An Investigation on Precipitation Behaviour of Cu-Rich Phase in Super304H Heat-Resistant Steel by Three Dimensional Atom Probe

Super304H, a Cu-containing 18Cr-9Ni-3CuNbN heat-resisting steel is wildly used as an superheater/reheater tube material for ultra-super-critical (USC) power plants all over the world. It is recognized that the Cu-rich phase is an important strengthening phase for Super304H. However, the detail precipitation behaviour and its strengthening effect are still not very clear. Investigated material was taken from routine production and was aged at 650 degrees C for different times. The precipitation of Cu-rich phase in Super304H was studied by three dimensional atom probe (3DAP) and TEM. Experimental results show that Cu-rich clusters have been formed at very early stage of 650 degrees C aging. The Cu-rich particle images have been clearly caught just after 650 degrees C aging for 5h. The Cu atoms gradually concentrate to Cu-rich particles and the other elements (such as Cr, Ni etc) diffuse away from Cu-rich particles to gamma-matrix with the increasing of aging time. The Cu-rich particle size and its density have been determined as a function of aging time. Cu-rich particles still keep nano-size and distribute homogenously in grains even after long time (1,000h) aging, which is one of the most important reasons for keeping good strength of Super304H heat-resistant steel at high temperatures.