微量元素偏析对厚大断面球铁石墨形态的影响

分析了厚大断面球铁件石墨畸变的形成原因及影响因素,浇注了400mm×400mm×450mm的试块研究微量元素偏析对石墨形态的影响。结果表明：开花状石墨的中心部存在Mg、Al、La、Ca、S等元素的富集,这些元素的富集破坏了石墨生长的稳定性;而晶界上V、Ti的偏析和球化元素Mg或RE等氧化形成的氧化夹杂,破坏了奥氏体壳的稳定性,造成石墨畸变;在铁液中添加微量Sb,凝固过程中Sb偏析于石墨—奥氏体界面上,可有效抑制或减缓C向石墨球扩散,限制石墨球生长,抑制石墨球畸变。     

复合使用低硅球化剂生产厚大断面球铁

介绍了低硅球化剂与普通稀土镁硅铁合金球化剂复合使用，采用一般普通生铁生产厚大断面球铁件的铸造技术，并对产品质量进行了分析。     

孕育处理对厚大断面球铁组织和性能的影响

为研究孕育处理对铸件组织和性能的影响情况,采用不同孕育处理方式制备厚大断面球墨铸铁试样.结果表明,多次孕育处理有助于改善厚大断面球墨铸铁的石墨形态、提高铸件力学性能.     

厚大断面球铁件工艺控制

以球墨铸铁容器件为例,介绍厚大断面球铁铸件的铸造工艺。在强制冷却条件下,通过优化化学成分,对铁液净化处理,并强化球化处理和孕育处理,选择合适的处理和浇注温度,并在模拟实验中进行工艺参数验证和修订,从而在生产中获得健全铸件。     

厚大断面RE-Mg球铁喷粉净化及球化效果的研究

利用自行研制的喷粉装置，首先探讨了厚大断面RE-Mg球铁喷粉脱硫净化的效果，然后研究了稀土镁球化剂在喷粉处理时的净化及球化效果，进而对喷粉脱硫、球化、孕育综合处理工艺进行了试验研究。结果表明：喷粉处理时活性CaO的脱硫率与CaC2的脱硫率相近，使得CaO取代CaC2成为可能，喷粉处理具有突出的净化作用，在脱硫的同时，使氧含量显著降低，同时使硫趋于均匀分布；喷粉球化是一种有效的新球化方法，可获得低硫、低氧、石墨圆整而细小的铸态铁素体球墨铸铁；综合喷粉球化工艺可将脱硫剂、球化剂、孕育剂同时喷入铁液，完成脱硫、球化、孕育过程，获得的球铁在强度较高的同时，使伸长率和冲击值也增大。     

厚大断面大型球铁花盘无冒口铸造

为实现厚大断面大型球铁实体花盘无冒口铸造,采取挂砂冷铁与石墨冷铁组成的型芯强制冷却;运用大流量快速浇注,创造均衡快速凝固条件及运用合金化等手段;有效地防止铸件缩孔缩松等缺陷。经多年生产验证,铸件合格率１００％,工艺出品率９３３％,属国内、外先进水平     

用亚共晶成分生产厚大断面铸态球铁油缸

金相、硬度、机械性能测试分析表明,在生产厚大断面油缸类球墨铸件时,用亚共晶成分（CE＝4．0％～4．2％）并加入合金化元素（Cu、Sn）,能完全满足铸态QT600－2的材质要求,并消除了铸件中心或热节部位的组织疏松、石墨漂浮等缺陷,解决了长期以来油缸类铸件渗漏问题,并取得了良好的经济和社会效益。     

厚大断面球铁飞轮的研制

针对厚大断面球铁飞轮容易产生球化衰退和石墨畸变的技术难点,先后开展了厚大断面球铁试块凝固时间测定实验和不同种类球化剂抗衰退情况的实验,依据实验结果和飞轮成型模拟试验,分别制定铸造工艺参数和合金熔炼工艺,成功生产出飞轮铸件,铸件品质达到了用户的验收标准。     

冷却条件对厚大断面球铁件凝固特性的影响

模拟壁厚400mm,模数M为20的大型球铁罐体,浇注了一个边长为400mm的立方体铸件,按不同冷却条件,沿一维方向凝固方式,测定了铸件不同部位的凝固曲线和铸件凝固动态曲线.分析了大型球铁件的凝固特征,讨论了铸件壁厚、凝固时间与球墨形态变化的规律.为保证铸件质量,在冷却条件方面提出了可靠的试验依据.     

