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WELDABILITY. The weldability of a material refers to its ability to be welded . Many metals and thermoplastics can be welded, but some are easier to weld than others. It greatly influences weld quality and is an important factor in choosing which welding process to use
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WELDABILITY The weldability of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others. It greatly influences weld quality and is an important factor in choosing which welding process to use Weldability is simply a measure of how easy it is to make a weld in a particular material without cracks. If it is easy to avoid cracking, the material is deemed 'weldable'. For a weld to be truly successful, however, it is also necessary for it to have adequate mechanical properties, and to be able to withstand degradation in service (e.g. corrosion damage).
Thus, weldability is a measure of how easy it is to: • Obtain crack free welds • Achieve adequate mechanical properties • Produce welds resistant to service degradation. • Weldability is not a fixed parameter for a given material, but will depend on joint details, service requirements, and welding processes and facilities available. • This variability in weldability is illustrated in the following examples:
Example 1 Which of these two C-Mn steels is most weldable? • The answer clearly depends on which type of cracking is of most concern: • Low restraint fillet onto thick steel - Hydrogen crack, steel 1 more weldable • Restrained high dilution MIG nozzle weld - solidification crack, steel 2 more weldable • Full penetration highly restrained T butt - lamellar tearing, steel 2 more weldable.
Example 2Which of these materials is most weldable? (welding a fairly thin walled (~3mm) pipe)Commercially pure titanium 316 L austenitic stainless steel 22% Cr duplex stainless steel 6% Mo high alloy austenitic stainless steel • The answer will depend on an individual's experience, and available facilities. • The titanium expert knows that it is one of the easiest materials to weld - but he is very familiar with very good back purges, and the use of a trailing shield. • The expert in austenitic stainless steel would see this level of control to be very difficult. He knows to watch out for solidification cracking, and is careful to check the penetration characteristics of each cast, and does not consider that these pose a significant risk. • An expert in duplex stainless steels will tell you that it is much easier to weld than austenitic stainless steel, because there is no real risk of solidification cracking, and less of a variable penetration problem. But now, you generally need a filler. • High alloy austenitic steel is similar to duplex, expect that with a Ni based filler there is a risk of microfissuring.
Example 3 • Consider Example 2, but now add that the weld will be operating in an acid, chloride containing environment at about 30°C. Had the concern been purely about freedom from cracking, then duplex and titanium would have been on an equal footing, with the high alloy austenitic being the least weldable because of the risk of solidification cracking. Now, however, the duplex stainless steel becomes more of a problem, as it becomes necessary to work within quite a narrow heat input window. It can be difficult to pass procedure qualification tests involving corrosion tests with duplex stainless steels. • Example 4 • Consider examples 2 and 3, but now add a toughness requirement. Now titanium is not so weldable, as near perfect shielding is necessary to avoid toughness degradation.
Example 5Is AISI 4130 weldable? The composition range for AISI 4130 is: • It is not possible to answer this question without knowing the intended service. The answer would be different for a gear component, to operate in a warm oil bath, and a piece of wellhead equipment to carry sour gas.
Weldability of materials- Steels • In arc welding, as the weld metal needs mechanical properties to match the parent metal, the welder must avoid forming defects in the weld. Imperfections are principally caused by: • poor welder technique; • insufficient measures to accommodate the material or welding process; • high stress in the component. • Techniques to avoid imperfections such as lack of fusion and slag inclusions, which result from poor welder techniques, are relatively well known. However, the welder should be aware that the material itself may be susceptible to formation of imperfections caused by the welding process. • In the materials section of the Job Knowledge for Welders, guidelines are given on material weldability and precautions to be taken to avoid defects.
Material types In terms of weldability, commonly used materials can be divided into the following types: • Steels • Stainless steels • Aluminium and its alloys • Nickel and its alloys • Copper and its alloys • Titanium and its alloys • Cast iron • Fusion welding processes can be used to weld most alloys of these materials, in a wide range of thickness. • When imperfections are formed, they will be located in either the weld metal or the parent material immediately adjacent to the weld, called the heat affected zone (HAZ). • As chemical composition of the weld metal determines the risk of imperfections, the choice of filler metal may be crucial not only in achieving adequate mechanical properties and corrosion resistance but also in producing a sound weld. • HAZ imperfections are caused by the adverse effect of the heat generated during welding and can only be avoided by strict adherence to the welding procedure.
Imperfections in welds • Commonly used steels are considered to be readily welded and can be at risk from the imperfections: • porosity; • solidification cracking; • hydrogen cracking; • reheat cracking. • lamellar tearing • Using modern steels and consumables, these types of defects are less likely to arise.
Welding Metallurgy and Weldability of Stainless Steels by John C. Lippold (Author), Damian J. Kotecki (Author)
New developments in advanced welding • Edited by N Ahmed, CSIRO, Australia • - discusses the changes in advanced welding techniques - looks at new technologies - explores mechanical and structural engineering examples • summarises some of the most important of these and their applications in mechanical and structural engineering. begins by reviewing advances in gas metal arc welding, tubular cored wired welding and gas tungsten arc welding. • A number of chapters discuss developments in laser welding, including laser beam welding and Nd:YAG laser welding. • Other new techniques such as electron beam welding, explosion welding and ultrasonic welding are also analysed. • The book concludes with a review of current research into health and safety issues. This is a standard guide for the welding community US $275.00
Contents • Gas metal arc welding • Tubular cored wire welding • Gas tungsten arc welding • Laser beam welding • Nd: YAG laser welding • New developments in laser welding • Electron beam welding • Developments in explosion welding technology • Ultrasonic metal welding • Occupational health and safety
