WELDING SHIELDING GAS
WELDING SHIELDING GAS PER AWS A5.32
What is a Welding Shielding Gas?
A welding shielding gas is a gas or a blend of gasses used to protect the molten weld pool from atmospheric contamination during welding. The gas may be supplied in either gaseous or liquid form, but when applied to the weld during welding it is always in the gaseous form. The gas provides a blanket of inert gas that provides a protective layer between the weld and atmospheric contamination.
Why are Shielding Gases Necessary for Welding?
Most metals, when heated to their melting point become susceptible to the absorption of nitrogen, oxygen, and hydrogen which leads to porosity, or embrittlement in the weld. Porosity is the presence of gas pores or voids in the weld after it has solidified. Porosity is an undesirable discontinuity that renders the weld inferior and may inhibit the ability of the weld to meet engineering and quality requirements.
What Code Governs Welding Shielding Gas?
AWS A5.32 and ASME SFA 5.32 are the specifications which govern American weld shielding gas supplies.
ISO 14175 includes gasses and gas mixtures for European standards.
Weld Shielding Gas Acronyms
Individual gas components are identified by the following:
Ar - Argon
C - Carbon Dioxide
H - Hydrogen
N - Nitrogen
O - Oxygen
He - Helium
Shielding Gas Classification
The shielding gas Classification is formed by the number of this standard, followed by the symbol for the gas or gas mixture (main group + sub-group)
The letter codes and numbers used for the main groups are:
MAIN GROUPS
I : Inert gases and mixtures of inert gases
M1, M2, M3 : oxidizing mixtures containing oxygen and/or carbon dioxide
C : highly oxidizing gas and highly oxidizing mixtures
R : reducing gas mixture, containing hydrogen
N : low reactive gas or reducing gas mixtures, containing nitrogen
O : oxygen
G : gas mixtures containing components not listed or mixtures outside the composition ranges listed in SFA-5.32 Table 2
SUB-GROUPS
The main groups (except G) are divided into sub-groups based on the presence and level of different shielding gas components having an influence on the reactivity of the shielding gas.
Shielding Gas Designation
The shielding gas Designation is formed by the “Classification” + the symbols for all chemical components + the nominal composition in volume percent.
Weld Process Shielding Gas Sources
GTAW, PAW, GMAW, FCAW, and EGW welding processes rely on compressed shielding gas to be delivered to the weld pool as part of the function of the welding equipment.
SMAW, SAW, and ESW welding processes rely on the gaseous atmosphere created from the flux as it is consumed during the welding process to protect the weld.
Welding Processes and the Source of Weld Shielding
Additional Weld Shielding Gas Features
In addition to atmospheric protection, shielding gas can also have a pronounced effect on the following welding attributes.
Arc Characteristics
Wetting or surface tension conditioning
Easier arc initiation
Ionization potential of the air gap between the electrode and work
Shape and depth of weld penetration profile
Spatter reduction
Weld metal mechanical properties
Cleaning action
Mode of Metal Transfer (GMAW or FCAW)
The ionization potential of the shielding gas influences the ease of arc initiation and stability. Thermal conductivity of a gas determines the voltage and energy constant of the arc.
Inert Shielding Gas
An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. For application in welding this means that the Inert gasses are those that do not actively react in the welding puddle.
SFA-5.32 definition of Inert Gas: A gas or gas mixture that does not react chemically with filler material, base metal, or weld metal.
Argon, and Helium are examples of an inert gas.
Reactive and Oxidizing Shielding Gas
For application in welding, reactive and oxidizing gasses are those that react in the high temperature welding puddle. Oxidizing gasses are often included as part of a blend, but can also be a major or sole component. For example 100% CO2 is used with E71T-1C FCAW wires which are designed for its use.
SFA-5.32 definition of Oxidizing Gas: A gas or gas mixture that reacts chemically with filler material or base metal and creates oxides.
SFA-5.32 definition of Reactive Gas: A gas or gas mixture that reacts chemically with filler material, base metal, or weld metal.
SFA-5.32 definition of Reducing Gas: A gas or gas mixture that reacts chemically with filler material, base metal, or weld metal by combining with oxygen and removing or depleting the oxygen.
The popularity of carbon dioxide is most often due to its lower cost per unit volume. The lower cost per unit of gas does not automatically translate to the lowest cost per foot of deposited weld and is greatly dependent on the welding application. The final weld cost with carbon dioxide shielding gas is influenced by bead contour, electrode spatter, and spatter removal. The lower deposition efficiency for carbon dioxide caused by fume and spatter loss will influence the final weld cost.
Carbon Dioxide, and Oxygen are examples of a reactive and oxidizing gas.
Shielding Gas Safety
Safety is of high importance when working with weld shielding gas as they displace the oxygen needed for respiration, and can cause asphyxiation. The air we breathe is roughly 21% Oxygen & 78% Nitrogen with the balance being miscellaneous other gasses. Breathing air with less than 18% Oxygen will cause dizziness, unconsciousness, and eventually death.
Shielding gas freely enters the localized atmosphere where the shielding gas is being applied. Heavier gasses such as Argon can pool, or build up in low areas in much the same way water would. Precautions must be taken to prevent the accumulation of Argon as it displaces Oxygen and will lead to dangerous or deadly breathing conditions. Ensure that adequate ventilation is provided in all work areas, but especially in confined spaces. Follow all safety procedures and best safety practices when working with shielding gasses.