US2098567A - Method of incorporating nitrogen in alloy steels - Google Patents

Description

Patented Nov. 9, 1937 UNITED STATES mz'rnon or mconrom'rmo NITROGEN m mor STEELS George F. Comstock and Viatcheslav V. Efimofl,

Niagara F alls, N. Y., assignors to The Titanium Alloy Manufacturing Company, New York, N. Y., a corporation of Maine No Drawing. Application July so, 1937, Serial No. 156,520

, 20 Claims.

Our,invention relates more particularly to improved methods of introducing nitrogen into alloy steels having a high chro'mium content to improve their physical properties. It is well-known that cast high-chromium steels as ordinarily produced consist of rather coarse crystals that result in weakness, and can not be refined by heat-treatment, and one of the objects of our invention is to devise improved and simplified methods to refine the grain of such steels without incurring the unsoundness which often results from theaddition of nitrogen, or requiring the superheating of the finished steel which islikely not only to create difiiculties in maintaining the furnace structure but. also in preserving the desired steel composition.

Our invention consists, therefore, in new and improved methods-for adding nitrogen to high chromium or other steels, in which nitrogen has been shown to have beneficial effects such as reducing the grain size andimproving the mechanical properties.

Nitrogen has been addedto these steels up to the present time chiefly as high-nitrogen ferrochromium. Although this method with proper precautions may be used successfully for obtaining the desired improvement in the properties of the steel, it is subject to certain limitations which interfere seriously with its use in many steel plants. v

One of these dimculties is the necessity for superheating the molten steel-for a considerable period of timeafter the addition of high-nitrogen ferrochromium, and this practice is harmful to the furnace refractories and also apt to alter the composition of the steel from that which it is desired to make, besides adding to the expense .by requiring longer operation of the furnace.

Another difliculty is the prevalence of gas .holes or porosity in the castings made from steel in which high-nitrogen ferrochromium hasbeen (TiCN) or zirconium cyanonitride (ZrCN) is used as the source of nitrogen for incorporating nitroto gen in.them, the advantages of grain refinement and improved mechanical properties are obtained without the necessity for superheating the melt, and without unsoundness in the castings.

56 The incorporation of titanium or zirconium melting and casting high-chromium steels, we have discovered that when titanium cyanonitride cyanonitride in these steels is not attended with any diiliculty, and is as simple as the addition of any other alloying ingredient. The former compound, (TiCN), as regularly produced for this and other uses by methods described by William M. Thornton Jr. in his book on Titanium published in 1927 by Chemical Catalog Company of New York, N. Y., page 53 contains over 75% titanium and less than 5% carbon, iron, silicon or other impurities, with about 10 to 20% of nitrogen.

A representative analysis of such titanium cyanonitride is as follows:

. Per cent Titanium 80.04 Carbon 3.06 Iron 1.94 Silicon 0.29 Nitrogen and impurities 14.67

Such alloy is a hard but rather brittle substance, and differs from titanium carbide in being more easily crushable to any size desired for adding to molten steel.

Zirconium cyanonitride, as produced by methods set forth in United States Letters Patent to Barton No. 1,351,091 of August 31, 1920, however, is a somewhat softer and more easily crushed compound, and is perhaps slightly less desirable for the purposes of our invention, since its nitrogen content is lower and more of. it-

therefore must be used. It is also more easily burned and lost by oxidation than titanium'cyanonitride. A representative analysis is as follows:

- Percent Zirconium--. 84.0 Car 3.0 Silicon 2.0

1 Titanium 0.2 Iron 0.1 Nitrogen and impurities; 10.?

Although the following expositiomof the advantages of our invention and description of our improved methods refers chiefly to titanium cyanonitride 'which in some waysmay be more desirable and practical to use, it should be understood that the invention applies with equal force to the of the similar compound of zirconium.

