Question

a. What may happen when a large section of steel is heated at a rapid rate? Why? b. Why are cold forming processes always ac- companied by work-hardening, whereas hot forming processes are not? C. Why are lubricants used in metal forming processes? List some useful effects. d. List some considerations that must be taken into account when designing a section for ex- trusion. e. For heavier sections, would you recommend larger angles of contact in rolling? Why? f. What should be avoided when heating large teel sections prior to hot forming?

Answer 1

A.What happens when a large section of steel is heated at a rapid rate? Why?

When a large section of steel is heated at a rapid rate means it will change the electrical, magnetic and structural properties of the metal.

When it is heated above the critical temperature various transformation occurs in the metal they are electrical resistance, thermal expansion, structure and magnetism

B.Why are cold forming processes always accompanied by work hardening where as hot forming are not

Because in hot working process shaping a metal much less force and power than in cold working.

Above its recrystaliation temperature a metal possesses far greater ductility than cold working

Cold forming process is conducted at temperatures below the metal's recrystaliation point and mechanical stress& not heat. It is used to affect change.

When these metals are cold worked, permanent defects change their crystalline makeup.

These defects reduce the ability of crystals to move within the metal structure and the metal becomes more resistant to further deformation.

The resulting metal product has improved tensile strength and hardness, but less ductility.

Cold rolling and cold drawing of steel also improve surface finish.

Work hardening also called cold working is one of the process of increasing the strength of the material

C.Lubricants used in metal forming processes

They are classified into 4 categories

1.Water-based or soluble oils contain emulsions or micro-emulsions.

They offer great cooling characteristics but lack lubricity.

These lubricants are ideal for high-speed applications and can be sold in concentrates.

2. Oil-based lubricants provide great lubricity characteristics and long service life but sometimes lack additive solubility.

3. Synthetic and semi synthetic lubricants are designed to deliver very high cooling capabilities. Synthetics can be emulsified with water to offer better lubricity.

4. Solid film lubrication is the use of solid materials in conjunction with or without oil or water. The best applications are in extreme conditions such as low shaft speeds, high and low temperatures, and high pressures.

D. Consideration that must be taken into account when designing a section for extrusion

1. Limit the Irregularities of shape as much as the function of the parts allows

2.Provide standard shapes that might serve the designer

3.Thumb rule can be used for cross check

4.Tolerances are advised to be given very caution to avoid interference.

5.Radius corners on a product reduces die stress and extends the life of the die