DE4436670C2 - Objects made of nickel-based superalloys with improved machinability and method for producing a machined workpiece from such a superalloy - Google Patents

Description

Overall, the invention relates to the improved cutting Machining of superalloy objects Nickel base. In particular, the invention relates to the Improving the machinability properties of the Superalloy by controlling its composition.

Nickel-based superalloys play an important role in meeting the requirements at high temperature and high voltages for components used in modern gas turbines and blasting machines can be used. In general it will High temperature and voltage requirements as achieved when the component has temperatures of at least 538 ° C (1000 ° F) and stresses of at least 553 MPa (80 KSI) is exposed. Although these super alloys based on such relatively high temperatures and Can withstand voltages, making them suitable for such applications are ideal, they are difficult to machine because the Superalloys almost as hard as the tools are that are usually used for their processing.

DE 12 50 642 B, for example, forges materials a chrome-nickel alloy known to have a composition made from 15 to 23% chromium, 2 to 6% molybdenum, which is partly through Tungsten can be replaced, 3 to 8% niobium, at most for Half can be replaced by tantalum, 0.4 to 2.5% aluminum and titanium with at least 0.2% of each of the two elements, 0.001 to 0.02% boron, 45 to 55% nickel, balance, from Impurities apart, iron and between 845 and annealed at 1200 ° C and between 650 and 735 ° C are cured.

From "Nickel and Nickel Alloys" by K. E. Volk, Springer Verlag Berlin, Heidelberg, New York 1970 it is known that  Rohnickel is commercially available in greater purity when this emerges from the ASTM B 39-67 standard (Ni ≧ 99.8%, Co  ≦ 0.15%, Cu ≦ 0.02%, C ≦ 0.03%, Fe ≦ 0.02%, S ≦ 0.01%, P ≦ 0.005%, Mn ≦ 0.005%, Si ≦ 0.005%, As ≦ 0.005%, Pb ≦ 0.005 %, Sb <0.005%, Bi <0.005%, Sn <0.005%, Zn <0.005%).

For some time, manufacturers have been trying to get work pieces out Manufacture castings made from super alloys Are based on nickel, thereby eliminating the need for that Eliminate forging. A casting is thus one Object with a shape close to the final shape of the Component made from the nickel-based superalloy. Then the component after suitable heat treatments, through which the for relatively high temperatures and voltages required material properties are favored up to machined the final dimensions.

In contrast, the first step for a forging process is that Pouring a raw slab, d. H. a casting with a Overall shape, close to a shape for editing the final shape of the component is suitable. The slab will processed by raising the slab to an elevated temperature is heated and subjected to compressive forces, under which the slab is deformed closer to the shape of the component. Then the component after suitable heat treatments, what internal stresses due to the deformation in the processed object, and heat treatments, through which the for applications at relatively high temperature and high stresses required material properties be favored from the processed item to the Machined end dimensions.

There are two principal advantages to making one One-piece component without any forging process. First, larger one-piece components can be more complex Geometry compared to components that are manufactured be made in a process which involves forging requires. The main reason that larger one-piece components be produced by a casting process and without forging lies in the ability of the forging press  are limited. It is also difficult to find an item to transform a single slab into a hollow body, such as a diffuser housing for a gas turbine engine. Is too the forging process in comparison with a casting process in which no forging takes place, labor intensive. Of a Casting process, which excludes forging, can be reached immediately for heat treatment on which the machining he follows. When forging, the item must be close to that desired shape of the component and then formed heat treated and machined. Besides, one is much stronger machining after forging than after a casting process required in the subsequent forging omitted, because the casting is much narrower in the casting process in the final dimensional tolerances of the component to be manufactured at the beginning of the machining process.

However, in the process using Super alloys based on nickel for the production of Castings that do not require forging, aggravates problems when the item is down to the finished one Component is machined because the machining of a Cast object is much more difficult than one Machining forgings. The inventors believe that the reasons why a blacksmith's item comes from a Nickel-based superalloy easier to machine is, in the finer grain size, a more homogeneous microstructure and smaller carbides. For explanation, the Forging larger carbides in the superalloys crushed so that smaller carbides are produced and the Machinability is improved. In comparison, contains a Cast object that does not undergo a forging process will, larger carbides and a larger grain size because of this Components not refined during a forging process were.

The machinability problem continues through the process of hot isostatic pressing (HIP), e.g. B. is known from "Metall", 38th year, issue 6, pp. 740 to 747, aggravated,  in which castings are exposed to heat and pressure to the Reduce porosity and segregation of the superalloy to form an object with high purity. tries show that the mentioned isostatic hot pressing the Machinability adversely affected. It is noted that some prior art documents disadvantageous impairment of workability due to the do not specify hot isostatic pressing, although this is in the Contradicting experiences during the development of the Invention were made.

The object is achieved by the invention, an object with good machinability and a method of making one To provide workpiece, with high strength and hardness and good workability combined become.

According to the invention, this is achieved by means of an object with the Features according to claim 1 and by means of the method for Manufacture of a workpiece according to claim 13 achieved.

The following aspects of the invention have developers in the prior art Technology not developed: 1) a cast object from a Super alloy based on nickel with a carbon content an ultra low level close to the final shape of the manufactured item manufactured; 2) tried one Component from such a cast object by machining Machining to generate or 3) attempted a component to such a cast object by machining generate after the item has been hot isostatically pressed. Therefore, developers in the prior art could Characteristics of improved machinability Cast object not found on a super alloy Nickel base with an ultra low carbon content Level is established.

