Manufacturing method of steel joint

Abstract

To provide a method for manufacturing a steel material joint body, which can easily and effectively increase the joining strength of steel materials.SOLUTION: This steel material joint body manufacturing method for joining a plurality of steel materials comprises: an arrangement step of arranging a carbonaceous material on the joined surface of at least one of steel materials to be joined; a stacking step of stacking the steel materials by self-weight or applying a pressing load only by pressing the joined surfaces of the steel materials to be joined to each other via the carbonaceous material; a heating step of heating the steel materials at a reached highest temperature generated by a liquid phase of the carbonaceous material by the self-weight or applying the pressing load only by pressing the steel materials the joined surfaces of which have been stacked; and a cooling step of cooling the heated steel materials.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a steel joint. [Background technology]

[0002] To date, a hot joining method for steel materials has been disclosed in which a carbonaceous substance is applied or sprayed onto the joining surfaces, hot steel materials are overlapped or butted together, and then heated and pressure-welded in a reducing atmosphere, with the aim of developing a technology that can join hot steel materials simply and efficiently in an actual factory and that can obtain a high level of joining strength that does not interfere with the subsequent rolling process (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-7970 Summary of the Invention [Problem to be solved by the invention]

[0004] However, it cannot be said that the joined steel member obtained by the technique described in Patent Document 1 effectively improves the joining strength between the steel members. Therefore, an object of the present invention is to provide a method for manufacturing a joined steel member that can simply and effectively improve the joining strength between the steel members. [Means for solving the problem]

[0005] The method for manufacturing a steel material joined body according to the present invention is a method for manufacturing a steel material joined body in which a plurality of steel materials are joined together, and includes: an arrangement step of arranging a carbonaceous material on at least one of the joining surfaces of the steel materials to be joined; a superposition step of overlapping the joining surfaces of the steel materials to be joined together by simply pressing them together via the carbonaceous material, either by their own weight or while applying a pressing load; a heating step of heating the steel materials whose joining surfaces have been overlapped together by simply pressing them together by their own weight or while applying a pressing load, to a maximum temperature at which a liquid phase of the carbonaceous material is generated; and a cooling step of cooling the heated steel materials. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide a method for manufacturing a joined steel member that can simply and effectively improve the joining strength between steel members. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 2 is a process flow diagram illustrating a method for manufacturing a steel joint according to the embodiment of the present invention. [Figure 2] FIG. 2 is a conceptual diagram for illustrating a method for manufacturing a steel joint according to a first specific embodiment of the present invention. [Figure 3] FIG. 10 is a conceptual diagram for illustrating a method for manufacturing a steel joint according to a second specific embodiment of the present invention. [Figure 4] FIG. 1 is a phase diagram of an iron-cementite system for explaining the effect of the present invention. [Figure 5] FIG. 1 is a conceptual diagram for explaining the effect of the present invention. [Figure 6] 1 is a photograph showing the appearance of a steel member joint obtained in Example 1. [Figure 7] 10A and 10B are diagrams illustrating an observation position when checking a joining rate and a method for calculating the joining rate. [Figure 8] 1A and 1B are diagrams for explaining the locations where the metallographic structure of the steel welded bodies obtained in Examples 1 and 2 was confirmed, and photographs of the metallographic structure of these locations. [Figure 9]1 is a diagram for explaining a location where the metal structure of the steel welded bodies obtained in Examples 3 to 5 was confirmed, and a photograph of the metal structure of the location. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a process flow diagram illustrating a method for manufacturing a steel joint according to an embodiment of the present invention. As shown in FIG. 1, the method for manufacturing a steel joined body according to this embodiment is a method for manufacturing a steel joined body in which a plurality of steel materials are joined together, and includes: an arrangement step (step S100 shown in FIG. 1) of arranging a carbonaceous material on at least one of the joining surfaces of the steel materials to be joined; a superposition step (step S110 shown in FIG. 1) of overlapping the joining surfaces of the steel materials to be joined together by simply pressing the joining surfaces of the steel materials to each other with the carbonaceous material interposed therebetween, using their own weight or while applying a pressing load; a heating step (step S120 shown in FIG. 1) of heating the steel materials whose joining surfaces have been overlapped together to a maximum temperature at which a liquid phase of the carbonaceous material is generated, using their own weight or while applying a pressing load, using only the pressing surface of the steel materials to each other; and a cooling step (step S130 shown in FIG. 1) of cooling the heated steel materials. The method for manufacturing a steel member joint according to the present invention includes the above-described process flow, and thus can easily and effectively improve the joint strength between steel members. Each of these steps will be described in detail below.

