The Influence of Austempering Conditions on the Machinability of a Ductile Iron

Austempering conditions such as temperature and time and their influence on austempered ductile iron machinability were analyzed. Austenitization at 910°C for 90 min and austempering into molten salt bath at 300°C, 360°C, and 420°C for 30, 60, and 90 min each were performed. Microstructures were analyzed by optical microscopy and hardness measurements. Samples were further machined in a lathe for machinability tests. The lathe was instrumented considering power and cutting time and machinability evaluation performed referring to cutting force and material removal. Microstructures at 300°C for 30 min showed ausferrite with retained austenite and martensite. Retained austenite decreased and acicular ferrite sheaves appeared at 60-min austempering time. Mixed bainite was also present at 90-min austempering. Ausferrite and retained austenite were observed in all austempering periods at 360°C, whereas at 420°C only bainite and fine pearlite were present. Hardness increased with increasing temperature at 30-min austempering and decreased with increasing time. However, an exception was observed at 420°C. The highest machinability performance was achieved at 360°C at 60-min austempering, and the lowest performance at 420°C at 90-min austempering.     

Globular-to-needle Zn-rich phase transition during transient solidification of a eutectic Sn-9%Zn solder alloy

The aim of this article was to investigate the microstructural evolution of a eutectic Sn-9%Zn solder alloy as a function of growth rate during transient unidirectional solidification. It was found that globular-like and needle-like Zn-rich phases prevail at growth rates ranging from 0.5 to 2 mm/s and 0.3 to 0.1 mm/s, respectively, with a transition region occurring between these growth rate ranges. The microstructure control in soldering processes can be accomplished by manipulating solidification processing variables such as the cooling rate and the growth rate, since the resulting morphological microstructure depends on heat transfer conditions imposed by solidification, and as a direct consequence will affect the final properties.     

Nanoausferritic matrix of ductile iron

Abstract

Austempered ductile iron is known for its excellent mechanical properties resulting from special phase composition and austempering heat treatment. Typical microstructure consists of ferrite plates of micrometre size submerged in untransformed austenite matrix. It has been recently shown that by use of appropriate chemical composition of cast iron and well targeted heat treatment parameters, it is possible to reduce ferrite plates width to submicron or even nanometric size. This creates the potential to achieve even higher mechanical properties of austempered ductile iron. The paper describes the influence of applied heat treatment parameters on microstructure of selected austempered ductile iron grades. Conditions necessary to reduce size of phases to a nanometric scale by heat treatment in austempered ductile iron are discussed.     

Shape rheocasting of unmodified Al-Si binary eutectic

It is demonstrated experimentally that by using the Council for Scientific and Industrial Research Rheo Casting System and high pressure die casting it is possible to semi-solid process and cast into a shape unmodified Al-Si binary eutectic without a solidification temperature range. Silicon leads the aluminium coupled crystal growth subjected to convection by induction during thermal arrest. The semi-solid structure during thermal arrest is captured after rheo-processing and casting.     

低合金高强焊丝退火工艺研究

研究了一种铁基合金焊丝从铸态到焊丝最终成型的制备过程。分析了焊丝制备工艺以及制备过程中合金的组织和性能。采用各种不同温度对材料进行热处理,寻求最佳的处理方案,以降低材料的硬度,并初步探求材料在不同温度参数下去应力软化处理的效果。合金钢丝经1020℃×15min去应力软化处理（空冷）后,硬度由29．8HRC降至21．1HRC,满足了后续拉拔工艺的要求。     

Improvement of tensile properties of Al-Si alloys through directional solidification

This study examines the tensile properties in directionally solidified (DS-route) Al-Si alloys and in conventionally die casting (DC-route) of the same composition. An attempt has been made to correlate the results with the fracture and microstructure of the alloys. One of the most important findings in this work is that a marked improvement in ductility is observed for DS samples over their DC-processed counterparts.     

A study on microstructure and toughness of copper alloyed and austempered ductile irons

In this study, ductile irons with and without 1 wt% copper alloy were austempered to become austempered ductile irons (ADIs). Microstructure, impact toughness, and fracture toughness were evaluated to determine how both the copper alloying and austempering treatments influenced the toughness properties of ductile irons. The results show that, because copper increases the retained austenite content in ADI, the Cu-alloyed ADI has better impact toughness and fracture toughness (K_IC value) than does the unalloyed one. In particular, the impact toughness and the fracture toughness of ADI could be efficiently improved by treating the Cu-alloyed ductile iron at a higher austempering temperature (360 °C) to obtain more retained austenite in its microstructure.     

