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

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

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

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.

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

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

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

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

(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

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

(1) 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

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:-

(1) 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

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)

IDENTIFICATION MARKING ON ELECTRODES Let’s take the first rod in the photo E6010

AWS ELECTRODE CLASSIFICATION SYSTEM

STEEL ARC WELDING

THE IDENTIFICATION SYSTEM USES A FOUR DIGIT NUMBER PROCEDED BY A LETTER

E-6010The 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.

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

HEAT AFFECTED ZONE IN A WELD JOINT

HEAT AFFECTED ZONEBASE METAL

UNDER BEAD CRACKINGCAUSED 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