EP1660266A1

EP1660266A1 - A method of attaching a fibrous insulation panel to a metal structure and a stud welding system for this

Info

Publication number: EP1660266A1
Application number: EP04762784A
Authority: EP
Inventors: Lars Erik Hansen; Kurt Munk Pedersen; Jan THERNØE
Original assignee: Rockwool International AS
Current assignee: Rockwool AS
Priority date: 2003-08-27
Filing date: 2004-08-24
Publication date: 2006-05-31
Also published as: WO2005021201A1; NO20061097L

Abstract

The invention concerns a method of attaching at least one fibrous insulation sheet panel onto a metal structure having a surface covering top layer, whereby a number of studs are placed through the sheet material and welded onto the metal structure by using a stud-welding system in which the stud is positioned in a stud-welding gun, said method comprising the steps of penetrating a stud including a hardened steel pin through the sheet material, providing an electrical contact between the stud and the underlying metal structure by penetrating the distal end of the stud pin through the top layer of the metal structure, and then applying a welding current to the stud, retracting the stud providing a gap between the stud and the metal structure and establishing a welding arc, and forcing the stud against the metal structure after a predetermined duration of time.

Description

A METHOD OF ATTACHING A FIBROUS INSULATION PANEL TO A METAL STRUCTURE AND A STUD WELDING SYSTEM FOR THIS

The present invention relates to a method of attaching a sheet panel onto a metal structure by placing a number of studs through the sheet material and welding said studs onto the metal structure by using a stud- welding system, a stud for use by attaching a sheet panel onto a metal structure comprising a head with a protruding pin member and a stud welding gun for welding on studs comprising spring-loaded stud holding means for receiving a stud and pinching this stud through a sheet panel and forcing the stud towards an underlying metal structure.

In steel structures covering panels of fibrous insulation material may often be fitted in order to provide fire, thermal and/or noise insulation, e.g. of steel structures in building constructions, metallic ventilation ducts, steel chimneys/stacks, pipes or metal structures for maritime purposes. This insulation is often required in order to provide an effective fire protection.

In order to fit an insulating layer to a metal structure, typically a steel structure, it is known to weld a number of needle pins onto the steel surface. On these needles, panels of fibrous insulation material are mounted. When the panels are mounted, the needles are pinched through the panel and a locking disc is secured to the ends of the pins, which protrude through the fibrous insulation panels. In order to ensure that the insulation panels are properly secured, the needle pins must be positioned relatively close.

This method of mounting the insulation layer is extremely time-consuming, due to the excessive amount of needle pins, which must be secured to the steel structure. For the personnel fitting the insulation panels, there is a risk of being injured on the protruding needle pins or spikes. Moreover, the panels must be mounted with tremendous precision since they cannot be moved sideways once they have been pressed onto the needles. It is of utmost importance that the panels must be closely fitted to each other. This means that there is a risk that a considerable amount of insulation panels must be ejected and remounted/refitted if they have not been properly mounted in the first attempt. Moreover, this method makes it difficult - if not impossible - to fit a fire insulation onto surfaces in a confined space.

Another method of attaching fire insulation to a steel structure is to use welding studs pinched through an insulation panel, which are fitted to the surface of the structure, and then welded to the structure by use of a welding gun. By this method, it is possible to position the attachments in relation to the assembling joints of the fibrous insulation panels. The mounting of the insulating panels is also easier and the danger of being injured on the needles is eliminated.

The studs are pinched through the insulation panels or layers and into contact with the underlying steel surface at desired positions, i.e. away from the joining lines between the panels. Thereby, the amount of attachment points may be reduced and the mounting is easier and less time-consuming. This method is also applicable to rolls of insulation material.

However, this method is significantly less applicable to non-conducting, corrosion protected surfaces, such as painted top layer, or a corrosion layer on the surface, since the welding must burn away the top layer before a metallic contact between the steel and the stud can be established. The welding attachment may be of a poor quality due to residues in the welding areas, since the worker needs to remove the paint prior to performing the welding operation. Furthermore, the corrosion protective layer cannot be re-established if it is damaged. The method demands a great power from the worker in order to shoot the studs into the panel and they may fall out or become damaged during the penetration due to the high resistance.

