Microstructure and mechanical properties of high boron white cast iron

In this paper, high boron white cast iron, a new kind of wear-resistant white cast iron was developed, and its microstructure and mechanical properties were studied. The results indicate that the high boron white cast iron comprises a dendritic matrix and an interdendritic eutectic boride in as-cast condition. The distribution of eutectic boride with a chemical formula of M₂B (M represents Cr, Fe or Mn) and with a microhardness of HV2010 is much like that of carbide in high chromium white cast iron. The matrix includes martensite and a small amount of pearlite. After quenching in air, the matrix changes to martensite, but the morphology of boride remains almost unchanged. In the course of austenitizing, a secondary precipitation with the size of about 1 μm appears, but when tempered at different temperature, another secondary precipitation with the size of several tens of nanometers is found. Both secondary precipitations, which all forms by means of equilibrium segregation of boron, have a chemical formula of M₂₃(C,B)₆. Compared with high chromium white cast iron, the hardness of high boron white cast iron is almost similar, but the toughness is increased a lot, which attributes to the change of matrix from high carbon martensite in the high chromium white cast iron to low carbon martensite in the high boron white cast iron. Moreover, the high boron white cast iron has a good hardenability.     

烧结Cr15高铬铸铁组织与性能的研究

为研发耐磨性能优良、成本相对低廉的高铬铸铁,本文分别以亚共晶、过共晶的水雾化Cr15高铬铸铁粉末为原料,采用超固相线液相烧结工艺制备了烧结高铬铸铁(SHCCI),并对其显微组织、力学性能和冲击磨粒磨损工况下的耐磨性能进行对比研究。结果表明,烧结高铬铸铁主要由M7C3碳化物、马氏体和奥氏体组成;在亚共晶烧结高铬铸铁中,通过电解腐蚀萃取的M7C3碳化物三维形貌呈珊瑚状,沿晶界均匀分布,材料抗冲击耐磨性能优良;在过共晶烧结高铬铸铁中,优先形成的初生碳化物可能成为共晶碳化物的生长基底,形成核-壳结构的M7C3碳化物,沿晶界相互连接呈网状,严重割裂基体。亚共晶、过共晶烧结高铬铸铁的力学性能分别为:硬度HRC63.9、HRC64.3,冲击韧性7.92、3.04 J/cm^2,抗弯强度2112.65、1624.87 MPa。     

Large scale testing and statistical analysis of dynamic fracture toughness of ductile cast iron

The present paper deals with the experimental determination and statistical analysis of dynamic fracture toughness values of ductile cast iron. K_Id data from 140 mm thick single edge bend specimens of two dynamic fracture toughness test series on ductile cast iron from heavy-walled castings were analysed.At first, the statistical analysis of data at −40 °C was done based on ASME Code Case N-670 using a two-parameter Weibull distribution function. Weibull analyses of three samples covering different pearlite contents (?4%, ?9%, ?20%) were performed and characteristics of the distribution functions as well as two-sided confidence intervals were calculated. The calculated characteristics show that K_Id of ductile cast iron decreases with increasing pearlite content.In a second step, the applicability of the Master curve procedure according to ASTM E 1921 to ductile cast iron materials was investigated and it was formally used for statistical analysis of ductile cast iron dynamic fracture toughness data. Although the Master curve method was originally introduced for static fracture toughness data of ferritic steels, the successful individual analyses performed here support the engineering way taken to apply the method to ductile cast iron materials too. The results of both methods, the Master curve procedure and the ASME Code Case N-670, show acceptable congruity. At the same time, it is concluded from the present study that further investigations and experiments are required to improve precision and for verification before the results could be applied within component safety analyses.     

