Tungsten Inert Gas

. SUMMARY OF PROPOSAL

Welding is a process used to join metal parts by producing a coalescence, called a weld, at a joint. Advances in welding technology have opened the way for innovative designs in structures and machinery. Of modern welding processes, the greatest number fall under the general classifications of arc welding, gas welding and cutting, resistance welding, and brazing.

Tungsten Inert Gas (TIG) welding is one of the arc welding methods. This method is suitable for many kinds of metals, but especially for welding of stainless steels, aluminum and magnesium.

In stainless steel industry, TIG welding has an important role on joining thin sheets, tubes and pipes. Depending on this important role, the mechanical behaviours of TIG welded joints and criteria affecting these behaviours are the main cared subjects in industry to obtain products with less failure and use them for longer service times. Mechanical behaviours of TIG welded joints in stainless steels are also the main subject of the senior project which will be submitted at the end of this year.

The objective of this proposal is to give information on the plan of the project about the mechanical behaviours of TIG welded joints in stainless steels, regarding on the research facilities, equipments, materials and the methods that will be followed and used during the preparation of this project.

2. PROBLEM STATEMENT

A) BACKGROUND

1. TIG Welding

TIG welding, which was developed in late 1920's and perfected in early 1940's, is a commonly used high quality welding process and has become a popular choice of welding processes when high quality, precision welding is required and nearly all metals can be welded by using this process. The main application areas of TIG welding can be mentioned as naval industry, industrial pipe lines, stocking tanks, aviation and shipping.

TIG welding uses a nonconsumable tungsten electrode that creates an arc between the electrode and the weld pool and the metal being melted. An inert shielding gas is used in the process at no applied pressure. Argon is most commonly used as the shielding gas, and the process may be employed with or without the addition of filler metal. Helium, carbon dioxide and hydrogen (mixed with argon) are other main examples of shielding gases for TIG welding.

TIG is widely used for thinner sections. It's extremely suited to join thin sheets, tubes and making root pass welding in pipes, since the heat input in this process is minimal.

Advantages of TIG Welding include its versatility, low equipment costs, control, and weld quality. It is widely used for the welding of light gauge stainless steel and aluminum and root passes in pipe butt joints. The TIG process can easily be set up as an automated process. Another positive attribute of TIG Welding is the very low fume formation rate (FFR). The filler wire is fed and melted into the weld pool allowing a lower FFR. This procedure is different from other processes that require the fill material to pass through the arc. Since filler is fed directly to the weld pool, operating variables have little effect on the FFR.

The sometimes over looked disadvantages of TIG Welding are its low speed and deposition rate which utilizes hot or cold wire feed and high heat input efficiency. By using shielding gas, these problems can be overcome. The TIG weld zone is also difficult to shield properly in drafty environments.

2. Stainless Steels

Stainless steels are a family of iron-base alloys having excellent resistance to corrosion. These steels do not rust and strongly resist attack by a great many liquids, gases, and chemicals. Many of the stainless steels have good low-temperature toughness and ductility. Most of them exhibit good strength properties and resistance to scaling at high temperatures. All stainless steels contain iron as the main element and chromium in amounts ranging from about 11% to 30%. Chromium provides the basic corrosion resistance to stainless steels. Thin chromium oxide layer, which is formed on the surface, gives corrosion resistance to the stainless steels.

There are five different types of stainless steels, according to the amount of other alloying elements. These are :

1. Austenitic Type

2. Ferritic Type

3. Martensitic Type

4. Duplex Type

5. Precipitation Hardenable Type

Austenitic stainless steels have Cr and Ni as basic alloying elements. They have about 45% higher thermal coefficient of expansion, higher electrical resistance, and lower thermal conductivity than mild-carbon steels. High travel speed welding is recommended, which will reduce heat input and carbide precipitation, and minimize distortion. 200 and 300 series are known as austenitic type stainless steels. Type 304 steels are the main examples of austenitic stainless steels. All of the austenitic stainless steels are weldable with most of the welding processes, with the exception of Type 303, which contains high sulphur and Type 303Se, which contains selenium to improve machinability.

Ferritic stainless steels are types of stainless steels that are not hardenable by heat treatment and are magnetic. The coefficient of thermal expansion is lower than the austenitic types and is about the same as mild steel. Type 405, 409, 430, 422 and 446 steels are the mostly preferred types of this kind of stainless steels. All of the ferritic types are considered weldable

with the majority of the welding processes except for the free machining grade 430F, which contains high sulphur content.

Martensitic stainless steels include higher amount of carbon and lower amount of Cr than ferritic stainless steels. They are hardenable by heat treatment and are magnetic. Type 403, 410, 416 and 420 are the most typical types of martensitic stainless steels. For some martensitic stainless steels, welding isn't recommended so it can be said that not all steels of this type are weldable.

Duplex stainless steels are obtained by forming a microstructure having nearly same amounts of ferrite and austenite. This type of stainless steels generally contain 24 % Cr and 5 % Ni. They have higher yield strength and higher tensile