GB2187988A - Electric arc welding - Google Patents

Abstract

A method of electric arc welding uses a consumable tubular filled electrode and the arc is sustained by a power source providing a series of substantially constant voltage pulses. <IMAGE>

Description

SPECIFICATION Electric arc welding This invention concerns electric arc welding, the power source used for it and the electrodes used in it.

Power sources providing pulsed current for gas-shielded metal-arc welding using solid electrode wires have been available for a number of years. In their most modern form, they allow welding with higher metal deposition rates and with less spatter than can be achieved using continuous current power sources. Especially when welding other than in the flat position or to produce a fillet between a horizontal plate and a vertical plate, substantially higher deposition rates are possible and the incidence of defects associated with lack of fusion is markedly reduced. Hitherto, however, such power sources have not offered corresponding advantages when used with flux-cored electrode wires.

An alternative means of lowering the amount of spatter, reducing the incidence of lack-offusion defects and improving deposition rates, especially in vertical welding, is to use a continuous current power source with a suitable formulated tubular flux-cored wire. Such wires contain slag formers and arc modifiers which improve their welding characteristics relative to those of solid wires. The more successfully this objective is achieved, the less additional advantage is to be gained from the use of a pulsed power source with such a flux-cored wire.

Tubular flux-cored wires are also formulated with other objectives: for example, to produce weld metals with exceptional low-temperature toughness or to operate without external gas shielding. Wires formulated with these aims are characterised by the presence of slag formers giving a highly basic slag: typically, if slag basicity is defined by the well known basicity index: CaO+ MgO + BaO+SrO+Na20+K20 + Li20+CaF+ } (MnO+ FeO BI SiO2+ -41(Ti02+Al2Q+Zr02) where amounts of the quoted compounds are expressed in weight percent, then wires designed to produce welds with high toughness or without external gas shielding will have basicity indices in excess of 2.Such wires tend to produce more spatter, to give lower deposition rates in vertical welding and to be more susceptible to weld defects than those designed with ease of use as a prime objective. Hitherto, it has not been possible to improve the welding characteristics of these wires significantly by the use of a pulsed current power source.

Tubular electrode wires with metal or metal and flux filling are inherently less homogeneous than solid wires and the rate at which they melt in the arc is not constant over short time intervals. If a pulsed power source is designed to produce a constant current during the pulse, variations in arc length may occur and to achieve stable conditions the average arc length must be longer than metallurgical considerations or the welder's preference would dictate. Active stabilising techniques such as varying the wire feeding speed or the interval between pulses in response to arc voltage have been proposed, but these are limited in their speed of response.

The present invention makes use of the inherently self-stabilising property of a constant voltage arc during the pulse.

The following terms used herein are defined as follows: Ip=pulse current Ib=background current (which is always less than Ip) pulse duration The invention provides a method of electric arc welding using a consumable tubular filled electrode in which the arc is sustained by a power source providing a series of substantially constant voltage pulses.

Preferably the filling of the electrode is a basic flux; each pulse has a slope not exceeding 5 volts per 100 amps up to a pre-determined current cut-off; the background current (Ib) for maintaining the arc between pulses is adjusted to lie between 30 and 100 amps; the time between pulses is substantially greater than the duration of each pulse (to}; and the relative movement between the workpiece and the torch through which the electrode passes can have a substantially vertical component.

In the accompanying drawings, Figs. 1, 2a and 2b show certain voltage-current characteristics of a power source used in a method according to the invention, and Fig. 3 is a sketch of apparatus according to the invention.

The voltage-current characteristic of the power source during the pulse is of the form shown in Fig. 1, with a slope not exceeding 5 volts per 100A up to a pre-determined current cut-off Imam, which for a 1.2 mm diameter wire should be at least 400A.

A given wire is characterised by a detachment parameter l2ptp which defines the conditions for detaching one droplet per pulse. For stable droplet detachment from a filled tubular wire with the same pulse time, the maximum pulse current available will typically need to exceed that for a solid wire of the same diameter because despite the reduced cross-sectional area available to carry the current, filled tubular wires usually have a lower electrical resistance per unit length than a solid wire of the same diameter. For a given electrode extension, flux-cored wires may thus require a higher current to produce the same degree of joule (12R) heating in the electrode stickout.

To allow a short arc to be maintained without extinguishing the arc if the electrode stubs into the weld pool, it is necessary to allow the current to increase rapidly at very low arc voltages during the background period. However to maintain a constant heat input, it is desirable to maintain the background current (lib) approximately constant as long as there is no danger of the electrode stubbing. Combining these requirements dictates an ideal background voltage-current characteristic of the form shown in Fig. 2a.

This characteristic curve comprises a "constant" voltage section in which the voltage is preselected by the designer or operator to be between 5 and 12V, and a constant current section in which the current is adjustable by the operator between 30A and 100A. However, experience has shown that this may be relaxed to the form shown in Fig. 2b without marked detriment to the welding performance.

A welding power source provided with the characteristics described above allows welding to be carried out using pulsed current with flux-cored wire so as to give free flight metal transfer with one droplet per pulse, if the appropriate pulse parameters are selected.

Furthermore, free flight transfer of fine droplets is possible with the electrode connected to the positive pole of such a power source even when the electrode is of the highly basic type. Such electrodes must generally be connected to the negative pole of a conventional welding power source in order to transfer fine droplets (of a diameter about the same as or only slightly larger than the diameter of the wire) as opposed to coarse globules. Welding with the electrode positive is known to reduce hydrogen and, especially when welding without an external gas shield, nitrogen levels in the deposited metal.

Fig. 3 shows apparatus according to the invention with a power source 10 with one terminal connected to a workpiece 11. The other terminal is connected to an electrode 12.

EXAMPLE 1 A basic flux-cored wire with a mild steel sheath was used in which more than 90% by weight of the non-metallic fluxing constituents of the core comprised calcium carbonate and calcium fluoride. Using a conventional welding power source and a 1.2mm diameter wire, a vertical weld was made in 20mm thick plate at a current of 125A and 6 layers were required to complete the joint. Considerable spatter was experienced.

EXAMPLE 2 Using the same wire, a welding power source as described above was programmed to give a pulse voltage of 36V and duration of 2ms and a background current of 80A with a time of 12ms. At a wire feed speed of 4.2m/s, the pulse current generated was approximately 390A and the mean current 125A. A vertical weld was made using the same joint preparation as before but this time in 25mm thick plate. The joint was still completed in 6 layers. Good fusion was easier to achieve and spatter was greatly reduced.

Claims (8)

1. A method of electric arc welding using a consumable tubular filled electrode in which the arc is sustained by a power source providing a series of substantially constant voltage pulses.

2. A method as claimed in Claim 1, wherein the filling of the electrode is a basic flux.

3. A method as claimed in Claim 1 or 2, wherein each pulse has a slope not exceeding 5 volts per 100 amps up to a pre-determined current cut-off.

4. A method as claimed in any preceding claim, wherein the background current for maintaining the arc between pulses is adjusted to lie between 30 and 100 amps.

5. A method as claimed in any preceding claim, wherein the time between pulses is substantially greater than the duration of each pulse.

6. A method as claimed in any preceding claim, wherein the relative movement between the workpiece and the torch through which the electrode passes has a substantial vertical component.

7. A method as claimed in any preceding claim, wherein the electrode is connected to the positive pole of the power source.

8. A method of electric arc welding substantially as herein described with reference to Example 2.