[0009] Fig. 2 is a conceptual diagram for explaining a method for manufacturing a joined steel member according to a specific first embodiment of the present invention. Fig. 3 is a conceptual diagram for explaining a method for manufacturing a joined steel member according to a specific second embodiment of the present invention. The first embodiment shown in Fig. 2 and the second embodiment shown in Fig. 3 differ only in the form of the carbonaceous material placed on the joining surfaces of the steel members.

[0010] <Regarding the placement process (step S100)> In the manufacturing method of the steel member joint 1 according to the first and second embodiments, in the placement step (step S100), as shown in Figures 2(a) and 3(a), a carbonaceous material (30 or 40) is placed on at least one of the joining surfaces (10a and / or 20a: one of the joining surfaces in Figures 2(a) and 3(a)) between the steel members (10, 20) to be joined. In this case, the carbonaceous material is preferably arranged so that a liquid phase L (see FIG. 4: described later) is generated on the joining surfaces (10a and 20a) at the maximum temperature reached during heating in a heating step (step S120) described later. By arranging the carbonaceous material in this manner, the joining strength between the steel materials can be more effectively improved.

[0011] The material of the steel materials (10, 20) to be joined is not particularly limited as long as it is a metal that can be integrated with each other, and examples thereof include low carbon steel, medium carbon steel, and high carbon steel. In the present invention, "medium carbon steel" refers to a steel material with a carbon concentration of 0.30 mass% or more and 0.50 mass% or less. Low carbon steel refers to a steel material with a carbon concentration of less than 0.30 mass%, and high carbon steel refers to a steel material with a carbon concentration of more than 0.50 mass%. When using low carbon steel, medium carbon steel, or high carbon steel, the alloying elements other than carbon are not particularly limited, but may be, for example, as specified in JIS G 4051, a composition consisting of approximately 1.5 mass% or less of Si, 1.0 mass% or less of Mn, and the balance being Fe and unavoidable impurities.

[0012] The shape of the steel materials (10, 20) is not particularly limited as long as they have joining surfaces (10a, 20a) and can be integrated by overlapping these joining surfaces (10a and 20a). For example, a cylindrical shape, a prismatic shape, a screw shape, a concave-convex shape, etc. can be used.

[0013] The carbonaceous material (30, 40) can be disposed on at least one of the joining surfaces (10a and / or 20a) of the steel materials (10, 20) to be joined, and the material and shape are not particularly limited as long as the joining surfaces (10a and 20a) can be integrated. For example, a powdered carbonaceous material (30) as shown in FIG. 2 or a sheet-like carbonaceous material (40) as shown in FIG. 3 can be used. When a powdered carbonaceous material (30) is used, it can be disposed by being applied to the joining surfaces (10a and / or 20a). When a sheet-like carbonaceous material (40) is used, it can be disposed directly on the joining surfaces (10a and / or 20a).

[0014] <Regarding the Overlaying Process (Step S110)> In the manufacturing method of the steel joint body 1 according to the first and second embodiments, in the overlapping step (step S110), the steel members (10, 20) to be joined are overlapped by simply pressing the joining surfaces (10a and 20a) against each other via the carbonaceous material (30 or 40) arranged in the "arrangement step (step S100)", as shown in Figures 2(b) and 3(b). In this overlapping step (step S110), when the joining surfaces (10a and 20a) of the steel materials (10, 20) are overlapped by simply pressing them together, it is preferable to fix the joining surfaces together by their own weight. In the overlapping step of this embodiment, by fixing the joining surfaces (10a and 20a) of the steel materials (10, 20) together by their own weight in this way, it is possible to increase the joining strength between the steel materials (10 and 20).

