Patent Application
20050155679
This invention relates to CoCr alloys, and more particularly relates to CoCr alloys having improved properties and the methods for manufacturing same.
Typically cobalt/chromium/molybdenum medical implants are produced by a method wherein the raw material is cast into ingots, the ingots are rolled and finished into bar stock of a predetermined diameter, and then the bar stock is cut and forged into its final shape through a multi-stage process. The present invention combines the advantages of forging and casting. To understand the concept, cast and forged products must be compared. The forging input material is typically expensive relative to the casting input material by approximately a 4/1 ratio. The chemistry is essentially the same between the two types of material used for casting and forging. However, the differences include the fineness of grain size and the densification of the material caused by either rolling and/or extrusion to the desired size. This rolling and reshaping adds cost, but it is necessary to refine the grain structure and density the material. In casting, the ingot is simply remelted and poured into a mold of the preformed shape.
Consequently, a forged product has more desirable physical/mechanical properties than a cast product due to its finer microstructure and the elimination of microporosity caused by casting. However, in forging, a multiple number of dies is required. The dies are expensive, and the process of forging the material to its final shape in a number of dies also adds expense. In casting, the final shape/product is realized in one casting step.
Casting and forging each have their advantages—forging provides more desirable physical/mechanical properties and casting is less expensive. Accordingly, the inventive process significantly reduces manufacturing costs by eliminating operations from the manufacturing process, reducing the amount and cost of material required. A need exists for a method and resulting product that combines the advantages of both processes. The problem has been attempted to be addressed in the past. However, parts which were first cast then forged have not been able to achieve the required properties of ASTM F-1537 and ASTM F-799. The present invention overcomes the above-described disadvantages and meets these ASTM specifications.
In accordance with one aspect of the present invention there is provided a method for producing a forged part. The method includes the steps of providing an ingot having a chemistry that meets the chemical requirements of ASTM forging and casting specifications, casting the ingot into a preform by molding, and subjecting the cast preform to one or more hot forging hits, thereby forming the part. In a preferred embodiment the method also includes an annealing process that involves vacuum annealing at between about 2005° F. and about 2235° F. for between about 3 hours and 45 minutes and 4 hours and 15 minutes under a partial pressure of argon. After the annealing process the part is preferably subjected to at least one more hot forging hit at a predetermined temperature. Preferably the final steel part meets the requirements of ASTM F-799 and ASTM F-1537 for warm worked product.
In accordance with another aspect of the present invention there is provided an alloy consisting essentially of:
In accordance with yet another aspect of the present invention there is provided a surgical implant produced by casting and forging. The implant meets the mechanical property and microstructural requirements of ASTM F-799 and ASTM F-1537. The implant can be a hip stem.
In accordance with another aspect of the present invention there is provided a method of producing a forging. The method includes the steps of casting a material having a predetermined shape, and forging the material to produce a product. The product meets the mechanical property and microstructural requirements of ASTM F-799 and ASTM F-1537. The product is preferably a surgical implant.
In accordance with another aspect of the present invention there is provided a method of producing a forged part. The method includes the steps of: preparing an ingot consisting essentially of:
(b) determining the ingot melting temperature; (c) determining the pouring temperature; (d) minimizing the difference between the pouring temperature and the mold temperature to no more than about 100° F.; (e) designing an investment casting preform geometry to achieve uniform reduction in all sections of the part; (f) preparing an investment mold; (g) molding the part by casting; (h) placing the part in a forging die; (i) subjecting the part to one or more forging hits at a forging preheat of between about 1500° F. and about 2200° F. to achieve a reduction of about 10% to about 60%; (j) vacuum annealing the part at between about 1800° F. and about 2400° F. for about 30 minutes to about 8 hours under a partial pressure of argon; (k) quenching the part to below about 1400° F. in about 5 minutes or less under a minimum of about 5 bars of argon; and (l) subjecting the part to one or more forging hits at between about 1200° F. and about 2200° F. to achieve a final about 5% to about 40% of reduction. The pouring temperature is preferably no more than about 100° F. from the ingot melting temperature. The investment mold has one or more gates. The forging die is designed to match the preform geometry to facilitate homogeneous percentage reduction in all areas and section thicknesses of the part.
