Role of Sulfur on the Transition from Graphite to Cementite Eutectic in Cast Iron

In the current study, an analytic solution is considered to explain the influence of sulfur on the transition from graphite to cementite eutectic in cast iron. The outcome from the current study indicates that this transition can be related to (a) the graphite nucleation potential (directly characterized by the cell count and indirectly by the nucleation coefficients; (b) the eutectic graphite growth rate coefficient; (c) the temperature range between the equilibrium temperature for graphite eutectic and the formation temperature for cementite eutectic; and (d) the liquid volume fraction, after pre-eutectic austenite solidification. In addition, the absolute and the relative chilling tendencies, as well as critical cooling rates including the chill width of the cast iron can be predicted from the current study. The analytic model was experimentally verified for castings with various sulfur contents. It is found that the main role of sulfur on the transition from graphite to cementite eutectic is through its effect on lowering the growth coefficient, and hence, the graphite eutectic growth rate. In addition, it is found that with the increasing sulfur content, the critical cooling rate is significantly reduced, thus increasing the absolute and the relative chilling tendency values, including the chill width.     

Mechanism for the Role of Silicon on the Transition from Graphite to Cementite Eutectic in Cast Iron

In this work, an analytical solution is proposed to explain the influence of silicon on the transition from graphite to cementite eutectic in cast iron. It is found that this transition can be related to (1) the graphite nucleation potential (directly characterized by the cell count N and indirectly by the nucleation coefficients N _s and b), (2) the growth rate coefficient of graphite eutectic cells ??, (3) the temperature range ??T _sc?=?T _s ?C T _c (where T _s and T _c are the equilibrium temperature for graphite eutectic and the formation temperature for cementite eutectic, respectively), and (4) the liquid volume fraction f _l after preeutectic austenite solidification. Analytical equations were derived that describe the absolute and the relative chilling tendencies (CT and CT_r, respectively) as well as the critical cooling rate Q _cr and, hence, the chill w of the cast iron. Theoretical arguments are experimentally verified for castings with various silicon contents. This work also describes the methods used in the determination of N _s, b, and ?? values. It is found that the main role of silicon on the transition from graphite to cementite eutectic is to raise the density of the graphite nuclei N and temperature range ??T _sc. In addition, it is shown that increasing the silicon content of cast iron leads to an increasing value of Q _cr and decreasing values of CT and CT_r, and of the chill width w. In particular, this work shows that the chilling tendency indexes and, hence, the chill all can be estimated from a simple thermal analysis using reference castings.     

Cementite Solidification in Cast Iron

Two hypereutectic cast irons (5.01 pct Cr and 5.19 pct V) were cast and the polished surfaces of test pieces were deep-etched and analyzed via scanning electron microscopy. The results show that graphite lamellae intersect the cementite and a thin austenite film nucleates and grows on the cementite plates. For both compositions, graphite and cementite can coexist as equilibrium phases, with the former always nucleating and growing first. The eutectic carbides grow from the austenite dendrites in a direction perpendicular to the primary plates.     

稀土变质铸铁中石墨—奥氏体共晶的生长方式

用Ｂｒｉｄｇｍ ａｎ 定向凝固技术制取了稀土变质共晶铸铁不同石墨形态时的液－固平界面试样。观察结果表明,稀土变质铸铁共晶凝固过程中,石墨相不论形态如何均为领先相;石墨与奥氏体共晶有共生生长和离异生长两种生长方式。在共生生长情况下,奥氏体紧贴石墨片两侧协同生长,形成锯齿状的液－固界面结构;在离异生长情况下,石墨相单独在液相中析出并充分长大,随后被奥氏体包围,液－固界面形态主要取决于奥氏体－液相界面的稳定性。分析表明,稀土在石墨－液相界面的富集是导致石墨－奥氏体共晶离异生长的主要原因。     

