Root-cleaning-free welding process for T-type fillet welds

Abstract

The present application relates to the field of welding technology, and in particular to a T-type fillet weld root-cleaning-free welding process, comprising: S1, completing the pretreatment and blanking of steel, and opening symmetrical double-sided grooves on both sides of the web of the T-type weld; S2, grinding and removing rust from the welded area and adjacent areas of the groove to expose the metallic luster of the parent material; S3, forming a T-type fillet weld, positioning welding the weldment, and ensuring that the weld is located at the horizontal fillet weld position; S4, using two welding guns on both sides of the weld, and using gas shielded welding to complete the base welding, with the two welding guns one in front and one behind; S5, using two welding guns on both sides of the weld, and using gas shielded welding to complete multi-layer and multi-pass filling and cover welding. It can be seen that through the above process steps, gas shielded welding double-sided root-cleaning-free welding can be achieved for the T-type fillet weld, and by adjusting the welding parameters, the welding process can adapt to a wider range of gap variation tolerances, so that the root-cleaning-free process can be effectively promoted.

Description

Technical Field

[0001] The present application relates to the field of welding technology, and in particular to a T-shaped fillet weld root cleaning-free welding process. Background Art

[0002] The existing T-type penetration fillet weld root cleaning-free process has very strict assembly requirements, that is, it can only adapt to a small range of root gap changes of 0 to 1 mm, and the weld quality is difficult to guarantee. Therefore, the process has a very narrow application range and can mostly only be used for welding regular H-shaped steel and other workpieces. The root cleaning-free process cannot be effectively promoted. Summary of the Invention

[0003] The purpose of this application is to provide a T-type fillet weld root cleaning-free welding process, which to a certain extent solves the technical problems in the prior art of the T-type full penetration fillet weld root cleaning-free process, such as strict assembly requirements, maximum adaptability to root gap changes in a small range of 0 to 1 mm, difficulty in ensuring weld quality, harsh process requirements, and poor adaptability to workpiece types.

[0004] The present application provides a T-type fillet weld root cleaning-free welding process, comprising the following steps:

[0005] S1. Complete the pretreatment and blanking of steel materials, and open symmetrical double-sided grooves on both sides of the web of the T-weld;

[0006] S2. Grind the welding area and adjacent areas of the groove to remove impurities that affect welding quality and expose the metallic luster of the base material;

[0007] S3. Complete the assembly between the web and the bottom plate, form a T-shaped fillet weld, and perform positioning welding on the weldment to ensure that the weld is located at the horizontal fillet welding position;

[0008] S4. Use two welding guns on each side of the weld to complete the bottom pass welding using gas shielded welding. The welding starting point and welding end point of the two welding guns are the same. During the bottom pass welding process, the two welding guns are set one in front and one behind. The welding gun closer to the welding end point is the front welding gun, and the welding gun closer to the welding starting point is the rear welding gun. During the bottom pass welding process, the arc in the form of jet transfer is used;

[0009] S5. Two welding guns are used on both sides of the weld to complete multi-layer and multi-pass filling and capping welding by gas shielded welding, and pulse droplet transfer arc is used in both the filling and capping welding processes.

[0010] In the above technical solution, further, when the web and the bottom plate are assembled, a K-shaped groove with a blunt edge is formed between the two.

[0011] In any of the above technical solutions, further, in step S1, flame cutting is used to process the grooves on both sides, and the angle of the single-sided groove is 45-50°, and the thickness of the blunt edge of the K-shaped groove is 0-2 mm.

[0012] In any of the above technical solutions, further, in step S1, the thickness of the web is 12 to 60 mm.

[0013] In any of the above technical solutions, further, in step S2, the grinding area includes the groove and an area with a distance of 20 to 30 mm therefrom.

[0014] In any of the above technical solutions, further, in step S3, the positioning weld is ground before welding to remove welding defects, and the end is ground to form a transition slope, and the gap between the root assembly of the web and the bottom plate is ensured to be 0 to 3 mm.

