Heat Treatment of Controlled Expansion Alloys Special Heat Treating Processing of Certain Nickel-Iron and Nickel-Iron-Cobalt Alloys
Controlled low expansion, nickel-iron and nickel-cobalt-iron alloys are the alloys that are characterized by:
- an inflection temperature of at least 330 °C,
- a coefficient of expansion between ambient and inflection temperature not greater than 1 x 10-6 per C.,
- high room temperature tensile strength,
- improved elevated temperature stress-rupture properties, including notch-rupture strength,
- good notch ductility where notch bar rupture life exceeds smooth bar rupture life
The controlled expansion alloys contain about 34% to 55% nickel, up to 25% cobalt, about 1% to 2% titanium, about 1.5% to 5.5% columbium, about 0.25% to 1% silicon, not more than about 0.2% aluminium, not more than about 0.1% carbon, with iron being essentially the balance. A more advantageous and preferred composition contains about 35% to 39% nickel, about 12% to 16% cobalt, about 1.2% to 1.8% titanium, about 4.3% to 5.2% columbium, about 0.3% to 0.5% silicon, not more than about 0.1% aluminium, not more than about 0.1% carbon, with iron again constituting essentially the balance.
The Heat Treatments
A number of heat treatments can be applied to the controlled expansion alloys that are described above is follows:
- Heat Treatment "A": anneal at 930 °C/1hr; AC; age at 720 °C/8hr; FC to 620 °C at 38 °C/hr, age at 620 °C/8hr; AC
- Heat Treatment "B": same as "A" except anneal at 980 °C.
- Heat Treatment "C": same as "A" except anneal at 1040 °C.
- Heat Treatment "D": same as "B" except first ageing at 775 °C.
- Heat Treatment "E": same as "C" except first ageing at 775 °C.
- Heat Treatment "F": same as "A" except first ageing at 775 °C.
- Heat Treatment “G”: same as "A" except first cooling step is a WQ
- Heat Treatment "H": same as “C” except first ageing at 775 °C for 24 hrs.
Note: AC = Air Cool; FC = Furnace Cool; WQ = Water Quench
These heat treatments utilise relatively extended periods of time. Hence, there is a basic need to reduce the processing time.
The heat treating parameters can be applied to the controlled expansion alloys where shorter processing periods can be utilised. This leads to lower production costs. Moreover, the aluminium level can be increased to about 1.25% without adversely impacting the coefficient of thermal expansion and the mechanical properties of the controlled expansion alloys. This also increases their tensile strength and rupture properties. Whereas it is considered that boron might not be significantly beneficial, actually it is boron that contributes to the improved smooth bar rupture strength particularly at levels from about 0.003% to about 0.008%.
Improving The Heat Treatments
The Inflection Temperature (IT) and Coefficient of Expansion (COE) can be approximated from the composition of the alloy. Thus to guarantee an IT of at least 330 °C and a COE no greater than 1 x 10-6 per °C measured at 415 °C from ambient temperature, the composition of the controlled expansion alloys must be restricted.
Nickel-iron and nickel-cobalt-iron alloys which are age-hardenable, controlled low expansion type of alloys and contain about 34 to 55% nickel, up to 25% cobalt, about 1% to about 2% titanium, about 1.5% to 5.5% columbium, about 0.25% to 1% silicon, up to about 1.25% aluminium, up to about 0.01% boron, up to about 0.1% carbon, the balance essentially iron, are:
- annealed over the range of 955 °C to 1038 °C for a period of from 1 minute to 9 hours, depending upon section size,
- cooled to ambient temperature as by an air cool or water quench,
- aged to about 1150 °C to 815 °C for about 1 or 2 hours to 12 hours, depending upon section size,
- air cooled to about 138 °C,
- aged at about 138 °C to about 1210 °C for up to 12 hours, and
- cooled to ambient temperature.
An annealing temperature as low as 930 °C can be used and an excellent overall combination of tensile and rupture properties are obtained. However, annealing at this temperature level may not fully recrystallise the alloys, depending upon their chemistry, or solutionise intermetallic phases, e.g., Ni3 (Cb,Ti). This in turn could render the controlled expansion alloys unnecessarily sensitive to prior processing history. While an annealing temperature up to about 1038 °C can be utilized, the alloys tend to grain coarsen and this is accompanied by a fall-off in rupture properties. To offset this, over-ageing may be required. Accordingly, it is advantageous to anneal at from 955 °C or 970 °C to 996 °C or 1010 °C.
