US2157902A - Impregnation of metals with silicon - Google Patents

Description

Patented May 9, 1939 UNITED STATES IMPREGNATION OF METALS WITH SILICON Harry K. Ihrig, Milwaukee, Wis., assignor to Globe Steel Tubes 00., Milwaukee, Wis., a corporation of Delaware No Drawing. Application January 4, 1938,

' Serial No. 183,321

8 Claims. (Cl. 148-14) This invention relates to the impregnation of metals with silicon, more particularly to the silicon cementation of ferrous base articles.

In my copending application Serial No. 86,823, filed June 23, 1936, now Reissue Patent No. 20,719, granted May 10, 1938, I have disclosed and claimed a process for the impregnation of metals with silicon. In accordance with that invention articles formed from metals capable of silicon cementation are heated to a suitable elevated temperature and are then contacted with a siliconizing reagent formed by high temperature reaction of a silicon-bearing material, such as pure, or substantially pure, silicon, ferro-silicon and silicon carbide, with a reagent of the group chlorine gas and chloride vapor. In the practice of that invention the article may be packed in the silicon-bearing material, or rotated in contact with it, and the chlorine gas or chloride vapor introduced into the container after it has been brought up to the treating temperature, the siliconizing reagent being thus formed in contact with the article.

The temperature at which impregnation is effected will depend in part upon the particular material used and the results to be atatined. With ferro-silicon it is possible to effect impregnation by heating the article to about 1300 F. and then introducing the chlorine gas. Somewhat higher temperatures are apparently necessary with silicon carbide, say 1500 F. and higher. Higher temperatures increase the rate at which siliconizing occurs; for most purposes and for the production of relatively thick cases rich in silicon it is preferred to operate at temperatures upwards of 1650" F., suitably 1800 to 1900 F.

Alternatively the article is heated in one container, and the siliconizing reagent formed in a separate container and passed into contact with the heated article. In this latter embodiment it suffices to heat the article to temperatures above about 1500 F., suitably 1800 to 1900 F., but to form the reagent apart from the article it is necessary to heat the silicon-bearing materialto a a substantially higher temperature, say 2300 F.

The invention disclosed in my aforesaid application is capable of providing siliconized products possessing particularly desirable characteristics. For instance, it is possible toproduce cases of substantial depth containing from about 12 to about 14 per cent of silicon, or to cause the impregnation to take place throughout the entire section of the article. 'The cases produced are especially desirable because they are coherent, satisfactorily adherent to the case, and highly resistant to the action of corrosive media, such as mineral acids, and to scaling at elevated temperatures. Also, and this is of particular advantage, the cases are extremely resistant to abrasion. The process is easily and simply performed, and the products are of such desirable properties and satisfactory character that not only has that invention achieved commercial success, but also application to commercial uses is being extended continuously. As far as I am aware, it is the only process known heretofore which produces commercially satisfactory siliconized products.

In the production of articles in accordance with that invention it occasionally happens that the surfaces of the treated articles are rough when ferro-silicon is used as silicon-bearing material. For many purposes this is unimportant, but for some purposes it is desirable to have the surfaces perfectly smooth. Again, where silicon carbide is used as the silicon-supplying material, it is characteristic of that invention that the cases produced contain, at least in their marginal layers, large amounts of carbon resulting, apparently, from the carbon contained in the silicon carbide. Although carbon does not aifect the satisfactory character of the cases for most purposes, it occurs largely in the graphitic state and thus may cause the cases to be somewhat too porous at the surface for some uses.

It is among the objects of the present invention to provide silicon-impregnated articles by means of siliconizing reagent formed by the reaction of silicon carbide or ferro-silicon with. chlorine gas or chloride vapor, which articles in the impregnated regions do not contain abnormal amounts of carbon when silicon carbide is used, and which when ferro-silicon is used possess smooth surfaces.

A further object is to provide a method of making such articles in which some control of the silicon content is possible.

Yet another object is to provide silicon-cemented cases in which the silicon content is substantially uniform to a substantial depth inwardly from the outer surface.

