The term weldability is the ability to obtain economic welds, which are good, crack-free and would meet all the requirements. What is of great importance is the chemistry and the structure of the base metal and the weld metal. The effects of heating and cooling associated with welding are experienced by the weld metal and the Heat Affected Zone (HAZ) of the core metal. The base metal surrounding the weld metal and the weld itself have unduly varying hardness distribution across a weld. The hardness in steel depends upon the rate at which steel is cooled near the fusion zone; the hardness is maximum due to the higher temperature at that location and also have the maximum rate of cooling. A higher value of hardness leads to cracks in HAZ or in the weld. There are chances of Cracks that might be formed during or after the welding process. Good design and standard welding procedure help minimise the cracking problem.
The main features that affect weld cracking during the welding processes are
• Joint restraint that builds up high stress in the weld (convex or concave)
• Carbon and alloy content
• Cooling rate
• Hydrogen and nitrogen absorption
The cracks in HAZ are mainly caused by high carbon content, hydrogen embrittlement and rate of cooling. In most of the steels, weld cracks become a problem as the thickness of the plate increases. Types of joints and welds By means of welding, it is possible to make continuous, load-bearing joints between the members of a structure.
A variety of joints is used in structural steelwork and they can be classified into four basic configurations are
The weld defects detected during inspection are acceptable for structures. • For joints welded from both the surfaces, incomplete perception with thickness up to 5% of the parent metal thickness, but not surpassing 2 mm and the expansion, more than 500 mm can be accepted. The aggregate expansion of stain shall not be more than 200 mm per meter length of the joint. Incomplete perception and cracks are not permitted at or near the end or beginning of a joint.
• For joints united from one side without backing strip, incomplete penetration with thickness up to 15% of parent metal thickness but not exceeding 3 mm at the root is permitted.
• Slag inclusion located along the weld as a chain or unbroken line is allowed if their whole length does not exceed 200 mm per meter of the weld length. Size of the slag may also be considered.
• Total of isolated gas pores and slag inclusion shall not exceed 5 in number per square centimetre of the weld.
• The incomplete penetration, slag inclusion on pores located separately or as a chain shall not exceed 10% of metal thickness but not greater than 2 mm when welding is done from both the sides and 15% of metal thickness, but not vaster than 3 mm when welding is done from one side.