MECH2636: Rolling Processes Quiz

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Questions and Answers

What is the relationship between roll separating force and workpiece distortion?

If the force is high enough to distort the rolls, the workpiece will also be distorted. (correct)

Workpiece distortion only occurs with low roll force.

Workpiece distortion is independent of the roll force applied.

Higher roll force always leads to less distortion of the workpiece.

Which variable does NOT affect the roll separating force according to the provided information?

Length of contact L

Friction coefficient μ

Roll radius R (correct)

Width and average thickness of the work w and hav

How can the torque required per roll be calculated?

By multiplying the total power by the roll speed.

By adding the workpiece thickness before and after rolling.

By multiplying the roll force F by the roll radius R.

By using the formula T ≈ F L / 2. (correct)

What does the power required in rolling depend on?

The torque and the rotational speed in radians. Signup and view all the answers

If the flow stress of the material increases, what is the expected effect on the roll separating force?

The roll separating force will increase. Signup and view all the answers

What is the main benefit of using a four high rolling mill compared to a two high mill?

Reduced distortion of the workpiece Signup and view all the answers

Which process involves pushing a billet of material through a die?

Extrusion Signup and view all the answers

Why is lubrication used in rolling mills?

To minimize friction between rolls and workpiece Signup and view all the answers

What is the total power required for two rolls if the power per roll is 304 kW?

608 kW Signup and view all the answers

What is a characteristic of a Sendzimir rolling mill?

It features many rolls for rigidity and accuracy Signup and view all the answers

What is the outcome of shape rolling?

Creation of a variety of structural shapes Signup and view all the answers

In the drawing process, what happens to the cross-sectional area of the material?

It decreases Signup and view all the answers

Which type of rolling mill is considered the simplest?

Two high Signup and view all the answers

What is the main factor that increases the ideal force in metal extrusion?

Friction and redundant work Signup and view all the answers

What is redundant work in the context of metal extrusion?

Plastic work used in unnecessary shearing Signup and view all the answers

Which of the following is a disadvantage of high friction during the extrusion process?

Increased likelihood of internal shearing Signup and view all the answers

Which variant of extrusion uses a pressurized fluid to reduce friction?

Hydrostatic extrusion Signup and view all the answers

When and why is die drawing generally performed?

Cold, to improve material properties Signup and view all the answers

What happens to the total force exerted during the extrusion process as the die angle increases?

It can both increase or decrease based on other factors Signup and view all the answers

Which characteristic is generally preferred in extrusion designs?

Solid shapes with uniform wall thickness Signup and view all the answers

What is the mathematical expression for calculating the minimum force required for an ideal extrusion process?

$F = A_o imes au imes ln(rac{A_o}{A_f})$ Signup and view all the answers

What is a common defect that can occur during the metal extrusion process?

Surface cracking Signup and view all the answers

What effect does increasing the die angle have on friction force in the extrusion process?

Reduces friction force Signup and view all the answers

What is the ideal drawing force required to draw a round rod of 10 mm diameter down to 8 mm?

17,571 N Signup and view all the answers

If the actual drawing force is 24,600 N, what is the percentage increase from the ideal drawing force?

51.5% Signup and view all the answers

What is the flow stress of the material being drawn?

785 MPa Signup and view all the answers

What is the calculated tensile stress in the drawn rod after completing the drawing process?

489 N/mm² Signup and view all the answers

How much power is required ideally to perform the drawing operation without considering friction?

8,750 W Signup and view all the answers

What percentage of the ideal work does the frictional and redundant work together represent?

40% Signup and view all the answers

What is the calculated area after the rod is drawn down from 10 mm to 8 mm diameter?

50.3 mm² Signup and view all the answers

What happens to the area reduction in die drawing under the tensile strength limit of the drawn product?

It decreases Signup and view all the answers

What is the primary characteristic of cold working processes?

They are conducted below about 30% of the absolute melting temperature. Signup and view all the answers

Which process can lead to improved mechanical properties by refining grain structure?

Hot working Signup and view all the answers

What does upsetting refer to in the context of metal forging?

Compressing the workpiece to change its shape. Signup and view all the answers

Which of the following statements regarding hot working is incorrect?

It results in higher forces being required for shaping. Signup and view all the answers

What effect does high friction have on the deformation of a workpiece during upsetting?

It can lead to barrelling of the workpiece. Signup and view all the answers

What is a primary benefit of cold working compared to hot working?

Better dimensional accuracy and surface finish. Signup and view all the answers

Which solid deformation process involves compressing metal between two surfaces?

Forging Signup and view all the answers

In which type of working is thermal contraction not a concern?

Cold working Signup and view all the answers

What is the condition that defines warm working?

Between 30% and 60% of the melting point Signup and view all the answers

What can be estimated using the formula F ≈ σy πr (1 + 2μr/3h)?

The upsetting force in forging Signup and view all the answers

What is the primary purpose of applying a flash in conventional closed die forging?

To improve material flow into the die cavities Signup and view all the answers

Which of the following statements about precision die forging is true?

It typically uses harder tooling than conventional forging. Signup and view all the answers

What is a common flaw seen in forgings caused by rapid cooling or over-straining?

