Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
  • C22C19/055—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 20% but less than 30%

Definitions

• ABSTRACT Nickel-base alloys containing correlated SC I N D I amounts of aluminum, titanium. columbium. molybdenum o and tungsten. adaptable for use as structural components [52] US. Cl 75/l7l. (notably steam turbine bolts) at temperatures on the order of l48/32.5. l48/l62 l000 F. Alloys, which also contain iron and usually carbon [5]] lnt.C
• nickel-base alloys have found considerable acceptance in innumerable commercial and industrial applications, applications requiring adherence to critical service requirements. For example, such materials have been gainfully utilized in the production of various components for steam turbine environments, including both rotors and blades. Nonetheless and as is so often the case, advanced structural designs brought about by increasingly severe operating conditions necessitate the development of new alloys capable of withstanding the more stringent demands imposed. Steam turbine bolting is illustrative.
• bolting alloys should be capable of consistently delivering in the aged condition a minimum room temperature yield strength (0.02 percent offset) of at least 85,000 pounds per square inch (p.s.i.) and a l000 F. minimum yield strength of better than 70,000 p.s.i. And provided other desired properties are not sacrificed, it is most beneficial that these minima be at least 90,000 p.s.i., and 75,000 p.s.i., respectively. 1
• notch sensitivity i.e., its load carrying ability is considerably impaired by the presence of a notch.
• notch sensitivity i.e., its load carrying ability is considerably impaired by the presence of a notch.
• a flaw ostensibly serves as a focal point for concentration of stresses. This localization of stress concentration promotes the notch as a point of self-propagtion. How well a material resists this propagation is a reflection of its notch sensitivity or notch toughness.
• a drawback of the above-mentioned notch sensitive alloy concerns the coefiicient of thermal expansion (CTE) parameter.
• CTE coefiicient of thermal expansion
• the turbine shell is formed from a ferritic steel having a CTE of between about 7.75Xl0 to 8Xl0 in./in./ F. at the operating temperature of l,000 F.; however, at such temperature this notch sensitive alloy has a CTE on the order of about 8.4Xl0" in./in./ F.
• a bolting alloy should exhibit a CTE as compatible as practicable with the shell material and to this end a CTE of below about 8 would be of considerable advantage.
• Steam turbines are often "torn down for-purposes of inspection, repair, etc. On cooling the bolts (and nuts) tend to contract and thus tighten. At best the bolt is difficulty removable, and the wider the gap between the CTEs of shell and bolt the more acute and tedious the problem becomes.
• alloys suitable for steam turbine bolting should also manifest a low creep rate at temperatures of about l,000 F. to l,200 F., otherwise, should a bolt undergo substantial deformation (extension) in use, serious consequences could ensue. More specifically, such alloys should be characterized by high resistance to what is commonly termed relaxation.” As treated in excellent fashion at page 604 of the Eighth Edition of THE METALS HANDBOOK, turbine bolts are tightened initially to a certain elastic strain and corresponding elastic stress. While in service at elevated temperature, bolt creep does occur. It is considered that some portion of elastic strain is thus transformed into plastic strain. As a result, there is reduction in stress and this is given the apt description relaxation. lf the degree of relaxation is too much, what was a leakproof joint is no longer.
• nickel-base alloys of special composition and containing interrelated amounts of aluminum, titanium, columbium, molybdenum, tungsten. etc. display an excellent combination of room and elevated temperature characteristics rendering the alloys particularly useful in the production of various articles of utility, particularly steam turbine bolts.
• Another object is to provide nickel-base alloys capable of manifesting yield strengths of at least 85,000 p.s.i. at room temperature and of at least 70,000 p.s.i. at l,000 F the alloys also being substantially notch insensitive and characterized by good ductility.
• alloys contemplated herein are age hardenable and, in accordance with the invention, contain (percent by weight) from 17.5 percent to 22 percent chromium, about 2.3 percent to 3.3 percent of columbium, about 2.5 percent to about 3 percent molybdenum, about 2.5 percent to about 3.25 percent tungsten, about 0.4 percent to about 0.75 percent aluminum, about 0.35 percent to about 0.7 percent titanium, up to about 0.l2 percent e.g., about 0.01 percent (advantageously 0.04 percent) to 0.l percent carbon, about 3 percent to about 12 percent, e.g., 5 percent to 9 percent, iron, up to about 0.1 percent, e.g., 0.0l percent to 0.05 percent, magnesium, up to about 0.01 percent, e.g., 0.003 percent to 0.008 percent, boron, up to 0.l percent, e.g., 0.0] percent to 0.05 percent, zirconium, up to about 0.4 percent silicon, up to 0.75 percent manganese, and the balance
