热处理工艺对COST-FB2钢组织和力学性能的影响

发布时间：2018-04-15 03:36

本文选题：COST-FB2钢 + 淬火工艺　；参考：《昆明理工大学》2017年硕士论文

【摘要】：燃煤发电在我国未来相当长一段时期内仍然是发电的主要形式,620℃超超临界发电技术具有高效率低能耗的优点,建设620℃超超临界电站对于我国掌握先进发电技术、提高综合国力具有重要的意义。我国现投入运营的620℃超超临界电站均选用COST-FB2钢作为汽轮机转子材料,大型转子锻件均需进口购买,而我国对COST-FB2钢的相关研究仍处在起步阶段,尚不具有工业生产的能力。因此,通过系统的研究热处理工艺和退火工艺对大型汽轮机转子锻件实际的生产具有一定的参考价值及指导意义。实验借助OM、SEM、TEM、XRD和相分析等手段,研究了热处理工艺对COST-FB2钢组织和性能的影响和等温退火过程对COST-FB2钢组织均匀性的影响,探讨COST-FB2钢的强韧化机制,得出如下结论:(1)在1000℃-1200℃随淬火温度的提高,使钢固溶态过饱和度提高,回火后析出碳化物数量增加,加强了沉淀强化的效果,同时晶粒发生粗化,导致细晶强化效果减弱。晶粒的粗化导致钢冲击韧性下降。(2)淬火冷却速度降低至1℃/min开始发生铁素体转变,使材料强度硬度明显下降,回火后晶界处碳化物数量较少,提高了材料的韧性;淬火冷却速度降低至0.5℃/min获得单一的铁素体组织,强度硬度急剧下降,晶界处呈网状的碳化物析出造成韧性的下降。(3)400℃-570℃回火,金相显微组织差异不大,位错密度较高,因此强度硬度较高;高于570℃回火,开始发生回复,随回火温度的上升回复现象加剧,位错密度降低,导致强度硬度下降,韧性提高。延长570℃回火时间,回火1h和2h差别不大,延长至4h碳化物开始在晶界处聚集,造成冲击韧性的降低。700℃回火时,随时间的延长,碳化物开始聚集长大,回复加剧,位错密度降低,造成强度下降,冲击值上升。(4)570℃和700℃两步回火的冷却速度主要影响碳化物析出过程中原子的扩散和聚集长大,冷却速度快,原子扩散时间短,容易得到细小的碳化物;冷却速度慢,原子扩散时间长,容易造成碳化物的聚集长大,得到大尺寸碳化物,降低力学性能。700℃回火时的冷却速度对析出碳化物尺寸的影响更大。570℃和700℃两步回火均采用空冷的冷却方式可得到最佳的力学性能。(5)将COST-FB2钢进行700℃或730℃等温退火可发生完全的铁素体转变,将粗大晶粒分割为大量细小的铁素体,同时析出碳化物在奥氏体化过程中可以阻碍奥氏体晶粒长大。等温退火后直接最终热处理或者进行预备热处理与最终热处理制度均会得到细小均匀的组织。730℃等温退火效果优于700℃。
[Abstract]:Coal-fired power generation in China for quite a long period of time is still the main form of power generation, has 620 degrees of ultra supercritical power generation technology with high efficiency and low energy consumption, the construction of 620 C ultra supercritical power plant for advanced power generation technology in China, has important significance to improve the comprehensive national strength. China is put into operation 620 C ultra supercritical power stations are used for COST-FB2 steel as the steam turbine rotor, large rotor forgings are required to purchase imported, and related research of COST-FB2 steel in China is still in the initial stage, there is no industrial production capacity. It has certain reference value and significance through the systematic study of heat treatment process and annealing the process of large steam turbine rotor forging production. The actual experiment by means of OM, SEM, TEM, XRD and phase analysis method, study the effect of heat treatment on the microstructure and mechanical properties of COST-FB2 steel and isothermal annealing The fire process on Microstructure of COST-FB2 steel inhomogeneity of COST-FB2 steel, toughening mechanism, draw the following conclusions: (1) at 1000 DEG -1200 DEG with increasing quenching temperature, the steel solid solution supersaturation increased after tempering, the carbide precipitation increased, strengthened the effect of strong precipitation, while grain coarse, lead to fine grain strengthening effect is weakened. The grain coarsening decreases the impact toughness of steel. (2) the cooling rate decreases to 1 DEG /min began the transformation of ferrite, the material strength and hardness decreased obviously after tempering, less grain boundary carbide quantity, improve the toughness of the material; the cooling rate decreases to 0.5 DEG /min to obtain a single ferrite, strength and hardness decreased rapidly, the grain boundary was caused by mesh carbide toughness decreased. (3) 400 DEG -570 DEG tempering, little difference in microstructure, higher dislocation density, so strong The degree of hardness higher than 570 DEG C; tempering, began to return, with the increase of tempering temperature recovery phenomenon intensified, the dislocation density decreased, resulting in decreased strength and hardness, toughness increased. 570 C prolonged tempering time, tempering little difference in 1H and 2H, extended to 4h carbide began gathering at the grain boundary, the impact toughness decreased tempering.700 C, with the extension of time, carbide begins to grow up, recovery intensifies, reduction of dislocation density, leads to the decrease of strength and impact value rise. (4) 570 degrees and 700 degrees two step tempering cooling rate mainly affects the precipitation of carbide in the process of atomic diffusion and coalescence, cooling speed, diffusion time is short and easy to get fine carbides; slow cooling rate, diffusion time is long, easy to cause the aggregation and growth of carbides, large size carbide, lower cooling rate and mechanical properties of.700 DEG C during tempering of precipitation Effect of carbide size and greater.570 degrees and 700 degrees two step cooled by air cooling and tempering are the best mechanical properties could be obtained. (5) the COST-FB2 steel was 700 degrees or 730 degrees Celsius isothermal annealing can occur completely ferrite transformation, will be divided into a large number of coarse grains of fine ferrite, at the same time in the process of precipitation of carbides in austenite can impede austenite grain growth. The isothermal annealing directly after final heat treatment or pre heat treatment and final heat treatment system will be better than.730 DEG C fine uniform isothermal annealing effect of 700 degrees.

【学位授予单位】：昆明理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1

【参考文献】