In this Welding, Cutting, and Brazing module, three specific types of welding are covered. These are listed below: Oxygen-fuel gas welding and cutting Arc welding and cutting Resistance welding.
Electric current and mechanical pressure Resistance to the flow of current heats the material. Pressure is simultaneously applied to the joint, forming a solidified nugget that attaches the pieces.
I. Speaker’s Notes: Acetylene is extremely dangerous because of its flammability range. Oxyacetylene torch cutting is very common throughout the industry as it has a wide flammable range. Acetylene is extremely unstable. Acetylene is generated in the cylinder from a reaction of acetone and calcium silicate. Relief valves for generating chamber shall be set to open at a pressure not in excess of 15 psi. Using acetylene at pressures in excess of 15 psig pressure (or about 30 psia pressure) is a hazardous practice. II. Speaker’s Notes: This slide lists basic rules to follow when welding with oxyacetylene. Before connecting to the regulator, always blow out the cylinder valve. Release the regulator by adjusting the screw before opening the cylinder valve. Always stand to the side of the regulator while opening the cylinder in case of accidental pressure release. Open the valve cautiously and slowly. Never use acetylene at pressures above 15 pounds per square inch.
I. Speaker’s Notes: Before lighting the torch, remember to purge acetylene and oxygen passages. Before opening the oxygen on the torch, light the acetylene. Never use oil or grease on regulators, tips, or any part that may come into contact with oxygen. Do not use oxygen as a substitute for air. Always keep your work area free of items that could ignite.
Speaker’s Notes: Oxygen : When oxygen is supplied to a service piping system from a low pressure oxygen manifold without an intervening pressure regulating device, the piping system shall have a minimum design pressure of 250 psig. A pressure regulating device shall be used at each station outlet when the connected equipment is for use at pressures less than 250 psig. Acetylene or Acetylene Compounds : Piping for acetylene or acetylenic compounds shall be steel or wrought iron.
Speaker’s Notes: Pressure Relief Devices : Relief valves will vent automatically at preset pressures or may be manually operated to relieve pressure in the system.
Speaker’s Notes: The three function : The protective equipment in fuel-gas piping systems shall be located either at the main supply line, at the head of each branch line, or at each location where fuel-gas is withdrawn
Speaker’s Notes: Local exhaust Ventilation have enough velocity to draw away the contaminants. Natural Ventilation : Welding is not done in a confined space, and Welding space does not contain partitions, balconies or structured barriers that obstruct cross ventilation Mechanical Ventilation: These systems consist of hoods positioned at a distance from the work area.
Joining Processes WELDING
WELDING Definition: Material joining process. Two parts connected at their contacting surfaces by suitable heat and pressure. Many welding processes are accomplished by heat alone, some others by heat and pressure, and some with pressure only. In some welding operations a filler material is used. Welding operation usually applied to metals but also used for plastics.
Parts produced by any of the manufacturing processes can be made into larger, more complex bodies via Joining processes
Creating a metallurgical bond by adhesion and diffusion
Joining by fusion with the use of various heat sources
Metal fumes from vaporizing of the work with the extremely hot arcs may be inhaled into the worker’s lungs.
Certain metals and metal oxide fumes, including zinc, cadmium and beryllium, produce serious illnesses when inhaled .
Fluxes used with welding to create inert atmospheres at the point of the weld also present inhalation hazards.
All welding and cutting must have adequate ventilation to protect the person doing the welding and those working around the welding area.
Weld Joint Structure Characteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding. Microhardness (HV) profile across a weld bead. A fusion joint is far from homogenous. Degree of inhomogeity increases from pure metals to multiphase alloys.
Preheating the weld zone – reduces energy input, cooling rates in the weld and HAZ, reduces differential shrinkage, residual stresses, and distortion.
Postwelding heat treatment of the entire welded structure
Stress-relief anneal reduces residual stresses to acceptable level.
Normalizing a steel wipes out most undesirable effects of welding.
Full heat treatment (quenching and tempering of steels)
Peening (hammering or rolling) of weld bead improves the strength of welds.
Oxyacetylene Gas Welding Three basic types of oxyacetylene flames used in oxyfuel-gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel-gas welding operation.
Oxyacetylene Torch The acetylene valve is opened first; the gas is lit with a spark lighter or a pilot light; then the oxygen valve is opened and the flame adjusted. Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all threads on acetylene fittings are left-handed, whereas those for oxygen are right-handed . Oxygen regulators are usually painted green, and acetylene regulators red.
Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial welding operations use this process. Consumable Electrode Arc Welding Shielded-Metal Arc Welding
Heat is produced from electric arc between workpiece and electrode material for melting the workpiece material.
AC and DC are used
An Inert gas shields both electrodes
Most heat energy is due to electron flow to metal
Direct Current Electrode Negative (DCEN): Deeper weld. Direct Current Electrode Positive (DCEP): Shallower and Wider Heat input H- heat input, E, Voltage, I, Current, and v the velocity of the arc travels along the weld line Electric Arc Welding
Consumable-Electrode Welding: Gas Metal-Arc Welding (GMAW) - MIG
Consumable gas metal-arc welding (MIG)
Consumable electrode is metal which melts to become part of the weld seam.
Weld zone is protected by a gas or a flux
No slag is formed
Several layers could be build with little or no intermediate cleaning
It is suitable for most metals
Wire electrode can be supplied in long, coiled lengths which allow uninterrupted welds in any welding position.
(a ) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert gas) welding. (b) Basic equipment used in gas metal-arc welding operations . Consumable Electrode Arc Welding Gas - Gas Metal-arc Welding Process-