High Yield Strength Ni-Cr-Mo Alloys and Methods of Producing the Same

This invention relates to high yield strength, corrosion resistant Ni-Cr-Mo alloys and methods of producing them and particularly to such alloys having substantially good ductility in combination with high yield strength produced by aging to produce an A.sub.2 B ordering reaction.

There are many situations where a high yield strength corrosion resistant material whose ductility is unimpaired is desirable. For example, shafts in centrifuges, marine shafts and propulsion parts, and a great variety of other parts which are subject to loading at low and intermediate temperatures, in corrosive environments, need high yield strength and unimpaired ductility.

I have discovered that certain Ni-Cr-Mo alloys containing low carbon contents can be given unexpectedly high yield strengths without substantially affecting their ductility by aging in the range 900.degree. to 1100.degree. F. to effect an A.sub.2 B ordering reaction. Aging below or above this level will not affect the yield strength to any significant degree. The corrosion resistance is essentially not drastically affected by this same aging treatment. It is expected that the A.sub.2 B ordering reaction may be effected beginning at about 50 hours at temperatures within the range 900.degree. to 1100.degree. F.

Preferably, I provide in a process for producing a high strength material having substantially good ductility to provide a ductile, high strength, corrosion resistant alloy, the steps comprising: (1) preparing a body of material having a composition consisting essentially of by weight, about 13% to 18% chromium, about 13% to 18% molybdenum, less than 0.01% carbon, less than about 6% iron, less than about 1.25% cobalt, less than about 4% tungsten, less than 0.5% aluminum, less than 1% manganese, less than 0.5% silicon and the balance nickel with usual transient metals and impurities in ordinary amounts, and (2) thereafter aging said body at a temperature in the range about 900.degree. and 1100.degree. F. to effect an A.sub.2 B ordering reaction in the composition. Preferably, aging is carried out at 1000.degree. F. for times of about 50 hours and up to about 8000 hours.

In the foregoing general description, I have set out certain objects, purposes and advantages of my invention. Other objects, purposes and advantages will be apparent from a consideration of the following description and the accompanying drawings in which:

FIG. 1 is a graph of yield strength vs. aging temperature for an alloy composition according to this invention;

FIG. 2 is a graph of elongation vs. aging time for the composition of FIG. 1;

FIG. 3 is a graph of yield strength vs. aging temperature for a second composition according to this invention;

FIG. 4 is a graph of elongation vs. aging time for the composition of FIG. 3;

FIG. 5 is a graph of yield strength vs. aging temperature for a third composition according to this invention; and

FIG. 6 is a graph of elongation vs aging time for the composition of FIG. 5 .

Several alloy compositions within the range of this invention were melted, cast and wrought into plates. A group of 5 inch .times. 5 inch samples of each was aged for various times and temperatures and the physical properties determined.

The compositions of these alloys are set out in Table I hereafter.

The samples were aged in static air, without stress for 1000, 4000 and 8000 hours. Each 5 .times. 5 inch specimen was then cut into standard samples for testing. The physical properties of the alloys in the annealed condition prior to aging (average of 3 tests) is set out Table II.

The room temperature properties of Alloy 3 after aging (average of three tests) are set out in Table III.

The room temperature properties of Alloy 2 after aging (average of three tests) are set out in Table IV hereafter.

The room temperature properties of Alloy 1 after aging (average of three tests) are set out in Table V.

The yield strength values on 8000 hours aging of Alloy 3 plate are plotted on FIG. 1 and the elongation ratio aged/annealed are plotted on FIG. 2. Similarly, the yield strength values on 8000 hours aging of Alloy 2 are plotted on FIG. 3 and the elongation ratio aged/annealed are plotted on FIG. 4. Finally, the yield strength values on Alloy 1 plate are plotted on FIG. 5 along with the elongation ratio aged/annealed on FIG. 6. The data from Tables III, IV and V and FIGS. 1 through 6 illustrate the surprising increase in yield strength on aging in the temperature range 900.degree. F. to 1100.degree. F. while no substantial degradation in ductility occurs.

A plate of Alloy 2 was subjected to a corrosion rate test (Streicher Test) in the annealed and aged conditions. The results are tabulated in Table VI.

To further explore the suitability of this discovery to increase the strength of Ni-Cr-Mo Alloys at elevated temperatures and to explore the effect of shorter aging times more economically feasible than 8000 hours, a series of tensile tests were conducted on Alloy 2 aged at 1000.degree. F. for only 1 week (168 hours). The results of these tests are given in Table VII along with comparative data for the same Alloy 2 tested in the commercially standard mill annealed condition (1950.degree. F. for 15 minutes and rapid air cooled). The data show that the improvement in strength obtained by proper aging as low as 168 hours are maintained at elevated temperature, illustrating that this invention could be economically useful for parts operating at conditions hotter than ambient temperature. These results suggest that aging for about 50 hours will effect an effective degree of A.sub.2 B ordering.

In the foregoing specification, I have set out certain preferred practices and embodiments of my invention, however, it will be understood that this invention may be otherwise embodied within the scope of the following claims.