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。     

Microstructure and mechanical properties of V–Ti–N microalloyed steel used for fracture splitting connecting rod

A new kind of V–Ti–N high strength microalloyed medium carbon steel has been developed, which is used for fracture splitting connecting rod. In this article, the characteristics of this carbon steel and its production process were studied. The microstructure, precipitated phases and their effects on mechanical properties were investigated by optical microscope, SEM, and TEM. The results showed that the steel was constituted of ferrite and pearlite. By reducing the finish rolling temperature and accelerating the cooling rate after rolling, microstructure with fine grain ferrite and narrow lamellar space pearlite could be obtained in V–Ti–N microalloyed medium carbon, and a large number of precipitated phases distributed over ferrite. These led the tensile strength to be more than 1000 MPa, yield strength (YS) more than 750 MPa. The impact fractograph showed typically brittle fracture characteristic.     

Investigation of microstructure and mechanical properties of steel fibre–cast iron composites

Abstract

The aim of the present experimental study was to investigate improvement of the toughness and strength of grey cast iron by reinforcing with steel fibres. The carbon content of the steel fibres was chosen to be sufficiently low that graphite flakes behaving as cracks were removed by carbon diffusion from the cast iron to the steel fibres during the solidification and cooling stages. To produce a graphite free matrix, steel fibres with optimum carbon content were used and the reinforced composite structure was cast under controlled casting conditions and fibre orientation. Three point bend test specimens were manufactured from steel fibre reinforced and unreinforced flake graphite cast iron and then normalising heat treatments were applied to the specimens at temperatures of 800 and 850°C. The fracture toughness and strength properties of the steel fibre reinforced material were found to be much better than those of unreinforced cast iron. The microstructures of the composite at the fibre–matrix transition zone were examined.     

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.     

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.     

不同含量改性纳米粉体对球墨铸铁拉伸性能的研究

通过冲入法将改性纳米粉体M加入到球墨铸铁中,研究了不同含量的改性纳米粉体M对球墨铸铁组织和拉伸性能的影响。实验结果表明,加入0.01%(质量分数)改性纳米粉体M后,石墨球的综合性能提升最明显,其尺寸减小且变得均匀,数量增加,圆整度提高,晶粒得到细化,珠光体含量降低,铁素体含量增加,拉伸性能得到提升。抗拉强度提高到493.82 MPa,延伸率提高到19.09%。通过断口分析,确定其拉伸断裂的机理为混合式断裂。     

Metallurgical investigation into the failure of an iron ore sintering car pallet

A failure analysis on premature cracking of a sintering car pallet in an integrated steel plant is presented. A detailed study on the microstructural and mechanical properties of the failed component has been carried out. Microstructurally, the amount of pearlite in the ferritic–pearlitic matrix was found to be significantly higher (~ 65.8%) than normally expected in materials used for elevated temperature application. A thin layer of flake and vermicular graphite (degenerate graphite) was found just beneath the casting skin although interior of the matrix contained normal spheroidal graphite. In few occasions, the presence of undesirable spiky and exploded graphites was also noticed. The material exhibited lower yield stress (312 MPa), tensile strength (457 MPa) and Charpy impact energy (3.4 J) indicating poor strength and toughness of the casting. The improper graphite morphologies in the sinter car pallet acted as stress raisers and produced cracks under dynamic thermal cycling and external loads experienced during the sintering process resulting in its premature failure.     

Mechanical properties of SG-iron with different matrix structure

Spheroidal graphite (SG) irons with a variety of matrix-structure have been produced. The correlation between tensile properties, impact toughness, hardness and pearlite content is investigated. The pearlite content is varied from 0 to about 95 per cent by the use of different heat treatment processes. The apparent variation in the properties with the pearlite level reveals the remarkable consistency in the relationships between mechanical properties and pearlite content. The study of the tensile properties showed that the yield and ultimate tensile strengths are increased with increasing pearlite level in the matrix structure. For matrix structure with 94.6% pearlite, the increases are about 91% and 98%, respectively, compared with those of the ferritic matrix material. The impact toughness of SG-iron is influenced significantly by matrix microstructure. Energy of about 230 × 10³J/m² is required to fracture a ferritic matrix SG-iron. On the other hand, when the matrix structure approaches a fully pearlitic matrix the fracture energy is decreased by an amount of 75.5%. The Brinell hardness value is found to increase with increasing pearlite content in the matrix structure of the present material. It increases from about 128 for a fully ferritic matrix to about 258 as the matrix structure approaches a fully pearlitic condition. This change in the hardness value reflects the change in the mechanical properties presented in this study.     

