Welding and NDT

WELDING AND NDT

WHY WE NEED NDT IN WELDING INSPECTION?
To find out welding defects in welding without destroying it is an NDT technique.

WELDING DEFECTS AND DISCONTINUITIES-
During the welding operation, many unavoidable volume changes occur that may cause
distortion, residual stresses or even cracking in the weldment. These changes are due largely to thermal expansion or contraction of the metal. If the rigidity of the structure Is high because of the stiffness of the members themselves, or support of previous welding or the use of fixtures, the amount of warping will be small. The forces set up by the weld contraction, however, will reach high levels by the time room temperature is reached. Cracking may result while the metal Is still hot or during cooling. Thus, the more highly con-strained the joint to be welded, the more plastic must be the weld metal that is used to complete the joint.


Hardening of the weld and adjacent base metal, when heated to above the transition temperature of the metal, can occur if the steel contains s carbon and the Cooling rate is rapid. Hardening can be avoided in most hardenable steels by pre-heating. or post-heating the weld. If air-hardening steels are welded, the best heat treatment is a full furnace anneal of the welded item. Hardness in the weld zone should usually be avoided because of the corresponding lack of ductility and susceptibility to cracking.

Another factor to be considered in welding is the possible tendency towards hot shortness of the material (a marked Joss in strength at high temperatures). Some of the cope base alloys have this tendency to a marked degree. Allowances should be made in the welding technique used with these materials, and the jigging and clamping should be done with caution. Proper welding sequence and multilayer welding with narrow stringer beads help to reduce the hot cracking.

Other cracking occurs in weld metal because of a combination of the above factors, stress risers and other causes such as shallow beads, unfilled crates at the end of a weld bead, welding at low- atmospheric temperatures, welding.in high winds or wet weather. Codes and specifications governing welding allow certain maximum size and number of defects, such as slag, porosity, blowholes, inclusions and in certain cases, slight lack of penetration and/or fusion. Cracking is never acceptable. The end-use of the welded product governs the limitations of allowable defects.

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