The forty years of vermicular graphite cast iron development in China (Part Ⅰ)

In China, the research and development of vermicular graphite cast iron (VGCI) as a new type of engineering material, were started in the same period as in other developed countries; however, its actual industrial application was even earlier. In China, the deep and intensive studies on VGCI began as early as the 1960s. According to the incomplete statistics to date, more than 600 papers on VGCI have been published by Chinese researchers and scholars at national and international conferences, and in technical journals. More than ten types of production methods and more than thirty types of treatment alloy have been studied. Formulae for calculating the critical addition of treatment alloy required to produce VGCI have been put forward, and mechanisms for explaining the formation of dross during treatment were brought forward. The casting properties, metallographic structure, mechanical and physical properties and machining performance of VGCI, as well as the relationships between them, have all been studied in detail. The Chinese Standards for VGCI and VGCI metallographic structure have been issued. In China, the primary crystallization of VGCI has been studied by many researchers and scholars. The properties of VGCI can be improved by heat treatment and addition of alloying elements enabling its applications to be further expanded. Hundreds of kinds of VGCI castings have been produced and used in vehicles, engines, mining equipment, metallurgical products serviced under alternating thermal load, machinery, hydraulic components, textile machine parts and military applications. The heaviest VGCI casting produced is 38 tons and the lightest is only 1 kg. Currently, the annual production of the VGCI in China is about 200 000 tons. The majority of castings are made from cupola iron without pre-treatment, however, they are also produced from electric furnaces and by duplex melting from cupola-electric furnaces or blast furnace-electric furnace. Examples of typical applications for VGCI castings are introduced in this paper. In China, the technologies such as rapid testing of the molten metal and non-destructive testing of casting microstructure still need to be improved. Several proposals are put forward in this paper in order to improve the production of VGCI. Generally speaking, in China, the research, production, and application of vermicular graphite cast iron are at the same level as in other developed countries and in some fields China even takes lead. (332 references and 5 Tables)

Quantitative models for microstructure and thermal conductivity of vermicular graphite cast iron cylinder block based on cooling rate

The relationships of cooling rate with microstructure and thermal conductivity of vermicular graphite cast iron(VGI) cylinder block were studied, which are important for design and optimization of the casting process of VGI cylinder blocks. Cooling rates at different positions in the cylinder block were calculated based on the cooling curves recorded with a solidification simulation software. The metallographic structure and thermal conductivity were observed and measured using optical microscopy(OM), scanning electrical microscopy(SEM) and laser flash diffusivity apparatus, respectively. The effects of the cooling rate on the vermicularity, total and average areas of all graphite particles, and the pearlite fraction in the VGI cylinder block were investigated. It is found that the vermicularity changes in parabola trend with the increase of cooling rate. The total area of graphite particles and the cooling rate at eutectoid stage can be used to predict pearlite fraction well. Moreover, it is found that the thermal conductivity at room temperature is determined by the average area of graphite particles and pearlite fraction when the range of vermicularity is from 80% to 93%. Finally, the quantitative models are established to calculate the vermicularity, pearlite fraction, and thermal conductivity of the VGI cylinder block.

Different thermal fatigue behaviors between gray cast iron and vermicular graphite cast iron

The initiation and propagation of thermal fatigue cracks in gray cast iron and vemicular graphite cast iron were investigated by Uddeholm method to reveal the complex thermal fatigue behaviors of cast iron.Differences of thermal fatigue behaviors of gray cast iron and vemicular graphite cast iron were observed and analyzed.It is found that the observed differences are related to the combination of graphite morphology and the oxidization of matrix.More oxidized matrix is observed in gray cast iron due to its large specific surface area.The brittle oxidized matrix facilitates the propagation of microcracks along the oxidization layer.By contrast,the radial microcracks are formed in vermicular graphite at the edge of graphite due to fewer oxidization layers.It indicates that the thermal fatigue resistance of gray cast iron is dominated by graphite content and morphology while that of vermicular graphite cast iron strongly relates to the strength of the matrix.

