Highly conductive copper-zirconium alloy material and method for producing the same

Abstract

The application discloses a high-conductivity copper-zirconium alloy and a preparation method thereof. The copper-zirconium alloy comprises Cu, a doping element and inevitable impurities, and the total mass percentage of all elements is 100%, wherein the doping element comprises Zr: 0.10-0.20wt%, P: 0.007-0.015wt%, Cr: 0.005-0.015wt%, and O: 8-20ppm, and the mass percentage of Zr and Cr is (15-20):1. The copper-zirconium alloy has suitable strength and high conductivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of copper alloy, and particularly relates to a high-conductivity copper-zirconium alloy material and a preparation method thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles, the charging efficiency of vehicles is an important index for consumers to choose, so the charging voltage of new energy charging platforms is continuously increased. In the closing process, high-voltage circuits are prone to arc ablation. In addition, according to Q=U^2 / R*t, it can be known that the increase of voltage will cause the heat of the charging device to increase exponentially, and the temperature to rise sharply. Based on the above situation, the material required for the current new energy charging device has high conductivity, high high-temperature resistance and high strength.

[0003] Although the red copper material has high conductivity, it is no longer suitable for use in high-voltage charging devices due to insufficient high-temperature resistance. The CuZr alloy takes copper as the matrix and adds an appropriate amount of Zr in the copper alloy. Since the precipitation of Zr in the material has little effect on the conductivity of the material, and the precipitation of Zr in the material forms a certain amount of processing hardening and aging strengthening, the CuZr alloy has high conductivity and strength. At present, the CuZr alloy can effectively improve the tensile strength and high-temperature resistance by increasing the content of Zr and other elements, but its conductivity is generally less than 90% IACS.

[0004] The invention patent application with the publication number CN111363937A discloses a copper alloy wire for a plug-in connector and a manufacturing method thereof. The copper alloy wire comprises the following components by mass percentage: silver: 5-15%, zirconium: 0.1-0.9%, rare earth metal: 0.05-0.2%, and the balance is copper. First, silver, copper, copper-zirconium intermediate alloy and rare earth copper intermediate alloy are mixed, melted and cast into a shape, and then subjected to homogenization treatment and mechanical processing to obtain a copper alloy ingot. The ingot is hot extruded, multi-pass cold drawn, and then subjected to solid solution treatment, and then micro-drawing and annealing treatment to prepare the copper alloy wire. The patent application improves the tensile strength of the material to 550 MPa by increasing the content of Zr and appropriately adding rare earth elements, but the conductivity is only 83% IACS.

[0005] The patent application with the publication number CN116000084A discloses a preparation method of high-strength and high-conductivity copper-zirconium alloy wire, the chemical composition of the alloy material includes Cu, Zr, O elements and impurity elements, and the weight percentage is: Cu is greater than or equal to 99.80%, Zr is 0.10-0.20%. The preparation method of the alloy wire is: batching and smelting, cold forging, hot extrusion, hot rolling, solid solution treatment and surface treatment, cold drawing, aging treatment and surface strengthening. After increasing the solid solution temperature and applying the strengthening agent, the tensile strength of the material can reach more than 540 MPa, but the conductivity is only 81-83% IACS.

[0006] For high-voltage charging devices, the strength is moderate, and the most important thing is how to improve the charging efficiency, and the improvement of the charging efficiency needs to further improve the conductivity of the existing material, so it is urgent to design a copper-zirconium alloy with moderate strength and high conductivity. SUMMARY

[0007] The application provides a copper-zirconium alloy material with appropriate strength and high conductivity.

[0008] The application provides a high-conductivity copper-zirconium alloy, which includes Cu, doping elements and inevitable impurities, and the total mass percentage of all elements is 100%, wherein the doping elements include Zr: 0.10-0.20wt%, P: 0.007-0.015wt%, Cr: 0.005-0.015wt%, and O: 8-20ppm, and the mass percentage of Zr and Cr is (15-20): 1.

[0009] The effects of the doping elements are as follows:

[0010] Zirconium: The precipitation of zirconium in the material is beneficial to improve the strength and hardness of the material, and the more sufficient the precipitation is, the smaller the influence on the conductivity of the material is, so appropriate Zr content and the promotion of the precipitation of Zr are beneficial to enhance the strength of the material and improve the conductivity of the material.

