Podcast
Questions and Answers
What characterizes the process of residual stresses in a material?
What characterizes the process of residual stresses in a material?
What is the primary transformation involved in the formation of Martensite?
What is the primary transformation involved in the formation of Martensite?
What effect does laser peening have on materials?
What effect does laser peening have on materials?
What usually happens to the material after the load is removed?
What usually happens to the material after the load is removed?
Signup and view all the answers
Which of the following describes a consequence of residual stresses?
Which of the following describes a consequence of residual stresses?
Signup and view all the answers
Which type of crystal structure does Martensite adopt after transformation?
Which type of crystal structure does Martensite adopt after transformation?
Signup and view all the answers
What causes strain in materials during deformation?
What causes strain in materials during deformation?
Signup and view all the answers
What is a significant negative consequence of residual stresses in the context of engineering?
What is a significant negative consequence of residual stresses in the context of engineering?
Signup and view all the answers
What can be a factor in the failure of structures, as noted historically?
What can be a factor in the failure of structures, as noted historically?
Signup and view all the answers
How does the machining process affect stress and strain in materials?
How does the machining process affect stress and strain in materials?
Signup and view all the answers
What is the primary result of carburizing in steel components?
What is the primary result of carburizing in steel components?
Signup and view all the answers
Which temperature is typically used in gas carburizing?
Which temperature is typically used in gas carburizing?
Signup and view all the answers
What is a characteristic of liquid carburizing processes?
What is a characteristic of liquid carburizing processes?
Signup and view all the answers
What advantage does nitriding have compared to other hardening processes?
What advantage does nitriding have compared to other hardening processes?
Signup and view all the answers
During the liquid carburizing process, what additional element is introduced to enhance hardness?
During the liquid carburizing process, what additional element is introduced to enhance hardness?
Signup and view all the answers
In solid carburizing, what is the primary substance used to surround the components?
In solid carburizing, what is the primary substance used to surround the components?
Signup and view all the answers
What effect does nitriding have on dimensional changes of steel components?
What effect does nitriding have on dimensional changes of steel components?
Signup and view all the answers
How long can gas carburizing typically take to achieve a 4 mm case depth?
How long can gas carburizing typically take to achieve a 4 mm case depth?
Signup and view all the answers
What is the typical hardness value achieved on the surface of carburized steel?
What is the typical hardness value achieved on the surface of carburized steel?
Signup and view all the answers
What is one major disadvantage of solid pack carburizing compared to other methods?
What is one major disadvantage of solid pack carburizing compared to other methods?
Signup and view all the answers
What is the primary purpose of nitriding in steel treatment?
What is the primary purpose of nitriding in steel treatment?
Signup and view all the answers
At what temperature can an iron-base alloy absorb the maximum amount of nitrogen during nitriding?
At what temperature can an iron-base alloy absorb the maximum amount of nitrogen during nitriding?
Signup and view all the answers
Which of the following parameters is NOT critical for the gas nitriding process?
Which of the following parameters is NOT critical for the gas nitriding process?
Signup and view all the answers
What happens to nitrogen levels in steel during carbonitriding?
What happens to nitrogen levels in steel during carbonitriding?
Signup and view all the answers
What is the maximum case depth typically produced by carbonitriding?
What is the maximum case depth typically produced by carbonitriding?
Signup and view all the answers
During the carburizing step in carbonitriding, what is the temperature range typically used?
During the carburizing step in carbonitriding, what is the temperature range typically used?
Signup and view all the answers
Why is carbonitriding preferred over liquid cyaniding?
Why is carbonitriding preferred over liquid cyaniding?
Signup and view all the answers
What type of steel can be treated by carbonitriding at a temperature of 845°C?
What type of steel can be treated by carbonitriding at a temperature of 845°C?
Signup and view all the answers
What effect does nitrogen have on the hardenability of steel when carbonitriding?
What effect does nitrogen have on the hardenability of steel when carbonitriding?
Signup and view all the answers
What is a primary benefit of using oil quenching during carbonitriding?
What is a primary benefit of using oil quenching during carbonitriding?
Signup and view all the answers
What is the primary purpose of heat treatment?
What is the primary purpose of heat treatment?
Signup and view all the answers
Which phase is characterized by a solid solution of carbon and other alloying elements in ferrite?
Which phase is characterized by a solid solution of carbon and other alloying elements in ferrite?
Signup and view all the answers
What is the cooling method used in quenching?
What is the cooling method used in quenching?
Signup and view all the answers
During which heat treatment process is a desired microstructure achieved by initially heating and then cooling a metal?
During which heat treatment process is a desired microstructure achieved by initially heating and then cooling a metal?
Signup and view all the answers
What effect does tempering have on hardened steel?
What effect does tempering have on hardened steel?
Signup and view all the answers
What is the result of spheroidizing in steels?
What is the result of spheroidizing in steels?
Signup and view all the answers
What is the critical temperature for austenite transformation defined as Ms?
What is the critical temperature for austenite transformation defined as Ms?
Signup and view all the answers
In process annealing, how is the hardness of steel affected?
