FACTORS AFFECTING MATERIAL PROPERTIES

Haseeb Ullah Khan Jatoi

Department of Chemical Engineering

University of Engineering and Technology, Lahore

Synopsis Factors affecting material properties are;1. Heat treatment2. Processing3. Environmental reactions (Deteriorative) Composition of materials Selection of engineering materials

Heat treatment This is the controlled heating & cooling of

metals to change their properties to improve their performance or to facilitate processing.

For example, take the example of hardening a piece of high carbon steel rod

Steps in heat treatmentThe steps of hardening the high carbon iron are; Heating to red hot Cooling

i. If cooled rapidly by plunging in cold water, it will become hard & brittle (quenching)

ii. If it is allowed to cool slowly, it will be softer & less brittle i.e. more tough (annealing)

Now annealed metal is too soft to machine it to a good surface finish but it is now best for the flow forming

Flow forming is a cold forming method which is used to manufacture dimensionally precise, round & hollow components.

Steps in heat treatment Then to prepare the material for machining, it

must be normalized.NormalizingThe metal is again red heated & then allowed to

be cooled in air. It improves the metal’s machining properties

& strength.

Processing Working metalsMetals are said to be worked when they are

squeezed or stretched or beaten into desired shapes. Metals which have been shaped in this manner are said to be in wrought condition.

Metals can be hot worked or cold worked depending upon the temperature at which it is flow formed to the desired shape.

Cold working When metals are examined under microscope, it

can be seen that they consist of very small grains. When most metals are worked (bent, squeezed,

stretched etc.) at room temperature, the grains are distorted and the metal become hard & brittle. This is called as cold working the metal.

Excessive cold working is avoided as the material may crack.

If considerable working is required to form the particular component, the metal must be softened from time to time during the processing by annealing.

Hot working When metal are worked (bent, squeezed, stretched

etc.) at high temperatures, the crystals are also distorted here.

But as the work temperature is high, the distorted crystals are reformed instantly in the normal crystals and the work hardening does not occur.

The examples are forging & rolling.

Forging is the shaping of metal using localized compressive forces

Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is passed through a pair (or pairs) of rolls.

Hot & cold working Recrystallization the formation of a new grain structure in a deformed material

by the formation and migration of high angle grain boundaries driven by the stored energy of deformation.

Recrystallization requires a minimum temperature for the necessary atomic mechanisms to occur., called recrystallization temperature. This recrystallization temperature decreases with annealing time.

So cold working is the flow forming of metals below the temperature of recrystallization.

And hot working refers to processes where metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation (dynamic). Recrystallization can be ‘static’ (occurring after deformation)

Nucleation and crystal growth

Recrystallization of a metallic material (a → b) and crystal grains growth (b → c → d)

Environmental reactionsThe properties of materials can also be affected

by reaction with the environment in which they are used.

Some examples are;

Dezincification of brassBrass is an alloy of copper & zinc. When brass is

exposed to marine environment for a long time, the salts in the sea water reacts with the zinc content of the brass so as to remove it & leave behind the spongy mass of copper. This obviously weakens the material.

Environmental reactions Rusting of steelThe rust will eat the metals, reduce its

thickness & therefore its strength. Rusting will occur unless the steel structures are regularly maintained by rust neutralization & painting processes.

Degradation of plasticsMany plastics degrade & become weak and

brittle when exposed to the ultraviolet content of sunlight. Special dyestuffs have to be incorporated into the plastic to filter out these harmful rays.

Composition The properties of the materials depend largely

on the composition of the materials.For example Low carbon steel is relatively soft & ductile

with moderate strength & it can not be hardened by heating & quenching.

On the other hand, high carbon steel is less ductile and stronger. It can be hardened by heating & quenching.

Composition For example Brass alloy containing 70% copper & 30% zinc

has very great ductility and can be cold worked into complicated shapes without cracking

On the other hand, brass alloy containing 60% copper & 40% zinc lacks ductility and can not be readily cold worked

Selection of materials Materials are selected on the basis of their

mechanical & physical properties. Let’s consider the basic requirements for

selecting materials that are suitable for a particular application.

Consider the case of1. Cable connector2. Connecting rod of the motor car engine

Selection of materials of cable connectorThere are two materials used in it; Plastic casing Metal joining pieces of the brass

Why plastic casing? i. Because its good electrical insulator.ii. Prevents electric shock if a person touches itiii. Its cheap, tough, easily molded to shapeiv. Also low cost material & readily available

Selection of materials of cable connector

Why Metal joining pieces of the brass?1. Good electrical conductivity2. Ease of extruding to shape3. Ease of machining4. Adequate strength5. Corrosion resistanceThe silver is even better conductor for this

application but It is too expensive Weak, soft and the scarce material.

Connecting rod of motor car engineMade of special steel alloyWhy?1. Because of its strength & toughness2. Readily forged to shape3. Finishing by machining is easy

Reasons for selecting materials Commercial factors

1. Cost2. Availability3. Ease of manufacture

Engineering properties of materials1. Electrical conductivity2. Strength3. Toughness4. Ease of forming by extrusion, forging & casting5. Machinability6. Corrosion resistance