[0015] Furthermore, in this overlapping step (step S110), when the joining surfaces (10a and 20a) of the steel materials (10, 20) are overlapped by simply pressing them together, it is preferable to apply a load (pressing load) to fix the joining surfaces together. In the overlapping step of this embodiment, by applying a load to fix the joining surfaces (10a and 20a) of the steel materials (10, 20) together in this way, it is possible to increase the joining strength between the steel materials (10 and 20). Furthermore, when applying such a pressing load, it is preferable because it allows the joining surfaces (10a and 20a) of the steel materials (10, 20) to be overlapped together in a horizontal direction (not shown) rather than in a vertical direction as shown in FIGS. 2 and 3.

[0016] Furthermore, in the overlapping step of this embodiment, it is preferable to adjust the inclination of the joining surfaces of the steel materials to be joined when they are pressed together. By adjusting the inclination of the joining surfaces in this manner, the joining strength between the steel materials can be more effectively improved.

[0017] <About the heating step (step S120)> In the manufacturing method of the joined steel product 1 according to the first and second embodiments, in the heating step (step S120), the steel materials (10, 20) whose joining surfaces (10a and 20a) have been overlapped in the "overlapping step (step S110)" are heated while fixing the joining surfaces together by simply pressing them together under their own weight, as shown in Figures 2(c) and 3(c). Here, the maximum temperature to which the steel materials (10, 20) are heated is set within a range in which a liquid phase L is generated at the joining surface 50 (see Figure 4, described later). Furthermore, the heating is preferably performed while applying a load (pressing load) in order to fix the joining surfaces (10a and 20a) together by simply pressing the overlapping steel materials (10, 20) together. In this way, by applying a load while heating the steel materials (10, 20) whose joining surfaces (10a and 20a) are overlapped to fix them together, it is possible to increase the joining strength between the steel materials (10 and 20). Furthermore, when applying such a pressing load, it is possible to heat the joining surfaces (10a and 20a) of the steel materials (10, 20) in a horizontal direction (not shown) instead of in a vertical direction as shown in Figures 2 and 3, which is preferable.

[0018] As described above, in the placement process (step S100), the carbonaceous material is placed with its mass adjusted so that a liquid phase L is generated at the joining surface 50 at the maximum temperature reached during heating in the heating process (step S120). Therefore, since such a carbonaceous material is disposed on the joining surface, the carbon concentration of the joining surface 50 increases. As a result, the tensile strength and bending strength of the joining surface 50 increase, and the joining strength of the joining surface 50, i.e., the joining strength between the steel materials, can be effectively increased.

[0019] In order to adjust and arrange the mass of the carbonaceous material so that a liquid phase L (see FIG. 4) is generated at the joining surface 50 at the maximum temperature reached when the carbonaceous material is heated, for example, a steel material (10, 20) having the same shape and carbon concentration as the steel joined body to be a product is used, and the mass of the carbonaceous material to be arranged at the joining surface 50 is changed, and a heating joining test is performed on each of them at the same maximum temperature reached, and the joining rate, which will be described later, is evaluated, thereby making it possible to determine the appropriate mass of the carbonaceous material at which a liquid phase L is generated.

[0020] The carbonaceous material is preferably disposed so that a liquid phase is generated over the entire joining surface at the maximum temperature. By disposing the carbonaceous material so that a liquid phase is generated over the entire joining surface, joining can be achieved over the entire joining surface, thereby more effectively improving the joining strength between the steel materials.

[0021] The heating means in the heating step (step S120) is not particularly limited as long as it does not impair the effects of the present invention, and various heating methods can be used, such as high-frequency induction heating (100) using a conventionally known heating coil 60, or a heating furnace other than high-frequency induction heating, laser heating, etc. The atmosphere during heating in the heating step (step S120) is not particularly limited. The atmosphere includes, for example, an oxidizing atmosphere (oxygen, air, etc.) and a non-oxidizing atmosphere (nitrogen, argon, etc.).

[0022] The heating step (step S120) of the present invention is preferably performed by high-frequency induction heating 100. The high-frequency induction heating 100 can accurately and rapidly heat up to a desired temperature, and therefore can produce the above-mentioned joined steel member 1.

[0023] Then, in the heating process (step S120) of the present invention, when the liquid phase L generated at the joining surface 50 of the steel materials (10, 20) disappears, the steel materials (10 and 20) are joined together (Figures 2(d) and 3(d)).

