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 cupolaelectric 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.     

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.     

热疲劳作用下球墨铸铁组织变化规律

按球化率优化了球墨铸铁的化学成分,分析了热疲劳作用下球墨铸铁的组织变化.结果表明,球墨铸铁最佳热处理工艺为890℃淬火+400℃回火,可获得最佳的强韧性配比.890℃淬火次数对石墨级别基本没有影响.初次时,基体组织以马氏体为主;随淬火次数增加铁素体数量逐步增加,约25次左右铁素体量最多;随后铁素体数量减少,直至50次时组织几乎都是马氏体+石墨.     

石墨蠕化率对蠕墨铸铁受热变形趋势的影响

通过模拟蠕墨铸铁制动受热变形试验,采用薄片试样,模拟列车蠕墨铸铁制动盘使用时的受力情况,设计受热变形试验工装,给予试样两端一个相同的、较小的变形量(弹性变形范围内),模拟列车制动工况温度150℃,加热保温30 h,自然冷却后,在自由状态下测量试样的永久变形量,对试验数据进行统计分析,通过重复做多组试验来研究不同石墨蠕化...     

定向石墨灰口铸铁的拉伸行为

提出了一种新的灰口铸铁(GCI)热加工工艺,即包覆压缩(CCC)。采用该工艺,经大于45%热变形压缩,制备了定向石墨灰口铸铁。经80%热变形的GCI拉伸性能显著提高,抗拉强度从117MPa提高到249MPa,伸长率从0提高到5.2%;热变形量超过45%的GCI,拉伸断口有分层现象,出现了一些韧性断裂的特征。     

Study of crack propagation behavior during low cycle fatigue in spheroidal graphite cast iron based on observation of surface and fracture sections

In this paper, in order to clarify the effect of crack coalescence for crack growth rate, low cycle fatigue tests were carried out using two kinds of spheroidal graphite cast iron (SGI). Crack propagation behavior from crack initiation to fracture was investigated by observing the surface of specimens and their fracture section. The main results obtained are as follows: (1) The variation range in the crack growth curves in SGI is larger than that in its matrix material. This tendency is caused by the occurrence of crack coalescence and the difference in crack initiation length. (2) In most of the cases where the crack growth rate accelerated, there was microshrinkage on the neighboring surface. (3) Crack coalescence is not the main factor in accelerating crack growth rate.     

Production technique of vermicular graphite iron cylinder head of vehicle diesel engine

The 25 years＇ production and application have proved that vermicular graphite iron cylinder heads with vermicularity ≥50% satisfy the machinability and performance demand of diesel engine. The method, in which using cupola-induction furnace duplex melting and pour-over process with rare earth-ferrosilicon or rare earthsilicon compound as vermJcularizing alloy plus rare earth-magnesium-ferrosilicon as stirring alloy, is an optimal vermicularizing process for obtaining satisfied vermicularity. Using top kiss risers, enlarging kissing areas and expanding covering width and making ingates to freeze earlier are the effective measures to eliminate shrinkage, blowhole and oxide inclusions in the vermicular graphite iron cylinder heads.     

Experimental assessment of interfacial contact thermal conductance between graphite and matrix in cast iron

Cast iron with good mechanical and thermal combination property is preferred material for the cylinder head of high power density diesel engine. Interface between graphite and matrix should play a key role in thermal conductivity of cast iron. However, it is difficult to measure the interfacial contact thermal conductance (ICTC) between graphite inclusion and matrix in cast iron. In this paper, a modeling experimental sample containing a macroscopic interface is designed and ICTC can be estimated with the aid of finite element (FE) calculation. At last the effect of ICTC on the thermal conductivity of cast iron is assessed according to the change of metallographic FE simulation results before and after considering ICTC. Results show that the magnitude of ICTC is between 1.0 × 10⁵ and 2.0 × 10⁵ W m^?2·K on the whole. The interface makes the theoretical thermal conductivity of cast iron drop down 34.6%.     

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。     

Mechanical properties of compacted graphite cast iron with different microstructures

Tensile strength, fracture toughness and impact properties were evaluated in compacted graphite (CG) cast iron with ferritic, pearlitic and ausferritic microstructures. Ultimate tensile strengths for the ferritic and pearlitic samples were 337 and 632 MPa respectively. The austempered samples showed a significant increment in the strength and recording values between 675 and 943 MPa. The fracture toughness test revealed that the stress–intensity factor K_IC was 34·0 MPa m^1/2 for the ferritic CG iron, 39·7 MPa m^1/2 for the pearlitic and between 51·0 and 58·0 MPa m^1/2 for the austempered irons. On the other hand, CG iron with ferritic matrix exhibited the best impact properties with absorbed energy of 33·3 J. The absorbed energy of the pearlitic CG iron was the lowest, 14·3 J, while the austempered samples showed values between 17·2 and 28·4 J. Complementing these results, the critical crack size was also analysed.     

