Page 181 - CITS - Welder - Trade Theory

WELDER - CITS

The welding torch used for self shielded wire is very simple in construction as there is no need for the gas nozzle.
Similarly the feed rollers used for flux cored wires have to ensure positive feeding of the wire without applying too
much pressure on the soft tubular wire.
Metal transfer in FCAW: The metal transfer in FCAW differs significantly from GMAW process. FCAW process
exhibits two distinctly different modes of metal transfer, namely large droplet transfer and small droplet transfer.
However, both are classified as free flight transfer. The FCAW process does not produce a stable dip transfer as
that of solid wire GMAW. The large droplet transfer occurs at the lower current voltage ranges. At higher current
voltage ranges, the transfer mode changes to smaller
droplet transfer. An important aspect to be observed during FCAW metal transfer is the presence of the ‘flux pole’
at the core of the arc column, protruding into the arc. The ‘flux pole’ appears only during welding with basic type
flux cored wire. Fig.2(a) However, with rutile wire ‘flux pole’ does not occur and the metal transfer is of spray type.
Fig.2(b)

Fig 2

Classification of flux cored wires: The basic functions of the flux contained within the tubular wire include
providing protective slag on the weld bead, introducing the required alloying elements and deoxygenates into the
weld pool and providing stability to the arc, besides producing the required shielding medium to protect the arc
and weld pool.
Flux cored wires are now available for welding of plain carbon steel, low alloy steel and stainless steel and also for
hard facing applications. These wires based on the nature of flux, may be classified as rutile gas shielded, basic
gas shielded, metal cored and self shielded.
Rutile gas shielded wires have extremely good arc running characteristics, excellent positional welding capabilities
and good slag removal and mechanical properties.

Basic gas shielded wires give reasonable arc characteristics, excellent tolerance to operating parameters and
very good mechanical properties.
Metal cored wires contain very little mineral flux, the major constituent being iron powder and ferro alloys. These
wires give smooth spray transfer in Argon/CO2 gas mixtures. They generate minimum slag and are suitable for
mechanised welding applications. Self shielded wires are avail-able for general purpose down hand welding.

The flux cored wires are available in both seamless and folded types. The seamless type is generally coated with
copper, whereas the folded type wires (i.e. close butt and overlapped type) are treated with special compounds.
Deposition rate and efficiency: Deposition rate is de-fined as the weight of metal deposited per unit time. The
deposition efficiency is defined as the ratio of weight of weld metal effectively deposited to the weight of wire
consumed.
In GMAW welding the deposition efficiency is generally between 93% to 97% and in FCAW the corresponding
figure is between 80% to 86%. These values are deter-mined by the spatter losses and slag formation. The low
deposition efficiency in the case FCAW is due to the slag formation Generally the spatter loss can be minimised
by using Argon/CO2 mixed gas instead of CO2 gas.

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CITS : C G & M - Welder - Lesson 49 - 60