WO1996027471A1

WO1996027471A1 - Smaw with continuous high frequency

Info

Publication number: WO1996027471A1
Application number: PCT/US1996/002214
Authority: WO
Inventors: Larry W. Raper
Original assignee: The Babcock & Wilcox Company
Priority date: 1995-03-08
Filing date: 1996-02-16
Publication date: 1996-09-12
Also published as: FI974362A; CA2218313A1; CN1181723A; FI974362A0; EP0813460A1; AU4927596A; EP0813460A4

Abstract

A method of electric arc welding which reduces startup porosity and arc wandering comprises applying a current between an electrode (14) and a workpiece (22) to be welded, alternating the current at a frequency of about 1 to about 1.5 megahertz, supplying a shielding gas (30) generated from a flux covering (28) of the electrode (14) to a vicinity between the electrode (14) and an area of the workpiece (22) to be welded, and bringing the electrode (14) sufficiently close to the area to be welded to strike an arc between the electrode and the area to be welded without touching the electrode (14) to area to be welded.

Description

SMAW WITH CONTINUOUS HIGH FREQUNCY

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates, in general, to welding, and in particular, to a new and useful method of shielded metal arc welding (SMAW) using high-frequency alternating current.

2. DESCRIPTION OF THE RELATED ART

Arc welding can be achieved by using either direct current (DC) or alternating current (AC) . DC is used for conventional welding, while AC is used when difficult to weld metals, such as aluminum, are involved. AC welding is generally used in conjunction with gas shielding and conventionally does not exceed frequencies of about 200kHz. See U.S. Patents 3,818,177 and 3,894,210. U.S. Patent 4,507,543 also teaches a pulse arc welding method where peal- current pulses range between 180-200A, at a pulse frequency of 5Hz . Higher frequencies of 5-31.7Hz can be achieved at lower average currents of 20A to 100A. Lower frequencies and generally higher currents are also disclosed in references such as U.S. Patent 5,030,812 and 5,130,514. U.S. Patent 4,366,362 discloses a TIG welding process which utilizes DC current pulses, having a frequency of 30-300Hz. Further, a gas metal arc welding process with a frequency range of 8-500Hz is disclosed in U.S. Patent 5,221,825.

SUMMARY OF THE INVENTION

An object of the present invention is to utilize extremely high frequency pulses at start-up and during a welding process, in particular shielded metal arc welding (SMAW) .

In conventional shielded metal arc welding, the arc is struck by tapping the end of the electrode on the workpiece near the point where welding is to begin, then quickly withdrawing it a small distance to produce an arc of proper length. Another technique for striking the arc is to use a scratching motion similar to that used for striking a match. When the electrode touches the work there is a tendency for it to stick to the workpiece. The purpose of the tapping and scratching motion is to prevent this. When the electrode does stick, it needs to be quickly broken free, otherwise it will overheat and attempts to remove it from the workpiece will only bend the hot electrode.

With the use of the continuous high-frequency of the invention, the electrode does not come into contact with the workpiece at all. According to the present invention, it has been found that drastically increasing the frequency of the pulses during an SMAW process improves the start-up and sustained characteristics of the welding process, as well as avoiding porosity at the beginning of a weld bead, which has historically been a problem associated with SMAW. It has been found that according to the present invention, arc start-up and arc stability is maintained in an extremely simple fashion, no visible porosity is present, and further subsequent tests have shown the integrity of welds produced according to the present invention.

The Effect of continuous high-frequency is to produce a "stiff" welding arc having exceptional directional properties and stability. Arc outage (that is extinguishing of the arc) and wandering (that is undesired lateral displacement of the arc) are also greatly reduced or completely eliminated by the present invention.

According to the present invention, a welding arc current of between about 50 to about 425A is utilized, depending on electrode size, with a frequency of from about 1 to about 1.5MHz being utilized. Below 1MHz the advantages of the invention may not be achieved, and above 1.5MHz other problems occur such as skin irritation.

With the invention, an initial current of as little as 100mA is required to bridge the air gap and begin the arc.

