RU2781403C1 - Method for producing aluminum-graphene composite material with improved plasticity - Google Patents

Abstract

FIELD: powder metallurgy.

SUBSTANCE: invention relates to powder metallurgy, in particular to the production of composite materials based on aluminum. It can be used in the electrical industry. A method for producing aluminum-graphene composites with improved plasticity is proposed, including the selection of a fraction of aluminum powder with dimensions of no more than 40.0 mcm and mixing it with thermally expanded graphite powder in ratios of 99.3-98.8 wt. % of aluminum powder and 0.7-1.2 wt. % of thermally expanded graphite. The resulting mixture is mixed and micromechanical splitting of thermally expanded graphite is performed by grinding in a planetary mill in the 400-450 rpm mode for 2.5-3 hours with reverse cycles lasting 3-5 minutes and a break for cooling of the grinding fittings lasting 25-30 minutes in the middle of the process. The blanks are pressed at room temperature and pressure 5.0-5.5 t/cm² for 15-20 minutes. The compressed blanks undergo hydrostating at a pressure of 1400-1500 atm and are subjected to heat treatment in a vacuum furnace at a residual pressure of less than 0.003 atm at a heating rate of 7-10°C/min, kept at a maximum temperature of 400-500°C for 1-1.5 hours and cooled down together with the furnace.

EFFECT: proposed method makes it possible to obtain aluminum-graphene composites with improved mechanical characteristics, primarily with increased plasticity.

2 cl, 3 dwg, 1 tbl, 1 ex

Description

The invention relates to powder metallurgy and can be used to obtain aluminum-graphene composites with improved mechanical characteristics, primarily with increased plasticity; these composites can be used, for example, in the electrical industry in the manufacture of cable products, as well as in the aviation and space industries.

A known method for the synthesis of metal-graphene nanocomposites (Elshina L.A. and Muradymov R.V., RF patent RU 2623410 dated 07/20/2015, publ. 06/26/2017 in classes C01B 32/184, B82B 3/00, B82Y 30/00 ), related to the production of aluminum-graphene, magnesium-graphene and aluminum-magnesium-graphene composites. In the case of the production of aluminum-graphene composites, this method is based on the direct chemical interaction of a carbon ion with molten aluminum. The synthesis of composites was carried out as follows. A refractory (for example, alundum) crucible was placed in a vertical heating furnace, on the bottom of which a finely dispersed dry mixture of alkali metal halides (for example, lithium, sodium, potassium, cesium chlorides) with aluminum halide (for example, aluminum fluoride), as well as a carbon-containing material (eg silicon carbide powder or carboxylic acids). A source of aluminum was placed on top of this mixture (for example, an aluminum disk of A999 purity), which was covered on top with a similar fine mixture of alkali metal halides with aluminum halide and carbon-containing material. Next, the furnace was heated to a temperature of 400-500°C, after which an isothermal exposure was carried out for 1-5 hours in an air atmosphere. During this exposure, the following physical and chemical processes took place: melting of aluminum, melting of salts (alkali metal halides), as well as the reaction of a carbon-containing additive with molten aluminum. The result of these processes was the transition of carbon to the molten metal phase, while the electrolyte layer on the melt surface, formed due to the melting of alkali metal halides, isolated the aluminum-carbon melt from exposure to atmospheric oxygen, preventing aluminum oxidation and carbon combustion. After the expiration of the isothermal holding period, the melt was cooled at a rate of no more than 1 deg/min; as a result of this process, carbon atoms in the metal matrix were combined into graphene films with an area of up to 1000 μm ² in the absence of an aluminum carbide phase.

The technical result of this method was to obtain a homogeneous aluminum-graphene metal composite with a reduced density and improved mechanical characteristics - hardness, strength, modulus of elasticity and relative elongation at break. However, the use of this method, firstly, does not allow to completely remove impurity contaminants from the resulting material, which are formed due to the presence of alkali metal halides in the melt, and secondly, it requires significant energy costs associated with the need to melt aluminum and long exposure at high temperatures, and , thirdly, it is characterized by a significant duration of the melt cooling time (up to 20 hours). In addition, the production of large billets by this method will inevitably require an additional increase in the isothermal holding time to obtain a uniform distribution of graphene in the aluminum matrix.