It has been believed heretofore that in the method disclosed in my above-identified application, the article should be maintained under nonoxidizing conditions during the period in which it is heated to the treating temperatures, as by passing a suitable non-oxidizing gas, such as hydrogen, into the container. I have now discovered, however, that when the article is heated in contact with the silicon-bearing material it is not only unnecessary to introduce a non-oxidizing atmosphere, but also that actual benefits are to be derived from the presence of an oxidizing influence. Stated otherwise, I have discovered that the objects of this invention may be realized in the practice of the process described in my aboveidentified application, by having an oxidizing agent present in the treating container either during the heating step (i. e., prior to introduction of chlorine gas or chloride vapor) or during the treating step, or both.

The invention may be described further and exemplified by tests which demonstrate the benefits to be derived from its practice. In all of the tests described hereinafter there was used, for convenience, SAE 1015 steel in the form of 1 inch round bars. It will be understood, however, that the invention is not restricted to the treatment of such steel, but is applicable generally to the siliconizing of iron and steels as well as other metals capable of impregnation with silicon. In all of the tests the bars were treated in accordance with the procedure of my above-identified application. They were placed in a rotary furnace together with silicon carbide, heated to an appropriate temperature, and chlorine then introduced and continued for a period of time sufficient to effect substantial cementation of the bars.

In tests A and B iron oxide in the form of mill scale was used as the oxidizing agent in accordance with the present invention, being mixed with the silicon carbide before heating the furnace. In test C no oxidizing agent was used, non-oxidizing conditions being maintained during heating, and the test being representative of the practice of and results attainable by the invention of my aforesaid application. The operating conditions of these tests are shown in Table I:

Table I Case after dissolving coreout with BNO: Test SiO Mill Ch Hours Temp. No. ii scale lbs run F.

per- Slpercent cent A 20 20 4. 76 2 1850 0.22 12.14 B 25 5. 00 2 1850 0. 13. 25 O 40 4.50 2.5 1835 0.56 12.61

Table I shows also the total carbon content of the cases formed in the three tests.- Comparison of tests A and B with test C shows clearly that the use of an oxidizing agent in accordance with the present invention greatly reduces the overall carbon content of the siliconized case. This characteristic is evidenced more clearly in Table II which gives the carbon and silicon contents of successive cuts taken from the cased bars.

As appears from Table II, the outermost region of the cases produced in accordance with my prior invention (test C, 1st cut) contains about 3.5 per cent of carbon. In contrast, the corresponding region of cases produced in accordance with the present invention (tests A and B) contains only one-seventh to one-tenth as much carbon. Intest C substantial amounts of carbon were present in the second cut, and carbon was present throughout the case. However, in the second and third cuts of bars made by tests A and B, in accordance with the present invention, no carbon could be detected by standard combustion methods; in the fourth and fifth cuts the carbon contents were below those of the corresponding cuts of test C bars. Generally there is an increase in carbon toward the inner boundary of the case due, I believe, to the migration of carbon ahead of the silicon as impregnation proceeds, and of course the carbon initially present in the steel may show this phenomenon in the practice of the present invention, as indicated by the last cuts of bars from tests A and B.

Table II illustrates also another result attainable in the practice of this invention. It will be observed that in test A there were used 20 pounds of mill scale, while only 8 pounds were used in test B. As shown by Tables I and II the over-all and individual cut silicon contents produced in test B were higher than in test A. This illustrates a trend shown by my tests for the silicon content to decrease with increasing amounts of iron oxide. Thus the silicon content may be varied by variation in the amount of mill scale or similar oxidizing agent used, the greater the proportion of oxidizing agent, the lower being the silicon content. This is illustrated further by test D, similar to tests A and B, in which bars of the steel were placed in a rotary furnace together with 25 pounds of silicon carbide and 40 pounds of mill scale and heated to 1840 F. At that temperature chlorine was introduced and continued for 3 hours, there being used a total of 9.5 pounds of chlorine. The outer 0.01 inch cut of the case produced in this manner contained 6.94 per cent of silicon and but 0.24 per cent of carbon. The carbon content, like that of tests A and B, was thus very low, while the effect of increasing the amount of oxidizing agent upon the silicon content of the case is directly apparent upon comparison with the data for the first cut of bars produced by tests A and B. In succeeding cuts of 0.01 inch depth of test B bar the carbon content varied from about .03 to 0.7 per cent, while there was a rather decided uniformity of silicon content through the outer 0.05 inch portion of the case. This uniformity of silicon content is evidenced also by the data for test B, from which it appears that the silicon content is rather uniform throughout the outer region of the case, the variation being less than per cent in the first three cuts, and only about 1 per cent in the first four cuts. For some purposes, as for maximum life under abrasive conditions, this ability to produce cases of approximate uniformity of silicon content to a substantial depth is clearly advantageous.

