Heat Treatment Process MCQ Quiz - Objective Question with Answer for Heat Treatment Process - Download Free PDF

Explanation:

Normalising:

• Normalising is a heat treatment process used to refine the grain structure of steel and to make its composition uniform. This process involves heating the steel to a temperature above its critical point, usually between 750°C and 950°C (depending on the type of steel), and then allowing it to cool in air. The purpose of normalising is to remove internal stresses, enhance mechanical properties, and improve machinability.
• In the normalising process, steel is heated to a temperature that is typically 30-50°C above its upper critical temperature. The steel is held at this temperature for a period sufficient to allow the transformation to austenite. After heating, the steel is removed from the furnace and allowed to cool in still air. This controlled cooling results in a fine pearlitic structure, which is more uniform compared to the structures obtained by other heat treatment processes.
• Stainless steel is typically not normalised because it contains alloying elements such as chromium, nickel, and molybdenum that provide corrosion resistance and other specific properties. These alloying elements can respond differently to heat treatment processes, and normalising may not be suitable for achieving the desired properties in stainless steel. Instead, stainless steel is often subjected to other heat treatment processes such as annealing or solution treating, which are more appropriate for its unique composition.

Advantages:

• Improves mechanical properties such as toughness and hardness.
• Refines grain structure, making the steel more ductile and machinable.
• Reduces residual stresses that may have been induced during previous processing.
• Enhances the uniformity of the steel's microstructure.

Disadvantages:

• May not be suitable for all types of steel, especially those with specific alloying elements that require different heat treatment processes.
• Requires precise control of heating and cooling rates to achieve the desired properties.

Applications: Normalising is commonly used in the production of structural steel components, automotive parts, and other applications where uniform mechanical properties are critical. It is also used to prepare steel for further heat treatment processes.

Concept:

Full annealing:

• Full annealing is a heat treatment process applied mainly to steels to soften the material, improve ductility, and relieve internal stresses.

Steps in Full Annealing:

1. The metal is heated to a temperature above its upper critical point.
2. It is held at that temperature for a specific time to allow transformation.
3. It is then slowly cooled inside the furnace to room temperature.

Cooling Method:

• Slow cooling inside a furnace ensures uniform and gradual cooling, allowing the structure to reform into a soft and ductile state.

Explanation:

Annealing

• Annealing is a heat treatment process primarily used to relieve internal stresses within materials, improve ductility, and reduce hardness.
• This process involves heating the material to a specified temperature, maintaining that temperature for a certain period, and then allowing it to cool slowly, usually in a furnace.
• The main goal of annealing is to alter the physical and sometimes chemical properties of the material to make it more workable and to reduce internal stresses that may have been introduced during prior processing.

Working Principle:
The annealing process typically involves three main stages:

• Heating: The material is slowly heated to a temperature where recrystallization can occur. This temperature varies depending on the material but is usually above its recrystallization temperature.
• Soaking: The material is held at this high temperature for a period, allowing the internal stresses to be relieved and the grain structure to become more uniform.
• Cooling: The material is then allowed to cool slowly, often within the furnace itself. This slow cooling is crucial as it helps prevent the reintroduction of stresses and ensures the material maintains its improved properties.

Advantages:

• Relieves internal stresses induced by processes like welding, machining, or cold working.
• Improves ductility and toughness, making the material easier to work with.
• Reduces hardness and brittleness, which can be beneficial for further processing.
• Enhances the homogeneity of the material by refining its grain structure.

Disadvantages:

• Requires precise control over temperature and cooling rates to achieve the desired properties.
• Can be time-consuming due to the slow heating and cooling stages.

Applications: Annealing is widely used in various industries, including:

• Metallurgy: To soften metals and make them more ductile for further processing.
• Manufacturing: To relieve stresses in components that have undergone significant machining or forming operations.
• Glassmaking: To remove internal stresses from glass products and improve their durability.

Additional InformationTempering:

• Tempering is a heat treatment process that follows quenching. It involves heating a quenched material to a temperature below its critical point, holding it at that temperature, and then cooling it. The primary purpose of tempering is to reduce the brittleness and increase the toughness of the material, rather than to relieve internal stresses. While tempering does help in reducing some internal stresses, its main goal is to achieve a balance between hardness and toughness.

Normalizing:

• Normalizing is a heat treatment process where the material is heated to a temperature above its critical range and then allowed to cool in air. This process refines the grain structure and improves the mechanical properties of the material. While normalizing can help in relieving some internal stresses, its main purpose is to enhance the uniformity of the material’s microstructure and mechanical properties.

