Casting and Heat Treatment Lesson 3

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Questions and Answers

What is the primary result of macrosegregation during casting?

Uniform distribution of alloying elements

Differences in composition throughout the casting (correct)

Concentration of low melting point constituents at the surface

Induction of grain nucleation

Which statement describes inverse segregation in solid-solution alloys?

Alloying elements distribute evenly throughout the casting

The center of dendritic structures has a lower concentration of alloying elements (correct)

High melting point constituents migrate towards the center

The center of the casting has a higher concentration of alloying elements

What is the primary function of inoculants in casting?

To enhance the density of the casting

To drive low melting point constituents to the surface

To increase the temperature of the liquid metal

To induce nucleation of grains throughout the liquid metal (correct)

What causes gravity segregation in cast materials?

Density differences within liquid compositions Signup and view all the answers

Which of the following best describes normal segregation in solidification processing?

Low melting point constituents migrate towards the center Signup and view all the answers

What is a characteristic of sand casting?

It can improve mechanical properties through mold ablation. Signup and view all the answers

Which option best describes shell-mold casting?

It may be more economical and offers high-quality finished castings. Signup and view all the answers

What benefit does mold ablation provide in sand casting?

It allows for rapid and directional solidification of the metal. Signup and view all the answers

Which statement about ceramic molds is correct?

They work well with ferrous and high-temperature alloys. Signup and view all the answers

What is a disadvantage of some casting processes mentioned?

They can lead to lower dimensional accuracy. Signup and view all the answers

What type of structure is formed through heterogeneous nucleation in casting?

Columnar dendritic structure Signup and view all the answers

Which phase in the iron-carbon binary system is characterized as soft, ductile, and magnetic?

Alpha ferrite Signup and view all the answers

What is the effect of alloying elements on eutectoid composition in steel?

They decrease the eutectoid composition Signup and view all the answers

How do higher cooling rates affect dendrite formation?

They result in finer dendrite structures Signup and view all the answers

What is a key feature of vacuum casting?

Suitable for thin walled complex shapes Signup and view all the answers

In the iron-carbon system, which phase is described as hard and brittle?

Cementite Signup and view all the answers

Which structure is typically obtained by slow cooling rates?

Coarse dendritic structure Signup and view all the answers

Which method is not associated with permanent-mold casting?

Expendable Pattern Casting Signup and view all the answers

What phenomenon occurs when a single solid phase transforms into two solid phases in the iron-carbon system?

Eutectoid transformation Signup and view all the answers

Which heat treatment process improves elasticity?

Tempering Signup and view all the answers

What is the definition of the freezing range in casting?

The temperature difference between solidification and melting Signup and view all the answers

What is a disadvantage of expendable pattern casting?

High labor and material costs Signup and view all the answers

Which casting method utilizes preheated molds for better heat dissipation?

Permanent-Mold Casting Signup and view all the answers

Which type of phase diagram is important for understanding two-phase alloys?

Eutectic diagram Signup and view all the answers

Which statement about the structure-property relationships of dendrites is correct?

Dendrites have a varying concentration gradient Signup and view all the answers

Which process involves rapid cooling in various mediums?

Quenching Signup and view all the answers

What does semisolid metal forming take advantage of?

Decreased viscosity when agitated Signup and view all the answers

What best describes the goal of annealing in heat treatment?

Restore original properties Signup and view all the answers

Which of the following casting processes is specifically meant for creating single-crystal components?

None of the above Signup and view all the answers

What is a characteristic of permanent-mold casting?

Utilizes metal molds Signup and view all the answers

Which theorem describes the relationship between pressure, velocity, and elevation in fluid flow?

Bernoulli’s theorem Signup and view all the answers

What is the primary purpose of using fluxes in melting practices?

To refine molten metal Signup and view all the answers

In which casting process is a pattern placed in sand to create an imprint?

Sand casting Signup and view all the answers

What does the term 'degree of superheat' refer to in casting?

The extra temperature above melting point Signup and view all the answers

Which type of flow is characterized by orderly layers of fluid with little to no mixing?

Laminar flow Signup and view all the answers

According to Chvorinov's rule, what primarily influences solidification time?

Surface area of the casting Signup and view all the answers

What type of furnace is primarily used for melting steel?

Electric-arc furnace Signup and view all the answers

Which characteristic of molten metal affects its ability to fill a mold?

Viscosity Signup and view all the answers

What is the most common type of sand used in sand casting?

Green sand Signup and view all the answers

What does the term 'fluidity of molten metal' refer to?

The ability of the metal to flow and fill molds Signup and view all the answers

Which casting method uses compressive stresses to reduce porosity?

Strip casting Signup and view all the answers

Which type of mold in sand casting incorporates inorganic and organic binders?

Cold-box sand Signup and view all the answers

What causes shrinkage cracking in a casting?

Cooling of the solidified metal Signup and view all the answers

Which component of the casting process is critical for limiting exposure of molten metal to the environment?

Sprue profile Signup and view all the answers

Study Notes

Two-Phase Alloys

Phase diagrams are essential for understanding the composition of various phases.
The lever rule calculates weight fractions of solids and liquids in two-phase systems.

