mss£ oxygen per second or 5*4 oxygen per cm^ surface per second.

The Fe-1.33$ Si alloys were examined at five heights of fall in air and the results are collected in Tables 88 to 92

and Figs. 80 and 81. A total of thirty four droplets were

analysed for oxygen. The results are extremely consistent despite the small number in each sample. Again, as with the Fe-0.75^ Si alloys, the oxygen pick-up is low and beyond the 37 cm fall in air the figures remain static suggesting that once an initial oxygen level is reached there is no further oxidation. Hence the rate constant over the time the droplet fell in air would be zero with a maximum initial rate of 0.14 mass% oxygen per second or

5 .0 mg oxygen per cm2 surface per second.

The results for all the silicon alloys are compared in

Figs. 82 and 83. These clearly show the marked reduction in oxygen pick-up with increasing silicon content in the alloy.

4 .3 Reduction Studies 4*3«1 Introduction

In the reduction experiments it was assumed that the change in mass was attributable only to loss of oxygen and the loss of oxygen could be determined as mass per cent by reference to the mass of the sample. Hence the per cent oxygen loss with tempera­ ture could be determined. Further if the final oxygen analysis on the hydrogen reduced droplet was known it was then possible to work backwards from this final oxygen figure to determine the original oxygen figure and record how the level of oxygen in the sample had changed with temperature and time. Where these

figures have been determined they are plotted with mass per cent - 113 -

oxygen as ordinate and temperature/time as abscissa. Since the heating rate was the same in all the reduction runs, i.e. 6°C per minute, the graphs were plotted such that the temperature intervals corresponded approximately to the time intervals so that, if required, the total heating time could be easily determined.

Of interest from the graphs are the final oxygen figures of the reduced droplet and the temperature at which onset of reduction occurs.

4*3.2 Reduction of Oxidised Iron Droplets

Oxidised iron droplets were reduced in hydrogen and the data is presented in Tables 93 and 94 and Figs.84 and 85. Sample

number A.3O4 bad been oxidised by falling 100 cm through air prior to reduction in hydrogen. There was no change in mass till about 500°C when there was a slight inflexion and no further change till 690°C. Again this was a single inflexion and there was no further change till 870°C. From this point there was a continual

reduction in oxygen level with increasing temperature to 1150°C. The gpecimen now held isothermal at 1150°C continued to be

reduced but at a reduced level until after 30 minutes no further reduction occurred. The final oxygen figure was O.O3S mass% (for

0 .7 g droplet).

Sample number A. 307 was similarly oxidised by a 100 cm fall in air. There was a minor inflexion before 800°G at about 570°C but, as in the previous sample, the major reduction of the oxide commenced at 800°C and continued to 1150°C and for about 20 minutes at 1150°C. The final oxygen figure was 0.O43 mass^ (for 0.7 g droplet).

A number of dewpoint measurements were taken during the heating 19 cycle of a sample similarly oxidised to the above two samples. Values ranging from -20°C(-4°F) to -25°C(-13°F) were recorded.

4.3.3 Reduction of Oxidised Iron-Manganese Alloy Droplets Six ironr-manganese alloy droplets, after oxidation in air, were reduced in hydrogen; four samples were Fe-0.5 mass^ Mn and two samples Fe-0.7 mass/6 Mn. The results are recorded in Tables 95 to 100 and Figs. 86 to 91.

Sanple number D.77, Fe-0.5^ Mn, (Table 96, Fig.87) was

oxidised by allowing it to fall through 50 cm of air before quench­ ing. The onset of reduction was around the 800°C temperature mark. Once reduction commenced it proceeded fairly rapidly and the

specimen was still losing oxygen at 1150°C, the maximum temperature although the rate was beginning to slow down. However, some

reduction occurred up to about 30 minutes at 1150°C.

A similar graph is given by speciment number D.53 (Table 95, Fig. 86). This droplet, of the same composition, was oxidised by falling through 100 cm air. The onset of reduction was about 800 to 820°C. Again once reduction had started it proceeded fairly quickly and by 1150°C was slowing down. Some reduction occurred up to about 20 minutes at 1150°C. Taking the mass loss figures for these two samples and projecting back to an initial oxygen figure, in both cases, gives a level which is somewhat higher than the rest of the samples in the same group.

Two other samples of the same composition and similar oxygen level, D.92 (Table 97, Fig.88) and D.94 (Table 98, Fig.89) are

presented. Although no final oxygen figure is available both show the onset of reduction near 800°C with the highest

reduction rate at the commencement of the reduction process.

Samples D.ll (Table 99, Fig.90) and D.32 (Table 100, Fig.9l), both Fe-0.7$ Mn, with falls in air of 50 cm and 100 cm respectively, gave similar results. Sample D.ll was an * overshoot* situation and the sample reached a temperature of 1300°C before the furnace was switched off. Reduction of the oxide continued up to 1300°C, from the mass-loss figures, and ceased immediately the cooling process commenced. Reduction appeared to commence at about 840°C. Sample D.32 showed a threshold temperature of 820°C for reduction; the reduction rate was greatest immediately after the onset and this rate reduced as temperature increased. Some reduction at H50°C appeared to occur for a few minutes.

4.3*4 Reduction of Oxidised Iron-Chromium Alloy Droplets

Eleven iron-chromium alloy droplets were reduced in hydrogen. Three samples containing 0.17$ Cr were investigated and the

results are given in Tables 101 and 102 and Fig.92. Sample C.109 (Table 101, Fig.92) indicates that reduction commenced at about 750°C. The reduction rate appeared fairly constant up to H 00°C when the rate decreased, however, there was a sharp decrease in

oxygen level after 30 minutes at 1150°C which is ■unusual. Table

102 gives the results of two samples reduced at the same time. Perhaps the only important feature here is the threshold tempera­ ture for reduction, 780°C. The oxygen mass loss recorded in the table is an average for the two samples since there was no means of establishing the rate of reduction of each sample.

Four samples at the0.25$ chromium composition were reduced in hydrogen. Sample number C.54 was oxidised by falling through

50 cm air prior to reduction, the other three samples were