A known method for producing an aluminum composite reinforced with graphene (patent CN108085524B, authors Wu Haifeng, Wang Shan, Gao Yuanyuan, Li Jianchao and Zhang Xue dated 11/22/2016, publ. 09/13/2019 class C22C1 / 05), selected for the prototype. In this method, the aluminum-graphene composite was obtained as follows. Grinding of graphene powders (1-5 vol.%) and aluminum was carried out in a ball mill in a two-stage mode: 10-12 hours at a rotation speed of 120-180 rpm, after each 1-1.5 hours grinding was interrupted for 15- 30 minutes to cool the grinding set. Lubricant (stearic acid) was also added to the initial mixture in the ratio of 1–1.5 g of stearic acid per 100 g of graphene-aluminum powder mixture. As a result, there was a uniform distribution of graphene sheets in aluminum powder, while the low-speed grinding mode guaranteed the prevention of graphene agglomeration into multilayer particles and rupture of graphene sheets. After that, high-speed grinding was carried out for 10-12 hours at a grinding speed of 200-300 rpm, after each 1-1.5 hours, grinding was interrupted for 15-30 minutes to cool the grinding set. Due to this stage of grinding, graphene sheets were partially pressed into the surface of aluminum particles. At the next stage of synthesis, the mixture of powders was converted into primary blanks using the cold pressing method at a pressure of 150–300 MPa for 10–20 minutes. At the final stage of synthesis, the aluminum-graphene composite was hot extruded from a steel mold using a mixture of graphite with a lubricant in the following mode: 420-460°C, extrusion speed no more than 6 mm/s, extrusion ratio varied from 10:1 to 40: one.

The technical result of this method is to obtain composites with a high density, close to theoretical, with the absence of the carbide phase Al ₄ C ₃ and with a high uniformity of graphene distribution in the aluminum matrix. However, this approach has a number of disadvantages. Firstly, the proposed use of graphene in a grinding mixture is difficult to implement on an industrial scale due to the well-known problems in the production and use of monolayer graphene, leading to the fact that in industrial technologies monolayer graphene is replaced by a multilayer or graphene-containing substances (graphene oxide or reduced graphene oxide). In turn, the use of multilayer graphene eliminates all the advantages of the two-stage grinding procedure in a ball mill and does not provide the claimed density of the final material. The use of graphene-containing substances will lead to the need for a significant modification of the process by introducing into it the operation of reducing these compounds to graphene. Secondly, the presence of a lubricant (stearic acid) in the initial mixture of powders, even in small amounts, requires a set of measures to prevent clumping of the lubricant, which leads to the formation of particles of large sizes, which, in turn, during hot extrusion can contribute to the formation of large pores in final material.

The technical result of the claimed invention is the possibility of obtaining aluminum-graphene composites with improved mechanical characteristics, primarily with increased plasticity, the technical result is achieved through the use of thermally expanded graphite as a source of graphene in the composite.

The achievement of the technical result is ensured by the inventive method for producing aluminum-graphene composites, including the preparation of a mixture of aluminum powders with a particle size of not more than 40.0 μm and thermally expanded graphite, in a ratio of 99.3-98.8 wt. % aluminum powder - 0.7-1.2 wt. % thermally expanded graphite; grinding the prepared mixture with mechanical activation in a planetary mill for 2.5-3.0 hours at a rotation speed of 400-450 rpm with reverse cycles lasting 3-5 minutes and a break for cooling the grinding set lasting 25-30 minutes in the middle of the process, pressing blanks at room temperature at a pressing pressure of 5.0-5.5 t/cm ² for 15-20 min, isostatic pressing in a liquid medium at a pressure of 1400-1500 atm. and subsequent firing in vacuum speech with the following parameters: firing temperature 400-500°C for 1-1.5 hours at a residual pressure of less than 0.003 atm., with a heating rate of 7-10°C/min and cooling to room temperature for 5-6 hours.

