蠕铁两步成蠕工艺及瞬时蠕化剂的研究

以球铁衰退过程中蠕墨存在时间较长为依据，提出先把原铁液处理成球铁后，通过加入LS蠕化剂使其衰退成为蠕铁的“两步成蠕法”，是一种方便可靠的蠕墨铸铁的生产工艺。试验表明：（1）在球铁衰退过程中．蠕墨存在时间长达10min,w（Mg）和w（RE）也有较大的变化范围；（2）用LS瞬时蠕化剂可使球化铁液迅速蠕化．并且蠕化剂加入量范围较宽；（3）调整瞬时蠕化剂加入量能较准确地控制蠕化率。     

蠕墨铸铁的生产

蠕墨铸铁的石墨形状介于灰铁的片状石墨与球铁的球状石墨之间，抗拉强度高于灰铸铁70％，略低于球墨铸铁，是结构件的优良材料。对蠕墨铸铁生产中应当如何选择、控制化学成分，蠕化剂的种类，蠕化处理方法及质量控制方法进行了介绍。     

合成蠕墨铸铁气缸盖的试制

通过严格控制铁液温度、化学成分及熔炼过程,生产的合成蠕墨铸铁气缸盖的各项技术指标均优于普通蠕墨铸铁气缸盖,而且合成蠕墨铸铁气缸盖的每吨原材料成本比普通蠕墨铸铁的低500～650元.     

蠕墨铸铁在发动机铸件上的应用

蠕墨铸铁作为一种新型铸造材料在国外发动机缸体等重要铸件上得到广泛使用,本文介绍了发动机铸件对蠕墨铸铁的具体要求以及目前国内外的应用状况,并介绍了蠕墨铸铁铸造生产工艺难点及其先进的生产工艺.在发动机关键铸件上的应用,蠕铁应当是这样的一种铸造材料,即蠕化率超过80%以上的蠕墨铸铁.蠕铁存在着较大的铸造生产工艺难点,为能生产出符合这一标准的蠕铁,必须采用具备有效过程控制手段的先进的蠕铁生产工艺,来稳定地、高标准地生产出符合要求的铸件.     

蠕墨铸铁的孕育处理

蠕墨铸铁孕育的主要目的是防止基体组织中出现莱氏体和自由渗碳体,调整铁素体和珠光体在基体中的比例。本文论述了采用固体孕育剂进行孕育,将引起"孕育促球"现象,它敏感地影响蠕墨铸铁蠕化率的高低,从而造成蠕墨铸铁性能的不稳定;指出"孕育促球"现象在特定条件下可以用来调整蠕化率,但在缺乏炉前快速测定铁液状态的情况下缺乏可操作性;提出了减少或避免"孕育促球"的若干措施,并推荐采用液体孕育剂进行孕育处理。     

蠕墨铸铁铁液状态的定量化测评

高质量蠕墨铸铁的最佳生产方式是在炉前预处理(蠕化和孕育)后,对铁液状态进行快速准确测评,并根据测评结果进行快速调整,实现铁液状态的闭环控制。在本研究中,用热分析曲线整体形状表征蠕墨铸铁铁液状态,当热分析曲线之间的差别很小时,相同凝固条件下试样组织的蠕化率差别可忽略。热分析曲线整体形状法与定量金相、模式识别和数据库技术相结合,能够实现蠕墨铸铁铁液状态的定量化预测,为铁液状态的闭环控制奠定基础。     

硅对蠕墨铸铁热分析特征值及蠕化率的影响

试验设计了 5种不同硅含量的蠕墨铸铁铁液,分析了硅含量对蠕墨铸铁凝固的热分析曲线特征值TEU、TER、△Tr和蠕墨铸铁蠕化率的影响.试验结果表明:(1)在本次试验的硅含量范围内,蠕墨铸铁热分析曲线的特征值TEU和TER先是随着硅的增加而上升,然后,变化不明显;(2)蠕墨铸铁共晶温度回升特征值△Tr随着硅含量的增加呈下降...     

