铸铁熔体的衰退及石墨形态的转变

用定向凝固装置,研究铸铁熔体的衰退及石墨形态的转变。对石墨形态连续衰变过程的观察,发现了以前从未报导过的蠕虫状向过冷石墨转变的分界线。这表明石墨生长机制在分界线处发生了突变。本文认为铸铁熔体衰退过程中,石墨形态经历了如下转变:球状→蠕虫状→D 型片状→A 型片状,石墨晶体生长机制发生了渐变→突变→再渐变的过程。其中球状石墨→蠕虫状石墨的转变是渐变;蠕虫状石墨→过冷石墨的转变是突变;过冷石墨→A 型片状石墨的转变是再一次渐变。     

单一稀土元素镧对铸铁石墨形态的影响

用定向凝固技术究研了单一稀土元素La对铸铁石墨形态的影响,发现随着加La量的增加,石墨形态从片状变到蠕虫状、球状;加入反球化剂Al,可使蠕虫状石墨范围扩大。     

奥氏体包围后球状石墨的生长特性研究

根据球墨铸铁凝固过程中石墨—奥氏体共晶团的结构,分析了球状石墨—奥氏体边界应力产生的原因以及奥氏体内应力的分布规律。进而从石墨生长的扩散驱动力出发,研究了球状石墨生长的影响因素。结果表明,石墨生长与Fe-C(石墨)合金及各组元的特性、奥氏体所处的力学状态有关,还与石墨—奥氏体共晶团的尺寸有关。     

铝铜共晶合金连续定向凝固自生复合材料的组织与性能

本文采用自制的连续定向凝固装置,考察了Al-Cu共晶合金的组织形貌与抗拉强度与生长速度的关系。研究表明,Al-33.7%Cu合金的微观组织随着生长速度增大,由不规则片状组织逐步向规则片状组织,胞状组织转变,其片间距与生长速度的1/2次幂呈线性关系。连续定向凝固铸绽的抗拉强度远高于金属型试样,并与生长速度的1/4次幂呈线性变化。文章还讨论了共晶合金由规则片状组织向胞状组织转变的条件。      

铸铁的热物性测定及其与显微组织的关系

用激光脉冲法和比较法测量了不同种类铸铁300-1300K的热物理参数(α,Cp,λ),并对铸铁热物性与显微组织的关系作了初步讨论。结果表明,片状石墨对铸铁导热、导温能力影响最大,蠕虫状石墨次之,球状石墨较小。石墨对铸铁的导热、导温能力的贡献最大,基体组织次之,而合金元素对热物性的影响大小则决定于其在基体中的存在形式;通常各种铸铁的导热、导温系数值按灰口铁、蠕虫铁、球铁、可锻铸铁、奥氏体铸铁和白口铸铁的顺序依次降低,且在810K以后开始趋于一致;除奥氏体铸铁和白口铸铁外大多数铸铁在1040K附近存在居里点,导温系数在该处出现最小值;除奥氏体铸铁外,多数铸铁在1040K附近比热容出现峰值,有些铸铁在490K附近存在渗碳体居里点,比热容在该处也出现峰值。     

热解碳涂层的烧蚀行为

研究涂复热解碳(PC)的石墨材料的氧化侵蚀作用,对其实际应用十分重要,然而至今为止,对这样腐蚀的研究进行得还很不够。一般地说,多晶石墨的氧化速度与其孔隙度、织态结构及石墨化程度紧密相关。随着石墨化程度的提高,由O_2和CO_2引起的氧化反应的速度相应降低。在氧化反应过程中,氧化剂首先对晶体的棱状边缘进行作用。900℃左右温度时,氧化反     

不同强度301L冷轧板激光对焊接头的组织和力学性能

研究了4级强度亚稳态奥氏体不锈钢301L-DLT、301L-ST、301L-MT和301L-HT冷轧薄板激光对焊接头的凝固组织和拉伸性能。激光焊缝以初始铁素体FA模式凝固,热裂敏感性较小;焊缝由垂直熔合线向内生长的柱状晶组成,没有中心等轴晶粒区。焊缝组织中有奥氏体和板条状、骨架状和蠕虫状铁素体,无杂质、热裂纹和析出相。一次铁素体枝晶臂的平均间距约为17.5 μm,平均铁素体量为5.7%(体积分数)。焊缝的硬度为208~241HV,低于301L-ST、301L-MT和301L-HT板材的硬度。301L-DLT和301L-ST板激光焊件的拉伸断裂位置在母材内,301L-MT和301L-HT板焊件的断裂位置在焊缝内,焊缝金属的断裂强度为886 MPa和921 MPa。301L-HT板焊件的塑性较低,其余三种强度冷轧板激光焊件的拉伸性能都达到了JIS G 4305标准中相应强度冷轧301L板材的力学性能。     