In addition, the prior art leads to the use of Super alloys based on nickel with a carbon content  at an ultra-low level for four main reasons. First  is featured in prior art publications such as Conaway, "Machining the High-Nickel Alloys" on page 254, and Zlatin et al., "Machining Characteristics of Difficult to Machine Materials "on pages 300 and 305-307 in 'Influence of Metallurgy an Machinability '(American Society for Metals) indicated that the machinability due to increasing hardness decreases. Reviews in the development of the invention showed that reducing the carbon content increased significantly greater hardness (Table III). So you had to expect that by reducing the carbon content the cutting Workability would decrease, however, is according to the invention quite the opposite.

Second, prior art documents teach how Conaway on pages 247-248 that workability with increasing strength decreases. Development tests the invention showed that by reducing the coal strength increased (Table IV and V). Therefore would have again been expected that by reducing the Carbon content would impair the machinability, whereas the reverse is the case with the invention.

Third, in prior art documents reports that by reducing the carbon content Stress crack properties can be reduced. An example of one such prior art document is Stroup et al., "How Low-Carbon Contents Affect Superalloys", Metal Progress (Feb 1968). Stoup et al. reported a minor Carbon content in "INCONEL 718", which is the Industry standard for high temperature and high temperature applications Is stresses that deteriorated stress cracking properties.

Meyer reported in "Extra Low Carbon Alloy 718", Superalloys 1984 (The Metallurgical Society of AIME 1984) also one adverse impairment in terms of Stress cracking properties, unless these are added is compensated by magnesium. The latest document in the booth of the technique that has been localized, Banix et al., "Ultra Fine  Grain / Ultra Low Carbon 718 ", Superalloys 718, 625 and Various Derivatives (The Minerals, Metals & Materials Society 1991) continues from using a carbon portion to ultra low levels in a superalloy Lead away nickel base. Not only does Banix et al. Teach that due to the reduced carbon content Stress crack properties are deteriorated, but that The document also teaches that the reduction in carbon content is not economically feasible because of the cost of Reduction in carbon content outweigh all benefits.

Finally, a major supplier of superalloy Cast nickel-based items raised concerns that the Reduction of the carbon content to an ultra-low Level the fluidity of the superalloy in their molten state before potting, which reduces possibly the porosity to an unacceptable level increases when the cast object is poured off.

Although developers in the state of the art forgings may have produced a chemical structure similar to have the invention, the forged items from the State of the art physically of the invention in at least distinguishable in three ways. First, in the state of the Forging was made using technology, i. H. there were Slabs produced, but not a casting in a form that is close to the shape of the component to be produced. On Blacksmith has a medium grain size, which is essential smaller in area, at least by one or two Orders of magnitude smaller than the average grain size of a Is cast object.

Second, blacksmiths have essentially none Segregation or porosity because the forging process is the same practically eliminated. In contrast, a cast object points at least some segregation and porosity, even if the Cast object was hot pressed isostatically, which is typical Segregations and pro-porosity can be reduced. molded items  are used to manufacture components for relatively high Temperatures and voltages should be suitable, isostatic hot pressed, if the process excludes forging, whereas blacksmiths are usually not isostatic be hot pressed.

Third, blacksmiths contain essentially none large carbides because the large ones are produced by the forging process Carbides are broken down into smaller particles. In comparison a casting according to the invention has large carbides because the forging is eliminated.

Finally, as can easily be seen by experts, Cast articles according to the invention from cast articles a nickel-based superalloy from the prior art, the close to the final shape of the object to be made approach, differentiate because the invention compared with Prior art castings provides: one reduced carbon content in the superalloy used and a significant improvement in machinability.

According to the invention, the molded article can be isostatic be hot pressed at a temperature and pressure that sufficient to segregate and porosity in the Significantly reduce nickel-based superalloy. After that the cast article can be heat treated to give a to achieve a fully precipitated, heat-treated state, through which the strength of the superalloy is essential is increased. Following the isostatic hot pressing and the heat treatment has the cast object a standard Machinability rate of at least 13% compared to one AMS 5010 steel on. A comparable blacksmith would have a machinability rate of 14 to 18%.

A multi-step process for producing a cutting machined component from a super alloy Nickel base with improved machinability also created according to the invention. The first step of  The process is a nickel-based superalloy produce a composition in wt .-% of up to 0.02 Carbon, 0-0.015 phosphorus, 0-0.015 sulfur, 17.00- 21.00 chrome, 50.00-55.00 nickel, 2.80-3.30 molybdenum, 4.40- 5.50 niobium and tantalum combined, 0.30-0.80 aluminum, 0.65- 1.15 titanium, 0-0.006 boron, 0-0.35 manganese, 0-1.00 cobalt, 0-0.35 silicon, 0-0.30 copper and iron as the rest.

Then a casting is made of the nickel-based super alloy manufactured, after which the casting machined in the last step is edited to produce the edited component. The method can also contain other steps, in principle the isostatic hot pressing of the casting, followed by a heat treatment of the cast article until complete precipitated condition before the casting to make the processed component is edited.

In addition, a machined by the invention Component created by a particular process was produced. Furthermore, the edited component suitable for applications where the component of a Exposed to temperatures above 538 ° C (1000 ° F) under tension where the casting has a standard machinability level of has at least 13% compared to an AMS 5010 steel. The process of making the machined Component contains the production of a as a first step Super alloy based on nickel, which is in% by weight up to 0.02 Carbon, 0-0.015 phosphorus, 0-0.015 sulfur, 17.00- 21.00 chrome, 50.00-55.00 nickel, 2.80-3.30 molybdenum, 4.40- 5.50 niobium and tantalum combined, 0.30-0.80 aluminum, 0.65- 1.15 titanium, 0-0.006 boron, 0-0.35 manganese, 0-1.00 cobalt, 0-0.35 silicon, 0-0.30 copper and iron as the rest having.