The time at annealing is dependent upon thickness of the material to be aged. Thin sheets may require a few minutes while rod products would require up to three or four hours. Practically, an annealing period of up to six hours or less normally suffices with the grain growth being a controlling factor.
Cooling rate can vary from a water quench to air cooling to a furnace cool. Cooling rate from the anneal can have a significant impact on mechanical properties developed upon ageing. This can require adjusting the ageing parameters to compensate. For example, water quenching tends to cause overageing, hence, ageing at lower temperatures is desirable. Slow cooling can also induce overageing, requiring similar precautions. Cooling rates of 10 °C to 150 °C/hr are generally suitable. It might be added that cooling to ambient temperature prior to ageing is deemed a normal procedure to follow although in some instances, e.g. when heat treating in atmosphere, the controlled expansion alloys may be cooled directly to the ageing temperature.
The first ageing treatment should be conducted within the range of about 1150 °C to about 1410 °C for about 2 to 12 hrs. Temperatures above 1410 °C and higher result in overageing 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 and the notch strengths obtained from ageing temperatures in the range of 720 °C to 1310 °C, notch strength increased by an order of magnitude, i.e., from 97 hrs to 975 hrs at the 800 °C age with a test temperature of 538 °C and stress being 14.5 ksi. For applications geared to elevated temperature notch strength, an ageing treatment of above 1410 °C and up to 815 °C is considered beneficial.
There also appears to be an interrelationship between aluminium content and ageing temperatures. For example, an ageing temperature of 720 °C together with an aluminium level of about 0.5% does not afford good results whereas satisfactory properties are obtained with an ageing temperature of 750 °C at the same percentage of aluminium. Similarly, an ageing temperature of 750 °C plus an aluminium content of 1% is not acceptable in terms of property characteristics; however, satisfactory results are obtained when the temperature is about 800 °C. Thus, the aluminium level can be increased above 0.2% and up to at least 1% provided the ageing temperature is increased from about 720 °C to about 800 °C or greater. It is possible that the aluminium content could be raised to levels as high as 1.25%.
When, for reasons of fabrication or otherwise, higher annealing temperatures are used, e.g., 1038 °C for brazing, an ageing temperature over the range of 750 °C to 800 °C should be used for good rupture strength.
With the presence of silicon, not only can an excellent combination of tensile and rupture properties be obtained, but ageing periods can also be reduced. This is particularly important in respect of applications requiring ageing in vacuum since such an operation is quite cost sensitive to total ageing time. 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.
Second Cooling Stage
While other cooling cycles can be employed after the initial age, it is preferable to directly cool to the second stage ageing temperature. This can be a furnace cool at a rate of about 10 °C/hr to 110 °C/hr. As for other cooling treatments, the controlled expansion alloys can be cooled to ambient temperature much in the same manner as the cooling cycle following the annealing stage.
Second Ageing Stage
The second ageing treatment should be carried out with the temperature range of about 138 °C to about 1210 °C for a period of about 2 to 12 hours. Temperatures much below 138 °C tend to increase the time necessary to develop the desired properties whereas temperature above 1210 °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 stages as well.
Final Cooling Stage
There is no particular substantive reason property wise which dictates the necessity of applying other than a simple air cooling in the final stage. Water quenching or furnace cooling could be employed without significantly altering resultant physical and mechanical properties.
Illustrative Heat Treatment
A 9,000 Kg commercial size heat of controlled expansion alloy was vacuum induction melted to two 18' dia. electrodes which in turn were vacuum arc remelted to a 20' dia. ingot. The ingot was homogenised at 1190 °C for 48 hrs and then hot worked to an 8' octagon. A portion of the octagon was heated to 2010 °C and hot rolled to a 1'x4' flat, the finishing step comprising of a 20% reduction at circa 930 °C.
Starting at 930 °C, a series of different annealing temperatures was employed up to 1038 °C, variation of 10 °C being used with the time interval being 1 hr followed by an air cool to minimise the possible sensitivity to water quench.
An overall treatment of ageing at 720 °C/8 hr, followed by FC 38 °C/hr to 1110 °C, ageing at 1110 °C/8 hr and AC was adopted. The test was done in a long transverse orientation through the hot rolled flat. The as-rolled yield strength was 91 ksi which increased to about 150 ksi after annealing at 930 °C - 1038 °C and ageing as described above. The resulting grain size was mixed and elongated ASTM 8. Recrystallization occurred at 955 °C - 980 °C and the grain growth proceeded at 1010 °C - 1038 °C (ASTM 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 °C - 1038 °C.
