Free thermal strain cycle experiments were undertaken with a high sensitivity dilatometer that provided the reference behavior and properties of this steel. Some other similar tests were conducted to better reflect actual field weld conditions. These additional tests served to acquire the experimental variables such as the internal stress value evolution during the weld cooling.

An innovative experimental approach was developed to fully characterize the metallurgical changes of the steel during thermal incursions, such as volumic changes due to phase transformation and thermal expansion of this two-phase material. Tensile tests to measure the mechanical properties were conducted during heating as well as during cooling. Free thermal strain cycles at various heating and cooling rates were then performed on samples machined in the short transverse and in the longitudinal orientation of the rolled steel plate. Austenite phase volume proportion at initial and final state of the tests was determined from X-Ray diffraction. All results were treated with the mathematic model to assess the current phase proportion and the dimensional change caused by the phase transformation during the process.

The mathematic model, whose estimations are based on the dilatometric curves and on the application of a simple rule of mixture, led to the observation that the strain associated with the martensitic transformation could likely have a preferred orientation. The austenite volume fraction at the end of the first thermal run, agrees fairly well with the X-Ray measurement. However, a revision is necessary to reflect the anisotropy of the austenite phase transformation. X-Ray pole figure determination also is required to show up the preferred orientation of the crystallographic planes of the austenite.