Such working procedures are costly, in particularly in maritime applications where up to 80% of the costs for mounting the insulation are labour related costs. In summary, in the known method, any paint or rust scales must initially be removed, e.g. by grinding, before the welding can take place. This results in inaccuracies since many areas are left exposed without any protection against corrosion.

With the known stud welding systems, the studs are often not sufficiently attached due to impurities on the top surface which in turn, results in a poor welding attachment, and the pins drop out and the insulation panels must be re-attached by more studs. This means that the fitting of fibrous insulation is still very labour intensive.

In this light, it is an object of the present invention to provide a method and a stud welding system, which allows for a better attachment of insulating panels onto a metal structure and to ensure a proper metallic contact for the welding process. Moreover, it is an object of the invention to provide a method and a system, which is less labour intensive and thus allows for a more cost effective way of attaching insulating panels.

The object is achieved by a method of attaching at least one fibrous insulation sheet panel onto a metal structure having a surface covering top layer, whereby a number of studs are placed tlirough the sheet material and welded onto the metal structure by using a stud-welding system in which the stud is positioned in a stud-welding gun, said method comprising the steps of penetrating a stud comprising a hardened steel pin through the sheet material, providing an electrical contact between the stud and the underlying metal structure by penetrating the distal end of the stud pin through the top layer of the metal structure, and then applying a welding current to the stud, retracting the stud providing a gap between the stud and the metal structure and establishing a welding arc, and forcing the stud against the metal structure after a predetermined duration of time.

By establishing an electrical contact and accordingly a gap with a welding arc between the distal end of the pin of the stud and the surface of the metal structure, any coating layer will be removed or at least burned off to a sufficient degree such that a secure stud welding process may be performed.

hi order to ensure that the distal end of the pin establishes a good electrical contact, it is found that the stud pin must be of hardened steel or steel alloy. This ensures that the abutment against the top layer of the metal structure is sufficiently penetrated without bending the pin or otherwise damaging the stud pin, e.g. damaging the stud pin by flattening the distal end geometry of the pin as this results in a poor welding contact. In particular, in relation to relative hard top layer surfaces, such as epoxy- based two-component corrosion protective paint, this has proven a particular problem to establish a good electrical contact. By the invention, it is realised that a stud consisting of a pin may be welded onto the structure and a stud head may subsequently be attached to the pin after the fibrous insulation material has been fitted.

According to the invention, the attachment of panels may be carried out in a single operation and with a better precision, since the panels may be placed in position before the attachment points are established. By the hitherto known techniques, the spikes must be welded onto the exposed surface before the panels are pressed into position. Moreover, by the method according to the invention, there are no exposed areas on the steel surface.

This method is particular advantageous for attachment of fibrous insulation to steel constructions in maritime insulation and fire insulation of steel columns and girders. The method is also advantageous in that insulation panels in certain occasions may be attached to confined spaces of a steel structure since a considerably smaller amount of space is required for performing the attachment.

Preferably, the electrical contact is provided by abutting the stud against the surface of the metal structure and repetitively tapping the stud against the surface of the metal structure until a sufficient contact is achieved. Alternatively, or as a supplement, the electrical contact may also be provided by rotating and substantially simultaneously forcing the stud onto the metal structure before the discharge of the welding current. Accordingly, the stud may be provided with top layer penetration means in the form of a drilling head formed in the end of the stud pin.

Throughout the top layer penetration process, the electrical conductivity is preferably measured by measuring means and when the electrical conductivity exceeds a predetermined value the surface coating is sufficiently broken or removed so that a good electrical contact between the stud and the metal structure may be established. The welding current is established and right after, almost simultaneously, the stud is moved back in order to initiate the small gap creating the welding arc.

