Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F28—HEAT EXCHANGE IN GENERAL
  • F28F—DETAILS OF HEAT-EXCHANGE AND HEAT-TRANSFER APPARATUS, OF GENERAL APPLICATION
  • F28F21/00—Constructions of heat-exchange apparatus characterised by the selection of particular materials
  • F28F21/08—Constructions of heat-exchange apparatus characterised by the selection of particular materials of metal
  • F28F21/081—Heat exchange elements made from metals or metal alloys
  • F28F21/087—Heat exchange elements made from metals or metal alloys from nickel or nickel alloys
• • 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
  • C22C19/052—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 40%
• • 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
  • C22C27/06—Alloys based on chromium
• • 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
  • C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon

Definitions

• the sum of the tungsten and molybdenum contents does not exceed 10% and one or more of zirconium, cerium, yttrium and hafnium in a total amount exceeding that required to combine with all the nitrogen present and sufiieient to form a eutectic with the nickel, but not exceeding 4%, the balance, except for impurities including nitrogen in an amount not exceeding 0.1%, being nickel in an amount of at least 25%, to break down its cast structure and then solution-heating it at a temperature within the range of 50 C. below its solidus temperature for from /2 to 20 hours.
• This invention relates to wrought nickel-chromium alloys for use at elevated temperatures in corrosive environments and which have an improved combination of stressrupture strength, ductility and corrosion resistance.
• Nickel-chromium alloys containing from 45 to 75% chromium are known to have good resistance at elevated temperatures, e.g. 600 C. and above, to corrosion by the combustion products of low-grade fuel oil, which products include highly corrosive compounds of sodium and sulphur, and in particular of vanadium.
• the alloys have a two-phase structure on solidification, consisting of the alpha-chromium and gamma-nickel phases. In the cast form they exhibit moderate strength at such temperatures, but suffer from lack of ductility, and they are not readily workable into wrought products.
• the workability and ductility of alloys containing from 28 to 75 chromium, with or without up to 49% iron, and including the two-phase alloys with 45% or more chromium, can be improved, as described and claimed in the specification of our Pat. No. 3,627,511, by incorporating in the alloy one of the eutectic-forming elements zirconium, cerium, yttrium and hafnium in excess of the amount required to combine with nitrogen in .the' alloy as nitride and adequate to form a eutectic with the nickel.
• These additions enable the cast alloys to be wrought and the resulting wrought products to be further worked.
• the stress-rupture strength of the alloys at temperatures above 600 C. remains fairly low, and it is desirable to produce wrought alloys having an improved combination of stress-rupture strength, ductility and corrosion resistance at temperatures in excess of 600 C.
• the need to obtain a single phase structure limits the amount of chromium that can be present or requires the presence of iron, both of which impair the resistance of the alloys to corrosion by vanadium-containing combustion products of impure hydrocarbon fuels.
• the well-known nickel-chromium superalloys containing up to about 35% chromium together with one or more precipitation-hardening or solid solution-strengthening elements also have a single-phase austenitic (gammaphase) matrix.
• the heat treatments commonly used to strengthen such alloys comprise a solution heating step followed by an aging treatment, the solution heating being performed at the lowest practical temperature in order to avoid loss of creep ductility at elevated temperatures.
• nickel-chromium alloys was made by Bloom and Grant in US. Specification No. 2,809,139. These patentees postulated the existence in nickel-chromium alloys with more than 40 or 45% chromium of a high-temperature chr0- mium phase which they called the beta-phase, stable above an eutectoid temperature of 1180 C.
• Their proposed method of improving the stress-rupture strength of the cast alloys at a temperature of 816 C. comprised heating the alloys above the eutectoid temperature followed by cooling, with or without subsequent heating below this temperature to develop a fine-grained high temperature structure.
• the present invention is based on the discoveries that certain two-phase nickel-chromium base alloys having compositions within a critically-controlled range can be worked to form wrought products after breaking down their cast structure, and that the resulting wrought products can be heat-treated to develop surprisingly improved stress-rupture strengths at 600 C. and above, while retaining excellent ductility, by solution-heating within a narrow temperature range immediately below the solidus temperature.
• the alloys contain, by weight, from 47 to 65 chromium, from 0 to 12% cobalt, from 0.02 to 0.1% carbon, from 0 to 0.01% boron, one or more of titanium, aluminum, molybdenum, tungsten, tantalum and niobium in amounts in the ranges from 1 to 6% titanium, from 0.5 to 5% aluminum, from 1 to 10% molybdenum, from 2 to tungsten, from 2 to 10% tantalum and from 0.5 to 10% niobium, and such that 3 (pe1'cent Ti) +3 (percent Al) +5 X (percent Mo) +2.5 X (percent W) +2.5 (percent Ta) +5 X (percent Nb)-1220 with the provisos that (i) the sum of the aluminum and titanium contents does not exceed 6%;
• the alloys are hot-worked, preferably by extrusion, to break down their cast structure, and then subjected to a solution heat-treatment at a temperature within the range of 50 C.
• alloy solidus temperature for from /2 to hours, if desired after further hotor cold-working to the desired shape.
• they are subsequently subjected to an aging heat-treatment at a temperature in the range 600 to 900 C. for from 4 to 20 hours. This latter aging will in any event occur when the alloys are heated above 600 C. in service, but preferably it is performed as a separate step before they are put into service.
• the resulting alloys have an excellent combination of sterss-rupture strength and ductility at temperatures of 600 C. and above combined with excellent resistance to the combustion products of impure fuels containing sodium, sulphur and vanadium. This makes them particularly suitable as materials for load-bearing furnace or turbine parts exposed in use to the combustion products of such low-grade fuels.
• the chromium content of the alloys must be at least 47% for adequate resistance to the combustion products of low-grade fuel oil containing vanadium, sodium and sulphur as impurities.
• the hot-workability of the alloys becomes inadequate and there is a tendency for the corrosion resistance to decrease.
• Increasing the chromium content also lowers the room-temperature ductility, and preferably therefore the chromium content does not exceed 53%.