[0011] Oxygen: Excessive oxygen is harmful to the material, but a small amount of oxygen can purify impurities in the copper alloy and is beneficial to the improvement of the conductivity of the copper alloy, but when the content of impurities is too high, the purification effect will lose its significance. The copper-zirconium alloy has less impurities, so a small amount of oxygen content is beneficial to improve the conductivity of the material, and the remaining small amount of oxygen is in the form of interstitial atoms and is beneficial to the improvement of the strength of the material.

[0012] Phosphorus: Phosphorus is the most effective and the lowest cost deoxidizer, which is beneficial to control the oxygen content in the alloy, and the Cu3P phase formed by phosphorus and copper can improve the high temperature plasticity of copper alloy, so that it can exhibit better plastic deformation ability in high temperature environment and avoid defects in the production process affecting the material performance. Because there is a certain solid solubility of phosphorus in copper, and the phosphorus solid-solved in copper significantly reduces its conductivity, therefore, in order to ensure the conductivity of copper alloy, the content of phosphorus element needs to be controlled.

[0013] Chromium: When the Cr content in the material is lower than the solid solubility, the Cr in the material will still have a tendency to precipitate, thereby forming a Cr-rich region, and Zr element has a certain inhibitory effect on Cr precipitation, so Cr is beneficial to control the size of the initial Zr phase, but when the Cr content is higher than the solid solubility, the Cr in the material will precipitate, thereby occupying the nucleation site of Zr, inhibiting the subsequent precipitation of Zr, causing insufficient Zr precipitation during aging process, thereby causing the conductivity of the material to decrease.

[0014] Preferably, the main strengthening phase of the copper-zirconium alloy material is the primary Zr-containing phase, the precipitated Zr-containing phase, the Cr-rich solid solution phase and the CuP intermetallic compound, wherein the average size of the primary Zr-containing phase is <2 μm, the average size of the precipitated Zr-containing phase is <1 μm, and the ratio of the primary Zr-containing phase and the precipitated Zr-containing phase is 1:10-15.

[0015] By controlling the size of the primary Zr-containing phase and the precipitated Zr-containing phase, the distance between the strengthening phases is shortened, so that the copper-zirconium alloy provided by the present application has suitable strength, and the content of the precipitated Zr-containing phase is further controlled, so that the content of the precipitated Zr-containing phase is in a higher range, so that the Zr phase solid-solved in the matrix is in a lower degree, and the copper-zirconium alloy provided by the present application has higher conductivity.

[0016] Preferably, the primary Zr-containing phase is elliptical. The elliptical primary Zr-containing phase is beneficial to the propagation of electrons, and improves the conductivity of the copper-zirconium alloy.

[0017] Preferably, the grain size of the copper-zirconium alloy is 0.02-0.04 mm, the conductivity is above 95% IACS, and the strength is above 410 MPa.

[0018] On the other hand, the present application also provides a preparation method of the high-conductivity copper-zirconium alloy, which comprises: the process flow of the preparation method comprises: batching and casting → continuous extrusion → solid solution → cold drawing → aging treatment.

[0019] In the preparation method, the batching and casting is carried out according to the percentage of each component of the high-conductivity copper-zirconium alloy.

[0020] The solid solution temperature T is:

[0021] T = 122ln(-30X + 0.12) + 1195

[0022] wherein X is the content of Zr;

[0023] The aging temperature is 400-420 DEG C.

