Transcript Chapter 16 Gas Tungsten Arc Welding of Plate © 2012 Delmar, Cengage Learning Objectives • Name the applications for which the gas tungsten arc welding process.

Chapter 16
Gas Tungsten Arc Welding
of Plate
© 2012 Delmar, Cengage Learning
Objectives
• Name the applications for which the gas tungsten
arc welding process is more commonly used
• Discuss the effects on the weld of varying torch
angles
• Explain why the filler rod end must be kept inside
the protective zone of the shielding gas and how
to accomplish this
• Tell how tungsten contamination occurs and what
should be done when it happens
© 2012 Delmar, Cengage Learning
Objectives (cont'd.)
• Explain what can cause the actual welding
amperage to change
• Determine the correct machine settings for the
minimum and maximum welding current for the
machine used, the types and sizes of tungsten,
and the metal types and thicknesses
• List factors that affect the gas preflow and postflow
times required to protect the tungsten and the
weld
© 2012 Delmar, Cengage Learning
Objectives (cont'd.)
• Determine the minimum and maximum gas flow
settings for each nozzle size, tungsten size, and
amperage setting
• Compare the characteristics of low carbon and
mild steels, stainless steel, and aluminum in
respect to GTA welding
• Describe the metal preparation needed before
GTA welding
© 2012 Delmar, Cengage Learning
Objectives (cont'd.)
• Demonstrate how to properly make GTA welds in
butt joints, lap joints, and tee joints in all positions
that can pass the specified standard
© 2012 Delmar, Cengage Learning
Introduction
• Gas tungsten arc welding
– Also called GTA welding
– Can be used to for nearly all types and thicknesses
of metal
– Fluxless, slagless, and smokeless
– Welders have fine control of the welding process
– Used when appearance is important
– Setup of equipment affects weld quality
• Charts give correct settings
• Field conditions affect the variables
© 2012 Delmar, Cengage Learning
Torch Angle
• Key points
– Torch should be held as close to perpendicular as
possible
– May be angled zero to fifteen degrees from
perpendicular for better visibility
– As the gas flows out it must form a protective zone
around the weld
– Too much tilt distorts protective shielding gas zone
– Velocity of shielding gas affects protective zone as
torch angle changes
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Filler Rod Manipulation
• Filler rod must be kept inside the protective zone
– If removed from the gas protection
• Oxidizes rapidly: oxide is added to weld pool
• Rod tip becomes oxidized: cut it off
• Weld is temporarily stopped
– Shielding gas must be kept flowing
• Rod should enter shielding gas as close to base
metal as possible
– Angles under 15 degrees prevent air from being
pulled in welding zone
© 2012 Delmar, Cengage Learning
FIGURE 16-2 The hot filler rod end is well within the protective gas envelope.
Larry Jeffus
© 2012 Delmar, Cengage Learning
FIGURE 16-5 Filler being remelted as the weld is continued.
Larry Jeffus
© 2012 Delmar, Cengage Learning
Tungsten Contamination
• Most frequent problem
– Tungsten becomes contaminated if it touches
molten weld pool or filler metal
– Surface tension pulls contamination up onto the hot
tungsten
– Extreme heat causes some of the metal to vaporize
and form a large oxide layer
© 2012 Delmar, Cengage Learning
FIGURE 16-8 Contaminated tungsten.
Larry Jeffus
© 2012 Delmar, Cengage Learning
Tungsten Contamination (cont'd.)
• Contamination forms a weak weld
– Weld and tungsten must be cleaned before any
more welding can be done
– Tiny tungsten particles will show up if the weld is Xrayed
– Contamination can be knocked off quickly by
flipping the torch head
© 2012 Delmar, Cengage Learning
FIGURE 16-8 Contaminated tungsten.
Larry Jeffus
© 2012 Delmar, Cengage Learning
Current Setting
• Amperage on machine's control is the same at the
arc when:
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–
–
–
–
Power to machine is exactly correct
Lead length is very short
All cable connections are perfect
Arc length is exactly right
Remote current control is in full on position
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Experiments
• Designed to help new welders learn basic skills
– Help troubleshoot welding problems
• Learn more
– Subtle changes will become more noticeable
– Even experienced welders make changes
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Figure 16-10 Melting first occurring.
Larry Jeffus
© 2012 Delmar, Cengage Learning
Gas Flow
• Gas preflow and postflow times depend upon:
–
–
–
–
–
–
–
Wind or draft speed
Nozzle size
Tungsten size
Amperage
Joint design
Welding position
Type of metal welded
• Maximum flow rates must never be exceeded
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Practice Welds
• Grouped according to weld position and type of
joint
– Mild steel
• Inexpensive
• Requires the least amount of cleaning
– Aluminum
• Cleanliness is a critical factor
– Try each weld with each metal
• Determine which metal will be easier to master
© 2012 Delmar, Cengage Learning
Low Carbon and Mild Steels
• Two basic steel classifications
– Most common
– During manufacturing small pockets of primary
carbon dioxide gas become trapped
• Do not affect strength
– Porosity: likely when not using a filler metal
– Most filler metals have some alloys (i.e.,
deoxidizers)
• Prevent porosity caused by gases trapped in base
metal
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Stainless Steel
• Setup and manipulation
– Nearly the same as for low carbon and mild steels
• Welds show effects of contamination
– Precleaning is important
• Most common problem
– Bead color after the weld
• Using a low arc current with faster travel speeds is
important
– Carbide precipitation
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Aluminum
• Molten aluminum weld pool
– High surface tension
• Preheat base metal in thick sections
– Preheat temperature is around 300 degrees
Fahrenheit
• Cleaning and keeping the metal clean
– Time consuming
• Aluminum resists oxidation at room temperature
– Rapidly oxidizes at welding temperatures
© 2012 Delmar, Cengage Learning
FIGURE 16-15 Aluminum filler being correctly added to the molten weld pool.
Larry Jeffus
© 2012 Delmar, Cengage Learning
Metal Preparation
• Base and filler metals
– Must be thoroughly cleaned
– Contamination will be deposited into the weld
• Oxides, oil, and dirt are the most common
• Contaminants can be removed mechanically or
chemically
© 2012 Delmar, Cengage Learning
Summary
• Position yourself to control the electrode filler
metal and to see the joint
– Experienced welders realize they need to see only
the leading edge of the weld pool
• Good idea to gradually reduce your need for
seeing 100% of the weld pool
– Increasing this skill is significant advantage
• Welding in the field
– May have to be done out of position
© 2012 Delmar, Cengage Learning