Formation of Microphases and Constituents from Remaining Austenite Decomposition in API X80 Steel Under Different Processing Conditions

Abstract

The main goal this work is to evaluate the occurrence of the constituents and microphases observed in as-received API X80 pipe through the microstructures transformed from rich-carbon remaining austenite obtained via heat treatments. These heat treatments comprised austenitization at 1000 °C for 30 minutes, followed by annealing at 700 °C, 623 °C, 542 °C and 462 °C for 15, 60 and 300 minutes and then cooling in water or still air. The effects of the increase in annealing parameters were: 1) the increase of microhardness values of the MA constituents and martensite islands, 2) the grain size of both ferritic phase and the martensite islands were increased, 3) the rise in the mean free path of ferrite and 4) the microstrains of samples were decreased. The cooling rates influenced significantly the transformation of carbon-rich remaining austenite to hard constituents and several microphases. After annealing at 700 °C during 60 min followed by quenching, the morphology of the MA constituents and its microhardness values were compatible for both heat-treated and as-received conditions.

Effect of Different Thermomechanical Processes on the Microstructure, Texture, and Mechanical Properties of API 5L X70 Steel

“…This distribution might be explained by the delayed pearlitic reaction that occurs during the continuous cooling due to the high manganese content of the studied steel. The blocky M-A constituents, or the remaining austenite coexisting with martensite, are formed under favorable conditions of chemical composition and appropriate continuous cooling rates of the untransformed carbon-enriched austenite by carbon partitioning from ferrite into the remaining austenite during austenitic decomposition (Ref 24,25). The formation of M-A constituents could also be explained by the manganese segregation from the prior austenite to the grain boundaries during cooling, leading to an increase in the stability of austenite at room temperature.…”

Revealing the mechanism of extraordinary hardness without compensating the toughness in a low alloyed high carbon steel

“…SFE is related to the chemical composition, such as, C and Mn content and therefore an elementary analysis in the retained austenite was conducted using EPMA. Both C and Mn elements are the strong austenite stabilizers 3,36 . The SFE depends on the chemical composition and temperature.…”

Quantitative Correlation between Thermal Cycling and the Microstructures of X100 Pipeline Steel Laser-Welded Joints

“…Transformed martensite is also the location of microcracks [27,28]. Therefore, controlling the shape and distribution of the M-A constituent in laser-welded joints of X100 pipeline steel is important [25,27,29,30]. Table 1.…”

“…The rate of formation of the austenite and the inhomogeneity of the austenite composition increases with a growing heating rate. The initial austenite grains are refined with the heating rate [22,25,30,44,45].…”

“…The relative proportions of these four microstructural components have important effects on the properties of the joint. The M-A constituent from carbon-enriched and untransformed austenite surrounded by bainite is known to deteriorate the toughness of bainitic steel [24,25,27,30]. In addition, when the AF and GB contents in Xl00 pipeline steel are too low, the original austenite grains are not separated, and the growth of the lath beams is not controlled, which results in large effective grain sizes for the lath bundles and diminished elongation and impact properties.…”