The invention may be more readily understood by referring to the accompanying figures in which
Generally, the present invention involves the forging of cast ASTM F-75 cobalt/chromium/molybdenum (CoCrMo) alloy preforms to achieve wrought product mechanical property requirements governed by specification ASTM F-799. In a preferred embodiment, the invention is used for producing medical industry surgical implant products made of CoCr material that exceeds minimum requirements for forgings, as specified in ASTM F-1537 and ASTM F-799 specifications, by starting with a casting instead of normal forging bar stock. The present invention can be used for CoCr surgical implants, such as hip stems, and can also be used for other types of implants, such as knee implants, etc. It will be understood that use of the materials and processes described herein in the surgical implant industry is only exemplary and not limiting and that the finished materials and the methods for making same can be used in other fields as well. The alloys may be useful in other industries and applications, for example, commercial or aerospace applications. Materials constructed according to the principles of this invention are formed using a casting and a forging process and have improved physical properties when compared to materials that are formed only by casting.
Additional benefits of the process include a reduction in material usage by approximately 20%-40% due to the geometric capabilities of the investment casting process to produce a preform with material distribution tailored to the final part geometry. The design of the preform can also be tailored to provide a uniform reduction in all sections of the part with resultant mechanical and microstructural property uniformity far superior to that obtained through the use of input barstock.
According to a preferred embodiment of the present invention, a method of producing a forging includes the following steps, which will each be described more fully below:
The above-described process results in a part that started as a casting and, when finished, meets or exceeds all forging specifications and was preferably made at a lower cost than traditional forging.
Step 1—Develop a properly designed casting preform geometry. The casting preform geometry is based on the final part design. Proper preform geometry design entails analyzing the final part design and generating a preform which equalizes the warm work (measured in percent reduction) throughout the part. Accordingly, in performing the process, one must determine the casting preform geometry based on the desired design of the final part.
Following is an example of how a proper preform design is typically chosen when producing a surgical implant. Start with the final desired implant shape. Then, utilizing an appropriate solid modeling CAD program, design a blocker forged part that imparts a uniform amount of reduction such that the mechanical and microstructural properties of the ASTM standards are achieved. Typically, this is a shape that has between about 1% and about 10% extra volume, and imparts a uniform reduction percentage of between about 5% and about 40% (with about 10% being most preferred) at a forging preheat temperature of between about 1200° F. and about 2220° F. (with about 1350° F. being most preferred). Next, the blocker forging shape is used to design a cast preform that imparts between about 10% to about 60% reduction with a target of 20% at a forging preheat temperature of between about 1200° F. and about 2220° F. (with about 1750° F. being most preferred). A shrinkage factor is added to each dimension so that a uniform reduction is achieved during the forge operation. Additionally, a shrink factor has to be added for the dimensional change that occurs from wax to metal for each dimension. Finally, if a rapid prototype (“RPT”) is being used as a master to construct the tool then a shrink factor has to be added to the RPT for this change.
Preferably, the casting method used is investment casting. The investment casting preform is designed to achieve uniform reduction in all sections of the part. For example, in a typical procedure it may be desirable for a thicker part section (preferably measured across the die parting line) to receive the same reduction percentage as a thinner part section in the first forging blow, as described below. By uniformly working all areas of the part regardless of section thickness, mechanical property uniformity and microstructural uniformity (grain size) is achieved.
As will be described below, the present invention preferably provides for an approximately 15-20% reduction in the first hit, followed by a recrystallizing anneal to refine the microstructure, and an approximately 3-20% reduction in the coin hit to provide mechanical properties. The importance of having this uniform reduction is to achieve the grain size requirements of ASTM F-799 throughout the casting. Accordingly, the casting preform geometry is designed to provide for the thickness desired, as described above.
