Thursday, 26 June 2014

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Plasma enhancing used in welding and allied processes

Before going into detail. I want to introduce you to some basic terms related to this topic.
SMAW-  An old welding process that uses a solid, flux coated electrode.
PLASMA- It is the fourth state of matter obtained after further heating of gas. It is the mixture of charged particles(Electrons & Ions) that satisfy basic conditions of plasma. Welding arc is a continuous plasma.
PINCH FORCE- This force acts at certain angle on the molten metal drop at the tip of the electrode before the detachment of it. It acts due to presence of electro-magnetic force around the arc.
IONIZATION POTENTIAL- It is measure of a metal to emit the electrons and is defined as energy per unit charge(eV) required for removing an electron from an atom. Metals like Ca, Na & K has low ionization potential(2.1-2.33 eV) therefore they are used in flux covering.
SYNTHETIC WELD- It is a weld that differs from the base metal in composition due to alloying elements. Suppose a base metal of mild steel on which a bead of stainless steel can be made then it is called synthetic weld & procedure is called synthetic welding.   

ACCIDENTAL  INVENTION OF PLASMA ENHANCING  PHENOMENON
Prof. Sunil Pandey was  working on making a synthetic weld at Welding laboratory of IIT Delhi with tubular electrode. He was trying to add alloying elements to the weld pool through the hollow tube. When he had made the bead. He examined it to know the penetration. When he cut it. He surprised to see the penetration. Penetration was deep. He repeated the experiments & found same results. The reason of deep penetration was unknown at that time. But he saw the scope of this invention & developed several welding & allied processes. 
Due to deep penetration, Plasma Enhanced Shielded metal arc welding can replace the
use of cellulosic electrode. Because cellulosic electrode has a problem of hydrogen
embrittlement.  He worked for the Indo-Uk project on “Commercialization of Plasma
Enhanced shielded metal arc welding process and development of  application specific flux”
for which he visited Brunel University, London. He used same principle for optimizing
surfacing and cladding through dilution control.

WHAT IS PLASMA ENHANCING??



Pinch force plays main role in plasma enhancing. Flow of current through the arc gap
develops the electro-magnetic field. Interaction of this EM field with charge carriers
produces a force which tends to pinch the drop hanging at the tip of the electrode also
called pinch force.
It acts at certain angle. It has two components. Role of horizontal component in case solid
electrode is limited to the cutting of throat of drop only.
In case of tubular electrode, hollow tube contain gases only. When this horizontal pinch
force acts on the tip of electrode then it produces a extra thrust on the drop. For better
understanding, we can take the analogy. When we press the tube of tooth paste , paste comes
out. The velocity of paste depends on intensity of applied force. Same situation is here.

  Prof. Sunil Pandey used this phenomenon in various welding & allied processes to attain desired results & found satisfactory results.
In PESMAW, There is an Universal Electrode Holder(UEH). That holds tubular electrodes. It has  supply of gas, Powder(Flux or alloying Elements).We can use these supplies as per the requirement  of the process.Prof. Pandey holds the patent of UEH.
During welding, gas supply may or may not be used as per requirement. But during synthetic welding, use of shielding gas supply is must. Because due to supply of gas only, powder can be supplied.
For the welding of steel, Shielding gas is carbon Di-oxide, whose Ionization potential is higher than air. So arc voltage is also higher there. Due to this Heat input is higher in PESMAW with CO2. 

PLASMA ENHANCING IN ALLIED PROCESSES
In Cutting, Simple tubular electrode without gas and powder supply can be used. But during powder cutting, Iron powder with oxygen supply must be used.
This technology is useful in concrete cutting during natural disaster.
Instead of gas, Air supply can also be used during cutting.
These are the allied processes  that is developed on this principle:-
PLASMA ENHANCED CUTTING
PLASMA ENHANCED LANCING
PLASMA ENHANCED GOUGING

 ADVANTAGE:
Deep penetration is achieved. No chances of hydrogen embritllement.
We can produce synthetic weld joints.
Quality of weld is high because of additional shielding.
One setup can be used for various operations(Welding & Allied). Thus reduces floor area
requirement. Thus becomes economical.
It is a high heat input process than SMAW. Thus increases productivity.
When bare , hollow electrode is used , then current limit is from short circuit mode to
rotational transfer mode because there is no flux pre-heating.

 WHERE & WHY TO BUY THIS TECHNOLOGY
Now, This Technology is fully developed. In future, It Will available in market.
Manufacturing of actual product is not started yet due to the lag of non material culture &
mind set of people.
For the growth of India, Manufacturing sector should be strengthened & to stop the export of
foreign technology we have to promote the use of indigenous developed technologies.
Industry persons are most important in this perspective. They should consider this
technology at least for trial.
Use of indigenous technology can save  money & creates employment.

REFERENCE:
1.  Sunil Pandey, New Technologies in Shielded metal Arc welding and Gas Metal Arc
Welding,National Welding Seminar, Indian Institute of Welding (IIW), Chennai, India, 2002.
2.   Sunil Pandey, Tahir I. Khan, S.Thiruchitrambalam,Surface modification of steels using
plasma enhanced shielded metal arc welding process (PESMAW), Indian welding journal, April
2003


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