The choice of the proposed approach is due to the fact that in the process of joint grinding of powders of aluminum and thermally expanded graphite in a planetary mill, the process of micromechanical splitting of thermally expanded graphite occurs, which is an effective mechanism for the conversion of thermally expanded graphite into graphene. Thus, due to the use of thermally expanded graphite as a source of graphene in the final composite, it becomes possible to solve a number of problems characteristic of alternative methods for producing aluminum-graphene composites. Thus, the production of graphene in the process of micromechanical splitting of thermally expanded graphite makes it possible to use cheap and available raw materials in the technological process (commercially available thermally expanded graphite instead of monolayer graphene) and solves the problems of possible agglomeration and damage to monolayer graphene sheets. At the same time, the use of thermally expanded graphite as a source of graphene instead of graphene-containing additives such as graphene oxide or reduced graphene oxide makes the high-temperature annealing step necessary to reduce these forms into graphene redundant. Along with this, the micromechanical splitting of thermally expanded graphite proceeds in parallel with the efficient mixing of the mixture of powders, as a result of which it is possible to significantly reduce the time of the mixing process, the duration of this stage of obtaining an aluminum-graphene composite in the proposed method is 3.0-3.5 hours versus 20-25 hours. the method described in patent CN108085524B (prototype).

The proposed method can be applied to obtain aluminum-graphene composites with improved mechanical characteristics, primarily with increased plasticity. The resulting material can be used in the electrical industry in the manufacture of cable products, as well as in the aviation and space industries. To implement the proposed method for obtaining aluminum-graphene powder composites with improved mechanical characteristics, means and materials known in the art can be used.