Generally, in forging, the rolled and extruded material by its method of manufacture ensures a fine structure. The steps in the typical forging process also provide a desirable microstructure. The present invention eliminates most of the forging steps, thus the starting blank needs to possess a homogenous and small grain, which is not a normal condition for a casting. To help provide a homogeneous and small grain several Δ's must be controlled. How these Δ's are controlled, and what the Δ's are, will be described below in the discussion of step 2.
Step 2—develop a casting mold configuration and establish proper casting parameters including mold and melt temperatures. As discussed above, in a preferred embodiment of the invention, the casting method used is investment casting. Investment casting is a casting method designed to achieve high dimensional accuracy for small castings by making a mold of refractory slurry, which sets at room temperature, surrounding a wax pattern which is then melted out to leave a mold without joints. The mold configuration is important, because it helps control the solidification process and thus, the grain size. As discussed above, the grain size and grain alignment are important.
In preparing the investment mold, the number and size of gates is minimized so as to achieve directional solidification in a directional line with the desired grain alignment. In a preferred embodiment a single gate is used. Preferably, the preform is set up in a circular configuration to allow for a uniform solidification rate in the casting. In an exemplary embodiment, the mold includes 1 gate. It should be understood that the goal is to achieve a directional solidification in the casting, and, therefore, it is preferred that the gating be kept small and in a direct line with the desired grain alignment.
The chemistry of the alloy is discussed below in step 3. However, it should be understood that before the inventive process can be started, ingots comprised of the desired materials must be provided. Knowing this chemistry, the ingot melting point and the pouring temperature must be determined. The ingot melting point is typically determined by the ingot supplier using DSC (differential scanning calorimetry).
The melt Δ is the difference between the melting temperature of the ingot and its pouring temperature (melt Δ=pouring temp−melting temp). Preferably, the pouring temperature is between about 20° F. and 100° F. from the ingot melting temperature (melt Δ between about 25 and 100). In a more preferred embodiment, the pouring temperature is between about 60° F. and 100° F. from the ingot melting temperature (melt Δ between 60 and 100). In the most preferred embodiment, the pouring temperature is between about 75° F. and 80° F. from the ingot melting temperature (melt Δ between 75 and 80).
In a preferred embodiment, the difference between the pouring temperature and the mold temperature is minimized to no more than 100° F. (sometimes referred to herein as the mold Δ). However, generally, the maximum mold temperature is dependent upon the heat limit of the oven used. In a preferred embodiment, the mold Δ=(between about 2150° F. to about 2500° F.)−(P+80° F.±20° F.). In a more preferred embodiment, the mold Δ=(2150° F.±25° F.)−(P+80° F.±20° F.), where P is the ingot melting temperature. In a most preferred embodiment, the mold Δ=(2150° F.±25° F.)−(P+80° F.±5° F.). Preferably, this spread is kept as small as possible because the smaller the Δ between the pour temperature and the mold temperature the finer the grain size. In typical casting operations there is a large Δ between the mold temperature and the pouring temperature (large mold Δ), which produces large grains of both the equiaxed and columnar type. As will be appreciated by those skilled in the forging art, to minimize the number of forging blows, the finest uniform homogenous grain size possible must be achieved in the as cast casting so as to simulate the condition in a starting forge stock material. Ovens with a higher maximum temperature, thereby providing a higher mold temperature, thus reducing the mold and melt Δ's, are beneficial in achieving a finer grain size.
Accordingly, the mold is configured to control solidification (using minimal and small gating) and casting parameters are established that will provide a small grain size (low pour temperature and high mold temperature).
