WO1996005935A2

WO1996005935A2 - Method, electrode and welding apparatus for depositing a welded facing on a member and an engine or machine part provided with said facing

Info

Publication number: WO1996005935A2
Application number: PCT/DK1995/000329
Authority: WO
Inventors: Bent Thomasen
Original assignee: MAN B&W Diesel GmbH; MAN B&W Diesel AS
Current assignee: MAN B&W Diesel GmbH; MAN B&W Diesel AS
Priority date: 1994-08-16
Filing date: 1995-08-15
Publication date: 1996-02-29
Anticipated expiration: 1997-02-16
Also published as: AU3219995A; DK94794A; WO1996005935A3

Abstract

A member (17) is provided with a welded facing by means of a method in which a strip electrode, the strip width of which is at least 12 times larger than the strip thickness, is fed and melted under gas shielding by means of a transmitted open arc. A feed device (10) moves the strip electrode at a constant, adjustable speed down through a welding head (15) with electric contact means (31, 32) and further towards the member, a power source at the same time supplying a voltage and current so that the electrode material is melted in a spray arc or a short arc. The melted material is shielded by gas supplied via an annular gas distributor nozzle (46, 49) positioned inside a gas cup (48), which may suitably have an outlet opening which is oblong in the welding direction. The welding is performed at a melting rate of about 8 kg/h, and the method is particularly applicable for welding a protective facing on to members of an irregular geometry, such as a disc valve (17) for an internal combustion engine.

Description

Method, electrode and welding apparatus for depositing a welded facing on a member and an engine or machine part provided with said facing.

The present invention relates to a method of depositing a welded facing on a member, such as a disc valve for a large internal combustion engine, an uncoated, strip-shaped electrode being fed and melted under gas shielding by means of a transferred open arc, the electrode being moved by means of a feed device at a substantially constant adjustable speed from a magazine, such as a spool with coiled electrode material, through a welding head with electric contact means and further towards the member on which the melted electrode material is deposited, a power source con- nected with the electric contact means supplying a welding current and shielding gas being supplied from gas means at the welding head.

Welding with a consumable electrode under shielding gas is known under the designation GMAW (Gas Metal Arc Welding, comprising both MIG and MAG welding) , in which an electrode in the form of a wire of a diameter ranging between 0.6 mm and 1.6 mm is used. The wire is fed continuously and is melted under gas shielding, where the shielding gas may, for example, be argon, C0₂ or mixtures thereof. The GMAW method is normally used for joining of structural elements, but can also be used for welding a facing onto a member.

Thus it is known to face the bottom side of a valve disc by means of MIG welding (Metal Inactive Gas welding) . The valve discs are provided by welding with a heat-resistant facing consisting of Inconel 625 supplied in the form of an electrode wire of a diameter of 1.6 mm, using a shielding gas with 85 per cent argon and 15 per cent helium. The melting rate at this method is just below 4 kg/hour. As a valve disc for a large two-stroke internal combustion engine has to have, for example, 22 kg of heat-resistant material welded on to achieve a desired facing thickness of 10 mm, the welding is a time-consuming process. A more substantial disad- vantage of the known method is that the MIG welding results in a relatively deep and concentrated penetra¬ tion with a subsequent heavy dilution of up to 30 per cent with the base material. Owing to the dilution there is a risk that the welded facing may locally deviate so much from the desired composition that hot cracking may occur in the outermost layer. The GMAW method also restricts the composition of the welded facing, as only few super-alloys with the desired high temperature properties can be drawn into wire shape. Submerged-arc strip welding is another known method of depositing a welded facing on a member. This method uses an uncoated consumable strip electrode being enveloped in the arc area by a protective powder which melts together into a protective layer of slag on top of the weld pool. Thus, this method does not use shielding gas. The method has an advantageously large melting rate, but is quite unsuitable for welding on members with edges as the powder drops off at the edges so that the weld pool is uncovered. Submerged-arc strip welding therefore cannot be used for depositing a welded facing near the periphery of a valve disc.

On a valve disc and similar circular members, the facing is welded on in a spiral pattern. Near the middle of the valve disc the weld beads are deposited so quickly following each other that submerged-arc strip welding is not applicable as the protective slag over a weld bead already deposited cannot be removed in time before the neighbouring weld bead is to be deposited. This problem occurs not only with valve discs, but with all members where the weld beads are deposited quickly one after the other. Submerged-arc strip welding also suffers from the disadvantage that owing to the protec¬ tive and heat-insulating slag, the member is exposed to a very large heat supply with subsequent deformations of the member, particularly if it is small or has a geometry with varying material thickness.

