Annealing MCQ Quiz in தமிழ் - Objective Question with Answer for Annealing - இலவச PDF ஐப் பதிவிறக்கவும்

Heat treatment: Heat treatment is the process of heating and cooling a metal in its solid state in order to obtain the desired changes in its physical properties.

Heat treatment or the amount of heat treatment an alloy can undergo essentially depends upon the alloying elements.

Once the metal is heated to certain temperature then it is the cooling rate which decides the properties obtained by the heat treatment, for example in the annealing process the steel it is allowed to cool in the furnace itself, the austenite changes into coarse pearlite, and in the normalising process the steel is allowed to cool in the open air then the austenite changes into fine pearlite.

Lower critical temperature: Steel is heat treated to mainly remove the internal stresses, reduce grain size or increase in toughness. For a steel alloy, the temperature below which, under equilibrium conditions, all austenite has transformed into ferrite and cementite phases. The heat treatment of the steel is done above the lower critical temperature, to convert it into the austenite.

Explanation:

Quenching is the rapid cooling of a work-piece to obtain certain material properties.

If the cooling rate is very high then the entire austenite is converted into martensite because the cooling curve does not enter the pearlite region as shown in figure below.

Explanation:

Process annealing:

• The purpose of process annealing is to remove the defects of cold working. So, that its ductility is restored and it can be again plastically deformed or put to service without any danger of its failure due to fracture.
• It is also known a slow temperature annealing or sub-critical annealing or commercial annealing. The process is extremely useful for mild steel and low carbon steel and is cheaper and quicker than full annealing. Also, less scale is produced during this process.
• The main output of this process is increased ductility and plasticity, improved shock resistance, reduced hardness, improved machinability, and removal of internal stresses. During cold working operations like cold-rolling, wire drawing, a metal gets severely strain-hardened.
• Due to this, the metal is heated to a temperature, generally in the range of 550°C to 650°C, held there for enough time to allow recrystallization of cold-worked metal and thus, softening to take place and then cooled at a slower rate (normally in the air).

Additional InformationFull annealing:

• The main objectives of this type of annealing are to soften the metal, relieve its stresses and refine its grain structure. It is also known as high-temperature annealing.
• In this process complete phase recrystallization takes place and all imperfections of the previous structure are wiped out.
• This involves heating of steel to a temperature about 30° to 50° above the higher critical point for hypereutectoid steels, and by the same amount above the lower critical point for hypereutectoid steels, holding it at that temperature for sufficient time to allow the internal changes to take place and then cooling slowly.
• The steel gets softened by this process, together with an appreciable amount of increase in its ductility and toughness.

Explanation:

Annealing:

• Annealing involves heating the steel to a suitable temperature, holding it at that temperature for some time, and then cooling it slowly.
• There are different methods of cooling.
• The main purpose of Annealing is to reduce the hardness of a material. 
• Besides this, it is also used -
  • To relieve the internal stress of a material
  • To restore ductility to perform the further operation on the material
  • To increase the machinability of the material
  • To induce softness

​There are the following processes in Annealing.

Explanation:

Based on the cooling rate, heat treatment processes can be classified and the cooling rate has the following impact on properties

​Slow cooling: The specimen is cooled slowly to obtain the coarse grain structure and it posses the following properties.

• High ductility and machinability.
• Low strength and hardness.
• Low residual stresses.

Fast cooling: The specimen is cooled at a higher rate to obtain the fine grain structure and it posses the following properties.

• High hardness and strength.
• Low ductility and machinability.
• High residual stresses.

Annealing: It refers to a wide group of heat treatment processes and is performed primarily for homogenization, recrystallization or relief of residual stress in typical cold-worked or welded components. Depending upon the temperature conditions under which it is performed, annealing eliminates chemical or physical non-homogeneity produced by phase transformations.

It is done to

• Reduce hardness/ Increase softness 
• Increase ductility
• Eliminate internal stresses

Important types of annealing are

• Isothermal annealing
• Process annealing.
• Recrystallization annealing
• Partial annealing
• Speroidize annealing.
• Diffusion annealing.
• Full annealing.

Isothermal annealing (cycle annealing):

• The specimen is heated above the upper critical point
• Cool rapidly to a predetermined level, hold there 
• Cool to room temperature.
• Used to achieve a more homogeneous microstructure within the steel 

Process annealing:

• In this process, the specimen is heated to 50°C below the lower critical temperature.
• After heating and holding it is left inside a furnace.
• It is applied to steel to increase the processability by reducing the residual stresses of a cold-worked component.

