishing pad (5-10 lb, 100/100 rpm, 1 min). The rigid composite discs (SIRIUS, ORION) at the 9/3 µm steps are essential: carbides (M6C, MC, M2C, M23C6, M7C3) polish at a different rate than the martensite matrix, and woven cloths at these intermediate steps produce visible relief around carbides. Don's procedure applies identically to nitrided steel (§11.8.2). For HSS and high-carbide grades (M2, M42, D2, T15), stay strictly within this recipe; substituting woven cloths anywhere in the sequence produces noticeable carbide relief.

  5. 5

    Etching

    2–5% nital is the workhorse; 4% picral for high-alloy grades, Vilella's for prior-austenite grain, tint etchants for retained austenite.

    More detail

    Nital (2-5%) is the standard etchant for most hardened steels; swab 5-10 seconds. 4% Picral is preferred for high-alloy tool steels (D2, M2, T15) because it reveals carbide boundaries more clearly without darkening the martensitic matrix as aggressively as nital. Vilella's reagent reveals prior austenite grain boundaries in quenched steels. For retained austenite identification, use a 10% sodium metabisulfite tint etch, which colors martensite dark while leaving austenite white.

    Common etchants by tool-steel family

    Cold-work tool steels (A, D, O, W)
    2–4% nital (matrix); 4% picral (carbides); Vilella's (tempered structures)
    High-speed steels (M, T)
    Vilella's (general); Marble's reagent; modified Murakami's for carbide-type ID
    Hot-work tool steels (H-series)
    2% nital; 4% picral; Vilella's for tempered martensite
    Shock-resisting steels (S-series)
    2% nital; Vilella's
    Nitrided / nitrocarburized cases
    2% nital (case structure); modified Marble's; Murakami's for nitride ID
    Carburized cases
    2% nital; 4% picral for cementite networks & case depth

    High-carbon & tool steel etchant guide → Learn about etchants → Shop etchants →

Quality Checks

  • Martensite structure clearly resolved without grinding-induced transformation artifacts
  • Carbides retained in place with no pull-out voids or edge rounding
  • Specimen edges sharp with no chipping (especially for case depth measurement)
  • No thermal discoloration or white-layer artifacts from sectioning
  • Case-core transition (if applicable) clearly visible with a measurable boundary