quenching

name	quenching
description	Quenching and hardening — TTT/CCT diagrams, hardenability (Jominy end-quench, Grossmann DI), quench media (water, oil, polymer, air, salt bath), heat transfer coefficient h_q [W/m²·K], Grossmann H-factor, Leidenfrost effect in quenching, quench distortion and cracking (tensile vs. compressive surface stress), Biot number, case/core hardness prediction, ASTM A255 Jominy test, Taguchi quench optimization, and IQ (intensive quenching) process.
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Quenching and Hardening — Complete Skill

Metallurgical Background

TTT and CCT Diagrams

TTT (Time-Temperature-Transformation): Isothermal diagram: temperature vs. log(time) at constant T; shows start (1%) and finish (99%) of transformation Nose of TTT curve: minimum time for diffusional transformation (pearlite/bainite); typically 500–600°C for steels

CCT (Continuous Cooling Transformation): Practical diagram: cooling paths superimposed; shows critical cooling rate for full martensite Critical cooling rate R_c: minimum rate to avoid pearlite/bainite nose on CCT → full martensite R_c (water quench): ~50–200°C/s for plain carbon steels; < 5°C/s for high-alloy steels (air hardening)

Martensite start/finish: M_s: martensite start temperature (°C); M_f: finish (°C) Andrews empirical (1965): M_s [°C] = 539 - 423C - 30.4Mn - 17.7Ni - 12.1Cr - 7.5Mo [C, Mn, etc. in wt%] M_f ≈ M_s - 215 [approximate; often below room temperature for high-C steels → retained austenite]

Hardness from microstructure: Martensite: HRC ≈ 20 + 60 × C% [wt% C]; H_martensite [HRC] = 30 + 50 × (%C)^0.5 (Krauss) Full martensite 0.45%C: HRC ≈ 55–57; 0.60%C: HRC ≈ 62–64

Hardenability

Jominy End-Quench Test (ASTM A255)

Test procedure: 25 mm (1 in) diameter × 102 mm bar; austenitize at standard temperature; quench from one end with water at 23°C Measure HRC every 1.6 mm (1/16 in) from quenched end → Jominy curve

Hardenability band: AISI grades have specified H-bands: H (hardenability) designation SAE 4140H, 4340H: guaranteed hardenability within band (max/min Jominy curves)

Reading Jominy curve: Distance from quenched end (J-distance) = cooling rate equivalent J1/16 = 1/16 in from end → cooling rate ≈ 480°C/s (fastest; max hardness) J4 (4/16 = ¼ in) → ≈ 70°C/s; J8 → ≈ 30°C/s; J16 → ≈ 10°C/s; J40 → ≈ 2°C/s (slowest)

Grossmann H-Factor and Ideal Diameter

Ideal diameter DI: Diameter of round bar that transforms 50% martensite at center when quenched in ideal (infinite) quench DI correlates with Jominy at J1/4 (quarter-length hardenability indicator) or from alloy content: DI = DI_base × f_Mn × f_Si × f_Ni × f_Cr × f_Mo × ... [multiplication factors from Grossmann charts]

Example: SAE 4340 (0.40C, 0.70Mn, 1.65Ni, 0.80Cr, 0.25Mo): DI ≈ 100–150 mm → very high hardenability

Actual diameter DA (hardening in real quench): DA = DI / (1 + DI / (H × 2.54)) [Grossmann; H = severity of quench = h_quench × diameter / k_steel] H_values: water agitated = 1.0–2.0; still water = 0.9–1.0; oil agitated = 0.35–0.7; air = 0.02–0.05

Center hardness from DI and quench severity: Use Grossmann's master chart: given DI and H → surface and center hardness from bar diameter

Quench Media

Heat Transfer Coefficients

Quench media comparison:

Medium	h_q [W/m²·K]	H factor	Distortion risk	Cracking risk
Brine (10% NaCl)	14,000–21,000	2.0	High	High
Water (20°C, agitated)	8,000–14,000	1.0	High	Moderate
Water (60°C)	3,000–6,000	0.6	Moderate	Low
Polymer (PAG, 10%)	3,000–8,000	0.4–0.8	Moderate	Low
Fast quench oil	1,800–3,000	0.35	Low	Low
Slow quench oil	800–1,500	0.25	Low	Very low
Air (still)	10–30	0.02	Very low	Very low
Pressurized N₂ (gas quench)	300–1,000	0.05–0.15	Very low	Very low
Salt bath (180°C)	2,000–4,000	0.5	Low	Low

Leidenfrost effect in water quenching: First stage: vapor film around hot part (T > 600°C) → low h_q; film/vapor blanket phase Second stage: T drops to 300°C → film collapses → nucleate boiling → h_q peaks (8,000–14,000 W/m²·K) Third stage: T < 100°C → convective cooling; h_q moderate Leidenfrost temperature: 100–300°C in water (affected by water temperature, agitation, additives) Agitation suppresses Leidenfrost → more uniform cooling → less distortion