Al-Cu 合金高梯度定向凝固过程中的形态转变

本文利用 ZMLMC 定向凝固装置,研究了 Al-Cu 合金系在不同温度梯度下定向凝固时凝固界面和组织形态的变化。发现存在着两种树枝状生长和胞状生长之间的转变,即在低速范围内胞状向树枝状转变,在高速范围内树枝状向胞状转变;当温度梯度足够高时,可以在整个生长速率范围内不出现树枝状生长,获得高度细化的胞状组织;合金的结晶温度间隔越宽,完全消除树枝状生长所需的温度梯度越高。     

高温共焦激光扫描显微镜在钢铁原位组织观察中的应用

简介了高温共焦激光扫描显微镜(CLSM)的基本结构和成像原理。针对其可进行高温下实时观察和记录的特点对YF45MnV非调质钢和12L14易切削钢进行了原位组织观察。结果表明:CLSM可清晰记录YF45MnV非调质钢在不同温度下的奥氏体晶粒变化情况以及12L14易切削钢液相→高温铁素体→奥氏体的相变过程;确定了YF45MnV非调质钢的奥氏体晶粒粗化温度为1 220℃;12L14易切削钢凝固过程中,奥氏体形核于铁素体相的三叉晶界处,初始为薄片状,后沿铁素体相晶界呈胞状或拇指状发展并迅速长大。     

膨胀石墨的孔隙结构-中国国家自然科学基金和日本学术振兴会联合研究项目报道

通过对膨胀石墨膨化容积，石墨蠕虫之间和内部孔隙尺寸的测定，详细研究了膨胀石墨的孔结构，建立了一些估测孔结构的新技术；为了研究膨胀石墨的孔结构，发展了一些测量新技术：包括石腊浸渍法，将石墨蠕虫浸渍在液态石蜡中，石蜡就会填充石墨间的大孔隙，通过断裂方法还可以看到蠕虫内部孔隙，这些方法均需要借助图像处理技术。本论文第一部分分析了市售膨胀石墨的孔隙结构，第二和第三部分分别讨论了胀化温度和插层剂对孔结构的一些影响。     

Effects of graphite shape on thermal fatigue resistance of cast iron with biomimetic non-smooth surface

In order to improve thermal fatigue resistance of cast iron and further study the effects of graphite shape on it, samples with different graphite shapes were processed by laser to create a biomimetic non-smooth surface. The results indicated that laser processed zone caused the longer crack initiation life and the slower crack propagation rate, which endowed biomimetic non-smooth surface with a beneficial effect on improving thermal fatigue behavior of iron with flake, vermiform or nodular graphite. Thermal fatigue resistance of samples with the same kind of surface (smooth or non-smooth) all were sorted as nodular graphite iron > vermicular graphite iron > flake graphite iron.     

Effect of spatial distribution of graphite and matrixon dry sliding vvear of cast irons

Abstract

In the selection of cast irons for sliding applications, the manner in which graphite and matrix are spatially distributed is an important factor. A Poisson distribution for graphite count and an exponential distribution for graphite size and matrix length along a line seem to be the ideal required characteristics. Of the five irons studied (three ductile irons, one flake graphite iron, and one vermicular graphite iron), only grey (flake graphite) iron satisfies the above criteria to a certain extent.     

高定向石墨/铜复合材料的制备和热物理性能

为了制备出具有优良热物理性能的石墨/铜复合材料,采用流延法将天然鳞片石墨定向排列在铜箔表面,并使用真空热压法制备具有层状结构的高定向石墨/铜复合材料。使用XRD和SEM等表征方法分析样品的微观形貌和成分,结果表明,在高温的作用下,流延所使用的溶剂充分挥发,热压后石墨仍高定向排列在相邻的两层铜箔之间,并相互搭接;部分熔化的铜在压力作用下渗透到石墨层的孔隙处,铜层之间相互贯穿。这种结构使石墨/铜复合材料具有优良的热物理性能。当石墨体积分数为20vol%~70vol%时,石墨/铜复合材料在高导热平面内热导率高达402~743 W/(m·K),抗弯强度达到126~48 MPa。深入讨论了石墨/铜复合材料的热传导机制,并建立了导热预测模型。      

Formation Mechanism of Chunky Graphite and Its Preventive Measures

The formation mechanism of chunky graphite has been reviewed and studied. The study consisted of a unidirectional solidification method, a small droplet method and a furnace cooling method. Four kinds of iron samples were prepared, namely, the pure Fe-C, Fe-C-S, Fe-C-Ce and Fe-C-Si-Ce alloys, and three kinds of nickel samples, namely the Ni-C, Ni-C-S and Ni-C-Mg alloys. The results of the unidirectional solidification of the Ni-C alloys showed that spheroidal graphite is not observed in the continuous solidified region, in which only flake-like graphite is observed, while spheroidal graphite is usually observed in the quenched liquid region. The existence of spheroidal graphite in the solidified phase is recognized only in the discontinuous growth mode of the Ni-C-Mg alloy solidified at 150 mm/h. This means that the spheroidal graphite is directly crystallized from the melt and entrapped by the flake-like chunky graphite that is formed by the continuous growth mode. In the small droplet method, a small piece of the Fe-C or Fe-C-Ce sample was melted on a pure graphite plate then cooled at a different cooling rate in a He-3%H2 atmosphere. The graphite in the Fe-C-Ce alloy is usually spherical. Nevertheless, the graphite morphology of the final solidified area changed from spherical to chunky and chunky to ledeburite with an increase in the cooling rate. This means that the chunky graphite is formed in the residual liquid region by the solidification into Fe-graphite system. The sample was cooled in a furnace, and the graphite morphology changes from spherical to chunky and chunky to ledeburite with the decrease in the Si content. These phenomena can be confirmed by the cooling curves of these samples.     