In obtaining the data given in Table I a section of the treated bar was boiled in nitric acid of about 30 per cent strength until the core was dissolved. The case remaining was then analyzed for silicon and carbon. This method does not necessarily give absolute results because the relatively low silicon content of the case next to the core permits the acid to remove more or less of the case at this point. Likewise, this procedure may also remove some of the graphitic carbon in the outer and inner regions of the case. Experimental difiiculties are encountered also in obtaining data such as given in Table II because these cases are commercially unmachinable. Cuts can not be taken for analytical purposes with a grinding wheel because the ground material would be contaminated by particles from the wheel. In obtaining the foregoing data,

and similar data given hereinafter, a high-speed steel tool was used for making the cuts. Such tools must be ground at very frequent intervals for each cut. Naturally, as the tool dulls the cut is not wholly accurate. Although the data given are thus not entirely absolute, they are relative and comparable, and they do accurately represent the character and trend of results obtained in the different tests.

The invention is applicable also to minimizing or eliminating the surface roughness occasionally encountered in theuse of ferro-silicon in the practice of my above-identified prior invention. This may be exemplified with reference to tests in which the steel bars were treated in the'manner described above, using mixtures of silicon-carbide and ferrosilicon, there being mixed therewith mill scale in accordance with the present invention. The operating data are 29 percent silicon.

Table III shows that the cases produced were very low in carbon, which is characteristic of the present invention. These bars were free from the roughness which may be encountered in the use of ferro-silicon in accordance with my use of materials containing free silicon, in accordance with that invention, is likewise applicable to the method of the present invention to increase the silicon content of the product, or to offset the tendency of large amounts of oxidizing agent to reduce the silicon content, while attaining the advantages of the present invention, such as low carbon content and smooth-surfaced case's. Also, such use of ferro-silicon causes the silicon content to become uniform to a substantial depth of the case.

The use of ferro-silicon as a silicon-supplying material may thus be advantageous where for any reason it is desirable to use relatively large amounts of mill scale, or similar oxidizing agent, to offset the tendency of large amounts of oxidizing agent to reduce the content of silicon in the case, while obtaining a case of low carbon content. This is demonstrated well and further by test G conducted in the manner of those described above. The steel bars were rotated in a furnace with 25 pounds of silicon carbide (47% Si), 10 pounds of mill scale, and 6 pounds of ferro-silicon. When heated to 1850 F. chlorine was introduced and passed through the furnace for 4 hours, using a total of 2 pounds of chlorine. Despite the factthat in this test there was used twice as much mill scale as in test F, the case had an over-all silicon content of 14.33 per cent, and the outer 0.01 inch layer of the case contained 14.99 per cent of silicon, an exceedingly high value for cases produced by this general procedure. This case likewise exhibited great uniformity of silicon content, as appears from the following table:

Table V.-Test G Si percent 14.90 prior invention. The carbon and silicon contents 11.22 throughout the outer regions of the cases pro- 14:43 duced in tests E and F are shown in Table IV. 14.13

Table IV 1st cut 2nd out 3rd out 4th out 5th out N Run 0 'Per- Per- Per- Per- Per- Per- Per- Pcr- Per- Percent C- cent Si cent 0 cent Si cent 0 cent 51 cent 0 cent Si cent 0 cent Si All cuts 0.010 inch.

The data of Table' IV exemplify particularly well a further advantage to be derived from the use of ferro-silicon in the practice of the present invention, namely assured high and uniform silicon content coupled with the low carbon content characteristic of products made according to the present invention.

In the practice of my aforesaid prior invention the maximum silicon content of the case which I have observed isabout 14.3 per cent. In a oo- The over-all carbon content of the case was but 0.32 per cent, and the carbon content of some of the cuts was as low as 0.01 per cent.

In the foregoing tests the mill scale was present, as will be observed, during both the heating and the silicon impregnating stages of the operation. The invention is not restricted to having the oxidizing influence present during both stages. Nor is it restricted to the use of mill scale, it being possible to use other iron oxides as well as various other oxidizing agencies which possess the function exemplified, by way of illustration and not of limitation, in the foregoing tests. For instance, copper oxide and air have been used satisfactorily as the oxidizing agent, and metallic oxides in other forms, 1. e., as salts, might be used.