At 930 °C plus ageing, stress rupture strength and ductility were quite good. The combination bar at 140 ksi was notch ductile and had good smooth bar ductility. Raising the annealing temperature to 955 °C and 980 °C resulted in higher notch strength ut smooth bar ductility and notch ductility fell off. Smooth bar life, ductility and notch bar life decreased with an annealing temperature of 1038 °C.
The initial ageing temperature was varied from 720 °C to 800 °C per 8 hrs using both an 425 °C and 1038 °C anneal. In essence, the results derived were that yield and ultimate tensile strength decreased with increasing initial ageing temperature. Similarly tensile ductility fell off as ageing temperature was increased up to 775 °C.
In contrast with the results for the 980 °C anneal, smooth bar rupture life increased with ageing temperature. While the explanation for this unexpected behavior is not fully understood, it is thought there is an increased sensitivity by reason of a course 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.
A reflection of the effect of short time ageing treatments, 4 hours, after both 980 °C and 1038 °C annealing temperatures, the ageing temperatures being varied. A comparison of total heat treating periods, i.e., the shorter cycle of 10 hours versus the longer cycle of 18 hours. Satisfactory properties can be attained with the shorter duration heat treating cycles. It might be added that the 980 °C/1 hr, AC, age 750 °C/4 hr, FC to 1110 °C/4 hr, AC gave good notch ductility with a Kt=3.6 combination bar.
A preferred silicon range is from 0.3 to 0.6% and the carbon level can be extended up to about 0.12% and the aluminium content can range from above 0.2 and up to 1.25%. The range of a given constituent of the controlled expansion alloys can be used together with the ranges of the other constituents. Similarly, a specific heat treating range can be used with other heat treating parameters.
- The process is for heat treating age hardenable, nickel-iron alloys and nickel-cobalt-iron alloys, the alloys consisting essentially of about 34% to 55% nickel, up to 25% cobalt, about 1% to about 2% titanium, about 1.5% to about 5.5% columbium, about 0.25% to 1% silicon, up to about 1.25% aluminium, up to about 0.01% boron, up to 0.1% carbon, the balance essentially iron. This process Controlled expansion alloys when balanced in composition in accordance with the details in the age-hardened condition, an Inflection Temperature of at least 330 °C and a Coefficient of Expansion no greater than 1 x 10-6 per C between ambient temperature and 780 °C which comprises of
- annealing the alloys at a temperature from about 930 °C to about 1038 °C for a period of up to about 9 hours depending upon section size,
- cooling the controlled expansion alloy,
- ageing the controlled expansion alloy at a temperature of from about 1150 °C to about 815 °C for up to about 12 hours, depending upon section size,
- cooling the controlled expansion alloy to a second ageing temperature,
- ageing at a temperature of about 138 °C to about 1210 °C for up to 12 hours, and
- cooling the controlled expansion alloy to ambient temperature
- The heat treatment is being characterised with an initial ageing temperature of 1150 °C to 815 °C and aluminium content are correlated such that as the aluminium content is increased above about 0.2% ageing temperature is also increased within the said temperature range of 1150 °C to 815 °C.
- The process of heat treatment: The controlled expansion alloy is heat treated consisting essentially of about 35% to about 39% nickel, about 12% to about 16% cobalt, about 1.2% to about 1.8% titanium, about 4.3% to about 5.2% columbium, about 0.3% to about 0.6% silicon, up to about 0.1% aluminium, up to about 0.1% carbon, the balance being essentially iron.
- The process of the respective ageing treatments are carried out for periods of less than about 8 hours. The process of the respective ageing treatments are carried out for periods of at least 3 hours.
- The controlled expansion alloy is characterised in the age-hardened condition by controlled expansion properties with an inflection temperature of at least 330 °C and a coefficient of thermal expansion between ambient temperature and 415 °C of 1 x 10-6 per C or less, high strength and good notch rupture strength consisting essentially of about 34% to 55% nickel, up to about 25% cobalt, about 1% to 2% titanium, about 1.5% to 5.5% columbium, about 0.25% to 1% silicon, from above 0.2% and up to 1.25% aluminium, up to about 0.12% carbon and the balance essentially iron. The controlled expansion alloy being in balance in composition to satisfy the alloy being further characterised that has been heat treated in accordance with the heat treatment process. Some controlled expansion alloys in this accordance with which the silicon content is 0.3% to 0.6%.