Recrystallization Annealing:

• It is used as an intermediate step in (cold) deformation processing.
• It consists of heating a steel component below A1 temperature, i.e. at a temperature between 625°C and 675°C (recrystallization temperature range of steel) 
• Holding at this temperature and subsequent cooling.
• This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multistep cold working operations.

Partial Annealing:

• The specimen is heated between upper and lower critical temperatures.
• Slow cooling
• Preferred for high-carbon steels as it results in fine pearlite and martensite instead of grain boundary brittle network of martensite. 
• Reduced chances of cracking during machining

Full annealing: 

• In this process, the specimen is heated to 50°C above the upper critical temperature for hypo-eutectoid steel and 500 below the lower critical temperature for hyper-eutectoid steel.
• After heating and holding the specimen is left inside the furnace to obtain a coarse grain structure.
• It is applied to increase ductility.

Speroidize annealing:

• In this process, the specimen is heated to just below the lower critical temperature.
• After heating and holding it is left inside a furnace.
• The resultant compound obtained is a spherical form of cementite surrounded by pearlite.
• It is applied to steel to increase machinability.

Diffusion annealing:

• In this process, the specimen is heated to diffusion temperature (1100°C) and hold for some time period.
• During holding the carbon atoms move from high concentration region to low concentration region thus generating uniform properties throughout the specimen.
• It is applied for casted specimens for obtaining uniform properties.

Explanation:

Annealing:

• Annealing involves heating the steel to a suitable temperature, holding it at that temperature for some time, and then cooling it slowly.
• There are different methods of cooling.
• The main purpose of Annealing is to reduce the hardness of a material. 
• Besides this, it is also used -
  • To relieve the internal stress of a material
  • To restore ductility to perform the further operation on the material
  • To increase the machinability of the material
  • To induce softness

​There are the following processes in Annealing.

Additional Information

EXPLANATION:-

Heat treatment processes - Heat treatment is defined as an operation involving the heating and cooling of a metal or an alloy in the solid state to obtain certain desirable properties without changing composition. 

The process of heat treatment is carried out to change the grain size, modify the structure of the material, and relieve the stresses set up in the material after hot or cold work.

Heat treatment consists of heating the metal near or above its critical temperature, held for a particular time at that temperature, and then finally cooling the metal in some medium which may be air, water, brine, or molten salts. 

Types of heat treatment processes:- 

1. Annealing - It is a softening process in which there is the heating of metal from 30^o - 50^o C above the upper critical temperature and cooling it at a very slow rate by seeking it in the furnace. The main aim of annealing is to make steel more ductile and malleable and to remove internal stresses. 
2. Normalizing - The main aim of normalizing is to remove the internal stresses developed after the cold working process. In this, steel is heated 30-50^o C above its upper critical temperature and cooling it in the air.  
3. Hardening - The main aim of the hardening process is to make steel hard and tough. In this process, steel is heated 30^o - 40^o C above the upper critical temperature and then followed by continuous cooling to room temperature by quenching in water or oil. It is the opposite process of annealing.
4. Tempering - The steel after being quenched in the hardening process is reheated to a temperature slightly above the temperature range at which it is to be used, but below the lower critical temperature. The temperature here varies from 100^oC to 700^oC. The reheating is done in a bath of oil or molten lead or molten salt. The specimen is held in the bath for a period of time till attains the temperature evenly, the time depends on the composition and desired quality of steel. Now the specimen is removed from the bath and allowed to cool slowly in still air.

Quenching - Quenching is the soaking of a metal at a high temperature, above the recrystallization phase, followed by a rapid cooling process to obtain certain desirable material properties. 

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:

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.

Explanation:

Spherodising or Spherodise annealing:

• This process is applied to high carbon steels which are difficult to machined. It causes the formation of all carbides of steel in the form of very small globules or spheroids like sphere thus increases the machineability.
• This process consists of heating the steel slightly above the lower critical temperature (730 - 770°C), holding at this temperature and then cooling slowly to 600°C. The rate of cooling in the furnace is 25 - 30°C/hr.  
• Varieties of heat treatment temperature an be used to produce a spheroidized temperature but all of them are relatively lengthy and costly.
• Another method is to use a high temperature isothermal transformation of Austenite.

Important Points

From the above given options, 680°C is near to 730°C, so option 4 is correct answer