Gas Tungsten Arc Welding of Plate 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

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

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

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 Learn more 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