12 Industrial Pipeline Welding
403
Fig. 12.14 Evolution of pipeline steel grades regarding yield strength, toughness, and microstructural constituents
Low-alloyed steel deposits begin solidification when the temperature drops to
T L as indicated in phase diagram (see Fig. 12.15). At this temperature, solid phase
begins to form with chemical composition designated from the constructed tie line. Its
composition is significantly lower than this of the liquid. Upon further cooling to T S ,
the composition of the solid phase is designated from the tie line intersecting solidus
line. However, the solid’s diffusion is slower than the liquid’s. The time for diffusion
is not adequate in order to reach the designated composition. Therefore, the chemical
composition in the solid slightly increases, and the liquid phase becomes enriched in
alloying elements. As the temperature decreases, multiple layers of different chemical composition form and the chemical composition progressively increases from
the core to the outer layer of the solid phase [28, 29]. This phenomenon is called
segregation.
Fig. 12.15 Schematic
representation of binary
phase diagram during
solidification
403
Fig. 12.14 Evolution of pipeline steel grades regarding yield strength, toughness, and microstructural constituents
Low-alloyed steel deposits begin solidification when the temperature drops to
T L as indicated in phase diagram (see Fig. 12.15). At this temperature, solid phase
begins to form with chemical composition designated from the constructed tie line. Its
composition is significantly lower than this of the liquid. Upon further cooling to T S ,
the composition of the solid phase is designated from the tie line intersecting solidus
line. However, the solid’s diffusion is slower than the liquid’s. The time for diffusion
is not adequate in order to reach the designated composition. Therefore, the chemical
composition in the solid slightly increases, and the liquid phase becomes enriched in
alloying elements. As the temperature decreases, multiple layers of different chemical composition form and the chemical composition progressively increases from
the core to the outer layer of the solid phase [28, 29]. This phenomenon is called
segregation.
Fig. 12.15 Schematic
representation of binary
phase diagram during
solidification