厚大断面球铁齿轮的无冒口铸造工艺设计与实践

根据均衡凝固理论提出的有限补缩、膨胀及收缩动态叠加的基本原理,利用铸铁的自补缩方式,结合铸件本身的结构特点,进行铸造工艺设计与控制。实现了厚大断面球铁齿轮的无冒口工艺铸造。     

Effect of Bi on graphite morphology and mechanical properties of heavy section ductile cast iron

To improve the mechanical properties of heavy section ductile cast iron, bismuth （Bi） was introduced into the iron. Five castings with different Bi content from 0 to 0.014 wt.% were prepared; and four positions in the casting from the edge to the center, with different solidification cooling rates, were chosen for microstructure observation and mechanical properties test. The effect of the Bi content on the graphite morphology and mechanical properties of heavy section ductile cast iron were investigated. Results show that the tensile strength, elongation and impact toughness at different positions in the five castings decrease with a decrease in cooling rate. With an increase in Bi content, the graphite morphology and the mechanical properties at the same position are improved, and the improvement of mechanical properties is obvious when the Bi content is no higher than 0.011wt.%. But when the Bi content is further increased to 0.014wt.%, the improvement of mechanical properties is not obvious due to the increase of chunky graphite number and the aggregation of chunky graphite. With an increase in Bi content, the tensile fracture mechanism is changed from brittle to mixture ductile-brittle fracture.     

Development of thermal simulation system for heavy section ductile iron solidification

A new reliable thermal simulation system for studying solidification of heavy section ductile iron has been developed using computer feedback control and artificial intelligent methods. Results of idle test indicate that the temperature in the system responses exactly to the inputted control data and the temperature control error is less than ± 0.5 %. It is convenient to simulate solidification of heavy section ductile iron using this new system. Results of thermal simulation experiments show that the differences in nodularity and number of graphite nodule per unit area in the thermal simulation specimen and the actual heavy section block is less than 5 % and 10 %, respectively.     

Low temperature impact strength of heavy section ductile iron castings：effects of microstructure and chemical composition

A foundry research project has been recently initiated at RTIT in order to better understand the fabrication of as-cast heavy section DI parts meeting high impact energy requirements at low temperatures.The experimental castings have the following dimensions 180 mm x 180 mm x 190 mm.The achieved as-cast Charpy impact strengths were as follows:17 J (RT),16 J (-20℃) and 11 J (-40℃).The foundry process,the chemical composition and the microstructure of this experimental casting are compared to the ones of various examples in order to show the detrimental effects of residual elements,microshrinkage and microcarbide on the impact properties.Finally,quality index empirical models (based on casting chemical compositions) are used to analyse the impact tests results.This paper illustrates that an adequate nodule count can contribute to reducing the detrimental effects of the residual elements and microsegregation.     

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined. The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.     

Austempering and austempered ductile iron microstructure in copper alloyed ductile iron

The variation in the austempered microstructure, the volume fraction of retained austenite, X_λ, the average carbon content of retained austenite, C_λ, their product X_λC_λ and the size of bainitic ferrite needles with austempering temperature for 0.6% Cu alloyed ductile iron have been investigated for three austempering temperatures of 270, 330, and 380 °C for 60 min at each temperature after austenitization at 850 °C for 120 min. The austempering temperature not only affects the morphology of bainitic ferrite but also that of retained austenite. There is an increase in the amount of retained austenite, its carbon content, and size of bainitic ferrite needles with the rise in austempering temperature. The influence of austempering time on the structure has been studied on the samples austempered at 330 °C. The increase in the austempering time increases the amount of retained austenite and its carbon content, which ultimately reaches a plateau.     