• chromium content not fall below 17.5 percent, for, as will be illustrated herein, inadequate tensile strength can easily be the result.
• the efiect of chromium in this regard is deemed somewhat surprising.
• chromium is considered to perform as a simple solid solutioning element without much effect on age hardening response.
• chromium in accordance herewith appears to impart a rather strong influence on the age hardening characteristics of the alloy. Whatever its exact role, it markedly contributes to achieving the necessary strength levels at both room and elevated temperatures.
• chromium can be lowered to about 15 percent, possibly l4 percent, as will be illustrated herein. But this opens up an area of risk. In any event, however, percentages above 22 percent are to be avoided lest alloy stability be impaired. In consistently achieving a sufficient strength plateau and with the view of minimizing loss of stability, it is, accordingly, quite beneficial that the alloys contain not less than l9 percent nor more than 21 percent chromium.
• Aluminum even in the small amounts contemplated, exerts a most potent influence in imparting hardness and in conferring high tensile strength (at both room and elevated temperature) and stress-rupture strength. Reducing the aluminum much below 0.4 percent entails the objectionable risk of inadequate tensile strength. At the other end of the range, amounts much above 0.75 percent can undesirably detract from stress rupture ductility characteristics, particularly when the other essential constituents, columbium, titanium, molybdenum, tungsten, are at the higher end of their respective ranges. Moreover, the higher levels may give rise to an additional complicating factor.
• alloys within the invention upon exposure to high temperature for extended periods of time tend to exhibit an increase in yield strength.
• titanium for example, contributes'to tensile strength and hardness and also improves stress rupture life, although to a lesser degree than aluminum. Unlike aluminum, however, it significantly impairs alloy stability as evident from a not insubstantial loss in the capability of absorbing impact energy upon prolonged exposure to elevated temperature. For best results, including yield strength, stability and high temperature ductility, neither the lower nor the upper limits, respectively, of both of these constituents should be used simultaneously. In this regard, the sum of the aluminum plus titanium should be at least above 0.9 percent, advantageously at least 1 percent, and up to 1.4 percent.
• Columbium, molybdenum and tungsten coact to confer hardness and strength.
• columbium enhances stress rupture life, but lowers stability, particularly in conjunction with molybdenum.
• loss in impact strength the copresence of columbium and molybdenum is synergistic in effect, the loss being greater than what it might be for these constituents individually. Accordingly, with the desideratum of reaching the best combination of strength and stability the combined columbium plus molybdenum should not exceed about 6 percent.
• Tungsten although it exerts a positive influence in terms of strength, when present to the excess detrimentally affects high temperature ductility and undesirably raises alloy density. In view thereof and for highly satisfactory results, it beneficially should not exceed about 3 percent.
• the SSF is from 5.4 percent to 6.2 percent.
• alloys falling within the following ranges: about 19 percent to 21 percent chromium, about 2.5 percent to 3 percent each of columbium, molybdenum, and tungsten, about 0.5 percent to 0.7 percent aluminum, about 0.4 percent to 0.6 percent titanium, the sum of the aluminum plus titanium being at least 1 percent, about 0.04 percent to about 0.1 percent carbon, about 5 percent to 9 percent iron, about 0.003 percent to 0.008 percent boron, about 0.01 percent to 0.05 percent zirconium, and the balance essentially nickel.
• a series of alloys both within and without the invention were prepared using vacuum melting techniques.
• the melt charges were made using the following type of ingredients: carbonyl nickel pellets, vacuum grade chromium, electrolytic iron, ferro-columbium, molybdenum pellets, tungsten powder, titanium sponge, aluminum rod, ferro-boron, and spectrographic carbon.
• Magnesium and zirconium were introduced in the form of nickel-magnesium and nickel-zirconium master alloys, respectively.