The gap which is provided is preferably between 0.5 mm to 4 mm, more preferably between 0.75 mm to 2.25 mm. The welding current is applied as, or after, the electrical contact is being provided, and the welding arc is established as the gap is being created. The welding arc forms a melting pool between the stud and the metal construction. The welding current has a magnitude of 50 A to 850 A, preferably 100 A to 500 A, and duration between 0.5 ms to 500 ms, preferably 10 ms to 200 ms.

In order to improve the mechanical properties of the weld metal and the surrounded heat affected zone, the welding current may be preceded by a current sequence, e.g. comprising different current levels, where a first period with a current of approximately 10 A to 100 A, is used for preheating the metal in the welding zone. This preheating current has duration of approximately the same order of magnitude as the welding current. The welding current is applied after the preheating current. If using a sequence of currents, the gap is created as the welding current is initiated.

After the welding arc has created a molten pool, the stud is forced into the pool by releasing the spring-loaded receiving means holding the stud in the stud-welding gun. The time delay before this release and the welding current to be applied are determined according to the type of stud, e.g. stud pin diameter and the material of the metal structure.

By the invention, the panel is a fibrous insulation sheet material. However, it is realised that equivalents to the invention may also be applied in relation to the attachment of other types of panels, such as for instance calcium silicate boards, gypsum boards or cementious boards. The panels are usually stiff, but soft materials may also be used as panels, e.g. rolls, batts, foam panels or other sound or vibration absorption panels.

In the preferred embodiment of the invention, the metal structure is a steel structure. However, the method may also be applied in connection with other metal structures such as e.g. aluminium structures. The method is preferably applied in connection with a coated steel structure, e.g. with a corrosion resistant coating. However, it is realised that the method according to the invention may also be advantageously used for uncoated structures for removing impurities such as rust or scales on the surface at the point of welding. Otherwise, impurities, such as scaling, heat residues etc., must be removed before the welding process, which is very time consuming. The structures may be coated with plastic coatings, paint or any other suitable protective coating.

The invention also relates to a stud welding system comprising a stud for use by attaching a sheet panel onto a metal structure comprising a head, preferably a disc- shaped head, with a protruding pin member, wherein said pin member is provided with top layer removing means at its free end. In the present context, the top layer corresponds to the corrosion protective layer or coating, e.g. paint.

• The stud according to this aspect of the invention is thus adapted for use by a method according to the above-described first aspect of the invention. The top layer removing means may preferably be a pointed end formed in the pin end or other mechanical penetration means on the distal end of the stud pin. Hereby, a metallic contact may be obtained by penetrating the top layer of a metal structure behind the fibrous material.

In an embodiment, the stud pin is made of hardened steel having a content of at least 0.04% Carbon, preferably at least 0.1% Carbon and even more preferably at least 0.2% Carbon. This ensures a steel which may be hardened to a sufficient hardness without compromising the mechanical properties of the heat-affected zone in the pin just above the welding. By the term steel, any kind of steel alloy is meant.

When attempting to penetrate a strong top layer, such as a corrosion protective coating, the distal end of the pin which may be either pointed, wedge-like or pyramid-like shaped, the distal end may be flattened by the repetitive tapping - or by the simultaneous pressing and rotation. This results in a large proportion of the stud are not being properly attached by the welding. This may be avoided or at least substantially reduced by a hardened steel pin. Although, the flattening of the tip is small, even a small flat end may influence the result.

The Vickers hardness (HVoi) of the stud pin is measured by pressing the tip of a diamond pyramid against the surface of the stud pins with a weight of 100 gram in accordance with the international standard "ISO 6507-1 Metallic Materials - Vickers hardness. Part 1 Test Method". Preferably, this hardness is in the range of HVoι=140-500, preferably HV₀₁=160-400 and even more preferably HV₀ι=180- 350."

