A CCT Diagram for an Offshore Pipeline Steel of X70 Type

By means of dilatometry and metallographic analyses, a continuous-cooling transformation (CCT) diagram valid for the heat-affected zone (HAZ) in welding operations with relatively rapid heating up to about 1200 degrees C on an offshore pipeline steel of X70 type has been established along with the linear thermal expansion coefficients for the austenite and bainite phases. A dilatometer was built for this purpose. For comparison reasons, the steel was also tested in a commercial dilatometer using larger samples than in the laboratory-built dilatometer. The importance of using relatively small dilatometry samples in order to minimize the inaccuracy associated with the temperature gradients has been substantiated by means of mathematical modeling showing that cylindrical samples of 20-mm length and 3-mm diameter are sufficiently small.