mig WELD PROCEDURE

WELDING MACHINE Welding environment can be improved with the use of an inverter controlled welding machine.

WELDING WIRE To reduce splatter, blow holes, pits, and other defects it is advisable to use welding wires developed for galvanised steel. Welding wires with flux suitable for carbon steel and structural steel up to 490N/mm2 class also work well.

SHIELD GAS Use CO2 gas. When the combination of pulse current and the Ar + 20% CO2 gas is used the splatter tends to decrease.

WELDING CURRENT AND VOLTAGE The welding current should be set higher as more heat is absorbed by the evaporation of coating material. Current raised by 5 to 10%.

WELDING SPEED When such defects as blowholes and pits are found, the welding speed should be set lower. Good beads can be made if welding is carried out by releasing zinc vapour from the surface of the molten metal pool at the same time.

INSTALLATION OF A GAP Lap fillet welding tends to cause defect such as blowholes and pits. The most effective way to prevent this is to create gaps between the steel sheets. A gap of 0.6mm to 1.5mm helps substantially reduce these defects. This is recommended by the WTIA for any pre-coated zinc-alloy steel.

NOTE: SiLiCON FREE ANTi-SPLATTER SPRAY mUST BE USED iN THE WELD SPLATTER ZONE TO REDUCE DAmAgE

OF THE mAXi-TUBE® COATiNg.

WELDING A FILLET jOINT The fillet weld can be more problematic than the butt weld because the galvanised edge of one plate is butted against another galvanised surface and the vapour cannot escape easily and instead can be blown into the weed pool and create porosity. Nevertheless, with optimum settings and skill there is no need for any additional preparation for fillet welds.

Should problems arise, alternative approaches are available. Creating a gap of approximately 1.5mm between the two parts will help the vapour escape. Alternatively, using a slight bevel of approximately 15o or even removing the coating from one surface will improve the weldability.

Various approaches to welding a fillet joint

a) No Preparation b) With a gap of 1.5m c) With 15o Bevel

ELECTRODE WELD PROCEDURE WELDING MACHINE Welding can be performed by single phase alternating current, inverter direct current, and condenser type welding machines.

ELECTRODE The CF & DR type electrodes can be used. In some cases a favourable result may be obtained by selecting a tip diameter which is one size smaller than conventional electrodes.

PRESSURE Pressure is determined based on sheet thickness. Standard values are shown in table below.

WELD TIME The weld time is based on sheet thickness.

Sheet Thickness (mm) Electrode Pressure (kg/f) Weld Time (Cycle)

0.8 200 12

1.2 250 20

1.6 300 30

2.3 400 40

3.2 500 50

Standard electrode pressure and weld time (Single-phase alternating current)

Example of standard electrode tip shapes

Recommended Measures

Removal of coating using welding heat during welding.

• Reduce welding speed as compared with hot and cold-rolled steel.

• Avoid lap joints where the removal of coating is difficult.

• Release zinc vapor so that it does not enter the welding metals (install

gap between sheets to release gas).

Recommended Measures

Selection of welding conditions conductive to the stability of the arc

• Use a pulse MIG welding machine.

• Use Ar + 20% CO2 composite gas.

• Select a wire appropriate for the range of electrical current being used.

• Try robot welding.

Invasion of zinc vapor into the welding metals

Negative effects on zinc evaporation of coating layers

Instability of the arc due to zinc vapor

CF Type (Tip diameter 4.5mm) DR Type (Tip diameter 6mm)