Question

Problem 1 (20 points) It has been noted that the more a metal has been cold worked, the less it strain hardens. Explain why. b) Two parts have been made of the same material, but one was formed by cold working and the other by hot working. Explain the differences you might observe between the two. c) Utilizing coldworking, a strip of metal is reduced from 40mm in thickness to 20mm; a similar strip is reduced in a similar way from 40mm to 30mm. Which of these coldworked strips will recrystallize at a lower temperature?

Answer 1

ANSWER 1

a)

Cold working is defined as metal working operation below recrystallization temperature.

Amount of cold work increases yield strength, tensile strength and hardness. It reduces the ductility.

When material's strength and hardness gets increased it becomes difficult to strain hardening take place. Cold worked materials dislocations become pinned and offer resistance against further movement.Thus making difficult to strain hardened.

b)

S/N	Cold Working	Hot Working
1.	Cold worked material has better tolerances and accuracy	Porosity was eliminated, grain structure was very fine
2.	It had better looking,great surface finish	Surface finish was not good but mechanical properties such as toughness was enhanced
3.	No heating was required	Heating was necessary

c)

Recrystallization temperature depends upon amount of cold work material has received.

More the cold work lower will be recrystallization temperature.Since40 mm to 20 mm has received more cold work compared to 40 mm to 30 mm. Recrystallization temperature for 40 mm to 20 mm will be lower.

• Recrystallization temperature varies from 0.33 to 0.25 of Melting Point temperature ( Kelvin)
• For alloys Recrystallization temperature is half the melting point temperature in Kelvin