© 2012 Delmar, Cengage Learning Chapter 16 Gas Tungsten Arc Welding of Plate.

© 2012 Delmar, Cengage Learning

Chapter 16

Gas Tungsten Arc Welding of Plate


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


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



• 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



• Demonstrate how to properly make GTA welds in butt joints, lap joints, and tee joints in all positions that can pass the specified standard


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


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


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


FIGURE 16-2 The hot filler rod end is well within the protective gas envelope. Larry Jeffus


FIGURE 16-5 Filler being remelted as the weld is continued. Larry Jeffus


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


FIGURE 16-8 Contaminated tungsten. Larry Jeffus


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


FIGURE 16-8 Contaminated tungsten. Larry Jeffus


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


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


Figure 16-10 Melting first occurring. Larry Jeffus


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


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


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


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


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


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


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


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 Chapter 16 Gas Tungsten Arc Welding of Plate.

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Transcript of © 2012 Delmar, Cengage Learning Chapter 16 Gas Tungsten Arc Welding of Plate.