[0015] In any of the above technical solutions, further, in step S4 and step S5, the front welding gun achieves uniform penetration and back forming of the root of the groove, and the rear welding gun performs remelting welding on the back root of the front weld.

[0016] In any of the above technical solutions, further, in step S4, the front welding gun adopts a non-swinging mode during the bottom welding process, and the end of the welding wire of the front welding gun points to the center line of the weld and deviates 1 to 2 mm outward, the welding current is 320-330A, the welding voltage is 32-35V, the welding speed is 320-330mm / min, and the angle of the front welding gun is 30-35°.

[0017] In any of the above technical solutions, further, in step S4, the rear welding gun adopts a swinging or non-swinging mode during the bottom welding process, and the end of the welding wire of the rear welding gun points to the center line of the weld; when the rear welding gun is not swinging for welding, the welding current is 300-310A, the welding voltage is 29-30V, the welding speed is 320-330mm / min, and the angle of the welding gun is 25-30°; when the rear welding gun is swinging for welding, the welding current is 260-280A, the welding voltage is 28-30V, the welding speed is 320-330mm / min, and the angle of the rear welding gun is 25-30°.

[0018] In any of the above technical solutions, further, in step S5, during the filling and capping welding process, according to the plate thickness and the form of the groove, the two welding guns are used to perform multi-layer and multi-pass welding in a swinging or non-swinging form, and the welding parameters of the two welding guns are as follows: welding current is 240-280A, welding voltage is 26-30V, welding speed is 320-340mm / min, and the angle of the welding gun is 30-50°.

[0019] In any of the above technical solutions, further, the welding shielding gas is 80% Ar+20% CO2, and the welding material is a solid welding wire with a diameter of 1.2 mm that matches the material of the base material.

[0020] In any of the above technical solutions, further, in step S4 and step S5, an industrial robot is used for welding, and welding parameters are set before welding.

[0021] In any of the above technical solutions, further, in step S4, the front-to-back distance between the two welding guns is L, and L≥30mm, the two welding guns can weld simultaneously or the welding operation of the rear welding gun can be performed after the front welding gun completes the front base.

[0022] In any of the above technical solutions, further, the following step is included between step S3 and step S4: preheating the base material by flame heating or electromagnetic induction heating.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] The T-type fillet weld root-cleaning-free welding process provided by the present application adjusts the position and angle of the front and rear welding guns, and uses gas shielded welding to directly perform double-sided bottoming and double-sided filling and covering. In the bottoming welding process, the arcs of the two welding guns adopt a jet transition form, and the arc is strong and powerful. Therefore, the bottoming can achieve effective penetration. In the filling and covering welding process, the arcs of the two welding guns adopt a pulse droplet transition form, which has high welding efficiency, small spatter, and beautiful forming. The peak current of the pulse arc can help to effectively melt the interlayer oxide scale, avoiding the interlayer grinding and cleaning work, and the strong arc force can bring out impurities at the root of the bottom weld, ensuring the quality of the bottom weld. It can be seen that the present method can achieve root-cleaning-free full penetration welding, and by adjusting the position and angle of the front and rear welding guns, gas shielded welding is used to directly perform double-sided bottoming and double-sided filling and covering, which can adapt to a larger range of gap variation tolerances, such as 0-3mm, and there is no need to adjust the welding parameters during the welding process, the operation difficulty is reduced, and the root cleaning process can be effectively promoted. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the specific implementation methods of the present application or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the specific implementation methods or the description of the prior art. Obviously, the drawings described below are some implementation methods of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0026] Figure 1 A schematic diagram of a groove provided in an embodiment of the present application;

[0027] Figure 2 A schematic diagram of a polishing area provided in an embodiment of the present application;

[0028] Figure 3 Another schematic diagram of the polishing area provided in an embodiment of the present application;

[0029] Figure 4 A schematic diagram of the welding position provided in an embodiment of the present application;

[0030] Figure 5 A schematic diagram of the distance between two welding guns provided in an embodiment of the present application;

[0031] Figure 6 A schematic diagram of the positions of two welding guns provided in an embodiment of the present application.