[0024] The more Zr element is solid-solved into copper, the lower the conductivity of copper alloy is, and the Zr element will continuously precipitate after the subsequent aging, thereby reducing the influence of Zr element on the conductivity of copper. However, in the prior art, the Zr content solid-solved into the matrix is increased by increasing the solid-solution temperature, and a large amount of dislocations are formed by using a large processing rate in the subsequent process, thereby providing a large number of nucleation sites for the precipitation of Zr. The more the nucleation sites are, the finer and more dispersed the precipitated phase is, thereby improving the strength of the material. However, the Zr content is increased after the aging precipitation due to the large amount of Zr solid-solved into the matrix, and the precipitated phase is fine, which finally leads to a low conductivity of the product. In another method in the prior art, the precipitated phase is coarsened by increasing the aging temperature, but the strength of the material is seriously reduced. Since the coarsening of the precipitated phase is random, the uniformity of the material is poor. In view of the above technical problems, the present application determines the relationship between the Zr addition amount and the solid-solution temperature in the copper alloy: T = 122ln(-30X + 0.12) + 1195, controls the Zr element content solid-solved into copper, and reduces the solid-solution base, so that a large amount of Zr in the matrix can be precipitated in the subsequent aging process, a large amount of precipitated Zr-containing phase with appropriate size is formed, and the conductivity is improved. Secondly, Cr element is added to control the size of the initial Zr phase, so that the initial Zr-containing phase and the aging precipitated Zr-containing phase cooperatively reduce the spacing between the secondary phases in the matrix, and the material is strengthened. Since the influence of the initial phase on the conductivity is reduced compared with the aging precipitated phase, the conductivity of the material is improved, and a copper-zirconium alloy with higher conductivity and medium tensile strength is obtained.

[0025] Preferably, the single cold drawing processing amount is 20-25%, and the total cold drawing processing amount is 60-70%.

[0026] The present application forms a large amount of dislocations by a large processing amount, reduces the nucleation energy of the precipitated Zr-containing phase, increases the nucleation sites, and under the induced precipitation of the initial Zr-containing phase, the Zr element solid-solved into the matrix is precipitated, thereby reducing the hindering effect of Zr in the matrix on electrons. At the same time, due to the existence of a certain coherent relationship between the initial Zr-containing phase and the matrix, the initial Zr-containing phase presents an elliptical shape under the action of a large processing amount, thereby reducing the hindering of electron conduction and improving the conductivity.

[0027] Preferably, the holding time of the solid solution is 30-60 min, and then water cooling is used to reduce the temperature to room temperature. By controlling the holding time, the blank is uniformly heated, and the grain coarsening caused by the solid-solution time process is avoided.

[0028] Preferably, the smelting temperature of the melting casting is 1200-1300℃, the drawing speed is 0.6-0.7mm / s, and argon protection is adopted in the drawing process to avoid oxidation loss of alloy elements.

[0029] Preferably, P is added in the melting casting process at 0.007%-0.015% to improve the high-temperature fluidity of the blank and avoid casting cracking of the blank, and the addition of P element achieves the effect of degassing, thereby controlling the O content at 8ppm-20ppm.

[0030] Preferably, the specific steps of the melting casting comprise:

[0031] Before smelting, the copper-zirconium is wrapped with copper sheet to prepare a cored wire. After starting smelting, a proper amount of oxygen-free copper rod is first added into the up-drawing furnace, argon is introduced, and heating is performed until the oxygen-free copper rod is completely melted. Then, automatic feeding of the oxygen-free copper rod and the copper-zirconium cored wire (copper-phosphorus and copper-chromium alloy prepared by mixing in proportion) is started, and the feeding speed is controlled at 220Kg / h. The automatic feeding is stopped when the copper liquid is 20cm away from the edge of the crucible. The final temperature is kept at 1200-1300℃, and the composition is measured after 10-20min of heat preservation and adjusted. After the composition is qualified, the up-drawing blank production is carried out. In the subsequent drawing process, the oxygen-free copper rod and the copper-zirconium cored wire (copper-phosphorus and copper-chromium alloy prepared by mixing in proportion) are automatically fed according to the predetermined design composition, and the composition of the furnace liquid is tested every 30min to ensure the composition of the up-drawing blank, and finally the required up-drawing blank is obtained.

[0032] Preferably, the processing amount of the continuous extrusion is 15-20%. The present application controls the processing amount of the continuous extrusion to ensure that the as-cast structure in the extruded blank is completely broken, and the grain size before solid solution treatment is relatively small.

[0033] Preferably, the heat preservation time of the aging is 4-7h, and the cooling mode is furnace cooling.

[0034] Compared with the prior art, the present application has the following beneficial effects:

[0035] The present application controls the proportion of Zr and Cr elements to make the matrix form a chromium-rich solid solution phase, controls the size of the initial Zr-containing phase by using the inhibition effect of Zr on the precipitation of the chromium-rich solid solution phase, makes the size of the initial Zr-containing phase small, reduces the distance between the initial Zr-containing phase and the precipitated Zr-containing phase, and thus makes the copper alloy have appropriate strength. If the content of Cr element is too high, Cr will precipitate, thereby occupying the nucleation site of Zr, making Zr not precipitate sufficiently, resulting in a large amount of Zr existing in the matrix and thus hindering electron conduction and affecting electrical conductivity. Therefore, the present application controls the proportion of Zr and Cr elements to make the copper alloy have appropriate strength and high electrical conductivity.

