Heat Treatment of Metals: Hardening and Annealing

Heat Treatment of Metals: Controlling Microstructure

The same piece of steel can be soft as butter or hard as a knife blade — the difference is heat treatment. Controlled heating and cooling rearrange atoms inside the metal and completely change its properties. This is not magic — it is the science of microstructure.

Why Heat Treatment Works

Steel is an alloy of iron and carbon. At different temperatures, iron atoms adopt different crystal arrangements:

• Ferrite (α): BCC (body-centered cubic) — soft, ductile, magnetic. Stable below 727°C.
• Austenite (γ): FCC (face-centered cubic) — dissolves more carbon. Stable above 727°C.
• Cementite (Fe₃C): an iron-carbon compound, very hard and brittle.
• Pearlite: alternating layers of ferrite and cementite — a balance of strength and ductility.
• Martensite: distorted BCT structure — extremely hard and brittle. Forms by rapid cooling.

The critical temperature of 727°C (for steel with 0.77% carbon) is called the eutectoid temperature — above it, everything transforms to austenite.

Hardening: Creating Maximum Hardness

Goal: obtain martensite — the hardest possible microstructure.

Steps:

1. Heating: above the critical temperature (typically 800-900°C for carbon steel)
2. Soaking: sufficient time to fully transform the structure to austenite
3. Quenching: immersion in a cooling medium — austenite has no time to transform to pearlite and instead becomes martensite

Quenching media:

Warning: after hardening, the part is hard but extremely brittle — it will shatter like glass. It must never be used in this condition directly.

Tempering: Restoring Toughness

Immediately after hardening, the part is reheated to a temperature below the critical point (150-650°C) and then cooled slowly.

What happens? Martensite releases some of its internal stresses. Part of it transforms into more stable structures. The result: a slight decrease in hardness in exchange for a large increase in toughness.

Annealing: Restoring Softness

Goal: soften the metal, relieve internal stresses, and improve machinability.

Steps:

1. Heat above the critical temperature
2. Soak for sufficient time
3. Very slow cooling inside the furnace (turn off the furnace and let it cool naturally)

Result: coarse pearlite — the softest possible condition for steel. Used before difficult forming and machining operations.

Types of annealing:

• Full annealing: heating above critical + furnace cooling — maximum softness
• Stress relief annealing: 550-650°C — does not change microstructure but removes stresses from welding or forming
• Spheroidizing: prolonged heating near the critical temperature — converts cementite into spheres — easiest machining for high-carbon steel

Normalizing: Homogenizing the Structure

Similar to annealing but with cooling in still air instead of the furnace.

Result: fine, uniform pearlite — slightly harder than annealed and more homogeneous. Used:

• After casting or forging to homogenize the structure
• Before final heat treatment
• To improve machinability

Comparing the Four Processes

Case Hardening: Hard Outside, Tough Inside

Sometimes we need a hard, wear-resistant surface and a soft core that absorbs impact — such as gears and shafts.

Carburizing

Low-carbon steel (0.1-0.2%) is heated in a carbon-rich atmosphere at 900-950°C. Carbon diffuses into the surface (depth 0.5-2mm), making the surface high-carbon and hardenable while the core remains soft.

Nitriding

Heating in an ammonia atmosphere at 500-570°C. Nitrogen diffuses into the surface and forms very hard nitrides. Advantages:

• No quenching needed — hardness forms directly
• Much less distortion (low temperature)
• Excellent wear resistance

Induction Hardening

An induction coil rapidly heats only the surface, followed by spray quenching. Ideal for gear teeth and shaft surfaces — fast, and the hardened layer depth can be precisely controlled.

Microstructure Changes

A metallographic microscope reveals the metal's structure after polishing and chemical etching:

• After annealing: large, clearly defined pearlite grains with ferrite at boundaries — regular and soft
• After normalizing: finer grains — more homogeneous structure
• After hardening: crossed martensite needles — sharp acicular structure
• After tempering: less sharp needles — more stable structure

The Jominy End-Quench Test

The key test for determining hardenability — the ability of steel to form martensite in depth.

Procedure:

1. A standard cylindrical specimen (25mm x 100mm) is heated to austenitizing temperature
2. Mounted vertically with water sprayed on the bottom end only
3. After cooling, a flat is ground along one side
4. Hardness is measured at increasing distances from the quenched end

Result: a hardness-distance curve. Steel with high hardenability retains hardness over greater distances.

Alloying elements (Cr, Mo, Mn, Ni) increase hardenability — this is why alloy steel can be hardened with oil or even air instead of water.

Practical Tips from the Shop Floor

• Do not harden steel with less than 0.3% carbon — it will not achieve adequate hardness
• Tempering must follow hardening immediately — do not let the part cool completely
• Cracking during quenching is mainly caused by: sharp corners, uneven cooling, or wrong steel grade
• Large parts need a slower quenching medium to avoid thermal stresses
• Record every heat treatment: temperature, time, quenching medium — repeatability is the key to quality