What is CO2 Shielded Arc Welding?

C02 shielded arc welding is a new welding technique which came into being in 1950 ’ s. For half a century, CO2 gas shielded welding has been widely used in shipbuilding, automobile manufacturing, petrochemical industry, agricultural machinery, etc.

As compared with other welding processes, C02 shielded arc welding is high in efficiency, low in cost and ready for automatization. It can ease the intensity of labour and improve the quality of welding. C02 shielded arc welding is mainly applied to low-carbon steels, low-alloy steels, etc.

With all its advantages C02 shielded arc welding will in many cases displace other types of welding processes. It promises a great prospect in welding 0.5-2.5mm sheets instead of gas welding and argon shielded arc welding.

Basic of CO2 Shielded Arc Welding

Gas shielded welding is a fusion welding method that uses an arc as a heat source. In the welding process, in order to obtain good quality welds, the welding area must be effectively protected against harmful gas intrusion in the air; in order to meet the needs of the welding metallurgical process, the form of gas protection is adopted. The use of gas-shielded arc welding can reliably guarantee the welding quality and make up for the limitations of manual arc welding.

Using an automatic wire feed mechanism to feed wire to the molten pool, an arc is formed between the welding wire and the workpiece, and the gas protection method of the melting pole under the protection of CO2 gas is called CO2 gas shielded welding, referred to as CO2 welding. It is an advanced fusion welding method that uses the CO2 gas sprayed from the nozzle to isolate the air and protect the molten pool. The welding process and working principle are shown in Figure 1.

Figure-1  Structure of CO2 Shielded Arc Welding Process

1- remote control box; 2- power supply; 3- decompression regulator; 4- gas cylinder; 5- wire feeder; 6- welding gun

CO2 gas shielded welding is an active gas shielded welding, so it is also called MAG welding or MAGC welding. The CO2 gas ejected from the nozzle decomposes into CO at high temperature and emits O2. Under welding conditions, CO2 and O2 will oxidize iron and other alloying elements, resulting in splashes and CO pores in the welding process. The higher the temperature, the higher the decomposition rate of CO2, the more O2 is released, and the more serious the problem is . Therefore, when performing CO2 gas shielded welding, measures must be taken to prevent the burning of alloying elements and other welding defects in the base metal and the welding wire.

Material of CO2 Shielded Arc Welding

There are many classification methods for CO2 gas shielded welding wire. According to the different manufacturing methods, it can be divided into solid cored wire and flux cored wire. Flux cored wire is divided into self-shielded flux cored wire and gas shielded flux cored wire. According to different materials to be welded, it can be divided into carbon steel welding wire, low alloy steel welding wire, stainless steel welding wire, non-ferrous metal welding wire, etc.

When selecting CO2 gas shielded welding wire, the chemical composition of the base metal of the weldment, the welding method, the mechanical properties of the welded joint, the degree of constraint of the welded structure, whether the weldment can be heat treated after welding, and the high temperature resistance and resistance of the weld metal Low temperature, corrosion resistance and other service conditions are considered comprehensively.

Wire Feed of CO2 Shielded Arc Welding

Selection of stainless steel welding wire. Stainless steel containing a certain amount of Cr and Ni elements is mostly used in the manufacture of boilers, petroleum industry oil refining equipment, synthetic chemical industry equipment, and high temperature and high pressure hydrogen resistant materials. During use, steel and welded joints are required to have chemical stability and sufficient creep resistance and long-term strength under the action of high temperature and pressure. When selecting the welding wire for Cr-Ni heat-resistant steel, the chemical composition and mechanical properties of the weld should be as consistent as possible with the base metal, so that the weld has a certain strength, good oxidation resistance and gas resistance at the working temperature Medium corrosion ability. Secondly, while considering the weldability of the welding wire, it should be avoided to choose welding wire with higher impurity content or too high strength. Usually MIG welding wires such as ER308, ER310, ER316 are used as welding materials for CO2 gas shielded welding.

