蠕化率对蠕墨铸铁组织及热疲劳性能的影响

采用自约束热疲劳试验机,对不同蠕化率下的蠕墨铸铁进行热疲劳性能试验并观察其组织变化。结果表明,试样的抗热疲劳性能随着蠕化率的升高呈现先上升后下降的趋势。当蠕化率为70%~80%时,蠕墨铸铁的热疲劳性能最好。蠕虫状石墨占基体比重有一个最佳值,蠕化率过低时,由于球状石墨数量较多分布较密集,导致热疲劳性能较差;蠕化率过高时,由于蠕虫状石墨过于粗大使裂纹扩展速度加快,导致热疲劳性能较差。在裂纹扩展后期,主裂纹变粗大导致其向前扩展所需驱动力不足以及主裂纹上形成分叉裂纹及二次裂纹,进而导致裂纹扩展呈现先快后慢、阶段性扩展的特点。     

Elevated temperature thermal conductivities of some as-cast and austempered cast irons

The aim of this study was to provide insight on thermal conductivity of three cast iron groups, namely lamellar, compacted and spheroidal graphite irons at elevated temperatures up to 673?K (400°C) in as-cast and austempered states. Austempering treatments increased mechanical properties of all the studied materials while decreasing thermal conductivity across the line. The effects of austempering on conductivity were lower for grey and compacted graphite iron than for spheroidal graphite irons. The results indicate that heat treating can be a viable option in increasing cast iron performance in thermally stressed applications. One ferritic low-silicon spheroidal graphite iron surpassed lamellar graphite iron in conductivity at elevated temperatures, while high-silicon spheroidal graphite irons exhibited low conductivities.     

The role of graphite morphology and matrix structure on low frequency thermal cycling of cast irons

Low frequency thermal cycling tests were carried out on four types of cast iron (viz., austempered ductile iron, pearlitic ductile iron, compacted/vermicular graphite iron and grey cast iron) at predetermined ranges of thermal cycling temperatures. The specimens were unconstrained. Results show that austempered ductile iron has the highest thermal cycling resistance, followed by pearlitic ductile iron and compacted graphite iron, while grey cast iron exhibits the lowest resistance. Microstructural analysis of test specimens subjected to thermal cycling indicates that matrix decomposition and grain growth are responsible for the reduction in hardness while graphite oxidation, de-cohesion and grain boundary separation are responsible for the reduction in the modulus of elasticity upon thermal cycling.     

Strength,microstructure and chemistry of ingot mould grey iron after different cycles of low frequency high temperature loads

A kind of grey iron for producing large steel ingot moulds has been analyzed by thermal simulation experiment, and its tensile strength, matrix microstructure, fracture morphology and chemistry after different cycles of low frequency high temperature loads with oxidizing atmosphere have been studied. The micro and macro-mechanisms of large mould fatigue cracking have been discussed. As the period of low frequency high temperature pretreatments increases, the carbon content in grey iron decreases, resulting in the decrease of pearlite fraction and also the tensile strength. A new kind of graphite flake in short and small size appears in the matrix after heating and cooling treatments, leading to the rise of graphite exposure rate in the fracture under low frequency high temperature periodic loads. Although the decarburization rate in the specimens with ferrite matrix is much lower than that with pearlite matrix, the carbon content has more influence on tensile strength, induced by the following grains coarsening and grain boundary weakening. Accordingly, some feasible measures for extending mould service life have been proposed.     

Affects of graphite morphology and matrix structure on mechanical properties of cast ions

A comparative study of the affects of graphite morphology and matrix structure on mechanical properties was carried out on spheroidal, compacted and flake graphite irons by a short austenitizing plus austempering treatment. Transformation kinetic data showed that compacted graphite iron had the fastest, spheroidal graphite iron the second and flake graphite iron the slowest austenitizing rate. In spheroidal and compacted graphite irons the strength increased, while ductility decreased, with increasing the amount of bainite in matrix due to prolonging austenitizing time. On the other hand, the increase of bainite structure in the matrix had no significant affect on the mechanical properties of flake graphite iron. Fractographic examinations showed that the fracture surface of spheroidal graphite iron changed from a ductile mode to a brittle mode when its matrix changed from ferrite to bainite dominant. The flake graphite iron ruptured with brittle mode no matter what matrix it had. Compacted graphite iron exhibited an intermediate type of fracture surface. With a short austenitizing plus austempering treatment, the mechanical properties of spheroidal and compacted graphite irons could be improved and extended to a very wide range.     

