HEAT TREATMENT OF NICKEL-IRON AND NICKEL-COBALT-IRON ALLOYS

Abstract: Age-hardenable, controlled low expansion nickel-iron and nickel-cobalt-iron alloys containing from 34 to 55% nickel, up to 25% cobalt, 1% to 2% titanium, 1.5% to 5.5% niobium, 0.25% to 1% silicon, up to 1.25% aluminum, up to 0.01% boron, up to 0.12% carbon, the balance, apart from incidental elements and impurities, being iron, are annealed at a temperature from 927 to 1038°C for a period of up to 9 hours, depending on section size; cooled; aged at a tempera­ ture from 704 to 816°C for up to 12 hours, depending on section size an aluminum content; cooled; aged at a temperature from 593 to 677° C for up to 12 hours; and cooled to ambient temperature.

Description: Heat Treatment of Nickel-Iron and Nickel Cobalt-Iron Alloys The present invention relates to age-hardenable nickel-iron based controlled low expansion alloys, and in particular to alloys exhibiting good tensile strength and notch strength. In our application No. 83 304 699.8 we have described alloys of this kind that contain from 34 to 55% nickel, up to 25% cobalt, 1 to 2% titanium, 1.5 to 5.5% niobium, 0.25 to 1% silicon, not more than 0.2% aluminum, not more than 0.11% carbon, the balance apart from incidental elements and impurities being iron, and exhibit an inflexion temperature of at least 330 C and a coefficient of expansion between ambient and inflexion temperature of not more than 9.9 x 10-6 per C (5.5 x 10-6 per F). These alloys are subjected to a heat treatment comprising a solution anneal followed by first and second ageing treatments at different temperatures. Several specific heat treatments of this kind are described, applied to specific alloy compositions. The present invention is concerned with further developments in the heat treatment of such alloys and of modifications of these alloys. In particular, it has been found that provided a suitable heat treatment is used, the aluminum content can be increased to about 1.258 without deleteriously adversely affecting the coefficient of expansion and mechanical properties. This lends to increased tensile and rupture properties. Furthermore, whereas it was considered that boron might not have been significantly beneficial, we have determined that boron contributes to improved smooth bar rupture strength, particularly at levels from about 0.003% to about 0.008%. According to the invention, age-hardenable, controlled low expansion nickel-iron and nickel-cobalt-iron alloys containing from 34 to 55% nickel, up to 25% cobalt, 1% to 2% titanium, 1.5% to 5.5% niobium, 0.25% to 1% silicon, up to 1.25% aluminum, up to 0.01% boron, up to 0.12% carbon; the balance,apart from incidental elements and impurities, being iron, are annealed at a temperature from 927 to 10380 C. for a period of up to 9 hours, depending on section size; cooled: aged at a temperature from 704 to 816 C for up to 12 hours, depending on section size and aluminum content; cooled; aged at a temperature from 593 to 6770 C. for up to 12 hours; and cooled to ambient temperature. All the percentages herein are by weight. Preferred alloys which may be heat treated in this way consist of 35 to 39% nickel, 12 to 16% cobalt, 1.2 to 1.8% titanium, 4.3 to 5.2% niobium, 0.3 to 0.6% silicon, not more than 0.1% aluminum, and less than 0.1% carbon, the balance apart from incidental elements and impurities being iron. Preferred ranges of specific constituents may be used with broad ranges of other constituents. Incidental elements and impurities which may be present in the alloys may include up to 0.01% calcium, up to 0.01% magnesium, up to 0.03% boron, up to 0.1% zirconium, up to 1% each of copper, molybdenum, chromium, tungsten and manganese, and not over 