Q1: Define Critical Temperature.
Ans: Critical Temperature is the temperature at which steel undergoes a phase change, such as the transformation from ferrite to austenite or vice versa.
Q2: What is Heat Treatment of Steel?
Ans: Heat Treatment of Steel is the process of heating and cooling steel in a controlled way to alter its physical and mechanical properties like hardness, toughness, and strength.
Q3: What is Cold Working?
Ans: Cold Working is the process of shaping or deforming metals at room temperature to improve hardness and strength without heating.
Q4: State the purpose of Heat Treatment.
Ans: Purpose of Heat Treatment:
- ➔ Increase hardness
- ➔ Improve strength
- ➔ Enhance toughness
- ➔ Relieve internal stresses
- ➔ Improve machinability
Q5: What is Case Hardening?
Ans: Case Hardening is a heat treatment process where only the surface layer of steel is hardened by adding carbon or nitrogen while keeping the core soft.
Q6: Name the constituents of steel.
Ans: Constituents of Steel:
- ➔ Iron (Fe)
- ➔ Carbon (C)
- ➔ Manganese (Mn)
- ➔ Sulfur (S)
- ➔ Phosphorus (P)
- ➔ Silicon (Si)
Q7: What is Cementite?
Ans: Cementite (Fe3C) is a hard and brittle compound of iron and carbon found in steel, forming pearlite with ferrite.
Q8: Define Allotropic Substance.
Ans: An Allotropic Substance is a material that can exist in more than one crystalline form at different temperatures, like iron which forms ferrite, austenite, and delta ferrite.
Q9: Define Crystalline Compounds.
Ans: Crystalline Compounds are substances in which atoms or molecules are arranged in a regular, repeating three-dimensional structure.
Q10: What is the role of heating rate on steel?
Ans: Heating rate affects the microstructure of steel; rapid heating can lead to uneven austenite formation while slow heating ensures uniform transformation.
Q11: Define Micro Structure.
Ans: Microstructure is the structure of a material observed under a microscope, showing grains, phases, and defects.
Q12: Define Macro Structure.
Ans: Macrostructure is the structure of a material visible to the naked eye, showing overall grain patterns and surface features.
Q13: What is the Austenite range?
Ans: Austenite range is the temperature range in which steel transforms into austenite (γ-iron), typically between 727°C and 912°C for plain carbon steel.
Q14: What is the Critical Temperature Line?
Ans: The Critical Temperature Line on the Iron-Carbon diagram represents temperatures at which phase transformations occur in steel.
Q15: What is Eutectoid Temperature?
Ans: Eutectoid Temperature is the temperature (about 727°C) at which austenite transforms into pearlite in steel.
Q16: State the importance of the rate of cooling of steel.
Ans: Importance of Cooling Rate:
- ➔ Controls hardness and strength
- ➔ Determines microstructure (pearlite, bainite, martensite)
- ➔ Prevents cracking or distortion
Q17: State different methods of Cooling of steel.
Ans: Methods of Cooling Steel:
- ➔ Water Quenching
- ➔ Air Cooling
- ➔ Furnace Cooling
- ➔ Fused Salt Cooling
- ➔ Oil Quenching
Q18: Briefly explain the Cooling of steel in Water.
Ans: Procedure of Water Cooling:
- ➔ Prepare the steel specimen.
- ➔ Heat the steel to the desired austenitizing temperature.
- ➔ Rapidly quench the steel in water.
- ➔ Observe that hard and brittle structures like martensite are formed.
Q19: Briefly explain the Cooling of steel in Air.
Ans: Procedure of Air Cooling:
- ➔ Heat the steel specimen to the required temperature.
- ➔ Remove the steel from heat and allow it to cool slowly in air.
- ➔ Soft microstructures like pearlite and ferrite are produced.
- ➔ Internal stresses are minimal due to gradual cooling.
Q20: Briefly explain the Cooling of steel in Furnace.
Ans: Procedure of Furnace Cooling:
- ➔ Heat the steel specimen to the desired temperature.
- ➔ Turn off the furnace and let the steel cool slowly inside.
- ➔ Very slow cooling produces soft microstructures.
- ➔ This method is commonly used for annealing.
Q21: Briefly explain the Cooling of steel in Fused Salt.
Ans: Procedure of Fused Salt Cooling:
- ➔ Heat the steel specimen to the required temperature.
- ➔ Quench the steel in molten salts.
