Engineering Materials: Properties and Heat Treatment

Posted on Feb 1, 2026 in Electromechanical Engineering

Ferrous and Non-Ferrous Materials Comparison

Hardening is a heat treatment process used to increase the hardness and strength of metal, especially steel, by changing its internal (micro) structure.
In this process, the metal is heated to a suitable temperature and then rapidly cooled (quenched) in water, oil, or air.

Types of Hardening

1. Through Hardening

• The entire cross-section of the metal is hardened. Heating above critical temperature followed by quenching. Application: Shafts, gears, machine parts.

2. Case Hardening

• Only the surface is hardened while the core remains soft and tough. Used where wear resistance is required. Application: Gears, cams, bearings.

Heat Treatment

Heat treatment is a controlled process of heating and cooling metals in the solid state to change their mechanical properties such as hardness, strength, ductility, and toughness without changing shape or size.

Applications and Uses of Heat Treatment

• To increase hardness and wear resistance (cutting tools, gears)
• To improve strength and toughness (shafts, axles)
• To relieve internal stresses caused by welding or machining
• To improve machinability and ductility
• To refine grain structure
• To increase fatigue life of components

Advantages of Heat Treatment

• Improves mechanical properties of metals
• Increases service life of components
• Enhances wear and corrosion resistance
• Reduces residual stresses
• Improves machinability and formability
• Economical for mass production

Disadvantages of Heat Treatment

• Improper treatment may cause distortion or cracking
• Requires skilled control of temperature and time
• Additional processing cost
• Oxidation and scale formation may occur
• Not suitable for all materials

Methods of Case Hardening

3. Carburizing

• Adding carbon to the surface by heating in a carbon-rich environment.
• Produces a hard outer layer.
• Example: Gear teeth.

4. Cyaniding

• Heating steel in a molten cyanide salt bath.
• Fast process, gives a thin hard layer.
• Example: Small machine parts.

5. Nitriding

• Nitrogen is added to the surface at a lower temperature.
• No quenching required.
• Example: Crankshafts.

6. Induction Hardening

• Heating by high-frequency electric current followed by quenching.
• Localized hardening.
• Example: Gear teeth, axles.

7. Flame Hardening

• Surface heated using gas flame then quenched.
• Simple and economical.
• Example: Large gears, rails.

Polymerization

Polymerization is a chemical process in which small molecules called monomers combine together to form a large molecule known as a polymer.

1. Strain Hardening (Work Hardening)

Strain hardening is the process of increasing the hardness and strength of a metal by plastic deformation without heating.
When a metal is cold worked (rolled, drawn, hammered, or bent), its internal structure resists further deformation, making it harder.

Key Points: No heat treatment required, hardness increases, ductility decreases, occurs at room temperature.

Examples: Cold rolling of steel sheets, wire drawing, hammering of copper.

2. Flame Hardening

Flame hardening is a surface hardening process in which the surface of steel is heated by a gas flame (oxygen–acetylene or oxygen–propane) to a high temperature and then rapidly quenched.

Key Points:

• Only surface becomes hard, core remains tough, simple and economical method, suitable for medium/high carbon steels.

Examples: Gears, camshafts, rails, crankshafts.

3. Induction Hardening

Induction hardening is a surface hardening method where the metal surface is heated using high-frequency alternating current (electromagnetic induction) and then quenched.

Key Points:

• Fast and accurate heating, localized hardening possible, no direct contact with heat source.

Examples: Gear teeth, axles, shafts, bearings.

Difference Between Hot Working and Cold Working

1. Ductile Metals

Definition: Metals that can be drawn into wires or stretched without breaking.

Characteristics: High tensile strength, can be hammered or rolled into thin sheets, usually malleable and tough.

Examples: Copper, Aluminum, Gold, Silver

Applications: Electrical wires, cables, jewelry, metal sheets

2. Non-Ductile Metals

Definition: Metals that cannot be drawn into wires or stretched; they break easily under tension.

Characteristics: Brittle, low tensile strength, not suitable for forming operations.