0.015% of sulphur or phosphorus. It will be appreciated that a small amount of tantalum, e.g. about 0.1 to 10% of the niobium content, will be present unavoidably in most commercial niobium sources. For purposes of the invention, tantalum acts as niobium, but since the atomic weight of tantalum is twice that of niobium, the weight percent of tantalum present is divided by two. Thus, "niobium" herein means "niobium plus half the tantalum present". From 0.003 to 0.008% boron is preferably present. As in our previous application, to ensure an inflexion temperature of at least 330 C and a coefficient of expansion no greater than 9.9 x 10 6 per OC measured between ambient and inflexion temperature, the composition of the alloys must be restricted by the following relationships: A = (%Ni) + .93 (%Co) - 1.46 (%Ti) + .54 (%Si + %Cr) + 1.37 (%Mn + %Cu) + 7.04 (% C) not more than 52.9. B = (%Ni) + .97 (% Co) - 1.15 (% Ti) - 1.21 (% Si + %Cr) + 9.35 (%C) at least 43.6 Preferably the composition is such that the value of A is not more than 49.2 and that of B is at least 47.4. The successive stages of the heat treatment will now be discussed in more detail. An annealing temperature as low as 9270 C can be used and an excellent overall combination of tensile and rupture properties obtained. However, annealing at this temperature may not fully recrystallize the alloys (depending upon chemistry) or solutionize intermetallic phases, e.g. Ni3(Nb,Ti). This in turn could render the alloys unnecessarily sensitive to prior processing history. While an annealing temperature of up to 10380 C can be used, the alloys tend to grain coarsen and this is usually accompanied by a fall-off in rupture properties. To offset this, overageing may be required. Accordingly, it is advantageous to anneal at from 9540C or 9680C to 9960 C or 10100 C. The time at anneal is dependent upon thickness of the material aged. Thin sheet may require but a few minutes. Rod products on the other hand would require up to three or four hours. As a practical matter, an annealing period of up to six hours or less will normally suffice, grain growth being a controlling factor. The cooling rate can vary from a water quench to air cooling to a furnace cool The rate of cooling from the annealing temperature can have a significant impact on mechanical properties developed upon ageing, and this can require adjustment of the ageing parameters to compensate. For example, water quenching tends to cause overageing, so that ageing at lower temperatures would be desirable. Slow cooling can also induce overageing, requiring similar precautions. Cooling rates of 280 C to 1670 C/hr are generally suitable. Cooling will normally be down to ambient temperature prior to ageing, although in some instances, e.g. when heat treating in a controlled atmosphere, the alloys may be cooled directly to ghe ageing temperature. The first ageing treatment should be conducted within the range of 7040C to 7880 C for from 1 to 2 hrs. to 12 hrs. Temperatures above 7880 C, say 8020 C and higher, result in overageing of alloys with less than 0.28 aluminum with a concomitant loss in room temperature (RT) tensile strength and ductility and smooth bar rupture strengths; however, elevated temperature rupture ductility and notch strength increase. Based on data generated to date and using the notch strengths obtained from ageing temperatures in the range of 7180 C to 7720 C for purposes of comparison, notch strength increased by an order of magnitude, i.e. from 97 hrs. to 975 hrs. at the 8020 C age (test temperature 5380C with 2 stress being 1000 MN/m ),Thus, for applications requiring elevated temperature notch strength, an ageing treatment of above 7880 C and up to 8160 C is considered