cold rolling brass
Experiment 5 - Cold Work, Recovery, Recrystallization and Grain Growth
Objective
To study the effects of cold working on the microstructure and mechanical properties of 70/30
Cartridge Brass.
Background
A small percentage of the energy expended in plastically deforming a material remains stored in the metal as an increase in internal energy. Changes are produced in both its physical and mechanical properties. Principally, there is a marked increase in hardness and electrical resistivity with the amount of cold working.
Microstructurally, this increment in internal energy is associated with an increase in the dislocation density as well as the density of point defects, such as vacancies and interstitials.
For most metals, the dislocation density increases from the values of 106-107 lines/cm2 (typical of the annealed state) to 108-109 after a few percent deformation and up to 1011 -1012 lines/cm2 after heavy deformation.
At a more macrostructural level, the grains become markedly elongated in the direction of working and heavily distorted. This distortion is evident from a bending of annealing twins and from unevenness in etching caused by local strain inhomogeneities. While the increased hardness and strength that result from the working operation can be important, it is often necessary to return the metal to its initial condition by annealing. This usually means holding the cold worked metal at a temperature above about 1/3 of the absolute melting point for a period of time. The annealing treatment is divided into three distinct regions:
1. Recovery: This usually occurs at low temperatures and involves motion and annihilation of point defects as well as annihilation and rearrangement of dislocations resulting in the formation of subgrains and subgrain boundaries (e.g., tilt and/or twist low-angle boundaries). A distinctive feature of the recovery process is that it does not involve any change in the grain structure of the cold-worked metal,
Objective
To study the effects of cold working on the microstructure and mechanical properties of 70/30
Cartridge Brass.
Background
A small percentage of the energy expended in plastically deforming a material remains stored in the metal as an increase in internal energy. Changes are produced in both its physical and mechanical properties. Principally, there is a marked increase in hardness and electrical resistivity with the amount of cold working.
Microstructurally, this increment in internal energy is associated with an increase in the dislocation density as well as the density of point defects, such as vacancies and interstitials.
For most metals, the dislocation density increases from the values of 106-107 lines/cm2 (typical of the annealed state) to 108-109 after a few percent deformation and up to 1011 -1012 lines/cm2 after heavy deformation.
At a more macrostructural level, the grains become markedly elongated in the direction of working and heavily distorted. This distortion is evident from a bending of annealing twins and from unevenness in etching caused by local strain inhomogeneities. While the increased hardness and strength that result from the working operation can be important, it is often necessary to return the metal to its initial condition by annealing. This usually means holding the cold worked metal at a temperature above about 1/3 of the absolute melting point for a period of time. The annealing treatment is divided into three distinct regions:
1. Recovery: This usually occurs at low temperatures and involves motion and annihilation of point defects as well as annihilation and rearrangement of dislocations resulting in the formation of subgrains and subgrain boundaries (e.g., tilt and/or twist low-angle boundaries). A distinctive feature of the recovery process is that it does not involve any change in the grain structure of the cold-worked metal,
References: 1. Van Vlack, Elements of Materials Science and Engineering, Chapter 6 2. Barrett, Nix and Tetelman, The Principles of Engineering Materials 3. ASM Handbook, Vol. 2, Heat Treating and Cleaning of Metals 4. Flinn and Trojan, Engineering Materials and Their Applications, Chapter 3