Submerged-arc plasma welding is another known method of depositing a welded facing on a member. This method uses powdery filler material transmitted to the member in a plasma arc. The method results in little dilution only with the base material and can thus be used for welding of facings of a very high quality. To avoid clogging of the plasma torch, the filler material has to be atomized into spherical shape. Some elements cannot be atomized, but only crushed mechanically into flake shape, which sets a limit to the analysis of the welded facing. The method also suffers from the disad¬ vantages that the melting rate is limited to about 3 kg/hour, and that the welding apparatus is complicated and very costly.

Soviet inventor's certificate No. 524636 describes a method of welding strip material onto a shaft member, the strip material being heated by resistance and passed below a consumable electrode which performs a pendular motion across the strip so that it is melted onto the member. This method results in an extremely large heat supply to the member, the current density being as high

₂ as 125-250 A/mm .

Soviet inventor's certificate No. 966999 describes a method of welding strip material onto a member. The strip material is here used as an electrode and is moved in an oscillating motion in relation to the member so that the point of contact between the strip electrode and the member is moved all the time, whereby melted metal is transmitted in micro-portions to the member at the point of contact. This method uses point-by-point resistance welding without applying a shielding gas. The resistance welding enables a material to be deposited at a voltage of only 11 volt and a current density of only 0.5-5 A/mm , which permits deposition of a very thin layer without any noticeable deformations of the member.

US patent No. 2,848,593 describes a method of the former type, and indicates that a strip electrode may be melted under gas shielding. All the examples in this patent concern submerged-arc strip welding, the strip electrode must have a thickness of at least 0.51 mm, a ratio between the strip width and the strip thickness

₂ of at least 1:16 and a current density of 30 A/mm are recommended.

The object of the present invention is to facili¬ tate and improve manufacturing conditions for and/or the properties of disc valves for large internal combustion engines or similar engine or machine elements requiring the deposition of a heat resistant or a hard welded facing.

According to one aspect of the present invention, the above-mentioned method is characterized in that welding is performed at a current density in the range of 50 to 120 A/mm with a strip electrode, the strip width of which is at least 12 times larger than the strip thickness, and the material of which is melted in a spray arc or a short arc.

It has surprisingly proved possible with the invention to use the uncoated strip electrode which is melted in a spray or short arc under gas shielding, for welding onto disc valves or similar members with edges or other jumps in the geometry. This results in a number of advantages: - Firstly, a substantially greater melting rate at something like 8-9 kg/h is achieved, which is especially an advantage in welding of protective facings, where large amounts of material often have to be melted. - Secondly, the strip electrode provides an advantageously small and homogeneous dilution with the base material, and at the welding parameters indicated a very even penetration profile with consequent approxi¬ mately rectilinear transition between the base material and the welded facing. For a disc valve with a welded heat-resistant facing on the bottom disc side this results in improved operating characteristics. The very even transition has probably been provided by the arc travelling back and forth at the current density stated from one side to the other of the strip electrode at a suitable frequency so that the melting and the dilution becomes completely homogeneous. This minimizes or removes the prior-art problems of heat cracking in the outermost part of the facing. If the strip width becomes substantially smaller than 12 times the strip thickness, the penetration becomes more concentrated and the dilution greater.

- Thirdly, despite the high melting rate, the member is only exposed to a rather limited heat influ- ence, which renders the method particularly applicable for facing of small members, particularly members of an irregular geometry, such as disc valves which have an upright valve spindle on the back of the disc.

- Fourthly, the method can be used with welding apparatuses that are substantially more simple and thus cheaper than the systems to be used for plasma arc welding.

- Fifthly, facings of any analysis can be deposited using the method, because strip electrodes, as mentioned below in connection with another aspect of the inven- tion, can be manufactured mainly on the basis of desired properties of the finished facing and without taking into account whether the material has to be capable of being drawn into strip shape or be manufactured from an atomized starting material.

Welding can suitably be performed at a current

2 density of at least 75 A/mm . At lower current den¬ sities, the arc begins to become unstable, which can reduce the quality of the welded facing. For reasons of economy the current density is preferably not higher

2 than 100 A/mm , which renders it possible for the power sources very widely used for GMAW welding to be used also for strip welding with gas shielding. At higher current densities it may be necessary to invest in a new and more powerful power source.