beneficial. Apart from the foregoing, there appears to be an interrelationship between aluminum content and ageing temperatures higher, higher aluminum contents requiring higher ageing temperatures. For example, with an aluminium level of about 0.5%, an ageing temperature of 718 C does not afford good results, whereas quite satisfactory properties are obtained with an ageing temperature of 7460 C. Similarly, with an aluminium content of 1%, an ageing temperature of 746 C is not acceptable in terms of property characterisitics, but satisfactory results follow when the temperature is about 82 C or higher. Thus, the aluminium level can be increased above 0.2% and up to at least 1% provided the ageing temperature is increased from about 718 C and up to about 8020 C or greater. It is possible that the aluminum content could be raised to levels as high as 1.25%. When, for reasons of fabrication or otherwise, the higher annealing temperatures are used, e.g. 1038 C for brazing, an ageing temperature over the range of 746 C to 8020 C should be employed in the interests of good rupture strength. It is believed that the presence of silicon not only gives an excellent combination of tensile and rupture properties, but also enables ageing periods to be reduced. This is particularly important, for example, for applications requiring ageing in vacuum, since such an operation is quite cost-sensitive to total ageing time. Tables VI, VII and VII reflect that good properties are readily achievable with ageing periods of four hours. In silicon-free and low silicon alloys of otherwise comparable chemistry, it does not appear that a similar response is experienced. An ageing period of from three to less than eight hours gives satisfactory results. While other cooling cycles can be employed subsequent to the initial age, it is preferred to directly cool to the second stage ageing temperature. This can be a furnace cool at a rate of, say, about 28 C to 83 C/hr. We have used a rate of 55.5 C/hr. with highly satisfactory results. Alternatively, the alloys can be cooled to ambient temperature as described for the cooling from the annealing stage. The second ageing treatment should be carried out within the temperature range of about 5930 C to about 0 677 C for a period of 2 to 12 hours. Temperatures much below 5930 C. tend to increase the time necessary to develop desired properties whereas temperatures above 6770 C. result in lowered tensile strength due to insufficient dispersion of fine gamma prime/gamma double prime particles. The comments with regard to ageing time made in connection with the first ageing treatment also generally apply to the second stage as well. For the final cooling, there is no particular substantive reason in terms of properties for using other than a simple air cooling. Water quenching or furnace cooling could be employed without significantly altering resultant physical and mechanical properties. The effects of variations in the heat treatment are illustrated by the results of numerous tests set forth below. A 20,000 lb (9072 kg) commercial s ize heat was vacuum induction melted to two 45.7 cm dia.electrodes which in turn were vacuum arc remelted to a 50.8 cm dia.ingot.of the composition reported in Table I. The ingot was homogenized at 11900 C for 48 hrs. and then hot worked to an 20.3 cm octagon. A portion of the octagon was heated to 11210 C and hot rolled to a 2.5 x 10.2 cm flat, the finishing step being a 20% reduction at about 9270 C. Starting at 9270 C a series of different annealing temperatures was employed up to 10380 C, variation of 280 C being used with the time interval being 1 hr followed by an air cool (this minimized possible sensitivity to water quench). An overall treatment