Nickel Base Alloy Forged Parts

This invention relates to forged parts fabricated from alloys such as nickel-base alloys, and more particularly, it relates to large forged parts having fine grain size fabricated from nickel-base alloys.

Nickel-base alloys, iron-nickel-chromium-base alloys nickel-cobalt or cobalt-base alloys and the like have been found to be difficult to fabricate, particularly into large forged parts, and consistently obtain fine grain size which aids in achieving mechanical properties in the forged parts. The large forged parts referred to herein include turbine discs or spacers which can weigh 7,500 to 24,000 pounds.

The present invention provides a product and a process for achieving fine grain size and improved mechanical properties in large forged parts fabricated from superalloys such as nickel alloys, iron-nickel, nickel-cobalt and cobalt-base alloys and the like.

It is an object of the present invention to provide large forged parts from heat resistant alloys such as iron-nickel, nickel and nickel-cobalt and cobalt-base alloys.

It is a further object of this invention to provide large forged parts from such alloys having improved mechanical properties.

It is a further object of this invention to provide large forged parts from such alloys having uniformly fine grain size.

It is yet another object of this invention to provide large forged parts from iron-nickel-base alloys and the like having grain size of ASTM grain size 3 or finer as measured by ASTM method E112, Heyn Intercept Method, and being substantially uniform throughout the forged part.

It is still a further object of this invention to provide the forging from such alloys, the forging having enhanced ultrasonic transmission characteristics of at least 15 to 20% reduction in signal attenuation of an ultrasonic signal operating at a frequency of 2.25 MHZ and by at least a 15 to 20% improvement in return frequency of the same signal operating at 2.0 to 2.25 MHZ.

In accordance with these objects, there is provided a process for fabricating recrystallized, large forged parts having fine grain size and improved tensile properties by providing a body of an alloy selected from one of a nickel-base alloy, an iron-nickel-chromium-molybdenum alloy, cobalt-chromium-nickel alloy, nickel-cobalt and cobalt base alloy. The body is heated to a temperature above the recrystallization temperature of the alloy and subjected to a first upset forging operation to provide a reduction of at least 50% of the original length of the body. The body is then drawn at least once before subjecting it to a recrystallization and cooling treatment. The body is then reheated to a temperature above the recrystallization temperature before subjecting to a second upset forging operation to provide a reduction of at least 40 or 50% of its original length. Thereafter, the second upset forging is subjected to another recrystallization and cooling treatment. The second upset forged body is heated above the recrystallization temperature and then subjected to a third upset forging operation which reduces its length at least 60%, preferably at least 75%. After the third upset, it may be solution heat treated, quenched and aged to provide a forged product having improved properties and fine grains distributed uniformly.

The sole FIGURE shows the preferred process steps of the invention.

In accordance with the invention, there is provided a process, as shown in the FIGURE, for fabricating large forged parts having fine grain size and improved tensile properties. Examples of the forged parts are turbine discs having a thickness of about 11 inches and a diameter of about 72 inches and weighing about 13,000 pounds. Examples of other parts which may be fabricated in accordance with the invention are spacers, about 13 inches thick and 60 inches in diameter, and even larger turbine disc, as high as 21,000 lbs or higher, for example.

Examples of alloys which may be processed in accordance with the invention include the AISI Standard Austenitic stainless steels, the higher-nickel austenitic alloys, the precipitation-hardening stainless steels, the iron-nickel-chromium-molybdenum alloys, the nickel-base alloys, the cobalt-chromium-nickel-base alloys and nickel-cobalt-base alloys, compositions of which are provided in the Metals Handbook, Vol. I, page 467, which compositions are incorporated herein by reference. The iron-nickel-chromium-molybdenum alloys, the nickel-base alloys and the cobalt-chromium-nickel-base alloys, referred to as superalloys, are provided herein for convenience as follows:

INCONEL 718 is an alloy containing 17-21 wt. % Cr, 0.65-1.15 wt. % Ti, 0.20-0.8 wt. % Al, 50-55 wt. % Ni, 2.8-3.3 wt. % Mo, up to 1 wt. % Co, up to 0.08 wt. % C, up to 0.006 wt. % B, up to 0.35 wt. % Mn, up to 0.35 wt. % Si, up to 0.015 wt. % P, up to 0.015 wt. % S, up to 0.30 wt. % Cu, the sum of Nb+Ta being in the range of 4.75 to 5.5 wt. %, the balance being Fe.

Iron-nickel-chromium-molybdenum which may be used can have (by wt. %) 10-50 Ni, 10-25 Cr, 0-10 Mo, the balance Fe, and may contain Mn, W. Cb, Nb, Ti, Al up to about 10 wt. %. Representative of such alloys are INCONEL 706, INCONEL 718 and A286.

Nickel-based alloys suitable for use in the invention can have (by wt. %) 10-30 Cr, 0-13 Co, 0-25 Mo, the balance nickel with elements such as W, Cb, Nb, Ta, Ti and Al which may be added up to 10 wt. %. Examples of such alloys are INCONEL 625, NIMONIC 90 and NIMONIC 901.

