The induction furnace for the induction melting experiments, employed a
15 kVA, 350-450 kc/s Electroheating power source, the output being varied by the operator using a hand wheel located on the front panel of the unit. As there was no output current meter on the unit, the power setting was gauged from the position of the hand wheel pointer which covered a circular scale marked 0-75. A meter giving the anode current to the valve oscillator was another indication of the power being supplied to the coil.
Water cooled, flexible, high frequency leads from the generator were
connected by brass couplings to a copper induction coil. The induction coil
was made from 12 mm external diameter bright annealed copper tubing, and
covered with a close fitting insulating sheath of vida flex. The coil had seven turns in total, the diameter of each turn being 9cm.
The melting assembly used is illustrated in Figure 3.1 and briefly comprises of an alumina refractory brick placed at the base of the induction coil, to act as a support for the crucible. The coil was lined with kaopaper and an
alumina (Al-sint) protection tube placed in the centre. The area between the kaopaper and the protection tube was tightly packed with coarse, granular magnesia (Thermag). A coarse grained, spinel bonded magnesia crucible containing a cylindrical charge of low carbon steel weighing approximately 500g was placed in the alumina protection tube. The composition of the refractory materials are given in Table 3.1(a).
The protection tube had two functions:
1. it acted as a protective barrier if the magnesia crucible failed during the experiment; and
2. enabled an inert atmosphere to be maintained above the melt to prevent
reoxidation of the melt by atmospheric oxygen.
The top of the protection tube was fitted with a porous alumina refractory plug. The plug had two openings; one held a 3mm external diameter alumina tube providing an inlet for the argon gas shroud, the composition of the argon is given in Table 3.1 (b). The second was used to gain access to the melt in order to take both the temperature of the melt and samples during the
experiments.
Temperature measurements in the high frequency furnace were made with the aid of a Pt/Pt13%Rh thermocouple, protected by a 5mm external
diameter silica sheath. The temperature was read from a chart recorder with the appropriate compensations being made.
3.2.2 VERTICAL TUBE FURNACE.
A Carbolite vertical tube furnace with Pyrox elements was used to melt steel samples under conditions of natural convection. The furnace consisted of an 855mm long, 440mm external diameter insulated case containing a central alumina (Al-sint) work tube, the composition of which is given in Table 3.1(a), (1000mm long and 75mm external diameter). Arranged around the work tube were eight Pyrox 2000 heating elements, the main constituent of the Pyrox element was lanthanum chromite. The furnace required a 240 volt single phase power supply and had a maximum power output of 7kW. The maximum working temperature in the vertical position was 1800°C. The
furnace controls were housed in a separate control cabinet which included an ammeter and a Eurotherm type 017 P1D 3 term controller with an integral temperature indicator.
A temperature profile of the furnace was determined using a chromel alumel thermocouple with the furnace temperature set at 1000°C, Figure 3.2. The temperature sensor subsequently used was a Pt30%Rh/Pt6%Rh
thermocouple which was placed adjacent to the Pyrox elements in the hot zone of the furnace. This acted as the control thermocouple as well as the furnace temperature monitor. The control temperature was in good
agreement with the monitoring thermocouple at 1000°C within the accuracy of the thermocouples. The temperature was read from a continuous digital display on the control cabinet.
At the base of the Pyrox Vertical Tube Furnace was a water cooled
aluminium plate on top of which the alumina (Al-sint) work tube was mounted, as illustrated in Figure 3.3. An aluminium spigot was screwed into the plate, this acted as a location point for a second 50mm external diameter and 50cm long alumina (Al-sint) tube. This functioned as a support and ensured that the crucible was located in the hot zone of the furnace. Argon gas, used to create the inert atmosphere during melting, entered the assembly through the aluminium spigot.
A coarse grained, spinel bonded, magnesia crucible of identical composition to the crucibles used in the high frequency experiments, was contained within a 65mm external diameter alumina protection tube. The protection tube was sealed at the base with sodium silicate bonded granular magnesia and rested on the top of the support tube. The main alumina work tube was
extended using a second slightly smaller diameter alumina tube, the junction between the two tubes was sealed with a kaopaper gasket. The extension was sealed with a water cooled aluminium end assembly. This contained one orifice with a stopper which could be removed to enable samples to be taken during the experiment and two outlets which allowed the argon gas to flow from the assembly.
3.3. EXPERIMENTAL PROCEDURES.