Study sheet: Heat Treatment of Steel

📋 Course Outline

1. Heat Treatment Fundamentals
2. Full Annealing and Grain Refinement
3. Special Annealing Treatments
4. Normalizing and Hardening
5. Tempering and Hardening Variables
6. Case Hardening Principles
7. Carburizing and Carbonitriding
8. Cyaniding and Nitriding
9. Flame and Induction Hardening
10. Isothermal Hardening Processes

📖 1. Heat Treatment Fundamentals

🔑 Key Concepts & Definitions

• Heat treatment : The combination of heating metals or alloys in the solid state and cooling them at a suitable rate to obtain desired properties.

📝 Essential Points

• The purposes of heat treatment are:

  • relieving internal stresses
  • refining grain size
  • improving ductility
  • increasing hardness or tensile strength
  • changing chemical composition
  • improving machinability

• The principal parameters are:

  • temperature
  • holding time
  • rates of heating and cooling

💡 Memory Hook

THR: temperature, holding time, rate

📖 2. Full Annealing and Grain Refinement

🔑 Key Concepts & Definitions

• Full annealing : Heats steel to the proper temperature and cools it slowly through the transformation range, preferably in a furnace or insulating material.

★ Must-know

• In hypoeutectoid steel, heating above A1 transforms pearlite into small austenite grains while ferrite remains unchanged, continued heating to above A3 transforms the ferrite into fine austenite, and furnace cooling produces fine ferrite and small pearlite areas.

📌 The proper annealing temperature is approximately 50°F above A3 for hypoeutectoid steel and approximately 50°F above the lower critical temperature line A3,1 for hypereutectoid steel.

Further detail

• Annealed hypereutectoid steel contains coarse lamellar pearlite surrounded by a proeutectoid cementite network, which is brittle, creates a plane of weakness, and causes poor machinability.

💡 Memory Hook

A1 crossed → ferrite transforms → A3 crossed → fine austenite → furnace cooling

📖 3. Special Annealing Treatments

🔑 Key Concepts & Definitions

• Spheroidising annealing : Produces spheroidal carbide particles in a ferrite matrix to improve the machinability and ductility of high-carbon and air-hardening alloy steels.
• Stress relief annealing : Heats steel uniformly below the lower critical temperature, typically from 1000°F to 1250°F, holds it sufficiently, and cools it uniformly to reduce residual stresses without microstructural change.
• Recrystallisation annealing : Softens cold-worked steel by forming strain-free grains, thereby decreasing hardness and strength and increasing ductility for further cold working.

📝 Essential Points

• The three spheroidising methods are:
  • prolonged heating just below the lower critical temperature followed by slow cooling
  • alternating heating and cooling just above and below the lower critical temperature
  • heating above the lower critical temperature, slow cooling below it, and prolonged holding

💡 Memory Hook

Spheroidising softens carbide; stress relief removes stresses; recrystallisation removes cold-work effects

📖 4. Normalizing and Hardening

🔑 Key Concepts & Definitions

• Normalizing : Heats steel approximately 100°F above A3 or Acm, holds it sufficiently, and cools it in still air to produce finer grains and higher strength and hardness than full annealing.
• Hardening : Heats steel to its hardening temperature, holds it, and rapidly cools it by quenching in water, oil, or a salt bath to produce high hardness.

★ Must-know

📌 Hypoeutectoid steel is hardened 30–50°C above A3, while hypereutectoid steel is hardened 30–50°C above A1.

Further detail

• Normalized 0.5% carbon steel contains approximately 10% proeutectoid ferrite and 90% pearlite, whereas annealed 0.5% carbon steel contains approximately 38% ferrite and 62% pearlite.

💡 Memory Hook

Annealing uses furnace cooling, whereas normalizing uses still-air cooling

📖 5. Tempering and Hardening Variables

🔑 Key Concepts & Definitions

• Tempering : Heats hardened steel below the lower critical temperature, typically from 150–630°C, and then cools it to relieve residual stresses and improve ductility and toughness.

★ Must-know

• During tempering, supersaturated martensite decomposes into body-centered cubic alpha ferrite and small carbide particles.

Further detail

• The tempering classifications are: low-temperature tempering at about 200°C for high-carbon and low-alloy steel, medium-temperature tempering at 200–400°C for coils and laminated springs, high-temperature tempering at 400–650°C for medium-carbon steel, shafts, and gears

📌 The recommended hardening holding time is one hour for each 25 mm of thickness or diameter.

💡 Memory Hook

Martensite formation → brittleness and residual stress → tempering → improved toughness

📖 6. Case Hardening Principles

🔑 Key Concepts & Definitions

• Case hardening : Produces a hard, wear-resistant surface layer called the case while retaining a relatively soft and tough interior called the core.

📝 Essential Points

• The case-hardening methods are:
  • carburizing
  • nitriding
  • cyaniding or carbonitriding
  • flame hardening
  • induction hardening

📌 Carburizing adds carbon, nitriding adds nitrogen, and cyaniding or carbonitriding adds both carbon and nitrogen, whereas flame and induction hardening do not change the chemical composition.