Step 3—Use a metal chemistry that meets the chemical requirements of both forging and casting specifications, but is optimized to exceed the greater forging mechanical properties. In a preferred embodiment, the chemistry of the ingots falls within the following ranges:
It is important to control the amount of C and N. These elements should be kept on the high side in order to achieve the desired physical properties. The amount of N in particular is preferably on the high side of the range shown above because N is lost upon the melting of the ingot. All of the elements listed above except P and S are requirements of the ASTM Specification. P and S are preferably included because they help to provide a clean casting.
As discussed above, one of the benefits of the inventive process is the reduction of the number of forging blows, thus reducing the amount of work put into the part. Because strength is typically added to a part by the work put into it during the steps in the forging process the alloy in the inventive process has to be strengthened by other means. Keeping the C and N high is one way of adding additional tensile strength.
The ability to control the chemistry of the material, within a tight range for C, Cr, and N, but still meet the broad limits of ASTM F-1537 is essential to the inventive process. The right chemistry provides the desired mechanical properties. This chemistry is controlled to restrict the liquidus point, which, in turn helps set a melt Δ that can be easily reproduced. It is important to note that it is the Δ between the liquidus point and the casting point and not a specific melt temperature per se that is important.
Step 4—Mold the preform by casting. After the chemistry of the ingot has been determined, the casting preform geometry has been designed, the casting parameters have been set and the cast has been configured, the preform can be cast. As the process of casting is well known a description will be omitted.
Step 5—Subject the cast blanks to a one or more hit hot forging step. This step is preferably done using a specially designed die to achieve approximately 90% of the metal movement required to meet the final product dimension and impart both the required grain structure and physical properties for the part. The forging die is made to match the preform geometry to facilitate homogeneous percentage reduction in all areas and section thicknesses of the part and to thereby provide microstructural and mechanical property homogeneity.
In the inventive process, the cast preform is placed in the die and subjected to one or more hits at a predetermined forging preheat with a subsequent soak in air or in nitrogen. In a preferred embodiment the forging preheat ranges between about 1200° F. and about 2200° F. In a more preferred embodiment the forging preheat ranges between about 1700° F. and about 1800° F. In the most preferred embodiment, the forging preheat is done at about 1750° F. at about 15% to 20% reduction. The temperature/reduction combination is important to achieve enough warm work in the material to insure driving force for recrystallization in the interstage annealing operation. Typically, the higher the forging preheat temperature, the larger the reduction required to achieve the threshold amount of warm work. Preferably, the one or more hit forging process is done with no pre, interstage or post annealing. However, annealing could be done to homogenize the microstructure to reduce coring during solidification during the casting process.
When a proper preform geometry has been designed, the die works in tandem with the preform geometry by imparting a uniform amount of warm work in both the first and final (described below) forging operations to produce a refined, homogeneous microstructure. The preform and die cavities are designed to provide the desired percentage reduction in all areas and section thicknesses of the part to provide microstructural and mechanical property homogeneity.
One of the goals of the inventive process is to reduce the total amount of work put into the part. Accordingly, a single forging blow is most preferred. With a cast preform with the desired physical properties, the single blow forging step can provide the desired material ductility and microstructural requirements in the final part.
As will be appreciated, the single blow is referring to imparting the final amount of warm work to produce mechanical properties such as ultimate tensile and yield strength. The grain size is not affected in this stage and ductility is actually decreased from the annealed condition as the warm work is imparted to raise strength. The microstructural and ductility requirements are determined earlier as a result of the first forging blow imparting warm work and the interstage anneal recrystallizing the material using the driving force created by this warm work to yield a wrought, fine grained microstructure.
Step 6—Subject the part to a special thermal process (heat treat) that redissolves the carbides that formed in the grain boundaries during the casting and forging processes. The goal of the anneal is to recrystallize the prior cast, dendritic microstructure to produce a fine, equiaxed, wrought microstructure that meets ASTM standard requirements. In an exemplary embodiment, the part is vacuum annealed at between about 1800° F. and about 2400° F. (preferably at about 2220° F.±15°) for about 30 minutes to about 8 hours (preferably for about 4 hours±15 minutes) under a partial pressure (500-700 microns) of argon. In another embodiment the annealing temperatures can be about 2175° F.±60° F. The quench speed is important. It is essential in order to achieve the correct microstructure and mechanical properties. The faster the cooling rate the less chance there is for the carbides to precipitate out of solution. If the carbides come out of solution mechanical properties will be reduced. Additionally, a fast cooling rate prevents the growth of the grain size.
