【作者】 于君燕；

【导师】 殷凤仕；

【作者基本信息】 山东理工大学 ， 材料加工工程， 2008， 硕士

【摘要】 利用扫描电子显微镜（SEM）、能谱分析（EDS）、透射电子显微镜（TEM）、X-射线衍射分析（XRD）、拉伸蠕变试验等方法试验研究了五种成分的9-12%Cr系铁素体/马氏体耐热钢的微观结构和力学性能以及不同热处理和650℃高温长期时效对微观结构和力学性能的影响。所得结论如下:所研究的9-12%Cr铁素体/马氏体耐热钢经过1100℃×1h正火处理+750℃×1h或700℃×1h高温回火处理后,其微观组织特征是板条状回火马氏体,在原奥氏体晶界和马氏体板条界上析出M₂₃C₆型碳化物,在板条内部的基体上析出纳米尺寸的MX型碳氮化物。M₂₃C₆型碳化物的数量随着碳含量的降低而减少,超低碳钢中几乎没有M₂₃C₆碳化物析出。两种纳米尺寸的MX相均匀密集的分布在超低碳钢的基体内,直径约为30～50nm的较大尺寸MX相富含Nb和Ti,而直径约10mm的较小尺寸MX相富含V。在含1.5wt.%Mo的钢中发现有少量M₃B₂型硼化物。高Ti含量（0.14wt.%Ti）的钢中有尺寸约为2μm的粗大TiC颗粒,在马氏体板条界附近区域内分布有高密度纳米TiC析出相。含高密度MX型纳米析出相的高铬耐热钢在高应力下具有较长的蠕变断裂寿命,但随着蠕变应力的降低,蠕变断裂寿命增加缓慢,表现出较快的高温强度退化速度。蠕变裂纹易于在粗大的TiC颗粒与基体的界面处形成,导致高Ti含量（0.14wt.%Ti）的钢的蠕变性能大大降低。钢样经1100℃×1h正火+700℃×1h或750℃×1h回火热处理后在650℃长期时效处理过程中,MX析出相向四方结构的Z-相转化,且高N含量的钢中Z-相的形成较早;由于Z-相和MX相有相似的组成元素,故Z-相的形成消耗了MX相,降低MX相的强化作用。650℃长期时效期和蠕变过程中,在板条和晶界析出大颗粒Fe₂Mo（含Mo,不含W钢）或Fe₂W型（含W钢）Laves相;在含W钢中,时效500h前,Laves相逐渐析出,弥散强化效应提高,钢样硬度增加;时效超过500h后,随着Laves相粗化和粗大的Z-相析出,基体中的固溶强化元素W的浓度降低,强化效果降弱,钢的硬度降低。更多还原

【Abstract】 Microstructure and properties of five 9-12%Cr ferritic/martensitic heat resistant steels have been studied by using the scanning electron microscope （SEM） equipped with energy dispersive spectrum analysis （EDS）, transmission electron microscope （TEM）, X-ray diffraction （XRD） and tensile creep test in this paper. The conclusions are made as follows:After normalizing at 1100℃for 1h and then tempering at 750℃/700℃for 1h, the microstructure of all steels studied is lath martensitic. M₂₃C₆-type carbide precipitates along grain boundaries and lath boundaries while nano-sized MX carbonitride precipitates within lath. The amount of M₂₃C₆ carbide reduces with the decrease of carbon content. And there are few M₂₃C₆-type carbide particles in ultra-low carbon steels. Two kinds of nano-sized MX phase distribute densely and homogenously in the matrix of the ultra low carbon steels. The larger nano-sized MX precipitate with the size of 30～50nm is rich in Nb and Ti, while the smaller one with the size of about 10nm is rich in V. Small amount of M₃B₂-type boride are found in the steel with 1.5wt.%Mo. Large TiC particles with a size of about 2μm distribute in grain boundaries and dense nano-sized TiC particles in martensitic lath boundaries are found in the steel with 0.14wt.% Ti .Under condition of high stress at 650℃, high chromium steels with high density nano-sized MX precipitates have longer creep rupture life. However, creep rupture life increases lowly with the decrease of creep stress indicating that the strength degradation at high temperature is fast. Creep crack tends to initiate at the interfaces between coarse TiC particles and the matrix and causes a decrease in the creep property of the steel with 0.14wt.% Ti.During long time aging at 650℃after normalizing at 1100℃for 1h and tempering at 700℃/750℃for 1h, MX phase has transformed into Z-phase, which has a tetragonal structure and is prone to coarsen. Z-phase formation occurs earlier in high nitrogen steels. The formation of Z-phase consumes the nano-sized MX carbonitrides because the composition of both MX carbonitrides and Z-phase are all consisted of the same elements of V and Nb, resulting in decrease of the strengthening effect of MX precipitates. During long time aging and creep deformation at 650℃, large Fe2Mo-type （in steels without W） and Fe₂W-type （in steel with W） Laves phase precipites along lath and grain boundaries. In steels with high tungsten, Laves phase precipitates gradually leading to the increase of dispersion strengthening effect and hardness of the steels before 500h aging. After 500h aging, the hardness of the steels decreases because of the coarsening of Laves phase and the formation of large Z-phase.更多还原