Effects of cooling rate on vermicular graphite percentage in a brake drum produced by onestep cored wire injection

In this research, a vermicular graphite cast iron brake drum was produced by cored wire injection in a one-step method. Silica sand and low-density alumina-silicate ceramic were used as molding materials in order to investigate the effect of cooling rate on percentage of vermicular graphite and mechanical properties of the brake drum casting. Several thermocouples were inserted into the casting in the desired positions to measure the temperature change. By means of one-step cored wire injection, the two residual concentrations of Mg and RE were effectively controlled in the ranges of 0.013%-0.017% and 0.019%-0.025%, respectively, which are crucial for the production of vermicular graphite cast iron and the formation of vermicular graphite. In addition, the cooling rate had a significant effect on the vermicular graphite percentage. In the case of the silica mold brake drum casting, there was an obvious difference in the cooling rate with the wall change, leading to a change in vermicular graphite percentage from 70.8% to 90%. In the low-density alumina-silicate ceramic mold casting, no obvious change in temperature was detected by the thermocouples and the percentage of the vermicular graphite was stable at 85%. Therefore, the vermicular graphite cast iron brake drum with a better combination of mechanical properties could be obtained.

“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。

铸件厚度与热处理对RuT450蠕墨铸铁微观组织与力学性能的影响

对重卡发动机的蠕墨铸铁缸体不同厚度部分进行取样,采用3D激光显微镜、扫描电子显微镜、电子万能材料试验机以及X射线应力分析仪等分析测试手段,研究了不同铸造厚度蠕墨铸铁RuT450的显微组织、蠕化率、表面残余应力、布氏硬度、拉伸强度等性能,还研究了去应力退火、不同时长的自然时效处理下蠕墨铸铁RuT450的组织变化和力学性能演变规律。结果表明:相对较厚的蠕墨铸铁铸件石墨蠕化率较高;蠕墨铸铁的铸态表层残余应力为压应力,且随试样厚度的增加而逐渐增大;在保证强度指标的前提下适度的退火处理比自然时效能更高效地释放表面残余应力;蠕墨铸铁表面硬度及强度随着试样厚度的增加而逐渐降低,退火处理可进一步降低表面硬度和强度。该研究结论对蠕墨铸铁切削性能评价及优选切削加工工艺提供了重要的理论依据。

Monitoring of Quality of Vermicular Cast Iron from the Front of the Furnace

Vermicular cast iron is used in certain fields because of its special physical properties. However, it is difficult to control the quality from the front of the furnace owing to the narrow range of vermiculizer and other elements that can be added to the iron. A real time method was developed to monitor the vermicular- graphite ratio of the cast iron based on fast measurements of the melt surface tension. The system includes a detector and a control unit that measure the amplitude and frequency of bubbles rising in the melt. This paper describes the methodology for measuring the surface tension of the melt and test results monitoring the vermicular-graphite ratio of the vermicular cast iron from the front of the furnace. The relationship between surface tension and graphite shape has been established. The results show that this system can quickly evaluate the vermicular-graphite ratio of the cast iron.

蠕化率对重型柴油机用蠕墨铸铁的磨损损伤机制的研究

采用Rtec-MFT5000型多功能摩擦磨损试验机对几种不同蠕化率的蠕墨铸铁进行了往复滑动摩擦试验,研究了蠕化率对蠕墨铸铁的磨损表面损伤行为和和磨损性能的影响。结果表明,在相同工况条件下,蠕墨铸铁的抗磨损性能随蠕化率的提高呈下降趋势,在载荷30 N的情况下,蠕化率61%的试样的磨损体积为0.049 mm^(3),比蠕化率83%、96%的试样耐磨性分别提高了16.9%、22.2%。在相同载荷下,试样的摩擦系数随着蠕化率的提高而降低。随着载荷的增加,蠕墨铸铁的平均摩擦系数呈下降趋势。在低载荷2 N的情况下,蠕墨铸铁的磨损机制主要为显微切削。随着载荷增加到15 N时,磨损机制为显微切削和黏着磨损的混合形式。当载荷增大到30 N时,蠕墨铸铁的磨损机制为显微切削、黏着磨损和疲劳磨损的混合形式。