In addition to these advantages, by never having to contact the electrode against the workpiece, the problems of sticking electrodes is completely avoided. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Fig. 1 is an illustration of a welding apparatus employed to carry out the present invention.

Fig. 2 is a schematic illustration, displaying in cross section, a weld being made in a workpiece in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to Fig. 1, the invention illustrated therein comprises an SMAW process utilizing an electrode holder generally designated 10 having an insulated handle 12. Electrode holder 10 is connected to a flux covered electrode or welding rod or wire 14 which is fed manually. A high- frequency power supply 16 has two terminals, one of which is electrically connected at 18 to the electrode holder 10 and the other at 20 to a workpiece 22.

By operating power supply 16, an arc 24 (see Fig. 2) is struck between the tip of electrode 14 and the workpiece 22 at weld area 26. The arc 24 melts the electrode 14 and causes the flux covering 28 to produce a shielding gas 30. The core metal of the electrode 14 is deposited on workpiece 22 beneath the shielding gas 30 and a weld 32 is thereby produced. According to the invention, the power supply 16 is, for example, a Miller Syncrowave 350 constant current AC/DC welding power supply, specially equipped with high-frequency capacity using known electrical techniques for increasing the cadence of the pulses sent out by the power supply. The material selected for electrode 14 was mil 7018 and mil 8N12 and mil 9N10, one eighth inch diameter welding electrode, during tests of the invention. The arc was started easily, was stable and produced a cleaning action. No visible porosity was present during the start of the weld. This is contrary to the history of mil 7018 which is known to produce starting porosity.

The electrodes of mil 8N12 and mil 9N10 also were used to check stability of the arc for out of position welding. As noted above, a stiff welding arc was produced with exceptional directional properties, little or no outage and little or no wandering. As a test of the invention, weld beads were made on carbon steel plate using mil 7018 with continuous high- frequency arc of the invention, and also without the high frequency, for comparison. The plates had several starts and stops to test the start-up characteristics. The welds were 15 inches long by 1 inch thick for UT, RT and destructive testing. No grinding was performed on the starts and stops. The continuous high frequency test of the invention was UT inspected and acceptable to NAVSEA 250-1550-1 PER 12-1-UT42 REV.7. The RT inspection was acceptable to NAVSEA 250-1500-1 PER 12-1-RT12 REV.2. A sample was also welded using the continuous high-frequency technique of the invention, in a self-induced magnetic filed to initiate arc blow. The arc blow was minimal to non-existent. The continuous high frequency appears to balance the magnetic fields around the arc .

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles .

Claims

CLAIMS WHAT IS CLAIMED IS;

1. A method of electric arc welding which reduces start-up porosity and arc wandering, comprising: applying a current between an electrode having an outer flux covering and a workpiece to be welded; alternating the current at a frequency of about 1 to about 1.5 megahertz; supplying, to a vicinity between the electrode and an area of the workpiece to be welded, a shielding gas produced by melting the outer flux covering of the electrode with the current; and bringing the electrode sufficiently close to the area to be welded to strike an arc between the electrode and the area to be welded without touching the electrode to the area to be welded.

2. A method according to Claim 1, including continuously maintaining the frequency of 1 to 1.5 megahertz throughout the welding operation.

3. A method according to Claim 2, wherein the current is supplied in a range of about 50 to about 425 amps.

4. A method according to claim 1, including SMAW with the invention.

5. A method according to claim 1, including using mil 7018 as the electrode.

6. An apparatus for electronic arc welding which reduces start-up porosity and arc wandering while facilitating striking of an arc, comprising: an arc welding electrode having an outer flux covering which serves as a means for supplying a shielding gas in the vicinity of an area of a workpiece to be welded; means for holding the arc welding electrode in the vicinity of an area of a workpiece to be welded; and a power supply connected to the means for holding the electrode and the workpiece for applying a high-frequency alternating current between an electrode held by the means for holding and the workpiece at a frequency of about 1 to 1.5 megahertz and at a current level of about 50 to 425 amps.