Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C27/00—Alloys based on rhenium or a refractory metal not mentioned in groups C22C14/00 or C22C16/00
  • C22C27/02—Alloys based on vanadium, niobium, or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
  • C22F1/18—High-melting or refractory metals or alloys based thereon

Definitions

• This invention relates to novel columbium base alloys and to a method for making such alloys, and more particularly to new and improved columbium base alloys and to a method for making such alloys that possess both superior stress-rupture strength at high temperatures and a satisfactory low-temperature ductility that gives to them a range of utility that they were previously denied.
• the principal limitation in gas turbine technology today is the maximum turbine inlet temperature.
• the maximum turbine inlet temperature is in turn set by the temperature that the turbine vanes and blades are able to Withstand without danger of failure.
• the best available high-temperature alloys were nickel and cobalt base superalloys, but critical structural components, such as turbine vanes and blades constructed from such alloys are limited to maximum operating temperatures of between 1600 and 1900 F.
• columbium is, therefore, potentially useful for fast aircraft and space flight vehicles and in nuclear reactors.
• Columbium is inherently a soft, ductile, readily fabricable material. Although its melting temperature is about 4380 F., pure columbium becomes too weak for structural use at temperatures above 1200 F. Columbi-um is also a very reactive metal in that it dissolves large quantities of oxygen, and probably nitrogen, on exposure to atmospheres containing even small amounts of these elements at modest temperatures.
• columbium suffers from oxidation, its oxide does not volatilize, and it is thus potentially possible to localize oxygen attack on columbium by coating the metal.
• Further advantages offered by columbium base alloys as compared with molybdenum base alloys are that columbium alloys are relatively more ductile and workable at low temperatures and columbium has a lower density than molybdenum.
• a further object of this invention is to provide new and improved columbium base alloys and a process for making them that achieves alloys of superior stress-rupture strength at temperatures up to at least about 2500 F. without sacrificing their ductility at room-temperature.
• columbium base alloy containing tungsten and/ or molybdenum in solid solution in amounts sufficient to significantly improve its stress-rupture strength at high temperatures, by heating the alloy, preferably after initial fabrication or breakdown and an intermediate annealing treatment have been completed, to a temperature in the range of from about 1800 to about 2800 F., and then subjecting the alloy in a specimen of a given thickness, while at such elevated temperature, to deformation forces so as to effect a final fabrication or plastic deformation of the alloy whereby the cross-sectional area of the alloy is substantially reduced.
• Columbium base alloys treated in the manner just described have been found to possess both satisfactory stress-rupture strength at high temperatures and a low-temperature ductility.
• initial fabrication or breakdown are used in their conventional sense and mean the first fabrication to which an ingot is subjected after casting.
• An object of this initial fabrication is to reduce the thickness of the ingot to a more manageable dimension (generally from 33 to reduction in thickness or cross-sectional area) preparatory to final fabrication.
• Initial fabrication of high temperature columbium alloys will usually be accomplished at a temperature of from about 1800 F. to about 3000 F.
• intermediate process annealing means the intermediate annealing heat treatment to which the alloy specimen is subjected after initial fabrication and before final fabrication. Unless otherwise indicated in this application, the process an nealing step is conducted at a high enough temperature (2200-3000 F.) and for a long enough time (1 hour) to effectuate substantially complete recrystallization or recovery of the alloy specimen thereby substantially eradicating any work hardening introduced during initial fabrication.
• the columbium base alloys treated in accordance with this invention are those which have tungsten and/or molybdenum incorporated therein to improve their stressrupture strength at elevated temperatures.
• tungsten and molybdenum are added to columbium for improving its stress rupture-strength tungsten is generally added in an amount from 5 to 35% by weight, and preferably 20 to 30% by weight for significantly improved stress rupture-strength at temperatures of 2200 F. or above, while molybdenum is generally added in amount from 5 to 25% by weight, and preferably to 20% by Weight for significantly improved stress rupture-strength at temperatures of 2200 F. or above.
• the amount of each employed may be lower than that indicated above, and in a preferred form of alloy the ratio of tungsten to molybdenum in percentage by weight is about three to one.