The load is then quenched in a minimum of about 5 bar (±1 bar) argon pressure with a fan assist such that the load is cooled down to below 1400° F. (±15°) in less than eight minutes, and preferably in less than five minutes. It will be appreciated that the standard specification does not call out a pressure value and allows eight minutes to achieve the cool down.
As discussed above, the quench rate associated with this treatment is important and is preferably faster than the standard rate of 8 minutes from 2220° F. to 1400° F. As will be appreciated, this can be accomplished by using high pressure quenching equipment with the capability of meeting or exceeding 5 bar. The annealing step corrects the effects that the forging had on the microstructure. For example, the forging step can cause carbide formation, internal stress, and recrystallized grains.
Step 7—subject the part to one or more forge hits (a single hit is preferred) to achieve the final dimensional requirements. Finally, the part is subjected to a single forged step at between about 1200° F. and about 2200° F. in air or in nitrogen to achieve the target final reduction of about 5% to about 40% (with about 1350° F. at about 10% reduction most preferred). The hit is preferably a coin hit. This provides the warm work required to achieve the mechanical properties of ASTM F-799. Under these conditions, the amount of flash drawn is minimal and the preferred design is for only about 1% to about 10% extra volume to minimize flash and subsequent machine profiling/conditioning operations. The goal in this step is to minimize excess material that will be expelled from the cavity as flash. This will minimize trimming/profiling/conditioning that must be done to the part reducing cost. This is illustrated in
After all seven steps have been completed the resulting part meets the grain size requirements of ASTM F-799 and ASTM F-1537 for warm worked product.
ASTM F-799 states that the grain size of forgings shall be five or finer when tested in accordance with Test Method E 112. In forgings it may not be possible to fully recrystallize the entire microstructure to a fine grain size. Duplex microstructures exhibiting areas of unrecrystallized grains as large as ASTM No. 2 (or ALA No. 2, as applicable, see E 930) may be acceptable provided a minimum of 50% of the area of each section examined displays an average grain size of ASTM No. 5 or finer; and the average microhardness of the larger grained regions is the equivalent of HRC 38 or greater. In quantities of 10% (by area of the metallographic section in question) or less, unrecrystallized grains as large as ASTM No. 0 (or ALA No. 0, as applicable) may be acceptable provided the average microhardness of the larger grained regions is the equivalent of HRC 40 or greater.
Additionally the parts will meet ASTM F-1537 for warm worked product, which is 170,000 PSI minimum tensile, 120,000 PSI minimum yield and 12% for both elongation and reduction in area.
The inventive alloys can be used in the surgical implant industry, and particularly for hip stem implants. The following are examples of the research and development of the present invention. The following test results have been achieved using Co/Cr/Mo orthopedic hip implant production forge tooling and a desirable cast preform part geometry. The wrought input stock preform geometry is limited by the input barstock size and the upsetting process producing a preform geometry which is not optimized for uniform volume or percent reduction in all sections of the part. By utilizing the geometry capabilities of the investment casting process, an optimized preform can be designed which distributes material within the part to produce a uniform percentage of extra volume (flash) and reduction of area during the subsequent forging blows. This produces a much more uniform flash, mechanical properties and microstructure than can be achieved in the wrought process.