Examples: Cast iron, Lead, Zinc (at room temperature)

Applications: Pipes (lead), heavy machinery (cast iron), construction materials

Hot Working

Hot working is a metal working process in which the metal is deformed above its recrystallization temperature.
At this temperature, new grains are formed, so the metal does not become hard and brittle.

Characteristics

• No strain hardening, high ductility, less force required, refinement of grain structure.

Advantages

• Large deformation possible, improved mechanical properties, no residual stresses.

Disadvantages

• Poor surface finish, oxidation and scale formation, less dimensional accuracy.

Examples

• Hot rolling, hot forging, hot extrusion

Cold Working

Cold working is a metal working process in which deformation is done below recrystallization temperature (usually at room temperature).

Characteristics

• Causes strain hardening, increased strength and hardness, reduced ductility.

Advantages

• Good surface finish, high dimensional accuracy, improved strength.

Disadvantages

• Requires more force, residual stresses developed, limited deformation possible.

Examples

• Cold rolling, wire drawing, cold forging

Ceramics

Ceramics are inorganic, non-metallic materials made by shaping and firing at high temperatures.
They are hard, brittle, heat-resistant, and corrosion-resistant.
Example: Glass, porcelain, alumina.

Composites

Composites are materials made by combining two or more different materials to obtain improved properties.
They have a high strength-to-weight ratio and good toughness.
Example: Fiberglass, carbon fiber reinforced plastic (CFRP).

Nanomaterials

Nanomaterials are materials whose particle size is in the range of 1–100 nanometers (nm).
At this scale, materials show unique physical, chemical, and mechanical properties different from bulk materials.

Cold Treatment (Cryogenic Treatment)

Cold treatment is a heat-treatment process in which steel is cooled to very low temperatures (below 0°C, often −80°C to −196°C) after hardening.
It is done to convert retained austenite into martensite, thereby improving hardness and dimensional stability.

Advantages

• Increases hardness and wear resistance
• Improves dimensional stability
• Increases tool life
• Reduces retained austenite
• Improves fatigue strength

Disadvantages

• Costly process
• Requires special equipment
• Risk of cracking if not properly controlled
• Not suitable for all materials

Applications and Uses

• Cutting tools (drills, taps, milling cutters)
• Dies and punches
• Gears and bearings
• Automotive and aerospace components
• Measuring instruments

Properties of Metals

1. Good electrical conductivity – Metals conduct electricity easily (e.g., copper, aluminum).
2. Good thermal conductivity – Heat passes quickly through metals.
3. Malleability – Can be beaten into thin sheets.
4. Ductility – Can be drawn into wires.
5. High strength and hardness – Suitable for load-bearing applications.
6. Metallic luster – Shiny appearance when polished.
7. High melting and boiling point – Except mercury.
8. Sonorous – Produce ringing sound when struck.
9. Form alloys easily – Improve properties (steel, brass).
10. Generally solid at room temperature – Except mercury.

Properties of Non-Metals

1. Poor electrical conductivity – Except graphite.
2. Poor thermal conductivity – Heat does not pass easily.
3. Brittle in solid form – Break easily when struck.
4. Non-malleable – Cannot be beaten into sheets.
5. Non-ductile – Cannot be drawn into wires.
6. Low strength and hardness – Generally soft.
7. Dull appearance – No metallic shine (except iodine).
8. Low melting and boiling point – Except diamond.
9. Non-sonorous – Do not produce sound when struck.
10. May exist as solid, liquid, or gas – At room temperature.

Polymers

Polymers are high-molecular-weight materials formed by the repetition of small units called monomers.
They can be natural or synthetic and are widely used due to their light weight, flexibility, and corrosion resistance.

Types of Polymers

1. Thermoplastics

• Soften on heating and harden on cooling repeatedly.
• Can be reshaped.

Examples: Polyethylene (PE), PVC, Nylon

Uses and Applications:

• Plastic bags, bottles, pipes and insulation, electrical components.

2. Thermosetting Plastics

• Become permanently hard after heating.
• Cannot be remolded.

Examples: Bakelite, Epoxy, Phenolic resin

Uses and Applications:

• Electrical switches, adhesives, heat-resistant components.

