Electric heating – Metal heating – Nonatmospheric environment at hot spot
Reexamination Certificate
1999-10-28
2001-10-16
Shaw, Clifford C. (Department: 1725)
Electric heating
Metal heating
Nonatmospheric environment at hot spot
C219S13700R
Reexamination Certificate
active
06303891
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to gas shielded electric arc welding, and more particularly to a universal shielding gas composition for use in gas shielded gas metal arc welding (GMAW) and flux core arc welding (FCAW) processes and to an improved gas shielded metal arc welding process.
2. Brief Description of the Prior Art
In the Gas Metal Arc Welding (GMAW) process, heat for welding is produced by forming an arc between a continuous consumable wire electrode and the workpiece which melts the electrode to form the weld bead. The wire electrode is constantly driven through a welding gun and the weld pool or puddle is protected by an atmosphere of shielding gas, also delivered to the arc through the welding gun. The shielding gas shrouds the weld pool and is effective in preventing the pool and the subsequent weld from being oxidized or corroded by air or other ambients. A similar process, Flux Cored Arc Welding (FCAW) uses a wire with a central core of protective flux.
The manner, or mode, in which the metal transfers from the electrode to the weld pool largely determines the operating features of the process. There are three principal metal transfer modes; short circuiting, droplet or spray, and pulsed. Short-circuiting and pulsed metal transfer are used for low current operation while spray metal transfer is used with high welding currents.
In addition to general shielding of the arc and the weld pool, the shielding gas performs a number of important functions: forms the arc plasma, stabilizes the arc roots on the material surface, and ensures smooth transfer of molten droplets from the wire to the weld pool.
Each of the gases, and the relative ratio, in a shielded gas mixture also has a substantial effect on the welding operation. For example, argon affects the arc transfer, ionization potential, and arc plasma. Carbon dioxide affects penetration, allowable arc voltages and, depending upon the amount present, will determine the type of material on which the gas can be used and whether it can be short circuit, spray or pulse welded. oxygen affects thermal conductivity, heat transfer, weld puddle fluidity in short circuit welding, and wetting of the toe of the weld in spray and pulse arc welding. Helium has typically been used to affect heat input.
Thus, the shielding gas will have a substantial effect on the stability of the arc and metal transfer and the behavior of the weld pool, in particular, its penetration. Most shielding gases for GMAW welding are either mixtures of argon and oxygen or argon and carbon dioxide, and some gas mixtures may contain helium.
Table 1 below is a list of common shielding gases and their proportions, by volume, that are used for conventional GMAW welding processes:
TABLE 1
WELDING PROCESS AND
Carbon
MATERIAL
Oxygen
Argon
dioxide
Helium
GMAW spray arc welding/stainless
1%-2%
98-99%
steel and carbon steel
GMAW spray arc welding/stainless
5%
95%
steel and carbon steel (Increased
heat and oxidizing)
GMAW High-Speed spray arc
8%
92%
welding/stainless steel and carbon
steel
GMAW spray arc and pulse arc
80-85%
15-20%
welding/carbon steel
GMAW short circuit and FCAW
75%
25%
welding/carbon steel
GMAW short circuit welding/
8%
2%
90%
stainless steel
Typical argon/oxygen mixtures previously used for GMAW spray arc welding on both carbon steel and stainless steel have contained, by volume, from 1% to 8% oxygen and from 92% to 99% argon, and no carbon dioxide. Typical argon/carbon dioxide mixtures previously use for GMAW spray arc and pulse arc welding on carbon steel have contained from 15% to 20% carbon dioxide and 80% to 85% argon, and no oxygen. Typical argon/carbon dioxide mixtures previously used for GMAW short circuit and FCAW (flux core) welding on carbon steel have contained 25% carbon dioxide and 75% argon, and no oxygen. The typical argon/carbon-dioxide/helium mixtures previously used for GMAW short circuit welding on stainless steel have contained 2% carbon dioxide, 8% argon, 90% helium, and no oxygen.
It can be seen that a different shielding gas composition having a critical ratio of gases is required for carbon steel and for stainless steel, and for each mode of metal transfer.
Some suppliers provide tanks or containers of shielding gases in the critical proportions as listed above, plus other mixtures, which requires the welder to have available, and often transport, a large number of heavy tanks in order to have the proper shielding gas for a welding operation. Prior to welding, the welder must determine what the proper mixture should be for the particular job at hand, and then obtain two or more tanks containing the gases, for example argon, oxygen and carbon dioxide, or select the tank containing the proper proportion of premixed gases.
Mixing of the shielding gases from the tanks is typically accomplished by a using a conventional gas mixer into which the two or more gases are fed from independent gas tanks. The welder must set the mixer and tank regulator to establish the gas mixture in the critical proportions, which is then fed to the gun or torch.
Poor gas shielding can result in porosity of the weld. Porosity is formed by entrapment of discrete pockets of gas in the solidifying weld pool. The gas may originate from the shielding gas, surface contaminants such as rust or grease, or insufficient deoxidants in the parent metal, electrode or filler wire.
The proportion of the gases contained in a shielding gas can also alter the chemical composition of the weld metal, which may lead to weld imperfections. For example, it has been found that changing the proportion of oxygen and carbon dioxide relative to the argon content by just 1.0% can cause the carbon dioxide to mix with the solidifying weld pool such that carbon content of the weld metal will be higher or lower than the base metal.
There are several patents which disclose various shielding gas mixtures that include oxygen, carbon dioxide, and argon (or an argon/helium mixture) in critical ratios.
Church, U.S. Pat. No. 4,463,243 discloses an electric arc welding gas system utilizing a welding gas comprising a mixture of from 40% to 70% argon, from 25% to 60% helium, from 3% to 10% carbon dioxide and from 0.10% to 1% oxygen and utilizing electrode currents in the range of 100 to 1100 amperes so as to form electrode metal globules of at least the size of the electrode diameter at the rate of 400 to 1200 globules per second.
Lebel, U.S. Pat. No. 4,529,863 discloses a metal-arc welding method that is particularly useful for out-of-position welding or vertical, inclined and overhead welds which utilizes a shielding gas formed of ratios of a minor proportion of a carbon dioxide and oxygen mixture combined with a major proportion of an argon and helium mixture. The approximate volumetric relationships of the gases are: carbon dioxide between about 2.5-8.5%, oxygen between about 0.1-0.8%, helium between about 25-60%, and argon the remainder, with the ratio between carbon dioxide and oxygen approximately 10:1 to about 20:1.
Church, U.S. Pat. No. 4,572,942, discloses a gas-metal-arc welding process utilizing a shielding gas mixture that produces a stable plasma formation. The gas is a mixture of major proportions of each of argon and helium and minor proportions of each of carbon dioxide and oxygen, which produces a stable, approximately dome-like plasma formation in the arc gap between the electrode melting end and the weld deposit. The gas comprises essentially between about 40-70% argon, between about 25-60% helium, between about 3-10% carbon dioxide and between about 0.1-2% oxygen.
Hilton, U.S. Pat. No. 4,626,646, discloses a shielding gas for electric arc welding which is a mixture, by volume, of 1.75% to 2.25% carbon dioxide and 0.25% to 1.0% oxygen with the remainder being helium and argon. Use of the gas enables slag-free welds that readily accept paint applied by an electrostatic deposition process.
De Vito, U.S. Pat. No. 4,645,903 discloses a gas metal arc welding process that extends the axial spray metal transfer range with a
Roddy Kenneth A.
Shaw Clifford C.
Taylor Robert B.
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