• the columbium base alloys treated in accordance with this invention may contain amounts of other metals that may be normally added to columbium base alloys for improving the properties thereof such, for example, as tantalum in an amount from 0.5 to 40% by weight, and preferably 20 to 40% by weight, and the alloys may also contain either with or without tantalum an element from the group consisting of zirconium, hafnium, vanadium, and beryllium, in amounts of from 0.2 to 5% by weight each, but the total of such added elements in the alloys should not be more than 10% by weight.
• the invention is particularly advantageous with respect to columbium base alloys having relatively high amounts of tungsten, e.g., over 20% by weight, or molybdenum e.g., over 10% by weight, or combined additions of these elements, e.g., over 10% expressed in terms of atomic concentration.
• Such alloys because of the inclusion of high amounts of tungsten and/ or molybdenum exhibit very superior stress-rupture strength at elevated temperatures up to at least 2500 P.
• these alloys when made by conventional melting and casting techniques, such as an induction furnace or am melting furnace using either consumable or non-consumable electrodes and not improved upon as provided by this invention, these alloys may be too brittle at room temperature to make them useful in such a primary application as turbine vanes or blades in a jet engine.
• the columbium base alloy is heated to a temperature from about 1800 to about 2800 F., preferably, but not necessarily, after an initial fabrication or breakdown and intermediate annealing have been completed, and then the alloy of given cross-sectional area is subjected to deformation forces to effect plastic deformation whereby a substantial reduction in cross-sectional area is effectuated.
• Excellent results have been achieved where the reduction in cross-sectional area is in an amount of about at least /3 of the initial cross-sectional area (e.g., 33% to 98% reduction).
• the plastic deformation step should provide a reduction of from about 60 to 90 percent.
• Best results concerning improved low temperature ductility compatible with structural stability and resistance to recrystallization for the alloys created by the method of this invention are 0 tained if a temperature of from about 2000 to 2600 F. and a plastic deformation or reduction of from about 60 to 95 percent are used.
• a temperature of about 2400 F. and a reduction of from about 60 to 95 percent provide the most preferred parameters and condition or stabilize the metallurgical structure and give the alloys superior properties of ductility at low temperature commensurate with stable strength characteristics during subsequent high-temperature fabrications or service.
• alloys of this invention When the alloys of this invention are treated by the process of this invention, their brittle-to-ductile transition temperature is significantly lowered and they thereby achieve superior low-temperature ductility.
• the alloys created by this process may thus attain an important range of utility previously denied to them, and be given a usefulness they could not achieve before, because in the past their relative brittleness at low-temperatures (e.g., room temperature) has prevented their use for many applications.
• the columbium base alloys treated in accordance with this invention have good low-temperature ductility.
• a convenient measure of low-temperature ductility is the 4T ductile-to-brittle transition temperature, defined as the minimum temperature at which an alloy strip can be bent without cracking or fracture through an angle of 105 degrees with an included radius of curvature equal to four times the strip thickness.
• the alloys treated in accordance with this invention show good low-temperature ductility when judged by the 4T cluctile-to-brittle transition temperature criterion described above.
• Unalloyed columbium exhibits a 4T transition temperature of about -320 F. Additions of tungsten and/ or molybdenum increase the transition temperature as shown in Table 1 (below). The data of this application are seen to agree well with the tensile transition data reported by Begley and Bechtold, despite probable structural differences. It has been established that the spread between the 4T bend transition temperature and the minimum temperature at which the alloys retain useful tensile ductility is about 200 F. (tensile transition is lower) for alloys strengthened with molybdenum and about 300 F.
• Table 1 also shows the beneficial effect of the presence of tantalum in decreasing the transition temperature at a given tungsten and molybdenum strengthening level, or, conversely, in allowing a greater strengthener level (tungsten and molybdenum) for a given transition temperature.
• a method of fabricating columbium base alloys containing signficant amounts of tungsten and/ or molybdenum is achieve-d in which the 4T bend transition temperatures of such alloys can be reduced by at least 200 F. and thereby imparting to the alloys some ductility at room temperature.