Then a casting is made from the nickel-based superalloy made and heat treated to a to achieve the precipitation hardened state, followed by the machining of the casting to the machined  Manufacture component. Before heat treatment, the Cast object to be hot pressed isostatically.

The invention also a cast object Super alloy created based on nickel, the improved Features for machining and a Composition in% by weight of up to 0.02 carbon, 0-0.015 Phosphorus, 0-0.015 sulfur, 17.00-21.00 chromium, 50.00- 55.00 nickel, 2.80-3.30 molybdenum, 4.40-5.50 niobium and tantalum together, 0.30-0.80 aluminum, 0.65-1.15 titanium, 0-0.006 Boron, 0-0.35 manganese, 0-1.00 cobalt, 0-0.35 silicon, 0- 0.30 copper and iron as the rest. The casting has the physical distinguishability features that there is a Machinability rate that is much larger than when the nickel-based superalloy has a carbon content of at least 0.038% by weight.

Furthermore, a cast object from a Super alloy based on nickel with improved Machinability properties and reduced carbon content in a composition of less than 0.020% by weight Carbon created. It also contains one or more of the following elements: titanium, niobium, tantalum or hafnium. The Amount of any of the elements in the aforementioned group individually or in combination with other elements of the group in sufficient of the superalloy to prevent the formation of carbides promote. The cast article has a machinability rate that is substantially larger than an essentially identical one Casting of a second nickel-based superalloy, the however a carbon content of at least 0.030% by weight having.

The invention is described below with reference to the Drawing explained.

Figure 1 is a photomicrograph of the standard, non-etched, investment grade "INCONEL 718" superalloy (with a carbon content in the range of 0.038-0.042% by weight) that was tested during the development of the invention at 50X magnification.

Figure 2 is a photomicrograph of the standard etched standard "INCONEL 718" superalloy (with a carbon content in the range 0.038-0.042 wt%) that was tested in the development of the invention at 100X magnification.

Figure 3 is a photomicrograph of the non-etched, "INCONEL 718" ultra low carbon superalloy (with a carbon content in the range of 0.014 to 0.017 wt%) that was tested in the development of the invention at 50X magnification.

Figure 4 is a microphotograph of the etched, ultra-low carbon "INCONEL 718" superalloy (with a carbon content in the range of 0.014 to 0.017) that was tested during the development of the invention at 100X magnification.

Figure 5 is a photomicrograph of the unetched, ultra-low carbon "INCONEL 718" superalloy (with a carbon content in the range of 0.009 to 0.013) that was tested in the development of the invention at 50X magnification.

Figure 6 is a photomicrograph of the etched, ultra-low carbon "INCONEL 718" superalloy (with a carbon content in the range of 0.009 to 0.013) that was tested during the development of the invention at 100X magnification.

Figure 7 is a photomicrograph of the non-etched, "INCONEL 718" ultra-low carbon super alloy. (with a carbon content in the range of 0.0057 to 0.008), which was tested during the development of the invention, in 50-fold magnification.

Figure 8 is a photomicrograph of the etched, ultra-low carbon "INCONEL 718" superalloy (with a carbon content in the range of 0.0057 to 0.008) that was tested during the development of the invention at 100X magnification.

Figure 9 is a micrograph of the standard etched "INCONEL 718" superalloy at 100X magnification, forged and heat treated to the precipitation hardened state.

Figure 10 is a photomicrograph of the standard etched standard "INCONEL 718" superalloy that has been heat treated to the precipitation hardened state at 100X magnification.

Fig. 11 is a photomicrograph of the standard etched standard "INCONEL 718" superalloy which has been isostatically hot pressed and heat treated to the precipitation hardened state at 100X magnification.

A key aspect of the invention is the reduction of Carbon content in nickel-based superalloys in order to Improve machinability properties. As in the state is known in the art, the carbon content in a Super alloy by heating the super alloy under controlled pressure conditions can be reduced Carbon escapes from the compound as a gas, or through Use of high-purity element materials.

Although the invention is expected to apply to all Super alloys based on nickel have been applied Examinations in this regard only with a super alloy Nickel base performed, which is essentially identical to is a super alloy made by the International Nickel Company is sold under the trademark "INCONEL 718" except that the carbon content is significantly reduced, which produces an improved form of superalloy which is here called "INCONEL 718" super alloy  ultra-low carbon content is called. (representative carbon content in the "INCONEL 718" - Superalloy with ultra low carbon content are below described in% by weight in the compound.)

Initial development efforts to achieve the Invention were made on the "INCONEL 718" super alloy ultra-low carbon content because the "INCONEL 718 "super alloy to the industry standard for applications high temperature and high voltages. Main user of the "INCONEL 718" superalloy specify that the Carbon content in wt .-% amount to at least 0.02 to 0.03 got to. Although the extreme minimum range of the Carbon content was specified in the prior art for the "INCONEL 718" superalloy considered acceptable the typical carbon content in the "INCONEL 718" - Superalloy, as used in industry, always slightly larger than the specified minimum proportion.

For example, attempts during the development of the Invention showed that a supplier of the "INCONEL 718" - Superalloy a lot of the "INCONEL 718" superalloy provided that the carbon content was approximately 0.042% by weight had (the supplier 's analysis indicated that the Carbon content was 0.038 wt .-%) when requested, the "INCONEL 718" super alloy with a minimal To supply carbon content of 0.03 wt .-%. Table I shows the range of elements for the "INCONEL 718" super alloy, that of a major user of the "INCONEL 718" super alloy is specified as acceptable, and the current laboratory analysis the standard "INCONEL 718" superalloy made by one Supplier when asked to the "INCONEL 718" super alloy in accordance with the to provide designated acceptable ranges. Additionally shows Table I the components in three compounds of the "INCONEL 718 "super alloy with ultra low carbon content, the was tested during the development of the invention.  