To achieve welding of uniform quality it is of importance that the arc is kept as constant as possible. Therefore, preferably, the strip electrode is moved towards the member at a substantially constant speed and at a substantially constant angle, preferably in the interval from 80 to 100°. It has turned out that the best results are achieved when the strip electrode is at a right angle to the surface of the member, but near jumps in the member geometry, such as at corners, it may be necessary to set the electrode at an acute angle in relation to the member.

Particularly when facing disc valves for large engines it may be suitable for the welding to be performed with a strip electrode which is at least partly manufactured from mechanically crushed powder, as this renders possible welding of a super-alloy with very desirable properties, which have until now in practice been unattainable for a disc valve.

According to another aspect the invention relates to an electrode for use in the method. The consumable electrode is characterized in comprising mechanically crushed powder sintered together into strip shape. The electrode may be manufactured exclusively on the basis of mechanically crushed powder or from a mixture of mechanically crushed and atomized powder. The composi¬ tion of the electrode at least partially from mechan¬ ically crushed powder renders it possible to manufacture electrodes from alloys which have not previously been capable of manufacture for commercial use in the facing of, for example, exhaust valves with a heat-resistant layer. It is thus possible to manufacture strip-shaped electrodes from the alloys Inconel 625, Inconel 671, Renee 220 or Stellite 6. Thus, the alloy can be selected exclusively on the basis of the desired properties of the welded facing and without taking into consideration the welding process itself. Such an option has not previously been available in the manufacture of, for example, exhaust valves, and this renders possible the manufacture of members with novel advantageous prop- erties.

According to a further aspect the invention also relates to a welding apparatus for use in the above method and comprising an electrode magazine, such as a rotatably journalled spool with coiled electrode material, a feed device having drive means for feeding the uncoated strip-shaped electrode at an adjustable, substantially constant speed, a welding head with contact means which, for contact and control of the electrode, define a slot of a thickness corresponding to the strip thickness of the electrode, a power source which is connected with the contact means and via them can supply the electrode with a voltage and current so that welding can be performed with a transferred open arc between the electrode and a member to be provided with a welded facing, and a shielding gas source with at least one cup located at the welding head for guiding shielding gas towards the area around the arc and the melted electrode material.

To achieve the above advantages, the welding apparatus according to the invention is characterized in that the gas cup or gas cups discharge(s) shielding gas in an area which is oblong in the welding direction and extends furthest away from the strip electrode in a direction opposite to the welding direction. Owing to the high melting rate, the weld pool extends relatively far to the back of the arc area. The rearwards oblong area with gas shielding protects the weld pool until it is suitably cooled, without the gas consumption becoming unsuitably large. The quality of the weld and thus the properties of the finished member are ensured by means of the extended gas shielding, which also reduces the risk of edge defects in the weld bead and thus permits rapid laying of a partially overlapping neighbouring bead. The manufacturing advantage thus provided is of particular importance at the welding on the bottom side of a disc valve where the weld bead can be applied in spiral shape.

In a very simple embodiment, the outlet opening of the gas cup is oblong in the welding direction, and it is positioned with the strip electrode located at the greatest distance from the rear edge of the cup. Alternatively, at least two gas cups may be used, of which the first cup extends around the strip electrode and the second one is positioned behind the first one in the welding direction. The two or more consecutive welding cups can each be provided with a smaller area than a single oblong cup, which may be an advantage if there is a draught around the operating point, but the gas shield covering the weld pool becomes less homogene- ous. In a preferred embodiment of the welding apparatus the gas cup contains an annular gas distributor nozzle.

The gas cup must necessarily have relatively large dimensions to provide the required gas shielding of the wide strip electrode and the large weld pool produced by welding with such an electrode. The gas distributor nozzle limits the gas consumption and promotes an even discharge of gas over the full area of the gas cup, whether or not the cup is at an oblique angle in relation to the member.

