Heat-Treatment Hardness and Microstructure Study of EN8 Medium-Carbon Steel

Heat-Treatment Hardness and Microstructure Study of EN8 is a standards-driven experimental study on the steel every Indian machine shop knows: EN8 medium-carbon steel (~0.40% C), the default material for shafts, axles, studs and gears, and the steel every local hardening-and-tempering shop claims to process — usually without telling you which cycle they actually ran.

The team prepares a coupon matrix from one single EN8 round bar to eliminate composition variation, then processes eight treatment groups — as-received, annealed (furnace-cooled), normalised (air-cooled), water-quenched, oil-quenched, and tempered at 300, 450 and 600 °C after quenching — with triplicate coupons per group. Hardness is measured per ASTM E18 (HRB for soft groups, HRC for hardened groups, with the standard's indent-spacing rules) and cross-checked with ASTM E92 Vickers readings; microstructures are prepared by mounting, polishing and 2% nital etching and photographed: ferrite–pearlite, sorbitic pearlite, martensite and tempered martensite.

The outputs are the plots that make this a metallurgy project rather than a lab repetition: hardness vs heat-treatment condition, hardness vs tempering temperature with a fitted trend, and mean ± standard-deviation tables that show which treatments scatter most and why (decarburised skin, quench severity, indent placement). Every number is traceable to a coupon register, a furnace log and a hardness-test record, and the discussion translates the data into shop-floor guidance — which cycle a shaft, a bolt and a wear plate actually need, and what a heat-treatment shop must be asked for.

Indian MSME fabricators specify EN8 steel for shafts, axles and fasteners and outsource heat treatment to local shops, but they receive only a hardness claim — no cycle record, no microstructure, no independent verification. Textbook hardness ranges are scattered across different heats, section sizes and testers, so neither the buyer nor the student can tell whether a 'quenched and tempered at 450 °C' shaft is actually what it claims. At the academic level, heat-treatment lab work usually stops at one quench and one hardness number, without replication, tempering series or microstructural correlation.

There is a need for a single-heat, standards-based heat-treatment study on EN8 that measures the hardness and microstructure of eight classical treatment conditions with triplicate replication per ASTM E18/E92, produces the hardness-vs-tempering-temperature curve and micrograph atlas on one documented heat of steel, and converts the results into shop-floor guidance that both a heat-treatment buyer and a metallurgy student can use and defend.

Abstract

Heat treatment is where metallurgy earns money: the same EN8 bar can be soft and machinable or hard and wear-resistant depending only on the thermal cycle. This study maps that span quantitatively — seven treatment conditions, triplicate coupons, hardness to ASTM E18/E92, microstructures to textbook standards — producing a hardness-vs-tempering curve and microstructure atlas for one widely used Indian steel.

Introduction

EN8 (≈0.40% C, ~0.7% Mn) sits in the sweet spot of carbon steels: enough carbon to harden usefully, cheap enough for commodity components. Indian MSME fabricators buy it by the ton for shafts and fasteners, then outsource heat treatment to local shops with informal process windows. A student-accessible study that measures what each classical cycle actually does — on the same bar, with standardised testing — gives both the department and the local industry a reference point.

Existing System / Literature Gap

• Textbooks (Avner; Raghavan; ASM Handbook Vol. 4A) give the classical cycles and typical hardness ranges, but values are spread across tables for different heats, sections and testers.
• Local heat-treatment shops quote outcomes ('40 HRC guaranteed') without publishing cycles; buyers have no independent check.
• Student lab work stops at one quench and one hardness reading — no tempering series, replication or statistics.
• Gap: a single-heat, triplicate, eight-condition study with standards-compliant hardness testing and matched micrographs — compact enough for one semester, rigorous enough to compare against handbook values.

Proposed System

• Coupon matrix: 24 coupons (8 groups × 3 replicates), 20 mm dia × 15 mm discs cut from one EN8 bar; each ground flat on both faces with the decarburised skin removed.
• Treatments: anneal (860 °C, furnace cool), normalise (860 °C, air cool), harden (840 °C, water quench; 840 °C, oil quench), temper (300 / 450 / 600 °C × 1 h, air cool, from the water-quenched condition) — verified against an independent calibrated thermocouple.
• Testing: Rockwell per ASTM E18 — HRB (100 kgf, 1/16" ball) for soft groups, HRC (150 kgf, brale) for hardened groups — five indents per coupon with the standard's spacing, plus HV30 Vickers per ASTM E92 on one coupon per group; microstructure per coupon set with 2% nital.
• Outputs: condition-wise hardness tables (mean ± SD), hardness vs tempering-temperature curve with fitted trend, micrograph atlas, and shop-floor interpretation per component class.