SiMoV蠕墨铸铁高温变形行为研究

通过光学显微镜、扫描电镜、高温力学性能测试等试验方法,研究了SiMoV蠕墨铸铁在500~800℃温度范围内的高温变形行为。结果表明,随着温度的升高,应力应变曲线中的屈服阶段应变范围并未发生明显的增长改变,而缩颈阶段至断裂过程的应变区间长度明显增加,材料出现软化现象。同时,抗拉强度和屈服强度随温度升高急剧下降,800℃下屈服强度和抗拉强度相较500℃降幅度高达740%和935%。而延伸却呈现相反趋势,增长479%。裂纹起源为蠕虫状石墨尖端,其高温强度的下降除了受铁素体基体组织特性影响,也与高温下蠕虫状石墨的氧化形成二次裂纹扩展存在关联性。

蠕墨铸铁的组织性能特点及稳定生产控制

鉴于蠕墨铸铁的微观结构和凝固特征等方面与球墨铸铁更相近,它的蠕化处理范围较窄,蠕化率过高或过低都影响铸件性能或导致报废,介绍了蠕墨铸铁的组织性能特点,说明了稳定生产一致性较高的蠕墨铸铁件的方法。

珠光体含量对蠕墨铸铁高温力学性能和热物理性能的影响

通过不同的热处理方法得到不同组织蠕墨铸铁,研究基体中珠光体含量对蠕墨铸铁不同温度下力学性能和热物理性能的影响。结果表明：珠光体含量为40%～60%时,珠光体的分散度高且分布均匀;随珠光体含量的增加,蠕墨铸铁的硬度、强度、密度和比热增加,塑性韧性、热扩散率和导热降低;当珠光体由5%增加到95%时,蠕墨铸铁500℃的抗拉强度增加,达到320 MPa;屈服强度增加,达到250 MPa,伸长率为4.05%,断面收缩率为1.68%;当珠光体含量小于60%时,蠕墨铸铁的导热随珠光体含量增加缓慢降低,当珠光体含量由5%增加到60%时,100℃的导热系数由48.3 W·m^-1·K^-1降低到47.1 W·m^-1·K^-1。

Sn对蠕墨铸铁组织与抗热疲劳性能的影响

研究了Sn含量对汽车制动鼓用蠕墨铸铁中蠕墨数量、形态、基体组织以及抗热疲劳性能的影响规律。结果表明，Sn可以明显改变蠕铁中的珠光体含量，对蠕化率无明显影响，可以改变蠕铁热疲劳性能。Sn含量为0．06％（质量分数，下同）时，珠光体含量最高为95％，片层间距为0．07μm。Sn含量增加时，石墨轴比率逐渐上升，石墨变得细长且分叉增多。在20～600℃循环条件下，含0．06％Sn时蠕铁的热疲劳裂纹长度最短，抗热疲劳性能最好。

Sn对蠕墨铸铁显微组织和切削加工性能的影响

试验浇注了蠕化率均为95%的加入0.08wt.%Sn和不加Sn两种蠕墨铸铁。对比研究了它们的微观组织、力学性能和切削加工性能。合金元素Sn的加入,使蠕虫状石墨得到了细化,并且分布更加均匀,同时,促进了珠光体的形成,珠光体的含量明显增加。在使蠕墨铸铁的强度和硬度提高的同时,也使切削抗力明显增大。