TABLE I

It should be noted that Nb + Ta (niobium + tantalum) in means further in the description and in the claims, that the combined total weight of niobium and tantalum together must be in the specified range. In other words by itself it is not enough that niobium and tantalum both are individually within the specified range.

Another major user is asking suppliers to supply the known one Standard "INCONEL 718" super alloy according to the following Provide ranges for the elements in% by weight: 0.02-0.08 Carbon, 0-0.015 phosphorus, 0-0.015 sulfur, 17.00- 21.00 chrome, 50.00-55.00 nickel, 2.80-3.30 molybdenum, 4.40- 5.40 niobium + tantalum, 0.30-0.70 aluminum, 0.65-1.15 titanium, 0-0.006 boron, 0-0.35 manganese, 0-1.00 cobalt, 0-0.35 Silicon, 0-0.30 copper and iron as the rest. Like evident, there is little variation in the acceptable Areas by two main users of the well-known standard "INCONEL 718" super alloy can be specified. For example can be combined according to Table I niobium and tantalum in the range of 4.75 to 5.50 wt .-%, whereas the other main user the "INCONEL 718" super alloy combined the same Specified elements in the range of 4.40 to 5.40 wt .-%.

In the first preferred embodiment of the invention, the "INCONEL 718" super alloy with low carbon content a composition as in the second column of Table I, except for the carbon content. In alternatives Embodiments has an "INCONEL 718" super alloy with low carbon a composition, again with Except for carbon, as in the one immediately above Paragraph.

The minimum proportion of carbon either after the preferred one Embodiment or according to the alternative embodiment for a Nickel-based superalloy according to the invention, such as the "INCONEL 718" super alloy with low carbon content, is not clearly defined, but can be less than 0.005%. However, the maximum carbon content must be less than 0.020% by weight  his. As mentioned earlier, current ones were used for comparison Laboratory tests with a nickel-based superalloy, which essential, except for the reduced Carbon content, identical to the "INCONEL 718" - Was super alloy, and with the "INCONEL 718" super alloy performed according to the industry standard, the one Has carbon content in the range of 0.038 to 0.042 wt .-% (as A manufacturing test was also discussed later with a "INCONEL 718" super alloy with low carbon content of about 0.007% by weight).

Laboratory analyzes showed that the highest proportion of carbon in the "INCONEL 718" super alloy with low carbon content, which was tested was 0.017 wt%, whereas an analysis of the supplier of the "INCONEL 718" super alloy with low Carbon content indicated that the highest value of the Carbon content was 0.014 wt .-%. There was a similar one Laboratory analysis indicates that the lowest levels of carbon in the "INCONEL 718" super alloy with ultra-low Carbon content was 0.008% by weight, whereas an analysis of the supplier of the "INCONEL 718" super alloy ultra low carbon content the lowest value of Carbon stated as 0.0057% by weight.

Studies have shown that a reduction in Carbon content in the "INCONEL 718" super alloy that of the prior art superalloy Improve machinability properties significantly. The Composition of the "INCONEL 718" super alloy with ultra-low carbon content during development the invention was examined and the least Had a carbon content, had a carbon content in the Range from 0.0057 to 0.008% by weight. Therefore data is not available through which the editability of an "INCONEL 718 "super alloy with a reduced carbon content be defined below 0.0057% by weight.  

Nevertheless, it is clear that significant improvements in the Machinability with a carbon content below that Minimum value of 0.020% for the standard "INCONEL 718" - Superalloy can be achieved by at least one Main users of the "INCONEL 718" super alloy specified is. In fact, there has been a continuous improvement in Machinability properties of the "INCONEL 718" super alloy observed when their carbon content over the range of 0.042 to 0.008% by weight was reduced. Therefore it is very likely a reduction in carbon to a value which is less than 0.0057 or 0.008% by weight, bring further improvements in machinability. The continuous improvement of workability with decreasing carbon content is shown in Table II.

TABLE II

From Table II it can be seen that on the basis of a Supplier analysis of carbon content a continuous Improvement in workability across the range itself when the carbon content was from about 0.038 to 0.0057% by weight was reduced.

In addition, manufacturing tests were performed, not just pure ones Laboratory analysis, with a nickel-based superalloy ultra-low carbon content during the development of the Invention carried out. The manufacturing tests were carried out on one Superalloy performed a composition according to the  Areas for the first preferred embodiment of the Invention had, the carbon content to 0.007 wt .-% was reduced. Manufacturing tests later in this Discussion defined, showed that the aforementioned "INCONEL 718" super alloy with ultra-low Carbon content a tested according to standard production Machinability level in the range of 20% to 30%. in the Contrary to the aforementioned "INCONEL 718" super alloy with ultra-low carbon content had the standard "INCONEL 718 "super alloy with a carbon content of 0.050% by weight a machinability level tested to standard manufacturing from an average of 8% to 13%.

At this point it may be helpful to discuss how the Degrees of workability can be determined objectively. An AMS 5010 steel becomes the standard when measuring machinability used. Failed at a constant reference speed a tool used to drill a hole in an AMS 5010- Steel is used after drilling a hole from about 381 mm (15 inches) in length in the steel. Become typical several holes drilled, the total depth of which is approximately 381 mm (15 inch) instead of drilling a single hole, this one Has length. An essentially identical tool becomes the Drilling holes in the superalloy used to make them Determine machinability. At a certain percentage The reference speed is the tool after drilling fail a total of 381 mm (15 inch). The percentage of Reference speed for drilling 381 mm (15 inch) in the super alloy is the standard machinability level for the super alloy.

