1987 Volume 5 Issue 4 Pages 504-511
For the analysis of microstructure in the transition zone occurring in weld metal near base metal in welding dissimilar metals, Transition Zone Transformation (TZT) diagram was proposed, which gives the distribution of liquidus, solidus and transformation temperatures in the transition zone. As an example, the distributions of Cr and Ni measured in the transition zone between 2 1/4 Cr-lMo steel and type 309 overlaid metal were combined to Fe-Cr-Ni phase diagram to draw its TZT diagram. Further, the combination of the TZT diagram with the temperature distribution during overlaying yielded the elucidation of formation mechanism of austenite coarse grain-boundary as the site of hydrogen disbonding. The proposed mechanism was confirmed experimentally, and could be stated as follows: The austenite (γ) grains at the fufusion boundary in HAZ formed in δ→γ transformation during cooling are going to grow into the overlaid metal. Before that, however, other γ grains has already nucleated and being growing into the transition zone by the reaction of liquid→liquid+δ→liquid+δ+γ during solidification. Therefore, when the γ grains from HAZ grow only a little into the overlaid metal both the γ grains from HAZ and in the overlaid metal collide with each other in the transition zone, and this collision makes γ grain-boundary parallel to the fusion boundary. This r grain-boundary shifts a little accompanying the disappearing of δ during cooling, and the zone between the γ grain-boundary and the carbide layer formed after PWHT is regarded as so-called γ coarse grain.
