Classification of Engineering Materials

Classification of Engineering Materials

Introduction

• Materials are an important aspect of engineering design and analysis. 
• The importance of materials science and engineering can be noted from the fact that historical ages have been named after materials.
• There is a wide variety of materials available which have shown their potential in various engineering fields ranging from aerospace to house hold applications.
• The materials are usually selected after considering their characteristics, specific application areas, advantages and  limitations.

Metals & Alloys

Metals

• Polycrystalline consisting of a great number of fine crystals 
• Possess low strength 
•  Do not have the required properties

Alloys

• are produced by melting or sintering two or more metals or metal and a non-metal together

"Faceted glass objects are sometimes called crystals."

Materials used in the design and manufacture of products -

1.  Plastics 
2.  Wood 
3.  Composites
4.  Ceramics
5.  Metals 
6.  Fabrics

Classification

Classification of Metallic Materials 

Classification of Steels

Ferrous Material - Steel

Steel - alloy of iron-carbon.

      - May contain other alloying elements.

Low Alloy (less than 10 wt% C)

- Low Carbon (less than 0.25 wt% C)
- Medium Carbon (0.25 to 0.60 wt% C)
- High Carbon (0.6 to 1.4 wt% C)

High Alloy 

- Stainless Steel (greater than 11 wt% Cr)
- Tool Steel

Low Carbon Steel

• Plain carbon steels - very low content of alloying elements and small amounts of Mn.
• Most abundant grade of steel is low carbon steel – greatest quantity produced; least expensive.
• Not responsive to heat treatment; cold working needed to improve the strength.
• Good Weldability and machinability 
• High Strength, Low Alloy (HSLA) steels - alloying elements (like Cu, V, Ni and Mo) up to 10 wt %; have higher strengths and may be heat treated.

Medium Carbon Steel

• Carbon content in the range of 0.25 – 0.6%. 
• Can be heat treated - austenitizing, quenching and then tempering. 
• Most often used in tempered condition – tempered martensite. 
• Medium carbon steels have low hardenability.
• Addition of Cr, Ni, Mo improves the heat treating capacity. 
• Heat treated alloys are stronger but have lower ductility. 
• Typical applications – Railway wheels and tracks, gears, crankshafts.

High Carbon Steel

• Carbon content in the range of 0.6 - 1.4%.
• High C content provides high hardness and strength. Hardest and least ductile.
• Used in hardened and tempered condition.
• Strong carbide formers like Cr, V, W are added as alloying elements to form carbides of these metals.
• Used as tool and die steels owing to the high hardness and wear resistance property. 

Stainless Steel

• A group of steels that contain at least 11% Cr. Exhibits extraordinary corrosion resistance due to formation of a very thin layer of Cr2O3 on the surface.

Categories of Stainless Steel -

1. Ferritic Stainless Steel
2. Martensitic Stainless Steel
3. Austenitic Stainless Steel
4. Precipitation-Hardening (PH) Stainless Steel
5. Duplex Stainless Steel

Main types of Iron

1.  Pig Iron
2.  Cast Iron
3.  Wrought Iron

Cast Iron

1. White Cast Iron
2. Gray Cast Iron
3. Malleable Cast Iron
4. Ductile Cast Iron
5. Meehanite Cast Iron
6. Alloy Cast Iron

Pig Iron

• Pig iron acts as the raw material for production of all kinds of cast iron and steel products. 
• It is obtained by smelting (chemical reduction of iron ore in the blast furnace. 
• It is of great importance in the foundry and in steel making processes. 
• The charge in the blast furnace for manufacturing pig iron is :-

(a) Ore - Consisting of iron oxide or carbonate associated with earth impurities.
(b) Coke - A fuel
(c) Limestone - A flux

Approximate Composition of Pig Iron

Carbon — 4 to 4.5%
Phosphorus — 0.1 to 2.0%
Silicon — 0.4 to 2.0%
Sulphur — 0.4 to 1.0%
Manganese — 0.2 to 1.5 %
Iron — Remainder