Effects of Alloying Elements on the Microstructures and Mechanical Properties of Heavy Section Ductile Cast Iron

The effects of alloying elements on the as-cast microstructures and mechanical properties of heavy section ductile cast iron were investigated to develop press die material having high strength and high ductility. Measurements of ultimate tensile strength, 0.2% proof strength, elongation and unnotched Charpy impact energy are presented as a function of alloy amounts within 0.25 to 0.75 wt pct range. Hardness is measured on the broken tensile specimens. The small additions of Mo, Cu, Ni and Cr changed the-as-cast mechanical properties owing to the different as-cast matrix microstructures. The ferrite matrix of Mo and Ni alloyed cast iron exhibits low strength and hardness as well as high elongation and impact energy. The increase in Mo and Ni contents developed some fractions of pearlite structures near the austenite eutectic cell boundaries, which caused the elongation and impact energy to drop in a small range. Adding Cu and Cr elements rapidly changed the ferrite matrix into pearlite matrix, so strength and hardness were significantly increased. As more Mo and Cr were added, the size and fraction of primary carbides in the eutectic cell boundaries increased through the segregation of these elements into the intercellular boundaries.     

An experimental and numerical analysis to correlate variation in ductility to defects and microstructure in ductile cast iron components

This paper describes a statistical test plan to determine tensile properties and fracture properties for ductile cast iron considered for the Swedish nuclear waste canisters and associated analysis. Large variations were found in the ductility between tested canister inserts and between specimens taken from different locations in each insert. A large number of tested tensile specimens were subsequently analysed by fractography and metallography to relate low ductility values to size and type of casting defects. Loss of ductility could be related to slag defects and to a lesser extent to high pearlite content, low nodularity and chunky graphite. Slag defects were modelled by an elasto-plastic fracture mechanics model for penny-shaped slag defects and semi-empirical models for the other defect types. The fracture model was incorporated into a probabilistic scheme to compute distribution of elongation for the inserts and the associated defect size. The computed ductility distribution agrees very well with measured data whereas the computed defect size distribution is underestimated. By including crack growth resistance and various aspect ratios of defects a much better agreement with observed defects can be achieved.     

Fuzzy neural network analysis on the compacted graphite iron with improved tensile and heat transport properties

In order to find out the most effective method for developing compacted graphite iron with a combination of high tensile strength, ductility and thermal conductivity, the superposed structural effects were investigated by experimental results and the relative significances were ranked on the basis of fuzzy neural network model. The concerned structural parameters consisted of graphite content, vermicularity and microhardness of the matrix. It was found that the relationships between properties and structural parameters become complex due to the mutual disturbances of various characteristics. Irregular and compossible corrections were both observed. The sensitivity level suggested that low microhardness of the matrix and low vermicularity are the optimal directions for improving simultaneously the tensile strength, thermal conductivity and elongation of compacted graphite iron.     

Mechanical behaviour of graphite in fracture of austempered ductile iron

Abstract

An investigation was carried out to examine the mechanical behaviour of graphite in the fracture of austempered ductile iron (ADI) by in situ tensile testing with an SEM. The results indicate that the graphite in ADI cannot be regarded as voids with no strength because graphite–matrix (G–M) interface cracking and the internal fracture of graphite were observed. Under tensile testing, microcracks always initiated at and propagated along the G–M interface first. Graphite nodules do not cause micronotch stress concentration thermselves in advance of the G–M interface cracking. The propagation of interfacial cracks along the G–M interface resulted in crack deflection. When the main crack propagated to a graphite nodule whose G–M interface had cracked and formed a void, it was obviously blunted. Graphite–matrix interface cracking occurred ahead of a propagating crack. The G–M interfaces and graphite nodules have a certain strength.     

Influence of chunky graphite on mechanical and fatigue properties of heavy-section cast iron

This research work aimed to find out the influences of the different amounts of chunky graphite on mechanical and fatigue properties of GJS 800 ductile cast iron. Chunky graphite has been a problem of heavy section thick-walled ductile cast iron components. Chunky graphite is branched and interconnected as a network within eutectic cells and has been observed to form in thermal centres of heavy ductile cast iron sections during solidification. This research work proved that chunky graphite in the microstructure decreases the ultimate tensile strength, the elongation to fracture and fatigue life significantly, but does not influence on the yield stress of ductile cast iron GJS 800 substantially. Low nodular count and nodularity rate also decrease the fatigue life of ductile cast iron, and the difference of fatigue life of specimens containing chunky graphite or having low nodular count and nodularity rate is not large. Influence of the amount of chunky graphite on fatigue life increases until 20% chunky graphite content, and above that the fatigue life does not decrease substantially.     