Cobalt-chromium-nickel-based alloys can have (by wt. %) 10-30 Cr, 10-30 Ni, 1-4 Mo, 1-15 W, the balance cobalt, with other elements which may be added up to 10 wt. % are Mn, Cb and Nb. Examples are WASPOLOY, UDIMET 700, UDIMET 720 and ASTROLOY.

It will be appreciated that modifications may be made to these alloy compositions and such modifications are contemplated within the purview of this invention.

A body of the alloy is provided as an ingot. Typically, the ingot is about 28 to 38 inches in diameter and about 70 to 120 inches in length. Ingots which are square or rectangular in cross section may also be used, depending on the end product desired. Ingots of such alloys are often characterized by coarse, equiaxed or columnar as-cast grains which may range from 0.1 to 1 inch or larger in the major dimension. For purposes of nomenclature, the length or height of the ingot is referred to as the A direction, the width is referred to as the B direction and thickness as the C direction. Obviously, in a round or square ingot, the dimensions are the same in the B or C direction.

By fine grain size as used herein is meant ASTM Grain size 3 or finer as measured by ASTM E112 Heyn Intercept Method where the largest dimension of the grain is measured and the grain size is calculated.

By recrystallized is meant that at least 90%, preferably greater than 95% of the material is recrystallized.

Typically, an ingot of the alloy employed is heated from about 50.degree. to 350.degree. F. above its recrystallization temperature. For INCONEL 706, for example, this is about 2000.degree. F. The ingot may be heated to about 2050.degree. F., for example. The ingot should be held at this temperature for a time sufficient to obtain substantially uniform temperature, which time can be as long as 2 to 10 hours and which is suitable for hot working.

After the heating treatment, the ingot is subjected to a first hot upset forging operation which is accomplished by squeezing the ingot to reduce its height or length, i.e., in the A direction. For INCONEL 706, preferably the forging upset is performed while the ingot is maintained in the temperature range of 1700.degree. to 2050.degree. F. The upset forging can be performed in one step so as to impart work to the ingot which reduces its length greater than 40% of its original length, preferably greater than 50% and typically in the range of 50 to 60%. During the forging operation, heated dies may be used to aid maintaining the ingot in the temperature range for purposes of hot working. For example, the dies may be maintained in the temperature range of 600.degree. to 1000.degree. F., typically greater than 800.degree. F. For purposes of the present invention, while reference is made to an ingot, it should be understood that part of an ingot may be used and reference herein to a body is meant to include a full ingot or part of an ingot or billet.

After the first upset forging, the body is redrawn or reworked. The redrawing is performed preferably in the range of 50.degree. to 350.degree. F. above the recrystallization temperature. For INCONEL 706, this temperature is about 1700.degree. to 2000.degree. F. The redrawing is provided in an amount which preferably brings the ingot or body back to about its original dimensions. For example, in a round ingot, the redrawing can be sufficient to work the ingot back to about its original diameter. However, it is within the purview of the invention to provide a working amount equivalent to redrawing to the original ingot dimension plus or minus 20% of the original length, for example.

It should be noted that the upset and redraw operation can be performed in one step or in a number of steps. That is, the body may be upset forged and redrawn several times to impart the desired amount of work or equivalent amount of work to the body.

By redrawing as used herein is meant working the upset forged body by rolling, drawing or forging, for example.

After the redrawing step, preferably the body is heated to a temperature sufficiently high to cause the redrawn body to recrystallize. For example, for INCONEL 706, the temperature is about 2000.degree. F. The temperature can be higher than the recrystallization temperature without any detrimental effects. Typical time at temperature is about 5 to 8 hours. The redrawn body is then cooled by air to ambient.

After recrystallization and cooling, the redrawn body is again heated above the recrystallization temperature, typically in the range of 50.degree. to 250.degree. F. above the alloy's recrystallization temperature, for example 2000.degree. F. for INCONEL 706. The time at temperature can be 2 to 10 hours. The redrawn body is then subjected to a second upset forging operation in the A direction which reduces it at least 40% of its original length, preferably 50 to 70% and typically about 60%. During this forging operation which can be performed in a single step, the body is preferably maintained in a temperature range of 1700.degree. to 2000.degree. F.

The forging dies can be heated, e.g., to 800.degree. F. or above, to aid in maintaining the body at temperature during the second forging operation.

After the second upset forging thermal operation, preferably the body is subjected to a second recrystallization thermal treatment. At a temperature of 50.degree. to 200.degree. F. above the recrystallization temperature of the alloy, the body can be held for 2 to 10 hours and then cooled to ambient.

