CHAPTER XIX

THE DETERMINATION OF OXYGEN IN STEEL

This important determination has not received as much attention as it should have, as the properties of steel are considerably affected by the presence of considerable amounts of oxygen. The oxygen exists in the steel as oxides of iron, manganese, silicon, titanium, aluminium, etc. The method here given depends upon the reaction between the oxide of iron and hydrogen at a high temperature, with the formation of water vapor which is absorbed in phosphorus pentoxide and weighed. Since hydrogen does not reduce the oxides of manganese, aluminium and silicon, the oxides combined with them are not determined with this method, that is, the method determines only the occluded oxygen and the oxygen combined with the iron and other easily reduced metals. However, when the metal is a very pure iron, containing only a few hundredths per cent of manganese and a trace of silicon, the result for oxygen obtained by this method must be fairly accurate. It is precisely in this kind of iron that the oxygen content must be closely watched. The oxides of easily reduced metals, such as copper, nickel and tungsten, could scarcely exist in steel, the oxides of the metals having a very high heat of oxidation, such as silicon, aluminium, chromium, vanadium, are undoubtedly present but are not reducible by hydrogen. The determination of oxygen in steel by the Ledebur method, using hydrogen, therefore, can only determine the oxygen present as iron oxide and probably this oxide when present as silicate is not completely reduced. The higher oxides of manganese, titanium and vanadium are reducible under the conditions of the method, but it is hardly conceivable that they are present. The oxygen which is present as occluded CO2 and CO is largely included in the determination.

The determination depends upon the reaction 2FeO+H2=2Fe+ H2O, the water formed by the reaction being absorbed by phosphorus pentoxide and weighed.

The drillings used for the determination of oxygen are quite certain to have enough oxide formed on the surface, due to the heat generated during drilling, to have a profound effect upon the results, unless the drilling is done under oil.

The accurate determination of oxygen in steel requires the very greatest care and experience. The method as given below is the Ledebur method modified by the American Rolling Mill Company for practical furnace control.

Process of Analysis.-Drill the block of steel under oil until about 100 grams are obtained, pour off the oil and wash with benzol until all trace of oil is removed, dry thoroughly in a desiccator through which a current of air dried over sulfuric acid is passed. The sample should be used for the determination of oxygen as soon as possible after drying, since the steel slowly oxidizes.

For mill practice, where samples can be taken from bars, they should first be cleaned from all mill scale or surface oxide by the use of an emery wheel. The sample should then be placed in a milling machine which should be run at very slow speed in order to avoid oxidizing the millings. A light transverse cut should be taken entirely across the bar and the millings discarded in order to remove any oxidized cavities which were not removed by the emery wheel.

The sample should be the average of the entire cross-section if possible, as there is some difference in gas content between the interior and exterior portions of bars. The sample must be free from all dirt, and samples should not be ground in the vicinity where a sample is being milled, on account of the danger of contamination from finely divided particles of oxide of iron.

The millings should be removed from the sample by the use of a magnet, and placed in a dry glass-stoppered bottle. It is of the utmost importance that millings of uniform size be used for analysis, those passing a 20-mesh and remaining upon a 40-mesh sieve being selected. The millings should remain in the unstoppered bottle for half an hour in a desiccator containing concentrated sulphuric acid. A 30-gram sample is placed in a 1⁄2 x 1⁄2 X6 in. platinum or pure iron boat (for cast iron use a porcelain boat), which is placed in the 7% × 30 in. silica tube T.

In most descriptions for determining oxygen by the Ledebur method, hydrogen is generated by the action of some acid upon zinc contained in a Kipp's generator. It has been found that hydrogen so prepared contains considerable carbon monoxide

and carbon dioxide, whereas hydrogen produced by the electrolytic process and stored in tanks is practically free from these two gases, is much easier to handle and is cheaper.

Apparatus. The apparatus used in making the oxygen determination is shown in Fig. 10.

A 4-liter Kipp generator may be used for generating the hydrogen. It should be charged with drillings of pure iron or mossy zinc, and dilute hydrochloric acid (1:1). Steel turnings should not be used in the generator, as the object is to generate the purest

possible hydrogen. Hydrochloric is preferable to sulfuric acid. Electrolytic hydrogen is much better. After its formation, the hydrogen is purified and dried by passing through the usual train, as shown in the figure. It passes first over stick potash, and next through a 30 per cent potash solution. This solution in the second bottle should be renewed as soon as it shows a tinge of yellow, due to the presence of sulfides. The hydrogen next passes through concentrated sulfuric acid to dry it, and then enters a silica tube with 14-in. bore, 30 in. in length, which contains a roll of platinum gauze or palladiumized asbestos. The 1/4-in. tube lies on top of a 1 X 30-in. fused silica tube contained in a suitable 12-in. gas-blast furnace. It is better to preheat the hydrogen in a separate furnace.