Preferably, the distal end of the stud pin is provided with an increased Vickers hardness relative to the rest of the pin. It is realised that not only the tip of the pin must be hardened. If the pin behind the distal end is too soft, the flattening may also occur or the distal end of the tip of the stud pin could bend or be destroyed, so it is found that in order to ensure a reliable penetration to the metallic surface, the pin itself must also be hardened. Though the hardness of the tip, i.e. the distal end, ensures the penetration, there is a limit to how hard the stud pin should be, as the stud pin would otherwise be too brittle and this would increase the risk of breaking the stud pin during the tapping and the pressing of the stud against the metal surface. Moreover, the Carbon content will also influence the brittleness of the finished welding. There is a maximum for the carbon content of approximately 0.25 percent. See also "Hάndbog i lysbuesvejsning" (Chapter 6) by Poul Ettrup Petersen, Teknisk Forlag A/S, Copenhagen 1963; and "Metallurgi for ingeniører" (page 405) by Conrad Vogel, Celia Juhl and Ernst Maahn; Akademisk forlag, 6 ed. 1993 (ISBN 87- 5003083-3).

In order to ensure a firm grip on the head of the stud so that the stud does not slide inside the welding gun, the head may be provided with a non-rotary symmetric shape. Alternatively or as a supplement, magnetic holding means may be provided and/or the disk-shaped head may be provided with indexing means, such as one or more indentations on the outer rim of the head. The indentations may also be provided in the interior of the head. However, it is realised that alternatively the stud comprise a subsequently attachable head, which may be attached after the hardened pin is welded onto the structure and the fibrous insulation material is fitted.

In a preferred embodiment, the pin is provided with a zinc deposit adjacent the cutting means at the end of the stud. Hereby, a corrosion resistant coating may be reapplied as the zinc deposit will set as a "sacrificing anode" and thereby protect the distorted area around the pin.

In a preferred embodiment, the pin and/or the underside of the head is provided with an electrically insulating sleeve. This is advantageous if the insulating panels are provided with a top layer of aluminium foil in order to isolate the aluminium foil from the welding process. According to another aspect of the invention, the invention comprises a stud- welding gun for welding studs onto an underlying metal surface for attaching an insulation panel to said metal surface, said stud-welding gun comprising first spring-loaded stud holding means for receiving a stud, which is pinched through a sheet panel, and biasing the stud towards an underlying metal structure, welding current application means for applying a welding current to the stud, wherein said stud receiving means are provided with lifting means for retracting the stud along its pin axis, when the stud is biased into contact with the metal structure for creation of a gap between the pin and the metal structure surface. This stud-welding gun is adapted for use in performing a method according to the first aspect of the invention.

The receiving means are preferably provided with magnetic means for retaining the stud in a predetermined position relative to the metal structure. Hereby, the stud can be placed in a predetermined position in the receiving means of the stud-welding gun and kept in this position before the action of attachment commences.

Moreover, the stud-welding gun is preferably provided with support members, such as support levers, said legs preferably being adjustable, for providing a predetermined distance and pressure during the welding process. The support legs are designed in such a manner, that the distal end of the stud when positioned in the spring-loaded receiving means protrude a certain protrusion distance in front of the legs. When the stud is penetrated through the insulation and abuts the metal surface underneath, the protrusion distance in cooperation with the spring-loaded receiving means ensures that a certain pressure is exercised on the stud during the welding process.

Different number of supporting members has different advantages. Three or more supporting members offer a good adjustment of the stud, always holding it perpendicular to the surface of the metal construction. However, three or more supporting members complicate application of the method in e.g. corners due to limited space. This problem is reduced by only having two legs. With two legs, however, there is a risk of installing the stud not perpendicular to the underlying metal construction surface. With only one supporting member, the application in limited spaces is easier, but also the risk of installing the stud not perpendicular to the underlying metal construction surface is higher. However, one supporting member should be enough for ensuring the correct distance during the welding process.