A further advantage derived from the employment of our new methods for adding nitrogen to high-chromium or other steels results from the action of the titanium which accompanies the nitrogen in the titanium cyanonitride. The practicability of our methods depends, of course, on the higher affinity of chromium than titanium for nitrogen. In passing from its combination with titanium into the chromium steel, the nitrogen leaves the titanium, with which it had been combined, free to exert the usual beneficial efiects.

of titanium as a deoxidizer and scavenger, thereby improving the cleanness and soundness of the finished steel.

Evidence of such action is afforded by the recoveries of nitrogen and titanium observed when titanium cyanonitride has been used in various ways in chromium steels. With nitrogen recoveries of 19 to 68% from this compound, the titanium recoveries in the steel were only from about 3 to 11%, thereby showing that titanium was released from combination with nitrogen and passed out of the steel. Since titanium is known not to be volatile at steel-making temperatures, such titanium must have been eliminated as the oxide, and therefore has acted to deoxidize the steel without leaving any appreciable titanium content in the resulting steel.

As an example of the use of titanium cyanonitride instead of high-nitrogen ferrochromium as the source of nitrogen in high-chromium steels, the following tests which We have made will now be described.

The materials indicated in the following table were melted in a magnesia crucible by heating with an Ajax-Northrup high-frequency induction furnace, the time for melting being about the same for each heat, with the contents of each heat being poured into a single mold when at temperatures of about 2810 F. No superheating was applied to any of the melts. The titanium cyanonitride, ground to 20 mesh andfiner, was mixed with the low-carbon ferrochromium in Heat2, and both were added together after the iron scrap was fused.

After the ingots had solidified, their top ends were nicked and broken to examine the fractures, and the grain size was estimated by counting the number of crystal facets observed within a one-inch length at several places on the fracture.

The average result, together with a note on the soundness of the fracture, is now given for each heat in the following table.

Heat No.

Pure iron scrap, lbs.--.. 10.5 10. 5 11 High-carbon ierrochromium, 1bs.. 0. 25 0. 25 0. 25 Low-carbon ferrochromium, lbs..-" 5. 75 5. 0 Higlrnitrogen ferrochromium, lbs. 0 I 0 5. 5 Titanium cyanonitride, gms. 0 300 0 Low-carbon i'erromanganese, gins.-. 20 20 20 50% Ierrosilicon, gms 35 -35 35 Per cent nitrogen added 0 0. 44 0. 23 Average grain-size, inches. O. 72 0. 09 0. 06 Shape of riser surlaca Shrunk Shrunk Expanded soundness of fracture Good Good Unsound Our invention is of course applicable to other methods of melting than the one described in the above example, and the titanium cyanonitride may also be added to the steel in other ways than the one described without departing from the essential features of our invention. For example, when making 25% chromium steel in an acid electric furnace of the direct arc type, the titanium cyanonitride was added to the molten iron scrap and covered immediately with lowcarbon ferrochromium, and similar results regarding soundness and grain-size of the castings were obtained as we have hereinbefore described in detail.

The titanium cyanonitride may also be added just before the iron scrap is completely melted, or even stirred into the completed melt after the ferrochromium, although adding it either directly before or with the ferrochromium is the preferred practice. The addition of TiCNshould be made in the furnace rather than in the ladle, since this compound is too refractory to mix well with the steel in the short time available in the ladle, and the reaction involving the liberation of nitrogen also requires a reasonable time.

For the best recovery of nitrogen the presence of some slag which will absorb titanium dioxide is desirable, so that in induction furnace melting where there may be very little slag on the melt, the addition of a small amount of lime we have found to be desirable when titanium cyanonitride is used as a source of nitrogen for grain refinement of chromium steels.

In using zirconium cyanonitride by the method of our invention, exactly the same procedure was carried out as described for Heat No. 2 of the previous example given in respect to titanium cyanonitride, except that a smaller charge was used (15 pounds instead of 17 pounds) and grams of zirconium cyanonitride were substituted for the titanium cyanonitride. This involved an addition of 0.28% nitrogen, and gave a good sound ingot with an average grain size of about I 0.17 inch, decidedly finer than the untreated steel. Zirconium, which is liberated in the steel to which our invention is applied in the same way as has been explained for titanium, has a similar effect in deoxidizing and improving the soundness of the steel, so that the results of the invention are equally beneficial no matter whether titanium cyanonitride, or the closely related compound zirconium cyanonitride, is used.

