Written Instructions

The objective of this experiment is to determine the increase in hardness of an engineering material as a function of the amount of cold work (also called strain hardening) imparted onto the material.

Cautions:

• Do not use your bare fingers to push the specimen through the rolling mill.

• Do not use the Rockwell hardness tester until you have received the necessary instructions. Make sure that the bottom of the specimen directly under the indenter is firmly seated on the platen before applying the major load.

Materials and equipment:

• Rolling mill

• Hardness tester

• Calipers

• Annealed copper sheet

PROCEDURE:

• The experiment is to be performed on the copper sheet that is provided to you. You should start out by measuring accurately the dimensions of the sheet and its hardness. The length and the width of the sheet can be measured with a ruler, but the thickness should be determined with calipers. The hardness should be measured with a Rockwell hardness tester with a 1/16-in. hardened steel ball indenter, corresponding to the B scale. Each measurement should be performed three times. Be sure to read the instructions of the hardness tester before using it.

Figure 1. Common metal forming operations that change the cross sectional area of metal, and thus produce cold work CW

Thickness reduction and measurement:

• The thickness of the metal sheet should be reduced to about 50% of its original value in four steps of rolling. If the original thickness is d mm, the thickness should be reduced to about 0.90d mm in the first step, and to 0.8d, 0.65d, and 0.5d in the subsequent steps. After each cold rolling step the hardness of the material is to be determined. At least three measurements should be made for each hardness determination. To properly deform the sample, start by completely closing the rolling mill. Open the rolling mill to the starting point. Close the mill by six markings for the proper spacing for the first cold working step. For the second cold