[0032] Reference numerals:

[0033] 1-web, 2-bottom plate, 3-single-side groove, 4-grinding area, 5-weld centerline, 6-front welding gun, 7-rear welding gun. DETAILED DESCRIPTION

[0034] The technical solution of the present application will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments.

[0035] The components of the embodiments of the present application generally described and shown in the drawings herein may be arranged and designed in a variety of different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed application, but merely represents selected embodiments of the present application.

[0036] Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative work shall fall within the scope of protection of this application.

[0037] In the description of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate the description of this application and simplify the description. They do not indicate or imply that the devices or components referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0038] In the description of this application, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integral connections; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; and internal connections between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on the specific circumstances.

[0039] Refer to the following Figures 1 to 6 A root cleaning-free welding process for T-type fillet welds according to some embodiments of the present application is described.

[0040] See also Figures 1 to 6 As shown, an embodiment of the present application provides a T-type fillet weld root cleaning-free welding process, comprising the following steps:

[0041] S1. Complete the pretreatment and blanking of steel materials, and open symmetrical double-sided grooves on both sides of the web 1 of the T-weld;

[0042] S2. Grind the welding area and adjacent areas of the groove to remove impurities that affect welding quality and expose the parent material;

[0043] S3. Complete the assembly, positioning welding and grinding between the web 1 and the bottom plate 2, and place the bottom plate 2 horizontally and the web 1 vertically to ensure that the weld is located at the horizontal fillet welding position;

[0044] S4. Two welding guns are used on both sides of the weld to complete the bottom welding by gas shielded welding. The welding starting point and welding end point of the two welding guns are the same. In the bottom welding process, the two welding guns are arranged one in front and one behind. The welding gun closer to the welding end point is the front welding gun 6, and the welding gun closer to the welding starting point is the rear welding gun 7. In the bottom welding process, the arc in the form of jet transfer is used;

[0045] S5. Two welding guns are used on both sides of the weld, and gas shielded welding is used to complete multi-layer and multi-pass filling and capping welding. In the filling and capping welding processes, pulse droplet transfer arc is used.

[0046] According to the above description, the T-type fillet weld root-cleaning-free welding process provided by the present application adjusts the position and angle of the front and rear welding guns, and uses gas shielded welding to directly perform double-sided bottoming and double-sided filling and covering. In the bottom welding process, the arcs of the two welding guns adopt a jet transition form, and the arc is strong and powerful. Therefore, the bottoming can achieve effective penetration. In the filling and covering welding process, the arcs of the two welding guns adopt a pulse droplet transition form, with high welding efficiency, small spatter, and beautiful forming. The peak current of the pulse arc can help to effectively melt the interlayer oxide scale, avoiding the interlayer grinding and cleaning work, and the strong arc force can bring out impurities at the root of the bottom weld, ensuring the quality of the bottom weld. It can be seen that the present method can achieve root-cleaning-free full penetration welding, and by adjusting the position and angle of the front and rear welding guns, gas shielded welding is used to directly perform double-sided bottoming and double-sided filling and covering, which can adapt to a wider range of gap variation tolerances, so that the root cleaning process can be effectively promoted.

[0047] In this embodiment, preferably, Figure 1 As shown, in step S1, when the web 1 and the bottom plate 2 are assembled, a K-shaped groove with a blunt edge is formed between the two. For the single-sided groove 3, it is a V-shaped groove. It can be seen that a blunt edge is provided between the two V-shaped grooves, which can effectively avoid the burn-through of the base weld and has better tolerance adaptability. At the same time, it can also reduce the heat-affected zone generated during welding, reduce welding stress and deformation, and improve the strength and toughness of the weld, thereby improving the welding quality.

[0048] Further, preferably, Figure 1 As shown, in step S1, flame cutting is used to form grooves on both sides, and the angle of the single-side groove 3 is θ, and θ is 45-50°, the thickness of the blunt edge of the entire K-shaped groove is a, and 0<a<2mm.

