Engineering Materials Science Test 2

Explanation:

Marquenching or Stepped Quenching

• Articles are first quenched in water to a temperature of 300-400°c and then quickly transferred to a less intensive medium like oil or air where they are held until they are completely cooled.
• This method is widely used in the heat treatment of steel tools like taps, disc, milling cutter, etc.
• A larger holding time will cause austenite decomposition. Austenite is transformed into martensite during the subsequent period of cooling to the room temperature.

Cold Working:

• Plastic deformation of metals below the recrystallization temperature is known as cold working
• It is generally performed at room temperature
• In some cases, slightly elevated temperatures may be used to provide increased ductility and reduced strength
   

Hot Working:

• Plastic deformation of metal carried out at a temperature above the recrystallization temperature is called hot working
• Under the action of heat and force, when the atoms of metal reach a certain higher energy level, the new crystals start forming; This is called recrystallization
• When this happens, the old grain structure deformed by previously carried out mechanical working no longer exist, instead new crystals which are strain-free are formed

Warm Working:

• Metal deformation carried out at temperatures intermediate to hot and cold forming is called Warm Forming

Explanation:

Heat treatment

It is defined as an operation of heating and cooling of metals in solid-state to induce certain desired properties into them.

Various types of heat treatment processes are:

Normalising

• The specimen is heated beyond upper critical temperature and is cooled in still air.
• The structure will now become fine equiaxial pearlite.
• It is used to restore the ductility of cold and hot worked material without appreciable alteration in their properties.

Annealing

• The specimen is heated beyond upper critical temperature and held it there for some time and then cooled slowly in furnace.
• It is used to refine grain size due to phase recrystallisation and produce uniformity.
• After Annealing structure became large-grained pearlite.
• We will be able to improve the properties of cast and forged steels before machining.

Spherodizing

• The specimen is heated slightly above the lower critical temperature and held at this temperature for some time and then cooled slowly in the furnace.
• It produces a round and globular form of carbides (cementite).
• It is mainly used for low carbon steels to improve its strength.
• High carbon steels can also be spherodized to have improved machinability during cutting operations.
• By using this process, abrasive resistance is also improved and the material can be easily cold worked.

Tempering

• The specimen is reheated to temperature below lower critical temperature followed by any desired rate of cooling.
• In this process, martensite is produced in which iron carbide will be present in the matrix of ferrite.

Case hardening:

• The surface of the steel will be made hard and the core remains soft and tough.
• The structure also becomes wear-resistant.

Carburising:

In this operation, the steel is heated to a suitable temperature in a carbonaceous atmosphere and kept at that temperature until the carbon has penetrated to the depth required.

Nitriding:

In the nitriding process, the surface is enriched not with carbon, but with nitrogen. There are two systems in common use, gas nitriding and salt bath nitriding.

Carburising is the process of saturating the surface layer of low carbon steels at 850° - 950°C with carbon from a carbonaceous source capable of giving up its carbon to the metal.

There are three methods of carburising:

• Solid or pack carburising
• Gas carburising
• Liquid carburising

Case hardening is a method used to harden the outer surface of low-carbon steel while leaving the centre or core soft and ductile. Case hardening involves heating the metal to its critical temperature in some carbonaceous material. The following methods are commonly used:

1. Pack hardening

2. Cyaniding

3. Nitriding

4. Induction hardening

5. Flame hardening

Induction Hardening:

• Induction Hardening is a production method of surface-hardening in which the part to be surface-hardened is placed within an inductor coil through which a high-frequency current is passed.
• Selective areas of the component heated by induction heating to the desired depth
• Quenched after the induction heating

Flame Hardening:

• Selected areas heated by an oxy-acetylene flame
• It is a type of surface hardening process

Nitriding:

It is a case hardening process in which nitrogen instead of carbon is added to the surface of the steel. Nitriding heats the steel part to 482-621°C (900-1,150°F) in an atmosphere of ammonia gas and dissociated ammonia. The time the part spends in this environment dictates the depth of the case. No quenching is done after nitriding.

Cyaniding:

• It also called liquid carbonitriding, involves the addition of carbon and nitrogen.
• This is achieved by heating the steel in contact with a molten bath of cyanide.
• The cyanide bath may consist of sodium cyanide, potassium cyanide with inert salts such as sodium chloride and sodium carbonate which provide the necessary fluidity to the cyanide bath.

Read the Quetion carefully it has been asked about the processes which are NOT used for surface hardening.