电脉冲对铸态高韧性球墨铸铁凝固组织及性能的影响

对铸态高韧性球墨铸铁QT400-18的凝固过程进行电脉冲处理,电脉冲参数为电压2 600 V,频率0.88 Hz,电容200μF,处理时间15min。通过对比试验可知:电脉冲处理可使铸铁的过冷度相比未处理的升高12K,改善了石墨形核和生长的动力学条件,从而使球状石墨的粒径减小,石墨球数量增加。铸铁经过电脉冲处理,球化率由未处理的平均80%提高到平均91%,石墨球数由平均172/mm~2提高到209/mm~2,石墨不易变态生长,圆整度得到改善。同时,球墨铸铁的凝固组织得到了改善,铁素体数量增加,达到83%,相比未电脉冲处理的试样提高了26.6%,铁素体晶粒小于未处理试样,基体中分布的珠光体数量和片层间距减少,在抗拉强度符合要求的条件下,伸长率提高了4.89%,-233K的冲击功Akv提高了3.56J。     

基于S35140钢的超低碳成分设计和力学性能研究

S35140钢是一种基于25Ni-20Cr的奥氏体耐热钢,为了获得高强度,通常会提高碳含量,但碳含量较高不利于高温时效稳定性和长期耐腐蚀性能.本文在S35140钢的基础上,大幅度降低碳含量,并通过调控N和Nb等微合金元素含量,以及加入Ti元素,促使析出新的强化相,弥补减少碳含量所导致的强度降低.同时引入一定量的Al元素...     

珠光体对ZG120Mn13钢拉伸断裂过程的影响

为探究珠光体降低高碳高锰钢机械性能的原因,本文采用金相组织分析、机械性能测试和断口微观形貌分析等实验方法,研究了奥氏体基体上含体积分数23%珠光体的ZG120Mn13高碳高锰钢的拉伸性能及其裂纹形核和扩展过程.结果表明:通过时效处理,在奥氏体基体上析出的条状、颗粒状以及沿晶界连续分布的珠光体将使ZG120Mn13钢的强度和塑性大幅度下降.机械性能的降低与其力学行为有关,当基体为单一奥氏体时,裂纹将在大量孪生变形后,在孪晶界、孪晶与晶界交界处形核,并沿孪晶界长大而相互连接、扩展.而奥氏体基体上存在珠光体时,裂纹主要在珠光体团内形核,并通过相邻珠光体间奥氏体的塑性耗竭、切断而得以扩展.     

短切炭纤维增强沥青基C/C复合材料的组织特征

利用新型、高效的模压半炭化成型工艺，在大气环境下制备出了短切炭纤维增强沥青基C／C复合材料制品，并借助光学显做镜和扫描电镜对其微观组织和断口形貌进行了观察。通过分析，解释了短切炭纤维增强沥青基C／C复合材料中炭纤维损伤的形成机制，提出了作为增强体相的短切炭纤维和焦炭颗粒与基体炭之间独特的界而结构模型。研究还表明：复合材料中明显存在着基体相和颗粒相一基体相的显微结构不仅呈层片状，而且层片状的结构好像数层桔子皮，将颗粒相包裹起来，这种“桔皮包裹”式的结构与炭纤维表面的POG结构基本相似。     

显微组织对球墨铸铁电阻率的影响

利用双臂电桥测量电阻的方法,研究了石墨球和基体组织对球墨铸铁电阻率的影响。研究结果表明,珠光体和混合基体球墨铸铁的电阻率随石墨球数增加而逐渐提高,而石墨球数对铁素体基体的球墨铸铁电阻率影响不显著;基体组织对球墨铸铁的电阻率有较大影响,随着铁素体量的增加,电阻率逐渐降低。石墨球数量增加导致球墨铸铁电阻率升高的原因是单位体积中石墨球与基体的晶界面积增多。     

Strengths of C/C composites under tensile, shear, and compressive loading: Role of interfacial shear strength

Various strengths of carbon–carbon composites (C/Cs) are comprehensively reviewed. The topics reviewed include tensile, shear, compressive, and fatigue strength as well as fiber/matrix interfacial strength of C/Cs. When data are available, high temperature properties, including creep behavior, are presented. Since C/Cs have extremely low fiber/matrix interfacial strength τ_d, the interfacial fracture plays important roles in all of the fracture processes dealt in this review. The low τ_d was found to divide tensile fracture units into small bundles, to seriously degrade both shear and compressive strength, and to improve fatigue performance. In spite of the importance of the interfacial strength of C/Cs, techniques for its evaluation and analysis are still in a primitive stage.