Quenching:

• Quenching is a heat treatment process that involves rapidly cooling a material from a high temperature, usually by immersing it in water, oil, or another cooling medium. The primary purpose of quenching is to increase the hardness and strength of the material. However, quenching can introduce significant internal stresses due to the rapid cooling, which often requires subsequent processes like tempering to relieve these stresses.

Annealing is a heat treatment process used primarily to alter the physical and sometimes chemical properties of a material, typically metal, to increase its ductility and reduce its hardness, making it more workable. The objectives of annealing generally include:

Removing internal stresses: These stresses can be induced during processes such as casting, welding, or deformation. Annealing helps to relieve these stresses, preventing potential failure or deformation in service.

Refining grain size: Annealing can help in the formation of a more uniform and finer grain structure, which can enhance the material's mechanical properties, including toughness and strength.

Improving machinability: Annealing reduces hardness and increases ductility, making the material easier to cut, shape, or machine.

Concept:

There are five heat treatment processes.

1. Annealing 
2. Normalizing   
3. Hardening   
4. Tempering 
5. Case hardening

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. Carburizing
2. Cyaniding
3. Nitriding
4. Induction Hardening
5. Flame hardening

Nitriding:

• In the nitriding process, the surface is enriched not with carbon, but with nitrogen.
• It consists of heating the part to a temperature of 480° to 650°C inside a chamber through which a stream of NH3 is passed.

           2NH3 = 2N + 3H2

• The atomic nitrogen so formed diffused into α – iron and saturates the metal.
• The nitriding parts acquire a very high surface hardness (730 to 1100 BHN). Nitriding increases the wear resistance, fatigue limit, and corrosion resistance in air, water, and water vapor.
• The parts are heat treated for a period of 40 to 100 hours.

​​Carburizing:

It is one of the most widely used surface hardening processes. The process involves diffusing carbon into low carbon steel to form a high carbon steel surface.

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.

Hardness level: Nitriding > Cyaniding > Carburizing 

Explanation:

In surface hardness treatment, Nitriding gives the hardest surface. The general hardness number for a different process is given below.

Carburizing – 800 VHN

Cyaniding – 900 VHN

Nitriding – 1000 VHN

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

Nitriding:

• In the nitriding process, the surface is enriched not with carbon, but with nitrogen.
• It consists of heating the part to a temperature of 480° to 650°C inside a chamber through which a stream of NH3 is passed.
• 2 NH3 = 2N + 3H2
• The atomic nitrogen so formed diffused into α – iron and saturates the metal.
• The nitriding parts acquire a very high surface hardness (730 to 1100 BHN). Nitriding increases the wear resistance, fatigue limit and corrosion resistance in air, water and water vapour.

Concept:

Heat treatment:

• Heat treatment operation can be defined as heating a metal or alloy to various definite temperatures, holding these for the various time durations and cooling at various rates to get the desired grain structure and property in the metal or alloy.

Explanation:

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

Carburizing:

• Carburizing is one of the most widely used surface hardening processes.
• The process involves diffusing carbon into low carbon steel to form a high carbon steel surface.
• The carburizing process is also referred to as case hardening or case carburizing process.
• It is a heat treatment process that produces a surface that is resistant to wear while maintaining the toughness and strength of the core.
• High Carbon Steels have higher strength and hardness.
• Low carbon steels have higher toughness.
• Increasing carbon only on the surface of low carbon steel can give a hard surface and tough core.

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.

Nitriding: 

• It is a type of case hardening process to enrich the surface of steel with interstitial nitrogen at elevated temperatures. The reaction of nitrogen with steel causes the formation of Nitrides which is very hard.
• Nitriding temperature: 500°C – 600°C
• Holding time: 20 – 100 hours.
• Applications: Gears, Fuel injection pump parts, Valves, Pump boring tools, etc.

Flame Hardening:

• The surface is heated with an oxy-acetylene torch, then quenched with water spray or other quenching media.
• It is a type of surface/case hardening used for improving resistance to surface indentation, fatigue, and wear.
• There is practically no distortion of the workpiece because only small sections of a workpiece are heated.
• The work surface remains clean as the heating rate is very high 2400°C - 3300°C.
• The process is more efficient and economical for large work as compared to induction heating.
• Example: Gear and sprocket, lathe beds, machine tool guideways, crankshaft, piston rod, etc.