The mechanical and physical properties of Compacted Graphite Iron

The mechanical and physical properties of compacted graphite iron (CGI) have been studied. While it is known that CGI provides at least 70% higher tensile strength, 35% higher elastic modulus and 80% higher fatigue limits than conventional grey cast iron, the objective of this paper was to determine how the properties change with varying nodularity (graphite shape) and pearlite content. Tensile properties, hardness, damping capacity and thermal conductivity were determined over the range 0–90% nodularity and 25–100% pearlite. Compressive properties, wear resistance and fatigue behaviour of CGI are also reported upon. Of greatest importance is the influence of patches of flake graphite in predominantly CGI microstructures. As soon as flake patches appear, which can occur with a loss of as little as 0.001% active magnesium, the mechanical properties of CGI abruptly decrease by 25–40%. Proper foundry control technology and quality procedures are therefore required to ensure the reliable production of this improved material.     

Fracture mechanics behaviour of austenitic compacted graphite cast iron

The fracture mechanics behaviour of high-nickel austenitic compacted graphite cast iron was studied and the effects of graphite morphology, alloying elements and specimen thickness on the mechanical properties, plane stress fracture toughness, and fatigue crack growth rate were evaluated. It was found that the graphite morphology, i.e. the percentage of compacted graphite present, was the major determinant of all properties of the materials investigated. The irons with a greater amount of compacted graphite (the balance was nodular graphite in austenitic matrix) resulted in lower tensile strength, yield strength, elongation and K _c fracture toughness but higher crack-growth index values (poorer crack-growth resistance). For 25 mm thick specimens, K _c values of the austenitic compacted graphite cast irons in this study were in the range of 58–64 MPa m^1/2. This is higher than ferritic/pearlitic ductile iron of 43–53 Mpa m^1/2, and is compatible to Ni-resist austenitic ductile iron of 64.1 Mpa m^1/2. The addition of cobalt not only contributed to slightly higher values of mechanical properties, but also higher plane stress fracture toughness and better crack growth resistance. Optical microscopy, scanning electron microscopy and X-ray diffraction techniques were applied to correlate the microstructural features to the properties attained.     

Mechanical, electrical, thermal performances and structure characteristics of flexible graphite sheets

The mechanical, electrical, and thermal performances of flexible graphite sheets (FGS) derived from HClO₄-graphite intercalation compounds were characterized by measuring their tensile strength, electrical resistivity, and thermal conductivity. Results show that these performances are closely related to the density of sheets and exfoliated volume (EV) of exfoliated graphite. Enhancing density of sheets tends to improve their in-plane tensile strength, electrical, or thermal performance while using the exfoliated graphite of low EV to prepare sheets is prone to resulting in low in-plane tensile strength but high electrical or thermal performance. The structure characteristics of FGS were characterized by using X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The X-ray diffraction results show that the average interlayer spacing of graphite microcrystals for sheets, d ₀₀₂, is 3.357 ? which is the same as that of natural graphite flake, but the stacking height of sheets, Lc, is lower than that of natural graphite. The Raman spectra indicate that sheets has both disorder peak and graphite one and the intensity ratio of them is ca. 0.06, which is almost the same as that of natural graphite, but higher than that of exfoliated graphite.     

