Welding Electrodes selection

Electrodes selection

1)    Composition of base metal.

2)    Tensile strength of joint

3)    Thickness of base ,metal for current setting

      4)    Weld position (flat, vertical, overhead).

American welding society (AWS) process a uniform system of classifying welding electrodes.

Three AWS specifications are of primary interest to marine fleet.

1)    AWS A5.1 for Mild steel

2)    AWS A 5.5 for low alloy steel

3)    AWS A5.4 for corrosion resisting Cr, Cr-Ni steels.

Under AWS A 5.1

Electrodes are classified by tour (4) digit number,. Preceded buy the letter ‘E’ which signifies electrode. The first two digits specify tensile strength of deposited metal.

Eg E 70 XX will produced a weld metal deposit of 70,000 PSI minimum tensile strength.

The 3^rd digit indicates welding position for which electrode can be used.

E XX1X – ALL POSITION

EXX2X – FLAT+ HORIZONTAL

EXX3X – FLAT ONLY

EXX4X – VERTICAL ONLY

The 4^th digit indicates the type of welding for which electrode can be used as well as the type of coating. However designation is better d3escribed by considering last two digits to gether.

EXX10-DCRP 9ELECTRODE + VE ONLY)

EXX11 – AC OR DCRP

EXX12 – AC OR DCSP

EXX13- - AC OR DCSP

EXX 14 – AC OR DC RP//SP (IRON POWDER)

EXX 16 – AC OR DC RP (IRON POWDER, LOW H₂ )

Further speciation A 5.1 limits the total alloy content of EXXXX type of electrodes to 1.50% of cr, mn, ni, va

Low alloy steel electrodes

AWS specification A 5.5 may produce weld metal deposits with tensile strengths in exceeds of 100,000 Psi. In which case the designation will contains 5 numerical digits. Eg E 11018. In addition, an alpha numeric code may follow a dash (-) eg E 70 10 – A1

A1 – ½ % Mo

B1 – ½ % Mo, ½ % Cr

B2 – 1(1/4) % Cr, ½ % Mo

B2 L – Low carbon version of b2, C- .05% or less

C- Indicates nickel- steel alloy

D- Mn, Mo

Corrosion resistant electrodes

AWS specification A 5.4 use a 3-digit number to indicate the composition of weld deposits same as AISI type numbers.

This is followed by dash 9-0 and a two-digit umber to indicate welding position and coating type.

The 3-digit alloy number may be followed by a letter code indicating special characteristic egf L-Low carbon, ELC-extra low carbon and Mo is addition of molybdenum.

E 312 – 16 coatings – 15 (LIM E) for DCRP

E 312 L –16 – 16 (titanium) for AC/DC

Vertical

1)    Electrode is held at an angle of about 20deg from vertical plane.

2)    Short arc length in chosen.

3)    Electrode is moved in and out of the puddle.

ARC Blow

An a/c arc once started is more stable but it is not so with the DC arc.

The predominant problems faced with DC arc is the arc how

Arc blow is the deflection of arc by means of magnetic fields set up due to flow of welding current.

All electrical conductors arc surrounded by magnetic flux lines.

These magnetic flux lines move easily in metal but not in air.

When the electrode moves forward these flux lines more with the electrode. But when the weld comes the edge of the plat or taking a turn the flux lines moves out of the base metal which is not possible.

This causes a high magnetic pull on the arc which results in a backward arc blow.

This result in spatter and incomplete fusion.

The problem of arc blow gets multiplied when welding is done on strong magnetic materials ie Ni alloys.

Methods to reduce arc blow.

a)     Change its A/C welding because of continuous change in polarity effect of magnetic field is mollified.

b)    Reduce the current so that strength of magnetic field is reduced

c)     Use a short arc length

d)    Put steel hocks bear the end of the plate in contact with the base metal.

e)     Place more than one ground lead from the base metal.