Mechanical properties and microstructural investigation of a quenched and tempered 2.25Cr-1Mo alloy for very thick forgings applications

Large forgings fabricated from heavy ingot can involve homogeneity challenges for the mechanical properties. The 2.25Cr-1Mo steel has been used for a long time for very thick and heavy forgings for the pressure vessels. Since it is only lightly sensitive to the macro segregation and exhibits good bainitic hardenability due to its chemical composition. However, for the quenched and tempered thick forgings, very few data on mechanical properties and microstructure have been reported for this grade of steel. In this study, different quenching and tempering conditions have been applied on forging blocks to simulate the actual heat treatment conditions at different depths in the thick forgings (thickness up to 700 mm at the stage of quenching). Low quenching rates, down to 150 °C/h, have been chosen to investigate the mechanical properties and the microstructure at the mid-thickness of thick forgings. The influence of the carbon content has also been analyzed for low quenching rate condition. For each metallurgical state, tensile tests and Charpy impact tests have been conducted. The microstructures have been characterized by optical and electron microscopies and by EBSD analysis. The micro-hardness has been measured. The results show that even with a quenching rate as low as 150 °C/h, a considerable proportion of bainite can still be formed for a 2.25Cr-1Mo steel with relatively low carbon. Its bainitic microstructure contains highly disoriented bainite blocks which is in favor of a good fracture toughness. At 0 °C, the upper shelf of the transition curve is basically reached with energies above 150J.