Cross-property connections for copper–graphite composites

Copper–graphite composite materials in the range of 0–10 vol% of carbon phase were prepared from the mixture of copper and graphite powders by hot isostatic pressing. The microstructure, mechanical (tensile strength, elongation to fracture) and physical (electrical and thermal conductivity) properties of composite samples were investigated, and the cross-property connections were calculated. It was shown that electrical and thermal conductivity cross-property (Lorenz number) is almost constant and increases only slightly (no more than 10 % increase was observed). This implies that in the investigated composition range the Lorenz number of a copper–graphite composite system behaves according to Franz–Wiedemann law for pure metals at constant temperature. On the contrary, the conductivity to tensile strength cross-property connections showed significant linear increase (over 200 % in the investigated composition range) for both electrical conductivity and thermal conductivity of composite materials. The cross-property connections of conductivity to the elongation to fracture exhibit a nonlinear dependence on the volume fraction of graphite.     

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.     

Strength and conductivity ofin situ Cu-Fe alloys

Alloys of Cu-Fe with iron contents from 10 to 30 wt % have been prepared by casting plus mechanical reduction. A series of heat treatments was done at various stages of the mechanical reduction to promote precipitation of the iron from the copper matrix with the hope of optimizing electrical conductivity at a given strength level. A curve of optimum tensile strength against electrical conductivity was determined. It was found to lie significantly below the available data for Cu-Nb alloys and it is suggested that further improvements may be possible in Cu-Fe alloys by improved thermal mechanism processing.     

The urea nitrocarburizing of cast irons with various graphite morphologies

The urea nitrocarburizing of nodular and compacted graphite cast irons has been investigated. Optical microscopy, EDAX analysis and hardness measurements have shown that a thick compound layer with high contents of nitrogen and silicon is formed, and a greater hardness in the area around the graphite beneath the surface is observed. The compacted cast iron with interconnected graphite has been found to exhibit a smaller effective diffusion zone, and formed Si₃N₄ precipitates within the hardened area. The nitrided nodular cast iron exhibited a higher fatigue strength increment and a smaller elongation decrement than that of compacted cast iron. Most fracture surfaces of compacted cast iron were found to be granular whereas the nodular cast iron was characterized as transgranular cleavage.     

An Investigation on the Cracking of Pallet Side Walls at a Sinter Plant

A failure analysis on the cracking of pallet side walls of a sintering machine in an integrated steel plant is presented. The pallets moving at a constant speed carry the base mix for sintering and enter an ignition hood furnace (temperature????1150°C) at a regular interval of time. The pallet side walls of a sintering machine are therefore subjected to continuous thermal cycling. The material of the pallet side wall is spheroidal graphite (SG) cast iron. Ten cracked side walls are collected and analyzed. The failure investigation involves field visit, visual observation of the cracked side walls, fractography, chemical analysis, microstructural characterization, tensile and impact tests. Most of the cracks are observed between the bolt?Chole locations of the lower side walls; bolt?Chole locations act as obstructions to thermal movement of the casting. The chemical analysis shows higher level of sulfur while the materials must be of higher purity for SG iron. Fractography shows predominantly intergranular fracture. Examinations of microstructures at the cross sections of the samples show the presence of primarily intergranular cracks. Matrix structure reveals pearlite along with ferrite surrounding the embedded graphite nodules. The amount of pearlite in the matrix is measured around 30?C35% whereas predominantly ferrite matrix is desirable at the elevated temperature application. Determinations of tensile and impact properties exhibit low values of elongation (10%) and impact energy (7?J), respectively, indicating poor toughness properties of the casting. The presence of pearlite and lower amount of graphite nodules deteriorate the thermal conductivity of the casting, thereby generating more thermal stress. The analyses show that the pallet side walls start cracking under cyclic high thermal stress due to embrittlement because of improper material.     

