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Welder’s Handbook
P
Plasma Arc Gouging – Gouging utilizing a plasma arc for metal removal. An electric arc contained inside a gas shield is passed through a constricting orifice in order to generate extremely high temperatures and a high velocity stream of ionized gas. This stream Rapidly melts the metal on which it is focused and then blows the molten material away.
Pearlite – Eutectoid alloy of iron and 85% carbon consisting of layers or plates of ferrite and cementite.
Peening – The mechanical working of metal by means of hammer blows to relieve stresses and reduce distortion. Peening is recommended for thicker sections (over 1” or 2”) of some alloys on each successive pass. Experience has shown that peening helps to reduce cracking. Peening may decrease the ductility and impact properties; however, the next pass will nullify this condition. For this reason, the last surface layers should not be peened. Penetration – (1) The depth below the surface of the base metal to which welding heat is sufficient for the metal to melt and become liquid or semi-liquid. Also called the depth of fusion. (2) The ability of arc or electrode to reach into the root of the groove between two members being welded. Phase Transformation – The changes in the crystalline structure of metals caused by temperature and time. Piercing – A method of starting a plasma arc cut in which the arc plunges into and through the workpiece before cutting begins. Pig iron – The product of the blast furnace cast into blocks convenient for handling or storage; iron alloy as recovered from the ore. A brittle material of high carbon content (5%). Pilot Arc – A low current arc between the electrode and the constricting nozzle of the plasma arc torch to ionize the gas and facilitate the start of the welding / cutting arc.
Plasticity – Ability of a metallic state to undergo permanent deformation without rupture.
Plunger – It is installed behind the removable seat in a plasma torch head to enable the safety interlock.
Porosity – The scattered presence of gas pockets or inclusions in a metallic solid.
Post Weld Heat Treatment – Reheating the weldment to 1100°F to 1350°F after welding and holding at that temperature for a specified length of time. Heat treating allows additional hydrogen to escape, lowers the residual stresses due to welding, and restores toughness in the heat affected zone. Post-heating – Heat applied to the base metal after welding or cutting, for the purpose of tempering, stress-relieving, or annealing.
Power Source – An apparatus for supplying current and voltage suitable for welding, cutting, etc.
Preheat – The heating of the parts of a structure to be welded before welding is started to minimize thermal shock and of slowing the cooling rate. Preheat Temperature – The temperature to which many of the low alloy steels must be heated before welding. Preheating retards the cooling rate, allowing more time for the hydrogen to escape, which minimizes under-bead cracking. Preheat temperatures can vary from 10°F to 500°F on ½” sections to 300°F to 600°F on heavy sections, depending upon the alloy. Preheating – The heating of parent metal prior to welding or cutting for the purpose of minimizing thermal shock and of slowing the cooling rate.
Pilot Hole – A punched hole at (near) the plate edge of which the plasma arc cut is started.
Plasma – A gas that has been heated by an arc to at least a partially ionized condition, enabling it to conduct an electric current. Plasma Arc Cutting (PAC) – An arc cutting process that uses a constricted arc and removes the molten metal in a high velocity jet of ionized gas issuing from the constricting orifice. Plasma arc cutting is a direct current electrode negative (DCEN) process. Plasma Gas – A gas directed into the torch to surround the electrode, which becomes ionized by the arc to form a plasma and issues from the torch nozzle as the plasma jet. (Also referred to as orifice gas or cutting gas).
Procedure – The detailed elements of a process or method used to produce a specific result.
Proton – Positively charged particles which are part of the nucleus of atoms.
Pulsed MIG Welding – Process is used mainly for welding aluminium and stainless steel. The method of controlling the transfer of the droplets by current pulses from the power source makes it possible to extend the spray range down. The process provides a stable and spatter free arc.
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