The invention is described in more detail in the following with reference to examples in the accompanying drawings, in which:

fig. 1 is an illustration of a steel structure with a fibrous insulation mounted thereon, fig. 2 is a principal cross-section view of a steel girder with an insulation fitted around it, fig. 3 is a schematic illustration of the method of attaching a fibrous insulation panel according to the invention, fig. 4 is a perspective drawing of a stud according to the invention, fig. 5 and 5 A are side views of a stud according to two embodiment, fig. 6 is a schematic detailed view of the stud as it engages the steel structure; fig. 7 is a schematic illustration of another use of the invention for assembly of panels of a plurality of layers; fig. 8 is a diagram of the welding current as a function of time during the welding process; and figs. 9 and 10 are detailed views of a stud- welding gun according to the preferred embodiment of the invention.

In figure 1 is shown a steel structure 2 with fire insulation 1, The insulation is typically made up of fibrous insulation panels 1, which are mounted on the steel girder structure side by side covering the entire surface of the steel structure. The fibrous insulation panels 1 are secured to the steel structure by studs 3, which are pinched through the insulation panel and brought in contact with the steel surface underneath, as shown in fig. 2 and welded onto the steel structure 2 by use of a stud welding gun 4 by an electric welding process (see fig. 3).

The welding gun 4 is connected to a capacitor unit or a transformer unit (not shown). The capacitor unit is charged by a main voltage-supplied transformer (not shown). By using a capacitor welding process a sufficient high amount of energy may be applied to the stud in a short time (a fraction of a second) so that the stud is welded onto the metallic surface. The welding gun 4 is pressed against a stud 3, which is pressed through the insulation panel 1, as shown in fig. 3. The stud 3 is accommodated in a receiving head 5 of the welding gun 4. The welding gun 4 is placed over the stud 3 so that the receiving head 5 is fitted over the head 31 of the stud 3 (see fig. 4). By the aid of the receiving means 5, the stud 3 is biased towards the metal structure 2 abutting or repetitively tapping the stud against the metal surface establishing an electrical contact. A welding current is applied and a gap between the stud and the metal surface is formed. This establishes a welding arc creating a molten pool. After a predetermined time the stud is forced into this molten pool and the insulation panel 1 is retained between the metal structure and the head ofthe stud 3.

As shown in figure 4, the stud 3 may consist of a disc-shaped head 31 from which an elongated protruding pin member or stud pin 32 extends. The head 31 may additionally be provided with indentations and the contour of the head 31 is designed to co-operate with the inner shape (non shown) of the receiving means 5 of the welding gun 4 to ensure a firm grip of the receiving means 5 is transferred to the head 31 and thereby the stud 3. As shown in figures 4, 5 and 5 A, at the free end of the stud 3 opposite the head 31, a top layer penetration means 35 is provided, preferably as a cone or pyramid shaped end of the pin 32 (see fig. 4 and 5) or a drilling head 35 as shown in fig. 5 A. Adjacent the penetration means 35, a small deposit 33 of zinc or similar corrosion protective material may be provided or the entire stud pin may be galvanised or provided with a similar coating. The amount of corrosion resistant deposit 33 is such that when the welding current is applied to the stud 3 as the welding process is activated, the deposit 33 distributes over the welding spot and (re-)establish a corrosion resistant coating.

In a preferred embodiment according to the invention, a magnetic grip by the receiving means 5 on the stud may prove sufficient in some circumstances in order to provide the necessary grip of the stud 3 (see figs. 9 and 10).

In fig. 5 and 5A, some embodiments of the stud 3 according to the invention are shown. According to the embodiment in fig. 5, the pin 32 is provided with a sleeve 36 of plastic or similar non-conductive material extending substantially along the length of the pin between the tip 35 and the head 31. In addition to the sleeve 36 or as a substitute therefore, an isolating coating 38 of non-conductive material may be provided on the underside of the head 31. This prevents damaging an aluminium foil coated insulation panel from the electrical current of the welding gun as the welding energy is released during the welding process.