稀土蠕化剂对蠕墨铸铁石墨形态和性能的影响

对以云南地方生铁为主要炉料的铁液,搭配使用稀土硅铁合金和稀土镁合金作为蠕化剂,研究了稀土蠕化剂加入量对蠕墨铸铁石墨形态和性能的影响。研究结果表明,随着稀土蠕化剂加入量的增加,铁液中稀土残留量逐渐增加,石墨形态从片状逐渐向球状过渡,抗拉强度和硬度逐渐上升。实践证明,通过合适的蠕化处理工艺,云南地方生铁铁液可以获得组织和性能较为理想的蠕墨铸铁。     

“两步法”蠕化处理工艺的开发与应用

蠕墨铸铁传统蠕化处理工艺采用冲入法或喂丝法,严格控制原铁液的化学成分、熔炼温度和蠕化处理过程,但因影响蠕化率的因素多铁液质量波动较大。为此采用预蠕化处理+第二次喂丝处理的“两步法”蠕化处理工艺,通过快速准确判断预蠕化处理铁液的状态,根据浇注铸件蠕化率和孕育要求,自动计算出二次喂丝的加入量,从而获得蠕化质量稳定的铁液,有利于提高蠕墨铸铁件质量的一致性。     

低硫铁液用蠕化剂及其生产性试验研究

介绍了供感应电炉熔炼低硫铁液生产蠕墨铸铁用的三种REMgTiSiFe合金蠕化剂的化学成分,并用其中的#1、#2和#3号蠕化剂进行了蠕墨铸铁的生产性试验。分析了铁液的蠕化衰退问题和铸造性能问题。指出:这种类型蠕化剂的蠕化效果随时间的衰退不明显;用该合金蠕化剂生产蠕墨铸铁,其铁液的铸造性能和线收缩率与HT200的线收缩基本相同,而体收缩率比HT200的要大一些。     

蠕铁缸体的生产试验

采用自制的蠕化剂,在炉前不采取脱硫处理对高硫铁液而直接进行蠕化处理的条件下,通过在自动流水线上工艺试验,小批量生产了蠕铁缸体.解剖并检测分析了RuT340缸体的本体性能与组织,完善了缸体蠕化处理工艺.结果表明,采用本文的工艺能浇注满足RuT340组织和力学性能要求的缸体.     

Modeling the effect of the microstructure of compacted graphite iron on chip formation

The unique mechanical and physical properties of compacted graphite iron (CGI) have awarded the material such desirable and increasing demands in both automotive and locomotive industries. The graphite round edges combined with irregular graphite boundaries highly enhance crack arrest resistance within the matrix and participate into the good adhesion of graphite–matrix interface, compared to gray cast iron. However, the praised mechanical performance of compacted graphite iron (CGI) compared to gray iron, and its superior thermal properties compared to nodular iron (ductile iron) have come with CGI's relative poor machinability. Finite element simulation of the microstructure of CGI will provide better understanding of the behavior of the metal during machining and will establish a good foundation of CGI machining optimization.Modeling of the microstructure of CGI chip considering the three main constituents of the metal; graphite, pearlite, and ferrite, was possible using the accumulated plastic strain fracture criterion. Although there is no distinctive boundary line between adiabatic shearing and surface crack initiation chip formation principles in real metal cutting, chip formation simulation showed that it was predominantly due to crack initiation and propagation. Cracks initiated at either the graphite particles or the graphite–matrix interface promoted by the fracture of surface graphite particles then progressed through the matrix. The characteristic segmental chip produced in CGI machining was mainly driven by the presence of graphite embedded in the matrix. Chip segments formation initiated at the graphite particles (or graphite–matrix interface) then progressed towards the chip-tool tip. Modeling of the graphite/matrix interface was based on the utilization of cohesive zone elements. Comparison between the modeled CGI microstructure and graphite-free modified microstructure highlights the significant role of graphite on chip characteristics. Comparison between the simulated segmental chip and real CGI chip validated the proposed simulation and proposes it as a valid foundation for further CGI machining optimization.     

蠕铁生产的过程控制

描述了基于热分析手段的蠕铁生产过程控制系统。该系统通过测量镁的损耗,以及在线调整铁液状态来防止片状石墨的产生。这种测量、调整的在线控制手段使生产蠕铁过程中的波动性降到最低,从根本上消除了蠕铁生产所带来的质量风险。     

Influence of fading on characteristics of thermal analysis curve of compacted graphite iron

In general,during the production of compacted graphite iron (CGI),the active residual magnesium reduces and the effect of inoculation fades after magnesium treatment.In this paper,characteristics of the thermal analysis curve of CGI are compared with those of ductile iron and grey cast iron.The fading effect on the compacted graphite percentage and thermal analysis curve were also studied.Results indicate that the undercooling of CGI is as low as that of ductile iron,but CGI shows evident recalescence.In fading process,the magnesium element acts with oxygen.For a decrease in magnesium content,both the compacted graphite percentage and the austenitic liquidus temperature increase.The temperature of eutectic undercooling (TEu) decreases before the flake graphite appears.After that,TEu increases quickly,up to as high as 20 ℃,and then gradually decreases.The evolution of recalescence degree is opposite to that of TEU.     

