Quantitative analysis of cast microstructures

If it is to be incorporated into an analytical model, the microstructure of a cast material must be described in unambiguous, quantitative terms. Three-dimensional microstructures have useful stereological properties which are common to features of any geometry and can be determined from measurements on metallographic sections. This paper features examples for determining microstructural fineness (e.g., dendrite arm spacing), constituent phase coarsening during heat treatment, and distribution analysis of graphite nodule sizes in ductile iron.     

Unambiguous classification of complex microstructures by their three-dimensional parameters applied to graphite in cast iron

Three-dimensional (3D) quantitative analysis is indispensable for the unambiguous characterization and objective classification of complex microstructures. Focused ion beam (FIB) nanotomography provides complete information of the spatial arrangement, chemistry and orientation of different phases of real microstructures on scales especially important in materials science (10 nm–100 μm). Complex graphite particles were analyzed in three-dimensions. Whereas nodular, vermicular and temper graphite particles can be characterized individually, the whole network of flake graphite has to be considered due to the high spatial interconnection of particles. The characterization method was verified in comparison to established two-dimensional stereological methods. The influence of anisotropy and image resolution was discussed. Basic stereological characteristics (volume, surface area, integrals of mean and total curvature) as well as 3D connectivity (Euler number) and shape parameters objectively differentiate these graphite morphologies and contribute to the understanding of their growth mechanisms and the properties of the cast iron.     

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.     

中板轧机高铬复合铸铁轧辊使用分析

简单论述了中板四辊轧机工作辊的材质选择原则及影响因素,对邯钢中板厂使用的高铬复合铸铁轧辊存在的问题及原因进行了分析,并提出了改进措施。     

Calculating the effective thermal conductivity of gray cast iron by using an interconnected graphite model

A new theoretical model of gray cast iron taking into account a locally interconnected structure of flake graphite was designed, and the corresponding effective thermal conductivity was calculated using the thermal resistance network method. The calculated results are obviously higher than that of the effective medium approximation assuming that graphite is distributed in isolation. It is suggested that the interconnected structure significantly enhances the overall thermal conductivity. Moreover, it is shown that high anisotropy of graphite thermal conductivity, high volume fraction of graphite, and small aspect ratio of flake graphite will cause the connectivity effects of graphite to more obviously improve the overall thermal conductivity. Higher graphite volume fraction, lower aspect ratio and higher matrix thermal conductivity are beneficial to obtain a high thermal conductivity gray cast iron. This work can provide guidance and reference for the development of high thermal conductivity gray cast iron and the design of high thermal conductivity composites with similar locally interconnected structures.     

Fluidity and solidification microstructures of flake graphite cast iron in thin sections

Fluidity determinations have been undertaken using a silica tube as the fluidity channel, metal being drawn in to the test tubes by means of a regulated vacuum system. Fe-Si and Ca-Si inoculants were added to a hypoeutectic cast iron melt which was then tested for fluidity. A linear relationship between fluidity length and tube diameter was obtained. Fluidity length increased with increasing suction pressure and with increasing temperature of the molten iron. Fluidity length is proportional to the square root of effective suction pressure. Structures within the fluidity specimens have been classified, from the tip backwards, into three zones, containing ledeburite, mottled and flake graphite structures. The length of each structural zone increases with increasing suction pressure. The total length of the ledeburite zone, plus the mottled zone, is constant for increasing tube diameter. Thus, the flake graphite zone increases linearly with tube diameter. The effects of inoculation on the structure and its fading were evaluated by the length of the flake-graphite zone.     

Effect of variable heat treatment modes on microstructures of Fe-Cr-B cast iron alloy

The effect of heat treatment mode on the microstructure of Fe-Cr-B cast iron alloys was investigated in this paper by comparing the difference of precipitation patterns of secondary particles after thermal cycling treatment （TCT） with those after normal heat treatment （NHT）. No obvious differences were found in precipitation patterns of secondary particles between TCT and NHT when experimental temperature was below Ar1. However, when temperature was over Ar1, there were significant differences, with secondary particles prominently segregated at the grain boundaries under TCT, while the particles evenly distributed in the matrix under NHT. The reason for the microstructure differences could be associated with the development of non-equilibrium segregation of boron during TCT.     