In some situations, the insulation 1 which has to be fastened to the metal structure is covered by a metal foil, e.g. aluminium, on the surface opposite the metal structure. The first welding pin 3 may therefore make up an electrical connection or short- circuit between the foil and the metal structure 2. This will make it more difficult to perform the welding of the second and following pins, as the pin holding means 5 on the welding gun 4 or the pin 3 itself will be in electrical contact with the foil. This will make it impossible to establish a welding arc.

This problem can be solved by having a substantially electrical insulating surface of the pin holding means 5 on the welding gun 4. Furthermore, it might also be an advantage if the end of the pin 3 has a substantially electrically insulating surface. The electrically insulating surfaces on the pin holding means 5 could be ceramic materials applied by e.g. thermal spraying, or some other temperature resistant material. The electrically insulating surface on parts of the pin 3 may be applied by any substantially electrically insulating paint or other substantially electrically insulating surface layer.

The tip 35 of the pin 32 is provided with a pointed distal end, e.g. a conical or a pyramid shape for facilitating a penetration of a top layer 21 on the steel surface of the steel structure 2 which is engaged by abutting or repetitive tapping.

In an alternative embodiment of the stud pin 3, it is realised that it may be without a stud head. The disc shaped head may be subsequently attached to the pin after the pin has been welded onto the metal structure by the welding gun.

The stud pin 32 is hardened in order to prevent, the distal end from being flattened during the tapping. It has been realised that many weldings have failed if the steel of the stud pin 32 is not sufficiently hard. The flattening of the end 35 cannot be seen by the naked eye, but if the stud pins from failed weldings are being studied in a microscope, the flattening may be observed.

By the invention, it is realised that the stud pin 32 may be hardened by a heat- treatment, i.e. a tempering process, followed by a rapid cooling and/or a deformation hardening. Moreover, a hard steel alloy could be used. In particular, for the rapid cooling and the deformation process, it is found advantageous that an extra hard distal end may be provided. However, as explained earlier, it is important that the pin material behind the tip (distal end) posses a certain hardness, as it otherwise would deteriorate during the welding process.

The steel type and the Micro Vickers hardness of two types of stud pins have been tested for comparison. The first one (pin A) proved too soft for the purpose whereas the second pin (pin B) was adequate for the purpose of attaching insulation material by a stud-welding process. Pin A had a carbon content of 0.037% and pin B had a carbon content of approx. 0.25 %. The Micro Vickers hardness was measured by pressing the tip of a diamond pyramid against the surface of the stud pins with a weight of 100 grammes. The lengths of the two diagonals in the square print in the surface is then measured in a microscope and used for the calculation of the Vickers hardness. These hardness measurements were performed in accordance with the international standard "ISO 6507-1 Metallic Materials - Vickers hardness. Part 1 Test Method". For both types of pins, the Vickers hardness that was measured close to the outermost part of the tip higher than the Vickers hardness on the pin itself. For pin A, the Vickers hardness at the tip was approx. 290-300, whereas the hardness on the other part of the pin was 130-150. For pin B, the Vickers hardness at the tip was 270-280, and for the other part of the pin 200-210.

Although a high Vickers hardness for the tip and the pin is essential for the welding result when the pins are to be attached, the required hardness may vary depending on the type of surface underneath. However, irrespective of the metal type below the top layer, it may often be the hardness and/or resilience of the top layer that determines the required Vickers hardness.

Another utility of the invention is shown in fig. 7 where several layers la, lb of panels 1 are attached to a steel structure 2 by one or more studs 3.