• the roomtemperature ductility is also adversely affected if the nickel content is reduced below Cobalt increases the stress-rupture lives of the alloys.
• more than 12% cobalt leads to inadequate corrosion resistance and preferably the co'balt content does not exceed 5%.
• At least 0.02% carbon is required so that the alloys possess good stress-rupture properties but more than 0.1% leads to lower room-temperature ductility, tends to reduce corrosion resistance and can lead to difficulties in working, e.g. by forging.
• the elements titanium, aluminum, molybdenum, tungsten, tantalum and niobium make an important contribution to the stress-rupture strength of the alloys, and one or more of these elements must be present in individual amounts of at least 1% titanium, 0.5% aluminum, 1% molybdenum, 2% tungsten, 2% tantalum and 0.5% niobium, the total amount being such that the relationship 3 x (percent Ti) +3X (percent Al) +5 (percent Mo) +2.5 X (percent W) +2.5 X (Percent Ta) +5 X (Percent Nb) 12 0 is obeyed.
• the presence of uncombined nitrogen in the alloys is highly detrimental as it leads to embrittlement of the alloys during service.
• the elements zirconium, cerium, yttrium and hafnium each have a very strong afiinity for nitrogen, and the alloys must contain one or more of these elements in an amount in excess of that required to combine with all of the nitrogen present but not more than 4% in all by weight.
• the excess amounts of these elements form a eutectic with nickel which serves to impart good workability to the alloys.
• the preferred eutecticforming element is zirconium, but cerium, yttrium or hafnium or a combination of any two or more of these four elements may be used up to the total of 4%, although preferably the total amount does not exceed 2%.
• the nitrogen content of the alloys must in any event not exceed 0.1%.
• Nitrogen is commonly introduced into the alloys by the chromium, as commercial chromium usually contains about 0.1% nitrogen, so that nitrogen is invariably present in alloys of this type as an impurity.
• the nitrogen content does not exceed 0.04%.
• the amount of eutectic-forming element in excess of the amount combined with nitrogen as nitride and in excess of any amount which may be dissolved in the nickel or chromium phases of the alloy may conveniently be referred to as the effective zirconium (or cerium, yttrium or hafnium). If there is no eutectic-forming.element in solution in the nickel or chromium phases, the effective zirconium is that in excess of 6.5 times the nitrogen content, the effective cerium is that in excessive of 9 times the nitrogen content, the effective yttrium is that in excess of 6 times the nitrogen content and the effective hafnium is that in excess of 13 times the nitrogen content.
• the amounts of impurities present other than nitrogen should also be as small as possible. Thus any silicon, iron and manganese present should not exceed 0.5% each.
• the contents of zirconium, cerium and yttrium preferably do not exceed 1.0% each and the hafnium content preferably does not exceed 1.5%
• the alloys contain both titanium and aluminum in the ranges 2.5 to 4.0% titanium and 3.0 to 4.0% aluminum, the combination of 3% titanium and 3.5% aluminum being particularly satisfactory.
• composition of the alloys is within the broad range set forth above, they can be hotor coldworked after their duplex cast structure, which comprises the gamma nickel and alpha chromium phases, has been broken down by extrusion or forging. Thus they can be rolled to rod or sheet, swaged, upset, drawn to wire or otherwise shaped to the desired wrought form.
• the wrought alloys must be solution-heated at a temperature within 50 C. below the alloy solidus temperature for from 1 to 20 hours. Depending on the precise composition, the solidus temperature is generally in the range from 1150 to 1348 C. This treatment serves to coarsen the grain structure and thus to reduce the area of the grain boundaries. This effect is surprising in a duplex alloy and is not fully understood, since the two phases would be expected to be in substantial equilibrium s0 that there would be no driving force available for massive diffusion. It is important to use a solution temperature as close as practicable to the solidus temperature and in any event not more than 50 C. below it, in order to develop the maximum driving force for grain growth.
• the alloys may then be subjected to an aging treatment at a temperature in the range from 600 to 900 C. for from 4 to 20 hours.
• an aging treatment at a temperature in the range from 600 to 900 C. for from 4 to 20 hours.
• the alloys contain at least one of the elements titanium, aluminum, niobium and tantalum which form precipitable inter-metallic phases that further contribute to the stress-rupture strength of the alloys, but it also serves to bring the alloy structure into equilibrium by dilfusion of nickel and chromium from the saturated alpha and gamma solid solutions.
• aging will take place during the initial heating in service, but preferably it is performed as a separate step.
• Test pieces machined from the bars were then subjected to one of the following three heat-treatments, of which numbers 2 and 3 were in accordance with the invention while number 1 was not, because the solution heating temperature employed was more than 50 C. below the solidus temperature of the alloys treated.
• Norm-The solidus temperature of Alloys 4, 6 and 7 was approximately 1,290 C. while that of the other alloys was approximately 1,260 G aging treatment may follow the solution-heating without any intermediate treatment. However to obtain the maximum ductility with a somewhat lower stress-rupture strength the alloy may be given an intermediate heattreatment comprising heating in the range 1100 to 1150 C. for from /2 to 8 hours before it is aged.
• the alloys are preferably vacuummelted.
• alloys free from titanium and aluminum may be melted at atmospheric pressure with the use of an inert gas shield and a dry basic slag cover in order to prevent the ingress of nitrogen.
• the excellent properties of the alloys according to the invention may be compared with those of two commercially-available wrought nickel-chromium alloys.
• the first of these which nominally contained 1% zirconium, 50% chromium, balance nickel, was solution-treated for 2 hours at 1140 C., which was within 30 C. of its solidus temperature of 1170 C.
• the second alloy which nominally contained 0.7% titanium, 0.6% aluminum, 0.4% zirconium, balance nickel, was not heat-treated.
• Each of these alloys had a stress-rupture life of less than 1 hour under a stress of 4.6 hbar at 900 C.
• resistance for example cobalt content does not exceed 2% and which contains sgiio iis fig 22 :3512:261 )?zgg gghi ggrg ilf ci one or more of titanium, aluminum, molybdenum, tungample turbine blades, rotors and the like, and particularly i g ig 29 2 rmges T1 from those which are exposed in use to the combustion products 0 mm to 0 mm to of low-grade hydrocarbon fuels, especially those contain- W from to 80%, Ta from to and Nb from ing vanadium, sodium and sulphur. to