[0024] <Regarding the Cooling Step (Step S130)> The cooling step (step S130) after the heating step (step S120) of the present invention is not particularly limited, and conventionally known methods such as natural cooling, gas cooling, and spray cooling using a quenching coolant such as a polymer can be used.

[0025] Each step of the present invention has been described above. Next, a specific example of the mechanism by which the joining strength of steel materials is increased by performing each step will be described with reference to the drawings. Fig. 4 is a phase diagram of an iron-cementite system for explaining the effects of the present invention. Fig. 5 is a conceptual diagram for explaining the effects of the present invention, specifically showing the reaction occurring at the joining interface. A method for producing the joined steel product 1 according to this embodiment involves placing the carbon powder (carbonaceous material) on at least one joining surface of the steel materials to be joined, the mass of which is adjusted so that a liquid phase L is generated at the maximum temperature reached during heating, and then heating the steel materials, with the joining surfaces of the steel materials to be joined via the carbonaceous material, in a predetermined atmosphere (for example, in the air), at a predetermined maximum temperature reached (see Fig. 3(a)).

[0026] From FIG. 4, the temperature at which the liquid phase L is generated at the joining interface between the steel material and the carbonaceous material is 1150°C or higher and 1500°C or lower. Specifically, when the maximum temperature near the joining surface of the steel materials reaches, for example, 1250°C, a liquid phase L with a carbon concentration of 3.5 mass% is generated at the interface between the steel materials and the carbonaceous material (see the area indicated by square in Figure 4). This liquid phase L increases until the carbonaceous material disappears (see Figure 5(b)).

[0027] At 1250°C, the carbon concentration at the interface between the "austenite γ" region and the "austenite γ + liquid L" region (see the circle in Figure 4) is 1.6 mass%. Therefore, at 1250°C, most of the steel is a single-phase austenite γ. Incidentally, when the carbon content exceeds 1.6 mass%, two phases, austenite γ and liquid L, coexist. Carbon diffusion is extremely fast at 1250°C, and when maintained at this temperature, carbon diffuses rapidly from the joint surface into the internal austenite γ. As a result, at the interface between the liquid L and austenite γ, the austenite γ side steals carbon from the liquid L side in an attempt to maintain its carbon concentration at 1.6 mass%, and the liquid L side also tries to maintain its carbon concentration at 3.5 mass%, resulting in a decrease in the amount of liquid L (see Figure 5(c)). The liquid phase L eventually disappears and the joining of the steel materials is completed (see Figure 5(d)).

[0028] The effects of the present invention in the heating step (step S120) of the present invention have been explained using a maximum temperature of 1250°C. However, based on the phase diagram of the iron-cementite system in FIG. 4 and the examples described later, it is believed that the above-mentioned effects of the present invention can be obtained in the temperature range in which the "austenite phase γ" region and the "austenite phase γ + liquid phase L" region are formed (maximum temperature of 1150°C or higher and 1500°C or lower). As described above, the mass of the carbonaceous material is adjusted so that liquid phase L is generated on the joining surface, and the carbonaceous material is arranged, and the steel materials with the joining surfaces overlapped are heated at a maximum temperature of 1150°C or higher and 1500°C or lower, thereby enabling the joining to be performed. Therefore, the maximum temperature is preferably 1150°C or higher and 1500°C or lower. This makes it possible to more effectively improve the joining strength between the steel materials.

[0029] The maximum temperature is more preferably 1150° C. or higher and 1300° C. or lower. By setting the maximum temperature to 1300° C. or lower, it is possible to efficiently cool a heating element (coil, etc.) for heating the steel materials, and also to suppress deformation of the joined steel material itself due to heat. The maximum temperature in the heating step (S120) can be measured using a platinum-rhodium thermocouple welded to the outer circumferential surface of the steel materials (raw materials) within 2 mm from the joining surface 50 of the steel materials to be joined.