The selection of the required fractions of aluminum powder materials (for example, PA-4 according to GOST 6058-73, the aluminum content is not less than 98 wt.% with a particle size of not more than 40 μm) can be performed, for example, using sieve vibration techniques. The preparation of initial powder mixtures with the required ratio of components (aluminum powder 99.3-98.8 wt.% - 0.7-1.2 wt.% thermally expanded graphite) can be performed both in an automated mode (for example, using dispensers), and by using an analytical balance of the required accuracy class. The required amount of thermally expanded graphite in the initial mixture is determined by the amount of graphene in the final composite required to fill the intergranular spaces of the metal matrix of the aluminum-graphene composite. For used in the claimed method, the dispersion of the original powders of aluminum and methods for producing a composite, this amount is defined as 0.7-1.2 wt. %. When the content of thermally expanded graphite in the initial mixture of powders is less than the claimed amount of 0.5 wt. %, obtaining composites within the proposed method is almost impossible. This fact is due to the fact that individual aluminum without additives of lubricants is not able to be compacted under the influence of cold isostatic pressing; this pressing method does not provide the possibility of obtaining workpieces suitable for further hydrostatic and vacuum sintering operations. The presence in the mixture of powders of thermally expanded graphite in quantities of more than 0.5 wt. % provides a lubricating effect and makes it possible to obtain workpieces suitable for further operations as a result of isostatic pressing at room temperature. However, the content of thermally expanded graphite in a mixture of powders in amounts of 0.5-0.7 wt. % does not fully provide the required density and porosity of the blanks, and therefore the mechanical properties of the resulting aluminum-graphene composites, primarily plasticity, are insufficient. The excess of the claimed amount of thermally expanded graphite in the initial mixture of powders in 1.2 wt. % leads to agglomeration of excess graphene between particles of aluminum powder; in the process of further production of the aluminum-graphene composite, the reverse transformation of graphene into graphite occurs. In this case, the resulting composite is characterized by the formation of pores in the thickness of the material with inclusions of graphite in them, along with this, multilayer graphene or graphite can also be realized in the form of inclusions in the intergranular space. Such inclusions are oriented in planes perpendicular to the pressing axis, their presence leads to cracking of the material under load. Mixing of a mixture of powders of aluminum and thermally expanded graphite with simultaneous conversion of thermally expanded graphite into graphene by micromechanical splitting by grinding with mechanical activation can be carried out using planetary mills (for example, Pulverisette-6) in the mode of 400-450 rpm with reverse cycles of 3-5 minutes for 2.5-3.0 hours with a break for cooling the grinding set lasting 25-30 minutes in the middle. Such grinding can be carried out using known materials of the required hardness, for example, in an agate glass with an agate set. Grinding speeds lower than in the claimed method (less than 400 rpm) for insufficient time (less than 2.5 hours) do not allow to achieve complete conversion of thermally expanded graphite present in the initial mixture of powders into graphene, as well as its effective mixing, ensuring distribution uniformity graphene in the mixture after grinding. The result of such a deviation of the grinding parameters from the claimed ones is the presence of residual thermally expanded graphite in the mixture of powders and the inhomogeneous chemical composition of the mixture of powders, which, in turn, leads to the heterogeneity of the final composite material and a decrease in its strength characteristics. Exceeding the claimed grinding parameters (speed more than 450 rpm for more than 3 hours) leads to partial deformation of graphene sheets and flattening of aluminum powder particles into plates. The result of the formation of a composite from a mixture of powders obtained in this mode will be a material with a disturbed geometric structure of the graphene phase and an increased porosity due to the lamellar shape of the matrix grains, which will also lead to a decrease in the mechanical strength of the resulting material. The duration of the reverse cycles is 3-5 minutes due to the fact that when the duration of the reverse cycle is less than 3 minutes, there is excessive wear of the grinding equipment; at the same time, if the reverse cycle time of 5 minutes is exceeded, undesirable agglomerates may form in the mixed powders. This agglomeration of particles during the milling and mixing step can lead to grain size inhomogeneity in the final material and to the presence of graphite inclusions, as a result of which the plasticity of the resulting material will be reduced. Provided in the claimed method, a break of 25-30 minutes in the middle of the grinding process is associated with excessive heating of the grinding set in the claimed grinding mode. The absence of such a break will lead to the failure of the grinding equipment, the time parameters of such a break are due to the fact that the duration of a break of less than 25 minutes is not enough to guarantee the cooling of the grinding set, while exceeding the time of such a break for more than 30 minutes is inappropriate in terms of increasing the total duration of the process production. Pressing of ground powder mixtures into blanks at room temperature and pressure of 5.0-5.5 t/cm ² for 15-20 minutes is carried out using standard presses in molds made, for example, of stainless steel. With insufficient pressing pressure (less than the claimed value of 5.0 t/cm ² ) and / or insufficient duration of the pressing operation (less than 15 minutes), the workpiece pressed from the powder mixture has a density and porosity insufficient for further operations with it - extraction from the press -shape and isostatic pressing; The blanks obtained in this mode are characterized by the presence of a significant number of cracks, which also makes further operations with it impossible. At the same time, the use of pressing modes with parameters exceeding those claimed (pressure over 5.5 t/cm ² and/or pressing time over 20 minutes) is redundant, since it does not improve the quality of the final aluminum-graphene composite. In addition, excessive pressure or pressing time can lead to partial extrusion of graphene into the pores of the workpiece, which will lead to excessive porosity of the final material and the appearance of graphite inclusions in it. Isostatic pressing in a liquid medium at a pressure of 1400-1500 atm. can be carried out using hydrostatic presses (for example, P9100.10-GidKh). The use of hydrostatic mode less than the claimed 1400 atm. does not provide the required density of the final material, while the use of pressure exceeding the declared 1500 atm. impractical, since it does not improve the properties of the resulting composite. Heat treatment of pressed blanks is carried out in vacuum furnaces (for example, Averon or Sirius series) at a firing temperature of 400-450 ° C for 1-1.5 hours at a residual pressure of less than 0.003 atm., at a heating rate of 7-10 ° C / min and cooling to room temperature for 5-6 hours. Exceeding the required residual pressure in the working volume of the vacuum furnace during heat treatment leads to burnout of graphene in the workpieces due to the reaction of carbon with residual oxygen in the air, as well as to oxidation of the surface of the formed grains of the aluminum matrix of the material. As a result, there is a deviation of the composition from the required one with a violation of both the structure of the graphene phase of the composite due to burnout and a change in the electrical properties of the material due to the appearance of poorly conducting aluminum oxide at the grain boundaries of the material. Exceeding the firing temperature relative to the claimed leads to a softening of aluminum, which results in a possible heterogeneity in grain size in the final material, as well as an undesirable change in the geometry of the workpieces; insufficient firing temperature does not ensure sintering of products. Exceeding the heating rate relative to the claimed one (more than 10°C/min) leads to inhomogeneous sintering of workpieces, while lower heating rates relative to the claimed ones (less than 7°C/min) lead to an increase in the duration of the process and are not economically feasible.

An example of a method for producing aluminum-graphene composites with improved ductility.