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• General Engineering & Computer Science (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Nonferrous Metals Or Alloys (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Forging (AREA)

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Cited By (12)

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• 0
  • BE BE794144D patent/BE794144A/xx unknown
• 1972
  • 1972-01-17 GB GB217072A patent/GB1383995A/en not_active Expired
• 1973
  • 1973-01-11 US US00322610A patent/US3811960A/en not_active Expired - Lifetime
  • 1973-01-12 AU AU51046/73A patent/AU467756B2/en not_active Expired
  • 1973-01-15 LU LU66830A patent/LU66830A1/xx unknown
  • 1973-01-16 DE DE2301985A patent/DE2301985A1/de active Pending
  • 1973-01-16 IT IT7347711A patent/IT976904B/it active
  • 1973-01-16 ES ES410675A patent/ES410675A1/es not_active Expired
  • 1973-01-16 CA CA161,341A patent/CA990188A/en not_active Expired
  • 1973-01-16 NL NL7300619A patent/NL7300619A/xx unknown
  • 1973-01-16 FR FR7301462A patent/FR2168402B1/fr not_active Expired
  • 1973-01-16 SE SE7300550A patent/SE384535B/xx unknown
  • 1973-01-17 JP JP48008241A patent/JPS4881718A/ja active Pending
  • 1973-01-17 IL IL41320A patent/IL41320A0/xx unknown
  • 1973-01-17 AT AT34973A patent/AT323437B/de not_active IP Right Cessation

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