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Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
Welding
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Welding

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welding, model engineering.

welding, model engineering.

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  • 1. OVERVIEW : WELDING PROCESSES AND THEIR USES FOR MODEL ENGINEERS MMA Manual Metallic Arc Welding (Stick Welding) MIG Metal Inert Gas Welding TIG Tungsten Inert Gas welding
  • 2.
    • COMMON ELECTRIC ARC WELDING PROCESSES
    • EQUIPMENT REQUIREMENTS
    • Step Down
    • (Welding Transformer)
    • Power sources:-
    • ( Static ) mains supply single / three phase
    • ( Generated / Rotating ) mains motor /
    • fuel oil mobile plant
  • 3.
    • COMMON ELECTRIC ARC WELDING PROCESSES
    • EQUIPMENT REQUIREMENTS
    • Step Down
    • (Welding Transformer)
    • Current Choices types for consideration :-
    • ( AC ) Alternating Current
    • ONE CYCLE ALTERNATING CURRENT THREE PHASE AC
    • ( DC ) Direct Current ONE CYCLE
    SINGLE PHASE FULL WAVE RECTIFICATION 3 PHASE FULL WAVE RECTIFICATION
  • 4. CONSTANT CURRENT OR CONSTANT VOLTAGE
    • Striking the arc -> 80
    • volt / ampercurve constant current
    • volts
    • 34 volts 290 amps
    • 40 32 volts 300 amps
    • 30 volts 308 amps
    • 10
    • 100 200 300
    • amperes
    • Constant current volt / ampercurve
    • CONSTANT CURRENT IS PRIMARILY USED WITH COATED ELECTRODES
    • (Stick Welding)
    • Amperage is set by the operator the voltage is automatically designed into the unit
    • CONSTANT VOLTAGE IS USED WITH SOLID AND FLUX CORED ELECTRODES (Metal Inert Gas Welding)
    • The voltage is set at the machine by the operator, the amperage is automatically determined by the speed that the wire is fed to the welding gun.
    • Increasing the wire feed speed increases the amperage.
    • Decreasing the wire feed speed reduces the amperage.
  • 5. COMMON ELECTRIC ARC WELDING PROCESSES (Stick Welding) The operator can adjust the arc length manually Consequently the voltage will increase and the amperage will decrease Diagram shows the electrode provides a gaseous shield from the decomposition or the coating around the electrode arc and produces a slag covering on the hot weld deposit
  • 6. AC verses DC current Heat in the work piece Heat in the electrode Straight polarity (ACEN) electrode negative Reverse polarity (ACEP) electrode positive Good for welding Stainless steel Deep penetration
  • 7. AC verses DC current
    • AC Alternating Current:-
    • Good for distance from the work source low voltage resistance drop in cable length. ie voltage drop of 5 volts over 205 feet inclusive of ground lead and electrode lead
    • Good for welding carbon steels less effective on stainless steels and alloys
    • Excellent for Tungsten arc welding of Aluminum (Removes surface oxides)
    • DC electrodes can be used successfully
    • Not good out of positional welding without specialist electrodes
    • Smoother welding arc using 3 phase
    • Deep penetration obtained with AC reverse polarity (electrode +)
    • AC Equipment readily available at modest prices
    • AC power source available in every household
    • AC Not prone to arc blow
  • 8. DC Direct Current:- Arc is established at lower voltages (50/55volts) enabling easier arc striking and arc maintaining qualities (especially on thin sheet steels) DC+) Versatile for heavy and light gauge materials Excellent for out of position welding (Lower welding currents) Greater range of electrode types Voltage drop across the Arc adjustable by means of a field regulator DC current electrode ve+ Excellent for welding Carbon steels, Stainless Steel, Alloys, and dissimilar metals, DC current excellent for TIG (Tungsten Inert Gas) Welding almost all metals ve+ and ve- Mild steel, low alloys, Stainless steel, Copper and Copper alloys, Aluminum and Aluminum Alloys, Nickel and Nickel Alloys, Magnesium and Magnesium Alloys, Titanium, and others. Welding equipment very expensive compered to AC plant Voltage drop higher than AC on long cable distance work Arc Blow can disrupt the weld area caused by magnetic fields (especially troublesome when welding in corners the arc may stray or fluctuate in direction AC verses DC current
