Introduction to Welding Processes
CONSTANT CURRENT (DROOPING) OUTPUT CHARACTERISTIC
Welding Inspection 10.6 WORLD CENTRE FOR
Direct Current. Electrode positive (DCEP / DC+).
When the electrode is on the positive pole of the welding circuit; the workpiece therefore becomes the negative pole. Electron flow direction is from the workpiece to the electrode.
When the electrode is positively charged (DCEP) and the workpiece is negatively charged this has the effect of generating two thirds of the available heat energy, at the tip of the electrode, with the remaining one third of the available heat energy being generated in the parent material this will result in an increase in the depth of the weld penetration.
Direct current.. Electrode negative (DCEN / DC-)
When the electrode is on the negative pole of the welding circuit, the workpiece therefore becomes the positive pole. Electron flow direction is from the electrode to the workpiece. The distribution of energy is now reversed. One third of the available heat energy is generated at the tip of the electrode; the remaining two thirds of the available heat energy is now in the parent material.
Direct current with a negatively charged electrode (DCEN) causes heat to build up on the electrode, increasing the electrode melting rate and decreasing the depth of the weld penetration.
The welding arc when using direct current can be affected by arc blow*. • The deflection of the arc from its normal path due to magnetic forces.
Alternating current (AC)
The current alternates in the welding circuit, flowing first in one direction and then the other. With alternating current, the direction of flow changes 100 to 120 times per second, 50 to 60 cycles per second (cps). Alternating current is the flow of current in two directions.
Therefore distribution of heat energy at the arc is equal, 50% at the electrode, 50% at the workpiece.
10.4.5
TYPE OF CONSUMABLE ELECTRODE
For manual metal arc welding there are three generic types of flux covering:
Rutile electrodes contain a high proportion of titanium oxide (rutile) in the coating.
Titanium oxide promotes easy arc ignition, smooth arc operation and low spatter. These electrodes are general-purpose electrodes with good welding properties. They can be used with AC and DC power sources and in all positions. The electrodes are especially suitable for welding fillet joints in the horizontal/vertical (H/V) position.
Features:
• moderate weld metal mechanical properties
• good bead profile produced through the viscous slag
• positional welding possible with a fluid slag (containing fluoride) • easily removable slag
Basic electrodes
• low hydrogen weld metal
contain a high proportion of calcium carbonate (limestone) and calcium fluoride (fluorspar) in the coating. This makes their slag coating more fluid than rutile coatings - this is also fast freezing which assists welding in the vertical and overhead position. These electrodes are used for welding medium and heavy section fabrications where higher weld quality, good mechanical properties and resistance to cracking (due to high restraint) are required.
Features:
• requires high welding currents/speeds
• poor bead profile (convex and coarse surface profile) • slag removal difficult
Cellulosic electrodes
• deep penetration in all positions
contain a high proportion of cellulose in the coating and are characterised by a deeply penetrating arc and a rapid burn-off rate giving high welding speeds. Weld deposit can be coarse and with fluid slag, deslagging can be difficult. These electrodes are easy to use in any position and are noted for their use in the 'stovepipe' welding technique.
Features:
• suitability for vertical down welding • reasonably good mechanical properties
• high level of hydrogen generated - risk of cracking in the heat affected zone (HAZ)
It should be noted that within these three generic groups there are sub-groups of covered electrodes providing a wide range of electrode choice.
MMA electrodes are designed to operate with alternating current (AC) and direct current (DC) power sources. Although AC electrodes can be used on DC, not all DC electrodes can be used with AC power sources.
Welding Inspection 10.8 WORLD CENTRE FOR *Operating Factor: (O/F) The percentage (%) of ”Arc On Time” in a given time span. When compared with semi automatic welding processes the MMA welding process has a low O/F of approximately 30% Manual semi automatic MIG/MAG O/F is in the region 60% with fully automated MIG/MAG in the region of 90% O/F. A welding process Operating Factor can be directly linked to productivity.
Operating Factor should not to be confused with the term Duty Cycle
1) A Transformer/Rectifier, generator, inverter. (Constant amperage type)
, which is a safety value given as the % of time a conductor can carry a current and is given as a specific current at 60% and 100% of 10 minutes i.e. 350amps 60% and 300amps 100%
10.5 SUMMARY OF MMA/SMAW
Equipment requirements
2) A power and power return cable. (Of a suitable amperage rating) 3) Electrode holder. (Of a suitable amperage rating)
4) Electrodes (Of a suitable type & amperage rating.
5) Correct visor/glass, all safety clothing and good extraction
Parameters & Inspection Points
1) Amperage
2) Open Circuit Voltage. (OCV) 3) AC/DC & Polarity
4) Speed of travel
5) Electrode type & diameter 6) Duty cycles
7) Electrode condition 8) Connections
9) Insulation/extraction
10) Any special electrode treatment
Typical Welding Imperfections
1) Slag inclusions caused by poor welding technique or insufficient inter-run
cleaning.
2) Porosity from using damp or damaged electrodes or when welding
contaminated or unclean material.
3) Lack of root fusion or penetration caused by in-correct settings of the amps,
4) Undercut caused by too high amperage for the position or by a poor welding
technique e.g. travel speed too fast or too slow, arc length (therefore voltage) variations particularly during excessive weaving.
5) Arc strikes caused by incorrect arc striking procedure, or lack of skill. These
may be also caused by incorrectly fitted/secured power return lead clamps.
6) Hydrogen cracks caused by the use of incorrect electrode type or incorrect
baking procedure and/or control of basic coated electrodes.
Successful welding with the MMA process is reliant on a number of factors, not least of which is the skill required to produce a sound weld. This is dependent on the welder’s ability to match the arc length (the distance from the tip of the electrode to the work piece), to the burn off rate, (the rate at which the electrode is consumed).
Advantages & Disadvantages Advantages
1) Field or shop use 2) Range of consumables 3) All positional
4) Very portable
5) Simple equipment
Disadvantages
1) High skill factor required 2) Arc strikes/Slag inclusions 3) Low Operating Factor
4) High level of generated fumes 5) Hydrogen control