[0049] In this embodiment, preferably, Figure 1 As shown, in step S1 , the thickness of the web 1 is t, and 12 mm ≤ t ≤ 60 mm, which meets the strength and use requirements. Of course, it is not limited to this.

[0050] In this embodiment, preferably, Figures 1 to 3 As shown, in step S2, the grinding area 4 includes the single-sided groove 3 and an area with a distance m therefrom of 20 to 30 mm. It should be noted that the distance range includes two end point values.

[0051] According to the above description, impurities such as rust that affect welding quality can be fully removed, the luster of the base metal can be exposed, and the quality of the welding area can be improved.

[0052] In this embodiment, preferably, Figures 1 to 3As shown, in step S3, the gap between the web 1 and the base plate 2 at their base is assembled with a gap of b, where 0 ≤ b ≤ 3 mm. The tack weld is ground before welding to remove any defects, and a transition slope is ground at the end to achieve a smooth transition during welding, thereby ensuring effective penetration of the front root weld at the joint. This process can accommodate gap variations of 0 to 3 mm, extending its practical application range.

[0053] In this embodiment, preferably, Figures 1 to 3 As shown, in this embodiment, preferably, the following step is further included between step S3 and step S4: preheating the base material by flame heating or electromagnetic induction heating.

[0054] According to the above description, it can be seen that preheating measures should be taken for the weld to be welded according to the specific plate thickness and material of the base material. Preheating the base material before welding can reduce the cooling rate of the weld joint, avoid the formation of hardened structure and reduce welding stress and deformation, thereby preventing the generation of welding cracks. Among them, flame heating is simpler to operate and can adapt to various welding conditions, while electromagnetic induction heating has high heating efficiency and a wide heating area, and can be selected according to actual needs.

[0055] In this embodiment, preferably, Figures 1 to 3 As shown, in step S4 and step S5, an industrial robot is used for welding, and the welding power source is CLOOS-A500FG, which has a "rapid weld" or deep penetration welding mode, corresponding to a welding arc in a jet transfer form, and a "speed weld" or rapid pulse welding mode, corresponding to a welding arc in a pulse droplet transfer form. Of course, it is not limited to this, and can also be selected according to actual needs.

[0056] In this embodiment, preferably, Figures 1 to 3 As shown, in step S4 and step S5, the front welding gun 6 is used to achieve uniform penetration and back-forming of the root of the single-sided groove 3, and the rear welding gun 7 is used to remelt the root of the penetration weld.

[0057] In this embodiment, preferably, Figures 1 to 3 As shown, in step S4, the front welding gun 6 adopts a non-swinging mode during the bottom welding process, and the welding wire of the front welding gun 6 points to the center line of the weld, that is, the weld center line 5, which is 1 to 2 mm outward, the welding current is 320-330A, the welding voltage is 32-35V, the welding speed is 320-330mm / min, and the angle of the front welding gun 6 is 30-35°. It should be noted that the aforementioned data range includes two endpoint values.

[0058] According to the above description, when welding the base, the power supply of the front gun selects the "rapid weld" mode, which can adjust a concentrated and stable high-intensity jet arc to achieve stable base penetration and back penetration.

[0059] In this embodiment, preferably, Figures 1 to 3 As shown, in step S4, the rear welding gun 7 is swung or non-swung during the root welding process, and the rear welding gun 7 is directed toward the weld centerline 5. When the rear welding gun 7 is not swung, the welding current is 300-310A, the welding voltage is 29-30V, the welding speed is 320-330mm / min, and the welding gun angle is 25-30°. When the rear welding gun 7 is swung, the welding current is 260-280A, the welding voltage is 28-30V, the welding speed is 320-330mm / min, and the welding gun angle is 25-30°. It should be noted that the aforementioned data range includes both endpoints.

[0060] According to the above description, during root welding, the power supply of the rear gun selects the "rapid weld" mode, which can adjust a concentrated and stable high-intensity jet arc, and can remelt the back of the root of the root weld of the front welding gun 6. The strong arc force can remove impurities at the root and ensure the quality of the root weld.