[0030] Next, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples in any way. [Example]

[0031] [Example 1] In Example 1, two S45C steel (medium carbon steel) round bars with a joining surface diameter of φ14.7 mm and an axial length of L47 mm were prepared. Then, the joining surfaces of these S45C steels were overlapped and thermally joined together via a carbonaceous material (PGS graphite sheet manufactured by Panasonic Corporation) measuring 20 mm in length, 20 mm in width, and 16 μm in thickness. The mass (grams) of this graphite sheet was adjusted to a carbon concentration that would result in the formation of a liquid phase L at the joining surface of the steels at the maximum temperature (1250°C) described below. The maximum temperature was measured by welding a platinum-rhodium thermocouple to the outer peripheral surface of the S45C steel material within 2 mm from the joining surface of the steel material.

[0032] In other words, the following steps were performed in order, as shown in the process flow of Fig. 1: a placement step (step S100), a lamination step (step S110), a heating step (step S120), and a cooling step (step S130). In this lamination step, the joining surfaces of the steel materials to be joined were simply pressed against each other via the carbonaceous material at a pressure of 7.0 MPa, i.e., while applying a pressing load. In this heating process, the S45C steel sheets were heated to a maximum temperature of 1250°C in 20 seconds using high-frequency induction heating at a frequency of 10 kHz in an air atmosphere, and then held at this temperature for 20 seconds before being air-cooled to room temperature. This heating in an air atmosphere involved simply pressing the joining surfaces of the S45C steel sheets together with the carbonaceous material between them at a pressure of 7.2 MPa (a load to fix the joining interface), i.e., applying a pressing load to heat and bond the steel sheets. In the cooling process, at least the joining surfaces of the heated steel sheets were allowed to cool naturally. In Example 1, the steel joint obtained through these steps was used as a test specimen.

[0033] The appearance of the joined steel member obtained in Example 1 was observed with a camera. Fig. 6 shows a photograph of the appearance of the joined steel member obtained in Example 1. In addition, the joining rate of the joined steel member obtained in Example 1 was confirmed. Here, the joining rate was confirmed by observing the joining interface of the joined steel member obtained in an unetched state with an optical microscope. Fig. 7 shows a diagram illustrating the observation position when confirming the joining rate and a method for calculating the joining rate. As shown in Fig. 7(a), the observation position for checking the joining rate was the state where the steel joint was cut at an L cross section parallel to the central axis α, as shown in Fig. 7(a), and the joining rate was calculated as "(d-ΣLv) / d×100" using the values ​​where d is the diameter of the steel joint and Lv is the length of the void, as shown in Fig. 7(b). Table 1 below shows the results of checking the joining rate together with other results.

[0034] Furthermore, in this Example 1, the metallographic structure of the resulting joined steel member was confirmed at the joint interface. Here, the metallographic structure was confirmed by observing, with a camera, predetermined locations at the joint interface of the surface of the heat-joined joined steel member cut at an L-section as shown in FIG. 7(a) without etching. FIG. 8 shows a diagram explaining the locations where the metallographic structure was confirmed and a metallographic photograph of the locations. The joined steel member shown in FIG. 8(a) is in a state where the joined steel member is cut at the joint interface as shown in FIG. 7(a), and the metallographic structure was confirmed at locations A, B, and C on the cross section of the joint interface. FIG. 8(b) shows the metallographic photographs of the locations A, B, and C together with other metallographic photographs.

[0035] [Example 2] In Example 2, the obtained steel welded bodies were examined for the "joining rate" and the "metal structure at the joint interface" in the same manner as in Example 1. The same methods as in Example 1 were used to examine these properties, and therefore, a description of these methods will be omitted.

[0036] In Example 2, similarly to Example 1, the arrangement step (step S100), overlapping step (step S110), and heating step (step S120) were performed in this order, as shown in the process flow of Figure 1. The conditions in Example 2 that differ from those in Example 1 are that the heating in this heating step was performed in a nitrogen gas atmosphere. In this heating in the nitrogen gas atmosphere, the flow rate of the nitrogen gas was set to 100 L / min, and the joining surfaces of the S45C steel materials, with the carbonaceous material interposed between them, were pressed together at a pressure of 7.2 MPa (load for fixing the joining interface) to perform thermal joining.

[0037] Table 1 below shows the results of confirming the joining rate, along with the results of Example 1. Fig. 8 shows a diagram for explaining the location where the metal structure was confirmed, and a metal structure photograph of this location, along with the metal structure photograph of Example 1.