Induction hardening:

• The process is employed to increase the hardness, wear resistance, and endurance limit of the surface of the workpiece.
• The surface is heated to the austenite range and then quenched immediately to form martensite where the structure of the core remains unchanged.
• The workpiece should contain 0.4-0.5 % Carbon or alloying elements as Chromium, Ni, and Mo.
• The work is placed in copper induction coils and heated by high-frequency AC Current
  It is a type of surface/case hardening.
• Disadvantage: Each workpiece requires different fixtures for its holding.
• Mostly used in industry for hardening surfaces of a camshaft, gear, sprocket, crankshaft, piston rod, lathe bed, etc.

∴ Carburizing, Cyaniding, Nitriding are methods that result in a change in the composition of a steel component.

Concept:

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

Confusion point:

Normalising:

• In normalizing, the cooling rate is faster. The steel is allowed to cool in the air rather than furnace cooling which is very slow.
• This results in fine pearlite, higher strength, and hardness, but lower ductility than the full anneal treatment.
• Cold worked parts are often annealed to reduce the effects of strain hardening and increase ductility.

Important Points

Explanation:

Heat treatment of the crankshaft: A crankshaft is made of forged and heat-treated alloy steel. It is machined and ground to provide suitable journals for the connecting rods and main bearings. The following methods are used to harden the crankshaft journals.

• Nitriding
• Carburising
• Chrome plating

In the above process the case of the crankshaft journal is hardened. These processes give very little depth of hardness. Some manufacturers recommend hardening of the crankshaft journals after regrinding.

Induction hardening: induction hardening involves passing a high-frequency alternating current through a suitably-shaped coil to induce rapid heating of the component surface situated appropriately within its electromagnetic field. The depth of hardening is controlled by the parameters of the induction heating equipment, time of application, and the hardenability of the material. 

Benefits of Induction hardening:

• Induction hardening imparts a hard, wear-resistant surface to the component whilst improving its fatigue strength through the development of residual surface compressive stresses in suitably deep cases.
• Only the surface is heated and quenched, therefore heat treatment distortion can be minimized.
• Faster localized cooling rates permit higher surface hardness values than might be achieved through hardening.
• Deeper hardening can be obtained than with thermo-chemical treatments. Depending upon process parameters, hardened depths can be in the range of 0.5-10mm.
• Localized hardening can be used to strengthen components at critical points while leaving other areas soft, without the need for the stopping-off procedures required in thermochemical case hardening.
• Induction hardening offers options for the treatment of exceptionally large components, where conventional furnace heating and cooling would be impractical and where only localised surface hardening is necessary.

Explanation:

Normalizing is used to refine the structure of steel casting and forging, to improve machinability, tensile strength and to remove stresses caused by cold working like rolling, forging, drawing, hammering, bending etc.

Normalizing is a quicker process and is generally performed on large casting, forging and low as well as medium carbon steels. The normal rate of cooling depends upon the size and shape of the component.

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 recrystallization 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 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.

Normalizing

• The specimen is heated beyond the 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.

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.

Steel is basically an alloy of iron and carbon in which the carbon content can be less than 1.7 % and carbon is present in the form of iron carbide to impart hardness and strength.

Two main categories of steel are (a) Plain carbon steel and (b) alloy steel.

Steels in which carbon is the prime alloying element are termed plain carbon steels, whereas alloy steels contain appreciable concentrations of other elements. Alloying materials like chromium, nickel, molybdenum, copper, aluminium, sulphur etc. are added to improve the properties of steel.

An alloy steel contains:

Explanation:

Annealing:

• Annealing is a heat treatment process that involves heating the material to a specific temperature and then cooling it slowly, usually in a furnace.
• It is primarily used for stress relief, improvement of mechanical properties, and increasing machinability.
• This process enhances ductility and toughness of the material, making it less brittle and more workable.
• By softening the material, annealing makes it easier to machine, which is particularly beneficial in manufacturing processes involving extensive machining.

Comparison with Other Options:

• Normalizing: This process is used to refine grain structure and improve uniformity in mechanical properties, but it is not primarily intended to relieve stress or improve machinability.
• Hardening: Increases the hardness and strength of the material but often leaves it more brittle, which can reduce machinability.
• Tempering: Done after hardening to reduce brittleness and increase toughness, but it doesn't primarily focus on relieving stress or enhancing machinability.

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.