Tuesday, April 1, 2008

TIG Welding

Advantages of TIG Welding

High quality welds, low distortion
Easy to mechanize and automate
Flexibility and ease of heat control
All metals, all positions

Limitations of TIG Welding
Lower deposition rates
Good skills required
Sensitive to cleanliness and contamination
Arc blow sensitive

The Tungsten Electrode in Gas Tungsten Arc Welding (GTAW or Tig) can make contact with a Direct Current (DC) power supply either at the positive (+) or at the negative (-) terminal. In the first case the connection is for DCEP (electrode positive) or reverse polarity. In the second one for DCEN (electrode negative) or straight polarity.

30% of the heat is concentrated in the workpiece
70% of the heat in concentrated in the tungsten electrode
Shallow penetration
Wide weld area

70% of the heat is concentrated in the workpiece
30% of the heat is concentrated in the tungsten electrode
Deep penetration
Narrow weld area

Tungsten Electrodes
A few tungsten alloys are used as electrodes for GTAW (tig welding). They are non consumable as they do not enter, intentionally, in the composition of the weld pool. Their function is limited to carry the currents employed, to stand the heat of the arc without melting and to provide electrons by thermionic emission.

Filler Wire
Manual GTAW is often performed autogenously (without added filler metal) when the joint requires only fusion of the abutting ends. However when needed, filler material can be added manually from the side, right in the weld puddle, by alternatively introducing the wire tip into the arc and withdrawing it.

Shielding Gases
The shielding gas is provided continuously through the torch to protect the molten weld metal, the tungsten electrode and the filler wire tip, if used, from air contamination. Backup shielding gas may be provided if necessary on the underside of the weld to purge and displace surrounding air.

Shielding is always needed but it may be critical especially for highly reactive metals like titanium that would be promptly oxidized and ruined if heated in air. To weld titanium by GTAW use is made of glove boxes purged with shielding gas of the highest purity before weld start or of trailing shields, specially designed accessories that provide a sufficient gas trail to protect the welded surface until cool enough.

The gases uses are mostly Argon and Helium or mixtures of the two. Argon is the standard shielding gas. Helium, which is lighter than air, must be supplied with a higher flow than argon because it tends to rise. It transfers more heat than argon for the same current and voltage. Thus it may be needed to join metals of higher conductivity or heavy plates.

When extinguishing the arc at the end of the weld, it is important to let the gas flow for a certain time to shield the hot metal from air. Equipment provides a delay before shutting off the gas flow, but the welder must linger with the torch upon the weld as long as needed.

Most weldable metals can be welded by the GTAW process. It is adaptable to thin and delicate joints but also to thick and demanding applications, always of good quality although possibly not the most fast and economic process.

Aluminum alloys of the weldable kinds are among those for which GTAW is most suited. One should always remember that not all aluminum alloys can be welded successfully or with the same ease. For a reminder of the reasons making some of the strongest alloys non readily weldable by fusion processes see Aluminum Welding.

Joining is made difficult by the surface formation of tenacious refractory aluminum oxides of melting point much higher than that of aluminum metal. The oxides though are broken up by the cathodic cleaning action of the Electrode Positive part of the alternating current cycle. Once broken they may float upon the molten metal and they no longer interfere with the welding process. Therefore high frequency stabilized alternating current power supply is the most used GTAW process for aluminum with pure tungsten electrodes. Exceptionally, for thin sections, DCEP can be used with thoriated tungsten. Argon is the most used shielding gas.


Materials and Welding for Cryogenic Service

Vessels and pipelines designed to operate at very low temperatures(cryogenic) must be fabricated from materials and with techniques suitable to the service conditions.

The problems connected with the behavior of normal steels at low temperatures were first brought to attention at the end of World War II, when eight ships of the Liberty class where involved in spectacular disasters by their sudden brittle fracture in two parts under conditions of cold weather and high seas.

The investigations that followed were instrumental in discovering a new property, unheard of up to that time, called Fracture Toughness. Furthermore it was found that, in certain materials, ductility is strongly dependent upon temperature.

A special Drop-Weight Nil-Ductility-Test was introduced to help rating materials and processes and to determine the temperature above which a dynamic crack is arrested.

In normal steels perfectly ductile at room temperature, a gradual passage to brittle fracture susceptibility upon impact was found, strongly dependent on decreasing temperatures. The temperature range where this phenomenon occurs has been called the Ductile to Brittle Transition Temperature (DBTT). Structures made for service at low temperatures must therefore be made of materials that maintain adequate ductility at sub freezing conditions. Cryogenic alloys include austenitic stainless steels, manganese stainless steels, 9 Nickel steels, maraging steels, titanium, aluminum and nickel alloys.

A low cost way to measure the resistance of a weldment to impact fracture is performed by means of an Impact Toughness Testing on a specimen of specified geometry like the Charpy V-notch (CVN) test (ASTM E23). The amount of energy absorbed at any specified temperature during fracture is measured and recorded.

This test does not measure an inherent material property but results in a relative measure of impact toughness. The results permit to rate different materials and procedures in a database to be used for design and for inspection.

The specimen is oriented to have the notch and the expected plane of fracture run longitudinally through the weld metal. The test is conducted on welded specimens at the specified low temperature. The CVN absorbed energy result should be at least as specified by the requirements.

Cryogenic Tanks are used for Liquefied Natural Gas (LNG), Cryogenic Ammonia and other products at cryogenic temperatures for storage and/or transportation.


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In-Demand Job in Oil & Gas Industry

Did you know that the most in-demand job in the Oil and Gas Industry are Welders and Welding Inspectors?

In Philippines alone, when you check the classified ads of Manila Bulletin every sunday, you can see a lot of vacancies for welders and Welding Inspectors (or in general, Inspectors). These vacancies are either local or overseas jobs.

Correct me if I am wrong. Check the Manila Bulletin every sunday to see it for yourself.

Some Welding Codes and Standards normally used in Construction

1.) AWS D1.1 - For Structural Welding
2.) API 1104 - For Pipeline
3.) ANSI B31.3 - For Process Piping
4.) ANSI B3.1 - For Power Piping
5.) ASME VIII Div. 1 - For Pressure Vessel
6.) API 650 - For Storage Tank