Hot Working and Cold Working MCQ Quiz in తెలుగు - Objective Question with Answer for Hot Working and Cold Working - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Explanation:

Ductility: A metal is said to be ductile when it can be drawn out into wires under tension without rupture. Wire drawing depends upon the ductility of a metal. A ductile metal must be both strong and plastic. Copper and aluminum are good examples of ductile metals.

Cold working or Cold Forming:

• Cold working is the plastic deformation process where metal is processed below the re-crystallization temperature.
• Most of the time cold forming is done at room temperature.
• The major cold-working operations can be classified basically as rolling, squeezing, bending, shearing, and drawing.

Advantages: 

• No heating required.
• A better surface finishes obtained.
• Superior dimension control.
• Better reproducibility and interchangeability of parts.
• Improved strength properties like fatigue strength and tensile strength.

Disadvantages: 

• Higher forces required for deformation
• Heavier and more powerful equipment required
• Less ductility available
• Metal surfaces must be clean and scale-free
• Strain hardening occurs

Materials used for cold working

• Low and medium carbon steels, low alloy steels, copper and light alloys, Titanium and beryllium

Additional Information

Hot rolling:

• Hot-working is the metal processing that is done above re-crystallization temperatures of metal.
• For the manufacturing of railways tracks stainless steel is used & it is manufactured using on I-section hot rolling as shown below.
• In the process of hot-working, plasticity of rail components increase and its tendency to fracture decrease.

Piercing: 

It is a process by which a hole is cut (or torn) in metal. It is different from punching in that piercing does not generate a slug.

In hole cutting operation, the metal cut out is the scrap. Hence any shear provided must be ground on the punch.

The main purpose of shear is to reduce the load placed on the punches to accomplish the piercing.

Piercing operation:

• Punch size = blank size
• Die size = blank size + 2 x clearance

So in piercing punch is of the exact size, clearance is provided on a die, and shear is provided on the punch.

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

Concept:

Shot peening 

• It is a cold work process used to finish metal parts to prevent fatigue and stress corrosion failures and prolong product life for the part.
• In shot peening, a small spherical shot bombards the surface of the part to be finished.
• The shot acts like a peen hammer, dimpling the surface and causing compression stresses under the dimple.
• The surface compression stress strengthens the metal, ensuring that the finished part will resist fatigue failures, corrosion fatigue and cracking, and galling and erosion from cavitation.

Theory:

• Annealing is a heat treating process used to modify the properties of cold-worked metal.
• To obtain large deformations by the cold working intermediate annealing is required to relieve the stresses set up in the metal.
• Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing.
• Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases.
• There comes a point when additional cold working is not possible without causing the metal to crack.
• At this point, it is necessary to anneal the metal if continued cold-working is required.
   

Explanation:

Recrystallization Temperature:

• It is defined as the temperature at which the formation of new grains starts appearing in the microstructure.
• It is also defined as the temperature at which materials are just transformed from solid to liquid.
• At this temperature there is a sliding takes place between the grains inside the materials.
• It is generally 0.3 to 0.7 times of melting point temperature of base materials.

Hot-working:

if the metal forming process is done above the recrystallization temperature then it is called hot working.

Cold working:

if the metal forming process is done below the recrystallization temperature then it is called cold working.

Explanation:

• The mechanical working of metal is defined as an intentional deformation of metals plastically under the action of externally applied force.
• Based on the temperature level of working, they are classifieds into two types:
  1. Hot-working
  2. Cold working

Hot-working:

• The working of metal above recrystallization temperature is called hot-working e.g. hot rolling, hot drawing, hot spinning, hot extrusion, hot forging, etc.
• Advantages:
  • the porosity of metal largely eliminated.
  • the grain structure of the metal is refined.
  • mechanical properties such as toughness, ductility improved.
  • The deformation of metal is easy.

