To understand why having the correct shielding gas is important there is a need to understand the requirements of the relevant standards, and the influence a shielding gas can have on a welded joint.
Shielding gases and gas mixtures
In the suite of AS/NZS 1554 welding fabrication standards, the clauses regarding shielding gases/gas mixtures generally state: a gas or gas mixture used for shielding during arc welding shall be of a welding grade and comply with the requirements of AS 4882, and shall be suitable for the intended application. The AS/NZS 1554 suite of standards have been around since 2014 and are currently under review.
In September 2022 the new standard: AS/NZS ISO 14175 2022: Welding consumables – Gases and Gas Mixtures for fusion welding and allied processes, was introduced and this standard has been identified as the preferred method for specifying/classifying shielding gases/gas mixtures
Section 5 of AS/NZS 14175 discusses the Classification and Designation of shielding gases and states that – Gases and Gas Mixtures shall be classified by the number found in this international standard, followed by the symbol for the gas in accordance with Table 2
The main groups of gas/gas mixtures identified by letter codes and numbers in Table 2 are:
- I: Inert gases and gas mixtures
- M1, M2 and M3: oxidising mixtures containing oxygen and/or carbon dioxide
- C: highly oxidising gas and highly oxidising mixtures
- R: reducing gas mixtures
- N: low reactive gas or reducing gas mixtures, contain nitrogen
- O: oxygen
- Z: gas mixtures containing not listed or mixtures outside the composition ranges listed in Table 2
Table 2 further breaks down the main groups into sub-groups by nominating the range of minor components (CO2, O2, Argon, Helium, H2, N2) that make up a gas mixture.
Example: AS/NZS ISO 14175 M21 covers all gases that are a mixture of 15-25% CO2 blended with 75-85% Argon as the balance
In some cases, a shielding gas can be a blend of more than two gases.
Example: AS/NZS ISO 14175 M24 covers all gases that are a mixture of 5-15%CO2 + 0.5-3%O2 blended with Argon as the balance
In shielding gases documentation (Welding Procedure Specifications, Product Data Sheets) the composition can be identified within the shielding gas Classification/Specification as in the below example of a shielding gas mixture containing 12% CO2 and 2% O2 bended with Argon as the remaining gas.
Classification: AS/NZS ISO 14175 – M24
Designation: AS/NZS ISO 14175 – M24 – ArCO – 12/2
The different components/and percentage used in a shielding gas/gas mixtures can have a major influence on the final weld chemical composition, mechanical properties and weldability of the consumable. The choice may influence:
- Depth of penetration
- Arc stability
- Spatter generation
- Oxidization potential
- Side wall fusion
- Weld profile
- Element burn-off (losing chemical elements from the parent metal)
- Weld pool fluidity
- Weld fume generation
Welding consumables (wires)
When a welding consumable manufacture produces a welding electrode (wire) they will specify in the consumables classification/designation the gases that are suitable for use within this welding process. The standard to which these consumables have been manufactured will define the range of shielding gases that can be used.
- Example of a flux cored wire prequalified with CO2(C) shielding gas
Classification: AS/NZS 17632 – B
Specification: T49 3 T12 C A U H10
2) Example of a flux cored wire prequalified with Argon/CO2(M21) shielding gas mixture
Classification: AS/NZS 17632 – B
Specification: T49 3 T12 M21 A U H10
Some welding wires are designed to operate with only one specific shielding gas, whereas some may be used with a variety of shield gases, depending upon the consumable standard, the required mechanical properties and welder usability for the combination.
Example of a solid Gas Metal Arc Welding (GMAW) wire usable with a range of gases
Classification: AS/NZS 14341 – B
Specification: G 49A 3U C1/M21/M24 S6
The consumables (wire) product data sheet will usually identify the range of gas/gas mixtures and flow rates to be used with their product.
