Cold work recovery recrystallization and grain growth PDF

COLD WORK, RECOVERY, RECRYSTALLIZATION AND GRAIN GROWTH
Object
Effect of recovery, recrystallization, and grain growth on 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. Thus, 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, for example, 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 an unevenness in etching caused by local strain inhomogeneities.
While the increased hardness and strength which results 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 deflects 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, the only changes taking place are the dislocation arrangements within the
existing grains. Small changes in hardness, which are sometimes observed during recovery, can
be attributed to the decrease in the dislocation and point defect density and to the growth of the
subgrains.
2. Recrystallization: If increased thermal activation is available (i.e., if the temperature is
raised) nucleation and growth of strain-free grains in the deformed matrix will take place. As
these grains grow, the dislocations in the matrix are annihilated at the boundaries of the newly-
formed grains. Strength and hardness decrease considerably and ductility increases.
The lowest temperature at which stress-free grains appear in the structure of a previously
plastically deformed metal is termed the recrystallization temperature. This depends upon the
grain size, the severity of plastic deformation, and the presence of solute atoms or second phase
particles. The recrystallization temperature is usually 1/3-1/2 the absolute melting point of the
material.
3. Grain Growth: If a recrystallized material is further annealed at the same temperature or
at a higher temperature grain growth usually occurs. Boundaries between annealed grains
migrate and larger grains grow by an increase in the average grain size (or a decrease in the
ASTM grain size number, n). Grain growth depends on the fact that the grain boundary energy
of the material is reduced due to the decrease in grain boundary area for a given volume of
material. The effect of recovery, recrystallization, and grain growth on grain size, internal stress
and strength (or hardness) of a plastically deformed material is illustrated, schematically, in
Figure 5-1.

Measurement of Hardness
Throughout this experiment, hardness measurements will be made using a Rockwell
hardness tester. A 1/16" steel ball indentor with either a 60 or 100 kg load corresponds to the
Rockwell F or Rockwell B scales. Hardness value may be calculated from the load applied
divided by the surface area of the indentation. However, the instrument you will use is calibrated
to read hardness values directly. Consult with the instructor before operating the hardness
tester. You should run a hardness test on the test standard for the particular scale being used
before and after each set of measurements. At least three hardness measurements should be
made on a smooth surface of each specimen to achieve statistical significance. If your hardness
values fall below 20 or above 100, you need to change hardness scales.
Procedures
You are provided with one rectangular strip of 70/30 brass (Cartridge Brass). The strip is
fully annealed.
On the fully-annealed strip, perform the following:
1. Measure the initial thickness and hardness
2. Reduce the thickness 50% by many small rolling steps. Make sure to measure the
thickness and hardness after each pass through the rolling mill. Tighten the rolls
only a small amount at each step or a safety pin will break.
3. Plot a curve of hardness vs. percent reduction in thickness.
On the cold-rolled sample, perform the following:
1. Measure the hardness using Rockwell B scale
2. Cut the specimen into eight pieces about 1/2 in. long
3. Heat the eight pieces for 30 minutes at the following temperatures
respectively:
oC 250 300 350 400 500 600 700oC