comprising ageing at 7180 C/8 hr, followed by FC at 55.5 C/hr to 6210 C, ageing at 6210 C/8 hr and AC was adopted. Test results (long transverse orientation through the hot rolled flat) are reported in Tables II and III. As can be seen, the as-rolled yield strength was 630 MN/cmê which increased to about 1034 MN/cmê after annealing at 9271038 C and ageing as described above. Grain size was mixed, elongated ASTM &num;8. Recrystallization occurred at 954 - 9820 and grain growth proceeded at 1010 - 10380C (ASTM &num;2). Room temperature yield and ultimate tensile strength were virtually unaffected over the annealing range in respect of grain size. Tensile ductility decreased at 1010-1038 C. With ageing at 927 C and above stress rupture strength and ductility (Table III) were quite good. The combination-bar at 965 MN/cm2 was notch ductile and had good smooth bar ductility. Raising the annealing temperature to 954 C and 982 C resulted in higher notch strength but smooth bar ductility and nothch ductility fell off. Smooth bar life, ductility and notch bar life (Kt = 2) decreased with an annealing temperature of 10380 C. In Tables IV and V, the initial ageing temperature was varied from 718 to 802 C (8 hrs) using both an 9820C and 10380 C anneal. In essence, the results derived were as indicated above herein, yield and ultimate tensile strength decreased with increasing ageing (initial) temperature. Similarly tensile ductility fell off as ageing temperature was increased up to 7740 C. The 5380 C stress rupture properties developed as follows: A. 982 C Anneal: Kt = 2 Notch Bar 1. only one notch bar failed in the notch section, all other tests having been discontinued or failed in smooth bar ii. the notch tests at 896 MN/m2 were discon tinued after 1000 hrs iii. of the notch tests at 1010 MN/m2, one fractured in the notch at approximately 100 hrs. life (718 C age). iv. tests given higher ageing temperatures broke in the smooth ligament Smooth Bar i. rupture strength decreased with increasing ageing temperature however, ii. rupture ductility increased Notch Ductility i. a comparison between smooth bar and Kt= 2 notch bar life indicates that only the 7180C age evidenced signs of notch brittleness ii. the notch bar to smooth bar rupture life ratio markedly inscreased at ageing temperatures above 718 C. B. 10380 C Anneal: Kt = 2 Notch Bar notch bar life at 538 C/827 MN/mê increased as ageing temperature was raised Smooth Bar In contrast with the results given for the 982 C anneal, smooth bar rupture life increased with ageing temperature. While the explanation for this unexpected behavior is not fully understood at present, it is thought there is an increased sensitivity by reason of a coarse grained structure to the mechanism of stress accelerated grain boundary oxygen embrittlement. But it should be mentioned that smooth bar, as in the case of notched bars, can be affected by machining marks, alignment, etc. Overageing tends to lessen the sensitivity to such factors. Tables VI and VII reflect the effect of short time ageing treatments, 4 hours, after both 9820C and 10380C annealing temperatures, the ageing temperatures being varied as in Table VI. Table VIII offers a comparison of total heat treating periods, i.e. the shorter cycle (10 hours) versus the longer cycle (18 hours). As can be seen, satisfactory properties can be attained with the shorter duration heat treating cycles. It might be added that the 982 C/1 hr, AC, age 746 C/4 hr, FC to 621 C/4 hr. AC gave good notch ductility with a Kt = 3.6 combination bar. TABLE I CHEMICAL COMPOSmON Element Wt.% Element Wt.