3. Elastomers

• Show rubber-like elasticity.

Examples: Rubber, Neoprene, Silicone

Uses and Applications:

• Tires, seals and gaskets, shock absorbers.

4. Fibers

• Strong and flexible polymers.

Examples: Nylon, Polyester, Rayon

Uses and Applications:

• Textiles, ropes, industrial fabrics.

Advantages of Polymers

• Light in weight
• Corrosion and chemical resistant
• Easy to mold and shape
• Good electrical insulation
• Low cost

Disadvantages of Polymers

• Low strength compared to metals
• Poor heat resistance
• Deform under load (creep)
• Environmental pollution (non-biodegradable)
• Aging under sunlight

Heat Treatment of Alloys

Heat treatment of alloys is a controlled heating and cooling process used to improve mechanical properties such as hardness, strength, ductility, toughness, and machinability without changing the shape of the component.

Objectives of Heat Treatment

• Increase hardness and strength
• Improve ductility and toughness
• Relieve internal stresses
• Improve wear and corrosion resistance
• Refine grain structure

Common Heat Treatment Processes for Alloys

1. Annealing

• Heating alloy to a suitable temperature, holding, then slow cooling. Softens the material and improves ductility.

Applications: Brass sheets, aluminum alloys, steel components.

2. Normalizing

• Heating above critical temperature followed by air cooling. Improves strength and grain structure.

Applications: Steel forgings, castings.

3. Hardening

• Heating above critical temperature and quenching. Increases hardness and wear resistance.

Applications: Tool steels, alloy steels.

4. Tempering

• Reheating hardened alloy below critical temperature. Reduces brittleness and improves toughness.

Applications: Springs, gears, shafts.

5. Solution Heat Treatment

• Heating alloy to dissolve alloying elements, then rapid quenching. Used mainly for non-ferrous alloys.

Applications: Aluminum, magnesium, and nickel alloys.

6. Age Hardening (Precipitation Hardening)

• Alloy is aged at room or elevated temperature after solution treatment. Increases strength and hardness.

Applications: Aluminum alloys (Duralumin), copper alloys.

7. Stress Relieving

• Heating below critical temperature and slow cooling. Removes residual stresses.

Applications: Welded structures, castings.

Classification of Materials

Materials are classified based on their composition, structure, and properties. This helps in selecting the right material for engineering, construction, and industrial applications.

1. Metals

• Characteristics: Good strength, ductility, thermal & electrical conductivity.
• Examples: Steel, aluminum, copper.
• Uses: Construction, machinery, electrical components.

2. Non-Metals

• Characteristics: Poor conductors, brittle, low density.
• Examples: Sulfur, phosphorus, graphite, plastics.
• Uses: Insulators, chemicals, packaging.

3. Alloys

• Definition: Mixture of two or more metals, or metal + non-metal, to improve properties.
• Examples: Steel (iron + carbon), brass (copper + zinc).
• Uses: Gears, tools, structural components.

4. Ceramics

• Definition: Inorganic, non-metallic, brittle materials made by heating.
• Examples: Porcelain, glass, alumina.
• Uses: Tiles, insulators, refractory materials.

5. Polymers

• Definition: High molecular weight materials formed by polymerization of monomers.
• Examples: Polyethylene, PVC, Nylon.
• Uses: Packaging, pipes, textiles, electrical insulation.

6. Composites

• Definition: Materials made by combining two or more different materials to improve properties.
• Examples: Fiberglass, carbon fiber reinforced plastics.
• Uses: Aerospace, automotive, sports equipment.

Optional Subclassification

• Metals: Ferrous (iron-based) / Non-ferrous (copper, aluminum) / Precious / Refractory / Light / Heavy
• Polymers: Thermoplastics / Thermosets / Elastomers / Fibers

Cold Treatment of Alloys

Definition: Cold treatment of alloys is a process of cooling alloys to very low temperatures (cryogenic treatment, usually −80°C to −196°C) after conventional heat treatment to improve mechanical properties such as hardness, wear resistance, and dimensional stability.

