Gas Tungsten Arc Welding of Plate

1
Chapter 16

• Gas Tungsten Arc Welding of Plate

2
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

3
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

4
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

5
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

6
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

7
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

8
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

9
FIGURE 16-2 The hot filler rod end is well within
the protective gas envelope. Larry Jeffus
10
FIGURE 16-5 Filler being remelted as the weld is
continued. Larry Jeffus
11
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

12
FIGURE 16-8 Contaminated tungsten. Larry Jeffus
13
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 X-rayed
• Contamination can be knocked off quickly by
  flipping the torch head

14
FIGURE 16-8 Contaminated tungsten. Larry Jeffus
15
Current Setting

• Amperage on machine's control is the same at the
  arc when
• 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

16
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

17
Figure 16-10 Melting first occurring. Larry
Jeffus
18
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

19
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

20
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

21
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

22
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

23
FIGURE 16-15 Aluminum filler being correctly
added to the molten weld pool. Larry Jeffus
24
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

25
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