Engineering Metals

By virtue of the iron content, engineering metals are broadly grouped into two kinds:

  1. Ferrous Materials: Materials in which iron (Fe) is the principal constituent.
  2. Non-Ferrous Materials: Materials that do not contain significant iron.

If materials are hard to form, they are fabricated by casting and are called cast alloys. If deformable, they are known as wrought alloys.

1 Ferrous Materials

Ferrous materials are produced in larger quantities than any other metallic material. Their main disadvantages are:

The main alloying element is carbon (C). Based on carbon content:

1.1 Plain Carbon Steels

Classified by carbon content:

1. Low Carbon Steels (< 0.3% C)

Also called mild steels. Not heat treatable but strengthened by cold working. Microstructure: ferrite + pearlite. Ductile, tough, easily machinable and weldable. Applications: bolts, nuts, sheets, wire fences, auto body sheets, fan blades.

2. Medium Carbon Steels (0.3%–0.6% C)

Stronger than low-carbon steels. Heat treatable, good machinability. Applications: railroad equipment, axles, chisels, punches, cutting tools, gears, turbine buckets, clutch discs.

3. High Carbon Steels (> 0.6% C)

Strongest and hardest among plain carbon steels. Limited ductility. Heat treated, wear resistant, holds sharp edges. Applications: hand tools, cutting tools, files, springs, etc.

1.2 Alloy Steels

Steels containing alloying elements besides carbon. Types:

1. Free Cutting Steels

Good machinability due to sulphur (0.08–0.35%) and lead. MnS inclusions aid chip breakage and lubrication. Mechanical properties increase with carbon (0.15–0.3%).

2. Low Alloy Steels

Microstructure: ferrite + martensite/austenite. Alloy content < 8%, carbon up to 0.3%. Applications: automobile bodies, bridges, buildings.

3. Stainless Steels

Form a protective chromium oxide layer. Requires 10–20% Cr. Higher carbon reduces corrosion resistance (due to Cr carbide formation). Caution: promotes galvanic corrosion.

4. High Speed Steels (HSS)

Used in high-speed cutting tools. Two types:

May be coated with TiN or TiC. Typical composition: W ≈18%, Cr ≈4%, V ≈1%.

5. Invar

Nickel-iron alloy with 36% Ni. Very low thermal expansion up to 230°C. Ductile, weldable, stress corrosion resistant. Commonly used in bimetallic strips with brass or nickel alloy.