Effect of nodule count on austenitising and austempering kinetics of ductile iron castings and mechanical properties of thin walled iron castings

Abstract

In the present work, the potential for producing thin walled ductile iron castings with an ausferritic matrix is presented. Experimentally, thin walled iron castings of 2 mm in thickness were obtained and characterised by a nodule count of 1992 mm^?2. In addition, a reference casting was produced with a 25 mm thick wall and a nodule count of 330 mm^?2. Austenitising was carried out at 920°C, whereas austempering was implemented in the 300–400°C temperature range. The austenitising and austempering transformation rates were determined by dilatometry, and the results were confirmed by microstructural analyses. It was found that in thin walled castings, the austenitising and austempering times were reduced by either one-half or one-third of the ones corresponding to the reference casting. The exhibited mechanical properties of the thin walled castings were also determined as a function of austempering time and temperature. It was found that austempering at 300°C for 1200 s leads to thin walled castings with a tensile strength of 1500 MPa. Accordingly, from this work, it is plausible to produce high strength thin walled castings that satisfy all the ASTM 897M grades of ausferritic ductile iron through proper heat treating.     

铸造新型铁镍基合金组织与性能研究

设计了用于“还原－氧化”复合介质的铸造新型铁镍基合金的化学成分，观察与分析了合金的金相组织，并研究了新型合金的均匀腐蚀、晶间腐蚀与电化学腐蚀行为。结果表明，低的含碳量和适量的Ｎｉ、Ｃｒ、Ｍｏ、Ｗ的复合作用，使该合金对还原性和氧化性介质均具有良好的耐蚀性和较高的抗点蚀性能。此外．该合金还具有良好的综合机械性能与满意的铸造性能。     

A Novel Process in Semi-Solid Metal Casting

In this research a new process for semi-solid casting of ductile iron based on the high nucleation rate combined with locally mechanical stirring is presented.In this process at first fully liquid ductile iron was poured on the peripheral surface of a wheel rotating against pouring direction.At this stage,the solid crystals nucleated at the chilling surface were pushed to the melt by a heat resistance steel cutter and finally the semi-solid slurry was generated.Reheating treatment was done on the samples to achieve more efficiency of semi-solid casting process.The effects of the travelling distance of solid particles during casting,the reheating time and temperature were examined.The results showed that the process effectively changes the dendrite structure to globular one.     

Modelling of Mould Filling and Solidification of Castings

An experimental casting for validation has been designed. The casting is composed of two 50×600×2.5 (width×length×thick) thin-wall pieces. One downsprue is located in the middle. A pouring cup with a stopper is used. This design allows to using two different types of moulds simultaneously. An Al-10%Si alloy has been poured at different temperatures. Two effects have been studied: one is the pouring temperature and the other is the moulding method (namely by machine or manually). The filling length is proportional to the pouring temperature. The influence of different moulding methods on mould filling is more complicated. The filling length in the manual-made mould is 1.5 times as long as the one in the machine-made mould due to the different thermal conductivities. Vents have little influence. A finite volume based computer code which can simulate fluid flow during mould filling coupled with heat transfer as well as solidification has been developed in WTCM Foundry Center.. The code can predict cold shut during mould filling and shrinkage defects during solidification. The simulated results are in good agreement with the experiments.In the second part of the paper, an example is given which illustrates how to use computer simulation to aid designing the casting system. The final computational result is compared with the industrial casting. The process of designing castings by using simulation is completely different from the traditional way. The computer aided casting design offers the possibility to obtain a sound casting from the first time.     

Characterization of tensile properties and microstructures in directionally solidified Al–Si alloys using linear roughness index

The tensile properties in directionally solidified Al–Si alloys and in gravity die casting of the same composition are presented. Examples of relating both the tensile and the microstructural properties of these alloys with the fracture roughness index are indicated. The roughness index was measured on vertical sections cut through the tensile fracture surface. The tensile properties examined were the yield and ultimate tensile strengths, strain at fracture, and Young's modulus. The analyzed microstructural features were porosity, dendrite area fraction, secondary dendrite arm spacing, and Si particle spacing. In almost all cases an unambiguous correlation was found between the roughness index and the tensile or the microstructural properties. A marked improvement in ductility was observed for directionally solidified samples over their gravity die casting processed counterparts. The roughness index diminished going from die cast to direction solidification, and this is likely accompanied by change in the fracture mechanisms.