heat-treatment

name	heat-treatment
description	Heat treatment of steels — annealing, normalizing, hardening, tempering, carburizing, nitriding. CCT/TTT diagrams, hardenability (Jominy), Larson-Miller, distortion control.
metadata	{"priority":7,"promptSignals":{"phrases":["heat treatment","annealing","hardening","quenching","tempering","carburizing","case hardening","hardenability"],"minScore":3}}

Heat Treatment of Steels — Complete Skill

Process Overview

Annealing

Full annealing: heat above Ac₃ (for hypoeutectoid) or Ac₁ (for hypereutectoid) → slow furnace cool Result: soft, ductile, coarse pearlite; eliminates cold work effects; machinability optimized Typical: 50°C above Ac₃; cool 10-30°C/hr to below 300°C

Process (subcritical) annealing: 550-700°C (below Ac₁); stress relief + softening for cold-worked steel Spheroidize annealing: 700-760°C long soak → carbides sphere up → maximum machinability + softness

Normalizing

Heat above Ac₃ + 50°C → air cool Result: fine pearlite, harder than full anneal but uniform; used to refine grain before hardening

Hardening (Austenitize + Quench)

Heat above Ac₃ (hypoeutectoid) + 30-50°C → quench Result: martensite (BCT crystal structure); hardness depends on carbon content Max martensite hardness: HRC ≈ 30 + 50 × %C (approximately) 0.4%C → HRC ~54; 0.8%C → HRC ~65 (theoretical max)

Quench media (severity H factor): Brine (10% NaCl): H = 2 (fastest); Water: H = 1; Fast oil: H = 0.4; Slow oil: H = 0.25; Air: H = 0.02 Critical cooling rate: must exceed to get 100% martensite at any radius

Tempering

After hardening (must temper within 1 hr to prevent quench cracking) Temperature: 150-650°C; time: 1-2 hr minimum (per 25mm thickness) Higher temp → lower hardness + higher toughness

Temper chart (approx, medium carbon steel): 150°C → HRC 62-63; 200°C → 58-60; 300°C → 50-55; 400°C → 45-50; 500°C → 38-42; 600°C → 30-35

Tempered martensite embrittlement (TME): avoid 260-370°C for alloy steels High-temperature temper embrittlement: avoid slow cool through 375-575°C (Sb, Sn, P segregation)

Larson-Miller Parameter: LMP = T × (C + log₁₀ t) × 10⁻³ (T in Rankine, t in hours, C ≈ 20) Equal LMP = equal metallurgical effect; trade temperature vs. time

Hardenability

Jominy End-Quench Test (ASTM A255)

Standardized bar 25mm × 100mm; one end water-quenched; hardness profile measured J = hardness at distance from quench end [HRC vs. mm from end]

Ideal diameter D_I: diameter of bar with 50% martensite at center in ideal quench D_I = f(carbon content, grain size, alloy additions) Actual critical diameter D_c = D_I × H (H factor for actual quench medium)

Grossmann H factors: see quench media above

Jominy correlation: J₁₅ (HRC at 15mm) ≈ hardness at center of 50mm diameter bar in oil quench

Case Hardening

Carburizing (Gas, Liquid, Solid, Vacuum)

Temperature: 900-950°C (below solidus; above Ac₃) Carbon potential: 0.8-1.1% C at surface (controlled atmosphere: CO/CO₂ ratio) Case depth (diffusion): x = D₀ × t (D₀ = diffusivity coefficient at temperature) For steel at 930°C: case depth ≈ 0.5mm per hour (rule of thumb) After carburizing: quench + temper 150-175°C; surface HRC 58-64, core HRC 25-35

Nitriding (Gas or Ion/Plasma)

Temperature: 480-570°C (below Ac₁) — no phase change, minimal distortion Surface hardness: HRC 60-70 (Fe₂-₃N white layer) WITHOUT quench Case depth: 0.1-0.5mm; diffusion zone extends further Distortion: very low (no quenching) — suitable for precision parts

Gas nitriding: NH₃ atmosphere; 48-100 hours; dissociation rate controls hardness Ion nitriding: plasma bombardment; faster, controllable; no white layer option

Carbo-nitriding

900°C in C+N atmosphere; faster than carburizing; thin cases; good for small parts

Induction Hardening

Local heating with electromagnetic induction coil → rapid quench Case depth controlled by frequency: f ↑ → shallower case 1 kHz → ~6mm depth; 10 kHz → ~2mm; 100 kHz → ~0.5mm Single-shot or progressive scanning; excellent for shafts, gears, camshafts

CCT Diagram Usage

Continuous Cooling Transformation: actual cooling paths plotted over transformation curves Cooling rate at center: use Grossmann charts or Jominy correlation Read off: microstructure formed (martensite, bainite, pearlite, ferrite) and hardness TTT (Time-Temperature-Transformation): isothermal, use for austempering/martempering processes

Distortion Control

Sources: non-uniform temperature gradient, transformation volume changes, residual stress Minimize: symmetric geometry, balanced quench, pre-stress relief before hardening, proper fixture Martempering: quench to Ms + 50°C → hold → air cool → reduces quench gradient → less distortion Austempering: quench to bainite nose → isothermal hold → lower bainite → no martensite, less distortion

Output

Provide: process selection (anneal/normalize/harden/case harden), temperatures [°C], time [hr], expected hardness HRC at surface and core, case depth [mm], quench medium recommendation, distortion risk level, tempering temperature for target properties.