In process annealing, how is the hardness of steel affected?
Signup and view all the answers
What occurs during stage II of tempering?
What occurs during stage II of tempering?
Signup and view all the answers
What is martempering primarily aimed at achieving?
What is martempering primarily aimed at achieving?
Signup and view all the answers
What is the key benefit of normalizing steel?
What is the key benefit of normalizing steel?
Signup and view all the answers
What property is significantly reduced in ferritic steel due to stress-relief treatments?
What property is significantly reduced in ferritic steel due to stress-relief treatments?
Signup and view all the answers
What defines the term 'full annealing' for hypoeutectoid steels?
What defines the term 'full annealing' for hypoeutectoid steels?
Signup and view all the answers
Which of the following is NOT a method of heat treatment commonly used for steels?
Which of the following is NOT a method of heat treatment commonly used for steels?
Signup and view all the answers
Study Notes
Metallurgy & Materials Science
- Presented by Dr. R. Vaira Vignesh
- Assistant Professor, Amrita School of Engineering, Coimbatore
- Topic is Corrosion is a Bliss
- Presentation materials are from internet/articles/books; for classroom use only
Heat Treatment of Steels
- Combination of heating and cooling metal/alloy
- Changes micro-constituents to modify properties
- Applied to ingots, castings, semi-finished products, welds.
- Changes micro-constituent's properties: nature, size, distribution
Stress and Strain
- Non-uniform plastic deformation--manufacturing sequence.
- Includes machining, welding and grinding
- Deformation processing (hot/cold work)
- Thermal variation
- Phase transformations
- Heterogeneity of chemical or crystallographic order
- Hooke's Law: Complication, muddle, misunderstanding, obfuscation
- Load removed: Tries to recover elasticity.
- Inhibited from full recovery - Adjacent material is deformed plastically.
Residual Stresses
- Stresses present in matrix after load removal
- Thermal or Mechanical
- Positive or Negative
- Laser peening--compressive residual stresses
- Strengthens thin sections.
- Toughens brittle surfaces
- Negative effects as well.
- Invisible to manufacturers unless significant distortion.
- Affects structural integrity
- Thick walled structures prone to brittle fracture.
- Undesired stresses affect fatigue performance.
- Often a cause of premature failure of critical components.
Heat Treatment
- Combination of heating and cooling operations timed and applied to a metal or alloy in a solid state.
- Changes in microconstituents—nature, size and distribution.
- Applied to ingots, castings, semi-finished products, welded joints, and various elements of machines and instruments.
Purpose of Heat Treatment
- Relieve internal stresses
- Refine grain structure (coarse to fine)
- Augment material properties (mechanical, corrosion, tribological)
Heat Treatment of Steels
- Steel is alloy of Fe and C (0.2% to 2% weight %)
- Other alloying components: up to 5% in low alloy steels
- More alloyed steels: tool steel and stainless steel
- Wide variety of properties
- Liquid phase is always avoided in heat treating.
Quick Review
- Austenite - Solid solution of C and/or other alloying elements in γ-Fe; not stable at room temperature, unless stabilizers added.
- Ferrite - BCC iron phase; limited solubility of C (α-Fe) - BCC
- Cementite - Fe3C (6.67 wt.% C) - Complex orthorhombic
- Ledeburite - Eutectic mixture of Austenite and Cementite
- Pearlite - Alternate lamellae of Ferrite and Cementite (austenite decomposition by eutectoid reaction)
Quick Review (cont.)
- Martensite - Austenite transformation below Ms (240°C)--shear type transformation
- Bainite - Austenite transforms below temp. at which pearlite is produced; above transformation temp of Martensite
- Troosite - Radial lamellae of ferrite and cementite, tempering below 450°C
- Sorbite - Ferrite and finely divided cementite, tempering above 450°C
- A1 – 727°C (Eutectoid transformation temperature)
- A2 - 768°C (Curie temperature); Ferro --> Para-magnetic
- A3 – γ-Fe / y-Fe + α-Fe boundary
- Acm
- Ar3
- AC3
Annealing
- Generic term—heating to a temperature and holding at that temperature.
- Cooling at an appropriate rate to soften or improve properties.
- Improve mechanical or electrical properties; promote dimensional stability
- Maximum temperature = A1, A3, Acm
- Austenite is present at temps above A1
- Subcritical, inter-critical, super-critical.
Annealing Cycles
- Subcritical (Stress-Relief, Process Annealing)--(Below A1)
- Inter-critical annealing
- Above A1, but below A3
- Above A1, but below Acm
- Super-critical (Full annealing, Normalizing)--(Above A3 or Acm)
Stress-Relief Annealing
- Relieve stresses, consequence of manufacturing sequence.
- Separates stress-relief from post-weld.
Annealing (cont.)
- Minimizing internal residual stresses.
- Uniform heating.
- Cooling after specified holding time
Normalizing
- Heating to ≥55°C above A3 for hypoeutectoid compositions.
- Above Acm for compositions higher than eutectoid.