The selection of the fraction of aluminum powder PA-4 according to GOST 6058-73 was carried out on a sieve analyzer A30, placed on a vibrating stand VT-30T, a fraction of less than 40 μm was taken, which was later used to prepare a mixture of powders. The resulting aluminum powder fraction was mixed with thermally expanded graphite powder (TRG according to TU 2573-001-91200348-2011) in a ratio of 99.0 wt. % aluminum powder - 1.0 wt. % thermally expanded graphite. Grinding of the prepared mixture with mechanical activation of the components and concomitant conversion of thermally expanded graphite into graphene due to micromechanical splitting of thermally expanded graphite was carried out using a Pulverisette-6 planetary mill in agate jars with agate balls at 450 rpm for 3 hours with 5-minute reverse cycles and a break for cooling the grinding set lasting 30 minutes, the beginning of the break corresponded to the middle of the grinding process, that is, a time mark of 1.5 hours. A mixed mixture of aluminum powders and thermally expanded graphite was loaded into molds made of stainless steel grade 08X18H10, which provided pressed blanks with a diameter of 35 mm and a height of 15 mm, after which they were pressed at room temperature using a KMG-50A hydraulic press at a pressure of 5.5 t/cm ² for 20 minutes. The preforms obtained with a diameter of 35 mm and a height of 15 mm were subjected to isostatic pressing in a liquid medium using a P9100.10-GidKh hydrostat at a pressure of 1500 atm. The hydrostatic blanks were loaded into a vacuum furnace and subjected to heat treatment at a residual pressure of 0.002 atm., the heating rate was 8°C/min, upon reaching a temperature of 500°C, the sample was held for 1 hour. The sample was cooled together with the oven (6 hours to room temperature).

The obtained samples of aluminum-graphene composites had the same dimensions as the hydrostatic blanks, that is, a diameter of 35 mm and a height of 15 mm. Visual inspection of the obtained composites showed the absence of cracks and macrodefects. Raman spectroscopy studies confirmed that graphene was the only carbon phase in the resulting composite. The resulting composites were tested for mechanical properties using a Shimadzu AG-X Plus universal mechanical testing machine.

Specific examples of the implementation of the method and the results of studies of aluminum-graphene composites with improved mechanical characteristics are shown in Table 1, as well as in Fig. 1-3.

Table 1. Examples of the implementation of the method for obtaining a copper-based powder composite with improved mechanical characteristics. The content of thermally expanded graphite in the initial mixture of powders, wt. % Stirring mode in planetary mill Mode of pressing and isostatic pressing in a liquid medium Heat treatment mode Brief characteristics of the composite and compressive resilience 0.3 450 rpm
3 hours 5.5 t/ ^cm2
20 minutes
1500 atm 7°C/min
500°C for 1.5 hours Failed to compress blanks by isostatic pressing at room temperature 0.5 450 rpm
3 hours 5.5 t/ ^cm2
20 minutes
1500 atm 7°C/min
500°C for 1.5 hours Insufficient compressive strength 0.7 400 rpm
2.5 hours 5.0 t/ ^cm2
15 minutes
1500 atm 7°C/min
400°C for 1 hour Maximum compressive resilience 400 rpm
2.5 hours 5.0 t/ ^cm2
15 minutes
1400 atm 10°C/min
450°C for 1 hour 450 rpm
2.5 hours 5.0 t/ ^cm2
20 minutes
1500 atm 8°C/min
500°C for 1 hour 1.0 400 rpm
2.5 hours 5.0 t/ ^cm2
15 minutes
1400 atm 7°C/min
400°C for 1 hour Maximum compressive resilience 450 rpm
2.5 hours 5.5 t/ ^cm2
20 minutes
1500 atm 10°C/min
450°C for 1 hour 450 rpm
3 hours 5.5 t/ ^cm2
20 minutes
1500 atm 8°C/min
500°C for 1 hour 1.2 400 rpm
2.5 hours 5.5 t/ ^cm2
15 minutes
1400 atm 8°C/min
400°C for 1 hour Maximum compressive resilience 450 rpm
3 hours 5.0 t/ ^cm2
15 minutes
1500 atm 10°C/min
450°C for 1 hour 450 rpm
3 hours 5.5 t/ ^cm2
20 minutes
1500 atm 10°C/min
500°C for 1 hour 1.5 450 rpm
2.5 hours 5.5 t/ ^cm2
20 minutes
1500 atm 7°C/min
400°C for 1 hour Decrease in the elasticity of the material under compression 400 rpm
3 hours 5.0 t/ ^cm2
20 minutes
1400 atm 10°C/min
450°C for 1 hour 450 rpm
2.5 hours 5.0 t/ ^cm2
20 minutes
1500 atm 10°C/min
500°C for 1 hour 3.0 400 rpm
3 hours 5.5 t/ ^cm2
20 minutes
1500 atm 7°C/min
400°C for 1 hour Low compressive strength 450 rpm
2.5 hours 5.0 t/ ^cm2
15 minutes
1500 atm 10°C/min
400°C for 1 hour 450 rpm
3 hours a 5.5 t/ ^cm2
15 minutes
1500 atm 10°C/min
400°C for 1 hour