[0036] The solid solution temperature is controlled based on the relationship between the Zr content and the solid solution temperature in the preparation method of the application, so as to dissolve appropriate Zr in the matrix, so that a large amount of Zr can be precipitated at a lower aging temperature, thereby reducing the Zr content dissolved in the matrix, improving the electrical conductivity, and ensuring appropriate strength; under the action of the chromium-rich region formed by a small amount of Cr, the size of the primary Zr-containing phase is small, and the size of the precipitated Zr-containing phase formed at a lower aging temperature is also small; since the sizes of the primary Zr-containing phase and the precipitated Zr-containing phase are small, and the precipitated Zr-containing phase is precipitated between the primary Zr-containing phases, the distance between the strengthening phases is reduced, so that the copper-zirconium alloy provided by the application has appropriate strength.

[0037] Drawings

[0038] Figure 1 The primary phase electron micrograph of the copper-zirconium alloy material prepared in Example 1 of the application.

[0039] Figure 2 The primary phase and precipitated phase electron micrographs of the copper-zirconium alloy material prepared in Example 1 of the application.

[0040] Specific embodiment mode

[0041] In order to make the purpose, technical scheme and advantages of the application more clear and explicit, the application will be further described in detail below in combination with examples. It should be understood that the specific examples described here are only used to explain the application and not to limit the application. Those skilled in the art can modify or replace equivalently without departing from the spirit and scope of the technical scheme of the application, which should be covered within the protection scope of the application.

[0042] The application provides three examples and four comparative examples, and the specific components are shown in Table 1

[0043] The example is prepared by the preparation method of the application, and the process flow is: raw material preparation→up-drawing continuous casting→continuous extrusion→solid solution→cold working→aging treatment. The raw materials are: oxygen-free copper rod, copper-zirconium alloy cored wire, and copper-chromium alloy and copper-phosphorus alloy are added in the cored wire in proportion.

[0044] Example 1

[0045] The copper-zirconium alloy, according to 100% by mass, includes components: Zr: 0.10%; P: 0.008%;

[0046] Cr: 0.006%; 0: 10 ppm, wherein the Zr and Cr content is within Zr: Cr = (15-20): 1, and the balance is Cu.

[0047] Step 1: using the up-drawing furnace for smelting and blank preparation, the smelting holding temperature is controlled at 1280°C, the drawing speed is controlled at 0.7 mm / s, argon protection is used during the drawing process, the cooling water flow is controlled at 11 m3 / min, and the cooling water temperature is controlled at 23°C, and finally the qualified up-drawing blank is prepared.

[0048] Step 2: the above up-drawing blank is extruded using a continuous extruder, and the extrusion processing amount is controlled at 18.5% to ensure that the as-cast structure in the extruded blank is completely broken;

[0049] Step 3: high-temperature solid solution treatment is performed on the extruded blank, the solid solution temperature is determined according to the zirconium content, the relationship is T = 122ln(-30X+0.12)+1195; the solid solution temperature is 901°C, the Zr content is 0.10%, i.e. X = 0.001. The solid solution holding time is 40 min, and after the holding is completed, water cooling is used quickly.

[0050] Step 4: cold drawing processing is performed on the solid solution blank, the single pass drawing processing amount is controlled at 20-25%, and the total cold drawing amount is controlled at 70%;

[0051] Step 5: low-temperature aging is performed on the drawn blank, the aging temperature is 420°C, the holding time is 5h, and the cooling method is furnace cooling.

[0052] Example 2

[0053] The copper-zirconium alloy, according to 100% by mass, includes components: Zr: 0.15%; P: 0.010%;

[0054] Cr: 0.008%; 0: 15 ppm, wherein the Zr and Cr contents are within Zr: Cr = (15-20): 1, and the balance is Cu.