Unidirectional Solidification of Eutectic Cast Iron

Abstract

High intrinsic damping in alloys finds many applications in the elimination of unwanted vibrations and reduction of acoustic noise. The increasing interest in these materials has resulted in the development of a new class of engineering alloys, the HIDAMETS (high-damping metals), specifically tailored for use in vibration-resistant structures. Numerous different physical mechanisms (usually controlled by the alloy microstructure) can be responsible for the level of internal friction or damping in a candidate alloy for a particular application. Consequently, alloy selection depends mainly upon the conditions to be encountered in service, i.e. temperature, frequency of vibration, amplitude of vibration, magnetic field, etc. Characterization of the internal friction in high damping alloys is described and some of the mechanisms involved are discussed. Results obtained for two different alloys are presented. The first is a commercial Mn-Cu alloy used for marine propellers and the second is a die-cast Zn-Al alloy with possible application in the automotive industry.     

The Microstructure of Graphite Spherulites in Cast Iron

The microstructure of graphite spherulites （G.S.） in Ce addition ductile cast iron was carried out using transmission electron microscopy （TEM） techniques. The structral characterization of the graphites is as following: there are platelets growing along the periphery as well as fan-like structure formed from platelet aggregates in the diameter section of spherulite, in which most of the platelets are with sizes ranging from several to tens nm in radial; the [001] direction of the platelets would have a tend to parallel to radial of the G. S. and meanwhile, they also a small misorientation deviated from the radial among the platelets each other. It shows that the anisotropy of graphites has been restrained by adding Ce element in cast iron and the structural characterization of the G. S. is consistent with the Double’ s model of the conical helixes formed in helical growing and branching.     

铸铁中石墨形成机理的再探讨

运用金属型人实验仪证实，进行冷却的铸造铁水内部存在着温差电势场，且为点电势场，其寿命约为０．０５～３．５８ｓ。我们发现，这种电势场是运动的，运动速率与其所的区域的温度梯度的变化率有关，分析并找出了决定铸铁中石墨形态的关键因素。     

Effect of Pressure on Graphite Morphology and Mechanical Properties of Squeeze Cast Hyper-Eutectic Grey Cast Iron

Squeeze casting is a process in which a high external pressure is applied and maintained on the molten metal during the whole solidification process resulting in the lower porosity, higher solidification rate and improved mechanical properties. In this research, the effects of applied pressure on the morphology of graphite flakes in a squeeze cast hyper-eutectic grey cast iron were investigated. The molten grey cast irons were solidified under atmospheric pressure as well as 25, 50 and 75 MPa externally applied pressures. The results indicated that the pressure increase during solidification decreased the amount of free graphite flakes, reduced the size of pro-eutectic Kish graphite flakes and increased the density of the castings. D-type graphite enclosing austenite dendrites, resembling that expected in hypo-eutectic grey cast iron, was observed near the surfaces of the castings solidified under externally applied pressures. Moreover, some compacted and spheroidal graphite particles were observed in the castings solidified under 50 and 75 MPa pressures. Furthermore, by increasing the squeeze casting pressure from atmospheric pressure to 75 MPa, the hardness, ultimate tensile strength and elongation increased by about 13, 68 and 128%, respectively. The results have been discussed in terms of increased cooling rate, expected change in the phase diagram of the alloy, different atomic structure of the interfaces of graphite and austenite as well as some proposed theories for formation of eutectic cells and spheroidal graphite.     

富镧蠕化剂对铸铁中的石墨形态和组织的影响

为了合理利用稀土资源中随着钕、铈的大量提取和应用后所得的副产品富镧混合稀土金属,本文直接在直流矿热炉中用碳热还原法制备富镧基稀土硅铁合金,并利用所制备的富镧稀土硅铁合金作为蠕化剂来制取蠕墨铸铁.主要研究了富镧稀土硅铁合金的加入量对铸铁中石墨形态和组织的影响.结果证明,用富镧基稀土蠕化剂,通过控制适当的工艺条件,能够得到蠕墨铸铁和球墨铸铁.     