Cold-working:

• The working of metal below recrystallization temperature is known as cold working e.g. cold rolling, cold drawing, cold spinning, cold bending, cold extrusion, squeezing and shot peening, etc.
• Advantages:
  • strength and hardness of metal are increased.
  • residual stress is set-up in the metal.
  • surface finish improved.
  • close dimensional tolerance maintained.

Concept:

Recrystallization Temperature:

• It is defined as the temperature at which the formation of new grains starts appearing in the microstructure.
• It is also defined as the temperature at which materials are just transformed from solid to liquid.
• At this temperature there is sliding takes place between the grains inside the materials.

Hot-working:

if the metal forming process is done above the recrystallization temperature then it is called hot working.

It is generally 0.5 to 0.7 times of melting point temperature of base metal.

Cold working:

if the metal forming process is done below the recrystallization temperature then it is called cold working.

It is generally 0.2 to 0.3 times of melting point temperature of base metal.

Explanation:

Advantages of cold working:

• Better Accuracy
• Closer Tolerances
• Strain hardening increases strength and hardness
• Grain flow during deformation can cause desirable directional properties in the product
• No heating of work required (less total energy)

Explanation:

• Strain hardening or work hardening is the strengthening of a metal by plastic deformation.
• This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material. 
• Due to strain hardening, yield strength increases and ductility decreases.

Explanation:

Cold heading is a manufacturing process, where the metal wire is shaped, by being forced into a punch and die set, without loss of material. It is known as the cold heading because the metal forming process is done at room temperature and no heat is applied to the metal.

Advantages:

• As there is no cutting required, the cold heading is chipless machining. Therefore, no scrap and no loss of material.
• Production is fast with no process required for cooling and creates pieces with closer tolerances without secondary operations.

Disadvantages:

• The machinery required is bigger and more expensive.
• More complex pieces will require secondary operations because cold heading machines cannot perform operations like rolling, stamping, or bending.

Applications:

Simple fasteners like screws and bolts and other complex fasteners. This includes fasteners used in bridges, automotive manufacturing, electronics, and specialty bolts.

Concept:

Shot peening 

• It is a cold work process used to finish metal parts to prevent fatigue and stress corrosion failures and prolong product life for the part.
• In shot peening, a small spherical shot bombards the surface of the part to be finished.
• The shot acts like a peen hammer, dimpling the surface and causing compression stresses under the dimple.
• The surface compression stress strengthens the metal, ensuring that the finished part will resist fatigue failures, corrosion fatigue and cracking, and galling and erosion from cavitation.

Explanation:

Recrystallisation Processes:

Recrystallization is a purification technique for solid compounds. Recrystallization is a process by which deformed grains are replaced by a new set of defect-free grains that nucleate and grow until the original grains have been entirely consumed and  the crystal lattice structure of the metal becomes reoriented. Consequently, the metal becomes more workable and ductile.

Recrystallisation temperature:

The recrystallization temperature is temperature at which recrystallization can occur for a given material and processing conditions. This is not a set temperature and is dependent upon various factors like purity, grain size, cold worked and heating rate etc. Some standard outcomes are given below.

• Increasing annealing time decreases recrystallization temperature
• Alloys have higher recrystallization temperatures than pure metals
• Increasing amount of cold work decreases recrystallization temperature
• Smaller cold-worked grain sizes decrease the recrystallization temperature

Note-The recrystallisation temperature for steels is typically between 400 and 700 °C.

Hot working:

If the metalworking process is carried out above its re-crystallization temperature, it is called as hot working. 

• In hot working process metals are plastically deformed above their recrystallization temperature. 
• Hot working improves the engineering properties of the workpiece because it replaces the microstructure with one that has fine spherical shaped grains. These grains increase the strength, ductility, and toughness of the material.

Examples: Rolling, Extrusion, drawing, hot forging etc

Cold Working:

If the metalworking process is carried out below its re-crystallization temperature, it is called as cold working.

• In cold working process metals are plastically deformed below their recrystallization temperature.
• In cold working hardness, yield strength increases but ductility decreases.

Examples: Bending, shearing, squeezing etc