[0055] Step 1: using the up-drawing furnace for smelting and blank preparation, the smelting holding temperature is controlled at 1280°C, the drawing speed is controlled at 0.7 mm / s, argon protection is used during the drawing process, the cooling water flow is controlled at 11 m3 / min, and the cooling water temperature is controlled at 23°C, and finally the qualified up-drawing blank is prepared.

[0056] Step 2: the above up-drawing blank is extruded using a continuous extruder, and the extrusion processing amount is controlled at 18.5% to ensure that the as-cast structure in the extruded blank is completely broken;

[0057] Step 3: high-temperature solid solution treatment is performed on the extruded blank, the solid solution temperature is determined according to the zirconium content, the relationship is T = 122ln(-30X+0.12)+1195; the solid solution temperature is 879°C, the Zr content is 0.15%, i.e. X = 0.0015. The solid solution holding time is 40 min, and after the holding is completed, water cooling is used quickly.

[0058] Step 4: Cold drawing process is carried out on the solid solution blank, the single pass drawing processing amount is controlled at 20-25%, and the total cold drawing amount is controlled at 65%;

[0059] Step 5: Low-temperature aging is carried out on the drawn blank, the aging temperature is 420℃, the holding time is 5h, and the cooling mode is furnace cooling.

[0060] Example 3

[0061] The copper-zirconium alloy, according to 100% by mass, comprises components: Zr: 0.20%; P: 0.007%; Cr: 0.011%; O: 20ppm, wherein the Zr and Cr contents are within Zr:Cr=(15-20):1, and the balance is Cu.

[0062] The copper-zirconium alloy, according to 100% by mass, comprises components: Zr: 0.20%; P: 0.007%; Cr: 0.011%; O: 20ppm, wherein the Zr and Cr contents are within Zr:Cr=(15-20):1, and the balance is Cu.

[0063] Step 1: Melting and blank preparation are carried out by using an up-drawing furnace, the melting holding temperature is controlled at 1280℃, the drawing speed is controlled at 0.7mm / s, argon protection is adopted during the drawing process, the cooling water flow is controlled at 11m^3 / min, and the cooling water temperature is controlled at 23℃, and finally a qualified up-drawing blank is prepared.

[0064] Step 2: The above up-drawing blank is extruded by using a continuous extruder, and the extrusion processing amount is controlled at 18.5% to ensure that the as-cast structure in the extruded blank is completely broken;

[0065] Step 3: High-temperature solid solution treatment is carried out on the extruded blank, the solid solution temperature is determined according to the zirconium content, the relationship is T=122ln(-30X+0.12)+1195; the solid solution temperature is 852℃, the Zr content is 0.20%, i.e. X=0.002. The solid solution holding time is 40min, and water cooling is adopted immediately after the holding is completed.

[0066] Step 4: Cold drawing process is carried out on the solid solution blank, the single pass drawing processing amount is controlled at 20-25%, and the total cold drawing amount is controlled at 60%;

[0067] Step 5: Low-temperature aging is carried out on the drawn blank, the aging temperature is 420℃, the holding time is 5h, and the cooling mode is furnace cooling.

[0068] Comparative Example 1

[0069] Compared with Example 1, the difference lies in that the solid solution temperature is 930℃.

[0070] Comparative Example 2

[0071] Compared with Example 2, the difference lies in that the Cr component is relatively high, according to 100% by mass, Zr: 0.20%; P: 0.007%; Cr: 0.10%; O: 20ppm, and the balance is Cu.

[0072] Comparative Example 3

[0073] Comparative to Example 3, the difference is that the P element is too high, Zr: 0.20%; P: 0.018%; Cr: 0.011%; O: 20 ppm, wherein Zr: Cr = (15-20): 1, the balance Cu, based on 100% by mass.

[0074] Comparative Example 4

[0075] Comparative to Example 3, the difference is that the O element is too high in step, Zr: 0.20%; P: 0.018%; Cr: 0.011%; O: 50 ppm, wherein Zr: Cr = (15-20): 1, the balance Cu, based on 100% by mass.

[0076] Comparative Example 5

[0077] Comparative to Example 2, the difference is that step 4: the total processing amount is 50%

[0078] Comparative Example 6

[0079] Comparative to Example 2, the difference is that the solid solution temperature is 930℃, and the aging temperature in step 5 is controlled at 470℃.