Machinability levels are usually based on the Percentage of speed because engineers with the Service life of tools work in time units and not with the amount of a super alloy that is with a tool was removed. Other types of machinability levels will be discussed used in industry. For example, one is common method used to determine the degree of machinability  the time span in which a tool can be used, before the tool at a certain speed a reference alloy, as described by Gorsler, "The Effect of HIP Densification on the Machinability of Cast Inconel 718 " (American Society of Metals 1983) on page 205, and by Cook "What is Machinability" (Influence of Metallurgy on Machinability) on page 13. Easily editable Alloys such as those that contain substantial amounts of aluminum typically have degrees of machinability of substantial more than 100%, whereas super alloys based on nickel in have general degrees of significantly less than 100%.

As previously mentioned, manufacturing tests were performed using an "INCONEL 718 "super alloy with ultra low carbon content of approximately 0.007 wt .-% carried out. The attempts consisted in the manufacture of a diffuser housing, which has a high Component exposed to temperature and high voltages modern gas turbine engines is, using the Fine casting. The manufacturing studies showed that the Machinability levels varied with the type of machining. For example, the average machinability levels were for turning about 25% and for milling about 18%. However stands the standard machinability level for drilling at which the Machinability level is 25% on average. For comparison was the mean machinability level of the standard "INCONEL 718 "super alloy with a carbon content of approximately 0.050% by weight for drilling 8% to 13%.

The machinability levels listed in Table II are on Laboratory tests, not related to manufacturing tests using a Precision casting of any super alloy that was made had been subjected to a standard heat treatment in order to to achieve precipitation hardened heat treatment condition. The heat treatment except for a precipitation hardened Condition is carried out after hot isostatic pressing. Otherwise the isostatic heat treatment Material properties deteriorated considerably for the Applications at relatively high temperatures and voltages  heat treatment up to the precipitation hardened state be favored. Were similar to those previously mentioned Manufacturing tests in which diffuser housing using the Investment casting was made, the isostatic hot pressing of the investment casting, and after that, the heat treatment except for the precipitation hardened condition included.

As is well known in the art, the precipitation hardened heat treatment condition achieved by the superalloy is subjected to a heating process by what precipitates, such as γ 'and γ "precipitates in the Super alloy are formed, which makes the super alloy very gets a lot harder. Therefore the machinability levels are in Table II for those of the super alloys in their so far known hardest, achievable state, measured after a Rockwell hardness standard or any generally accepted one Hardness measurement standard. As can be seen from Table 2, seems the reduction in the carbon content an increase in the Producing hardness, which according to information from the prior art, as according to Conaway and Zlatin et al. a deterioration in the Would show editability. The opposite however became apparent found the invention.

Isostatic hot pressing is typical by heating the Superalloy to 1190 ± 14 ° C (2175 ± 25 ° F) at one pressure of 103 MPa (15 KSI) for three to four hours carried out. In addition, at least some users submit the "INCONEL 718" superalloy is generally the precision casting an initial heat treatment, before the isostatic Hot pressing, known as isostatic preheating Homogenization. The isostatic preheating Homogenization treatment of a major user of the "INCONEL 718 "super alloy is made by the casting is brought into a partial vacuum and the cast object is opened heating approximately 1135 ° C (2075 ° F) for eight hours, followed by a heat treatment at approximately 1150 ° C (2100 ° F) for sixteen hours. Following the isostatic Hot pressing can be the precision casting of a standard  Heat treatment, generally in a multi-stage process, be subjected to the precipitation hardened state receive. Usually the precipitation hardened state is in achieved a two-stage process in which the first stage a solution heat treatment by heating the superalloy 955 ± 14 ° C (1750 ± 25 ° F) for one hour and then that Cooling in the air requires. After that, in the second Stage of the precipitation hardened state by heating the Super alloy at 732 ± 14 ° C (1350 ± 25 ° F) during 8 Hours followed by cooling at a rate of approximately 56 ° C / h (100 ° F / h) to about 663 ° C (1225 ° F) for eight Hours and then cooling the superalloy in air. earlier Experiments indicate that conventional superalloys Nickel base which have undergone the above treatment have very low levels of machinability.

As stated earlier, an investment casting can be one Nickel-based superalloy is more difficult to machine are made as a forgings from the same super alloy. An investment casting from the is even more difficult to machine Superalloy, which is used to achieve a precipitation hardened Condition has been heat treated. Experience finally shows that the aforementioned heat-treated investment casting, which before Heat treatment to achieve the precipitation hardened Was hot pressed isostatically, most difficult of is editable by everyone, although some voices in the state of the Technique indicate that hot isostatic pressing does not has an adverse effect on machinability. The experience also shows that isostatic in general Preheat homogenization treatment before the isostatic Hot pressing the machinability of the investment casting from the Superalloy not noticeably affected.

The laboratory analysis of the standard "INCONEL 718" super alloy in precipitation hardened state and after the isostatic Hot pressing indicated that the highest standard Machinability level that could be achieved, about 12% scam. Production experience with the aforementioned  Superalloy showed standard machinability levels like this as low as 6%. In contrast, there was a medium one Standard machinability level of the "INCONEL 718" super alloy with ultra-low carbon content from 0.0057 to 0.008% by weight in the precipitation hardened state and after the isostatic Hot pressing of 18%. Production experience with the aforementioned "INCONEL 718" super alloy with ultra-low Carbon content showed machinability levels from 18% to 25% with an average of approximately 20%.

The improvement in the level of machinability at "INCONEL 718 "super alloy with ultra-low carbon content is like this significant that the machinability levels for castings this superalloy the degrees of machinability for comparison bare forgings made of standard "INCONEL 718" super alloy overlap. For example, a forging from the Standard "INCONEL 718" super alloy, which except for hardened condition, typically one Standard level of machinability ranging from 14% to 18%.

