distortion-heat-treatment

name	distortion-heat-treatment
description	Distortion from heat treatment — quench distortion mechanisms, thermal and transformation stresses, residual stress, straightening, case hardening distortion, AMS 2759, ASTM A255, prediction and mitigation strategies.
metadata	{"priority":7,"promptSignals":{"phrases":["heat treatment distortion","quench distortion","hardening distortion","warping heat treatment","residual stress heat treatment","distortion carburizing"],"minScore":3}}

Distortion from Heat Treatment — Complete Skill

Mechanisms of Distortion

1. Thermal Stresses (Quenching)

Mechanism: surface cools faster than core → thermal gradient → surface contracts first During cooling: T_surface < T_core → surface shrinks, core resists → surface in TENSION; core in COMPRESSION If surface stress > σ_y (hot) → plastic deformation → residual distortion on cooling

Quench severity (H value): H = h / (2k) [h = surface heat transfer coefficient [W/m²K]; k = thermal conductivity [W/mK]] H: still air = 0.02; oil agitated = 0.3–0.5; water agitated = 0.6–1.0; brine = 1.0–2.0; salt spray = 1.5–2.0

Severity of distortion α quench severity: Higher H → faster surface cooling → larger ΔT → more thermal stress → more distortion

2. Transformation Stresses (Phase Change)

Martensite transformation: austenite → martensite; volume INCREASE of 2–4% (bcc vs. fcc packing) If core transforms after surface: Core expands → puts surface in COMPRESSION → may cause surface compressive residual stress Transformation strain: ε_transf = ΔV/3V ≈ 0.005–0.015 (0.5–1.5% linear strain) Higher C content → more expansion (martensite more tetragonal)

3. Machining Residual Stresses (Pre-existing)

Parts with residual stress from machining → release and redistruct during austenitizing heating Pre-existing tensile → distort to compressive balance; pre-existing bending stress → bending on heating

4. Case Hardening Distortion (Carburizing + Quench)

Unique features: Surface has more C (0.8–1.1%) → martensite harder, more expansion Core has low C (0.15–0.25%) → less expansion Net: surface in COMPRESSION (beneficial for fatigue) But: asymmetric geometry + case depth variation → net bending/warping

Dilatometry: measure volume change as function of T → quantify expansion at each transformation

Distortion Types

Type	Cause	Example
Size change (growth/shrinkage)	Martensite expansion; thermal contraction	Shaft grows in length
Bending/warping	Asymmetric cross-section; thermal gradient	Long thin shaft bends
Twisting	Asymmetric microstructure; off-center holes	Crankshaft twists
Ovality	Non-uniform quench around circumference	Ring becomes elliptical
Volume change	Phase transformation	Gear tooth pitch change

Prediction Methods

Empirical (Rakhit's rules): Circular cross-section shaft: δ_bending = k × L² / D [k from material/quench table] Ring ovality: δ_oval ≈ D² × t / (16 × E_material × moment_compensation) [simplified]

FEA simulation (DEFORM, DANTE software): Coupled: thermal + phase transformation + mechanical FEA Required inputs: TTT/CCT diagram, dilatometry data, temperature-dependent properties Output: residual stress field, final distorted shape Accuracy: ± 20–50% (good); better with calibrated material data

Heat transfer coefficient effect: Higher h at surface vs. bore → asymmetric quench → distortion in bored parts Vapor blanket (initial water quench) → patchy contact → large local distortion

Mitigation Strategies

Quench Process Modifications

Martempering (marquenching): Quench into bath at M_s temperature (100–250°C depending on alloy); hold to equalize T; air cool Result: reduced thermal gradient; delayed martensite formation → more uniform → less distortion Disadvantage: lower hardenability requirement; bath must be precisely controlled

Austempering: Quench into bainite range (250–400°C); hold to completion of bainite transformation; air cool Result: no martensite (bainite instead); better toughness; excellent distortion control Disadvantage: not applicable for through-hardening of thick sections

Press quench (for gears): Part held in fixture during quench; external pressure prevents warping Used for critical hypoid ring gears, thin washers Requires dedicated fixture per part; high setup cost

Fixture / die quench (hot press): Part between dies during quench; controlled deformation

Pre-heat Treatment

Stress relief before hardening: 570–620°C for 1–2 hr → relieve machining stresses before austenitizing Reduces contribution of pre-existing stress to distortion

Normalizing: Air cool from austenitize temperature → remove prior quenching stress; uniform microstructure before critical quench

Post-treatment Straightening

Hot straightening: at temperature (200–400°C, in temper range); plastic deformation + tempering → hold shape Risk: stress concentration at bend if over-straightened

Cold straightening: after full heat treatment; press straightening Danger: cracking in high-carbon or high-alloy materials; check for H₂ embrittlement risk (48 hr delay after quench for high-strength steel) Allowable: ±0.3% of diameter for most steels

Cryogenic treatment: -196°C for 24 hr → complete martensite transformation → reduce retained austenite → dimensional stability Especially valuable for tool steels, high-carbon bearing steels

Residual Stress from Heat Treatment

Beneficial compressive residual stress (surface): From case hardening or shot peening after hardening Magnitude: σ_res_surface = -300 to -800 MPa (compressive) in carburized gears

Detrimental tensile residual stress (core): Equilibrium requires tensile at core → fatigue crack nucleation in core possible

Measuring residual stress: X-ray diffraction (XRD): measures surface 10–20 μm; ASTM E915 Neutron diffraction: through-thickness; subsurface RS at neutron facility Blind hole drilling: ASTM E837; 1–2 mm depth; limited accuracy

Material-Specific Distortion Behavior

Low-alloy steel (4140): Moderate distortion; best in oil quench; high H₂O quench causes excessive distortion Tool steel (D2, M2): Very low alloy content (M2) → high hardenability; air cool → minimum distortion Stainless (440C): Oil quench; careful austenitizing (1065°C); slow heat-up prevents distortion Titanium alloys: No quenching typically; precipitation hardening at 480–600°C; very low distortion

Standards

Standard	Scope
AMS 2759	Heat treatment of steel; distortion limits
AMS 2770	Heat treatment of aluminum
ASTM A255	Hardenability of steel (Jominy)
ASTM E915	XRD residual stress (stress measurement)
ASTM E837	Blind hole drilling residual stress
ASM Heat Treater's Guide	Industry reference for distortion data

Output

Provide: material and quench method, quench severity H [m⁻¹], transformation volume expansion ΔV/V [%], thermal gradient ΔT during quench [°C], dominant distortion type (size change/bending/ovality), magnitude of distortion expected [mm] at critical dimensions, residual stress at surface [MPa] (tensile/compressive), mitigation strategy (martempering/press quench/stress relief), post-straightening method and limits [mm], retained austenite [%] before/after cryo treatment, and applicable standard (AMS 2759, ASTM A255, ASTM E915).