SHEET NON-FERROUS METALS 761 



seen that blocks would also be required to cover the nickel-silver and 

 the phosphor-bronze alloys. To prepare new series of blocks espe- 

 cially for these materials would have resulted in a wide variety of 

 blocks and would have been found burdensome to the laboratories. 

 Consequently, a single series of test blocks covering the brass, nickel- 

 silver, and phosphor bronze alloys was selected. Thisseries of test blocks 

 started with the original blocks to which were added a few additional 

 blocks made necessary in order to cover the range of the nickel-silver 

 and phosphor-bronze alloys. The data upon which the calibration 

 of these blocks was based have been submitted to the Section on In- 

 dentation Hardness of the A. S. T. M. Committee E-1 on Methods of 

 Testing. 



Hardness of Thin Sheet. — The Rockwell hardness test has certain 

 limitations in its application to the testing of thin sheet stock. Mate- 

 rial less than 0.020 in. thick gives hardness readings dififerent from 

 thicker sheet of the same temper. Referring to the curves shown 

 hereinafter of Rockwell hardness plotted against tensile strength for 

 materials thinner than No. 24 B. & S. gage the points fall below the 

 curve. This apparently is due to lack of support of the metal about 

 the penetrator and consequently a low reading is given. For still 

 thinner material the penetrator passes nearly through the metal and 

 the hardness reading recorded is inaccurate due to the effect of the 

 supporting anvil in addition to lack of support of the metal. 



Cleaning Anvil and Specimen. — Inasmuch as the Rockwell hardness 

 tester measures hardness in terms of penetration, any movement 

 of the penetrator affects the hardness reading. In other words, if 

 the metal has a roughened surface or if the anvil is not polished smooth 

 the metal will flow under the high unit pressure involved and this will 

 cause the Rockwell hardness reading to be lower than its true value. 

 It is considered necessary therefore that the anvil should be polished 

 flat and the material tested should be reasonably free from surface 

 imperfections and oxide film. Table III shows the effect on the 

 hardness readings of polishing the anvil. 



In addition to the need of polishing the anvils, if close agreement is 

 to be had, some refinement is also needed in the use of the standard 

 test blocks. It is difficult to obtain test blocks that will not show a 

 variation in hardness. It has been customary, therefore, in calibrating 

 test blocks to take five readings, one in each corner of the test block 

 and one in the center. These readings are made on a machine which 

 has been calibrated with the standard test blocks both before and 

 after calibration of the secondary blocks. Experience in calibrating 

 the Rockwell hardness machine with standard test blocks has empha- 