[0038] [Table 1]

[0039] (Results and Evaluation) Fig. 6 shows how the round bars are joined together in the joined steel material obtained in Example 1. Table 1 also shows that there is no significant difference in the joining rate of the joined steel material between Example 1 and Example 2. Fig. 8 also shows that there is no significant difference in the metal structure of the joined interface of the joined steel material between Example 1 and Example 2, and that a pearlite phase is formed in both.

[0040] [Examples 3 to 5] Heat bonding was performed using the same process and conditions as in Example 2, except that the maximum temperature was changed to 1150°C (Example 3), 1200°C (Example 4), and 1300°C (Example 5). The steel joined bodies obtained by these heat bonding processes were used as test specimens at each of the maximum temperatures. For each of the resulting steel joined bodies, the "bonding rate" and "metal structure at the bonded interface" were confirmed, as in Example 1. Here, the same methods as in Example 1 were used to confirm these. Therefore, a description of these methods will be omitted.

[0041] Table 2 below shows the results of confirming the joining rate for Examples 3 to 5. Fig. 9 shows a diagram for explaining the location where the metal structure was confirmed, along with photographs of the metal structure of each of Examples 3 to 5 at that location.

[0042] [Table 2]

[0043] (Results and Evaluation) From Table 2, it can be seen that there was no significant difference in the joining rate of the joined steel materials between Examples 3 to 5. Furthermore, there was no significant difference between Examples 3 to 5 and Examples 1 and 2. Furthermore, from Fig. 9, it can be seen that there was no significant difference in the metal structure of the joining interface portion of the joined steel materials between Examples 3 to 5, and between Examples 3 to 5 and Examples 1 and 2, and it was confirmed that a pearlite phase was formed in all cases.

[0044] [Comparative Example 1] In Comparative Example 1, the maximum temperature reached was set to 1100°C, and thermal bonding was performed under the same process flow and conditions as in Example 2. As a result, bonding as shown in Examples 1 to 5 was not observed, and the bonding rate was 0%.

[0045] Comparative Example 2 Heat bonding was performed using the same process flow and conditions as in Example 2, except that the graphite sheet of Example 1 was not used. As a result, no bonding as shown in Examples 1 to 5 was observed, and the bonding rate was 0%. From the above results, it can be seen that the manufacturing method of the joined steel member according to the present invention effectively improves the joining strength between steel members by preferably setting the maximum temperature to 1150°C or more and 1300°C or less. [Industrial Applicability]

[0046] The method for manufacturing a steel member joint according to the present invention can simply and effectively improve the joint strength between steel members, and therefore can be suitably employed in the manufacture of various structures including steel member joints. [Explanation of symbols]

[0047] 1 Steel joint 10 Steel material 10a Joint surface 20 Steel 20a Joint surface 30 Carbonaceous materials 40 Carbonaceous materials 50 Joint interface 100 High frequency induction heating γ austenite phase L liquid phase S100 placement process S110 Layering process S120 Heating process S130 Cooling process

Claims

1. A method for manufacturing a steel joint in which a plurality of steel materials that are medium carbon steel are joined together, comprising: a disposing step of disposing a carbonaceous material on at least one of the joining surfaces of the steel materials to be joined; a laminating step in which the steel materials to be joined are laminated by simply pressing the joining surfaces of the steel materials to be joined against each other via the carbonaceous material, using their own weight or while applying a pressing load; a heating step of heating the overlapping steel materials whose joining surfaces are joined together by high-frequency induction heating using their own weight or while applying a pressing load, to a maximum temperature at which a liquid phase of the carbonaceous material is generated; a cooling step of cooling the heated steel material by natural cooling, a pearlite phase is formed at a joining interface portion of the joined steel material, The method for manufacturing a joined steel member, wherein the maximum temperature reached is 1150°C or higher and 1500°C or lower.

2. The method for manufacturing a steel member assembly according to claim 1 , wherein the overlapping step adjusts an inclination of the joining surfaces of the steel members to be joined when the steel members are pressed against each other.

3. 2. The method for manufacturing a joined steel member according to claim 1, wherein the carbonaceous material is disposed so that a liquid phase is generated at the joining surface at the maximum temperature.

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