Heat Flow in Welding

Heat Flow in Welding

Lesson ObjectivesWhen 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

Learning Activities1. Read Handbook pp

66-842. Look up Keywords3. View Slides; 4. Read Notes, 5. Listen to lecture6. Do on-line

workbook7. Do homework

Keywords:Conduction, Convection, Radiation, Arc Heat Flow, Thermal Conductivity-Gas, Heat Flux, Fourier’s Law, Conservation of Energy, Moving Heat Source, Thermal Cycles

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

Plasma

Electron

Ion

Neutral Gas Atom

Ionization Free

Recombination T>10,000K

Thermal

Cathode

Anode

Electrons Emitted

Electrons Absorbed

ConvectionRadiation

ThermionicRadiation

Conduction in the Arc

Plasma

Electron

Ion

Neutral Gas Atom

Ionization Free

Recombination T>10,000K

Thermal

Cathode

Anode

Electrons Emitted

Electrons Absorbed

Argon

Argon

Helium

Helium

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?

Y t < 0

t = 0

Small t

Large t

T (y,t)

(Initial Transient)

Steady State

Y t < 0

t = 0

Small t

Large t

T (y,t)

(Initial Transient)

Steady State

sec

sec

sec2

calflowheatAqq

Kcm

caltyconductivithermal

cm

calfluxheatq

dy

dq

y

o

y

y

dy

Thot

Tcooler

yq

......

dyy

qqq yydyy

dyyq

Heat In - Internal Energy + Heat Generated = Heat Out

capacityheatC

density

QGeneratedHeat

Adyt

TCEnergyInternal

AqInHeat

p

gen

p

y

.

d

d

Start

Finish

Thermocouples

Arc

TEMP

Slow Travel Speed

Fast Travel Speed

1 2 34 5

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?

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

WE300 Website