Effect of elongated graphite on mechanical properties of hot-rolled ductile iron

The mechanical properties of hot-rolled and then annealed ductile iron were evaluated. The deformation of this two-phase material and the effects of the elongated graphite spheres on the mechanical properties and the development of anisotropy of mechanical properties were studied. An attempt was made to describe the anisotropy of tensile strength in terms of the deformation of graphite spheres and of the root curvature, area fraction, and the interbridges that result in disproportional changes of stress concentration, loading capacity, and the tendency to break the interbridges and link the neighboring deformed graphite spheres.     

等温淬火对含铬球墨铸铁组织及性能的影响

研究了等温淬火工艺对含铬球墨铸铁组织、硬度、冲击性能和耐磨性的影响。结果表明：奥氏体化温度升高,能促进球状石墨长大,增加残留奥氏体含量。淬火后组织主要为球状石墨、针状贝氏体、含铬碳化物及残留奥氏体。当淬火等温温度在240～270 ℃,随着等温温度升高,试样硬度和耐磨性均降低;在240 ℃等温时冲击韧度较低,继续升高等温温度,冲击韧度先增大后降低;当试样经910 ℃×80 min奥氏体化、270 ℃×180 min等温淬火后,含铬球墨铸铁的硬度可达54.1 HRC、冲击韧度αk可达8.1 J·cm-2,有较好的耐磨性。     

The influence of cast structure on the austempering of ductile iron. Part 3. The role of nodule count on the kinetics,microstructure and mechanical properties of austempered Mn alloyed ductile iron

Austempering kinetic measurements and mechanical property measurements are reported for irons with different Mn contents and different nodule counts after austenitising at 870 °C and austempering at 375 °C. It is shown that increasing nodule count, which reduces segregation and changes the size and distribution of intercellular boundaries, increases the interphase boundary area between graphite and matrix and decreases the continuity of the unreacted austenite in the intercellular boundary. This accelerates the stage I reaction which broadens the heat treatment window and moves it to earlier austempering times. A high nodule count can be used to counter the delay of the stage I reaction caused by Mn additions used to increase the hardenability of the iron. A high nodule count produces a finer, more uniform ausferrite structure that increases the strength, ductility and impact energy of the austempered iron.     

球铁光纤激光-MIG复合焊接头的裂纹倾向与力学性能

以308L不锈钢焊丝作为填充材料,采用光纤激光-MIG电弧复合焊在5mm厚的球墨铸铁上进行焊接,重点关注了工艺参数对裂纹倾向的影响,获得了成形良好且无裂纹的焊接接头．结果表明,随着激光功率的减小和电弧电流的增加,接头熔合比减小,裂纹倾向降低．接头显微硬度和组织的分析结果表明,由熔合比带来的碳含量变化是影响裂纹倾向的直接原因．在厚度10mm的球墨铸铁试件上开X形坡口进行多层多道焊,所得焊接接头的强度和断后伸长率分别为母材的73％和20％,接头断裂机制为脆性断裂,半熔化区的莱氏体是造成断裂的原因．     

Effect of austempering time on mechanical properties of a low manganese austempered ductile iron

An investigation was carried out to examine the influence of austempering time on the resultant microstructure and the room-temperature mechanical properties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane strain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductile cast iron with low manganese content and with a ferritic as-cast (solidified) structure. These specimens were then austempered in the upper (371 °C) and lower (260 °C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h. Microstructural features such as type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of this material. The results of the present investigation indicate that for this low manganese austempered ductile iron (ADI), upper ausferritic microstructures exhibit higher fracture toughness than lower ausferritic microstructures. Yield and tensile strength of the material was found to increase with an increase in austempering time in a lower bainitic temperature range, whereas in the upper bainitic temperature range, time has no significant effect on the mechanical properties. A retained austenite content between 30 to 35% was found to provide optimum fracture toughness. Fracture toughness was found to increase with the parameter (XγCγ/d)^1/2, where Xγ is the volume fraction of austenite, Cγ is the carbon content of the austenite, and d is the mean free path of dislocation motion in ferrite.