Fracture analysis of chilled cast iron camshaft

The fracture of a camshaft made of chilled cast iron, installed in a home-made Fukang car, happened only after running over a distance of 6,200 km. The fractured camshaft was received to conduct a series of failure analyses using visual inspection, SEM observation of fracture section, microstructure analysis, chemical composition analysis and hardness examination and so on, while those of CKD camshaft made by Citroen Company in France was also simultaneously analyzed to compare the difference between them. The results showed that the fracture of the camshaft mainly results from white section in macrostructure and Ledeburite in microstructure; the crack in the fractured camshaft should be recognized to initiate at the boundary of coarser needle-like carbide and matrix, and then propagate through the transverse section. At the same time, the casting defects such as dendritic shrinkage, accumulated inclusion and initiated crack and abnormal external force might stimulate the fracture of camshaft as well. Based on failure analysis, some measures have been employed, and as a result, the fracture of home-made camshafts has been effectively prevented.     

灰铸铁和球墨铸铁金相检验标准变更比对分析

2008年ISO颁布了ISO 945-1:2008标准[1](即<铸铁金相组织第一部分石墨分类 目测法>),该标准将石墨分为6种类型,见表1.     

A study of several factors governing the fatigue limits of austempered ductile cast iron with various microstructures

The effects the leading factors bearing on fatigue limits were investigated with three kinds of ductile iron specimens various microstructures. As a result of examination, the fatigue limits in relationship to hardness and tensile strength, the expected higher improvement for fatigue limits in the case of the high strength specimens that experienced austempering treatment are not observed in comparison with the specimens treated with stress relief treatment. The estimated maximum defect size is one of the important parameters in predicting and evaluating fatigue limits for three different heat-treated ductile cast irons. Also, a quantitative relationship can be established between the fatigue limit and maximum defect size. Moreover, it is possible to explain the difference in fatigue limits in the three ductile cast irons by application of the rates of non-propagating crack which connects the adjacent graphite nodules before it stops.     

铸铁焊条在大直径铸铁阀门修补上的应用

我公司在阀门安装过程中,由于紧固时用力不均,导致与循环冷水泵出口相连的Dg500 PN1．6单向铸铁阀门的出口法兰根部沿周向出现裂纹,经检查发现裂纹长度有圆周长度的三分之二还多,裂纹深度基本上是贯穿性的,部分裂到法兰密封面上(图1中粗黑线),还有部分裂穿法兰螺栓孔之间(图1中虚黑线)．针对     

Removal of manganese from molten cast iron using potassium sulphide

Small melts of Fe-C-Mn (1.5%) alloy were treated with up to 20 wt% of potassium sulphide to assess its effect on the removal of manganese. The manganese and sulphur contents were determined on chill-cast samples, whilst microscopic observation and electroprobe X-ray microanalysis (XMA) procedures were used to analyse the reaction products. It was found that up to 80% of the original Mn content was removed by an addition of 7.5 wt% of K₂S; the rate of removal from molten iron into the slag was very high. Large quantities of polygonal reaction products and blow holes with diameters above 200 mm were found in the quenched samples. The reaction product was shown to be manganese sulphide and evidence indicates that this is assisted to the surface of the melt by bubbles of potassium vapour.     

Flow analysis of semi solid processing for grey cast iron

Abstract

Fluidity tests for semi solid processed grey cast iron were conducted at different degrees of primary fraction solid using fluidity strips with 2·5, 5 and 14 mm wall thickness. The microstructure of the fluidity strip castings was examined. Fluidity parameters, such as slurry velocity, critical solid concentration and choking range were studied. It can be concluded that critical solid concentration increases with the increase of wall thickness. The slurry velocity drops suddenly at a certain fraction solid very near to the critical solid concentration. Choking range zone can be measured by observing microstructure. A solidification model for fluidity has been developed based on the literature and microstructure study of semi solid processed grey cast iron. A mathematical equation has been obtained from the solidification model by modifying Flemings' fluidity model in order to analyse cavity filling of semi solid grey cast iron.