- ➔ Achieves uniform cooling across the specimen.
- ➔ Produces controlled hardness, ideal for large components.
Q22: Briefly explain the Cooling of steel in Oil.
Ans: Procedure of Oil Quenching:
- ➔ Heat the steel specimen to the desired austenitizing temperature.
- ➔ Quench the steel in oil to reduce the risk of cracking.
- ➔ Moderate hardness and toughness are achieved.
- ➔ Safer than water quenching for thicker sections.
Q23: Define the process of Annealing.
Ans: Annealing is a heat treatment process in which steel is heated to a specific temperature and then cooled slowly to soften the material, relieve stresses, and improve ductility.
Q24: State the purpose of Annealing.
Ans: Purpose of Annealing:
- ➔ To soften steel for machining
- ➔ To relieve internal stresses
- ➔ To improve ductility and toughness
- ➔ To refine grain structure
Q25: Define Hardening.
Ans: Hardening is a heat treatment process in which steel is heated to a high temperature and then rapidly cooled (quenched) to increase hardness and strength.
Q26: State the purpose of Hardening.
Ans: Purpose of Hardening:
- ➔ To increase hardness
- ➔ To improve wear resistance
- ➔ To enhance strength
Q27: State the steps taken for Hardening.
Ans: Steps for Hardening:
- ➔ Heating steel to austenitizing temperature
- ➔ Holding at that temperature for uniform heating
- ➔ Rapidly quenching in water, oil, or another medium
- ➔ Optionally tempering to reduce brittleness
Q28: What is Martensite?
Ans: Martensite is a hard, brittle microstructure formed in steel when austenite is rapidly quenched, having a body-centered tetragonal (B.C.T) crystal structure.
Q29: What is B.C.T?
Ans: B.C.T (Body-Centered Tetragonal) is a crystal structure of martensite in steel where atoms are arranged in a tetragonal lattice.
Q30: Define Quenching.
Ans: Quenching is the rapid cooling of heated steel in water, oil, or other media to form hard microstructures like martensite.
Q31: State the purpose of Quenching.
Ans: Purpose of Quenching:
- ➔ To increase hardness
- ➔ To improve strength
- ➔ To achieve desired microstructure (martensite)
Q32: State different media used for Quenching.
Ans: Quenching Media:
- ➔ Water
- ➔ Oil
- ➔ Air (for slow cooling)
- ➔ Fused salts
- ➔ Brine solution
Q33: Name the Ideal Quenching medium.
Ans: Oil is considered an ideal quenching medium as it provides moderate cooling rate, reducing cracks while achieving hardness.
Q34: What is the Vapor Blanket Stage?
Ans: Vapor Blanket Stage is the initial stage of quenching where a vapor layer forms around the hot steel, slowing cooling temporarily.
Q35: What is the Vapor Transport Stage?
Ans: Vapor Transport Stage is the stage during quenching when bubbles collapse and heat transfer increases, accelerating cooling.
Q36: What is the Liquid Cooling Stage?
Ans: Liquid Cooling Stage is the final stage of quenching when the steel is fully immersed in the liquid medium and rapid cooling occurs.
Q37: Define Tempering.
Ans: Tempering is a heat treatment process in which hardened steel is reheated to a moderate temperature and cooled to reduce brittleness and improve toughness.
Q38: State the purpose of Tempering.
Ans: Purpose of Tempering:
- ➔ To reduce brittleness of hardened steel
- ➔ To improve toughness and ductility
- ➔ To relieve internal stresses
Q39: State the process of Tempering.
Ans: Tempering Process:
- ➔ Hardened steel is reheated to a temperature below critical point
- ➔ Held for a specific time
- ➔ Cooled slowly in air or furnace
Q40: Briefly explain the structural changes during Tempering.
Ans: During tempering, martensite transforms partially into ferrite and cementite, reducing hardness but increasing toughness and relieving internal stresses.
Q41: What is Temper Brittleness?
Ans: Temper Brittleness is the loss of toughness in steel when tempered at certain temperatures (around 230 — 270°C) due to formation of brittle structures.
Q42: What is notched Bar Toughness?
Ans: Notched Bar Toughness is the ability of a material to resist fracture when a notch is present, measured by impact tests like Charpy or Izod.
Q43: Define Metallography.
Ans: Metallography is the study of the microstructure of metals and alloys using optical or electron microscopes to understand their properties and behavior.