The electrode may suitably have a cross-sectional

2 area of not more than 7 mm , and preferably a cross- sectional area ranging from 4 mm 2 to 6 mm2. With a

2 typical current density of about 85 A/mm , these cross- sectional areas of the electrode result in a need for a current of just below 600 A and suitably from 340 A to 510 A, which can be produced by the power sources usually applied in GMAW welding with a wire electrode. With a reconstruction of the electrode feed system and the use of a new welding head, the prior-art, very widely used welding apparatus may be converted for use according to the invention. This conversion requires only a fairly small investment, and it is a further advantage of the welding apparatus according to the invention that the GMAW process is already very well- known, which means that even minor engineering workshops will directly be able to exploit the invention.

Preferably the strip thickness of the electrode is in the range of 0.35-0.5 mm. The small strip thickness facilitates ignition of the arc and lowers the threshold current to obtain a material transmission in the arc consisting of fine droplets, i.e. a so-called spray arc. Furthermore, the small strip thickness helps to keep the heat input to the member low in relation to the melting rate. Strip thicknesses around 0.40 mm, such as from 0.37 to 0.43 mm, are particularly advantageous in case of sintered strips, which on one hand have good mechan¬ ical properties and on the other hand are so suitably thin that largely all the bonding agent used to combine the powder evaporates in the arc and in the weld pool before the melt solidifies.

According to a further aspect, the invention relates to an engine or machine part provided with a welded facing, particularly a disc valve for a large internal combustion engine with a heat-resistant welded facing on the disc bottom side and/or a welded hard facing in the seat area on the disc upper side, charac¬ terized in that the facing has been welded on using the above method and preferably also by means of the above welding apparatus. The member manufactured by the method exhibits an extremely even transition between the welded facings and the base material, and very low dilution in the welded facing, which provides the part with superior operating properties in relation to prior-art parts with a welded facing. The high melting rate further renders possible the deposition of a weld run of a large thickness, which contributes to the welded facing having the desired analysis at the outer side of the facing. As the electrode manufacture sets no limits to the composition of the welded alloy, the part may have a welded facing having an analysis which has not so far been achievable. As examples of typical parts may be mentioned disc valves and seat sections for valve housings for suction or exhaust valves in an internal combustion engine, valve parts for the off-shore industry, pressure rollers for industrial plants, pipe sections with an internal or external protective facing as well as shaft portions and wearing parts in an engine or a machine. A working example and an example of an embodiment of the invention will now be described in further detail below with reference to the schematic drawing, in which

Fig. 1 shows a rough draft of an electrode feed system with associated welding head,

Figs. 2 and 3 are partial sectional cuts in the longitudinal and transverse directions, respectively, through the welding head of Fig. 1,

Fig. 4 shows the welding head seen from below, and Fig. 5 is one half of the welding head.

The very simplified perspective view of Fig. 1 shows part of a welding apparatus 1 having an electrode magazine 2 in the form of a spool rotatably mounted on a central shaft end 3, which is mounted through a holder 4 on a supporting column 5 fastened by a fitting 6 on the upper side of a mounting unit 7. The spool has two circular side pieces 8 arranged at a mutual distance, and a strip electrode 9 is coiled on the spool. For the sake of clarity, only one side piece 8 is shown. From the spool, the strip electrode is passed down into a feed device 10 with drive means for feeding the strip electrode, in the form of two rolls positioned on respective sides of the strip electrode. One roll, not shown, is fastened on a driving shaft positioned by a rolling bearing fastened on the mounting unit 7 and driven by a motor, such as a DC shunt motor which can rotate the shaft at a constant angular velocity despite varying load on the driving roll. The speed of the driving motor is adjustable, for example by adjustment of the armature voltage of the shunt motor. The periph¬ eral surface of the driving roll in contact with the strip electrode is provided with a friction-increasing coating or profile providing a sure grip on the strip so that it is fed at the peripheral speed of the driving roll without slipping. The second roll 11 is a pressure roll supported by a shaft mounted in a U-shaped swinging part 12 which is swingably hinged to the mounting unit 7 at the top of its upward branches. At the bottom of the U there is an adjustment knob 13, which presses the swinging part and thus the pressure roll 11 towards the driving roll against a spring bias. Because the strip electrode passes down between the two rolls, the force of contact of the rolls against the electrode can be adjusted by turning the adjustment knob 13, which also permits setting for different electrode thicknesses.

