Selecting the correct shielding gas and gas delivery system
Proper shielding gas selection is a critical step in successful welding, but setting the flow rate likewise is a necessary consideration—one that is frequently overlooked.
Requirements for selecting shielding gas, filler metal, and welding processes are well- documented to help welders ensure high quality and consistency in their welding applications. However, identifying the correct pressure and flow control system to use is just as vital to the process as the other components if you are trying to produce a finished product cost-efficiently.
Every fabricator should implement welding procedure specifications for their operations. These specifications should cover filler metal, shielding gas, welding machine (power source) as well as a gas delivery system that can supply the proper volume of shielding gas needed for the application.
The American Welding Society (AWS) developed the AWS 5.18 Specification for Carbon Steel Electrodes and Rods for Gas Metal Arc Welding (GMAW) classification system for identifying filler metals. These specifications are American National Standards Institute (ANSI-) approved. Following these guidelines can help fabricators produce a quality finished product.
The AWS A5.32 Specification for Welding Shielding Gases also is ANSI-approved and covers the chemical makeup of each gas and the possible impurities.
To maintain quality in a welding procedure, it is important that the gases supplied meet these specifications. Table 1 shows the requirements for the common shielding gases and the AWS classification and Compressed Gas Association (CGA) class for each gas.
|Table 1: Gas Type, Purity, and Dew Point for Shielding Gas Components|
|Argon||SG-A||99.997||10.5||Gas||Type I, G-11.1, Grade C||Liquid||Type II, G-11.1, Grade C||Carbon
|SG-C||99.8||32||Gas||G-6.2, Grade H||Liquid||G-6.2, Grade H||Helium||SG-He||99.995||15||Gas||Type I, G-9.1, Grade L||Liquid||Type II, G-9.1, Grade L||Hydrogen||SG-H||99.95||32||Gas||Type I, G-5.3, Grade B||Liquid||Type II, G-5.3, Grade A||Nitrogen||SG-N||99.9||32||Gas||Type I, G-10.1, Grade F||Liquid||Type II, G-10.1, Grade L||Oxygen||SG-O||99.5||N/A||Gas||Type I, G-4.3, Grade B||Liquid||Type II, G-4.3, Grade B|
|Note: Helium and hydrogen normally are supplied in gas state only;
all other gases can be supplied in either gas or liquid state.
Contact your gas supplier for any questions concerning adherence to these specifications; a quality gas supplier should be able to produce test results from individual cylinders or from one cylinder filled from a manifold. The test should verify the purity (or total impurities), the moisture content (sometimes specified as dew point), and the mix accuracy if the gases are a blend.
Gas Delivery System
Unlike the specifications for selecting filler metals and shielding gases, no specifications cover pressure and flow control equipment. Too little shielding gas flow can produce weldments lacking sufficient gas coverage of the weld area; too much shielding gas can blow away part of the weld puddle. In either case, if the proper shielding gas flow and pressure are not used, an inferior weld may result.
Items to consider when choosing a gas delivery system for your shielding gas application include:
- Type of gas or gases used. If you are welding a variety of materials or thicknesses, different gases or gas blends may be required.
- Mode or state in which the gases are supplied. As shown in Table 1, shielding gases may be supplied in gas or liquid form.
- Flow range of the gases used. Some applications require a wider range of flow rates than others.
To select the best system for the delivery of shielding gases, you must understand flow versus pressure and how each is controlled.
In all welding applications, the shielding gas should be supplied at a specific flow rate. For example, let's say the procedure requires 30 standard cubic feet per hour (SCFH) of an argon/carbon dioxide mixture for proper shielding gas coverage. The correct flow rate can be supplied in one of two ways:
- A set pressure through a predetermined orifice
- An adjustable orifice set to the desired flow rate
The following chart shows the orifice sizes for selected gases and flow rates. It is important that the correct pressure is supplied to the orifice to produce the required shielding gas flow rate.
|Table 2: Orifice Sizes and Flow Rates|
|Orifice Size||Argon||Helium||Carbon Dioxide||Helium 75/Argon 25||0.010||4||12||3.5||6.6||0.0135||6.3||20||6||11||0.016||8||25||7.5||14||0.018||10||31.5||9.5||17.5||0.021||11.4||36||10.5||20||0.025||15||47.5||14.3||26||0.025||20||63.2||19||35||0.028||22.5||72||21||40||0.0312||30||95||28||52.5||0.040||50||158||47.5||88|
|All flow rates are specified in standard cubic feet per hour (SCFH).|
|Note: These flow rates can be achieved when the set pressure to the orifice is 30 PSI. If the inlet set pressure (calibration pressure) is different, a pressure correction calculation must be used to determine the actual flow rate. The correction factor is determined by the square root of the quotient of the difference between the operating pressure and the calibration pressure in pounds per square inch atmosphere (PSIA). For example, if the calibration pressure is 30 PSI, but the operating pressure is 50 PSI, the correction factor would be 1.2.
Therefore, if the if the selected orifice is for 20 SCFH flow, the actual flow would be 24 SCFH (1.2 x 20 = 24 SCFH).
Adjustable Orifice. In the second method, which is more popular, the required gas flow is set from an adjustable flowmeter. If requirements change, the flowmeter can be adjusted.
This system requires a pressure control device (pressure regulator) and a gas flow meter. The regulator normally is set to a certain pressure, called calibration pressure or compensated pressure.
The flowmeter should be selected for the specific gases to be used-for example, a flowmeter with various gas scales and the desired calibration pressure listed ("This flowmeter is compensated at 30 PSI for the gases listed"). The regulator and flowmeter can be supplied together or as individual components.
When gases are supplied from high-pressure cylinders (3,000 or 4,500 PSI), the regulator-flowmeter combination is preferred; when gases are supplied via pipeline from liquid sources, individual pressure control and flow control equipment can be used. Always verify the pressure supplied to the flow control devices.
In summary, selecting the correct filler metal and shielding gas is crucial, but controlling shielding gas is equally important to ensure quality in your welding applications.