Heat Flow in Welding

Heat Flow in Welding Lesson Objectives Learning Activities Read Handbook pp 66-84 Look up Keywords View Slides; Read Notes, Listen to lecture Do on-line workbook Do homework Lesson Objectives When you finish this lesson you will understand: Heat transfer through the welding arc & factors affecting it Heat flow in welded parts & conservatio of energy Thermal profiles around moving heat sources & cooling rates & stresses around welds Keywords: Conduction, Convection, Radiation, Arc Heat Flow, Thermal Conductivity-Gas, Heat Flux, Fourier’s Law, Conservation of Energy, Moving Heat Source, Thermal Cycles

HEAT FLOW (TRANSFER) CONDUCTION - Transfer of Thermal Energy between one part of a body and an Adjacent part of the same body or between one body and another which is in physical contact with it. CONVECTION - Transfer of Thermal Energy through mass movement. RADIATION - Transfer of Thermal Energy by the emission and absorption of Electromagnetic Radiation.

Heat Flow in the Arc Convection Radiation Radiation Thermionic Cathode Electrons Emitted Thermal Ionization Free Electron Convection Radiation Ion Plasma T>10,000K Recombination After breakdown of the gas in the arc gap is created, current flow through the gas commences and rapidly grows. Since electrons are much lighter than ions, and therefore pick up speed and energy more rapidly in the potential difference, current is carried mainly by electrons. This is accompanied by intense resistive heating of the gas from collisions with the accelerating electrons, and a rise in temperature of the gas. As the gas heats up, so-called thermal ionization maintains ionization as charges are lost by recombination of free electrons and ions at the cool edges of the arc. If the source supplying the voltage is capable of suitably delivering the high current demanded by the arc, then a stable mode of current flow will be reached which is called an arc. At atmospheric pressure, this means that the power source must be able to deliver tens to hundreds of amperes at several tens of volts. If the power source cannot deliver current under these conditions, then only a transient spark will occur. The temperature of gasses in the arc typically rise to 10,000oK and higher as the arc develops. These temperatures are considerably higher than that of any other type of heat source, being well above temperatures achieved in conventional resistive heaters or combustion flames. Such high temperatures are necessary for the sustaining of ionization processes if the arc is to persist. The hot, ionized gas of the arc is sometimes referred to as a plasma. As current flows, electrons must enter the arc from the solid conductor at the negative electrode (cathode), and be reabsorbed at the positive electrode (anode). Physical processes in the arc must cause this to occur for a stable arc to be maintained. Anode Neutral Gas Atom Radiation Thermionic Electrons Absorbed

Conduction in the Arc Argon Helium Helium Argon Cathode Electrons Emitted Thermal Ionization Free Electron Argon Ion Plasma Helium T>10,000K Recombination After breakdown of the gas in the arc gap is created, current flow through the gas commences and rapidly grows. Since electrons are much lighter than ions, and therefore pick up speed and energy more rapidly in the potential difference, current is carried mainly by electrons. This is accompanied by intense resistive heating of the gas from collisions with the accelerating electrons, and a rise in temperature of the gas. As the gas heats up, so-called thermal ionization maintains ionization as charges are lost by recombination of free electrons and ions at the cool edges of the arc. If the source supplying the voltage is capable of suitably delivering the high current demanded by the arc, then a stable mode of current flow will be reached which is called an arc. At atmospheric pressure, this means that the power source must be able to deliver tens to hundreds of amperes at several tens of volts. If the power source cannot deliver current under these conditions, then only a transient spark will occur. The temperature of gasses in the arc typically rise to 10,000oK and higher as the arc develops. These temperatures are considerably higher than that of any other type of heat source, being well above temperatures achieved in conventional resistive heaters or combustion flames. Such high temperatures are necessary for the sustaining of ionization processes if the arc is to persist. The hot, ionized gas of the arc is sometimes referred to as a plasma. As current flows, electrons must enter the arc from the solid conductor at the negative electrode (cathode), and be reabsorbed at the positive electrode (anode). Physical processes in the arc must cause this to occur for a stable arc to be maintained. Anode Neutral Gas Atom Helium Argon Electrons Absorbed

Questions? Turn to the person sitting next to you and discuss (1 min.): The thermal conductivity of Argon and Helium increases continuously with increasing temperature, but that of nitrogen and hydrogen show a drop in conductivity where the diatomic gas atom dissociates. What do you think would happen if a significant amount of these gases were in the arc plasma?

Temp Profile After Arc Start

Fourier's Law of Heat Conduction Heat Flux Thot dy Tcooler

Conservation of Energy Heat In - Internal Energy + Heat Generated = Heat Out

Moving Heat Source Arc Finish Start Thermocouples

TEMP

Fast Travel Speed Slow Travel Speed 1 2 3 4 5

Questions Turn to the person sitting next to you and discuss (1 min.): This curve was generated when a fast moving arc passed by the thermocouples. What curves would a slow moving arc produce?

Homework Do the Homework Assignment 5 “Heat Flow in Welding” from the Assignment Page of the WE300 Website