As can be seen from Table II, the overlaps aforementioned range of machinability level with that of "INCONEL 718" super alloy with ultra-low Carbon content, tested in the laboratory, which standard Machinability levels from 14% to 19%.

Photomicrographic comparisons of nickel-based superalloys that were forged, investment-cast, or both investment-cast and hot-pressed show structural differences. Figure 9 is a photomicrograph of the standard "INCONEL 718" superalloy in forged form at 100X magnification; as a contrast, Figure 10 is a photomicrograph of the standard investment "INCONEL 718" superalloy magnified 100 times. The first important difference is that the cast "INCONEL 718" super alloy has a coarse grain size, whereas the forged "INCONEL 718" super alloy has a fine grain structure.

Numerous grains are visible in FIG. 9, but not a single complete grain is visible in FIG. 10. The grain sizes in cast nickel-based superalloys are at least one order of magnitude and usually at least three to four orders of magnitude larger than the same forged form of the same superalloy. As can be seen from Fig. 9, the grains have approximately the shape of irregular polygons.

Typically, the effective diameter of the grains ranges from 0.076 to 0.127 mm (0.003 to 0.005 inch) in forged articles made of the "INCONEL 718" superalloy, which corresponds to an overall ASTM grain size number of 3 to 5. In comparison, the effective diameter of the grains in cast articles made of the "INCONEL 718" superalloy ranges from 2.03 to 6.35 mm (0.08 to 0.25 inch) corresponding to an ASTM grain size number greater than 0 the largest value of the Grain size that is provided in the ASTM standard. For example, the mean grain area in forged articles made of the "INCONEL 718" superalloy is about 0.0339 mm ² (0.00006 square inch) compared to a mean grain size range of 13.55 mm ² (0.021 square inch) for cast articles the "INCONEL 718" super alloy. Usually the mean grain size range in castings is at least 0.065 mm ² (0.0001 square inch).

Fig. 11 is a photomicrograph of the "INCONEL 718" super-alloy, which has been isostatically hot pressed, at 100x magnification. In comparison of FIG. 11 with FIG. 10 it can be observed that the isostatic hot pressing reduces the segregation, ie the superalloy becomes more homogeneous. More precisely, a precision casting made of a nickel-based superalloy which has not been hot isostatically pressed has higher concentrations of a single element or individual elements, as are present in inhomogeneous segregations, other than evenly distributed in the superalloy.

Forged articles made of a nickel-based superalloy are also significantly more homogeneous than investment articles made of the same superalloy. If, for example, FIG. 9 is compared with FIG. 10, FIG. 10 shows significantly larger darker areas, of which segregation is indicated. Further comparing Fig. 9 with Fig. 11, it can be seen that although hot isostatic pressing has reduced the segregation from Fig. 10 to Fig. 11, the darker areas of segregation in Fig. 11 are still significantly larger on average than the dark segregation areas in FIG. 9.

The "INCONEL 718" superalloy contains the segregations primarily niobium. Due to the isostatic pre-homogenization Heat treatment, which was described earlier, is the Segregation also decreased. To determine whether a Investment casting has been suitably hot pressed isostatically Superalloy users typically use a photomicrograph of the casting with a series of photomicrographs compare that define an acceptable norm and specify different segregation levels.

It is noted that Figures 9 through 11, although showing the standard "INCONEL 718" superalloy, are not figures of the standard "INCONEL 718" superalloy compound that was tested during the development of the invention. Next, Figures 9 to 11 a "INCONEL 718" -. Superalloy was heat-treated to reach the precipitation hardened state. Finally, Fig. 11 is a photomicrograph of the "INCONEL 718" superalloy that has been isostatically hot pressed and subjected to isostatic pre-homogenization heat treatment, as previously described, which also reduces segregation in a nickel-based superalloy.

Photomicrographs of the current superalloys tested during the development of the invention are shown in Figures 1-8. Figs. 1 to 8 representing the tested superalloys in the precipitation hardened condition after heat treatment after the super alloys were hot isostatically pressed. However, the superalloys tested were not subjected to isostatic pre-homogenization heat treatment. Therefore, the Fig. 1 show more segregations to 8 bit, as when the superalloys an isostatic Vorhomogenisations heat treatment would have been subjected prior to the hot isostatic pressing.

Figures 2, 4, 6 and 8 are photomicrographs of the etched superalloys referred to in Tables I through IV which have been tested in the laboratory during the development of the invention. The acid used to etch the superalloy attacks different phases of the superalloy to different extents, revealing the microstructure of each superalloy. Fig. 2 shows the typical microstructure of the standard "INCONEL 718" superalloy with a carbon content of 0.038 to 0.042% by weight, whereas Figs. 4, 6 and 8 show the typical microstructure of the "INCONEL 718" superalloys with ultra-low carbon content from 0.014 to 0.017, 0.009 to 0.013 and 0.0057 to 0.008% by weight, respectively.

In contrast, Figures 1, 3, 5 and 7 are photomicrographs of unetched superalloys referred to in Tables I through IV which were laboratory tested during the development of the invention. To prepare the microphotographs, the surface of the superalloys was polished with a fine abrasive. Since the carbides in the superalloys are correspondingly more resistant to abrasion than the rest of the superalloy, the microphotographs of the unetched superalloys best illustrate the presence of carbides. Fig. 1 is a photomicrograph of the unetched standard "INCONEL 718" - superalloy, 7 photomicrographs of the unetched while Figures 3, 5, "INCONEL 718" -. Superalloys having ultra-low carbon content from 0.014 to 0.017, 0.009 to 0.013 or 0, 0057 to 0.008 wt .-% represent.