As shown in fig. 9, the stud 3 is retained in magnetic receiving means 5 in the stud- welding gun 4. The magnetic receiving means retains the stud 3 at its head 31 so that the stud pin 32 points towards the surface to which it is to be attached. When retained in the welding gun 4, the stud 3 is penetrated through the insulation layer (not shown in figs. 9 and 10) and positioned so its distal end tip abuts the metal structure 2. As it can be seen in fig. 9, the distal end of the stud 3 protrude beyond the support members, e.g. support legs, 40 of the welding gun 4 when the spring-loaded receiving means 5 are in the spring means are unloaded. The support member 40 also penetrate the insulation and makes contact with the coated metal surface 2, as the spring-loaded receiving means 5 are compressed by a distance D, whereby the distal end of the support members 40 also makes contact with the underlying surface. The stud-welding gun is kept in a predetermined position by the support members 40, e.g. support legs, so that the correct distance D is maintained and controlled during the securing action of the stud pin 3. The spring-loaded receiving means 5 may comprise a compression spring means 41 which in an advantageous embodiment may be provided with adjustment means for regulating the spring force and the protrusion distance D. Moreover, the receiving means 5 may preferably be provided with reciprocating means which may be activated for repetitively knocking or tapping the stud against the surface of the metal structure for establishing a sufficiently good electrical contact. Measurement means (not shown) may preferably be provided for measuring the electrical resistance between the stud and the metal surface in order to automatically determine when a sufficiently good electrical contact is established.

During the tapping for establishing the electrical contact, a measurement means is applied for determining when the electrical resistance has fallen below a predetermined level. The welding current is applied and almost simultaneously the stud 3 is lifted slightly and the gap L appears as indicated in fig. 10. The welding arc is now established. In addition, the second welding current is applied. Hereby, the melting pool is created by the welding arc and after a predetermined amount of time (the welding time), the welding current is cut off and the stud is pressed into the molten pool and the stud is attached to the metal structure.

Examples:

In the tables below, the results of tests are shown where the welding process has been performed with different sized gaps (lifting distances) L and different protrusion distances D applying different welding currents. The welding time used in all tests was 10 ms. However, it is realised that this parameter may also be varied. In particular, it is realised that a longer welding time may improve the result, depending on the type of stud, the materials involved, etc. The results of the welding processes are rated according to strength of the weld. The ratings are: 0: No or weak weld 1: Reproducible weld of suboptimal strength, e.g. due to a reduced cross-section 2: A strong and firm weld which does not break even after several bendings.

Table 2: Welding results with welding current of 325 A

Table 3: Welding results with welding current of 350 A

It appears that the combination of a higher current, a larger protrusion distance and a gap which is not too small in size provides the best welding results.

It is realised that the results may vary depending on the stud pin diameter and other parameters, such as material, etc. The welding time may be longer than 10 ms. Generally, the result of the welding process may be enhanced by using a longer welding time. However, the pin size limits the duration of welding time. It is realised that a significant improvement in the welding quality may be achieved making the insulation panel attachment considerably better and less labour intensive.

The above description relates to some of the preferable embodiments of the invention. However, it is realised that other embodiments of the various aspects of the invention may be provided without departing from the scope of the invention as set forth in the accompanying claims.

Claims

PATENT CLAIMS:

1. A method of attaching at least one fibrous insulation sheet panel onto a metal structure having a surface covering top layer, whereby a number of studs are placed through the sheet material and welded onto the metal structure by using a stud- welding system in which the stud is positioned in a stud-welding gun, said method comprising the steps of penetrating a stud comprising a hardened steel pin through the sheet material, providing an electrical contact between the stud and the underlying metal structure by penetrating the distal end of the stud pin through the top layer of the metal structure, and then applying a welding current to the stud, retracting the stud providing a gap between the stud and the metal structure and establishing a welding arc, and forcing the stud against the metal structure after a predetermined duration of time.

2. A method according to claim 1, whereby the electrical contact is provided by abutting the stud against the surface of the metal structure and repetitively tapping the stud against the surface of the metal structure.

3. A method according to claim 1 or 2, whereby the electrical contact is provided by rotating and substantially simultaneously forcing the stud onto the metal structure before the discharge of the welding current.

4. A method according to claim 1, whereby the gap which is provided is between 0.5 mm to 4 mm, preferably between 0.75 mm to 2.50 mm, more preferably between 1.75 to 2.50.

5. A method according to any of the claims, whereby the welding current is preceded by a first current of approximately 10 A to 100 A for preheating the metal in the welding zone, before the welding current is applied.