The Iron-Carbon System

Key phases in the iron-carbon binary system include:
- Ferrite (α, δ): Body-centered cubic (BCC), soft and ductile, magnetic properties.
- Austenite (γ): Face-centered cubic (FCC), ductile at high temperatures, non-magnetic, excellent formability.
- Cementite (Fe3C): Also known as carbide, hard and brittle.
Eutectoid transformations involve a solid phase changing into two distinct solid phases.
Alloying elements lower both the eutectoid composition and temperature in steel.

Cast Structures

Solid-solution alloys and how grain structures are formed through heterogeneous nucleation.
Dendrites: Tree-like crystal formations critical during solidification.
Types of crystal structures formed in casting:
- Columnar dendritic
- Equiaxed dendritic
- Equiaxed non-dendritic

Effects of Cooling Rates

Slow cooling leads to coarse dendritic structures.
Rapid cooling results in smaller dendrite arm spacing.
Extremely high cooling rates produce amorphous (non-crystalline) structures.

Structure-Property Relationships

Variations in composition within dendrites can include cored structures (concentration gradient).
Types of segregation observed during solidification:
- Macrosegregation: Variances in composition throughout the casting.
- Normal segregation: Low melting point constituents move towards the center.
- Inverse segregation: Lower alloying element concentration at the center in solid-solution alloys.
- Gravity segregation: Density-driven composition differences.

Fluid Flow and Heat Transfer

Gate design minimizes exposure of molten metal to the environment.
Key principles:
- Bernoulli’s theorem relates pressure, velocity, and height in fluid flow.
- Mass continuity equation ensures flow rate consistency through different areas.

Molten Metal Flow Characteristics

Viscosity, surface tension, inclusions, and solidification behavior affect fluidity.
Critical casting parameters include mold design and superheating (temperature above melting point).

Shrinkage Effects

Shrinkage during solidification can lead to cracking.
Important phases of contraction involve:
- Molten metal contraction during phase change from liquid to solid.
- Further contraction as solidified metal cools to room temperature.

Melting Practices and Furnaces

Fluxes are inorganic compounds used to refine molten metal by removing impurities.
Types of furnaces include electric-arc (for steel), induction, crucible, cupolas, and levitation melting.

Casting Alloys and Methods

Sand Casting: Most common method, involving sand molds created using patterns which are then filled with molten metal.
Sand types used include:
- Naturally bonded (bank sand)
- Synthetic (lake sand), preferred for consistent composition.

Sand Casting Techniques

Core sand generally composed of sand aggregates.
Various sand molds include:
- Green sand (most common, least expensive).
- Cold-box (increased mold strength).
- No-bake (utilizing synthetic resins for binding).

Sand Casting Operations

Allows for most alloys but has lower precision than other methods.
Mold ablation can enhance mechanical properties by inducing rapid solidification and reducing porosity.

Additional Casting Methods

Shell-Mold Casting: Can be more economical, yielding high-quality castings.
Ceramic Mold Casting: Effective for ferrous and high-temperature alloys like stainless steel and tool steel.### Expendable-Mold, Permanent-Pattern Casting Processes
Vacuum Casting: Ideal for thin-walled complex shapes with uniform properties; commonly utilized in gas turbine manufacturing.
Expendable Pattern Casting:
- Simple process requiring minimal cleaning and finishing after casting.
- Labor and material costs can be high but allows for intricate shapes, offering good surface finish and tolerances.

Permanent-Mold Casting

General Process: Utilizes metal molds, resulting in a higher cooling rate that significantly affects microstructure and grain size.
Mold Preparation:
- Mold preheating and lubrication are essential for heat dissipation.
Casting Steps:
- Molds are closed with inserted cores if necessary.
- Pouring of molten metal occurs, leading to solidification.
- After cooling, the mold is opened to remove the finished part.

Types of Permanent-Mold Casting Techniques

Slush Casting: Involves creating hollow parts by rotating molds during the metal pouring process.
Pressure Casting: Derives benefits from high-pressure injection to produce detailed designs.
Die Casting:
- Hot Chamber: Metal is fed from a ladle to the die without solidifying.
- Cold Chamber: Molten metal is poured into the chamber before injection into the die.
Centrifugal Casting: Metal is poured into a rotating mold, using centrifugal force to enhance density and surface quality.
Squeeze Casting: Combines elements of die casting and forging, applying pressure to decrease porosity.
Semisolid Metal Forming:
- Thixocasting: Utilizes thixotropic behavior of alloys that decrease in viscosity upon agitation.
- Rheocasting: Employs a "mushy state" metal for enhanced forming processes like extrusion and forging.

Heat Treatment Processes

Tempering: A heating treatment aimed at reducing hardness while improving elasticity and toughness.
Quenching: Rapid cooling method which can involve various agents like water, oils, or gases to harden the material quickly.
Annealing: Restoration of a part’s original properties post cold working or heat treatment.
Carburization: A specific heat treatment process leading to carbon loss, affecting material characteristics.

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Description

Explore the fundamental concepts of casting and heat treatment in Lesson 3. This quiz examines two-phase alloys, phase diagrams, and the lever rule which helps in understanding the composition of various phases. Test your knowledge and grasp the critical aspects of materials science.