Effects of Casting Size on Microstructure and Mechanical Properties of Spheroidal and Compacted Graphite Cast Irons: Experimental Results and Comparison with International Standards

The aim of this research was to investigate the effects of casting size (10-210 mm) on the microstructure and mechanical properties of spheroidal (SGI) and compacted (CGI) graphite cast irons. A comparison of the experimental mechanical data with those specified by ISO standards is presented and discussed. The study highlighted that the microstructure and mechanical properties of SGI (also known as ductile or nodular cast iron) are more sensitive to casting size than CGI (also known as vermicular graphite cast irons). In particular, in both types of cast iron, hardness, yield strength and ultimate tensile strength decreased, with increasing casting size, by 27% in SGI and 17% in CGI. Elongation to failure showed, instead, an opposite trend, decreasing from 5 to 3% in CGI, while increasing from 5 to 11% in SGI. These results were related to different microstructures, the ferritic fraction being more sensitive to the casting size in SGI than CGI. Degeneration of spheroidal graphite was observed at casting size above 120 mm. The microstructural similarities between degenerated SGI and CGI suggested the proposal of a unified empirical constitutional law relating the most important microstructural parameters to the ultimate tensile strength. An outstanding result was also the finding that standard specifications underestimated the mechanical properties of both cast irons (in particular SGI) and, moreover, did not take into account their variation with casting size, at thicknesses over 60 mm.     

Identification and evaluation of modification level for compacted graphite cast iron

The shape of the freezing zone of a thermal analysis cooling curve not only contains the information about the modification level of compacted graphite cast iron (CGI) right after vermicularizing treatment, but also reflects the following fading process during holding. When the freezing zones of two cooling curves are approximately the same, graphite morphologies of the samples cast from the two corresponding melts right after vermicularizing treatment are similar, and those of the two corresponding samples cast after holding for the same period are similar too. Based on the pattern recognition method and database established from a large amount of experimental results, the shape of the freezing zone of a cooling curve can be used to identify the modification level of CGI melt and on-line prediction of a CGI melt quality has been realized.     

A reliable and consistent production technology for high volume compacted graphite iron castings

The demands for improved engine performance,fuel economy,durability,and lower emissions provide a continual challenge for engine designers.The use of Compacted Graphite Iron（CGI）has been established for successful high volume series production in the passenger vehicle,commercial vehicle and industrial power sectors over the last decade.The increased demand for CGI engine components provides new opportunities for the cast iron foundry industry to establish efficient and robust CGI volume production processes,in China and globally.The production window range for stable CGI is narrow and constantly moving.Therefore,any one step single addition of magnesium alloy and the inoculant cannot ensure a reliable and consistent production process for complicated CGI engine castings.The present paper introduces the SinterCast thermal analysis process control system that provides for the consistent production of CGI with low nodularity and reduced porosity,without risking the formation of flake graphite.The technology is currently being used in high volume Chinese foundry production.The Chinese foundry industry can develop complicated high demand CGI engine castings with the proper process control technology.     

Compacted graphite iron: Cast iron makes a comeback

Although compacted graphite iron has been known for more than four decades, the absence of a reliable mass-production technique has resulted in relatively little effort to exploit its operational benefits. However, a proven on-line process control technology developed by SinterCast allows for series production of complex components in high-quality CGI. The improved mechanical properties of compacted graphite iron relative to conventional gray iron allow for substantial weight reduction in gasoline and diesel engines or substantial increases in horsepower, or an optimal combination of both. Concurrent with these primary benefits, CGI also provides significant emissions and fuel efficiency benefits allowing automakers to meet legislated performance standards. The operational and environmental benefits of compacted graphite iron together with its low cost and recyclability reinforce cast iron as a prime engineering material for the future.     

On the thermal conductivity of CGI and SGI cast irons

The thermal conductivity of Compacted Graphite Iron (CGI) and spheroidal graphite iron (SGI) was established in the temperature range from room temperature up to 500 °C using the experimental thermal diffusivity, density and specific heat values. The influence of nodularity, graphite amount, silicon content and temperature on the thermal conductivity of fully ferritic high-silicon cast irons was investigated. It was found that the CGI materials showed higher thermal conductivity than the SGI materials. The thermal conductivity tended to increase with increasing temperature until it reached a maximum followed by a subsequent decrease as temperature was increased up to 500 °C. Conventional models were applied to estimate thermal conductivity and the predictive accuracy of each model was evaluated. The thermal conductivity could be estimated by the Helsing model. The Maxwell model, Bruggeman model and Hashin–Shtrikman model were also in fair agreement using the thermal conductivity value of graphite parallel to the basal planes in graphite.