Before subjecting the body to a third upset forging operation to provide the final product, the body is heated to a temperature above the recrystallization temperature. The heating may be to a temperature about 50.degree. to 100.degree. F. above the recrystallization temperature. For INCONEL 706, preferably this would be in the range of 1900.degree. to 1950.degree. F. For purposes of the forging operation, it is important that the body is maintained at a high temperature, e.g., in the range of 1800.degree. to 1950.degree. F. for INCONEL 706, during forging in order to aid in developing the fine grain size as strain energy is imparted in this temperature range. To aid in maintaining the body at temperature during the third forging operation, the forging dies can be heated to a temperature range of 1100.degree. to 1400.degree. F. and preferably not less than 1200.degree. F. Further, the body may be insulated to maintain its temperature in the desired range during forging. A suitable insulating material that can be used which does not interfere with the forging operation is fiberglass. The fiberglass can also function as a lubricant. The fiberglass may be covered with a foil in a layered arrangement. Stainless steel foil or similar foil may be used. Thus, the foil-covered insulation may be wrapped around the body and maintained there during the third deformation process.

The amount of deformation provided during the third upset forging is also important. That is, in this step, the amount of work applied is sufficient to reduce the length of the forging by 50 to 70%, preferably sufficient to reduce the length by at least 60% and typically to reduce the length by 55 to 65%.

For purposes of further developing the fine grain structure which is important in the present invention, the forged product may be subjected to a third recrystallization treatment wherein the product is heated to a temperature typically 50 to 200.degree. F. above the recrystallization treatment where the product is held for 2 to 10 hours and then permitted to cool either at air cooling rate or controlled cooling.

The forged product which has the configuration of a disc may be air cooled to about 1300.degree. to 1600.degree. F. and further cooled at a controlled rate of about 10 to 100.degree. F./hr, preferably 20.degree. to 60.degree. F./hr and typically about 50.degree. F./hr to room temperature. Alloys such as INCONEL 706 may be air cooled to room temperature. Thereafter, the forged product may be machined to an appropriate configuration before being solution heat treated, quenched and aged.

For a forged product produced from INCONEL 706, the solution heat treatment is performed in the range of 1800.degree. to 1900.degree. F. for 4 to 6 hours, quenched in oil and aged by heating to 1350.degree. F. and held for 16 hours. Then the product is cooled at about 100.degree. F./hr to 1150.degree. F. and held for 22 to 30 hours.

This process is particularly useful in producing large forgings, e.g., 5000 lbs or more, having improved mechanical properties. The process imparts a uniform level of deformation strain to the product. Further, the process recrystallizes large as-cast grains and produces a fine grain size, e.g., ASTM grain size 3 or finer. This aids in producing enhanced fatigue and tensile properties and ultrasonic transmission characteristics.

The forged disc provided by the process of the invention is useful as turbine discs and spacers on which the turbine blades are mounted. Typical dimensions for such components are about 60 to 90 inches in diameter and 8 to 15 inches thick. When finish machined, the discs are used in land-based turbines and power generators where temperatures can reach 600.degree. to 1300.degree. F. under loads.

INCONEL 706 forgings produced in accordance with the invention can have an ultimate tensile strength in the range of 170 to 190 KSI, yield strength in the range of 135 to 165 KSI and Charpy Impact Values of 25-45 ft/lb.

Further, with respect to the forging and recrystallization steps, it is preferred that each successive step is carried out at a lower temperature than the step before.

The following example is further illustrative of the present invention.

Nickel-base alloy INCONEL 706 (composition in wt. %, 41.4 Ni, 16 Cr, 1.7 Ti, 2.9 Cb and Ta and 0.2 Al, the balance Fe was provided as an ingot 31 inches in diameter and 71 inches in length. The ingot was first heated to 2050.degree. F., which was above the recrystallization temperature, and then worked or upset forged an amount which reduced the length to 35 inches (about 50%) in one step. During the forging operation, the forging dies were maintained at a temperature of about 800.degree. F. The forged body was then redrawn to its original ingot diameter by multiple draw forging steps in the temperature range of 1700.degree. to 2050.degree. F. The redrawn ingot was then placed in a furnace set at about 2050.degree. F. to recrystallize the internal structure of the ingot. After the forged body had reached temperature, it was held at temperature for 2 hours and then permitted to air cool to ambient temperature. For the second upset forging operation, the ingot was heated to about 2000.degree. F. and then upset forged to reduce the length of the body 50% using dies heated to 800.degree. F. The forged body was then placed in a furnace at 2000.degree. F. and held for 1 hour after reaching 2000.degree. F. and then permitted to air cool to ambient temperature. The forged body was then wrapped in an insulating material comprised of stainless steel foil and fiberglass to aid in maintaining the body at temperature and in providing lubrication for the last forging operation. The wrapped-forged body was heated to about 1950.degree. F. and then upset forged to reduce its length by 60% using dies heated to 1200.degree. F. to provide a disc-shaped product. The upset product was placed in a furnace at 1950.degree. F. for about 1 hour and then air cooled.

The recrystallization steps referred to herein can be optional as noted; however, it is believed that such recrystallization treatments aid in allowing uniform fine grain size which is important to properties of the turbine disc or spacer.

The disc product was rough machined, solution heat treated by heating to 1870.degree. F., oil quenched and aged at 1350.degree. F. for 16 hours.

The resulting disc product has an ASTM grain size of 3 to 4, ultimate tensile strength of 180 KSI a yield strength of 140 KSI and a Charpy Impact of 30 to 40 ft/lbs for tangential grain orientation.