The inherent strain method is known to be very efficient in predicting plate deformation due to line heating. However, in the actual line heating process in a shipyard, the rapid quenching changes the phase of steel. In this study, when calculating inherent strain, material properties of steel are applied differently in heating and in cooling, considering phase transformation. In this process, a new method which can reflect the thermal volume expansion of martensite is suggested. By this method, the plate deformations by line heating could be predicted more precisely.
Plate deformation byline heating has been studied mainly through 2 approaches. One is thermal elastoplastic analysis, which uses the FEM code through direct heat input; the other is the equivalent loads method based on inherent strain. Of the two, the latter is widely used from the viewpoint of efficiency and accuracy(Jang, Ko and Seo, 1997). However, there is a limitation to this method, which is that there should be an appropriate assumption of the inherent strain region. Satoh, Matsui, and Terai (1969) presented the depth and breadth of the HAZ (heat affected zone) obtained from welding experiments, and they assumed the region to be elliptical. In many studies, the HAZ has played a role in the inherent strain region. In line heating, Jang, Ko and Seo (1997) suggested that the inherent strain region can be substituted with the mechanical melting region. This region is very similar that whose maximum temperature is over Ac1 (the temperature at which austenite begins to form during heating). They obtained the inherent strain by adding residual plastic strains in heating and in cooling by the spot heat source. By using this inherent strain, Nomoto, Ohmori, Satoh, Enosawa, Aoyama and Saitoh (1990) estimated equivalent forces.