[0061] In this embodiment, preferably, Figures 1 to 3 As shown, in step S5, during the filling and capping welding process, according to the plate thickness and the form of the groove, the two welding guns perform multi-layer and multi-pass welding in a swinging or non-swinging form, and the welding parameters of the two welding guns are as follows: welding current is 240-280A, welding voltage is 26-30V, welding speed is 320-340mm / min, and the angle of the welding gun is 30-50°. It should be noted that the aforementioned data range includes two endpoint values.

[0062] According to the above description, pulse arc is used for filling and cap welding, and welding is carried out according to the above welding parameters, which can adjust a concentrated and stable pulse droplet transfer arc. The pulse peak current is used to break the oxide scale and the stable swinging effect of the robot welding gun can realize cleaning-free welding between layers, and the welding efficiency is high, the spatter is small, and the shape is beautiful.

[0063] In this embodiment, preferably, Figures 1 to 3 As shown in the figure, the welding shielding gas adopts 80% Ar + 20% CO2. The above argon-rich shielding gas can reduce the critical conditions of the molten droplet jet transition, ensure the stable and reliable molten droplet transition, and reduce the oxidation burning during the welding process to ensure the quality of the base weld; the welding material is a solid welding wire with a diameter of 1.2mm that matches the material of the base material, which can take into account the stability of the welding arc and the welding efficiency.

[0064] In this embodiment, preferably, Figures 1 to 3 As shown, in step S4, the distance between the two welding guns is L, and L≥30mm. The two welding guns can weld at the same time or the welding operation of the rear welding gun 7 can be carried out after the front welding gun 6 completes the front bottoming.

[0065] According to the above description, during the welding process, the two welding guns are staggered by an arc distance, which can effectively avoid arc interference and thus ensure welding quality.

[0066] Combined with the above, it can be seen that the angle between the front welding gun 6 and the horizontal plane is 30-35 degrees, and the center of the arc points to a position 1-2 mm outside the intersection line of the single-sided groove 3 of the web 1 and the bottom plate 2. The angle between the rear welding gun 7 and the horizontal plane is 25-30 degrees, and the center of the arc points to the position of the intersection line of the single-sided groove 3 of the web 1 and the bottom plate 2. This allows this process to adapt to a gap variation tolerance of 0-3 mm. Therefore, the heat of the front gun arc is mainly used to melt the base material of the bottom plate 2, avoiding burn-through of the bottom weld, and increasing the tolerance adaptability. It can be seen that the use of this process can complete the root cleaning-free welding of T-type penetration fillet welds, and at the same time can adapt to root gap changes of 0-3 mm without adjusting the process parameters, which can simultaneously improve the weld quality and increase the welding efficiency by 2.5-3 times.

[0067] In summary, the T-type fillet weld root-cleaning-free welding process provided by the present application has the following advantages: the T-type fillet weld root-cleaning-free welding process provided by the present application adjusts the position and angle of the front and rear welding guns, and uses gas shielded welding to directly perform double-sided base welding and double-sided filling and covering. The angle setting of the front and rear welding guns can adapt to the gap variation tolerance of 0-3mm, and cooperate with the blunt edge between the grooves to effectively avoid burning through the base weld.

[0068] The above-mentioned effective effects are specifically illustrated by detailed experiments:

[0069]

[0070]

[0071] It can be seen that the use of this process for T-type penetration fillet welding can improve the welding efficiency by 2.5-3 times.