Surface cracks Signup and view all the answers

What method is typically used to determine the upsetting force in open die forging?

Mathematical formula involving yield stress Signup and view all the answers

What does the term 'barrelling' refer to in the context of open die forging?

The uncontrolled flow of material Signup and view all the answers

Which of the following best describes sequential forging?

Carrying out forging operations in stages due to load limitations Signup and view all the answers

Which forging operation is typically associated with forming bolt heads?

Heading Signup and view all the answers

What factor significantly increases the forces required in precision die forging?

Increased flow stress of the material Signup and view all the answers

Which design rule is critical for ensuring the successful removal of a forging from its die?

Appropriate draft angles Signup and view all the answers

What is an outcome of choosing poor die parting lines in forging operations?

Increased likelihood of material fold-back Signup and view all the answers

Study Notes

Course Information

Course title: MECH2636: Design and Manufacture 2
Course topic: Solid Deformation Processes
Course Institution: University of Leeds

Rolling

Steel is the most common material in engineering structures and components.
Steel production is primarily through ingot casting or continuous casting.
Rolling is a steel processing step where the material is forced through rolls.
Rolling generates plates, bars, rods, pipes, and various other shapes.
Rolling is typically sequential on multiple rolls to reduce force and deformation at each stage.
Rolling processes use hot or warm metal, with finishing often done cold.
Rolling speed at exit must be higher than the speed at entry.
Contact between roll and workpiece has two areas where the roll is faster than the workpiece and vice-versa respectively.
The area of contact length is ~√(roll radius × (initial thickness - final thickness)).
Force generated on the rolls increases with the flow stress of the material, contact length, width of work, average thickness, and the friction coefficient.
Formula for force: F ≈ σy(LW)(1 + μL/2h_avg)
High forging force can distort the workpiece and the roles.
Torque is equal to ~ F x (length of contact /2),
Power required = Torque × Rotational speed.

Rolling Mills

Rolling mills have different arrangements (two high, three high, four high, Sendzimir).
Two high mills are the simplest.
Three high mills offer higher productivity per roll.
Four high mills use a small inner roll, reducing forces and distortion.
Sendzimir mills use many rolls for a very rigid process, usually for cold rolling.
Lubrication is used to reduce friction between rolls and workpiece.
Shape rolling produces various structural shapes across multiple passes.

Extrusion & Drawing

Extrusion and drawing are related processes used to produce long lengths of material with specific cross-sections.
Extrusion pushes a billet through a shaped or tapered die.
Drawing pulls the material through the die.
Cross-sectional area reduces during either process and the shape of the billet emerges as per the shape of the die.
Forward extrusion applies pressure.
Pressure applied to the billet is expressed by: F = σy A0 ln (A0/Af), where σ is the plastic flow stress of material, A0 is the initial cross-sectional area and Af is the final cross-sectional area.
Factors that increase the ideal force in reality involve friction and redundant work.
Friction is minimized by using lubricants or tapered dies.
Redundant work from unnecessary shearing increases as the die angle increases.

Backward Extrusion

Backward extrusion is a variation of forward extrusion where the direction is reversed.
Hydrostatic extrusion uses pressurized fluid on the billet to force it through a tapered die, reducing friction.

Die Drawing

Die drawing is similar to forward extrusion, except the work is pulled rather than pushed.
Drawing is often used as a finishing operation and is mostly employed for small cross-sections, such as tubes, wires, and rods.
Ideal drawing force = σ y A0 ln (A0/Af) (where σ is material flow stress, A0 is initial and Af is final area)

Forging

Forging is a bulk deformation process, typically by compressing metal between two surfaces.
Forging can be at room temperature or elevated temperature.
Open die forging, the most common process, involves placing a workpiece between two die surfaces and compressing it.
The resultant shape depends on the die surfaces' friction.
High friction leads to 'barrelling' of the workpiece.
Hot workpiece between cold dies also causes barrelling
Upsetting force formula: F ≈σy (πr²) (1+(2µr)/3h)).
Sequential forging may occur when a press is unable to perform all deformation in one go.
Heading and Coining are other forging operations.
Flaws in forging can include surface cracks from over-straining, laps from buckling thin sections, and cold shuts when the die design improperly guides the metal.
Design rules for forgings involves choosing fillet and corner radii, good draft angles and understanding of material anisotropy.
Parting line should pass through greatest cross section area in the die when possible. Minimum thicknesses for ribs & webs depends on the material. Punchout holes are to reduce weight & add clearance.

Cold Working

Cold working operates below ~30% of the material's absolute melting temperature.
Cold working increases strength and hardness but decreases ductility.
Cold working gives better dimensional accuracy and surface finish better than hot working.

Hot Working

Hot working operates above ~60% of the absolute melting temperature.
Hot working improves mechanical properties, refining the grain structure, and decreasing the forces required by increasing the flowability of the material.

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Description

Test your knowledge on the rolling processes in steel manufacturing in the MECH2636 course. Understand how steel is processed, the mechanics of rolling, and the characteristics that affect deformation. This quiz covers important concepts such as contact areas, rolling speed, and material characteristics.