rolling and bending processes requires heavy machinery and skillful people. But they cannot produce more complex shapes especially for double curve plate forming. And also after the forming process the plate may be slightly thinner. And also it is difficult to predict how the plate will be over bend. Line heating process helps to overcome these problems.
Line heating is a method of double curved plate by means of local heat treatment. This method is specially used in shipyard industries. In this case no problems of reduction of plate thickness therefore the plates can be obtain the approval of classification societies beyond doubt. Equipment used for this method is very simple. Those are heating bed, water line, heating torch (LP gas or Oxy-acetylene) and some other gadgets to be fixed and holed the plate.
This is very difficult to control therefore more skillful and well experienced people are recommended to be involved into this. In this process plates are subjected to heat and rapid cooling in order to obtain the desired shape or curvature from the plate. To do this heating lines are selected according to the curvature and heating is continued along those line.
When the plate is being heated; the heated area become more soft (lower yield limit) and same time expands. But it has its original strength yet due to its expansion. Then the plate is cooled using water steam. Due to this cooling regain the strength and the thermal contraction bends and shrinks the plate. But it is recommended to control the heat and to cooling rate to obtain desired shape accurately.
Heating and rapid cooling of steel plates changes its composition and atomic structure. Therefore its properties may change. Normally when a steel is heated over its lover critical temperature (for mild steel (0.16–0.29% of Carbon) 730oC) and cooled rapidly; it become more brittle because of stabilization of Cementite (FeC3). Therefore heat must be controlled so that temperature will not exceed critical temperature of steel. Normally hull plates are mild steel; therefore it should be made sure that heat is not exceeding 650oC.
To check the temperature of heated metal plates Heat Sticks are used. Heat Sticks are a very simple to used. It can only show temperatures higher than the melting temperature (rated temperature) of heat stick material.
To check the temperature heat stick should be rubbed on the heated object to be made a mark. Once the surface reaches the rated temperature, the mark would melt and show liquid smear appearance. Then we know that temperature is exceeding relevant temperature.