From the feed device 10, the strip electrode passes down through a link guide 14 consisting of two rod- shaped halves with an internal guide track which guides the strip electrode down to a burner head or welding head 15. By means of bolts, not shown, the upper end of the link guide is fastened to the mounting unit 7, and the welding head 15 is screwed onto the link guide at its lower end so that the mounting unit supports the welding head via the link guide. As the link guide only has to guide the strip electrode down to the welding head, the guide track in the rods may be designed with such an excess measure in relation to the electrode dimensions that one and the same link guide can be used for electrodes of different widths and thicknesses. Of course, it is also possible to arrange the feed device immediately on top of the welding head which can then be connected with the mounting unit via a separate console, but the design shown in the figure is preferred because owing to the distance to the welding head, the feed device is here protected against the high tempera¬ tures developed by the welding.

The mounting unit 7 is supported by a horizontal console 16 which is supported in a known manner, not shown in detail, by a frame standing on the floor and comprising a jig device for the member to be provided with a welded facing. The figure shows the end of a disc valve 17, such as an exhaust valve for a large two- stroke main engine of a ship. The spindle of the disc valve is fixed with its longitudinal axis being vertical in the jig device so that the bottom side 18 of the valve faces upwards toward the welding head. When the welding is being performed, the jig device rotates the disc valve in the direction of the arrow 19 at a speed which is so adapted to the position of the welding bead on the bottom side 18 that the relative speed between the welding head and the bottom side of the valve at the operating point is substantially constant, but adjust¬ able in dependency of the other welding parameters. During the welding, the welding head has to be positioned in relation to the member. This may be done, for example, by an adjustment drive, not shown, a so- called tractor, either displacing the console 16 in the latter's transverse direction indicated by arrows 20 or in the longitudinal direction indicated by arrows 21, or displacing the mounting unit 7 in the longitudinal direction of the console, or swinging the console about a vertical axis 22 positioned at a distance from the member, or by combinations of such movements. The setting movement of the welding head may be either continuous or discontinuous. In the latter case, the jig device may be provided with a signalling device emitting at each full revolution of the member a control signal which produces a stepwise adjustment of the welding head.

If the member is cylindrical and has to be faced on the outer side, it may be set up rotatably about a horizontal axis in the jig device, and the welding head may then during the facing be displaced in parallel with the longitudinal axis of the member. The console 16 may furthermore be movable in the vertical direction as indicated by arrows 23 to permit setting of the welding head height above the member and to move the welding head between the active position and an inactive position where the member may be set up.

The welding head 15 is shown in a larger scale in Figs. 2-4 and comprises a central member 24 constructed from two halves 24a, 24b joined together by means of two fixing means, such as machine screws 25. The halves are positioned in relation to each other by means of guide pins 26, 27 inserted in associated bores in the two halves. The half 24b has an upright flange 28 allowing the welding head to be fastened to the link guide 14 through fastening means, such as two machine screws 29. A recess 30 in the half 24a guides the strip electrode down between two electric contact means in the form of mutually facing plate-shaped contact parts 31, 32, of which part 31 is fastened in a recess at the bottom of the half 24b by means of a screw 33, and the part 32 is suspended via a groove about a projection 34 in a recess at the front end of the half 24a. A spring 35 presses the contact part 32 towards the part 31, but can be moved away from it with simultaneous compression of the spring 35 when the strip electrode passes down between the contact parts. To facilitate the penetration of the electrode through the contact parts, the latter are provided with an oblique cut at the upper corner facing the recess 30. As the contact parts are pressed towards both sides of the strip electrode, they ensure good and stable power supply to it.

The power supply to the welding head is provided through an electric wire 36 carrying at its outer end a shoe indicated by 37, which is bolted on to the side of the central member 24 creating the current transmit- ting connection between the shoe and the contact parts. The welding head is cooled by coolant supplied and drained away through conduits 38, 39 connected to respective sockets 40, 41 on the central member. Through cooling channels 42 in the halves 24a, 24b, the coolant is passed down into the area around the contact parts 31, 32.

The welding apparatus also comprises a shielding gas source which in a well-known manner comprises gas flasks, not shown, which pass gas at an adjustable delivery pressure and in a predetermined mixing ratio, if several types of gas are used, through a gas conduit 43 to an electrically controlled adjustment valve 44, which after fine adjustment of the gas pressure passes the gas through a gas hose 44' to a branch pipe 45 on the welding head. As shown in Fig. 3, the branch pipe 45 opens out into an annular gas chamber 46 surrounding the central member 24. The housing 47 of the gas chamber supports a screen or cup 48 positioned around and extending down past the lower part of the central member. The gas chamber 46 is defined downwards by an annular distributor plate 49, which, as shown in Fig. 4, is provided with many through holes for discharge of gas. The gas chamber and the distributor plate together constitute a gas distributor nozzle ensuring even discharge of gas over all the annular area positioned between the central member and the cup.

