Study Of Precipitates And Boundaries In Martensitic Heat Resistant Steels

The aspiration of cost reduction for advanced power plants can only be achieved by enhancing the plant efficiency through raising the steam pressure and temperature.It is well known that due to the creep property,the elevated working pressure and temperature tend to limit the service life of structural materials,hence,there is a strong motivation to develop high creep strength heat resistant steels.The fourth generation 9%?12%Cr martensitic heat resistant steels have been developed in recent years by increasing contents of high bonding energy elements such as W and Co.Although the creep property is superior to the previous generation,the cost increases correspondingly.More importantly,the precipitation of Fe2W Laves phase takes place at boundaries from the supersaturated solid solution of the high-W steels during the creep,and it has been generally believed that Laves phase is responsible for the impact brittleness of martensitic heat resistant steels.All these problems have restricted the fourth generation heat resistant steels to be put into commercial use.However,it is an intractable problem how to significantly improve the creep strength of martensitic heat resistant steel without substantially increasing contents of W and Co.In this thesis,a strategy to circumvent this through optimization of the precipitate morphologies and the number of low/high angle boundaries in martensitic heat resistant steels has been developed.In present study,four martensitic heat-resistant steels were designed for different precipitate morphologies.Steel NS1(nitride strengthened,NS)had a low density of fine nitrides with sizes of 50?80 nm,Steel NS2 was strengthened by a high density of fine nitrides with sizes of 20?60 nm,Steel CNS(carbide and nitride strengthened,CNS)contained large size carbides of 100?300 nm and fine carbonitrides,and Steel CS(carbide strengthened,CS)mainly possessed large size carbides of 200?500 nm.Aging tests at 650?750 ? were employed to evaluate the stability of the microstructure during high-temperature exposure as a function of the precipitate morphology.It was found that Steel NS1 and Steel NS2 displayed poor microstructure thermostability with quick recrystallization when the Larson-Miller parameter,T(20+logt),reached 21000 and 21500,respectively.While Steel CS and Steel CNS did not undergo recrystallization until the Larson-Miller parameter reached 23500.The large size M23C6 carbides produced high pining force to boundaries and eliminated the recrystallization in Steel CNS and Steel CS aged at 650?750?.On the contrary,the fine nitrides did not stop the recrystallization in Steel NS1 and Steel NS2.Though Steel CS and Steel CNS successfully maintained the tempered martensitic microstructure aged at 650?750?,they gave poor performance in the short-term creep at 650?.Under aging condition,M23C6 carbides played a role in pinning subgrain boundaries,however,this pinning role could give rise to stress concentration under creep condition,and then activated the dislocation source around subgrain boundaries.The creep strength decreased as a result of the multiplication of free dislocations.An extrapolation based on Larson-Miller parameters indicated that in the long-term creep at 650?,the M23C6 particles coarsened severely and they gradually losed the role in holding back the microstructure recrystallization.While the fine MX particles remained stable during the creep,which effectively pinned dislocations interior subgrains.As a consequence,in order to obtain a stable microstructure,M23C6 particles should be elimated from 9%?12%Cr heat resistant steels.Roles of high angle boundaries(HABs)and low angle boundaries(LABs)in the martensitic heat resistant steel were also studied.A martensite transformation can result in a complex microstructure,consisting of hierarchically arranged substructures such as packets,blocks and laths.Block/block and block/packet junctions are separated by HABs,while lath/lath junctions are separated by LABs.Different heat treatments were used in order to adjust the numbers of HABs and LABs in the steel,and then mechanical properties at room temperature and elevated temperature were measured.Results indicated that the steel with fewer HABs tended to achieve higher creep strength,which was mainly due to the facilitation of atomic diffusion by HABs.While the steel with fewer LABs tended to achieve both lower room termperature strength and high temperature creep strength,which was due to the weakened subgrain strengthening effect.On the basis of the above studies,two novel martensitic heat-resistant steels were successfully developed by reducing the carbon content so as to inhibiting the precipitation of M23C6 particles and reducing the number of HABs.The low cost Steel N1 presented a much better creep property than the T/P92 steel,which was the most advanced third generation martensitic heat resistant steel and is widely used in ultra-supercritical power plant.The creep test conducted at 600? indicated that within a 16000 h range,fracture time of the Steel N1 was 3 to 5 times longer than that the T/P92 steel.The Steel N2 appropriately increased the Mo and Co contents,which was strengthened by a high density of fine MX nitrides.Within a 4000 h range,fracture time of the Steel N2 was 15 to 20 times longer than that of the T/P92 steel at 650?.In the long-term aging test up to 12000 h,the microstructure of the Steel N2 was barely evolved.The present study has fulfilled the aim of developing low cost martensitic heat resistant steels to be used at 650?.