Metal articles for use at elevated temperatures

2,763,584

Patented Sept. 18, 1956

United States Patent Office

2,763,584

METAL ARTICLES FOR USE AT ELEVATED TEMPERATURES

Frank S. Badger, Kokomo, Ind., assignor to Union Carbide and Carbon Corporation, a corporation of New York

No Drawing. Application April 16,1952, Serial No. 282,677

Claims. (Cl. 148—32)

This invention relates to articles for use at elevated 15 temperatures arid refers more particularly to articles, such as blades for gas turbines, which are heated and cooled quickly many times in service. The invention has for its principal object the provision of such articles with improved resistance to failure by thermal shock. 20

There are many alloys now available for use under the extreme conditions of temperature encountered in the gas turbine, the jet engine, heat-treating furnaces, combustion nozzles and similar devices. A common characteristic of such alloys is that a relatively high carbon 25 content seems to be required to develop their best strength and resistance to rupture under load at high temperatures. Many of them contain boron.

While alloys in this general category exhibit excellent properties under load at elevated temperatures in tests such as the stress-rupture test where a load is applied to a specimen and maintained while the specimen is heated at a selected high temperature until the specimen fails, under conditions of actual use where an article is subjected to repetitive cycles of heating, loading and cooling, service life is frequently disappointingly short. Failure often occurs before the expected time and is evidenced by the formation of minute cracks. Such cracks are propagated by fatiguing conditions and eventually may lead to rupture resulting in property damage 40 and even loss of human life.

I have observed that this phenomenon of cracking is most often encountered in articles composed of highcarbon alloys and seems to be somewhat accelerated in articles composed of such alloys which also contain 45 boron. I believe the cracking is caused by thermal shock, that is, rapid heating or cooling to or from operating temperatures. I have discovered that it can be substantially prevented by surface decarburization and deboronization of articles composed of such alloys before ⁶⁰they are put into service.

The invention comprises an article, which in its normal use is subjected to repetitive heating to temperatures in excess of 1000° F. and rapid cooling to atmospheric temperatures, which article is provided with a skin or ⁵⁵case containing less carbon and less boron than the interior portions of the article. Generally, such articles are composed of cobalt-base or nickel-base alloys containing 10% to 30% chromium; up to 25% in the aggregate of at least one metal selected from the group consisting of molybdenum and tungsten; up to 8% in the aggregate of at least one metal selected from the group consisting of columbium, tantalum and titanium, titanium not exceeding 3%; up to 30% iron; up to 1% carbon; and up to 0.5% boron; the remainder nickel or cobalt or mixtures thereof together with incidental impurities.

In accordance with the invention there is provided on the surface portions of such articles a decarburized layer or skin about 0.001 inch to 0.01 inch in depth dependent „„ on the cross sectional thickness of the article. At the ⁷outermost portion of this decarburized layer the metal is substantially carbonless. The carbon content gradually increases toward the innermost portions of the layer. Generally the carbon content of the decarburized layer may range from about 0.001% to 0.1%. In this layer 5 the boron content of the alloy, if any, is also reduced to trace amounts to like depth. The effect of the treatment may be noticeable under the microscope for about 0.03 inch, but the interior portions of the treated article retain their full carbon and boron contents.

Decarburization and de-boronization may be accomplished simply by heating the article in a hydrogen atmosphere at a temperature in the range of about 2100° F. to just below the melting point of the alloy of which the article is composed. Heating is usually continued for at least about 1 hour, the time depending on· the temperature and size of the article. After heating, the article is cooled in air. If desired, it may then be aged in conventional manner for instance by heating at a temperature of about 1300° to 1500° F. for 8 to 48 hours.

Tests have shown that surface-decarburized alloy articles according to the invention have greatly improved thermal shock resistance as compared to articles composed of the same alloy but not provided with a decarburized skin. For example, in one series of tests cast turbine blades composed of an alloy containing about 18% chromium, 10% nickel, 15% tungsten, about 0.4% carbon and 0.03% boron, remainder cobalt, were evaluated. One group of blades was heated in hydrogen at a temperature of about 2350° F. for one hour. After cooling, examination showed that the carbon and boron contents of the surface portions of these blades had been reduced to a trace and that the decarburized and deboronized skin extended about 0.005 inch in depth.

Both the decarburized specimens and the untreated specimens were then tested in accordance with the test described in Technical Note 2037, published February 1950 by the National Advisory Committee for Aeronautics. In this test a wedge-shaped specimen %₂ inch thick at its thin edge is held in a furnace at 970° C. (1750° F.) for 10 to 15 minutes. The specimen is then quenched in a rack which holds the thin edge of the wedge in contact with water. The specimen is cooled to room temperature, oxide is removed from the quenched edge, and the quenched edge is examined for cracks. The cycle is repeated until cracks progress across the full width of the quenched edge. When this occurs, the specimen is considered to have failed. Three untreated specimens cracked after 6, 7 and 7 cycles respectively but three decarburized specimens failed only after 47, 109 and 122 cycles respectively.

By confining decarburization to a very thin skin, the articles are not seriously impaired from the standpoint of stress-to-rupture strength. At the same time, as shown, the surface-decarburized articles of this invention have substantially improved resistance to failure by thermal shock.

Related subject matter is described and claimed in the application of H. R. Spendelow, Jr. and R. M. Fowler, gQ Serial No. 282,678, filed April 16, 1952, now U. S. Patent No. 2,679,466, issued May 25, 1954.