• the expected 100 hour stress-rupture strength of both alloys is between 20,000 and 22,000 p.s.i. at 2200 F. Both alloys are thus classified as superior high-temperature stress-rupture strength alloys. These alloys were selected as examples from a very broad group of alloys which exhibited good high-temperature stress-rupture strength. Among the other alloys that could have been optionally selected for use as examples, on the basis of demonstrated good high-temperature stress rupture strength, and for which improved low-temperature ductility commensurate with stability of high temperature strength can be achieved by the method of this invention are Alloys 3-18, described above in detail.
• the alloy specimens were re-encapsulated as described previously, using steel pack assemblies for 1800 F. and 2400 F. final fabrication, and molybdenum assemblies for 3000 F. final fabrication, as required.
• additional inner covers of molybdenum sheet separated the steel covers from the alloys, to better distribute the normal rolling stresses during final fabrication.
• Final rolling was accomplished utilizing 10 percent reductions in each pass, the assemblies being reheated after each pass. Total final reductions, predetermined by experimental design, Varied from 33 to 89 percent. 30 mils.
• strip specimens of the alloys were recovered from the rolling assemblies and quality graded. They were cleaned by pickling and grinding as required, and were then given a uniform stressrelief annealing treatment at 2200 F. in vacuum on the order of 10 mm. of mercury for /2 hour. (The likelihood of this anneal producing partial recrystallization of more-severely-worked specimens was acknowledged, and considered in evaluation of results.) From the fabricated and annealed alloy strips, multiple bend test specimens, nominally 1 inch long x inch wide, were cut.
• Fabrication temperature 1800 F.severe, i.e., the expected degree of retained work hardening is greatest. 2400 F.moderate (intermediate retained work hardening). 3000 F.slight (least retained work hardening).
• Process annealing temperature 2200 F.stress relief. 2600 F.partial recrystallization (except perhaps for 3000 fabricated examples, where only stress relief may have been achieved). 3000 F.recrystallized.
• Examples El and E2 show moderate (2400 F.) initial fabrication, coupled with stress-relief (2200 F.) intermediate annealing and severe (1800 F.) final fabrication, all of which combined to efiect a very heavy degree of over-all work hardening and an effective total reduction of about 95 percent.
• the alloys of these examples were thus unable to resist at least partial recrystallization or recovery during the final stress-relief anneal (2200 F.), and the expected beneficial effects of the very heavy degree of work hardening were partially lost as a result of the structural instability exhibited during the final annealing process.
• examples E1 and E2 thus failed to achieve the desired low-temperature ductility or 9. 4T transition temperature of 300 F. or less, commensurate with adequate structural stability at high temperatures.
• Examples F1 and F2 which were final-fabricated under moderate (2400 F.) thermal conditions, but were like Examples E1 and E2 in all other respects, were beneficially conditioned during final fabrication, and exhibited good structural stability, resisting recrystallization during the final stress-relief anneal. Much improved low-temperature ductility of the degree desired resulted (Table 2).
• Examples L1 and L2 with only slight (3000 F.) initial fabrication were essentially only stress-relieved (2600 F.) by the intermediate anneal, and were then subjected to severe (1800 F.) final fabrication, all of which again combined to effect a very severe over-all fabrication (rather than the merely appreciable condition indicated) and consequent poor resistance to recrystallization structural instability during the final stress-relief treatment.
• the result was inferior low-temperature ductility (Table 2).
• Final fabrication temperature is a process variable of major importance. Fabrication at temperatures of about 1800 F. (2000 F. and lower) may result in metallurgical structures that are oversensitive to recrystallization during subsequent fabrication (secondary) or service, although total reductions during fabrication of from 60 to 90 percent may be combined with a final fabrication temperature of 1800 F. to achieve a lowtemperature ductility within the desired range. Fabrication at somewhat higher temperatures (2000 F. to 2600 or 2800 F.; and preferably at 2400 F., as shown in the examples) conditions or stabilizes the metallurgical structure of the alloy for superior properties during subsequent high-temperature fabrication or service yet still provides markedly improved low-temperature ductility.