In test H the steel bars were placed in a rotary furnace together with 500 grams of silicon carbide and heated to 1860 F. Air was passed into the container during the heating period. The air current was then shut off and chlorine intro duced into the container for 2 hours, there being used a total of 0.05 pound. In test I the steel bars and 500 grams of silicon carbide were heated in the rotary furnace to 1850" F. No gas was used during the heating period but after the contents of the furnace reached temperature a mixture of air and chlorine was passed into the furnace for 4 hours, a total of 1.5 pounds of chlorine being used.

The cases produced in both tests possessed the low-carbon characteristics described and exemplifled hereinabove. The over-all silicon content of the case resulting from test H was 12.8 per cent, and that of test I was 12.9 per cent. Similarly, the over-all carbon content produced by test H was 0.14 per cent, and that of test I was 0.15 per cent. It thus appears that oxidizing agent may be present during the entire procedure (tests A to G), during the heating step (test H) alone, or during the treating step (test I) alone, with attainment of benefits such as characterize the present invention.

According to the provisions of the patent statutes, I have explained the principle of my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. In a method of treating metallic articles capable of impregnation with silicon which includes the steps of heating such articles to an elevated temperature and contacting them with siliconizing reagent formed by the action of chlorine or chloride vapor on silicon-supplying material of the group silicon carbide, silicon, and ferro-silicon, the step comprising providing oxidizing conditions and thereby producing a siliconized case having a low carbon content.

2. In a method of treating metallic articles capable of impregnation with silicon which includes the steps of heating such articles to an elevated temperature and contacting them with siliconizing reagent formed by the action of chlorine or chloride vapor on a mixture of sili; con carbide and ferro-silicon, the step compris ing supplying iron oxide to said mixture and thereby producing a siliconized case having a low carbon content, and of'approximately uniform silicon content to a substantial depth.

3. That method of impregnating metals with silicon which comprises the steps of heating the article in a container and in contact with an agent of the group silicon carbide, silicon and ierro-silicon to a temperature of at least about 1300 F., passing chlorine-supplying atmosphere into the container, and during at least such heating step providing oxidizing conditions in said container and thereby producing a silicon ized case of low carbon content.

4. That method of impregnating metals with silicon which comprises the steps of heating the article in a container and in contact with an agent of the group silicon carbide, silicon and ferro-silicon to a temperature oi. at least about 1300 R, passing chlorinesupplying atmosphere into the container, and while supplying such atmosphere providing an oxidizing agent in said container and thereby producing a siliconized case of low carbon content.

5. That method of impregnating metals with silicon which comprises the steps of heating the article in a container and in contact with a mixture of iron oxide and an agent of the group silicon carbide, silicon and ferro-silicon to a temperature of at least about 1300 F., and passing chlorine-supplying atmosphere into the container, and continuing to pass said atmosphere into the heated container for a time to eiIect silicon penetration to a desired depth of the article, and thereby producing a siliconized case of low carbon content.

6. That method of impregnating metals with silicon which comprises the steps of heating the article in a container and in contact with a mixture of silicon carbide and mill scale to a temperature of at least about 1650 F., and then passing chlorine-supplying atmosphere into the container heated to such temperature to cause penetration of silicon into said article, and thereby producing a siliconized case of low carbon content.

7. That method of impregnating metals with silicon which comprises the steps of heating the article in a container and in contact with a mixture of ferro-silicon and iron oxide to a temperature of at least about 1300 F., and passing chlorine-supplying atmosphere into the container heated to such temperature for a time to effect penetration of silicon into the article, and thereby producing a smooth siliconized case of low carbon content, and of approximately uniform silicon content to a substantial depth.

8. That method of impregnating metals with silicon which comprises the steps of heating the article in a container and in contact with a. mixture of silicon carbide, ferro-silicon and iron oxide to a temperature of at least about 1650 F., and then passing chlorine-supplying atmosphere into the container for a time to effect penetration of silicon into the article, and thereby producing a smooth siliconized case of low carbon content, and of approximately uniform silicon content to a substantial depth.

HARRY K. IHRIG.