AU2020277590A1

AU2020277590A1 - Dual laser-hybrid process

Info

Publication number: AU2020277590A1
Application number: AU2020277590A
Authority: AU
Inventors: Heiko DRAWE; Roman LITSCHMANN
Original assignee: Stahlkontor & Co KG GmbH
Current assignee: Stahlkontor & Co KG GmbH
Priority date: 2019-05-22
Filing date: 2020-04-27
Publication date: 2021-03-18
Also published as: EP3817887A1; DE102019113697A1; AU2022201942A1; WO2020233945A1

Abstract

The invention relates to a method for non-detachably joining at least two structural components made of steel with a tensile yield strength of at least 1000 MPa and a hardness of at least 430 HBW by welding and producing at least one joint, characterized in that the area around the joint is inductively heated to 300°C to 750°C.

Description

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Stahlkontor GmbH & Co. KG, Hagen

DESCRIPTION

Dual Laser-Hybrid Process

The invention relates to a welding process, such as e.g. a laser beam MIG hybrid process, for the non-detachable joining of two or more components made of high strength steel by means of inductive high-temperature heat treatment of the areas around the joint.

For the non-detachable joining of two components made of high-strength steel generally laser beam welding methods, metal inert gas welding methods (MIG welding methods) or a combination of these methods are used, such as laser-beam MIG hybrid welding methods. These methods are sufficiently well known to a person skilled in the art.

With high-strength steels, such as e.g. fine-grained steels, when using these methods there is a rapid cooling of the joint and the surrounding areas (referred to in the following as the "heat-affected zone" (HAZ). In this way very hard structural constituents can be formed, wherein the material of the two components to be joined becomes brittle in this area. The more rapidly this area directly adjacent to the joint cools, the greater the hardening in the HAZ. The hardened and brittle joint areas formed in this way have worse mechanical-technological properties than the high strength steel used and under extreme stress are prone to cracking.

To avoid this type of unwanted post-hardening it is known to minimise the temperature gradients of the HAZ relative to the unprocessed areas of the parts to be joined and/or to slow down the cooling. Thus from Lahdo, Rabi (et al.): Laser beam MIG hybrid

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welding of fine-grade steels for use in steel construction (in "Stahlbau", volume 84, 2015, issue 12, pages 1016 to 1022 ISSN 0038-9145) a laser beam MIG hybrid welding process is known for joining high-strength steel, in which the area around a joint is heated inductively to a temperature of 190 degrees Celsius.

Similarly, Bach, Fr.-W. et al. in "Improved forming behaviour by means of serial inductive post-heating of laser beam welding seams" in: "Materialwissenschaft und Werkstofftechnik", volume 33, 2002, issue 7, pages 410 to 414, ISSN 0049-8688) and KOgler et al. in "Laser MIG hybrid welding of steel sheets, 31.12.2015" respectively describe laser beam welding process with inductive post-heating of welding seams for light-weight parts in motor vehicles.

Furthermore, W02018033310A1 describes a laser beam MIG hybrid welding method for high-strength steel components with an inductive heat treatment.

These publications propose heat treatment at a maximum of 3000 C, which is substantiated on the one hand by rapid and inexpensive heating and on the other hand by avoiding areas prone to cracking from the formation of very hard structural constituents at high temperatures. In addition, steel manufacturers always specify a temperature of 200 0C as an upper limit for heating, which must be adhered to, in order according to current knowledge to avoid a weakening of the joint, in particular caused by heating performed after the welding.

The disadvantage of these method is the narrow temperature window for the heat treatment, which in view of the high welding temperatures requires careful process control. This is particularly difficult in the case of thin components and/or where the mechanical strength requirements are high.

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The objective of the present invention was therefore to establish a process for welding components made of high-strength steel by heat treatment that is easier to control.

Surprisingly, it has been found that the heat treatment of such a welding process can also be performed at much higher temperatures than 3000 C, without forming areas that are prone to cracking or areas with undesirably hard structural constituents.

The subject-matter of the present invention is therefore a method for non-detachably joining at least two components made of steel with a tensile yield strength of at least 1000 MPa and a hardness of at least 420 HBW by welding forming at least one joint, wherein the area around the joint is heated inductively to 3000 C to 750°C.

Steels with a tensile yield strength of at least 1000 MPa and a hardness of at least 420 HBW are also referred to in the following as "high-strength steels".

Preferably, the process of joining according to the invention is performed by the laser beam MIG hybrid welding process.

By means of the process according to the invention, the areas around the joint may be prewarmed (but do not have to be), in particular around the joining seam, so that the joint area does not cool down too rapidly after performing the welding process. In this way the formation of hardened, brittle areas which are thus prone to cracking is effectively avoided.