• the major charge components with about 0.05 percent carbon were melted in a magnesium oxide crucible and held about one-half hour at about 2,900 F. to effect oxygen removal.
• alloys 1 through 8 represent alloys formulated in accordance with the invention whereas alloys A through H are beyond the scope thereof. These later alloys, however, afford a good basis for comparison with alloys 1 through 8 in terms of mechanical characteristics, the results being reported in table 11. Too, it should be noted that alloys 7 and 8 are included to demonstrate that it is possible to obtain an acceptable level of properties with alloys containing chromium percentages of about I5 percent to 16 percent, i.e., below 17.5 percent or 19 percent. The mechanical characteristics were obtained on specimens which had been annealed at l,800, F. for 1 hour and thereafter aged for about 24 hours at l,300 F.
• each of the alloys contained about 5 percent to 9 percent iron, not more than (a) 0.04 percent zirconium, (b) 0.0l percent boron, (c) 0.03 percent magnesium, (d) 0.1 percent manganese, (e) 0.1 percent ployed as reflected by alloys 7 and 8.
• failure came about at 190 hours exposure at the still very high stress of 80,000 p.s.i.
• the failure that l5. did occur was ductile in manner, the elongation being'37 perg: cent and the reduction of area being 62 percent.
• the notched E. portion of this specimen was put back in test and was exposed for an additional 1,502 hours (making a total of 1,692 hours) 11... whereupon the test was discontinued.
• Subsequent testing revealed the specimen was not weakened as a consequence of 0 the notch since the room temperature tensile test showed the n be noted from the data given in tables I and H that ultimate tensile strength to be l89,900 p.s.i.
• alloy 2 was double aged, an SSF of at least 5.5, beneficially at least 5.7, alloys containi.e., after aging at 1,300 F. for2 4 hours, it was furnace cooled ing down to 15 percent or 14 percent chromium can be emto 1.150" F. at a rate of about 25.20 F. per hour and held (aged) for 24 hours.
• alloy 2 exhibited a yield strength (0.02 percent offset) of 118,000 p.s.i., approximately 15,000 p.s.i. above that obtained with the single aging treatment conducted for 24 hours at l,300 F. This value of 1 18,300 p.s.i.
• Alloy 2 (also alloy 1) was the most heavily alloyed in terms of aluminum. titanium. columbium, molybdenum. and tungsten and. thus. would be compositionally most susceptible to manifest increased strength upon prolonged exposure at elevated temperature.
• alloy 2 was tested for notch sensitivity by exposing the alloy for approximately 1,2 hours at l,000 F. under a stress of 1 15,000 p.s.i. No failure was encountered whereupon the stress was raised to 125,000 p.s.i. and the test was continued for an additional 1 19 hours. When failure had not yet occurred, the stress was raised to 135,000 p.s.i., failure being brought about after 95.4 hours at temperature. Again failure was experienced in the smooth section. This cycle of testing confirmed that the alloys within the invention exhibited excellent resistance to notch sensitivity.
• the coefficient of thermal expansion for alloys used for turbine bolting purposes should be as close as possible to the coefficient of thermal expansion of the metal from which the turbine shell is formed.
• Alloy 2 manifested a coefficient of thermal expansion, after heating to 1,000 F. and holding for minutes, of 7.8 in./in./ F. This value compares quite favorably with the value of 7.75 l0" to 8 l0 in./in./ F. discussed previously herein.
• alloy 2 was compared against an alloy known to exhibit satisfactory resistance to relaxation as a bolting material.
• three separate determinations were made using three different strain values.
• the initial strain was 0.15 percent which was maintained over the full course (1,000 hours) of the first test.
• the stress at the beginning of this test was about 38,080 p.s.i., the temperature being 1,1 12 F. (same temperature used in all three tests).
• the final stress value was then determined, a level of approximately 30,200 p.s.i. being obtained. This value was virtually identical with that manifested by the standard alloy of comparison under the same conditions of test.
• the strain was increased to and maintained at 0.257 percent, the load in this instance being initially about 65,000 p.s.i. After approximately 144 hours, the stress was determined to be 56,100 p.s.i. 1n the last experiment the strain was maintained at 0.30 percent, the initial stress being 75,600 p.s.i. After nearly 170 hours, the stress was measured to be 65,500 p.s.i. As with the first, the second and third tests indicated that alloy 2 compared quite favorably with the standard alloy. These data, although ascertained by way of simulated test conditions, indicate that alloys contemplated within the invention will afford a more than satisfactory degree of resistance to stress relaxation.