[0080] The chemical components of the examples and comparative examples are summarized in Table 1; the copper alloy prepared by the examples and comparative examples is subjected to grain size, second phase size, tensile strength, and conductivity performance test results as shown in Table 2, and the test methods are as follows:

[0081] Tensile strength: detected according to GB / T228.1-2021 "Metallic materials tensile test Part 1: room temperature test method";

[0082] Conductivity: detected according to GB / T32791-2016 "Copper and copper alloy conductivity eddy current test method";

[0083] High temperature softening resistance: detected according to GB / T 33370-2016 "Copper and copper alloy softening temperature determination method".

[0084] As shown in Figure 1 , the spacing between the initial phases is relatively large, which is not conducive to the improvement of the strength of the material, and by Figure 2 , it can be seen that the existence of precipitated phase can greatly reduce the spacing of the strengthening phase, and the strength of the material is positively correlated with the spacing of the strengthening phase, while the initial phase has little effect on the conductivity of the material, so that the material has medium strength and high conductivity.

[0085] Table 1 Composition of Comparative Examples and Examples

[0086]

[0087] Table 2 Comparative Examples and Examples Physical Properties

[0088]

[0089] From Table 2, it can be seen that, under the present patent scheme, by controlling the Zr content dissolved into the matrix, thereby controlling the residual Zr content in the matrix after aging, the products prepared in Examples 1-3 have high electrical conductivity; and after aging precipitation, the sum of the initial phase and the precipitated phase reaches more than 45000 / mm 2 , thereby ensuring that the products have relatively high strength; and in the comparative examples, by increasing the solid solution temperature, increasing the aging temperature, and reducing the cold working amount in Comparative Examples 1-6 and excessive Cr / P / O elements in the material, the electrical conductivity and even the strength of the products prepared are reduced, thereby failing to meet the customer's requirements for medium strength and high conductivity.

Claims

1. A highly conductive copper-zirconium alloy, characterized in that, The mixture includes dopant elements, with the balance being Cu and unavoidable impurities, and the total mass percentage of all elements is 100%. The dopant elements include Zr: 0.10-0.20 wt%, P: 0.007-0.015 wt%, Cr: 0.005-0.015 wt%, O: 8-20 ppm, and the mass percentage of Zr and Cr is (15-20):

1. The main strengthening phases of the copper-zirconium alloy are primary Zr-containing phase, precipitated Zr-containing phase, Cr-rich solid solution phase and CuP intermetallic compound, wherein the average size of the primary Zr-containing phase is <2μm and the average size of the precipitated Zr-containing phase is <1μm, and the ratio of the primary Zr-containing phase to the precipitated Zr-containing phase is 1:10-15.

2. The high conductivity copper-zirconium alloy according to claim 1, characterized in that, The primary Zr-containing phase is elliptical.

3. The high conductivity copper-zirconium alloy according to claim 1, characterized in that, The copper-zirconium alloy has a grain size of 0.02-0.04 mm, a conductivity of 95% IACS or higher, and a strength of 410 MPa or higher.

4. A method for preparing a high-conductivity copper-zirconium alloy according to any one of claims 1-3, characterized in that, include: The process flow of the preparation method includes: batching and casting → continuous extrusion → solution treatment → cold drawing → aging treatment; The high-conductivity copper-zirconium alloy is prepared and melted according to the percentage of each component. The solution temperature T is given, where T is in °C. T = 122ln(-30X + 0.12) + 1195 Where X represents the Zr content; The aging temperature is 400-420℃.

5. The method for preparing the high conductivity copper-zirconium alloy according to claim 4, characterized in that, The single-pass drawing amount of the cold drawing is 20-25%, and the total cold drawing amount is 60-70%.

6. The method for preparing the high-conductivity copper-zirconium alloy according to claim 4, characterized in that, The solution treatment time is 30-60 minutes, and the cooling method is water cooling.

7. The method for preparing the high conductivity copper-zirconium alloy according to claim 4, characterized in that, Adding 0.007%-0.015% P during the casting process controls the O content to 8ppm-20ppm.

8. The method for preparing the high conductivity copper-zirconium alloy according to claim 4, characterized in that, The processing volume of the continuous extrusion is 15-20%.

9. The method for preparing the high-conductivity copper-zirconium alloy according to claim 4, characterized in that, The aging holding time is 4-7 hours, and the cooling method is furnace cooling.

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