At least at its lower rim, the cup 48 is provided with a cooling pipe 51 (not shown in Fig. 4) , which is in flow communication with the cooling channels 42. The cooling pipe may, for example, be soldered onto the cup, and this ensures the cup against damage from the strong heat development at the operating point. Alternatively, the cup may be provided with internal cooling channels, being designed, for example, as a double-wall cup with internal guide plates for controlling the coolant flow. The cup may be adjustable in the height in relation to the central member 24, for example by the central member having recesses 52 on its outer side located at various heights. On opposite sides of the cup, a locking pin 53 may be pressed into an associated recess 52 by mounting screws 54 so that the cup is fixed at the desired height on the central member 24. The outlet opening delimited by the lower rim 55 of the gas cup is oblong in the welding direction and is positioned so that the strip electrode projecting from the contact parts 31, 32 at welding, has the greatest distance from the rear edge 56 of the cup. As the melting pool becomes somewhat wider than the electrode width and is oblong in the welding direction, the outlet opening may suitably have a width corresponding to double the electrode width and a length corresponding to about 8 times the electrode width. Other width/length ratios are also possible, but the oblong outlet opening contributes to a limitation of the gas consumption, as the gas is only discharged where gas shielding is needed.

The wire 36 is connected in a known manner to a power source of a type well-known from GMAW welding with a wire electrode. The power source may, for example, be a DC source with a flat or slightly drooping static characteristic and a maximum current of, for example 500-600 A. The slightly drooping characteristic provides a possibility in a manner known per se of achieving a self-adjusting arc, the adjustment of which is based on the changes in the melting rate arising when the ohmic resistance of the arc is changed at variations in the arc length. If the length of the arc is increased, for example, the arc voltage rises, and the welding current drops, which reduces the melting rate until the length of the arc is again reduced to the predetermined value. The self adjustment occurs rapidly and has proved advantageous for the establishment and maintenance of a stable arc at strip electrode welding under gas shielding. It is thus sufficient to set the constant feed speed of the strip electrode and the voltage supplied from the power source, the current automati¬ cally being adjusted in relation to the prevailing ohmic resistance in the arc.

The arc voltage may suitably be controlled within the interval of 23-45 V, and the current density be con-

2 trolled within the interval of 50-100 A/mm or possible

2 slightly higher, up to 120 A/mm . If the arc voltage drops below 23 V, the electrode will tend to short- circuit with the member, and at a voltage exceeding 45 V, the arc becomes too long and unstable. The current density is adapted to the feed speed of the electrode and the desired depth of penetration. Outside the interval indicated it may be difficult to achieve a weld of the desired quality.

A working example will now be described. The welding was performed on a circular disc-shaped member having a diameter of 25 cm and a thickness of 3 cm. The disc was made of stainless steel of the type E 304. The strip electrode was made of the material Inconel 625 and had a strip thickness of 0.5 mm and a strip width of 10 mm. The shielding gas used was a mixture of 85 per cent argon and 15 per cent helium which was supplied at a flow amount of 27 1/min. The negative pole of the power source was connected with the welding head and the positive pole with the member. The welding head was positioned with the lower rim of the gas cup at a height of 10 mm above the member in a position so that the strip electrode formed an angle of 90° with the surface of the member.

For the welding, a current of 420 A and an arc voltage of 29.2 V were used, and the strip electrode was fed at a speed of 3.14 m/sec. corresponding to a melting rate of 8.3 kg/hour. The rotation of the disc was controlled so that the welding speed, i.e. the relative movement between the welding head and the member surface was 45 cm/min. The welding head was controlled so as to provide an overlap of 7 mm between beads. The welding bead produced turned out to have a width of 19 mm and a height of about 4 mm. A total of 4 layers were welded on with a total thickness of 15-16 mm. The member was then cut through and ground in the usual manner. The penetration line proved to be largely horizontal, and no welding defects were observed. The test was repeated several times with the same result. During the welding, the arc was completely stable, and it was observed visually that the arc travelled back and forth across the full strip width.