6. A method according to claim 5, whereby the welding current is within the range of 50 A to 850 A, preferably 100 A to 500 A, and with a duration between 0.5 ms to 500 ms, preferably between 10ms to 100 ms.

7. A method according to any of the claims, whereby the electrical resistance is measured by measuring means and the welding current can be established when the electrical conductivity exceeds a predetermined value.

8. A method according to any of the claims, where the metal structure is a steel structure.

9. A method according to any of the preceding claims, where the stud pin is made of hardened steel having a content of at least 0.04% Carbon, preferably at least 0.1% Carbon and even more preferably at least 0.2% Carbon.

10. A method according to any of the preceding claims, whereby the Vickers hardness of the stud pin is in the range of HV₀ι=140-500, preferably HV₀₁=160-400 and even more preferably HV₀ι=180-350.

11. A method according to any of the preceding claims, whereby the distal end of the stud pin is provided with an increased Vickers hardness relative to the rest of the pin.

12. A method according to any of the claims, where the metal structure is an aluminium structure.

13. A method according to any of the claims, where the top layer of the metal structure is a corrosion resistant coating.

14. A method according to any of the claims, where the stud welding gun is supported in a position relative to the metal structure by at least one support member, such as a support lever.

15. A method according to claim 14, where said at least one support member is penetrated through the insulation sheet and is in contact with the surface of the metal structure for supporting the stud-welding gun through the stud-welding process.

16. A stud for use for attaching a fibrous insulation sheet panel onto a metal structure comprising a head with a protruding electrically conductive pin member, c h ar a c t e r i s e d b y said pin member being made of hardened steel and provided with top layer penetration means at its distal end.

17. A stud according to claim 16, wherein the top layer penetration means is a pointed or wedge-shaped distal end of the stud pin.

18. A stud according to claim 16, wherein the top layer penetration means is a drilling head formed in the end of the stud pin.

19. A stud according to any of the claims 16 to 18, wherein the stud pin is made of hardened steel having a content of at least 0.04% Carbon, preferably at least 0.1% Carbon and even more preferably at least 0.2% Carbon.

20. A stud according to any of the claims 16 to 19, wherein the Vickers hardness of the stud pin is in the range of HV₀₁=l40-500, preferably HV₀ι=160-400 and even more preferably HV₀ι=180-350.

21. A stud according to any of the claims 16 to 20, wherein the distal end of the stud pin is provided with an increased Vickers hardness relative to the rest of the pin.

22. A stud according to any of the claims 16 to 20, wherein said head is magnetic metal plate disc.

23. A stud according to any of the claims 16 or 22, wherein the pin is provided with a corrosion protective material deposit adjacent the top layer removing means.

24. A stud according to any of the claims 16 or 23, wherein the pin is provided with a coating of corrosion protective material.

25. A stud according to claim 23 or 24, wherein the corrosion protective material is zinc.

26. A stud according to claim 23 or 25, wherein the corrosion protective material is aluminium.

27. A stud according to any of the claims 16 to 26, wherein the pin is provided with an electrically insulating sleeve.

28. A stud- welding gun for welding a stud onto an underlying metal structure for attaching a fibrous insulation panel to said metal structure, said stud-welding gun comprising first spring-loaded stud holding means for receiving a stud, which is pinched through a sheet panel, and biasing the stud towards an underlying metal structure, welding current application means for applying a welding current to the stud; c h ar a c t e r i s e d i n th a t said stud receiving means are provided with lifting means for retracting the stud along its pin axis for creation of a gap between the pin and the metal structure surface.

29. A stud welding gun according to claim 28, wherein the receiving means are provided with magnetic means for retaining the stud in a predetermined position relative to the metal structure.

30. A stud welding gun according to claim 28 or 29, wherein the stud welding gun is provided with at least one support member, such as a support lever, said at least one support member preferably being adjustable, for providing a predetermined distance and pressure during the welding process.

31. A stud welding gun according to any of claims 28 to 30, wherein the stud receiving means is provided with a substantially electrically insulating surface.