- Cooling practice through transformation for desired properties.
- Refine the grains, decrease average grain size.
Full Annealing
- Heating to 50°C above A3, for hypoeutectoid or A1, for hypereutectoid steels.
- Hold at this temp. for specified time.
- Uniform cooling to achieve the desired structure.
Carbo-Nitriding
- Modified form of gas carburizing.
- Introducing ammonia into the gas carburizing atmosphere.
- Lower temp, shorter time, shallower case than gas carburizing.
Carbo-Nitriding (cont.)
- Avoids cyanide-bearing waste.
- Improves hardenability, wear resistance, case depth.
- Adds Nitrogen for austenite stabilization.
Spheroidizing
- Strength of ferrite depends on its grain size and cooling rate.
- Carbides in pearlite or spheroids affect formability of steel.
- Spheroidized steels: globular carbides in a ferritic matrix.
Spheroidizing (cont.)
- Prolonged holding at just below A1.
- Alternating heating and cooling.
- Heating to just above A1, cooling slowly or holding below A1.
- Prevent carbide reformation.
Process Annealing
- During cold working.
- Hardness of steel increases, and ductility decreases.
- Must be annealed to restore ductility.
- Restore steel's ductility; merely soften steel.
- Intermediate treatment
Quenching
- Rapid cooling from austenitizing temperature; minimizes grain boundary carbides.
- Improves ferrite distribution, control martensite in microstructure.
- Achieving toughness, hardness, minimization of residual stress, distortion and cracking.
Tempering
- Reheating previously hardened steel below the critical temp.
- Increasing ductility.
- Increasing toughness.
Martempering
- Interrupted quench from austenitizing temp.
- Hold temp. above martensite transformation to equalize temp. throughout the piece.
- Minimize distortion, cracking, and residual stress.
- Essentially primary martensitic, untempered and brittle.
Martempering (cont.)
- Quench into hot fluid medium (oil, salt, etc.)
- Holding until uniform temp.
- Avoids large temp. difference.
Austempering
- Isothermal transformation below pearlite formation and above martensite.
- Heating to a temp. in austenitizing range, usually (790 to 915 °C.)
- Quenching in a bath maintained at a constant temp (usually 260-400 °C).
- Transforming isothermally to bainite in the bath.
- Cooling to room temperature.
Austempering (cont.)
- Molten salt is preferred quenching medium.
- Eliminates problem of a vapor phase barrier.
- Increased steel toughness, ductility, and strength.
- Reduced distortion.
- Reduced machining time, stock removal, sorting, and inspection.
- Shortest overall time cycle possible
Surface Hardening of Steels
- Methods vary based on diffusion and related processes.
- Includes surface engineering, heat treatment, and surface modifications.
Carburizing
- Diffusing carbon into the surface, increasing hardness.
- Used on low-carbon steels.
Carburizing (cont.)
- High carbon content in surface due to rapid diffusion and high solubility of carbon in austenite.
- Quenched and tempered surface becomes high-carbon tempered martensite - ferritic center is soft and ductile.
- Thickness is smaller in carburized steels than flame or induction hardened steels.
- Increased carbon content at the surface gives a martensitic layer. (wear resistant)
Carburizing (cont.)
- Uses methane (or propane) for decomposition into atomic carbon and hydrogen.
- Time is 2 hours to 36 hours for 1 mm to 4 mm depth case.
Carburizing (cont.)
- Liquid carburising/cyaniding
- Salt bath (cyanide-chloride-carbonate) at 845°C to 955°C.
- Cyanide salts introduce nitrogen, improving hardness.
- Fastest carburising process for small batch sizes.
Carburizing (cont.)
- Components surrounded by a carbonaceous medium
- Process of either gas (CO->CO2) or solid carburising
- Appropriate temps (790-845C) and times (2-36 hrs)
Nitriding
- Ferritic thermochemical method for diffusing nascent nitrogen into the surface of steels.
- Diffusion process is based on solubility of N in Iron.
- At 450°C, Iron-base alloy will absorb up to 5.7% to 6.1% of Nitrogen.
- Beyond this, the surface phase is predominantly ɛ phase.
- High carbon gives more potential for ɛ phase formation.
Nitriding (cont.)
- No molecular size change, no dimensional change.
- Slight growth due to volumetric change.
- Distortion induced by surface stresses being released.
Nitriding (cont.)
- Process parameters: Furnace temp., process control, time, gas flow, gas activity control, process chamber maintenance.
Carbo-Nitriding
- Modified form of gas carburizing.
- Introducing ammonia for nitrogen addition to the carburized case.
- Nitrogen diffuses with carbon simultaneously for a shorter time.
- Shallower case.
- Similar to liquid cyaniding.
Carbo-Nitriding (cont.)
- Preferred over liquid cyaniding due to cyanide disposal problems.
- Hard, wear-resistant case, with better hardenability.
- Nitrogen increases the hardenability of steel.
- Increases retained austenite (for alloy steels).
Studying That Suits You
Use AI to generate personalized quizzes and flashcards to suit your learning preferences.