Polymer Quenching (PAG — Polyalkylene Glycol)

Properties: Reverse solubility: PAG precipitates from solution above critical temperature (~70°C) → forms polymer film on part Film insulates → slower, more uniform cooling than water → less distortion; lower cracking risk Concentration control: 10–25% PAG by volume → cooling rate between water and oil Monitoring: viscosity or refractive index → maintain concentration

Advantage: Less fire hazard than oil; adjustable cooling rate; environmentally preferred over oil Used for aluminum alloys (precipitation hardening: T6 treatment), alloy steel large parts

Heat Transfer Analysis

Biot Number and Cooling

Biot number: Bi = h_q × L_c / k_steel [L_c = characteristic length = volume/area = R/3 for sphere, R/2 for cylinder] Bi >> 1: surface-controlled cooling; temperature gradient inside part significant Bi << 1: lumped capacitance; uniform T throughout (valid for Bi < 0.1)

Lumped capacitance (Bi < 0.1): T(t) = T_q + (T_i - T_q) × exp(-h_q × A / (ρ × V × c_p) × t) [exponential cooling] Time to cool from austenitize T_a to M_s: t_critical = ρVc_p/(h_qA) × ln((T_a - T_q)/(M_s - T_q))

Finite difference (for thick parts): Discretize part → solve heat equation; compare center cooling curve with CCT → predict microstructure at each node

Quench Distortion and Cracking

Mechanisms

Thermal stress: Non-uniform cooling → temperature gradient → differential thermal contraction → residual stress Rapid surface quench: surface compressive (cools and contracts first, supported by hot core); then tensile as core cools

Transformation stress: Martensite formation: volume expansion ≈ 4% → compressive stress in newly formed martensite zone Timing: if surface transforms before core → compressive surface stress (beneficial, like shot peening) If core transforms after surface (typical in through-hardened parts): core expansion pulls surface into tension → cracking risk

Distortion types: Size change (dimensional): uniform shrinkage/expansion; predictable; compensate in drawing Shape change (distortion): asymmetric cooling or geometry → bowing, oval, etc.

Cracking factors: Surface tensile stress after quench → cracks at stress concentrators (keyways, threads, cross-holes) High carbon (> 0.6%C) → brittle martensite → more cracking; temper immediately after quench

Distortion prevention: Symmetrical fixturing (vertical hanging vs. horizontal laying) Interrupted quench (marquenching): quench to above M_s (~200°C salt bath) → hold for equalization → air cool through M_s; reduces stress gradient

Intensive Quenching (IQ)

Principle: Very high quench rate throughout (h_q > 10,000–20,000 W/m²·K) → compressive residual stress at surface IQ produces compressive surface after quench (opposite of typical through-hardened tensile surface) Mechanism: very rapid surface hardening → core still austenitic when surface forms martensite → core contracts on martensite; surface in compression

IQ benefits: Fatigue life improvement: 2–5× vs. conventional quench; equivalent to carburizing surface treatment No distortion increase (compressive residual stress arrests distortion) Materials: works for medium carbon steels (0.35–0.65%C); IQ replaces carburizing + quench in some applications

Tempering After Quench

Mandatory: do not leave quenched steel in martensite state → brittleness; cracking risk Temper within 2 hours of quench for high-C steels

Tempering temperatures: 100–200°C: stress relief; precipitation of ε-carbide; small hardness drop; high retained hardness 200–400°C: tempered martensite (TM); toughness and ductility improve significantly; hardness drops to HRC 50–55 for 0.45%C steel 400–650°C: sorbite (fine pearlite); lower hardness; good toughness; structural steels

Secondary hardening: at 500–550°C for high-speed steels (M2, M35) and tool steels → carbide precipitation → hardness recovers after temper dip

Standards and References

Standard	Scope
ASTM A255	Standard test methods for determining hardenability of steel (Jominy)
ASTM A370	Mechanical testing of steel products
AMS 2759	Heat treatment of steel parts (general)
SAE J406	Methods of determining hardenability
AGMA 2004	Gear material and heat treatment manual
ASM Handbook Vol. 4A	Steel heat treating; quenching and distortion

Output

Provide: steel grade (AISI/SAE; C [wt%]; alloy composition; AMS designation if applicable), austenitizing temperature T_a [°C] and time [min], M_s [°C] and M_f [°C] (Andrews formula), ideal diameter DI [mm] (Grossmann; hardenability calculation), Jominy position equivalent for critical section (J-distance [1/16 in]; H [HRC] from band), quench medium selected (H-factor; h_q [W/m²·K]; agitation; justification vs. hardenability + distortion risk), Biot number Bi (surface vs. core cooling; uniform vs. non-uniform T), cooling time to M_s [s] and predicted surface/center hardness [HRC], tempering (T [°C]; time [hr]; final hardness HRC; microstructure: TM/sorbite), distortion risk assessment (cracking tendency; tensile vs. compressive surface; fixturing recommendation), and applicable standard (ASTM A255 Jominy, AMS 2759, ASM Handbook Vol. 4A).