The inductive heating can be performed in a narrow area around the joint. In principle the components to be joined together could be heated as a whole, e.g. if the components have complex geometries, in particular the joining seams. Preferably however, the area of inductive heating extends (in width) 5 to 25 mm around the joint/joining seam. Particularly preferably, this is an area of 5 mm to 15 mm around the

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Stahlkontor GmbH & Co. KG, Hagen

joint/joining seam. The narrower this area the better the structure after joining, as the joining seam (the joining area) basically weakens the starting material. If the joining area is narrower the starting material is weakened less by the joining and heating process performed.

It should be ensured here that the heating is not performed at temperatures which would cause structural change to the high-strength fine-grained steels. The heating is thus not only dependent on the high-strength steel used, but also on the geometric extension (area) of the components to be joined and their material strength (thickness).

Preferably, the inductive heating is performed in a range of 350 0 C to 7500 C, in particular 400 0C to 750 0C or further in particular 450 0C to 7500 C. To avoid hardened or brittle areas, which are thus prone to cracking, it is preferred that the inductively heated area reaches a temperature of at least 4000 C for at least 5 seconds.

In this context heating means that the two components are joined in a non-detachable manner (by a suitable, aforementioned welding process). Before this joining process however it is possible but not necessary to heat (preferably inductively) up to the said temperatures, which can also be referred to as preheating, and then perform the welding process. After the welding process is complete cooling is performed. This is performed approximately to ambient temperature (dependent on the environmental conditions, for example in a range of 100 C to 300 C) and can be performed passively (simply by waiting) or actively (by supplying suitable coolant, such as for example air). Afterwards the described heating is performed, which can also be referred to as post heating.

It has become clear that to achieve the desired properties of the subsequently finished welding assemblies, which consist of the at least two non-detachably joined components, the high-strength steel from which the components are made has a

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tensile yield strength of at least 1000 MPa and a hardness of at least 420 HBW. This means that according to the invention high-strength steels are used with a tensile yield strength of at least 1000 MPa and a hardness of at least 420 HBW.

In one development of the invention the heating is performed specifically and/or after performing the welding process. In all three cases it is ensured that the components to be joined have such energy by which the slow cooling effect is achieved after performing the welding. It is a particular advantage and especially important in practice that the heating is performed to the predefined temperature range only after performing the welding process to prevent the high-strength steel from forming very hard structural constituents after the completion of the welding process in the heat affected zone.

In one development of the invention the welding method is performed at a defined speed as a function of the material strength (thickness) of the components to be joined. In this way, it is not only possible in an advantageous manner to perform the welding very rapidly and thus at a much higher welding speed than conventional welding along a joining seam, but also to achieve by means of inductive heating the required mechanical-technological properties of the joined components, which are adjusted in an optimum manner to the material thickness. Thus in an advantageous manner two objectives are achieved and implemented: high welding speed and high loading capacity under extreme stress compared to partly mechanised and/or fully mechanised MAG welding processes.

For inductive heating, preferably an induction coil is used, which in turn is preferably moved at the same speed of the laser-beam before and/or behind the laser beam. In this way, in an advantageous manner, the device for laser beam MIG hybrid welding can be combined with the device for heating (generally induction coil). This means that the area to be heated leads and/or lags behind the welding device, so that in this way the required heating is always performed to avoid the cooling being too rapid after

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performing the welding process. In a simple manner the devices for welding and for heating can thus be coupled to one another.

The heating of the joint also has the advantage that above all longer welding seams can be formed. Previously, it was not possible in known welding processes to weld longer welding seams, which extend in particular over the whole length of the components to be joined together, in one run. Welding always had to be carried out in sections (e.g. pilgering process), in order to minimise the unwanted warping of the components. The advantages are thus in summary in the optimal adjustment of the mechanical-technological properties of the joining area, the business-economic view in terms of the higher welding speed and the virtually warp-free welding of large-format components.

It is particularly advantageous to use the aforementioned laser beam MIG hybrid welding process with inductive heating for military use, as hereby components made from a high-strength security steel are used and when in use are subject to the highest stresses, in particular from firing or explosions. The components to be joined are used in stationary or mobile devices, such as for example armoured vehicles.

Examples of groups of high-strength steels (without restriction) for military use include assemblies with material properties up to TL 2350-0000 quality Z, as well as according to standards: CEN ISO/TR 15608, Tab. 1, Group 3.