The invention is illustrated by the following drawings:

- in Fig. 1 shows the results of studies characterizing the limiting relative compression of samples obtained at a heat treatment temperature of 400°C;

- in Fig. 2 shows the results of studies characterizing the limiting relative compression of the samples obtained at a heat treatment temperature of 450°C;

- in Fig. 3 shows the results of studies characterizing the limiting relative compression of samples obtained at a heat treatment temperature of 500°C.

The results of the conducted studies confirm that the proposed method provides the possibility of creating an aluminum-graphene powder composite with improved ductility. As follows from the presented figures, for all three annealing temperatures (400, 450 and 500°C) the values of the limiting relative compression for samples containing 0.7-1.2 wt. % of graphene is 2 or more times higher than for samples with graphene content outside the above range. At the same time, a lower content of graphene (0.5-0.7 wt.%) does not provide a sufficient amount of lubricant additive when pressing the initial blanks, which leads to insufficient density of the blank, this fact, in turn, is the reason for the reduced plasticity of the final composite. When the required amount of graphene is exceeded (more than 1.2 wt %), the surface area of graphene becomes excessive compared to the total area of the obtained grain boundaries of the metal matrix of the composite, graphene is concentrated in the grain boundaries and pores of the material, as a result of which its plasticity also significantly decreases. As follows from the comparison of figures 1-3, the greatest effect of increasing plasticity is achieved at a heat treatment temperature of 500°C. A direct comparison of the obtained plasticity values with the data for pure aluminum is impossible due to the inability to manufacture comparison samples according to the claimed method. However, a comparison of the plastic properties of aluminum-graphene composites obtained for samples manufactured by the claimed method with the results obtained for known aluminum composites (J. Wang, Z. Li, G. Fan, H. Pang, Z. Chen and D. Zhang / / ScriptaMaterialia 66 (2012) 594 Z. Li, G. L. Fan, Z. Q. Tan, Q. Guo, D. B. Xiong, Y. S. Su, Z. Q. Li and D. Zhang // Nanotechnology 25 (2014) art. Pan, A. T. Tang and M. Asif // Progress in Natural Science: Materials International 24 (2014) 101; S. F. Bartolucci, J. Paras, M. A. Rafiee, J. Rafiee, S. Lee, D. Kapoor and N. Koratkar // Materials Science & Engineering A 528 (2011) 7933) shows that the proposed method provides an increase in the plasticity of aluminum-graphene composites in comparison with known analogues. Thus, it is confirmed that the proposed method provides the possibility of creating aluminum-graphene composites with improved ductility.

Claims (2)

1. A method for producing aluminum-graphene composites with improved plasticity, including the selection of an aluminum powder fraction with dimensions not exceeding 40.0 μm, mixing it with thermally expanded graphite powder in ratios of 99.3–98.8 wt.% of aluminum powder - 0.7 -1.2 wt.% of thermally expanded graphite, mixing of the prepared mixture and micromechanical splitting of thermally expanded graphite by grinding in a planetary mill in the mode of 400-450 rpm for 2.5-3 hours with reverse cycles lasting 3-5 minutes and a break for cooling of the grinding set for 25-30 minutes in the middle of the process, pressing of blanks carried out at room temperature and a pressure of 5.0-5.5 t/cm ² for 15-20 minutes, their hydrostating at a pressure of 1400-1500 atm and subsequent heat treatment carried out in a vacuum furnace at a residual pressure of less than 0.003 atm at a heating rate of 7-10 °C/min, holding at a maximum temperature of 400-500 °C. 2. The method according to p. 1, characterized in that exposure in a vacuum furnace is carried out for 1-1.5 hours, then cooling to room temperature occurs along with the furnace.

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