Factors affecting material properties

1. Factors affecting material properties HaseebUllah Khan Jatoi Department of Chemical Engineering University of Engineering and Technology, Lahore

2. Synopsis • Factors affecting material properties are; • Heat treatment • Processing • Environmental reactions (Deteriorative) • Composition of materials • Selection of engineering materials

3. 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

4. Steps in heat treatment The steps of hardening the high carbon iron are; • Heating to red hot • Cooling • If cooled rapidly by plunging in cold water, it will become hard & brittle (quenching) • 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.

6. Steps in heat treatment • Then to prepare the material for machining, it must be normalized. Normalizing The metal is again red heated & then allowed to be cooled in air. • It improves the metal’s machining properties & strength.

7. Processing • Working metals Metals 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.

8. 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.

10. 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.

14. 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 temperaturedecreases with annealing time. • So cold workingis the flow forming of metals below the temperature of recrystallization. • And hot workingrefers 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)

15. Nucleation and crystal growth • Recrystallization of a metallic material (a → b) and crystal grains growth (b → c → d)

16. Environmental reactions The properties of materials can also be affected by reaction with the environment in which they are used. Some examples are; • Dezincification of brass Brass 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.

18. Environmental reactions • Rusting of steel The 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 plastics Many 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.

20. 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.

21. 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

22. 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 of • Cable connector • Connecting rod of the motor car engine

24. Selection of materials of cable connector There are two materials used in it; • Plastic casing • Metal joining pieces of the brass Why plastic casing? • Because its good electrical insulator. • Prevents electric shock if a person touches it • Its cheap, tough, easily molded to shape • Also low cost material & readily available

25. Selection of materials of cable connector Why Metal joining pieces of the brass? • Good electrical conductivity • Ease of extruding to shape • Ease of machining • Adequate strength • Corrosion resistance The silver is even better conductor for this application but • It is too expensive • Weak, soft and the scarce material.

27. Connecting rod of motor car engine Made of special steel alloy Why? • Because of its strength & toughness • Readily forged to shape • Finishing by machining is easy

28. Reasons for selecting materials • Commercial factors • Cost • Availability • Ease of manufacture • Engineering properties of materials • Electrical conductivity • Strength • Toughness • Ease of forming by extrusion, forging & casting • Machinability • Corrosion resistance