% Si 0.39 C 0.01 Ni 38.46 Mn 0.04 Al 0.05 Cu 0.24 Ti 1.59 Cr 0.12 Cb 4.80 Mo 0.12 Co 13.36 Be Bal* * S, B, Ca, P=0.005% or leas. TABLE II EFFECT OP ANNEALING ThMPERATURE ON ROOM TEMPERATURE TENSILE PROPERTIES Product: 2.5 x 10.2 cm flat, hot rolled Test Orientation: Long Transverse Anneah Temp. shown hr/AC 0 Age: 718 C/8 hr/FC(55.5 C/hr); 621 C/8hr/AC ASTM Annealing Temp. Grain Size 0.2% YS TS EI RA ( C) &num; (MN/mê) (MN/mê) % % As Rolled 8ME 630 965 36.0 52.0 927+ Age 8ME 1024 1302 14.0 33.0 954+ Age 8ME 1038 1310 15.5 34.5 982+ Age 8M 1021 1313 16.0 32.0 1010t Age 5 1062 1341 15.0 32.5 1038 t Age 2 1069 1324 12.0 17.0 NOTE: M = Mixed ASTM 7-11. E = Elongated Grain. TABLE III EFFECT OF ANNEALING TEMPERATURE ON 538 C STRESS RUPTURE Product: 2.5 x 10.2 cm flat, hot rolled Test Orientation: Long Transverse Anneal: Temp. shown/1 hr/AC Age: 718 C/8 hr/FC (55.5 C/hr); 621 C/8 hr/AC 897 MN/mê 965 MN/mê 1000 MN/mê Kt=2 Comb. Kt=2 Annealing Smooth Life EL RA Notch Life Bar Life EL RA Smooth Life EL RA Notch Life Temp C hr % % hr hr % % hr % % hr 927 759.2 6.5 16.5 D1002.8 166.9 3.5 36 102.8 9 21.5 D1339.2 954 1032.9 5.0 15.5 D1000.3 512.1 Notch 215.9 7.5 7.5 D1153.0 982 D1000.3 - - D1153.0 240.2 Notch 199.8 4.0 9.5 97.0 1010 329.0 0.5 8.0 39.6 1038 15.6 2.5 11.0 7.5 NOTES: D=Discontinued Combination (Comb.) bar = Smooth bar and Kt = 3.6 Notch bar TABLE IV EFFECT OP AGING HEAT TREATMENT ON ROOM TEMPERATURE TENSILE PROPERTIES Product: 2.5 x 10.2 cm flat, hot rolled Test Orientation: Long Transverse Anneal: Temp. sh wn/l hr/AC Age: Temperature shown C ) Time shown (hr) FC(55.5 C/hr); 621 C/8 hr/AC Anneal Age .2% YS TS EI RA C C/hr MN/mê MN/mê (%) (%) A. 982 + 718/8 1021 1313 16 32 732/8 1003 1293-17 36 746/8 949 1245 16.5 35.5 774/8 876 1213 16 28 802/8 814 1203 14 20 774/12 831 1189 16 20 B. 1038 + 718/8 1069 1324 12 17 746/8 1007 1252 11.5 15 774/8 914 1227 11 12 802/8 814 1196 6 6 802/16 690 1097 6 5.5 TABLE V EFFECT OF AGING TEMPERATURE ON 538 C STRESS RUPTURE PROPERTIES Product: 2.5 x 10.2 cm flat, hot rolled Test Orientation: Long Transverse Anneal: Temp. shown/1 hr/AC *Age: Temperature shown ( C)/Time shown (hr) FC (55.5 C/hr); 621 C/8 hr/AC At 897 MN/mê At 1000 MN/mê Initial Aging Smooth Bar Kt=2 Smooth Bar Kt=2 Annealing Temp. Temp./Time* Life El. RA Notch Life Life El. RA Notch Life C C/hr (hr) (%) (%) (hr) (hr) (%) (%) (hr) A. 982 + 718/8 D1000. - - D1153. (1) 199.8 4 9.5 97 732/8 454.7 5 9.5 D1050. 12.3 15.5 24 975.6 S 746/8 277.9 4 6.5 D1121. 4.5 17 15.5 1035.7 S 774/8 160.8 9.5 21 D1002. 4.1 24.5 46 446. S 802/8 23.7 18.5 31 D1121. 0.7 23 32.5 219.5 S 774/12 12.7 25 37 D1121. - - - At 828 MN/mê B. 1038 + 718/8 100.(2) NA NA 20.9 746/8 129 2 10 123.7 774/8 133 3 8.5 207.7 802/8 695.7 6.5 7 D1000.3 NOTES: 802/16 38.5 6.5 10.5 D1003.2 (1) Comb. bar Kt=3.6 notch discontinued at 1205.7 hrs. (2) Estimated from tests at 897 MN/mê (15-23 hr) D=Test discontinued at duration shown S=Test broke in smooth ligament NA=Not Available TABLE VI EDDECT OP SHORT TIME AGING TREATMENTS ON ROOM TEMPERATURE TENSILE PROPERTIES Product: 2.5 x 10.2 cm flat, hot rolled Test Orientation: Long Transverse Anneal: temp. shown: ( C) /1 hr/ACO Age: Temperature shown ( Cl/4hr/FC (55.5 C/hr); 621oC/4hr/AC Heat Treatment Anneal Initial Age .2% YS TS El. RA C C/hr MN/mê MN/mê (%) (%) A. 982 + 718/8(1) 1021 1313 16 32 + 718 1052 1365 15.5 37.5 746 979 1290 15.5 37 760 941 1234 17 38 774 914 1220 17 33.5 B. 1038 + 774/8 (1) 914 1226 11 12 + 774 944 1231 14 17 802 975 1265 7 11.5 830 961 1255 7 9.5 NOTE: (1) Comparison ages are 8 hr at temp. shown PC to 621 C/8hr/AC TABLE M EFFECT OF SHORT TIME AGING TREATMENTS ON 330 STRESS RUPTURE PROPERTIES Product: 2.5 x 10.2 cm flat, hot rolled Test Orientations Long Transverse Anneal: Temp. shorn ( C) /lhr/AC Age: Temperature shown ( C) /4hr/FC(55.5 C/hr); 621 C/4hr/AC Heat Treatment Smooth Bar Kt = 2 Anneal Initial Age Life