First Iteration Testing
The table below illustrates the cast mechanical property requirements per ASTM F-75, the forged mechanical property requirements per ASTM F-799 and the first iteration mechanical properties achieved on “coarse grained” high carbon (0.20%) standard nitrogen castings using both single and two blow forging processes with interstage anneal with the following results (where UTS is ultimate tensile strength, YS is yield strength and Ra is reduction in area):
The second test was performed utilizing “fine grained” high carbon (0.20%) castings and both single and two blow forging processes with the following results:
The single strike properties listed were achieved by forging one blow at a forging preheat temperature of 2050° F. As can be observed, the strength requirements of both the cast and forged specifications were met, but were significantly below the forging specification for ductility as measured by elongation and reduction of area.
The two strike process comprised forging the parts to within 0.015″ of final thickness using a forging preheat temperature of 1750° F. Annealing the parts for approximately 1 hour at 2050° F. to regain ductility by recrystallization and break up the cast dendritic microstructure, followed by coining the parts the remaining 0.015″ (5%) of die closure at 1200° F. to achieve the yield and ultimate strengths. The actual finish reductions on the fine grained parts tested yielded a 0.009″ (3%) coin reduction in forge and the resultant ultimate strength was under the forged specification while the yield strength was within specification. The ductility of the parts is substantially improved over the single strike process as measured by elongation and reduction of area (Ra) but is still slightly below the forged specification requirements.
Second Iteration Testing
After completion of the first iteration testing it was determined that the 2050° F., one hour anneal that was performed in the two strike process was not sufficient to fully recrystallize the microstructure and homogenize the material after the initial forging blow. Fine grained, high carbon castings were used for this iteration. The material met the requirements of ASTM standard F-799 for high carbon grade material. When the test cast/forge process was completed, the resulting materials also met the mechanical property requirements. The microstructure was ASTM 5 with grains as coarse as ASTM 0 and were marginal to the specification. The castings were forged in a two blow process. In the first blow the parts were forged to within 015″ of the final part dimensions with a forging preheat temperature of 1750° F. The parts were then vacuum annealed at 2220° F. for approximately 4 hours at temperature, cooling to 1400° F. in 8 minutes or to recrystallize the microstructure and homogenize the material.
The parts were then broken into two groups with one group final forged with a reduction of 0.015″ (5%) of die closure in the stem (tensile bar) location at a forging preheat temperature of 1350° F. The second group was forged with the same 0.015″ of die closure in the stem location at a forging preheat temperature of 1500° F. The mechanical property results were as follows:
All parts exhibited a wrought, recrystallized microstructure with a grain size of primarily ASTM 6 with grains as coarse as ASTM 3 in the stem area of the part and grains as coarse as ASTM 1 in the trunnion area of the part. The properties and microstructure of this test illustrated that the properties required by ASTM F-799 could be met and that the microstructural requirements of a wrought microstructure with ASTM grain size of 5 or finer could almost be met.
Photomicrographs of the cast preforms in the stem and trunnion areas were taken prior to the first forging blow and exhibit a cast dendritic microstructure as can be seen in
Photomicrographs of the final forge parts exhibited a wrought recrystallized microstructure as can be seen in
The testing data on high carbon cast/forge material indicates that Co/Cr hip stems can be produced which meet all of the mechanical property and microstructural requirements of ASTM F-799. With an optimized cast preform geometry, the microstructure will be refined and become more uniform throughout the part allowing achievement of the grain size requirements of ASTM F-799. The optimized cast preform will also provide increased process flexibility to refine grain size and increase final coin forge blow reductions which will both contribute to the improvement of the yield and ultimate strengths of low carbon material. Additionally, the optimized alloy including increased nitrogen levels will help the low carbon material to achieve similar acceptable mechanical property levels.
While specific examples have been disclosed and illustrated, it is to be understood that cast-forged alloys constructed in accordance with the principles of this invention can have one of a number of different chemical make-ups, depending on the particular application for the final product. Furthermore, it should be understood that the temperatures, times and pressures, etc. described herein are only exemplary and those skilled in the art will be able to configure the process and chemistry differently than disclosed and illustrated without departing from the spirit of this invention.