The object of the preliminary heating over platinum foil is to free the hydrogen from the small quantity of oxygen which it

always contains. If this precaution is not taken, the results will be too high. The water formed in the small-bore tube is caught in a U-tube shown in the figure, which contains phosphoric anhydride opened up with glass wool. This drying tube has rubber stoppers. The connection is made with pure gum tubing and is permanent, the sample being introduced from the opposite end of the combustion tube. All rubber connections should be made tight, as directed on page 328.

Blanks should be run from time to time to make sure that the apparatus is in good order and everything working properly. Samples should not be introduced into or removed from the combustion tube when it is more than hand-hot, but silica tubes may be quickly cooled with perfect safety by turning off the gas and allowing the cold-air blast to play on the tube.

Process of Analysis. Weigh 20 or 30 grams of finely divided borings into a silica boat 11⁄2 x 1⁄2 X 6 in. The boat with its charge is quickly inserted into the combustion tube at the right end and pushed to the middle zone by means of a rod of suitable length. The stream of hydrogen should be passing freely when the tube is opened for the insertion of the sample. After the stopper is replaced, the weighing tube and guard tube are finally connected up with pure gum tubing. The weighing tube is a 4in. U-tube, with ground glass stoppers, containing phosphoric anhydride opened up with glass wool. The guard or trap tube is similarly charged and is intended to prevent the drawing back of moisture from the air of the laboratory. After the apparatus is all connected and in good order, the pure dry hydrogen should be allowed to sweep through a few minutes until all traces of air are removed from the entire system. The gas is then lighted, the blast turned on and the temperature run up to a yellow heat, about 1,000°C. This heat is maintained for 30 minutes while. the hydrogen is passing through the apparatus at the brisk rate of about 100 c.c. per minute. After the combustion is completed, the gas is turned off the furnace, leaving the blast playing upon the hot tube. The stream of hydrogen should continue to pass until the tube is cool enough to bear the hand upon it.

Immediately after the tube is cool enough, the weighing tube, with its guard tube, is disconnected and connected with a suitable aspirator, so as to suck out the hydrogen gas and replace

it with air dried over P2O5. A suitable aspirator consists of a 4-liter aspirator bottle filled with water. The upper tubular of the bottle is guarded with a calcium chloride tube to which the weighing tube is connected. A gas washing bottle containing concentrated sulfuric acid follows the phosphoric anhydride tube which is connected to the other side of the weighing tube. Place the weighing tube in the balance with an exactly similar tube and allow them to remain for 15 minutes to come to the temperature of the balance and to attain the same degree of moisture on the surface of the glass. Weigh, using the other tube as a counter poise. The weighing tube should, of course, be weighed in exactly the same way at the beginning of he determination. The increase in weight is H2O produced by the determination. Results ought to check within 0.003 per cent. Blank determinations and proper corrections must be made.

Notes on the Process.-The aspirator may be roughly calibrated by allowing about 500 c.c. of water to run out of the lower tubular of the aspirator. A sufficient quantity of perfectly dry air is drawn through to displace all the hydrogen thoroughly. After all is ready, the weighed tube is closed by its glass stop-cocks, disconnected from its guard tube and placed in a desiccator for 15 minutes before being weighed. Eightninths of the increased weight of the tube is oxygen. The blanks on the apparatus establish the average correction to be subtracted from the weight found. The correction on an apparatus in good order should not exceed 2 mg. On damp days the blank is usually a little higher than when the air is dry.

If the stop-cocks on the weighing tube are ground so as to fit very tightly, it is not necessary to displace the hydrogen with air. If the cocks do not fit very tightly, some hydrogen will diffuse out.

In charging the weighing tube with phosphoric anhydride and glass wool, take care to remove any specks of phosphoric anhydride from the upper portions of the tube.

The following points should be given careful attention in order to attain the highest degree of accuracy:

Samples must be clean, absolutely dry and free from oil. They should be cut, preferably with a milling machine tool running at a low rate of speed. The samples must not heat in cutting. Sheet samples are first. cleaned from oxide on an emery wheel, avoiding heating as much as possible. The sheet should be milled on the edge.