In this article we will discuss about:- 1. Meaning of Arc Welding 2. Shielded-Arc Welding 3. Shielded Metal Arc Welding (SMAW) 4. Precautions to be Taken in Arc Welding 5. Arc Energy 6. Starting an Arc 7. Arc Stability 8. Puddle Control for Fusion Welding 9. Welding Adjustments to Correct the Bead 10. Good Arc by Proper Length Adjustment.
Meaning of Arc Welding:
In this case, heat is liberated at the arc terminals and this heat is used to melt the metals to be welded at the points of contact, so that they will flow together and form an integral mass. Thus, different parts may be joined. A filler material is also added to the surface of the metal.
A tremendous heat is liberated and the temperature of the arc is of the order of 3600°C. This heat causes a small pool of metal to melt in the work. When additional metal is required for welding, a welding rod is used which is melted by the heat of the arc and deposited into this small pool in the molten state.
The molten metal in the pool is agitated by the action of the arc, and thus the parent and added metal are thoroughly mixed and refined so that after cooling, a sound joint is formed Fig. 9.30 shows the welding circuit for an electric-arc welding process. Arc is created by low voltage, high current supply.
Flux coated electrodes are used which on melting form a protective gas shield around the electrode tip and molten weld pool. On cooling, the reduce of this flux solidifies to form a slag on the surface of the weld, which is subsequently chipped away.
Arc initiation voltage is of the order of 60—100 V and arc maintenance voltage is of the order of 25—40 V. Power source may be a.c. or d.c. supply. In a.c. supply, it does not matter whether work or electrode is positive but with d.c. source, polarity is important. A gap of about 3 mm is maintained for producing sound weld.
It is well known fact that molten steel when exposed to air, forms oxides and nitrides in the steel due to chemical combination with oxygen and nitrogen in air. These impurities weaken and embrittle the steel and reduce its resistance to corrosion. Further the ideal welded joint is one which has same properties as of the parent metal.
To achieve this, the molten filler metal in the arc stream and the weld metal is effectively protected from the oxidising and nitriding effects of the air during the entire range of liquefaction and solidification.
Arc can be shielded by completely enveloping it with an inert gas which prevents the molten metal from coming into contact with the surrounding air.
Shielding can be in following four forms:
i. Coated or covered electrode welding
ii. Gas-Shielded welding
iii. Tape-Shielded welding
iv. Flux-Shielded welding.
Shielded Metal Arc Welding (SMAW):
It is a manual arc welding using coated electrodes. Since the electrode melts and joins the weld pool, it is classed under consumable arc welding method. The coating on the electrode burns along with metal of electrode and produces a dense smoke which covers or shields the weld pool, thus preventing oxidation and absorption of nitrogen by the metal. The electrode after melting forms a slag cover over the molten pool. This process is very versatile and flexible and can be used in difficult places.
SMAW can be performed both on AC and DC source with drooping characteristic, highest current rating being about 600 am-peres. Selection of proper electrode is essential for best results. A rough guide is given in Fig. 9.33 (b) about the size of electrode and current for different thicknesses of plates to be welded. However it is advisable to consult recommendations of electrode supplier.
In DC welding, arc starting is easy and the arc is steady and smooth. Straight polarity (electrode – ve) is used for welding thin sheets and for plates with wide gaps. Reverse polarity (electrode + ve) produces maximum penetration and is used for root passes and out-of-position welding. Due to high voltage drop in cables, cable length should be short.
AC arc welding requires more skill on the part of operator. It is difficult to start. Less drop in cable, less arc blow are suited for thicker plates.
Precautions to be Taken in Arc Welding:
Arc welder should be clear of the hazards likely to occur from arc welding and should take all precautions to avoid them.
These hazards are:
i. Radiation (ultraviolet and infrared rays) from the arc,
ii. Flying sparks,
iii. Globules of molten metal,
iv. Electric shock,
One should never look at arc till eyes are protected with welders helmet or face shields with approved lenses.
Face, hands, arms and other skin surfaces must be covered by wearing gloves and clothing of sufficient weight to shut off the rays from the arc. Flammable materials like matches, celluloid combs, fountain pens etc. should be removed from pockets. Floor should be dry and electrode holders insulated. Proper ventilation equipment should be used. Fuses of right capacity should be fitted in the main switch and ampere adjustment should be set to minimum. Correct size electrodes should be selected.
Before starting arc, the machines should be carefully inspected, cleaned and oiled. All connections should be tight. Electrode holder and ground electrode cables should be inspected and electrode holder hung on a wood or fibre hanger away from the work to be welded. Brushes should not arc heavily in case of motor generator unit.
The machine should never be started with the electrode holder on a table or base metal, since this closes the arc welding circuit and causes the arc welder to start under a full load, which is hard on most machines. The machine should be adjusted for proper current setting depending on base metal and electrode size, by switching off main power supply. After running a few beads, it may be necessary to make fine adjustment to obtain more perfect weld.
Arc energy is expressed in kilo joules per millimeter length of weld (k.J/mm) and is equal to:
arc voltage X welding current / welding speed (mm/s) X 100
The greater the voltage drop across the arc the greater the energy liberated in heat for a given current. The voltage drop can be varied by altering the gas shield produced by the electrode covering. For instance hydrogen gives a higher volts drop than CO2.
Long arcs are difficult to control and maintain. They also lower the efficiency of gas shield. For sound welds, it is desirable that arc length be as short as possible.
