tempering

name	tempering
description	Tempering of steel — stages (I–IV martensite decomposition), temper embrittlement, tempered martensite embrittlement (TME), secondary hardening, temper charts, Hollomon-Jaffe parameter.
metadata	{"priority":7,"promptSignals":{"phrases":["tempering steel","tempered martensite","tempering temperature","temper embrittlement","secondary hardening","Hollomon-Jaffe","quench and temper"],"minScore":3}}

Tempering — Complete Skill

Purpose and Overview

Tempering: heat quenched (martensitic) steel below A₁ → relieve residual stress, improve toughness at some strength cost Standard sequence: Austenitize (900–950°C) → Quench (water/oil/air) → Temper (150–650°C)

Martensite: supersaturated C in BCT lattice; very hard (up to 65 HRC); brittle as-quenched Tempering systematically decomposes martensite through four stages

Stages of Martensite Decomposition

Stage I: 80–200°C — ε-Carbide Precipitation

• C segregation to dislocations
• Precipitation of ε-carbide (Fe₂.₄C or η-Fe₂C; hexagonal)
• Slight hardness decrease; significant relief of quench stresses
• Tetragonality (c/a ratio) begins to decrease

Stage II: 200–300°C — Retained Austenite Decomposition

• Retained austenite (γ_R) transforms to bainite-like products
• Only significant when γ_R > 5% (high-C or high-alloy steels)
• Hardness may temporarily increase (transformation of γ_R contributes)
• Tempered Martensite Embrittlement (TME) most pronounced at this stage (see below)

Stage III: 250–350°C — ε-Carbide → Cementite

• ε-carbide dissolves; cementite (Fe₃C) precipitates along lath boundaries and habit planes
• Significant softening; martensite becomes ferritic matrix with fine carbides
• Hardness drops noticeably

Stage IV: 350–700°C — Carbide Coarsening and Recovery

• Cementite spheroidizes and coarsens (Ostwald ripening)
• Lath boundaries dissolve → equiaxed ferrite grains
• Low temperature tempering (350–450°C): good strength-toughness balance (engineering applications)
• High temperature tempering (550–650°C): maximum toughness (AISI 4340: 150 ksi, very tough)

Hollomon-Jaffe Tempering Parameter

Combines temperature and time: P = T × (C + log t) × 10⁻³

T = absolute temperature [K]; t = time at temperature [hours]; C = material constant (≈ 14–22; often C = 20 for steels)

Equivalent tempering cycles: If P_1 = P_2 → same expected hardness/strength

Example: 400°C × 2h vs. ??? °C × 1h (same C = 20): P_1 = 673 × (20 + log 2) × 10⁻³ = 673 × 20.301 × 10⁻³ = 13.66 Solve for T_2: 13.66 = T_2 × (20 + log 1) × 10⁻³ = T_2 × 20 × 10⁻³ → T_2 = 683 K = 410°C

Practical use: Double-tempering: two equal cycles; ensures complete decomposition (especially after cryogenic treatment) Multiple temper: P_total not simply additive; each cycle is independent exposure

Tempered Martensite Embrittlement (TME)

Also called "350°C embrittlement" or "500°F embrittlement" Occurs when tempering between 250–400°C (especially ~350°C) Not temperature-dependent in service; permanent embrittlement from processing

Mechanism: cementite films on prior austenite grain boundaries (PAGBs) Impurity segregation (P, Sb, Sn, As, Mn) enhances grain boundary weakness Fracture: intergranular (IG) along PAGBs

Indicators: sharp drop in Charpy toughness; ductile-to-brittle transition shift upward Susceptible steels: plain carbon (0.4–0.6%C), Mn-Si steels, medium alloy Less susceptible: Mo steels (Mo inhibits P segregation); V-containing steels

Avoidance: temper above 400°C or below 250°C; use Mo-bearing steels; minimize P (< 0.010%)

Temper Embrittlement (TE)

Different from TME: occurs on exposure to 375–575°C RANGE during tempering or service Reversible embrittlement: anneal above 575°C + fast cool → properties restored Mechanism: equilibrium grain boundary segregation of P, Sb, Sn, As (especially in presence of Mn, Cr, Ni)

Step cooling test: specimen cooled through embrittlement range in steps → measures ΔDBTTt J-factor = (Si + Mn) × (P + Sn) × 10⁴ [< 100 = low risk; > 250 = high risk] X-factor = (10P + 5Sb + 4Sn + As) / 100 (alternate; < 15 recommended)

Effect: Charpy transition temperature shift upward by 30–100°C Design implication: petroleum refinery vessels operating 350–500°C; temper embrittlement awareness for step-load analysis

Secondary Hardening

Occurs in alloy steels with W, Mo, V, Cr during tempering at 500–600°C Mechanism: coherent alloy carbide precipitation (M₂C, MC, M₆C) replaces iron carbides → precipitation strengthening overcomes softening from iron carbide coarsening

Steels showing secondary hardening: H13 tool steel (5%Cr, 1.5%Mo, 1%V): secondary peak at ~550°C; strength recovers M2 high-speed steel (6W-5Mo-4Cr-2V): secondary hardening preserves hardness to 600°C 9Ni-4Co steels: secondary hardening at ~500°C; aerospace applications

Double-temper: required for tool steels; first temper decomposes retained austenite → second temper tempers newly formed martensite from γ_R transformation

Typical Tempering Practices

Application	Temper T [°C]	Expected Hardness	Expected UTS [MPa]
Spring (AISI 5160)	400–450	42–52 HRC	1500–1900
Gear/shaft (AISI 4140)	450–550	38–48 HRC	1200–1600
Die steel (H13)	540–595	44–52 HRC	1500–1900
Pressure vessel (SA-508 Gr3)	620–660	280–320 HB	600–800
Low-alloy high-strength	550–660	25–35 HRC	800–1200

Atmosphere and Time

Furnace atmosphere: Neutral (N₂ or endothermic): preferred; no decarburization Air atmosphere: acceptable for low-temperature tempering (< 250°C) but may discolor Hydrogen atmosphere: not needed for tempering (unlike carburizing)

Time at temperature: 1 h per 25 mm cross-section minimum (to reach uniform T) Total hold: 1–4 h typical; longer → more complete carbide transformations

Heating rate: slow for highly stressed or complex geometry (< 200°C/hr for large sections) Cooling: air cool after tempering is sufficient; water quench after tempering above 600°C can avoid TE range

Output

Provide: tempering temperature [°C] for target hardness/UTS, predicted hardness from Hollomon-Jaffe parameter P (with C = 20), risk assessment for TME (tempering range) and TE (service range), secondary hardening check (alloy carbide-forming elements present?), required temper cycles (1× or 2×), furnace atmosphere recommendation, time at temperature [h/25mm], and applicable specification (AMS 2759 for aerospace heat treatment).