碳纤维含量对短碳纤维－铜复合材料性能的影响

用粉末冶金法制造了碳纤维分布均匀的碳纤维一铜复合材料，测定了复合材料的力学性能和物理性能，表明在碳纤维与铜基体之间存在界面结合，碳纤维含量对复合材料性能影响极大。     

基于SLS的高强度低密石墨陶瓷复合隔热材料快速制备

本文率先利用选择性激光烧结技术快速制备了高强度石墨陶瓷复合隔热材料,重点研究了二次固化、真空压力浸渍、碳化和高温烧结等后处理工艺以及材料配方组成对其密度、抗压强度和导热系数的影响。研究发现加入适量的硅粉和可膨胀石墨可以对石墨陶瓷复合隔热材料的密度、抗压强度和导热系数进行调控,采取合适后处理工艺路线可以改变石墨陶瓷复合隔热材料的综合性能。最终实现了低密度(<1.2 g/cm^3)、高抗压强度(>10 MPa)、低的导热系数(<2 W/(m·K))和耐高温(>1650℃)等多个性能指标的统一,满足了工业应用需求。     

Fe和C元素对铜合金组织结构及力学性能的影响

目的 增强铜合金的强度、硬度,拓展该合金的应用范围.方法 在铜粉中添加不同比例共析成分的铁粉和石墨粉,利用真空热压烧结法,制备Cu-Fe-C合金.结果 热压烧结可以原位生成渗碳体;随着铁粉、石墨粉添加量的增加,铜基体的硬度持续增加,Cu-Fe-C合金的抗拉强度先上升后下降;当铁粉和石墨粉的质量分数为5％时,抗拉强度由1...     

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     

碳纤维含量对短碳纤维-铜复合材料性能的影响

用粉末冶金法制造了碳纤维分布均匀的碳纤维一铜复合材料,测定了复合材料的力学性能和物理性能,表明在碳纤维与铜基体之间存在界面结合,碳纤维含量对复合材料性能影响极大。     

含有Cu、Mo、Sn的高强度蠕墨铸铁的蠕变行为

研究了一种含有Cu、Mo、Sn的高强度蠕墨铸铁在623~823 K、40~150 MPa的蠕变行为,观察了不同形态的蠕变损伤组织并分析了蠕变变形及断裂机理。当T/T_m＞0.5(T为使用温度,T_m为蠕墨铸铁熔点)、载荷大于150 MPa时这种蠕墨铸铁的蠕变变形显著,且变形主要来自基体变形、蠕变空洞的形核长大以及石墨/基体界面的开裂。随着温度的提高和载荷的增加,蠕变变形逐渐由晶界移动转变为晶内变形。在蠕变过程中有两种开裂机制：(I)微裂纹在石墨/基体开裂处形核长大并优先沿铁素体向基体扩展,与邻近石墨/基体开裂连接而逐渐形成主裂纹;(II)晶界处的蠕变空洞形核长大转变成蠕变裂纹。氧原子通过石墨的连通性向组织内部扩散,造成上述两种裂纹表面氧化。由于,石墨、铁素体、珠光体三者性能的差异,石墨/铁素体界面比石墨/珠光体界面更易发生开裂。另外,在773 K、823 K组织中的珠光体分解明显,层片状渗碳体逐渐转变为短棒状,在晶界附近则以颗粒状为主。     

Mechanical, thermal and electrical properties of aluminum nitride/polyetherimide composites

The mechanical, thermal and electrical properties of modified AlN/polyetherimide (PEI) composites were investigated. It revealed that the surface of AlN modified by silane could effectively increase the adhesion with matrix, which was beneficial for AlN to reinforce the polyetherimide matrix. After silane modification, the AlN showed good dispersion and wetibility in the polyetherimide matrix and imparted excellent mechanical, electrical and thermal properties. The tensile strength, modulus, electrical and thermal stability were improved with the increasing of AlN content. The tensile strength of AlN/PEI composites increased by 27% when 12.6 vol.% AlN was added to neat polyetherimide. The thermal conductivity of the 57.4 vol.% AlN/PEI composites increased three times compared with neat polyetherimide. Test results indicate that the silane grafted AlN incorporated into the polyetehetimide matrix effectively enhance the thermal stability, thermal conductivity and mechanical properties of the polyetherimide composites.