Heat Treatment & Materials — Comprehensive Study Pack Summary & Study Notes

🔬 Phase diagrams & TTT basics

Fe–Fe3C (iron–cementite) system and TTT (Time–Temperature–Transformation) diagrams are central for predicting microstructure after thermal cycles. TTT diagrams show isothermal transformation paths for austenite and indicate regions where pearlite (P), bainite (B) and martensite (M) form. Key temperatures: A1, A3, Acm, Ms, Mf mark phase boundaries and transformation start/finish.

⚙️ Interpretations and transformations

Austenite (A) on cooling can transform diffusively to ferrite + cementite (F + P) or non-diffusively to martensite (M) if cooled rapidly past Ms and Mf. Bainite forms at intermediate temperatures and combines hardness and toughness. The diagrams also differentiate metastable Fe–Fe3C (long cooling) and Uptonov/continuous cooling behaviors.

🧭 Practical notes

Understand how TTT curves shift with alloying and how continuous cooling paths cross pearlite/bainite regions. Recognize the notation Ap (undercooled austenite), and how overcooling (deep quench) can yield 100 % martensite or martensite + carbides for hypereutectoid steels.

🔥 Hardening, tempering and temper categories

Quenching (hardening) transforms austenite to martensite by rapid cooling. Tempering (popuštanje) follows quenching and is performed at temperatures below A1 to reduce residual stresses and adjust toughness. Distinguish low-, medium- and high-temperature tempering ranges (e.g., 20–220 °C; 220–400 °C; 400 °C < θp < A1) and their typical microstructural results (tempered martensite, carbides formation, reduction of tetragonality).

🔁 Purpose & effect

Tempering reduces hardness and increases ductility and impact toughness, and can precipitate secondary carbides (Kp). Austempering and martempering are variants that control bainite/martensite formation via isothermal holds.

📚 Heat treatment objectives & common processes

Annealing, normalizing, spheroidizing, and recrystallization anneal are heat treatments used to relieve stress, refine grain size, and improve machinability. Normalizing: heating above A3/A1 then air cooling to produce a fine, uniform grain structure. Spheroidizing (soft anneal) transforms lamellar carbides into spheroidal particles to improve machinability.

🧾 General principles

The choice of treatment depends on composition, initial microstructure, and final property targets (toughness, hardness, wear). Time and temperature control are crucial: slow cooling promotes diffusion-controlled products; fast cooling favors martensite.

⚗️ Precipitation hardening & carbides

Precipitation hardening (age hardening) uses solution treatment, quenching and ageing to form fine precipitates (β, AxBy) that strengthen alloys. Conditions: solute B must have temperature-dependent solubility and form coherent/semi-coherent precipitates on ageing.

🧪 Carbide chemistry and effects

Carbide formers (Ti, V, Mo, W, Cr, Mn) create hard phases like TiC, V4C3 or Cr7C3 with HV hardness up to several thousand. Carbides increase wear resistance and hardness but can reduce toughness. Distinguish substitutional solutes (affecting austenite stability) and carbide-formers (forming discrete compounds).

♻️ Normalizing and homogenization

Normalizing refines grains and homogenizes structure; homogenization annealing reduces chemical segregation after casting. These are essential preparatory steps before final heat treatments.

🧪 Hardenability, Jominy test & Burns diagram

Hardenability (kaljivost) is a material property describing depth/ability to form martensite during quenching. The Jominy end-quench test measures hardenability: a standard specimen is quenched at one end and hardness vs. distance is plotted.

📈 Practical influence factors

Hardenability depends on carbon content, alloying elements (Mn, Cr, Ni, Mo, etc.), section size, and quench severity. The Burns diagram and Jominy curves help predict hardness gradients in parts and guide selection of steel grade and quench method.

🛡️ Surface treatments and coatings

Surface thermochemical processes like carburizing (cementation) and nitriding enrich surface with C or N to form hard layers. CVD/PVD coatings (TiN, TiAlN, CrN, DLC) increase surface hardness, wear and corrosion resistance; CVD operates at higher temperatures (~1000 °C) while PVD is at 200–500 °C.