Carbon dioxide as a Shielding Medium

Quite early in the development of gas metal arc welding(GMAW) a search was made for a cheaper gas than argon to use for steel, but of the commercially available gases many could be rejected immediately because they would cause metallurgical damage to the weld, e.g. loss of ductility, oxidation, gross porosity with nitrogen and porosity with hydrogen.

Apart from the slightly oxidizing nature of carbon dioxide (chemical symbol C02), which increases with temperature as the gas dissociates, the gas would appear attractive as it is inexpensive and in plentiful supply. With a suitable filler wire composition it would be expected to deposit weld metal of good mechanical properties like argon-oxygen shielding. Moreover, the flow characteristics of C02 are such that the gas would issue in a nonturbulent manner from a MIG gun and effectively sweep the area to prevent

contamination by the air. A suitable filler wire would therefore only have to be effective in compensating for oxygen and possibly carbon, instead of nitrogen, hydrogen and oxygen of the air.

The absence of a “finger” type penetration characteristic with C02 shielding would obviously be an added advantage over argon-oxygen mixtures for deep penetration welding.

There were however a number of problems which prevented C02 being used as a cheap alternative shielding gas for steel. At the time these limitations seemed characteristic of the gas for welding and were not really thought to be surmountable. The need to combat rising labour costs and also to produce more welders focused the need for a semi-automatic welding process for steel, and the potential of C02 led to considerable research activity.

Three basic problems had to be solved before C0₂ could be used for gas metal arc welding of steel and “C0₂ shielded arc welding” became an every-day welding process.

Porosity in the Welded Metal

If a reactive gas such as C02 shielding was used with the rimming steel filler wire normally used for covered electrodes, the weld metal contained extensive porosity. This was caused by the evolution of bubbles of carbon monoxide, which resulted from the reaction in the molten metal of the carbon in the steel with iron oxide or oxygen from the CO2 atmosphere.

C02 shielded arc welding wires now contain sufficient silicon, manganese and other elements to provide an “over-killed” weld composition so that porosity is not caused when C02 is used for gas metal arc welding of steel.

The Problems of Spatter

The manner in which droplets of metal are detached from the electrode in an atmosphere of C02 differs markedly from that in argon or argon-oxygen mixtures for the same welding conditions. Whereas in argon-oxygen the droplet rate is very rapid, and achieved at fairly low currents, droplets are only infrequently transferred in C02 and in a non-axial manner. By increasing the current to around 400A (with 1/16 in. diameter wire) an improvement is obtained, but these currents cannot be used for welding sheet thicknesses because abnormally high welding speeds would be required.

The “short circuiting arc” or ‘dip transfer” has now effectively improved the metal transfer characteristics in C02 at low currents.

Positional Welding

When C02 shielded arc welding was first tried for vertical and overhead welding, the high currents required to produce a smooth metal transfer resulted in a weld pool too large for the operator to control, so that metal ran out of the joint.

The “short circuiting arc” has now made positional C02 shielded arc welding a simple procedure because low currents can be used to make a small, quickly freezing weld pool; filler metal is added each time the wire tip automatically touches the weld pool.

These three problems were sufficient to inhibit the application of C02 shielded arc welding in the early 1950s but have now all been overcome by developments in filler wire composition and power source design. Industry all over the world has not been slow to apply this new process and it has now become the most popular semiautomatic process for welding steel. The overwhelming success of the semi-automatic form arises from its high metal deposition rate coupled with excellent arc visibility—a combination not previously achieved by any other semi-automatic system for welding steel.

C02 shielded arc welding is not, however, the universal process that is intended to replace more established conventional methods of welding steel. If a prospective purchaser of additional welding equipment had already built up years of experience in submerged arc welding and wished to augment his capacity for mechanized downhand welding of pressure vessels he would certainly not be advised to buy C02 shielded arc welding equipment,  since the submerged arc process retains its superiority for high quality welding over a range of plate thicknesses; thereafter electroslag welding becomes more attractive. If, however, fully mechanized machines were being designed to weld ferrules into sheet metal domestic radiators or put nozzles into vessels by multi-pass welding, or if there was a considerable amount of semi-automatic welding of brackets, stiffeners and details that could not be mechanized, then C02 shielded arc welding would be an attractive welding method.