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Chapter 7: Problem 35

Explain the differences in grain structure for a metal that has been cold worked and one that has been cold worked and then recrystallized.

Short Answer

Expert verified
Answer: A cold worked metal has elongated grains with a high dislocation density, leading to increased strength and hardness but reduced ductility. A cold worked and recrystallized metal has a more equiaxed, smaller, and uniform grain structure with lower dislocation density, offering a balance between strength, hardness, and ductility.

Step by step solution

01

Introduction

Cold working is a process where metals are deformed at room temperature, whereas recrystallization is a heat treatment process where the cold-worked metal is heated to a temperature high enough to allow new grains to form without causing melting. The grain structure of a material directly affects its mechanical properties such as strength, ductility, and toughness.
02

Grain Structure of Cold Worked Metal

When a metal is cold worked, the applied force causes the existing grains to deform, leading to an elongation of the grain structure in the direction of the applied force. This results in an increase in dislocations, which are defects in the crystal lattice of a material. The increased dislocation density causes the grains to strain harden, which increases the strength and hardness of the material while reducing its ductility. However, excessive dislocation density can lead to material failure because the metal becomes brittle and more prone to crack propagation.
03

Grain Structure of Cold Worked and Recrystallized Metal

When a cold worked metal is heated to an appropriate temperature, recrystallization occurs. New grains start to nucleate from the existing grains' boundaries, growing until they consume the entire cold-worked grain structure. The new grains are typically more equiaxed (having equal dimensions in all directions) and have lower dislocation density. This process helps to restore the metal's ductility without significantly sacrificing its strength and hardness. Recrystallized grain structure is also more uniform and has smaller grains compared to the cold-worked grain structure.
04

Summary

In conclusion, a cold worked metal has elongated grains with a high dislocation density, resulting in increased strength and hardness at the expense of reduced ductility. On the other hand, a cold worked and recrystallized metal has a more equiaxed, smaller, and uniform grain structure with lower dislocation density, providing a balance between strength, hardness, and ductility.

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Most popular questions from this chapter

Briefly explain why small-angle grain boundaries are not as effective in interfering with the slip process as are high-angle grain boundaries.

(a) What is the approximate ductility (\%EL) of a brass that has a yield strength of \(275 \mathrm{MPa}\) \((40,000 \mathrm{psi}) ?\) (b) What is the approximate Brinell hardness of a 1040 steel having a yield strength of 690 \(\mathrm{MPa}(100,000 \mathrm{psi})\) ?

Consider a metal single crystal oriented such that the normal to the slip plane and the slip direction are at angles of \(43.1^{\circ}\) and \(47.9^{\circ}\), respectively, with the tensile axis. If the critical resolved shear stress is \(20.7\) MPa (3000 psi), will an applied stress of 45 MPa (6500 psi) cause the single crystal to yield? If not, what stress will be necessary?

An uncold-worked brass specimen of average grain size \(0.008 \mathrm{~mm}\) has a yield strength of 160 MPa \((23,500\) psi). Estimate the yield strength of this alloy after it has been heated to \(600^{\circ} \mathrm{C}\) for \(1000 \mathrm{~s}\), if it is known that the value of \(k_{y}\) is \(12.0 \mathrm{MPa} \cdot \mathrm{mm}^{1 / 2}\left(1740 \mathrm{psi} \cdot \mathrm{mm}^{1 / 2}\right)\)

The lower yield point for an iron that has an average grain diameter of \(5 \times 10^{-2} \mathrm{~mm}\) is 135 MPa (19,500 psi). At a grain diameter of \(8 \times\) \(10^{-3} \mathrm{~mm}\), the yield point increases to \(260 \mathrm{MPa}\) \((37,500 \mathrm{psi})\). At what grain diameter will the lower yield point be \(205 \mathrm{MPa}(30,000 \mathrm{psi})\) ?
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