灰铸铁件激光熔覆NiCuFeBSi合金的气孔行为

通过对比分析45钢与HT250两种基体成形熔覆层内气孔形态,研究了熔覆层内气孔气体类型、分布规律以及影响因素.结果表明,熔覆层内气孔气体为CO,由石墨和氧或氧化物反应生成,气孔形状不规则、具有棱角状特征.气孔上浮形式并非垂直上浮,而是弥散式上浮.熔池内部强对流是影响气孔分布的内在因素.气孔运动幅度取决于熔池对流加速度,熔池温度越高,熔池表面张力梯度也越大,熔池对流的加速度越大.激光熔覆工艺是影响气孔分布的外在因素,适当降低激光功率和扫描速度、提高灰铸铁预热温度可有效消除熔覆层气孔缺陷,降低孔隙率.     

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.     

浅谈近年来铸铁件的发展

介绍了近年来我国铸铁件的产量和新制订、修订的铸铁件国标:(1)灰铸铁的发展在产量增速上虽有所放缓,但质量上却有很大提高,体现在:新国标中牌号的细化,掌握了HT300和HT350的生产技术,专用灰铸铁的开发。(2)球墨铸铁不仅在产量上快速发展,而且更有质的飞跃,概括为:发展速度令人惊异,从无到有,从少量生产到占世界球铁产量的49.5%;球铁应用领域不断扩大;开发出各种球化处理方法;系列化生产了球化剂和孕育剂。重点介绍了Si固溶强化铁素体球铁、高强度高伸长率球铁、低温铁素体球铁、珠光体基体球铁、ADI、奥氏体球铁、高强韧TWIP铸铁,并指出了生产高端球铁件的关键点——优化石墨、净化晶界、强化基体。(3)蠕铁的进步体现在:新的国标替代了旧的部标,蠕铁缸体缸盖的生产渐入佳境,蠕铁制动盘的扩大应用正在稳妥推进。     

铁素体球墨铸铁与20钢的闪光对焊

试验使用铁素体球墨铸铁和20钢的实心棒料进行了闪光对焊，通过改变次级电压4～8级，通电时间3～5s，观察其焊接接头组织的变化并测定其抗拉强度。结果表明，接头由三个区组成；当通电时间一定时，抗拉强度随次级电压的升高先减小后增大；当次级电压一定时，抗拉强度随通电时间的增加而增大。在次级电压为8级、通电时间为5s时，抗拉强度达最大值，为370．8MPa，是20钢母材的90.4％，是球墨铸铁母材的83.7％。     

Eutectic solidification mode of spheroidal graphite cast iron and graphitization

The shrinkage and chilling tendency of spheroidal graphite (abbreviated SG) cast iron is much greater than that of the flake graphite cast iron in spite of its higher amount of C and Si contents. Why? The main reason should be the difference in their graphitization during the eutectic solidification. In this paper, we discuss the difference in the solidification mechanism of both cast irons for solving these problems using unidirectional solidification and the cooling curves of the spheroidal graphite cast iron. The eutectic solidification rate of the SG cast iron is controlled by the diffusion of carbon through the austenite shell, and the final thickness is 1.4 times the radius of the SG, therefore, the reduction of the SG size, namely, the increase in the number, is the main solution of these problems.     

不同基体组织的蠕墨铸铁热疲劳性能的研究

采用板状带V型缺口的试样，研究了蠕墨铸铁的基体组织对其热疲劳性能的影响。结果表明，当热循环温度的上限超过珠光体相组织变化温度时，蠕墨铸铁的热疲劳性能随基体组织中珠光体量的增加而下降；相反，当热循环温度的上限低于珠光体相组织变化温度时，稳定的珠光体组织可以有效地阻止热疲劳裂纹的扩展，蠕墨铸铁的热疲劳性能会随基体组织中珠光体量的增加而增强。     

Material and technique of Si-Mo heat-resistant vermicular iron exhaust manifold

E xfrhoamu satnm eanngiifnoeld. Bisecaanu soeuthleigtho ftewmapsteeragtuasrewoitfhw paasstesaggeass exhausted from engine,thisthin wallcom ponentwith m ultiple m anifold passages is usually m ade of cast iron type of m aterials except a quite few of them still fabricated of stainless steelplate.Gray castiron was applied a long tim e ago,butitwas easily therm aldam aged orcracked underthe alternative therm alloading conditionsofcooling and heating. To solve this problem , verm icular graphite c…     

气缸盖裂纹产生的原因分析及其解决措施

针对气体机气缸盖使用过程中出现裂纹的问题,分析了气缸盖裂纹产生的原因,选择适合气缸盖性能要求的材料,改进了浇注系统和冒口工艺,并确定合理的炉料配比和熔炼工艺.试生产结果表明,气缸盖材料性能符合技术要求,装机耐久试验取得满意效果,铸件的综合废品率大幅降低,有效节约了生产成本.