Microstructural and fracture analysis of aged cast duplex steel

The effect of increased carbon content and heat treatment parameters on the microstructure and selected properties of ferritic-austenitic duplex cast steel is discussed. Test results show that the cast steel microstructure after the solution heat treatment changes substantially with increasing carbon content. Ageing after the solution heat treatment results in approx. 20% increase in hardness and a few-times decrease in impact strength. Fractographic examinations show that fracture surfaces of specimens of steel with low carbon content are typically of transcrystalline ductile micromechanism. An increase in carbon content is accompanied by a decline in ductility areas, while fracture of specimens is of mixed nature: ductile and brittle. After ageing, only cases of mixed fracture were observed. __________ Translated from Problemy Prochnosti, No. 1, pp. 133–136, January–February, 2008.     

Reinforcement of Hadfield steel matrix by oriented high‐chromium cast iron bars

To attain a wear‐resistant material compatible with high hardness and high toughness, Hadfield steel matrix was reinforced by oriented high chromium cast iron bars, through inserting high chromium alloys flux‐cored welding wires into Hadfield steel melt at 1500 ± 10 °C. The obtained composites were investigated by XRD, SEM, micro‐hardness, three‐body abrasion wear and impact toughness testers. The results show that the alloy powders inside the flux‐cored welding wires can be melted by the heat capacity of Hadfield steel melt and in situ solidified into high chromium cast iron bar reinforcements tightly embedded in the matrix. The micro‐hardness of reinforcements of the water‐quenched composite is about four times higher than that of the matrix. The impact toughness of the water‐quenched composite is higher than that of the as‐cast composite and lower than that of Hadfield steel, and its fracture mechanism is very complicated and refers to brittle and ductile mixture fracture mode. The excellent impact toughness and better wear resistance of the water‐quenched composite are attributed to combine fully the advantages and avoid the drawbacks of both Hadfield steel and high chromium cast iron. Additionally, in industrial application, the pulverizer plate produced by this composite, has also better wear resistance compared to the reference Hadfield steel pulverizer plate.     

The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron

The fracture toughness of a high chromium, reduced carbon white cast iron was measured using the K_Ic fracture toughness test. The toughness was found to increase with increasing heat treatment temperature for the temperature range of 1273–1423 K. Increases in the fracture toughness were due to crack deflection into the dendritic phase. Cracking in the dendrites was promoted by the presence of secondary carbides which formed during the high temperature heat treatment employed. The characteristic distance for brittle fracture as calculated by the Ritchie–Knott–Rice model correlated well with the centre to centre mean free path of the secondary carbides on the fracture plane.     

Scanning electron microscopic fractography of permanent moulded cast iron

The fracture susceptibility of chill-free permanent moulded cast iron is discussed in terms of graphite configuration. The details of fracture surface topography as revealed by scanning electron microscope examination on areas of slow crack growth and fast fracture are presented and the usefulness of these SEM fractographs in categorizing the fracture modes is demonstrated.     

Fracture toughness and growth of short and long fatigue cracks in ductile cast iron EN‐GJS‐400‐18‐LT

Heavy components of ductile cast iron frequently exhibit metallurgical defects that behave like cracks under cyclic loading. Thus, in order to decide whether a given defect is permissible, it is important to establish the fatigue crack growth properties of the material. In this paper, results from a comprehensive study of ductile cast iron EN‐GJS‐400‐18‐LT have been reported. Growth rates of fatigue cracks ranging from a few tenths of a millimetre (‘short’ cracks) to several millimetres (‘long’ cracks) have been measured for load ratios R=?1, R= 0 and R= 0.5 using a highly sensitive potential‐drop technique. Short cracks were observed to grow faster than long cracks. The threshold stress intensity range, ΔK_th, as a function of the load ratio was fitted to a simple crack closure model. Fatigue crack growth data were compared with data from other laboratories. Single plain fatigue tests at R=?1 and R= 0 were also carried out. Fracture toughness was measured at temperatures ranging from ?40 °C to room temperature.     

