The Research On The Optimized Design Of Heat Treatments For SCRAM Steels

Reduced-activation ferritic/martensitic(RAFM) steels are currently considered as one of the realistic structural materials for fusion blanket systems because of their high thermal conductivity, low thermal expansion coefficient, good irradiation resistance and good compatibility with tritium breeding and coolant. But, the RAFM steels still have face with some problems, such as the irradiation swelling, irradiation hardening and the increment of DBTT, we should find some way to improve the irradiation properties and mechanical properties of the RAFM steels.The SCRAM(Super-clean Reduced Activation Martensitic) steels developed by Huazhong University of Science and Technology was experimented in this research. In order to improve the comprehensive mechanical properties and the resistance to irradiation of SCRAM steel, the SCRAM steel was processed by different heat treatments in this research. The effect of heat treatment on the microstructure, mechanical properties and irradiation properties of SCRAM steel was investigated. The main results are as follows:The precipitates of the 9Cr-WVTi N steels have better stability than that of Cr-WVTa steels by the Thermo-Calc calculation. The results showed that the MX phase such as Ti N, VN can start to precipitate first rarther than Ta C and VC respectively. That means the Ti N and VN can be more stable than Ta C and VC respectively. At the same time, the calculations also showed that, the M23C6 phase started to precipitate at the temperature below 870?,and the MX started rather earlier than M23C6 phases.A new heat treatment process with twice quenching and once tempering was desighed to refine the martensitic lath width. The result showed that the mean size of lath with can be decrease from 0.51?m to 0.42?m. But the mean size of precipitates were increased from 83 nm to 86 nm. The DBTT can decreased from-40? to-61? by the new designed heat treatment process. The results indicated that the refinement of martensitic lath width can have good effect on the mechanical properties. The two continuous quenching processes may cause the formation of micro-crack in the marterials and this process can not be applied for the industrial application.By applying an intermediate heat treatment with 870? for 1 h wihle after the austenitizing at 1050? for 1h, the precipitates can be refined. And the calculation results showed that, the MX phases can started to precipitate first than M23C6 phase. During the intermediate heat treatment process, the MX can precipitate sufficiently and the fraction of C would be decreased in the matrix. And the volume fraction and size of M23C6 would be decreased by the quenching process. By applying this new process, the mean sizes of precipitates for SCRAM-3 and SCRAM-6 steels can decrease from 80 nm, 83 nm to 64 nm and 71 nm respectively. But the mean sizes of martensitic lath width increased from 0.44?m,0.51?m to 0.86?m and 0.72?m. By using the new heat treatment processes, the DBTT of SCRAM-3 and SCRAM-6 steels decreased from-23?,-41? to-40 ? and-43?. The refinement of precipitates can have good effect on the mechanical properties, but the coarsening of martensite lath will decrease the interfaces of the steels.By conbining the two heat treatment processes, we designed an optimized processes with a furnace cooling processes after the second austenitizing processes. The martensitic lath width is 0.75?m for the traditional heat treatment(Q&T), while that is 0.61?m and 0.66?m for twice quenching twice tempering processes(2Q&2T) and the optimized processes(I&2Q&2T) respectively. The mean sizes of precipitates for Q&T, 2Q&2T and I&2Q&2T is 94 nm, 70 nm and 60 nm. That means the new designed heat treatment can reduce the the size of the martensitic lath and the precipitates. And the DBTT values for the Q&T, 2Q&2T and I&2Q&2T is-24?,-40? and-43?. The results indicated that the decrement of martensitic lath width and the precipitates can have good effect on the mechanical properties.We use the SCRAM-3 steels by Q&T, 2Q&2T and I&2Q&2T processes for the He ion irradiation tests. And a new designed impact tests with slice specimen was used to measure the irradiation hardening. The results showed that, by applying the new designed heatments, the rate of irradiation hardening would decrease from 18.86% to 7.92%. Becasuse the refinement of the microstructure for SCRAM steels can produce more interfaces which can help trap the He bubbles and impede the growth of helium bubbles. The SCRAM-9 steel with Q&T, 2Q&2T and I&2Q&2T processes were used for the creep teats at 550?, 270 MPa. The results showed that by applying the nea designed heatment, the elongation of the SCRAM-9 steels is 1.6% and 0.8% for 2Q&2T and I&2Q&2T, while that is 2.3% for Q&T. The resuls showed that, the refinement of martensites and precipitates can have benefits on the mechanical properties, irradiation properties and creep properties. By applying the new designed heat treatment processes, the SCRAM steels can better service the fusion reactor.