Heat Treatment Process MCQ Quiz in मल्याळम - Objective Question with Answer for Heat Treatment Process - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

Concept:

Case hardening is a method used to harden the outer surface of low-carbon steel while leaving the center 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 method
2. Cyaniding
3. Nitriding
4. Induction Hardening
5. Flame hardening

Cyaniding:

• In this process of surface hardening, both carbon and nitrogen are added to the surface layer of steel (ferrous material, usually low carbon grade).
• The process is based on the decomposition of cyanide compounds that easily release the cyan group (CN). Cyaniding involves heating the steel in a liquid or solid medium.
• The steel is heated in a molten cyanide salt bath maintained at 950°C, followed by water or oil quenching.
• Salt bath compositions may vary according to the temperature of the salt, the thickness of the case to be obtained, type of steel to be heat-treated, and period of operation.
• Case thickness from 0.075 – 1.5 mm can be obtained in the process.

Heat treatment is an operation involving heating and cooling of a metal or alloy so as to obtain certain desirable properties. A few important heat treatment processes are:

1. Annealing
2. Normalizing
3. Hardening
4. Tempering

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:

Normalizing: Heat the steel from 30°C to 50°C above its upper critical temp, held about fifteen minutes and then allowed to cool down in still air. The homogeneous structure provides a higher yield point, ultimate tensile strength and impact strength with lower ductility to steels.

Main objective:

• Refine grain, improve machinability, tensile strength, and structure of weld.
• Remove cold worked stress.
• Remove dislocations due to hot working.

Tempering:

• In tempering, the component is heated to a below critical temperature (low temperature) followed by air cooling produces a relatively small size of grains compared to annealing. Hence brittleness is reduced and little ductility enhances without significant loss of hardness.

Annealing: 

• The component is heated to an above upper critical temperature (high temperature) followed by furnace cooling produces the largest size of grains. Hence achieves the highest ductility due to significant loss of hardness.

Explanation:

Annealing:

• Annealing is heating of steel to austenite temperature and then cooling slowly in the furnace.
• Annealing results in the formation of ferrite, spheroidal cementite and coarse pearlite.
• All the phase constituents are relatively soft and therefore this is known as softening treatment and produces relatively lower hardness values while ductility increases.
• Purpose of annealing:
  • to reduce hardness, improves material ductility, relieve internal stress
  • improves machinability
  • it refines grain size.

Nitriding:

• Nitriding is a case of the hardening process in which nitrogen (Ammonia) 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.

Normalizing: 

• Heat the steel from 30°C to 50°C above its upper critical temp, hold about fifteen minutes, and then allow to cool down in still air. 
• The homogeneous structure provides a higher yield point, ultimate tensile strength, and impact strength with lower ductility to steel.
• The main purpose of normalizing is to refine grain size.
• Main objective:
  1. Refine grain size in metal, improve strength and hardness, reduce ductility
  2. Remove cold work stress.
  3. Remove dislocations due to hot working.

Martempering:

• In this martensite is formed and quenching is done in two medium
• 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.
• The purpose of the second medium is to reduce internal stress associated with the austenite to martensite transformation.
• Used for the heat treatment of C-steel tools like taps, disc, milling cutters, etc. to improve hardness of the whole mass

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:

Explanation:

Annealing:

• It is a heat treatment process which alters the microstructure of a material to change its mechanical or electrical properties.
• Typically, in steels, annealing is used to reduce hardness, increase ductility and help eliminate internal stresses.
• In annealing, Furnace cooling is required.

Normalising:

• Normalizing involves heating steel and keeping it at that temperature for a period of time followed by cooling it in air.
• The resulting microstructure is a mixture of ferrite and cementite which has higher strength and hardness, but lower ductility.

Tempering:

• It involves heating steel that has been quenched and hardened for an adequate period of time so that the metal can be equilibrated.
• The hardness and strength obtained depend upon the temperature at which tempering is carried out.
• Higher temperatures will result in high ductility, low strength and hardness while Low tempering temperatures will produce low ductility, high strength and hardness.

Explanation:

Martensite is formed in carbon steels by the rapid cooling also known as quenching of the austenite at a high rate such that carbon atoms do not have time to diffuse out of the crystal structure to form cementite(Fe₃C).

Austenite is gamma-phase iron (γ-Fe), As a result of quenching, the face-centered cubic austenite transforms into to a highly strained body-centered tetragonal form called martensite that is supersaturated with carbon. The highest hardness of a pearlitic steel is 400 Brinell whereas martensite can be as high as 700 Brinell. In terms of Rockwell hardness it is about RC 65. RC abbreviation means Rockwell Hardness measured on the C scale.

Explanation:

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 method

2. Cyaniding

3. Nitriding

4. Induction Hardening

5. Flame hardening

Heat treatment is an operation involving heating and cooling of a metal or alloy so as to obtain certain desirable properties. A few important heat treatment processes are:

1. Annealing

2. Normalising

3. Hardening

Explanation:

Case hardening is a process that is used to harden the outer layer of steel while maintaining soft inner metalcore.

Carburizing and Hardening

• The steel is heated in contact with a substance that has a high carbon content
• Then quenched rapidly to produce a hardened surface layer or “case” over a softer and tougher core
• Carburising is done on low C steel (between 0.1 and 0.3 wt% C)
• In pack carburising, the steel piece is packed in a steel container and completely surrounded with charcoal

Induction Hardening

• 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

Nitriding

• In nitriding, the steel piece is heated in a furnace between 500 – 600^∘C and at the same time is exposed to ammonia gas (NH₃)

Nitrogen reacts with elements in the steel to form nitrides in the outer layer of the steel providing high hardness and wear resistance