• the examples show that the low-temperature ductility of stress-rupture-resistant, solid solution strengthened columbium-base alloys can be markedly improved by the control of fabrication parameters as taught here. Specifically this invention embodies the following teachings.
• this invention includes within its scope the concept of conducting fabrication at a temperature just insufiicient to result in recrystallization (or recovery) during processing to thereby achieve both a low-temperature ductility, not readily attainable with higher-temperature fabrication, and a superior metallurgical stability compared with lower-temperature fabrication.
• This invention thus achieves stress-rupture-resistant solid-solution-strengthened columbium-base all-oys having superior low-temperature ductility and thereby prossessing a utility previously denied them.
• a recrystallized metallic structure is defined as a strain-free structure, i.e., one that has had all residual work hardening removed from it, and in this connotation, a recrystallized metallic structure includes the as-cast structure of the alloys of this invention.
• One of the important advantages of the invention is that its new and useful result is achieved by imposing a final reduction of the proper amount and temperature on a recrystallized or strain-free structure and that a strain-free state inthe alloy specimen being processed may be introduced at any time during the processing by the use of a recrystallizing annealing treatment.
• the invention provides great flexibility in the manner in which the final reduction parameters are imposed on the alloys.
• the important thing is the total amount and temperature of final reduction imposed on the alloy commencing from a strain-free or recrystallized base structure. In some instances, it may be desirable to impose the desired amount and temperature of final reduction in one step. In other instances, it may be desirable to break down the imposi tion of the desired amount of final reduction into two, three, or more steps.
• the final reduction imposed is partially dependent upon interaction between the process variables.
• the alloy specimen has a prior history of severe work hardening (e.g., initial fabrication at 1800 B)
• it is more susceptible to recrystallization and is capable of being recrystallized :at lower annealing temperatures (or subsequent fabrication or service temperatures) than where the alloy has been subjected to moderate work hardening (e.g., fabrication at 2400 R).
• moderate work hardening e.g., fabrication at 2400 R.
• the recrystallization temperature of various alloy specimens may vary depending upon the prior history of the specimens (e.g., a severely work hardened specimen will have a lower recrystallization temperature than a moderately work hardened specimen), the new and useful result of the invention is obtained in each instance by warm-Working of the alloy specimen, or working at a temperature slightly below the recrystallization temperature, to effect the desired final reduction.
• reduction means reduction in cross-sectional area of the alloy specimen.
• the new and useful result of this invention can be achieved by use of any of the known methods of reduction employed in fabrication of alloys similar to the alloys of this invention.
• the reduction is crosssectional area is achieved by strip rolling, sheet rolling, tensile straining, forging, extrusion, wire drawing, or any other method of fabrication, so long as the desired amount of reduction in cross-sectional area is achieved.
• a process for fabricating a high-temperature strength columbium-base alloy consisting essentially of columbium as the principal component and an additive selected from the group consisting of tungsten, molybdenum and mixtures thereof in an amount of from to 35% by weight of the alloy with the total molybdenum content not exceeding 25% by weight of the alloy
• the improvement that comprises the step of subjecting the alloy from a strain-free state to a reduction of at least 33% at a temperature of from 1800 to 2800" F., said reduction being sufiicient to impart to the alloy 21 4T-bend transition temperature not in excess of about 300 F., said 4T-bend transition temperature being defined as the minimum temperature at which an alloy ship can be bent without cracking or fracture through an angle of 105 with an included radius of curvature equal to four times the strip thickness, whereby substantial work hardening is introduced into the allow, said step comprising the final reduction step on said alloy, and thereafter maintaining the alloy free from substantial recrystallization.
• the alloy also includes tantalum in an amount of from about 0.5% to 40% by weight.
• the alloy also includes tantalum in an amount of from about 20% to 40% by weight.
• the alloy also includes at least one element selected from the group consisting of zirconium, hafnium, vanadium, and beryllium, with a total of the added elements from this group not exceeding 10% by weight of the alloy.

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• 1962
  • 1962-12-21 US US246364A patent/US3296038A/en not_active Expired - Lifetime
• 1963
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