Welding Practice - The Hong Kong Polytechnic University

Welding Practice
metal. By this continuous process of oxidation, the metal can be cut through very
rapidly.
3.2 Plasma Arc Cutting
Plasma arc cutting can be used for cutting all electrically conductive materials,
including stainless steel and aluminium. In this process, when an arc is struck
between the electrode in the torch and the workpiece, gas is heated to a high
temperature, and change into positive ions, neutral atoms and negative
electrons. When matter passes from one state to another, latest heat is
generated and for. a high velocity plasma gas jet, which can melt the metal and
blow it away to form a kerfs The basic arrangement and terminology for a
plasma arc torch are shown below. (see Fig 11)
4. Quality Control and Testing of Welds
Two main methods are employed in the quality control and testing of welds,
namely, destructive testing and non-destructive testing. The former method is
mainly used in a mass production situation where it is acceptable to lose a few
pieces of a workpiece from a large batch so as to obtain sufficient information
concerning the quality of the welds. Non-destructive testing, however, is aimed
at checking the quality without causing any damage to the workpiece, and this is
often employed on work such as in shipbuilding and civil engineering steel
structures.
Destructive testing is essentially in determining the mechanical properties of the
weld. Non-destructive testing is for spotting internal defects that impair the
soundness of the weld during production. The degree of acceptance of quality
thus has to be carefully worked out.
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