Other Standards that can influence the shielding gas selection
In AS/NZS 1554 Part 1/Part 5/Part 7 a consumable may be accepted as prequalified if it has a Ships Classification Societies’ Approval matching the minimum grades identified in table 4.6.1(A) applicable columns. Under the construction/fabrication standards NZS 3404 and AS/NZS 5131 it specified that joints welded that are subject to earthquake loads or effect, fabricated from steel types 2S,5S,3 and 6, the welding consumable shall have a Ships Classification Societies’ Approval of grade 3.
In cases where a Ships Classification Societies’ Approval is used to meet the requirements of a standard, the shielding gas used for the joint MUST be the same as the shielding gas used in achieving the approval. This information can usually be found on the Ships Classification Societies’ Approval website.
Changing shielding gas/gas mixtures or suppliers and effect on Welding Process Specifications (WPS)
Not all shielding gases/mixtures perform the same and some are preferred for specific applications.
Different shielding gas manufacturers offer different ranges of products that may or may not fully align with your current WPS or some wire consumable manufacturers recommended gas options.
If you are welding to a WPS, it will have been developed to align with a welding standard such as AS/NZS 1554 Part1. Within the standard there will be an allowable tolerance for changes in the shielding gas component percentages, outside which the WPS will require requalification e.g. Table 4.11(D). In this table the allowable variation is based on the minor component and is:
- For minor gas component between ≥5% to <50% the allowance is ±10% relative
e.g. WPS was 16%CO2 so variance equals ± 1.6% CO2 or 14.6% to 17.6% CO2
- For minor gas component <5% the allowance is ±0.5% relative
e.g. WPS was 4%CO2 so variance equals ± 0.5% CO2 or 3.5% to 4.5% CO2
If the new gas mixtures minor component(s) fall outside the calculated tolerances, or the new mixture has an additional/removed component from that used when developing the WPS you will be required to requalify the WPS as per the welding standards requirements
Checklist for selecting the correct shielding gas/gas mixture
1) Identify the fabrication standard you are welding under e.g. AS/NZS 1554.1: 2014
2) Identify the prequalified consumable for the steel type e.g. For Steel type 2s in Table 4.6.1 (A), Column 4, Flux Cored (AS/NZS 17632), B-T493U
3) Find a wire consumable that matches the classification requirements e.g. AS/NZS ISO 17632 B T49 3 T1A C/M U H10
4) Check the consumable manufacturer’s Product/Technical Data Sheet (PDS/TDS) to confirm recommendation shielding gas e.g. C=CO2, M=M21 (15-25% CO2 blended with remainder Argon)
5) If welding to a construction standard where seismic load restrictions may be applicable (NZS 3404, AS/NZS 5131), check if the consumable manufacturer’s PDS/TDS shows a Ships Classification Societies’ Approval (LR, DNV, BV, ABS etc)
6) Refer to the Ships Classification Societies website and in their online register, confirm the shielding gas that was used in the approval process. Note: the approval is specific to the factory the consumable was manufactured in, and may only qualify one of the consumable manufacturer’s recommended shielding gases e.g. CO2
7) Check the shielding gas supplier data sheets for a shielding gas/gas mixture matching the Ships Classification Societies’ Approval e.g. CO2
Summary
Shielding gas/gas mixtures can have a major impact on the final quality of a welded joint/connection.
The choice of shielding gases is usually determined by the welding consumable (wire) manufacturer.
If changing shielding gas mixtures/suppliers confirm the shielding gas/gas mixture composition still falls within the allowable tolerances of all applicable WPSs
If relying on a Ships Classification Societies’ Approval to meet compliance of a fabrication/construction standard, then the shielding gas must match the composition of the shielding gas/gas mixture that was used in the approval process
If in doubt, email the Welding Centre at HERA – Welding@HERA.org.nz
For other publications and reports check out the HERA website for:
- HERA Report R8-41: HERA Guide to Shielding Gases for Seismic Welds
- Welding consumable selection for structural welding – coming soon
