Objectives
1. To obtain an understanding of the effects of cold working on the mechanical properties of engineering materials and alloys.
2. To obtain an understanding of the effects that post-heating treatments will have on some mechanical properties of engineering materials and alloys which have been previously work-hardened.
3. To gain experience in operating a rolling mill.
4. To understand the principles of recovery, recrystallization and grain-growth.
Apparatus
Rolling mill, Rockwell hardness tester, electrical resistance furnaces, micrometers, bars of 70% Cu; 30% Zn (cartridge) brass.
Introduction
The two primary reasons for plastically deforming engineering metals and alloys are to change their shape for some particular purpose, or to change their properties. The temperature at which the deformation takes place will be an important determinant of the eventual properties. If the temperature is relatively low with respect to the melting point of the material, the deformation process is termed "cold-working". The upper temperature limit of cold working is roughly one-half the melting point on an absolute scale, but varies with such factors as the rate and amount of deformation, and the composition of the material. A material which is plastically deformed above this temperature is said to be hot-worked. There are significant differences between the effect of cold and hot working on the properties and structure of materials.
While cold working a metal will tend to increase its strength, other properties such as ductility or corrosion resistance may be negatively affected. Therefore, to remove internal stresses of cold work, it is sometimes considered desirable to post-het the metal. If this post-heating, or annealing, is conducted at a sufficiently high temperature, a reduction of the stress necessary to further deform the material may be achieved as recrystallization occurs.
The objective of the experiment is to study the relations between cold work and recrystallization processes and their associated properties.
Experimental Procedure
For each sample, perform the procedure given below and summarize the results in a table.
1. Measure the thickness and width of the sample with a micrometer.
2. Take a hardness reading on the as-received (annealed) specimen with a Rockwell hardness tester using the "B" scale (take the average of at least three readings).
3. Roll the specimen in stages using the rolling mill. Reduce the thickness by rolling from .....to.................
4. Take hardness data for the specimens following each stage of the thickness reduction of approximately 0.020".
5. When the thickness has reached approximately 0.125", test for final hardness and then cut the specimens) into 1-1/2 long pieces with a hacksaw or cut-off machine.
6. Place the samples in furnaces which were pre-heated to 400ºF, 800ºF and 1200º F for 30 minutes and then quench them in water.
7. Take the hardness of the annealed samples. A minimum of three-five readings should be taken on each of the samples and report the average of three independent Rockwell hardness reading on each of the annealed samples.
Report
1. Record thickness and Rockwell B hardness values specimen initially and after each stage reduction.
2. Record Rockwell B hardness values for the specimens after heating for a constant amount of time at the selected annealing temperatures.
3. Plot hardness vs. % reduction in thickness.
%RT = (to - tf/to) x 100
4. Plot Rockwell hardness vs. annealing temperature.
Attachment:- OAA-LAB.xlsx