🔥 Heat Treatment of Steel
A correctly selected type of heat treatment or thermo-chemical treatment is the key to achieving the required properties – from high hardness and resistance to toughness and dimensional stability.
⚙️ Heat Treatment (HT)
It affects the internal structure of steel, changing hardness, strength, toughness, and machinability.
The process consists of heating to a precise temperature followed by controlled cooling.
🔁 Basic Types of Heat Treatment
🔹 Annealing – reduces hardness and removes internal stresses.
🔹 Hardening – increases hardness and strength.
🔹 Tempering – reduces brittleness after hardening and increases toughness.
💡 Temperatures and times are approximate – exact values depend on the chemical composition and dimensions of the material.
♨️ Annealing
The goal is to reduce hardness or remove stresses after machining or forming.
Typical feature: long holding time at temperature followed by slow cooling.
Types of annealing:
🧩 Stress-relief annealing: 600–650 °C / 1 h → air cooling
⚙️ Soft annealing: 650–720 °C / holding time 4 h → cooling at 10–20 °C/h
🔄 Normalizing: 800–920 °C / holding time 1 h → air cooling
🔬 Homogenizing annealing: 1000–1200 °C → equalization of chemical composition after casting
⚡ Hardening
Goal: to form martensite or bainite and increase hardness.
The steel is heated to the austenitizing temperature (800–920 °C), held, and then rapidly cooled.
Medium: 💧 water, 🛢️ oil, 🧂 salt bath, 🌬️ air.
Properly hardened steel → at least 50% martensite in the core.
🔧 Tempering
Always performed after hardening.
It reduces internal stresses, modifies martensite, and increases toughness.
Temperature < Ac₁ (approx. 150–700 °C) depending on the steel grade.
Classification:
🌡️ Low tempering (150–300 °C): for cold work tools
🔥 High tempering (350–700 °C): for steels stressed at elevated temperatures
Alloy steels are tempered twice, high-speed steels three times.
Oxide colors on the surface indicate temperature (yellow → blue).
⚗️ Thermo-Chemical Treatment
This changes the chemical composition of the surface layer, while the core remains tough.
The elements used include carbon (C), nitrogen (N), boron (B), chromium (Cr) – or combinations of these.
Most common processes:
🧱 Carburizing
Temperature: approx. 920 °C
Medium: gaseous / liquid / solid
Layer: 0.5–1.5 mm, hardness up to 800 HV
Followed by: hardening + low-temperature tempering
Use: gears, shafts, pins
💨 Nitriding
Temperature: approx. 500 °C / duration 10–60 h
Surface hardness: up to 1200 HV, depth 0.2–0.6 mm
Performed in gaseous, liquid, or plasma atmosphere
Advantage: hardness is retained even at elevated temperatures
Use: precision parts, molds, shafts, pistons
📘 Summary:
A correctly selected type of heat treatment (HT) or thermo-chemical treatment (TCT) is the key to optimal steel properties – combining hardness, toughness, precision, and long service life.
