Fig. 28.—Differential Method for Determining Critical Points of Steel.
The determination of the “critical” points of steel call for special mention. In cooling down a mass of steel the fall of temperature is arrested at one or more points, observations of which are frequently of service in deciding the subsequent treatment of the steel. A method commonly employed is known as the “differential method,” and is indicated in [fig. 28]. The sample of steel, A, is placed side by side with a piece of nickel, B, of equal dimensions, in the tube of an electric furnace. A naked junction, C, is placed in a hole drilled in A, and is connected to the galvanometer G, which is calibrated to read temperatures. A two-junction circuit, formed of a junction D placed in the hole in A, and another junction E located in the hole in B, are connected to a delicate galvanometer H. The furnace is heated until the galvanometer G indicates 900° C., when the arrangement is allowed to cool. As A and B, under normal circumstances, cool at an equal rate, the junctions D and E will be at the same temperature, and no deflection will be observed on H. When, owing to recalescence, the cooling of A is arrested, B, not being thus affected, will continue to cool, thus producing a difference between the temperatures of D and E, and consequently a deflection on H. The temperature of A at the time this occurs is read off on G.
Fig. 29.—Electric Tube-Furnace.
The furnace illustrated in [fig. 29] is suited to this determination. It consists of a silica tube 1 foot long, wound with a special resistance wire and efficiently lagged, and may be heated in safety to 1000° C. for long periods, and to 1200° for a short time. It may be placed across the electric mains directly, and reaches 900° C. in less than half an hour. It consumes 600 to 700 watts at the highest temperatures, and the cost of re-winding is small. This furnace is useful as a general laboratory appliance, and may be kept at a given steady temperature by the use of an external resistance.
The wires in this experiment should be platinum and iridioplatinum or rhodioplatinum, or a good pair of base metals, and the junctions in A should be separated from each other and from the specimen by asbestos; the same precaution being taken to prevent the junction E from touching B. A thin layer of mica should be used beneath A and B, to avoid contact with the furnace tube, which, when hot, allows of leakage of current from the heating coil. Both A and B may be 1½ in. long, ¾ in. diameter, with a hole ¼ in. diameter drilled to a depth of ¾ inch.
An alternative method is to insert a junction in a hole in the specimen, and to take direct readings as the temperature slowly rises or falls, when an arrest in the movement of the pointer of the indicator shows that the change-point has been reached. Special sets are made for this purpose.