It is necessary to control weld residual stress which has negative influence on fracture strengths. In structural steel welds, complex residual stress fields are formed due to phase transformation that occur according to the thermal cycles. Therefore, it is important to evaluate the stress generating process during phase transformation in welding. In this study, in-situ evaluation of phase transformation and transitional stress simultaneously during welding is discussed. In the test using SM490A, after cooling process, stress evaluated by this system showed good agreement with that evaluated by lab X-ray. During austenite to ferrite transformation in weld metal, tensile stress occurred in austenite and compressive stress occurred in ferrite. These behavior are agree with internal stresses behavior depend on phase transformation strain and the difference of thermal expansion rate between ferrite and austenite. Moreover, stress concentration was occurred in ferrite phase immediately after the start of phase transformation. Also, stress concentration was occurred in austenite phase just before the end of phase transformation. Thus in-situ observation phase transformation and stress simultaneously during welding and phase transformation process can be done in weld metal.