Comparing Figure 7, which shows the "INCONEL 718" superalloy with ultra low carbon content of 0.0057 to 0.008% by weight, with Figure 1, which shows the standard "INCONEL 718" superalloy it can be seen that the proportion of carbides in the "INCONEL 718" superalloy with ultra-low carbon content is significantly reduced. Specifically, about 2 carbide particles are visible in Fig. 7 (the "INCONEL 718" superalloy with ultra low carbon content), whereas in Fig. 1 (the standard "INCONEL 718" superalloy) about 21 carbide particles are visible.

The area shown in each of FIGS. 1, 3, 5 and 7 is approximately 3.88 mm ² (0.06016 square inch). The carbide particles which are visible in FIGS . 1, 3, 5 and 7 have an average diameter which typically ranges from 5 to 15 μm. Therefore, in Figure 7 (the "INCONEL 718" ultra-low carbon superalloy) there are approximately 333 carbide particles per 645 mm ² (1 square inch) that have an average diameter of at least 5 µm. In comparison, in Fig. 1 (the standard "INCONEL 718" superalloy) there are about 3491 carbide particles per 645 mm ² (1 square inch) which are at least 5 µm in diameter.

A similar trend can be observed by comparing FIGS . 3 and 5, both of which show the "INCONEL 718" superalloys with ultra-low carbon content, with FIG. 1 of the standard "INCONEL 718" superalloy. For example, in Fig. 5, of which the "INCONEL 718" superalloy with ultra-low carbon content from 0.009 to 0.013 is shown, three carbide particles or about 499 carbide particles per 645 mm ² (1 square inch) with an average diameter of at least 5 µm are visible , In Fig. 3, of which the "INCONEL 718" superalloy with ultra-low carbon content of 0.014 to 0.017 wt.% Is shown, there are 16 carbide particles or about 2660 carbide particles per 645 mm ² (1 square inch) with an average diameter of at least 5 µm visible.

It should be noted that this is not a The standard estimate of the amount of carbide particles is that typical are present in super alloys. A standard way of It would estimate the amount of carbide particles present be a collection of at least 50 drawing files each Use superalloy instead of a representative drawing arbitrarily select. In addition, each drawing file would be one Standard photomicrograph as used by those skilled in the art is made in the usual way.

In relation to this thing, carbides in a forged article made of a nickel-based superalloy are significantly smaller than carbides that are present in an investment casting of the same superalloy. For example, in Fig. 9, which shows the standard forged "INCONEL 718" superalloy, no carbide particles are visible at 100X magnification. In contrast, numerous carbides are visible in Figures 10 and 11, of which castings of the same superalloy are shown, which are present in the small white areas within the darker areas of segregations. Typically, carbide particles in a cast article, such as those shown in Figures 1 to 8, 10, 11, have an average diameter ranging from 5 to 15 µm. However, carbide particles in forgings like that of Fig. 9 are much smaller in size.

Preliminary tests during the development of the Invention with an "INCONEL 718" super alloy ultra-low carbon content of approximately 0.01% by weight Carbide reduction of about 75% compared to the standard "INCONEL 718" super alloy, which has a carbon content of approximately 0.06% by weight. The reduction of approximately 75% is based on 40 to 60 partial images "INCONEL 718" super alloy with ultra-low Carbon content compared to the standard "INCONEL 718" - Superalloy. During the preliminary examinations  Standard machinability levels for the superalloys are not determined, however, the tool life was found when machining the "INCONEL 718" super alloy with ultra-low carbon content was about three times longer than when machining the standard "INCONEL 718" super alloy. Therefore, the level of workability for the "INCONEL 718" - Superalloy with ultra-low carbon content clearly less than the standard "INCONEL 718" super alloy.

The elements of which the formation of carbides in Superalloys are usually titanium, Niobium, tantalum and hafnium. Niobium is in the "INCONEL 718" - Superalloy the element that is primary for the formation of Carbides is responsible.

As previously discussed, isostatic hot pressing will Porosity as well as segregation in an investment casting reduced. However, the degree of porosity is all in one Investment casting is a function of the dimensions of the casting. Smaller castings have lower porosity, and that The opposite is the case for larger castings.

An isostatic hot pressing has an insignificant effect on the grain size in investment castings made of nickel-based superalloys, as by the comparison between the "INCONEL 718" superalloy, which was precision-cast and hot-pressed isostatically, as shown in FIG. 11, with the same investment casting as shown in FIG. 10 can be seen which was not isostatically hot pressed. According to both FIGS. 10 and 11, the grain sizes are so large that a single grain is not completely visible in the microphotographs. Table III lists the mean Rockwell hardness and the grain size for various fine cast isostatically hot pressed superalloys that have been heat treated to the precipitation hardened state.

TABLE III

As stated earlier, a decrease in Carbon content does not have a significant impact the resistance of the superalloy to high Temperatures and stress cracks. In fact give the data increased strength and significantly improved Stress cracking properties. These improvements are surprising for two reasons in particular. First, in State of the art as for Conaway on pages 247 to 248 taught that machinability through increased strength decreases. Therefore, one would not see improved machinability expect if the strength properties are increased.

Second, contrary to what Strop et al. Moyer and Banix et al. a significant improvement in the Stress cracking properties with reduced carbon content. Table IV shows the tensile properties 650 ° C (1200 ° F) and the moderate cracking properties at 650 ° C (1200 ° F) and 620 MPa (90 KSI) required for the standard "INCONEL 718 "super alloy were observed, as were the three Compositions of the "INCONEL 718" super alloy with ultra-low carbon content, to which the Machinability levels were tested in the laboratory. each Composition of the "INCONEL 718" super alloy with ultra-low carbon content and the standard "INCONEL 718" - Super alloy was checked three times and the data in table IV indicate an average of the test results.  

