INFLUENCE OF MICROSTRUCTURE ON IMPACT TOUGHNESS OF G18CrMo2-6 STEEL DURING TEMPERING

Abstract

Low alloy CrMo steels are widely used for high temperature applications in the power and petrochemical industries as structural materials. The most crucial mechanical properties for these steels are sufficient strength to withstand internal pressure and high impact toughness to assure safety from momentary shock owing to unexpected accidents. Particularly, impact toughness deteriorates because of continuous high temperature during the operation of a high pressure vessel and embrittlement can occur. Thus, the use of steels with high impact toughness is extremely important to guarantee sufficiently the safe operation of the nuclear reactor. Usually low alloy CrMo steels enter service after the normalized and tempered treatment or annealed treatment with a mixed ferritebainite or full bainite microstructure. The G18CrMo2-6 steel is one of the most popular materials with the mixed microstructures of ferrite and bainite for the pressure vessel in nuclear industry due to its good impact toughness,high strength and good creep resistance. In this work, the influence of microstructures, including the parent phases and precipitates, on the impact toughness is investigated in detail. The experimental results show that the constituent of the parent phases, namely the ferrite, pearlite or bainite, is not the reason resulting in the ultra-low impact energy. The microstructure characterization implies that the morphology and the distribution of precipitates play the key role in controlling the impact toughness of the G18CrMo2-6 steel. The lower tempering temperatures result in the blocky martensite/austenite(M/A) island and lathy M3C carbides with the large particle size. The finely granular M3C carbides with the uniform distribution on the bainite matrix can be found at the higher tempering temperatures. As the tempering temperature increased, the Charpy absorbed energy at room temperature increased. After the tempering below 600 ℃, Charpy absorbed energy has the ultra-low value of 17 and 29 J. Generally speaking,the weak softening of matrix during the lower tempering temperature increases the accumulative residual stress at particle-ferrite interface. The other important factor should be attributed that the blocky M/A island and lathy M3C carbides result in the lower critical fracture stress of a particle-ferrite interface.