Classification of electrodes - Welding Consumables .1 Introduction

Welding Consumables

7 Welding Consumables .1 Introduction

7.4 Classification of electrodes

Characteristics of basic electrodes are:

 Basic slag forms when the covering melts reacts with impurities, such as sulphur and phosphorus and also reduces the oxygen content of the weld metal by de-oxidation.

 Relatively clean weld metal that is deposited gives a very significant improvement in weld metal toughness (C-Mn electrodes with Ni additions can give good toughness down to -90°C).

 Can be baked at relatively high temperatures without any of the compounds present in the covering being destroyed, thereby giving low moisture content in the covering and low hydrogen levels in weld metal.

 To maintain the electrodes in a low hydrogen condition they need to be protected from moisture pick-up by:

Baking before use (typically at ~350°C), transferring to a holding oven (typically at ~120°C) and issued in small quantities and/or using heated quivers (portable ovens) at the work station (typically ~70°C).

Use of vacuum packed electrodes that do not need to be re-baked before use.

 Basic slag is relatively viscous and thick which means that electrode manipulation requires more skill and should be used with a short arc to minimise the risk of porosity.

 Surface profile of weld deposits from basic electrodes tends to be convex and slag removal requires more effort.

7.3.3 Metal powder electrodes

Contain an addition of metal powder to the flux coating to increase the maximum permissible welding current level. Thus, for a given electrode size, the metal deposition rate and efficiency (percentage of the metal deposited) are increased compared with an electrode containing no iron powder in the coating.

The slag is normally easily removed. Iron powder electrodes are mainly used in the flat and H/V positions to take advantage of the higher deposition rates.

Efficiencies as high as 130-140% can be achieved for rutile and basic electrodes without marked deterioration of the arcing characteristics but the arc tends to be less forceful which reduces bead penetration.

7.3.4 Applications of basic electrodes

Basic electrodes have to be used for all applications that require good fracture toughness at temperatures below -20°C.

To avoid the risk of hydrogen cracking basic electrodes have to be used for welding hardenable steels (most C-Mn and all low alloy steels) and for most steels when the joint thickness is greater than about 15mm.

7.4 Classification of electrodes

National standards for electrodes that are used for welding are:

EN 2560 Covered electrodes for manual metal arc welding of non-alloy and fine grain steels.

AWS A5.1 Specification for carbon steel electrodes for shielded metal arc welding.

AWS A5.5 Specification for low-alloy steel electrodes for shielded metal arc welding.

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Electrode classification is based on tests specified by the standard on weld deposits made with each type of covered electrode. The standards require chemical analysis and mechanical tests and electrode manufacturers tend to dual certify electrodes, wherever possible, to both the European and American standards.

7.4.1 EN 2560

EN 2560 - Covered electrodes for manual metal arc welding of non-alloy and fine grain steels (see Figure 7.2).

This is the designation that manufacturers print on to each electrode so that it can be easily identified. The classification is split into two sections:

Compulsory section - this includes the symbols for:

 Type of product.

 Strength.

 Impact properties.

 Chemical composition.

 Type of electrode covering.

Optional section - this includes the symbols for:

 Weld metal recovery.

 Type of current.

 Welding positions.

 Hydrogen content.

The designation, compulsory (strength, toughness and coating including any light alloying elements) must be identified on the electrode, however the optional (position, hydrogen levels, etc are not mandatory and may not be shown on all electrodes.

Figure 7.1 Covered electrode with manufacturers designation.

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Figure 7.2 Electrode classification system of EN 2560.

7.4.2 AWS A5.1/5.1M: 2003

AWS A5.1/5.1M: 2003 - Specification for carbon steel electrodes for shielded metal arc welding (see Figure 7.3).

This specification establishes the requirements for classification of covered electrodes with carbon steel cores for MMA welding. Requirements include mechanical properties of weld metal; weld metal soundness; and usability of electrodes.

Requirements for chemical composition of the weld metal, moisture content of low hydrogen electrodes, standard sizes and lengths, marking, manufacturing and packaging are also included.

A guide to the use of the standard is given in an appendix. Optional supplementary requirements include improved toughness and ductility, lower moisture contents and diffusible hydrogen limits.

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The AWS classification system has mandatory and optional designators and requires that both the mandatory classification designators and any optional designators be printed on each electrode.

The last two digits of the mandatory part of the classification are used to designate the type of electrode coating/covering and examples of some of the more widely used electrodes are shown in Table 7.1.

Table 7.1 Examples of some of the commonly used AWS A5.1 electrodes.

AWS A5.1

classification Tensile strength,

N/mm2 Type

E7024 Rutile high recovery

Figure 7.3 Mandatory classification designators.

Designates: An position the type of covering the kind of current

Typical electrode to AWS A5.1

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Table 7.2 Common electrodes that classified to BS EN 2560 and AWS A5.1 / 5.5.

General description EN 2560 AWS A5.1 / 5.5

Cellulosic electrodes (For vertical-down welding Stovepipe welding

of pipeline girth welds)

E 38 3 C 21

* P = specially designated piping electrodes

Rutile electrodes

(For general purpose fabrication of low strength steels – can be used for all positions except vertical-down)

E 38 2 R 12 E 42 0 R 12

E6013 E6013

Heavy coated rutile electrodes (Iron-powder electrodes)

(For higher productivity welding for general fabrication of low strength steels – can generally only be used for downhand or standing fillet welding)

E 42 0 RR 13 E 42 0 RR 74

E6013 E7024

Basic electrodes

(For higher strength steels, thicker section steels where there is risk of hydrogen cracking; for all applications requiring good fracture toughness)

* Vertical-down low hydrogen electrodes

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In document CSWIP 3.0 New Book (Page 99-104)