Portable hardness tester

CRUCIBLE FURNACE

The crucible furnace is one of the oldest furnaces for melting non-ferrous metals, and it is being used a great deal at the present time. High carbon steel is made by melting scrap steel in a crucible too. The construction of a crucible furnace consists of a shell which is made of a steel plate about ³/₁₆ in. thick. It rests on a plate made of cast iron. The legs support the furnace high enough from the floor to allow for draft and ashes. The grate works on hinges so that it can be raised and dropped. The grate is turned by the handle. The cover holds down the flame. The furnace should be connected by the flue with a chimney. The melting capacities of the crucible in one heat are from 20 to 300 lbs of brass or from 10 to 100 lbs of aluminium. The linings are made of brick and are applied about the same as in the cupola furnace.

Sometimes the furnaces are set on the floor, but, in most cases, they are set in a pit because they are more easily operated in such a setting. Shavings and kindling are placed on the grate with about 50 lbs of coke on the kindling. After the coke bed is well lighted, one should build up the coke bed so that the top of the crucible, when put in, will be even with the top of the flue. The bottom of the furnace and grates should be kept free from ashes and cinders, or the draft will not get through, causing much trouble in getting the metal enough to run the castings.

The crucible way is used for making high carbon steel and special steels. The percentage of carbon the scrap steel contains must be known.

Fig.56. Portable Tester

In use, the portable tester is applied to the work, as shown in Fig. 56, and a handpiece is depressed, with the result that a diamond indentor is brought into contact

with the surface. Continued movement of the handpiece causes the indentor to be pressed into the work under a pre-determined load for a maximum depth of 0.005 in. A reading which corresponds to the hardness value is then obtained on a large-diameter dial-type indicator. Scales of different colours are provided which give direct readings in Vickers pyramid, Brinell, and Rockwell values. The instrument is available in different types, one of which gives full scale readings for Vickers pyramid numbers from 80 to 1200, Brinell from 80 to 630, and Rockwell from 30 to 70. A second instrument gives full scale readings in Vickers pyramid numbers from 35 to 300. Another type provides readings of Rockwell A, B, and С values from 30 to 90, 40 to 100, and to 70. On the fourth hardness tester the full scale readings obtainable range from 80 to 1200 Vickers pyramid, 80 to 420 Brinell, 35 to 99 Rockwell B, and 20 to 70 Rockwell C.

Дата добавления: 2014-12-27