• Alloys of the subject invention can be produced in accordance with usual and conventional processing techniques as already indicated and as those skilled in the art will readily appreciate. It is preferred that vacuum induction techniques be employed although the alloys can be readily air melted. After forming ingots and prior to hot working, the ingots should be thoroughly homogenized at, say, a temperature on the order of about 2,100 F. This contributes to achieving uniform distribution of the alloying constituents and also better mechanical properties.
• the cast ingots can be initially hammered or press forged and subsequently hot rolled or the ingots can be hot rolled directly to plate or sheet with suitable intervening reheat treatments in order to maintain the temperature above about l,700 F. Where used, annealing treatments should be conducted within the temperature range of approximately 1,750 F. to l,850 F. as opposed to higher temperatures. It has been found that the lower annealing temperatures confer higher strength characteristics.
• the alloys of the present invention can be produced in the form of bar, rod, sheet, plate, extruded tubing, and forgings and are useful at elevated temperatures on the order of about l,000 F. for such applications as steam piping, tubing, etc.
• the alloys are particularly adapted for use as fasteners in steam turbine assemblies, particularly bolting for fastening the outer shells or casings (usually flanged) of such assemblies. This follows from the excellent minimum yield Strengths (0.02 percent offset) afiorded at both room temperature and at 1.000 F. in combination with other desired characteristics discussed herein. (The aging treatment criterion used in determining the minimum yield strength is 24 hours at l,300 F. followed by air cooling.)
• a nickel-chromium alloy adapted for use at elevated temperatures on the order of about l,000 F. and characterized in having a yield strength at room temperature of at least about 85,000 p.s.i. and a yield strength at l,000 F. of at least about 70,000 p.s.i.
• said alloy consisting essentially of about 17.5 percent to 22 percent chromium, about 2.3 percent to 3.3 percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5 percent to 3.25 percent tungsten, about 0.4 percent to 0.75 aluminum, from 0.35 percent to 0.7 titanium, the sum of the aluminum plus titanium at least 0.9 percent and up to 1.4 percent, the aluminum, titanium columbium, molybdenum, and tungsten being correlated such that the strengthening and stability factor, SSF, expressed by the following relationship is satisfied 2.2 %Al+l.2 %Ti-i-O.6X%Cb+ about 5.25% to 6.4%, about 0.01 percent to 0.12 percent carbon, about 3 percent to 12 percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium, up to 0.4 percent silicon, up to 0.75 percent manganese, and the balance essentially nickel.
• SSF strengthening and stability factor
• a fastener for bolting the outer shell sections thereof and formed from an alloy consisting essentially of at least 14 percent to 22 percent chromium, about 2.3 percent to 3.3 percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5 percent to 3.25 percent tungsten, about 0.4 percent to 0.75 percent aluminum, from 0.35 percent to 0.7 percent titanium, the aluminum, titanium, columbium, molybdenum and tungsten being correlated such that the strengthening and stability factor, SSF, expressed by the following relationship is satisfied about 5.25% to 6.4%, with the further provisos that when the chromium content is less than 17.5 percent (a) the aluminum content is at least 0.5 percent, (b) the sum of the aluminum plus titanium is at least 1.1 percent and (c) the SSF is at least 5.5 percent, about 0.01 percent to 0.12 percent carbon, about 3 percent to 12 percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium, up to 0.4 percent silicon, up to

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Turbine Rotor Nozzle Sealing (AREA)
• Heat Treatment Of Steel (AREA)
• Adornments (AREA)
• Heat Treatment Of Nonferrous Metals Or Alloys (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=24871351

Family Applications (1)

Country Status (6)

Cited By (8)

Families Citing this family (4)

Citations (2)

• 1968
  • 1968-03-21 US US714764A patent/US3619183A/en not_active Expired - Lifetime
• 1969
  • 1969-03-07 GB GB02263/69A patent/GB1201448A/en not_active Expired
  • 1969-03-20 DE DE19691914230 patent/DE1914230A1/de active Pending
  • 1969-03-20 AT AT279869A patent/AT288039B/de not_active IP Right Cessation
  • 1969-03-21 BE BE730243D patent/BE730243A/xx unknown
  • 1969-03-21 FR FR6908459A patent/FR2004465A1/fr not_active Withdrawn

Patent Citations (2)

Also Published As

Similar Documents