💡 Memory Hook

Carburizing, nitriding, and cyaniding change composition; flame and induction hardening do not

📖 7. Carburizing and Carbonitriding

🔑 Key Concepts & Definitions

• Carburizing : Increases the carbon content of low-carbon steel by heating it in a carbon-containing medium, usually at 1700°F or 900–930°C, so carbon diffuses into the austenite.
• Carbonitriding : Diffuses carbon and nitrogen into plain carbon steel at 800–870°C using a gas mixture containing 5% methane, 15% ammonia, and the remainder natural gas.

★ Must-know

• 🔄 Double heat treatment consists of: slow cooling to room temperature, reheating above the upper critical temperature and quenching, reheating just above the lower critical temperature and quenching, tempering to relieve stresses and brittleness

Further detail

• Pack carburizing uses approximately 80% charcoal coke and 20% barium carbonate, produces a case depth of 1–2 mm, and requires approximately 6–8 hours.

• Liquid carburizing uses a fused bath of sodium cyanide, sodium chloride, and barium chloride at 815–900°C for 5 minutes to 1 hour and produces a clear case up to 0.08 mm.

• Gas carburizing is widely used for mass production at about 930°C and commonly produces a carbon case 0.2–0.5 mm thick.

💡 Memory Hook

Carbon addition → diffusion → quenching → tempering

📖 8. Cyaniding and Nitriding

🔑 Key Concepts & Definitions

• Cyaniding : Diffuses carbon and nitrogen into low-carbon steel using a molten sodium cyanide bath at 800–870°C, producing a case 0.025–0.25 mm deep with surface hardness up to 65 HRC.
• Nitriding : Diffuses atomic nitrogen into alloy steel containing elements such as aluminum, chromium, vanadium, and molybdenum by circulating ammonia at about 550°C.

★ Must-know

• Nitriding requires no quenching, produces a case depth of 0.1–0.5 mm with hardness up to 1100 VHN, and may form a brittle white layer of Fe4N and Fe2N.

Further detail

• Nitriding generally requires 50–90 hours to produce a maximum case depth of 0.5–0.8 mm and retains hardness up to 500°C, whereas carburized hardness begins to fall at about 200°C.

💡 Memory Hook

Cyaniding is rapid and salt-based; nitriding is slower, ammonia-based, and needs no quench

📖 9. Flame and Induction Hardening

🔑 Key Concepts & Definitions

• Flame hardening : Heats the steel surface to the austenitizing temperature with an oxy-acetylene flame and then quenches it so the austenite transforms into martensite.
• Induction hardening : Rapidly heats the steel surface by alternating current and quenches it to form fine-grained martensite, usually followed by tempering at 160–200°C.

📝 Essential Points

• The flame-hardening methods are:

  • stationary
  • progressive
  • spinning
  • progressive spinning

• Steel for flame hardening generally contains 0.3–0.6% carbon, and the hardened zone is commonly 1/8–1/4 inch deep.

📌 The depth of induction hardening is controlled by the frequency of the alternating current, and steels containing 0.4–0.5% carbon are especially suitable.

💡 Memory Hook

Flame uses oxy-acetylene; induction uses alternating-current heating

📖 10. Isothermal Hardening Processes

🔑 Key Concepts & Definitions

• Austempering : Heats steel above the austenitizing temperature, quenches it into a bath at 200–400°C just above Ms in the bainitic region, holds it until austenite transforms completely to bainite, and then cools it to room temperature.
• Martempering : Heats steel to the austenitizing temperature, quenches it into a constant-temperature bath at 180–250°C above Ms until the part is temperature-uniform, and then cools it in air to form martensite.

★ Must-know

• Austempering reduces distortion and cracking in intricate components and may produce upper or lower bainite, with lower bainite preferred for better mechanical properties.

Further detail

• Martempering reduces quenching stresses, cracking, and distortion and is especially suitable for high-carbon and alloy steels, after which the steel is generally tempered.

💡 Memory Hook

Austempering transforms austenite to bainite; martempering equalizes temperature before martensite forms

📊 Synthesis Tables

Annealing and Normalizing Comparison

Feature	Normalizing	Annealing
Cooling	Still air	Furnace
0.5% carbon steel structure	10% ferrite and 90% pearlite	38% ferrite and 62% pearlite
Pearlite lamellae	Fine	Coarse
Strength and hardness	Higher	Lower
Typical machinability improvement	Low-carbon steel	Medium-carbon steel

Case Hardening Methods

Method	Chemical change	Key condition
Carburizing	Adds carbon	Low-carbon steel; 900–930°C
Nitriding	Adds nitrogen	Ammonia; about 550°C; no quench
Cyaniding	Adds carbon and nitrogen	Molten cyanide bath; 800–870°C
Flame hardening	No chemical change	Oxy-acetylene flame and quench
Induction hardening	No chemical change	Alternating-current heating and quench