Inoculation mechanism of rare earth–Mg alloy in molten cast iron

The dissolution region of RE–Mg (RE=rare earth) alloy in the reaction chambers of in-mould inoculated castings was studied by scanning electron microscopy and on electron microprobe. The dissolution region of the inoculant was divided into 7 zones according to its microstructure. The RE–Mg alloy is composed of Mg2Si, a Mg2Si–Si eutectic, FeSi, RE–Mg–Si and other silicides. The dissolution of the alloy is a process in which the low melting point phases such as Mg2Si, a Mg2Si–Si eutectic and RE–Mg–Si dissolve first, and the high melting point phases such as FeSi and FeSi2 dissolve later. In addition, some intermediate products form during the dissolution process. This causes a concentration gradient of Mg and RE in the melt which results in a transition of the graphite morphology from normal spheroid to open nodule, vermicular and flake graphite. The dissolution of the FeSi phase in the melt forms local sites of high silicon concentration which promote the nucleation and growth of the graphite. © 1998 Chapman & Hall     

定向凝固条件下含磷蠕墨铸铁凝固特性研究

在定向凝固过程中向试样内加入纯稀土金属进行变质处理，获得了蠕墨铸铁的平界面生长试验结果；考察了磷含量和凝固速度对蠕墨铸铁液－固界面形态的影响；与同样条件下的灰铸铁试验结果相比较，阐明了蠕墨铸铁不同于灰铸铁的凝固特点，解释了含磷蠕墨铸铁易产生缩松倾向的原因。     

The Effect of Graphite Aspect Ratio on the Mechanical and Microstructural Properties of Cast Iron

Tensile test specimens of flake graphite (FG), compacted/vermicular (VG) and spheroidal graphite (SG) structures at five different cooling rates (RC) have been produced by deliberate techniques. The tensile strength (R_m), elongation percentage (A) and aspect ratio (AR) of the three graphite structures have been studied. The relationship between the aspect ratio range and mechanical properties of grey cast iron, compact graphite as well as spheroidal graphite were investigated. A comparative curve for the three graphite structures has been traced so as to correlate the aspect ratio values and the mechanical properties for each one.     

基于聚类共晶团算法的蠕墨铸铁形核率预测模型

蠕墨铸铁是一种性能优良的结构材料,蠕化效果对力学性能有重要影响,形核率的准确预测是精准调控蠕化过程的重要前提,进而实现高效蠕化。为了更准确地通过金相照片计算金属材料的形核率,本文采用计算机图形学中的DBSCAN聚类算法来统计试样中的共晶团数,建立了直接描述形核率的模型及相对应的生长模型,并与通过统计金相照片中的石墨个数得到形核率的方法作为对照,对比分析发现,聚类算法计算后得到的冷却曲线和蠕化率与实验结果吻合良好,证明了该算法对应的形核率预测模型的合理性。此外,采用温度场模拟计算,通过对比石墨数统计共晶团数目、聚类数统计共晶团数目的温度场变化,进一步验证了聚类数统计共晶团方法的合理性。     

Relationship between structure and thermal fatigue in cast iron

Abstract

Thermal fatigue of a material is determined by rupture stress, the elasticity modulus, heat conductivity, and thermal expansion. In addition to thermal expansion, one has to consider also the volume changes as a result of phase transformations. It is known that high rupture stress and high heat conductivity result in high resistance to thermal fatigue. A high Young's modulus and high thermal expansion give low resistance to thermal fatigue. Cast iron is a composite material, consisting mostly of graphite, ferrite, and cementite. The graphite can occur in a number of different morphologies. It can be spherical, as in ductile cast iron, it can be flakelike, as in flake cast iron, but it can also be rodlike, as in vermicular or undercooled graphite. Many of the properties important for thermal fatigue are influenced by the shape of the graphite. By using various models to explain the properties of composite materials, the changes in the properties of cast iron as a function of graphite shape are analysed. The analytical results are compared with experimental results. It is shown that the elasticity modulus and thermal expansion are lowest for flake graphite and that thermal conductivity is highest for this material. The conclusion is that grey cast iron has a better resistance to thermal fatigue than vermicular as well as nodular cast irons, in spite of its lower rupture stress.

MST/783