不同表面处理对灰铸铁摩擦学性能的影响

分别对灰铸铁样件进行磷化处理、磷化+纳米铁基离子表面处理、磷化+二硫化钼粘结固体涂层处理,在浸油润滑条件下,采用万能摩擦试验机,考察灰铸铁在不同表面处理下的摩擦磨损性能。试验结果表明,磷化+二硫化钼粘结固体涂层试件的摩擦系数最低,稳定在0.125左右,并出现少量的粘着磨损和磨粒磨损。     

Potentiostatic etching of duplex stainless steels and high chromium white cast irons

Abstract

The reasons for potentiostatic etching and the way in which it is carried out are explained. The technique was applied to some commercially important materials, and the information obtained is described. It was found that the chromium content of duplex stainless steels had a significant effect on the etching potential, but did not greatly influence the etching time. In high chromium white cast irons (HCWCIs), the composition of the various alloys influenced the etching potential very little, and it was possible to deduce a generally suitable etching range of 950–1050 mV (SCE) for the whole group. Manganous sulphate proved to be a superior etching agent, and very fine detail, e.g. different types of carbide, could be detected in HCWCIs after potentiostatic etching in this solution. Quantitative phase determinations were also possible for all materials investigated, provided that suitable etchants were used. Optimum etching times were from 10 to 15 s, regardless of the group of materials investigated.

MST/1665     

两种高铬铸铁在热强碱中受冲刷时的腐蚀行为

为考察不同材料在热强碱中的腐蚀和磨损腐蚀行为，用电位线性扫描方法测定了Cr13和Cr26两种高铅铸铁在静态和动态的模拟介质中的极化曲线．研究结果表明：动态条件下的腐蚀电流密度比静态条件下约大两个数量级；在动态冲刷条件下，铸铁材料的表面始终处于活化状态，磨损腐蚀速率远大于纯腐蚀速率；Cr26抗磨损腐蚀的性能优于Cr13．     

锻造中铬合金白口铸铁组织和性能研究

利用锻造方法并结合锻后适当的热处理工艺可大幅度提高铬合金白口铸铁的冲击韧性．采用热望性镦粗试验方法,对不同碳含量中铬合金白口铸铁的热塑性、碳对中铬白口铸铁热塑性的影响规律进行了研究,分析了不同热处理工艺对其锻后组织和性能的影响,并提出了可明显提高锻造中铬合金白口铸铁抗冲击磨损性能的热处理工艺参数．研究表明：含碳量为1.85％-2．59％的中铬合金自口铸铁在850-1130℃的温度范围内,具有良好的塑性变形能力,且随碳含量的增加,中铬白口铸铁的热塑性下降,其主要原因是碳含量不同所引起的组织中共晶碳化物的数量的改变所致．     

粉末冶金制备工艺对TiC增强高铬铸铁基复合材料性能的影响EI北大核心CSCD

采用高能球磨和真空烧结的方法制备TiC增强高铬铸铁(HCCI)基复合材料。利用SEM,DSC等方法对不同球磨时间的粉末进行分析,研究不同烧结温度对高铬铸铁基复合材料的显微组织、硬度及密度的影响,比较相同工艺下复合材料与高铬铸铁材料的耐磨性。结果表明:球磨12 h后的粉末颗粒大小趋于稳定,粉末活性提高,烧结性能改善,烧结试样中TiC均匀地分布在基体中。随着烧结温度的升高,复合材料内部晶粒逐渐长大,密度和硬度逐渐提高。在1280℃超固相线液相烧结的条件下烧结2 h后,致密度达94.17%,硬度和抗弯强度分别为49.2HRC和980 MPa。在销盘磨损实验中复合材料的耐磨性为单一高铬铸铁材料的1.52倍,磨损机制为磨粒磨损+轻微氧化磨损。     

Influence of nitriding on the fatigue behavior and fracture micromechanisms of nodular cast iron