Cast Iron

• alloy of iron and carbon 
• Obtained by re-melting pig iron with coke, limestone and steel scrap in a furnace known as cupola. 
• The carbon content in cast iron varies from 1.7% to 6.67%.
• Wide range of applications (including pipes, machine and car parts, such as cylinder heads, blocks and gearbox cases) due to: 
§ low melting point,
§ good fluidity,
§ relatively easy to cast,
§ excellent machinability,
§ resistance to deformation
§ wear resistance
• Cast iron tends to be brittle, except for malleable cast irons, so shaping these by deformation is very difficult.

Grey cast iron (grey in color)
It contains:
C = 2.5 to 3.8%.
Si = 1.1 to 2.8 %
Mn = 0.4 to 1.0%
P = less than 0.15%
S = less than 0.1%
Fe = Remaining

White cast iron (White in color) 
C = 3.2 to 3.6%
Si = 0.4 to 1.1 %
Mg = 0.1 to 0.4%
P = less than 0.3%
S = less than 0.2%
Fe = Remaining

Ductile cast iron
Carbon = 3.2 to 4.2%
Silicon = 1.0 to 4.0 %
Magnesium = 0.1 to 0.8%
Nickel = 0.0 to 3.5%
Manganese = 0.5 to 0.1%
Iron = Remaining

Grey Cast Iron

• Grey cast iron is named after its grey fractured surface that occurs when the graphitic flakes deflect a passing crack and initiate many new cracks as the material breaks. 
• Graphite flakes surrounded by a-ferrite or pearlite matrix 
• Weak & brittle in tension (the graphite flake tips are sharp; act as stress raisers) 
• Stronger in compression 
• Excellent vibrational dampening 
• Wear resistant

Applications of Grey Cast Iron - 

1. Machine tool structures such as bed, frames, column etc. 
2. Household appliances etc. 
3. Gas or water pipes for under ground purposes. 
4. Rolling mill and general machinery parts.
5. Cylinder blocks and heads for I.C. engines. 
6. Frames of electric motor. 
7. General machinery parts.

White Cast Iron

• White cast iron is named after its white surface when fractured due to its carbide impurities that allow cracks to pass straight through; the crystalline fractures are shiny compared to the dull gray fractures of graphite irons. 
• < 1 wt% Si, rapid cooling rates 
• pearlite + most of the carbon forms cementite, not graphite. 
• very hard and brittle; 
• thickness may result in nonuniform microstructure from variable cooling; white iron develops from faster cooling; slower cooling rate yields grey iron.
• limited applications; used as intermediate to produce malleable cast iron.

Applications of White Cast Iron -

1. For producing malleable iron castings. 
2. For manufacturing those component or parts which require a hard, and abrasion resistant surface such as rim of car. 
3. Railway brake blocks.

Nodular (Ductile) Cast Iron

• Adding Mg and/or Cerium to grey iron before casting produces a distinctly different microstructure and mechanical properties. 
• graphite forms nodules not flakes 
• Normally a pearlite matrix 
• Photo (nodular) shows ferrite matrix that was heat treated for several hours at 700˚C. 
• Castings are stronger and much more ductile than grey iron.

Malleable Cast Iron

• Malleable cast iron formed by heat treating white iron at 800-900ºC for a prolonged period causes decomposition of cementite into graphite. 
• Graphite forms clusters or rosettes that are surrounded by a ferrite or pearlite matrix. 
• Reasonably strong and ductile (malleable)
• Carbon content: 2.3 – 2.7 wt% 
• Silicon content: 1.0 – 1.75 wt %

Applications for Malleable Cast Iron -

• Automobile parts
• Agriculture implementation
• Hinges
• Door keys
• Cranks,
• Waned components of sewing machines
• Textiles machine parts.

Classification of Engineering Materials Part 2
Classification of Engineering Materials Part 3