Purpose and Objectives

• Convert retained austenite into martensite (in steels)
• Increase hardness and wear resistance
• Relieve internal stresses and improve dimensional stability
• Improve fatigue strength of components

Process

1. Heat-treat the alloy (hardening if required).
2. Slowly cool to cryogenic temperatures.
3. Hold for a specified time.
4. Slowly bring back to room temperature.

Applications and Uses

• Cutting tools (drills, milling cutters, taps), dies and punches, bearings and gears, automotive and aerospace components, measuring instruments.

Advantages

• Increases hardness and wear resistance
• Improves dimensional stability, reduces retained austenite, extends tool life, enhances fatigue resistance.

Disadvantages

• Requires special cryogenic equipment
• More costly than normal heat treatment
• Not suitable for all types of alloys
• Risk of cracking if not properly controlled

Smart Materials

Smart materials are materials that can sense and respond to external stimuli such as stress, temperature, electric or magnetic fields by changing their properties in a controlled way.

Examples: Shape memory alloys (Nitinol), piezoelectric materials.

Applications: Sensors, actuators, robotics, aerospace systems.

Biomaterials

Biomaterials are materials that are designed to interact with biological systems for medical and healthcare applications.

Examples: Titanium implants, biodegradable polymers.

Applications: Artificial joints, dental implants, heart valves.

Nanomaterials

Nanomaterials are materials whose particle size ranges from 1 to 100 nanometers (nm) and exhibit unique properties due to their extremely small size.

Examples: Carbon nanotubes, graphene, nano-silver.

Applications: Electronics, medicine, coatings, energy storage.

Magnetic Properties

Magnetic properties describe how a material responds to an external magnetic field. Based on this behavior, materials are classified as diamagnetic, paramagnetic, and ferromagnetic.

1. Diamagnetic Materials

Definition: Diamagnetic materials are weakly repelled by a magnetic field.

Characteristics: Magnetic susceptibility is small and negative; they do not retain magnetism after removal of the field.

Examples: Copper, silver, gold, bismuth, water.

Applications: Magnetic levitation, non-magnetic components.

2. Paramagnetic Materials

Definition: Paramagnetic materials are weakly attracted by a magnetic field.

Characteristics: Small and positive magnetic susceptibility; magnetism disappears when the field is removed.

Examples: Aluminum, platinum, chromium.

3. Ferromagnetic Materials

Definition: Ferromagnetic materials are strongly attracted by a magnetic field and can be permanently magnetized.

Characteristics: High magnetic susceptibility, exhibit hysteresis, retain magnetism.

Examples: Iron, cobalt, nickel.

Applications: Permanent magnets, transformers, electric motors.

1. Mechanical Properties of Metals

Mechanical properties describe the behavior of metals under applied forces.

• Strength – Ability to withstand applied load without failure.
• Hardness – Resistance to indentation, scratching, or wear.
• Ductility – Ability to be drawn into wires.
• Malleability – Ability to be hammered into thin sheets.
• Toughness – Ability to absorb energy before fracture.
• Elasticity – Ability to regain original shape after removal of load.
• Plasticity – Ability to undergo permanent deformation.
• Brittleness – Tendency to fracture without much deformation.
• Fatigue strength – Resistance to repeated cyclic loads.
• Creep – Time-dependent deformation under constant load at high temperature.

2. Electrical Properties of Metals

Electrical properties describe the ability of metals to conduct electricity.

• High electrical conductivity.
• Low electrical resistivity.
• Temperature dependence – Resistance increases with temperature.
• Free electrons available for conduction.
• Ohmic behavior – Follow Ohm’s law.
• Good current-carrying capacity.
• Suitable for wiring and contacts.

Examples: Copper, aluminum, silver.

3. Thermal Properties of Metals

Thermal properties describe the behavior of metals under heat.

• High thermal conductivity – Heat flows easily.
• Thermal expansion – Expand on heating, contract on cooling.
• High melting point (generally).
• Heat resistance in alloys.
• Specific heat capacity – Amount of heat required to raise temperature.
• Good heat dissipation.
• Thermal stability at high temperatures.