Cerium Distribution in Spheroidal Graphite of Cast Iron

Six thin sections of spheroidal graphites in cast iron have been investigated by TEM and SEM.Thespheroidal graphites consist of two or three layers and have nuclei which are composed of Ce_2O_2S and Ce_2O_3.Cerium is existed ont only in nuclei,but also in central layer and middle layer of graphite.     

Effect of Rare Earths and Nitrogen on Graphite Structure of Gray Cast Iron

ＥｆｅｃｔｏｆＲａｒｅＥａｒｔｈｓａｎｄＮｉｔｒｏｇｅｎｏｎＧｒａｐｈｉｔｅＳｔｒｕｃｔｕｒｅｏｆＧｒａｙＣａｓｔＩｒｏｎＺｈａｉＱｉｊｉｅ（翟启杰），ＺｅｎｇＱｉ（曾奇）（ＦｏｕｎｄｒｙＩｎｓｔｉｔｕｔｅ，ＵｎｉｖｅｒｓｉｔｙｏｆＳｃｉｅｎｃｅａ...     

稀土对高Ni-Cr铸铁石墨的变质作用

采用金相和电镜等手段对稀土在高Ｎｉ－Ｃｒ铸铁中的行为进行了研究。结果表明：稀土可以改善高Ｎｉ－Ｃｒ铸铁石墨的形态及分布，使之由长条状向短厚状过渡。并且随着稀土加入量增多，石墨数量增加。     

蠕墨铸铁生产工艺控制

以Z14号生铁为原料,用中频炉熔炼原铁液,加入1.1%的FeSiMg4Re17作为蠕化剂,1%的FeSi75作为孕育剂,采用平底包冲入法蠕化处理原铁液,控制出铁温度为1420～1450℃,待反应结束后在10min内浇注完毕,可获得合格的蠕墨铸铁工件.检测表明,所生产的铸件成分稳定,石墨蠕化率为62%,机械性能达到国家标准.     

球墨铸铁轧辊保温冒口研究

球墨铸铁轧辊的补缩,一般是采用砂型明冒口和浇注后每隔20-25分钟用过热铁水点冒口1-2次的办法解决,这样的工艺使冒口金属消耗量占轧辊毛重的15%左右。为了节约金属和提高轧辊上辊颈质量,本研究工作是采用铝型膨胀发热剂单独使用或与绝热套组合成的保温冒口,以取代原工艺以稻草灰覆盖的干砂型冒口。经过实验室及现场对此实验表明:采用保温冒口以后,球墨铸铁共晶凝固时间延长了33-62%,缩孔形状由倒锥形变为盆形,轧辊冒口的致密段高度增加,省去了用过热铁水点冒口的工序。冒口高度可以降低25~50%,节约冒口金属量20~50%。研究工作表明:采用保温冒口是解决球铁轧辊补缩的最合理、经济而简便的措施。     

Improvements in the Characteristics of Nodular Graphite Cast Iron for Crankshafts

This study deals with spheroidal graphite cast iron metallurgy for the mass production of engine crankshafts. Pieces have been made in three different grades under industrial conditions. CCT diagrams and micrographs show that it is possible to remove the undesirable “bull's eye” ferrite from the matrix as well as to improve the mechanical properties of series parts by only altering the chemical composition. A 100% pearlitic matrix and up to a 14% increase in the tensile strength have been obtained by lowering the amount of silicon and increasing the share of pearlitizing elements such as copper and manganese without altering the cooling speed of the parts. The effect of the cooling speed on the matrix microstructure and the mechanical features has also been revealed. For an “improved” grade, the matrix is 100% pearlitic, its hardness is about 390Hv30kg for a cooling speed of around 150°C/min.     

蠕墨铸铁在焦炉护炉铁件设计中的应用

蠕墨铸铁具有较高的机械性能和良好的抗热疲劳、耐磨损、耐热腐蚀的性能。采用蠕墨铸铁代替灰铸铁作护炉铁件材料 ,经对炉门、炉框、大保护板强度进行核算 ,证明蠕墨铸铁适合于护炉铁件 ,并具有可观的经济效益