In one development of the invention, the high-strength steel has a tensile yield strength of at least 750 MPa and a hardness of at least 300 HBW. This means that high-strength steels with these material properties are used in order to join them in an non detachable manner by means of the said method. Due to the improved, i.e. higher material properties, the protective effects are increased further in this way.

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Stahlkontor GmbH & Co. KG, Hagen

By having a particularly preferred selection of high-strengthen steels to be used the high-strength steel of each component has a tensile yield strength in a range of at least 1000 MPa to a maximum 1750 MPa and a hardness in a range from at least 475 HBW to a maximum of 550 HBW. Due to the use of the high-strength steel with these material properties the inductive heating can be adjusted optimally before and/or after joining to the components that need to be joined. By using steels with the said material properties (tensile yield strength and hardness) welding assemblies with such joined components can be obtained which meet particularly high specifications.

Alternatively, a welding assembly (general device) made of high-strength steels according to the invention can be formed, when the high-strength steel has a tensile yield strength in a range of at least 1100 MPa to a maximum of 1650 MPa and a hardness in a range of at least 420 HBW to a maximum of 530 HBW. Likewise to achieve the required stabilities of devices under fire or exposure to explosives thus an alternative material is available which can be used to form the device.

Preferably, at least one component to be joined has a material thickness of at least 1 millimetre, preferably at least 3 millimetres. In one development of the invention the components to be joined have a material thickness of at least 3 millimetres (three millimetres, 3 mm). By means of this minimum material thickness it is ensured that devices, such as for example vehicle components for civil or military used, have sufficiently large dimensions, if such devices, such as for example vehicles, are exposed to fire or explosions. The material properties of the high-strength steels provided according to the invention from which the components are made, the non detachable joining and the minimum material thickness lead overall to an advantageous total protection of the device, which is sufficient to satisfy even the highest safety requirements.

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Stahlkontor GmbH & Co. KG, Hagen

The tensile yield strength Re is a material property and denotes the tension up to which a material exhibits no permanent plastic deformation under uniaxial and torque-free tensile stress. This is a flow limit. If the value is not reached the material returns elastically after relief to its original form, if it is exceeded however there is a change in form, in a test sample thus an extension. Depending on the material behaviour either the tensile yield strength or the yield strength are used to determine the elasticity limit of a material. The tensile yield strength is simple to determine by means of established and standardised tensile tests and has the highest technical significance. It is specified by units MPa (Megapascal) or N/mm 2 (Newton per square millimetre).

Hardness is the mechanical resistance by which a material resists the mechanical indentation of another body. Different types of hardness are defined according to the type of effect. Thus hardness is not only the resistance to harder bodies, but also to softer and equally hard bodies. It is defined by the unit HB (Brinell hardness) or HBW (Brinell hardness, W stands for the material of the test ball: tungsten carbide hard metal) and is determined according to established measuring methods.

For the conversion of hardness data in unit HB or HBW into unit HV (Vickers hardness) or vice versa there are corresponding conversion tables which have been available for a long time.

Examples

High-strength steels are joined by laser beam MIG hybrid welding, once without (attempt V2) and once with (attempt V3) inductive post-heating according to the invention at a feed rate of 0.9 m/min (metres per minute). Fig. 1 shows a comparison of the material hardnesses as a function of the distance from the joint.

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It has been shown that by means of the heating according to the invention there can be significant reduction of the hardness of the weld metal and HAZ with a levelling of hardness peaks. In addition, there can be a local structure homogenisation, wherein in a tensile test there was no established influence of the tensile yield strength Re and only a slight lowering of Rm and a slight increase in the elongation at break. In a notch impact test there was shown to be no effect from the heat treatment.

Claims

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1. Method for non-detachably joining at least two components made of steel with a tensile yield strength of at least 1000 MPa and a hardness of at least 420 HBW by welding and forming at least one joint, characterised in that the area around the joint is inductively heated to 3000 C to 750°C.

2. Method according to claim 1, characterised in that the joining is performed by laser beam MIG hybrid welding.

3. Method according to claim 1 or 2, characterised in that an area of 5 mm to 25 mm, preferably 5 mm to 15 mm, around the joint is heated inductively.

4. Method according to any of claims 1 to 3, characterised in that the inductively heated region has a temperature of at least 300 0 C for at least 5 seconds.

5. Method according to any of claims 1 to 4, characterised in that the inductive heating is performed before executing the welding process.

6. Method according to any of claims 1 to 4, characterised in that the inductive heating is performed after performing the welding process.

7. Method according to any of claims 1 to 6, characterised in that for the inductive heating an induction coil is used.

8. Method according to any of claims 1 to 7, characterised in that at least one component to be joined has a material thickness of at least 1 millimetre, preferably at least 3 millimetres.