【作者】 王海；

【导师】 杨柯； 单以银； 严伟；

【作者基本信息】 东北大学 ， 材料加工工程， 2018， 博士

【摘要】 提高超超临界火力发电机组的运行蒸汽参数可提高发电效率,而目前使用的9%～12%Cr马氏体耐热钢的蠕变强度制约着蒸汽参数的提升。在第三代马氏体耐热钢的基础上,通过大幅提高W、Co含量发展出来的第四代马氏体耐热钢虽然具有较高的蠕变强度,但其制备成本高昂,长期服役后的Laves相析出致脆问题无法得到很好的解决,限制了其大规模的推广应用。在不大幅提高W、Co含量的前提下,如何显著地提高马氏体耐热钢的蠕变强度是人们面对的一大难题。在此背景下,本文对马氏体耐热钢的蠕变强化机制进行研究,通过优化析出相形貌和界面构成,开发出具有优异蠕变性能的新型马氏体耐热钢。本文首先研究了不同类型、不同形貌的析出相对马氏体耐热钢的组织热稳定性和蠕变性能的影响。设计并冶炼了 4种具有不同析出相形貌的马氏体耐热钢:NS1钢采用少量尺寸在50～80 nm之间的MX氮化物强化;NS2钢采用大量尺寸在20～60 nm之间的MX氮化物强化;CNS钢采用大量尺寸小于100 nm的MX碳氮化物和尺寸介于100～300 nm之间的M23C6碳化物强化;CS钢采用大量尺寸小于100 nm的MX碳化物和尺寸在200～500 nm之间的M23C6碳化物强化。对这四种实验钢在650～750℃进行时效,NS1钢和NS2钢的组织分别在Larson-Miller参数(T(20+logt))大于21000和21500后开始发生再结晶,而CS钢和CNS钢的组织在Larson-Miller参数大于23500后才开始出现再结晶。大尺寸M23C6相能有效阻碍时效过程中界面和位错的运动,提升组织的热稳定性,而小尺寸MX相只能钉扎位错而无法阻碍界面的运动,所以采用MX相强化的实验钢易于发生再结晶。虽然采用M23C6相强化的CS钢和CNS钢在650～750℃时效具有优异的组织热稳定性,但是它们的650℃短时蠕变性能却明显低于仅采用MX相强化的NS1钢和NS2钢。这是因为在短时蠕变的过程中,大尺寸的M23C6相降低了亚晶界面的可动性,界面周围的应力集中难以及时地松弛,并最终激活了位于亚晶界上的位错源。随着可动位错的大量增殖,材料的短时蠕变性能显著降低。如果在650℃长时蠕变,根据Larson-Miller参数外推,M23C6相几乎不具有任何强化作用,它们的粗化十分严重,组织也在长时蠕变过程中发生再结晶,此时材料的蠕变强度主要来源于亚晶强化。在确保马氏体耐热钢组织稳定的前提下,应尽可能地减少M23C6相的数量,以提高材料的蠕变性能。本文随后以CS钢为研究对象,对马氏体耐热钢的大角度界面(原奥氏体晶界、板条群界、板条束界)和小角度界面(板条界)的作用进行研究。通过提高正火温度和奥氏体化后冷至MS点以上长时保温,减少大角度界面的数量;通过提高回火温度,减少小角度界面的数量。对大、小角度界面数量的变化对耐热钢的室温及高温力学性能的影响研究结果表明,减少大角度界面的数量虽然降低了材料的室温冲击韧性和拉伸强度,但抑制了原子沿大角度界面的扩散,提高了组织的热稳定性,使材料的蠕变强度显著提高;减少小角度界面的数量虽然提高了材料的室温冲击吸收功,但降低了亚晶强化效果,使材料的室温强度和高温蠕变强度下降。应在不降低小角度界面数量的条件下,减少大角度界面的数量,以提高材料的蠕变性能。最后,根据以上研究结果,本文通过降低碳含量来抑制M23C6相的析出,并减少大角度界面的数量,成功地开发出两种新型马氏体耐热钢:N1钢和N2钢。N1钢中的W、Mo含量低,材料成本低于目前商业应用最广泛、蠕变性能最优异的第三代马氏体耐热钢T/P92钢。在16000 h的实验时间范围内,其600℃的蠕变断裂时间是T/P92钢的3～5倍。N2钢是在N1钢成分的基础上添加了少量的Mo和Co,并利用大量小尺寸的MX氮化物进行强化,在4000 h的实验时间范围内,其650℃的蠕变断裂时间是T/P92钢的15～20倍。在650℃时效12000 h的时间范围内,N2钢表现出了优异的组织热稳定性。在不大幅提高材料成本的条件下,本文成功地将马氏体耐热钢的使用温度提高至650℃。更多还原

【Abstract】 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℃.更多还原