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。

蠕墨铸铁组织性能均匀性研究

蠕墨铸铁是制造高功率发动机机体及气缸盖等部件的理想材料。缸盖等部件属于结构复杂且壁厚不均匀铸件。为此，本文通过对阶梯件的蠕墨铸铁组织性能测试，研究壁厚对蠕铁组织性能的影响。通过金相分析发现：试样壁厚在10-50mm范围内，随着壁厚的增加，蠕化率陡增后缓慢增加。壁厚不同，试样各部位的冷却速率不同，冷速越快，蠕化衰退时间越短，球状石墨不能充分衰退为蠕虫状石墨，蠕化率降低，铁素体数量越少。通过力学性能测试发现：随着壁厚由10mm增加到50mm，抗拉强度先降低后基本不变，伸长率逐渐增大，硬度对壁厚敏感性小，变化不大。

两种蠕墨铸铁显微组织与切削加工性能

采用金相显微镜、力学性能测试、加工性能测试等手段,观测和测试了不同合金化处理与不同蠕化效果的两种蠕墨铸铁的显微组织、拉伸强度、布氏硬度、基体显微硬度以及切削抗力。分析了两种蠕墨铸铁的切削加工性能和力学性能与显微组织之间的关系。研究结果发现:在蠕墨铸铁的石墨含量相同的条件下,蠕化率低的蠕墨铸铁中珠光体组织多,抗拉强度和硬度较高;蠕化率高的蠕墨铸铁显微组织更趋于均匀,切削抗力小,切削加工性能好。

蠕墨铸铁中石墨的三维空间结构表征

石墨空间结构形貌的准确表征,对于预测和改善蠕墨铸铁的性能至关重要。以蠕化率61.0%、68.9%、82.0%和92.2%蠕铁为研究对象,采用X射线三维扫描技术重构石墨的三维空间结构形貌,定量描述石墨的三维连通性,并建立连通性与蠕铁力学和导热性能之间的关联。结果表明,蠕铁中尺寸越大的石墨,珊瑚状特征越明显,随着蠕化率的提高,蠕铁中石墨的连通性也越来越高,连通性相对于蠕化率这一石墨的二维表征参数,能够更准确全面地评价蠕铁中石墨的形貌,这对制备高性能蠕铁具有重要参考价值。

不同孕育剂对蠕墨铸铁显微组织和切削加工性能的影响

试验浇铸了不加孕育剂、加入0.30%硅铁、加入0.30%硅钡孕育剂的3种蠕墨铸铁。对比研究了它们的切削加工性能、力学性能与微观组织之间的关系。研究表明:相同的切削条件下,3种铸铁切削力基本相同;二次孕育处理提高了蠕铁的屈服强度和抗拉强度,并且使珠光体内部的组织更加均匀。

蠕墨铸铁导热性能的数值计算及分析

为进一步研究高功率密度柴油发动机缸盖材料的导热性能.本文基于金相和X射线三维扫描重构技术,构建了蠕墨铸铁二维和三维有限元导热模型,采用有限元软件ANSYS仿真计算了不同蠕化率蠕墨铸铁的导热性能.计算结果表明:不考虑界面和石墨导热各向异性,蠕铁的导热性能随着蠕化率的升高而增大,三维模型计算蠕化率40%的蠕铁导热性能较二维模型高出约7.4%,石墨连通性对铸铁导热性能有促进作用.

工艺参数对蠕墨铸铁激光熔凝淬火带裂纹率和形貌的影响

采用3 kW固体光纤激光器在蠕墨铸铁基体上进行了激光熔凝淬火试验,分析了淬火带的裂纹率、熔凝区形貌、显微硬度和组织.结果表明,当保护气体流量一定时,提高激光功率或降低扫描速率有利于降低激光熔凝淬火带的裂纹率;而激光功率一定时,不同的扫描速率对应的最佳保护气体流量大小不同.随着激光功率的增加,淬火带形貌会向基体扩展,扩展规律是先主要沿熔凝区宽度和深度方向扩展,然后沿其它方向扩展,到一定程度后,又沿着宽度和深度方向扩展,如此循环,熔凝区总面积不断增加,淬火带形貌也由扁平状逐步变化为帽子状.