Surface modification processes are increasingly used to fully exploit material potential in fatigue critical applications because fatigue strength is sensitive to surface conditions. Nitriding is extensively adopted with ferrous materials because it forms a hard and strong surface layer and a system of superficial compressive residual stresses. Fatigue, however, is strongly dependent also on defects and inhomogeneity. When nitriding is applied to nodular cast iron, the relatively thin hardened layer (about 300 μm) contains graphite nodules (diameter of the order of 30 μm), casting defects and a heterogeneous matrix structure. The paper presents and discusses the influence of nitriding on the fatigue response and fracture mechanisms of nodular cast iron. A ferritic nodular cast iron and a synthetic melt with different content of effective ferrite were initially gas-nitrided. Then, (i) structural analysis of nitrided layers, (ii) fatigue testing with rotating bending specimens, and (iii) fatigue fracture surface inspection were performed. Performance and scatter in fatigue performance is discussed by selective inspection of fracture surfaces and identification fracture micromechanisms. A semiempirical model explains observed trends in test results and is used for the process optimization. __________ Translated from Problemy Prochnosti, No. 1, pp. 85–88, January–February, 2008.     

16Cr2Mo1Cu高铬铸铁在亚临界处理中硬化行为研究

研究了亚临界处理对16Cr2Mo1Cu高铬铸铁的组织转变和性能的影响,并利用X射线衍射分析、磁性法和硬度测定法分析了硬化机制.研究表明:16Cr2Mo1Cu高铬铸铁的铸态组织由残余奥氏体、马氏体和M7C3型共晶碳化物组成,其相对含量分别为77.0%,7.2%和15.8%;在亚临界处理过程中,基体组织中的残余奥氏体析出二次碳化物并在冷却过程中转变为马氏体,使该合金在560～600℃的亚临界处理过程中出现二次硬化;在适当的处理温度和保温时间下,16Cr2Mo1Cu高铬铸铁可得到最高的硬度.     

Improving fracture toughness and hardness of Fe2B in high boron white cast iron by chromium addition

The effect of chromium containing 0%, 0.49%, 1.02%, 2.1%, 3.2% (in wt.%) on the morphology, fracture toughness and micro-hardness property of Fe₂B in high boron white cast iron was investigated. The results indicated that, with an increase of chromium addition, the morphology of Fe₂B becomes larger and changes from the block to rod shape, its micro-hardness increases and the fracture toughness increases first and then decreases. Compared with the fracture toughness (3.8 MPa m^1/2) of Fe₂B without chromium addition, the toughness at 2.1 wt.% chromium addition can be improved by above one time, achieving 7.8 Mpa m^1/2, and the result was also qualitatively testified by the micro-cracks in Fe₂B based on scanning electron microscope micrographs.     

Effect of fluctuation and modification on microstructure and impact toughness of 20 wt.% Cr hypereutectic white cast iron. Einfluss durch Partikelzugaben und Modifikationen auf die Mikrostruktur und die Kerbschlagzähigkeit von übereutektischem weißen Gusseisen mit 20 Gew.‐% Cr

In the present study, Fe‐Cr‐C hypereutectic high chromium white cast iron were prepared from industry‐grade materials and subjected to the treatment of modification using Fe‐Si‐RE alloy, aluminum and a self‐made intermediate alloy, fluctuation (ferroalloy powder), and the combination of the fluctuation and the modification respectively. The structures of the treated alloy were investigated by means of the optical microscopy (OM). The impact toughness of the specimens was also examined. The fractographs of the samples were examined by scanning electron microscopy (SEM). The results showed that, with the addition of fluctuation or modifying agents, the primary carbides were refined and the impact toughness of the alloys was improved, especially with the combination of them.     

Wear and oxidation behaviour of high-chromium white cast irons

High-chromium white cast irons can attain very hard microstructures composed of intergranular chromium carbides dispersed in a tempered martensitic matrix. Two heat-treated high-chromium white cast irons with different carbon contents were studied in this research work using compression at 500 °C, and wear and oxidation tests at 500 and 600 °C. The hot compression behaviour of these products is excellent, with high strengths and significant ductility. They also exhibited good wear and oxidation properties. Fracture micromechanisms are discussed in relation to the chemical composition and microstructure of the two alloys.     

球墨可锻铸铁与可锻铸铁的冲击韧性对比

研究了硅量和温度对球墨可锻铸铁冲击韧性的影响。得出了球墨可锻铸铁的室温和低温冲击韧性高于含硅较低的可锻铸铁的结论。