Study On The Hot Ductility Of 2.25Cr1Mo Steel

2.25Cr1Mo steel is a kind of low-carbon and low-alloy structural steel which iswidely used for pressure vessels in the power, nuclear energy and petrochemicalinductries, especially used in main steam and heat steam pipes in the power industry.This steel has high strength and oxidation-resistance at high temperatures, and wellweldability.The quality of 2.25 Cr1Mo steel is seriously impacted by the transverse crackinggenerated in the straightening process of continuous casting and rolling. There are threereasons for the formation of cracks caused by hot ductility deterioration: (1) formationof a thin pro-eutectoid ferrite layer along austenite grain boundaries; (2) segregation ofundesirable elements, such as S, Sb, Sn, As, P, Cu and others, at ferrite/austeniteinterfaces or at austenite grain boundaries; and (3) intergranular precipitation ofcarbides, carbonitrides, or nitrides. The ferrite is divided into two kinds, pro-eutectoidferrite and deformation-induced ferrite. At present, the effect of ferrite on the hotductility was discussed only with qualitative investigation. There are few studies todiscuss the effect of two kinds of ferrites separately with quantitative analysis.In this work, two groups of specimens were prepared. One group was used forhot tensile tests, and the other was undeformed and used merely for metallographicobservation after undergoing the same thermal history as the former for comparison.Hot tensile tests were carried out between 650℃to 950℃with a strain rate of 10-3 s-1 ,and the reduction in area was employed to evaluate the hot ductility. Fractographicobservations were performed with SEM. Microstructures near the fracture surfaces forthe deformed and undeformed specimens were metallographically examined usingoptical microscopy. In addition, the fraction of ferrite in the microstructure wasmeasured.It is indicated that there was an evident ductility trough in the range 750℃to900℃and the minimum ductility occured at 825℃. All fractures observed with SEMwere ductile. The number and depth of dimples on the fracture could also reflect the changes of hot ductility. The SEM fractographs is well coincident with the hot ductilitycurve. At 825℃, almost perfect ferrite networks along austenite grain boundaries areformed by ferrite films which lead to stress concentration during deformationcorresponds to the deterioration of hot ductility. Cracks origins from austenite grainboundaries or second-phase particles and propagates easily in the ferrite layer. Therewas only some pro-eutectoid ferrite distributed in the matrix uniformly of theundeformed specimens without deformation-induced ferrite generated. The content ofdeformation-induced ferrite in the tensile test was got from the comparison of results inthe two tests. There is a maximum value around 820℃and almost no deformationinduced ferrite present at 800℃and 900℃. It is concluded that the deformation-inducedferrite plays a primary role in the appearance of the hot ductility trough compared withthe hot ductility curve. There is a phosphorus concentration peak in the range 800℃to850℃from the calculation result of model for deformation induced non-equilibriumsegregation. Phosphorus segregation at grain boundaries can result in a low grainboundary cohesive strength, accelerating crack propagation along the grain boundary,and reducing the hot ductility of the steel.