TABLE IV

Average data of superalloys used in manufacturing were tested by manufacturing diffuser housings given in Table V. Show the data in Table V. also no significant adverse effect on the Superalloy properties with an ultra-low Carbon content in terms of resistance to high temperatures and voltages and confirm that Strength and Stress Crack Improvements Properties of the superalloy with reduced Carbon.  

TABLE V

Claims (17)

1. Objects which are characterized by good machinability, obtainable by a process which is characterized in that in the process
a casting that out
0.0057 to 0.015% by weight of carbon,
0 to 0.015% by weight of phosphorus,
0 to 0.015% by weight of sulfur,
17 to 21% by weight of chromium,
50 to 55% by weight of nickel,
2.8 to 3.3% by weight of molybdenum,
4.4 to 5.5% by weight of niobium and tantalum combined,
0.3 to 0.8% by weight aluminum,
0.65 to 1.15% by weight of titanium,
0 to 0.006% by weight boron,
0 to 0.35% by weight of manganese,
0 to 1% by weight of cobalt,
0 to 0.35% by weight of silicon,
0 to 0.3% by weight copper,
0 to 0.1% by weight of zirconium and
the rest is iron,
annealing at 955 ± 14 ° C for one hour,
then annealed in a second stage at 732 ± 14 ° C for 8 hours,
then cooled at a rate of 56 ° C per hour to 663 ° C,
then held at 663 ° C for 8 hours, and
then cooled in air. 2. Object according to claim 1, characterized in that the casting is hot isostatically pressed at one temperature and a pressure that is sufficient, the segregation and the Porosity in the nickel-based superalloy from which the Cast piece exists to decrease significantly, and then under Achievement of a precipitation hardened condition and increased Strength of the superalloy is heat treated, according to  hot isostatic pressing and after heat treatment the casting has a standard machinability level of at least 13% compared to an AMS 5010 steel having. 3. Object according to one of claims 1 and 2, characterized characterized in that the maximum carbon content in the Superalloy 0.014% by weight or 0.013% by weight or 0.009% by weight or 0.008% by weight or 0.0057% by weight. 4. Object according to claim 2, characterized in that the maximum carbon content in the superalloy 0.013% by weight and the casting is a standard Machinability level of at least 14%. 5. Object according to claim 2, characterized in that the maximum carbon content in the superalloy 0.008% by weight and the casting is a standard Machinability level of at least 15%. 6. Object according to claim 2, characterized in that the maximum carbon content in the superalloy 0.0057% by weight and the casting is a standard Machinability level of at least 16%. 7. Object according to claim 6, characterized in that the casting has a plurality of carbide particles with a has an average diameter of at least 5 microns, wherein in Four times as many carbide particles with a medium one Diameters of at least 5 µm are present as in one essentially identical casting from a second Nickel based superalloy except that this has a carbon content of at least 0.042% by weight having. 8. Object according to claim 2, characterized in that the casting has an average grain size range of at least 0.065 mm ² (0.0001 square inch). 9. Object according to claim 2, characterized in that the casting has a plurality of carbide particles with a contains average diameter of at least 5 microns. 10. Article according to one of claims 1 to 9, characterized in that the casting has an average grain size range of at least 0.065 mm ² (0.0001 square inch). 11. The article of claim 1, characterized in that the casting has a level of machinability that much larger than with a super alloy Is nickel based, which has a carbon content of at least 0.038% by weight. 12. Object according to one of claims 1 to 11, characterized characterized in that at least one of the elements titanium, Niobium, tantalum and hafnium in one to promote carbides sufficient quantity are provided, and the casting one Machinability level that is much larger than at a substantially identical casting from a second Super alloy is based on nickel, which one Has carbon content of at least 0.030 wt .-%. 13. A method for producing a machined workpiece from a nickel-based superalloy with improved machinability, characterized in that

• a) a nickel-based superalloy is provided which has, in% by weight: 0.0057 to 0.015 carbon, 0 to 0.015 phosphorus, 0 to 0.015 sulfur, 17.00 to 21.00 chromium, 50.00 to 55, 00 nickel, 2.80 to 3.30 molybdenum, 4.40 to 5.50 niobium and tantalum combined, 0.30 to 0.80 aluminum, 0.65 to 1.15 titanium, 0 to 0.006 boron, 0 to 0 , 35 manganese, 0 to 1.00 cobalt, 0 to 0.35 silicon, 0 to 0.30 copper, 0 to 0.10 zirconium and iron as the balance;
• b) a machinable casting is made from the nickel base superalloy, and then the casting is annealed at 955 ± 14 ° C for one hour,
  then annealed in a second stage at 732 ± 14 ° C for 8 hours,
  then cooled at a rate of 56 ° C per hour to 663 ° C,
  then held at 663 ° C for 8 hours, and
  then cooled in air;
• c) the cast object is machined to the workpiece.

14. The method according to claim 13, characterized in that the casting isostatic before machining is hot pressed. 15. The method according to claim 14, characterized in that the casting to achieve a precipitation hardened Is heat treated and then the casting to the Workpiece is machined. 16. Process for producing a machined Workpiece made of a super alloy based on nickel Claim 13, characterized in that the casting under Achieve a precipitation hardened condition is heat treated and used for components that a standard level of machinability of at least 13% Compared to an AMS 5010 steel and have temperatures Above 538 ° C (1000 ° F) while exposed to There are tensions. 17. The method according to claim 16, characterized in that  the casting before its heat treatment to achieve of a fully precipitation hardened condition isostatic is hot pressed.