El. RA Notch Bar Life C) ( C) (hr) (%) (%) (Hr) 538 C/1000MN/mê A. 982 + 718/8(1) 199.8 4 9.5 97 718 240.4 3 8 139.1 746 22.7 7 10 1894.6 S 760 4.7 19 21 736. D 774 3.5 24.5 44.5 866.5 S 5380C/828MN/m2 B. 1038 + 774/8 (1) 133 3 8.5 207.7 774 122.1 2.5 0.5 1406.3 802 133.4 1* 12 82.9 830 122.5 1.5 11 76.6 NOTES: (1) Comparison ages are 8 hr at temp. shown PC to 621 C/8hr/AC * = Broke in punch mark. S = Fractured in smooth ligament. D = Discontinued test. TABLE VIII COMPARISON OF SHORT TIME AND STANDARD AGES Stress Rupture Total Aging Initinal Aging RT Tensile Smooth Rupture Notch Bar Anneal Time Temp. 0.2% YS TS El. Test Temp. Test Stream Life El. RA Life ( C) (hr) ( C) (MN/mê) (MN/mê) (%) ( C) (MN/Mê) (hr) (%) (%) Kt (hr) X 982 10 746 979 1290 15 538 965 270 4 10 2 > 270 3.6 > 270 649 448 209 28 68 3.6 > 209 18 718 1021 1310 16 538 965 403 4 5 2 115 3.6 < 115 649 448 209 26 70 3.6 > 209 Y 1038 11 774 945 1227 14 538 827 122 3 8.5 2 1406 19 774 910 1227 11 538 827 133 2.5 .5 2 207 DISCLAIMER Application No. 83 304 699.8 describes the specific heat treatments "A" to "H" set forth below, applied to particular alloy compositions. We make no claim herein to any of the combinations of these specific heat treatments and alloy compositions disclosed in the said specification. "A": anneal at 9270C/1 hr; AC; age at 9190 C/8 hr; FC to 6210C at 55.50C/hr; age at 6210C/8 hr; AC "B": same as "A" except anneal at 9820C "C": same as "A" except anneal at 1038 C "D": same as "B" except fist ageing at 774 C "E": same as "C" except fist ageing at 774 C "F": same as "A" except fist ageing at 774 C "G": same as "A" except first cooling step is a WQ "H": anneal at 10380C/1 hr; AC; age at 7740C/24 hr; AC; age at 7190C/8 hr; FC 55.5%/hr to 621OC/8 hr; AC.

Claims: CLAIMS Subject to the foregoing disclaimer what we claim is: 1. A process for heat treating age-hardenable controlled low expansion nickel-iron and nickel-cobalt-iron alloys containing from 34 to 55% nickel, up to 25% cobalt, 1% to 2% titanium, 1.5% to 5.5% niobium, 0.25 to 1% silicon, up to 1.25% aluminum, up to 0.01% boron, up to 0.12% carbon, the balance apart from incidental elements and impurities being iron which comprises: : (i) annealing the alloys at a temperature from 927 to 10380 C for a period of up to 9 hours, depending on section size; (ii) cooling tha alloy; (iii) ageing the alloy at a temperature from 704 to 816 C for up to 12 hours, depending on section size and aluminum content, with the proviso that the ageing temperature is at least 746 C when the aluminium content is 0.5 % and at least 802 C when the aluminium content is 1% or more; (iv) cooling the alloy; (v) ageing the alloy at a temperature from 593 to 6770C for up to 12 hours and (vi) cooling the alloy to ambient temperature. 2. A process according to claim 1 wherein the composition of the alloys is such that (%Ni) + .93 (%Co) - 1.46 (%Ti) + .54 (%Si + %Cr) + 1.37 (%Mn + %Cu) + 7.04 (%C) not more than 52.9, (%Ni) + .97 (% Co) - 1.15 (% Ti) - 1.21 (% Si + %Cr) + 9.35 (%C) at least 43.6. 3. A process according to claim 1 or claim 2 in which the alloy heat-treated contains from 0.3 to 0.6% silicon. 4. A process according to any preceding claim in which the alloy treated contains not more than 0.1% aluminum. 5. A process according to any preceding claim in which the first and second ageing treatments are carried out for periods of less than 8 hours. 6. A process according to any preceding claim in which the first and second ageing treatments are carried out for periods of at least 3 hours.

Inventors: SMITH, DARRELL FRANKLIN, JR., SMITH, JOHN SCOTT
Applicants: INCO ALLOYS INTERNATIONAL, INC.
Application year: 1984
Main classification: C21D6
IPC codes: C22C30, C22F1, C22C38, C21D6, C22C19