Our invention may also be applied to other kinds of steel than the plain chromium steel mentioned above, such as steels containing b th chromium and nickel, or other alloying elements, in which a fine grain-size, and absence of gas cavities, are desired. The tensile properties in the resulting steels are substantially those which would be expected from the nitrogen contents.

We claim as our invention:

1. A method of incorporating nitrogen in alloy steels which consists in adding a cyanonitride of an element chosen from the group consisting of titanium and zirconium as an alloy to the other ingredients to form the steel.

2. A method of incorporating nitrogen in chromium steels which consists in adding a cyanonitride of an element chosen from the group consisting of titanium and zirconium as an alloy to the other ingredients to form the steel.

3. A method of incorporating nitrogen in alloy steels which consists in adding titanium cyanonitride as an alloy to the other ingredients to form the steel.

4. A method of incorporating nitrogen in chr0- mium steels which consists in adding titanium steels which consists in adding zirconium cyano- 75 nitride as an alloy to the other ingredients to form the steel.

7. A method of incorporating nitrogen in chromium steels which consists in adding zirconium cyanonitride as an alloy to the other ingredients to form the steel. 1

8. A method of incorporating nitrogen in high-chromium steel castings which consists in adding zirconium cyanonitride as an alloy to the other ingredients to form the steel.

9. A method of incorporating nitrogen in chromium steels which consists in adding titanium cyanonitride and low-carbon ferrochromium to the steel-forming ingredients before melting the charge to produce the steel.

10. A method of incorporating nitrogen in chromium steels which consists in adding titanium cyanonitride and low-carbon ferrochromium to the molten steel-forming ingredients.

11. In the method of making chromium steel containing nitrogen, the step which consists in adding to the steel forming ingredients titanium cyanonitride before melting.

12. In the method of making chromium steel containing nitrogen, the steps which consist in adding to the steel forming ingredients including iron and scrap titanium cyanonitride after the iron scrap has melted, and then adding ferrochromium to the melt.

13. In the method of making chromium steel containing nitrogen, the steps which consist in adding'to the molten steel-forming ingredients titanium cyanonitride, and then stirring the molten mass.

14. In the method of making chromium steel containing nitrogen, the steps which consist in A adding to the molten steelforming ingredients ferrochromium mixed with titanium cyanonitride, and then stirring the molten mass.

15. In the method of making chromium steel containing nitrogen, the steps which consist in adding to the molten steel-forming ingredients in the furnace titanium cyanonitride, and then stirring the molten mass.

16. In the method of making chromium steel containing nitrogen, the steps which consist in adding to the molten steel-forming ingredients in the furnace ferrochromium mixed with titanium cyanonitride, and then stirring the molten mass.

17. In the method of making chromium steel containing nitrogen, the step which consists in adding, titanium cyanonitride as an alloy to the other ingredients to form such steel, the nitrogen acting to refine the grain size with the titanium deoxidizing and degasifying the steel.

18. A method of incorporating nitrogen in chromium steel which comprises adding to the molten steel-forming ingredients titanium cyanonitride, and also lime to form a slag to absorb the titanium dioxide produced in the molten mass.

19. A method of incorporating nitrogen in chromium steel which comprises adding to the molten steel-forming ingredients ferrochromium mixed with titanium cyanonitride, and also lime to form a slag to absorb the titanium dioxide produced in the molten mass.

20. In the method of making chromium steel containing nitrogen, the step which consists in adding to the steel forming ingredients zirconium cyanonitride before melting.

GEORGE F. COMSTOCK. VIATCHESIAV V. EFIMOF'F.