Feasibility Study

• Technical: all equipment — muffle furnace, Rockwell and Vickers testers, mounting and polishing stations, microscope — exists in a standard metallurgy department; the procedures are spelled out in ASTM E18/E92 and textbook practice.
• Economic: one 1 m EN8 bar ≈ ₹900; consumables (SiC papers, alumina, nital reagents, mounting resin) ≈ ₹800; total under ₹2,000.
• Operational and safety: furnace work follows department hot-work rules (tongs, gloves, face shield); quench oil is kept below its flash point with a lid and a Class-B extinguisher; 2% nital is mixed fresh and used only in the fume hood; all handling by trained team members only.

In scope

• EN8 round-bar stock, one heat, 20 mm diameter; eight classical treatment conditions with triplicate coupons; Rockwell and Vickers hardness testing and optical metallography.
• Statistical treatment (mean ± SD, tempering trend fit) and comparison with published handbook ranges.

Out of scope

• Tensile, impact (Charpy) and fatigue testing — noted as future scope; hardness and microstructure only.
• Jominy end-quench hardenability, section-size effects beyond the 20 mm coupon, and alloy steel comparisons.
• Atmosphere-controlled or vacuum heat treatment; salt-bath practice; surface hardening (carburising, induction).
• SEM/EDS or quantitative metallography — optical microscopy with qualitative phase identification only.

This is an experimental research design with controlled treatments, triplicate replication and standards-anchored measurement — phased to fit the 14-week VTU major-project calendar.

Quality assurance: all coupons come from one bar so composition is a controlled variable; the furnace setpoint is verified against an independent calibrated K-type thermocouple at working temperature; hardness testing follows E18's minimum indent spacing (≥ 3× indent diameter centre-to-centre, ≥ 2.5× from edges) and every group is read by two team members; microscope magnification and etch time are recorded on each micrograph; any furnace excursion, mis-quench or dropped coupon is recorded in the deviation log rather than silently discarded.

As a research bundle, the architecture is the study design — how coupons, treatments, measurements and analysis interlock.

1. Specimen layer — 24 numbered discs from one EN8 bar, plus the coupon register: ID, cut position along the bar, dimensions, surface-prep record and baseline hardness of the as-received condition.
2. Treatment layer — a muffle furnace with an independently verified thermocouple; two quench media (water ~25 °C, oil ~50 °C, both logged) and tempering cycles held to ±5 °C for one hour; every cycle recorded with setpoint, soak time, transfer time to quench and coolant temperature — because transfer delay is a real variable in hardening.
3. Measurement layer — Rockwell testing per ASTM E18 with scale selection driven by the expected hardness band (HRB for annealed/normalised, HRC for quenched/tempered), five indents per coupon; Vickers (HV30) per ASTM E92 as the cross-check; optical metallography with a fixed preparation recipe and 2% nital.
4. Analysis layer — Python scripts compute per-group means and standard deviations, plot hardness vs condition and hardness vs tempering temperature with a fitted trend, and produce the correlation table linking each microstructure to its hardness band and to handbook values.

flowchart TD
  BAR["One EN8 bar (single heat)"] --> CUT["24 discs, 20 x 15 mm, stamped IDs"]
  CUT --> PREP["Grind faces, remove decarb skin,<br/>record in coupon register"]
  PREP --> G1["As-received (baseline)"]
  PREP --> G2["Annealed: 860 °C, furnace cool"]
  PREP --> G3["Normalised: 860 °C, air cool"]
  PREP --> G4["Water-quenched: 840 °C"]
  PREP --> G5["Oil-quenched: 840 °C"]
  G4 --> T["Tempered: 300 / 450 / 600 °C,<br/>1 h + air cool (Groups 6-8)"]
  G1 --> HT["Hardness: ASTM E18<br/>HRB soft groups, HRC hard groups"]
  G2 --> HT
  G3 --> HT
  G4 --> HT
  G5 --> HT
  T --> HT
  HT --> HV["ASTM E92 HV30 cross-check"]
  HT --> MET["Metallography: mount, polish,<br/>2% nital, photograph"]
  MET --> ANA["Statistics + tempering curve +<br/>microstructure correlation"]
  HV --> ANA
  ANA --> OUT["Report: curves, atlas,<br/>shop-floor guidance"]

(The matrix holds eight conditions: the as-received baseline; annealed and normalised; water- and oil-quenched; and three tempering temperatures drawn from the water-quenched condition.)

The design's key discipline is traceability: every hardness number maps to a coupon ID, a furnace-log entry and a test record, so the viva question "where does this 46 HRC come from?" always has a one-minute answer.

• Jominy end-quench arm — a hardenability study on the same bar to map hardness vs distance from the quenched end.
• Mechanical-property correlation — tensile and Charpy tests on duplicate coupons to link the hardness curve to strength and toughness trade-offs.
• Alloy comparison — repeat the matrix on EN9 or EN19 to put the carbon effect and hardenability side by side.
• Section-size effects — 12 mm vs 20 mm vs 32 mm coupons through the same quench to demonstrate mass effect empirically.
• Quantitative metallography — image-analysis phase fractions correlated with hardness instead of qualitative identification.
• Industry verification round — collect shafts heat-treated by two local shops and audit their hardness claims against this study's tempering curve.