[0072] In addition, here is a set of data, for example: the time comparison between the web 1 and the bottom plate 2 welded by this process and the ordinary manual welding process:

[0073]

[0074] It can be seen that the process of using this process for T-type penetration fillet welding takes less time than the ordinary process and can improve the welding efficiency by at least 2.5-3 times.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some or all of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A T-type fillet weld root cleaning-free welding process, characterized in that: The steps include: S1. Complete the pretreatment and blanking of steel materials, and open symmetrical double-sided grooves on both sides of the web of the T-weld; S2. Grind the welding area and adjacent areas of the groove to remove impurities that affect welding quality and expose the metallic luster of the base material; S3. Complete the assembly between the web and the bottom plate, form a T-shaped fillet weld, and perform positioning welding on the weldment to ensure that the weld is located at the horizontal fillet welding position; S4. Use two welding guns on each side of the weld to complete the bottom pass welding using gas shielded welding. The welding starting point and welding end point of the two welding guns are the same. During the bottom pass welding process, the two welding guns are set one in front and one behind. The welding gun closer to the welding end point is the front welding gun, and the welding gun closer to the welding starting point is the rear welding gun. During the bottom pass welding process, the arc in the form of jet transfer is used; S5. Using two welding guns on both sides of the weld, gas shielded welding is used to complete multi-layer and multi-pass filling and capping welding, and pulse droplet transfer arc is used in both the filling and capping welding processes; In step S1, the thickness of the web is 12 to 60 mm; In step S3, the tack weld is ground before welding to remove welding defects, and the end is ground to form a transition slope, and the gap between the root of the web and the bottom plate is ensured to be 0-3 mm; In step S4, the front welding gun is not swung during the bottom welding process, and the end of the welding wire of the front welding gun is directed to the center line of the weld and deviated 1-2 mm outward, the welding current is 320-330 A, the welding voltage is 32-35 V, the welding speed is 320-330 mm / min, and the angle of the front welding gun is 30-35°; In step S4, the rear welding gun adopts a swinging or non-swinging mode during the bottom welding process, and the end of the welding wire of the rear welding gun points to the center line of the weld; when the rear welding gun is not swinging welding, the welding current is 300-310A, the welding voltage is 29-30V, the welding speed is 320-330mm / min, and the angle of the welding gun is 25-30°; when the rear welding gun is swinging welding, the welding current is 260-280A, the welding voltage is 28-30V, the welding speed is 320-330mm / min, and the angle of the rear welding gun is 25-30°.

2. The T-type fillet weld root cleaning-free welding process according to claim 1 is characterized in that: When the web plate and the bottom plate are assembled, a K-shaped groove with a blunt edge is formed between the web plate and the bottom plate.

3. The T-type fillet weld root cleaning-free welding process according to claim 2 is characterized in that: In step S1, the bevels on both sides are processed by flame cutting, and the angle of the single-side bevel is 45-50 degrees, and the thickness of the blunt edge of the K-shaped bevel is 0-2 mm.

4. The T-type fillet weld root cleaning-free welding process according to claim 1, characterized in that: In step S2, the grinding area includes the groove and an area with a distance of 20 to 30 mm therefrom.

5. The T-type fillet weld root cleaning-free welding process according to claim 1, characterized in that: In step S4 and step S5, the front welding gun realizes uniform penetration and back-forming of the root of the groove, and the rear welding gun performs remelting welding on the back-root of the front weld.

6. The T-type fillet weld root cleaning-free welding process according to claim 1, characterized in that: In step S5, during the filling and capping welding process, the two welding guns are used in an oscillating or non-oscillating manner to perform multi-layer and multi-pass welding according to the plate thickness and the form of the groove. The welding parameters of the two welding guns are as follows: welding current of 240-280A, welding voltage of 26-30V, welding speed of 320-340mm / min, and the angle of the welding gun is 30-50°; and / or The welding shielding gas is 80% Ar+20% CO2, and the welding material is a solid welding wire with a diameter of 1.2 mm that matches the material of the base material.

7. The T-type fillet weld root cleaning-free welding process according to claim 1, characterized in that: In step S4 and step S5, an industrial robot is used for welding, and welding parameters are set before welding.

8. The T-type fillet weld root cleaning-free welding process according to any one of claims 1 to 7, characterized in that: In step S4, the front-to-back distance between the two welding guns is L, and L≥30mm; and / or The following step is also included between step S3 and step S4: preheating the base material by flame heating or electromagnetic induction heating.

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