© COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON III 3.2.1 Functions of Electrode Coatings - The ingredients that are commonly used in coatings can be classified physically in a broad manner as liquids and solids.  The liquids are generally sodium silicate or potassium silicate.  The solids are powdered or granulated mate- rials that may be found free in nature,  and need only concentration and grinding to the proper particle size.  Other solid materials used are produced as a result of chemical reactions, such as alloys or other complex synthetic compounds. The particle size of the solid material is an important factor.  Particle size may be as coarse as fine sand, or as minute as sub-sieve size. The physical structure of the coating ingredients may be classified as crystalline, fibrous or amorphous (non-crystalline).  Crystalline materials such as rutile, quartz and mica are commonly used.   Rutile is the naturally occurring form of the mineral titanium dioxide and is widely used in electrode coatings.  Fibrous materials such as wood fibers, and non-crystal- line materials such as glasses and other organic compounds are also common coating ingre- dients. The functions of the coating on covered electrodes are as follows: a)    Shielding of the Weld Metal - The most important function of a coating is to shield the weld metal from the oxygen and nitrogen of the air as it is being transferred across the arc, and while it is in the molten state.  This shielding is necessary to ensure the weld metal will be sound, free of gas pockets, and have the right strength and ductility.  At the high tem- peratures of the arc, nitrogen and oxygen combine readily with iron to form iron nitrides and iron oxides that, if present in the weld metal above certain minimum amounts, will cause brittle- ness and porosity.  Nitrogen is the primary concern since it is difficult to control its effect once it has entered the deposit.  Oxygen can be counteracted by the use of suitable deoxidizers.  In order to avoid contamination from the air, the stream of molten metal must be protected or shielded by gases that exclude the surrounding atmosphere from the arc and the molten weld metal.  This is accomplished by using gas-forming materials in the coating that break down during the welding operation and produce the gaseous shield. b)    Stabilization of the Arc - A stabilized arc is one that starts easily, burns smoothly even at low amperages, and can be maintained using either a long or a short arc length. c) Alloying Additions to Weld Metal - A variety of elements such as chromium, nickel, molybdenum, vanadium and copper can be added to the weld metal by including them in the coating composition.  It is often necessary to add alloys to the coating to balance the expected loss of alloys of the core wire during the welding operation, due to volatization and
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Lesson 1
The Basics of Arc Welding
Lesson 2
Common Electric
Arc Welding Processes
Lesson 3
Covered Electrodes for Welding
Mild Steels
Lesson 4
Covered Electrodes for Welding Low Alloy Steels
Lesson 5
Welding Filler Metals for Stainless Steels
Lesson 6
Carbon & Low Alloy
Steel Filler Metals -
Lesson 7
Flux Cored Arc Electrodes Carbon Low Alloy Steels
Lesson 8
Hardsurfacing Electrodes
Lesson 9
Estimating & Comparing Weld Metal Costs
Lesson 10
Reliability of Welding Filler Metals