If the member to receive a facing consists of a material which only allows a small heat influence, the welding parameters may be adjusted in a manner known per se so that the welding of the first layer is performed by melting the electrode material in a short arc, where the drop rate is lower than in a spray arc. Although this reduces the melting rate, the dilution is also reduced, and particularly the heat supply to the member is reduced.

Advantageously higher melting rates may be achieved with strips being 30-40 times wider than the strip thickness, for example strips with the dimensions 0.4 x 12 mm, 0.4 x 16 mm or 0.5 x 20 mm. In these cases, the power source may have a maximum current of 750-1000 A.

Claims

P A T E N T C L A I M S

1. A method of depositing a welded facing on a member (17) , such as a disc valve for a large internal combustion engine, an uncoated, strip-shaped electrode being fed and melted under gas shielding by means of a transferred open arc, the electrode being moved by means of a feed device (10) at a substantially constant adjustable speed from a magazine (2) , such as a spool with coiled electrode material, through a welding head (15) with electric contact means (31, 32) and further towards the member on which the melted electrode material is deposited, a power source connected with the electric contact means supplying a welding current and shielding gas being supplied from gas means (45-50) at the welding head, c h a r a c t e r i z e d in that welding is performed at a current density in the range

2 of 50 to 120 A/mm with a strip electrode (9) , the strip width of which is at least 12 times larger than the strip thickness, and the material of which is melted in a spray arc or a short arc.

2. A method according to claim 1, c h a r a c ¬ t e r i z e d in that welding is performed at a current

2 density ooif at least 75 A/mm and preferably at the most

3. A method according to claim l or 2, c h a r ¬ a c t e r i z e d in that the strip electrode (9) is moved towards the member (17) at a substantially constant speed and at a substantially constant angle, preferably in the interval from 80 to 100°.

4. A method according to any one of claim 1-3, c h a r a c t e r i z e d in that the welding is performed with a strip electrode at least partially manufactured from mechanically crushed powder.

5. An electrode for use in the method according to claim 4, c h a r a c t e r i z e d in that the consum- able strip electrode (9) comprises mechanically crushed powder sintered together into strip shape.

6. A welding apparatus for use in the method according to any one of claims 1-4, comprising an electrode magazine (2) , such as a rotatably journalled spool with coiled electrode material, a feed device (10) having drive means for feeding the uncoated strip-shaped electrode at an adjustable, substantially constant speed, a welding head (15) with contact means (31, 32) which, for contact and control of the electrode, define a slot of a thickness corresponding to the strip thickness of the electrode, a power source which is connected with the contact means and via them can supply the electrode with a voltage and current so that welding can be performed with transferred open arc between the electrode and a member (17) to be provided with a welded facing, and a shielding gas source with at least one cup

(48) located at the welding head for guiding shielding gas towards the area around the arc and the melted electrode material, c h a r a c t e r i z e d in that the gas cup or gas cups (48) discharge(s) shielding gas in an area which is oblong in the welding direction and extends furthest away from the strip electrode in a direction opposite to the welding direction.

7. A welding apparatus according to claim 6, c h a r a c t e r i z e d in having one gas cup, the outlet opening of which is oblong in the welding direction and is positioned with the strip electrode (9) located at the greatest possible distance from the rear edge of the cup.

8. A welding apparatus according to claim 6, c h a r a c t e r i z e d in that at least two gas cups are used, of which the first cup extends around the strip electrode and the second cup is positioned behind the first one in the welding direction.

9. A welding apparatus according to claim 7 or 8, c h a r a c t e r i z e d in that the gas cup contains an annular gas distributor nozzle (46, 49) .

10. A welding apparatus according to any one of claim 6-9, c h a r a c t e r i z e d in that the

₂ electrode (9) has a cross-sectional area of 7 mm at the most, preferably from 4 mm 2 to 6 mm2.

11. A welding apparatus according to any one of claims 6-10, c h a r a c t e r i z e d in that the strip thickness of the electrode (9) is in the interval of 0.35 - 0.5 mm, preferably 0.37 - 0.43 mm.

12. An engine or machine part provided with a welded facing, particularly a disc valve (17) for a large internal combustion engine with a heat-resistant welded facing on the disc bottom side and/or a welded hard facing in the seat area on the disc upper side, c h a r a c t e r i z e d in that the facing has been welded on using the method according to any one of claims 1-4, and preferably by means of a welding apparatus according to any one of claims 6-11.