HOT SHOP PRACTICE
Title: Oxy Acetylene Welding
Code: HS 301 and 302
Objectives: See your laboratory Manual
General Theoretical Knowledge:
Welding is process for joining separate pieces of metal in a continuous metallic bond. Cold-pressure welding is accomplished by the application of high pressure at room temperature; forge welding (forging) is done by means of hammering, with the addition of heat. In most processes in common use, the metal at the points to be joined is melted; additional molten metal is added as a filler, and the bond is allowed to cool. In the Thomson process, resistance to an electric current, passed through the sections to be joined, causes them to melt. Other notable methods include the thermite process, oxyacetylene, electric arc, oxyhydrogen, and the atomic hydrogen flame. In this last-named method, molecules of hydrogen gas passing through an electric arc are broken up into atoms of hydrogen by absorbing energy; when outside the arc, the atoms reunite into molecules, yielding in the process enough heat to weld the material. Another process, the argon-arc method, is widely used with metals such as stainless steel, aluminum, magnesium, and titanium, which require an inert atmosphere for successful welding. The use of argon prevents slag from forming in the weld and greatly increases the speed of the welding.
Classification. Modern methods of welding metals can be classified into two large groups: fusion welding, or liquid-state welding, and pressure welding, or solid-state welding. In fusion welding, the molten metal of the parts being joined is combined into one mass spontaneously, without applying any external force, as a result of melting and wetting in the weld zone and of mutual coalescence of the material. In pressure welding, substantial pressure is applied to join the parts without melting. The division between these two groups is not always distinct. It is possible, for instance, to weld by first fusing the parts partially and then pressing the parts together, as is done in resistance welding.
In arc welding, the oxygen and nitrogen of the atmosphere interact with the molten metal, forming oxides and nitrides that lower the strength and plasticity of the welded joint. There are internal and external methods of protecting the weld zone, such as introducing various substances into the electrode material or covering the electrodes (internal protection) or introducing inert gases or carbon dioxide into the weld zone or covering the weld zone with a flux (external protection). In the absence of external protection, the welding arc is called an unshielded arc; if external protection is used, it is called either a shielded arc or a submerged arc. Arc welding with an unshielded arc and consumable, covered electrodes is the most important method in actual practice. The high quality of the weld produced makes it possible to use this method for the fabrication of critical load-bearing parts.
Electric methods of fusion welding include electroslag welding. As in arc welding, this process begins with the formation of an arc and uses consumable electrodes. The process is then continued without an arc discharge, and a substantial amount of slag covers the weld puddle. The heat dissipated by the passage of electric current through the slag serves to heat the metal. This method was developed at the E. O. Paton Institute of Electric Welding and has been used commercially in industry since the late 1950’s. Electroslag welding with a single electrode is feasible for metals up to 200 mm thick; the use of several electrodes operated simultaneously makes it possible to weld metals up to 2,000 mm thick. The method is appropriate and economically advantageous for base metals with a thickness greater than 30 mm. Electroslag welding can be used for repair work and for hard facing where the required thickness of the surface layer is substantial. It is also being used in the manufacture of steam boilers, press beds, rolling mills, metal structural members, and other product
Oxyacetylene gas welding is commonly used to permanently join mild steel. A mixture of oxygen and acetylene, burns as an intense / focussed flame, at approximately 3,500 degrees centigrade. When the flame comes in contact with steel, it melts the surface forming a molten pool, allowing welding to take place. Oxyacetylene can also be used for brazing, bronze welding, forging / shaping metal and cutting.
This type of welding is suitable for the prefabrication of steel sheet, tubes and plates.
PREPARING THE CYLINDERS FOR WELDING
Slowly open the main valve of the acetylene tank and adjust the acetylene pressure regulator to 5PSI (pounds to square inch).
Open the needle valve on the torch and adjust the pressure on the acetylene regulators to show 5PSI. Close the needle valve.
Go through the same procedure for the oxygen cylinder.
IGNITING THE ACETYLENE / OXYGEN MIXTURE
TURNING ON:
Acetylene slowly turned on (quarter/half turn of the needle valve) and ignited, producing a small flame. At this stage, a small amount of soot/smoke is given off the end of the flame.
Acetylene increased and oxygen turned on slowly.
Acetylene increased slowly and oxygen more rapidly, to produce an intense, localised flame, capable of precise welding.
TURNING OFF:
Turn off the oxygen first, followed by the acetylene
1) 1/2 clamps ( you'll make your own)(I'll show you how, plus their usage)
2) MIG welder pliers (special pliers that do multiple jobs on your wire feed welder)
3) Slag Hammer (It's a NEEDED tool and a SPECIAL tool)
4) "Rattle Gun" (an air-driven multi-tip slag hammer, usually for those that have their own business & do LOTS of welding)
Assignments:
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