  • 9. (Experimentation of the arc began in the early 1800's) Development of Covered Electrodes First electric weld British patent 1865 During the 1890's welding was accomplished using bare metal electrodes Welds produced where porous and brittle due to large absorption of oxygen and nitrogen from the atmosphere Observations of improvements were noticeable by using rusty electrodes or wrapping the rod with paper These improvements shielded the arc from the atmosphere to some degree In the early twentieth century the importance of shielding the arc from the atmosphere was recognized
  • 10. COATED ELECTRODES
    • ( IMPROVEMENTS AND ADVANCEMENTS )
    • Around 1920 development of electrodes spirally wrapped with paper (soaked in sodium silicate, and then baked)
    • In 1924 improvements on coatings where extruded over the core wire with additional flux ingredients to further improve or modify the weld metal
    • Two major elements make up the electrode
    • The core wire or rod usually hot rolled (with added elements)
    • (2) The flux covering is extruded over the core containing an unlimited number of ingredients
    • ie:- sodium silicate or potassium silicate. Other solid materials added produced chemical reactions such as alloys or other complex synthetic compounds
  • 11. FUNCTIONS OF THE ELECTRODE COATING
    • Shield the weld metal from the oxygen and nitrogen from the atmosphere as it is being transferred across the arc in the molten state (Causes brittleness and porosity of the weld)
    • The outer flux breaks decomposes gives off gasses which:-
    • Stabilization of the Arc a stabilized arc is one that starts easily, burns smoothly even at low amperage, and can be maintained using either a long or a short arc length.
    • (2) Additional Alloying of elements such as chromium, nickel, molybdenum, vanadium and copper can be added to the weld metal by including them in the coating composition Lost during the welding operation
    • (3) Concentration of the Arc Stream
    • (4) Furnish Slag for Fluxing
    • (5) Characteristics for Welding Position
    • (6) Control of Weld Metal Soundness
    • (7) Specific Mechanical Properties to the Weld Metal
    • (8) Insulation of the Core Wire
  • 12. CLASSIFICATION OF WELDING RODS (Selection Criteria) Type of Base Metal (Carbon Steel/ Stainless steel etc) Position of the Weld (All position/ flat etc) Available Equipment AC or DC :- For deepest penetration, use DC reverse polarity (Electrode Positive) For lower penetration and higher deposition rate, use DC straight polarity (Electrode Negative) For freedom of arc blow, use AC. Plate Thickness (Heavy gauge and lower gauge electrodes) Fit-Up (Gaps and poor Fits etc) Welding Costs (Dependent on quality)
  • 13. IDENTIFICATION MARKING ON ELECTRODES Let’s take the first rod in the photo E6010
  • 14. AWS ELECTRODE CLASSIFICATION SYSTEM
    • STEEL ARC WELDING
    • THE IDENTIFICATION SYSTEM USES A FOUR DIGIT NUMBER PROCEDED BY A LETTER
    • E-6010
    • The symbol “ E” indicates that the electrode is intended for use in electrical welding.
    • The first two (or three) digits of the number indicate the tensile strength (the resistance of the material to forces trying to pull it apart), in thousands of pounds per square inch, of the deposited metal.
    • The third (or fourth) digit indicates the position of the weld
    • The number 0 in either one of these positions indicates the classification is not used.
    • The number 1 indicates the electrode may be used for all welding positions.
    • The number 2 indicates that the electrode may be used only in the flat and horizontal positions.
    • The number 3 indicates the electrode is to be used only in the flat welding position.
    • The fourth (or last) digit indicates the type of coating on the electrode, and the power supply (either alternating current (ac) or direct current (dc), straight polarity (sp), or reverse polarity (rp)) to be used with the pertinent electrode.
  • 15. HEAT AFFECTED ZONE IN A WELD JOINT HEAT AFFECTED ZONE THE HEAT AFFECTED ZONE IS THE AREA OF THE WELD JOINT THAT DID NOT BECOME MOLTEN IN THE WELDING PROCESS THIS ZONE NOW BECOMES WEAK DUE TO THE CHANGE IN GRAIN STRUCTURE SOLIDIFIED WELD METAL HEAT AFFECTIVE ZONE UNAFFECTED BASE METAL
  • 16. HEAT AFFECTED ZONE IN A WELD JOINT HEAT AFFECTED ZONE BASE METAL UNDER BEAD CRACKING CAUSED BY HYDROGEN BEARING COMPOUNDS (WATER& MINERALS) OR CHEMICALS PRESENT IN THE ELECTRODE COATING THE HYDROGEN IS CONVERTED INTO ATOMIC HYDROGEN INISHIATED BY THE HEAT OF THE ARC 4000°C TREMENDAS PRESSURE IS DEVELOPED WHICH OPENS CRACK DEFECTS IN THE HEAT AFFECTED ZONE WELD METAL HYDROGEN INDUCED CRACKS

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