Starting an Arc:
Starting an arc, i.e. producing an arc between the electrode and base metal initially is an important aspect for beginners and requires some practice. The best way is to touch the electrode with metal and then lift it up to the desired arc length. Usually what happens is that the electrode tends to stick to metal and weld itself with the metal.
Or else, the electrode may be withdrawn by a distance greater than the arc length and the voltage available then can’t maintain the arc and arc breaks. These problems can be overcome only by experience. The arc should always be started by drawing a rather long arc momentarily which helps in preheating the metal before deposition of filler material, thereby insuring complete fusion while beginning to weld.
This very effect can also be accomplished by moving a short arc rapidly back and forward over a limited area while starting the arc. However arc should always be initiated above the exact spot where the weld is to start.
Another method of starting the arc is by producing glancing or scratching motion with the end of the electrode, moving the electrode in arc by 10—15 mm.
The quality of weld is affected by changes of motion, angle and arc length. The arc may progress either in a straight forward motion or in an oscillatory motion depending on the size of the joint to be filled. For metals, the electrode is moved in some definite pattern, to secure the proper fusion of the electrode metal with the base metal.
Nice ripples are produced by using a small oscillating (back-and-forth) motion along the line of the weld. Depending on the type of seam and position of welding, other motions, viz. small circular motions, back-and-forth motion along with the half-circle across the weld at a 45° to it, figure-eight (8) motion, triangular motion, etc. can be used.
It may be mentioned that it is easier to start DC arc than AC arc because of the tendency of the apparatus to break the arc on the change of the cycle (100 times per sec. in a 50- cycle circuit). In AC arc welding, current setting should be done correctly to match the electrode diameter, thickness of metal, type of joint, etc. Too-low current adjustment would lead to continual breaking of arc. High current adjustment would lead to continual breaking of arc. High current setting results in heavy electrode spatter and overheating.
For restarting the weld if first electrode is consumed, the last portion should be cleaned with brush to remove slag and arc reinitiated at the point where it was left. Similarly in multi pass welding, first portion of weld should be cleaned before welding for second pass.
For obtaining sound welds it is necessary to maintain stable (uniform and steady) arc during welding process. Unstable arc may result in intermittent fusion, trapping of slag, production of blow holes. Arc stability is influenced by factors like the characteristics of power supply source, nature of the electrode, proper manipulation by the operator, etc.
The electric power supply source should be such that when arc has tendency to extinguish due to lengthening of arc, the voltage should automatically rise instantly.
If electrode covering is made of proper composition and has uniform consistency, it aids in promoting arc stability. The covering not only protects the arc but also assists is maintaining stability. The chemical composition of the filler metal also plays an important role in arc stability.
The arc is extremely sensitive to the condition of the electrode surface. Materials like iron oxide, calcium oxide, calcium carbonate, manganese oxide, and ferrous sulphide have tendency to stabilize the arc while aluminium, aluminium oxide, silicon, silicon oxide and ferrous sulphate tend to decrease the arc stability. Thus their presence on the electrode wire or surface affects arc stability.
Steam or vapour formed during welding can cause the arc to become unstable or even extinguish.
While welding with base electrodes on direct current, arc may have a tendency to waver from its intended path (arc blow). It is caused by a magnetic disturbance in the neighbourhood of the arc. It causes lack of fusion, porosity, spattering and uneven welded joints. It can be minimised by using a.c. welding, or using covered electrodes.
Puddle Control for Fusion Welding:
The torch adjustment (distance of torch from puddle and torch angle w.r.t. vertical plane) is the single factor responsible for controlling puddle and hence acceptable weld. If torch distance is more, puddle will be cooler and smaller whereas it will be hotter and wider when torch distance is close. Metal will oxidise if flame is too close to puddle.
Steep torch angle (5—10° of torch tip w.r.t. vertical plane) will produce deep penetration and hot puddle in thick sections, whereas shallow angle (45°) will produce mild penetration and less heat input because higher angle allows much of the flame heat to reflect off the plate into the air.
With oxidising flame, puddle will boil and become full of slag due to oxidation of metal. With carburising flame, the; puddle develops slowly and absorbs carbon into the weld making it brittle.
Welding Adjustments to Correct the Bead:
The right bead can be obtained by adjusting following five variables:
(i) Electrode size:
While too big electrode may stick and be hard to start, too small electrode will spatter and even can catch fire.
If amperage or heat is too much, spatter, and/or undercut will take place. If heat is too less, arc will be difficult of strike and the bead will be overlapped.
(iii) Electrode angle:
Too great travel angle results into a bead with poor penetration. Angle of attack should be about half the joint angle otherwise the leg length will be uneven, fusion poor and undercut on the top leg.
(iv) Travel speed:
Too slow speed results in overlap and piling up of head. If too fast, bead will be sparse and have poor fusion.
(v) Arc length:
Too long arc length results into erratic arc and spatter. If electrode is too close, rod will not arc properly and result into improper shielding and sticking.
Good Arc by Proper Length Adjustment:
A long arc is unstable and causes small explosions occurring at steady intervals which scatter larger globules of metal. It makes hissing sound. Short arc is surrounded by the protecting arc flame and ejects a steady shower of fine sparks. Short arc also makes a sharp rapid crackling sound resembling the sound of grease flying in a pan.
With a long arc the electrode is fused quite rapidly but the directional control of the molten metal is poor, and some of the metal is not deposited where it is needed.
By experience operator can maintain proper arc length so that arc is stable. He is guided by both the appearance and the sound of the arc. Arc power is dependent on the length of arc. From Fig. 9.37, it will be seen that there is an optimum length of arc for which the arc power is maximum.