Shielding gases for arc welding and cutting (May 2001)

Lucas, DSc, PhD, CEng, FIM, FWeldI, is Consultant & Technology Manager, Arcs at TWI in the Metallurgy, Corrosion, Arc & Surfacing Group. He is also visiting Professor at Liverpool University.

In recent years, there have been substantial changes in the range of shielding gases available for arc welding and cutting. Equally significant has been the re-branding of gases by some of the major gas supply companies. The range of shielding gases currently available in the UK are listed in Table 1together with their designation in accordance with the requirements of BS EN 439: 1994, Welding consumables - Shielding gases for arc welding and cutting.

Classification of shielding gases

BS EN 439 classifies shielding gases into six main groups according to their chemical properties and reaction behaviour. The groups and the prefix letter are as follow:

The groups, apart from Group S, are then sub-divided with an Identification Number which specifies the components and the compositional ranges. The Identification Number and the components in percent volume are as follows:

R	reducing gas mixtures - those mixtures containing hydrogen
I	inert gases and mixtures - argon, helium and their mixtures
M	mixed gases - argon based gases with additions of oxygen or carbon dioxide or both
C	carbon dioxide rich mixtures
F	unreactive or reducing gases and mixtures - nitrogen and nitrogen based mixtures
S	specials - any gas or mixture not covered in the other groups

Table 2. Classification of shielding gases for arc welding and cutting

The shielding gases in Groups R and M containing helium have a suffix identifying the amount of helium in the mixture:

Symbol		Components in percent volume
Group	Identification No.	Oxidising		Inert	Reducing		Unreactive
		CO 2	O 2	Ar	He	H 2	N 2
R	1			Balance		> 0 to 15
	2			Balance		> 15 to 35
I	1			100
	2				100
	3			Balance	> 0 to 95
M1	1	> 0 to 5		Balance		> 0 to 5
	2	> 0 to 5		Balance
	3		> 0 to 3	Balance
	4	> 0 to 5	> 0 to 3	Balance
M2	1	> 5 to 25		Balance
	2		> 3 to 10	Balance
	3	> 0 to 5	> 3 to 10	Balance
	4	> 5 to 25	> 0 to 8	Balance
M3	1	> 25 to 50		Balance
	2		> 10 to 15	Balance
	3	> 5 to 50	> 8 to 15	Balance
C	1	100
	2	Balance	> 0 to 30
F	1						100
	2					> 0 to 50	Balance

(1) contains >0 to 33% helium
(2) contains >33 to 66% helium
(3) contains >66 to 95% helium

Applying the classification

The reaction behaviour of the shielding gas will determine its suitability for arc welding and cutting processes. For example, an oxidising gas is normally required to stabilise the arc in MIG/MAG welding but an inert or reducing gas is required for TIG and plasma processes.

Typical applications of the shielding gases are given in Table 3. The Classification Code can be used to differentiate between, for example, slightly oxidising (Group M1), intermediate (Group M2) and more pronounced oxidising gases (Group M3).

Table 3. Typical applications of shielding gases for arc welding and cutting

The Classification Code can be used to identify the shielding gas on the Welding Procedure Approval Record (BS EN 288) but the range of approval is normally restricted to the type of gas (nominal composition) used in the welding procedure test. In approval testing of welders to BS EN 287, the Classification Code is shown on the Manufacturer's Welding Procedure Specification (WPS) but the test will confer approval on the welder to use any similar shielding gas.

Group	Identification No.	Remarks	Typical applications
R	1	Reducing	TIG plasma arc welding, plasma arc cutting, back shielding
	2
I	1	Inert	MIG, TIG, plasma arc welding, back shielding
	2
	3
M1	1	slightly oxidizing ↓ More pronounced oxidation	MAG
	2
	3
	4
M2	1
	2
	3
	4
M3	1
	2
	3
C	1
	2
F	1	Unreactive	Plasma arc cutting, back shielding
	2	Reducing

Acknowledgements

The author would like to acknowledge the information provided for the table by:

A Williams	Air Products Plc
D Yates	BOC
R Eden	Linde Gas
G Bruton	Messer UK Ltd