Austempering of spheroidal graphite cast iron

We study gray spherulitic cast iron, its microstructure, hardness ultimate strength in tension, elongation, and impact toughness as functions of the duration of isothermal austenitizing in the bainite region at 350 and 400°C after austenitizing at 900°C. As the temperature of quenching increases from 350 to 400°C, the microstructure of the analyzed cast iron changes from lower to higher bainite and the amount of retained austenite increases (its maximum is attained after 1 h). At the same time, the ultimate strength in tension and hardness decrease, whereas the elongation and fracture toughness increase.Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 4, pp. 79–83, July–August, 2004.     

High-temperature properties of ferritic spheroidal graphite cast iron

The behaviour of fracture mode and intermediate temperature embrittlement of ferritic spheroidal graphite cast iron is influenced by many factors. From the experimental results, intermediate temperature embrittlement can be considered to be dominated by dynamic strain ageing and the triaxial stress field developed in the ferrite matrix amongst the graphite particles. In order to understand the effect of dynamic strain ageing on high-temperature properties, tensile properties, push-pull low-cycle fatigue properties, rotary bending fatigue properties and creep-rupture properties were investigated from room temperature to 500° C. It was found that all the properties investigated were influenced by dynamic strain ageing. The intermediate temperature embrittlement of ferritic spheroidal graphite cast iron found in different load conditions is reported.     

Heterogeneous nuclei of graphite in grey cast iron

Abstract

In the present paper, the initial nuclei of flake graphite in grey cast iron are investigated. Using liquid quench, deep electrolytic erosion, and electron microanalysis, it is shown that the initial graphite in grey cast iron is nodular and involves a heterogeneous nucleus which can be one of several types: complex oxides, MnS, sulphides of rare earth, etc. The elements Si, Ca, Mn, Ce, La, Al, etc. can promote heterogeneous nucleation of flake graphite in grey cast iron.

MST/709     

A useful technique for studying graphite in cast iron

A new experimental procedure for studying eutectic graphite in cast iron is described. It consists of slow dissolution of the matrix, which is simulteneously replaced by a transparent one in which the graphite remains entrapped. This provides advantages for the study of nucleation using light microscopy. Afterward, the complete graphite lattice can be released through dissolution of the amorphous matrix, retaining unaltered the structure of the eutectic grains and the spatial distribution produced during the solidification process. In this way, the crystallography of the phase can be followed by different methods in zones with a well-defined morphology and also in zones where there are transitions between different morphologies. In the last case, the close bonding with the surroundings in which they were produced is maintained. Finally, the study of the particles included exclusively in the graphite phase can be optimized because of the elimination of matrix interference in microanalysis.     

Fracture mechanics behaviour of austenitic compacted graphite cast iron

The fracture mechanics behaviour of high-nickel austenitic compacted graphite cast iron was studied and the effects of graphite morphology, alloying elements and specimen thickness on the mechanical properties, plane stress fracture toughness, and fatigue crack growth rate were evaluated. It was found that the graphite morphology, i.e. the percentage of compacted graphite present, was the major determinant of all properties of the materials investigated. The irons with a greater amount of compacted graphite (the balance was nodular graphite in austenitic matrix) resulted in lower tensile strength, yield strength, elongation and K _c fracture toughness but higher crack-growth index values (poorer crack-growth resistance). For 25 mm thick specimens, K _c values of the austenitic compacted graphite cast irons in this study were in the range of 58–64 MPa m^1/2. This is higher than ferritic/pearlitic ductile iron of 43–53 Mpa m^1/2, and is compatible to Ni-resist austenitic ductile iron of 64.1 Mpa m^1/2. The addition of cobalt not only contributed to slightly higher values of mechanical properties, but also higher plane stress fracture toughness and better crack growth resistance. Optical microscopy, scanning electron microscopy and X-ray diffraction techniques were applied to correlate the microstructural features to the properties attained.     

A realistic computer-based fatigue comparison of spheroidal graphite cast iron and cast steel

Loads measured in a wheeled loader during service are used to compare two casts of SG iron with a cast steel to BS 592 grade A. Fatigue lives are predicted using computer based analytical methods. The conclusions reached are contrasted to those inficated by the more conventional static and fatigue tests. The suitability of SG iron as a material for cast axle housings is demonstrated.     

Perspectives on research on the formation of nodular graphite in cast iron

Current views on the mechanism of the formation of nodular graphite in cast iron are examined with respect to their explanatory power. The aim of this review is to find, if possible, a rationale for further studies. Arguments are presented in favour of a comprehensive model, encompassing the characteristics of both the nucleation and the growth of the graphite. A plea is made for focusing on the most salient features rather than on phenomenological aspects.     

The fatigue strength of centrifugally cast spheroidal graphite iron pipe

The fatigue strength of centrifugally cast spheroidal graphite (SG) iron pipe was investigated. A parallel series of tests were carried out both on plain plate specimens which were extracted from an iron pipe, and on notched iron specimens. These results were compared with results for rolled steel beams, which were made from a steel with a tensile strength similar to the SG iron [1]. It was found that the strengths at a life of $\text{2 × 10}{}^6$ cycles differed only by 20% between the SG iron pipe and the rolled steel beam, whereas those of plain plate specimens of the two materials differed by 38%. The fatigue failure in the SG iron pipe initiated from the inherent gas pores existing in the inner surface of the pipe, while the fracture in the rolled steel beams originated from external notch defects. Thus, the steel beam appeared more sensitive to the external notches than the SG iron pipe, when the notch size was smaller than 1 mm. However, it was revealed from the fatigue tests on notched plates that, as the notch became severer, the fatigue strength of SG iron became more affected by the notches than did that of the steel. A fracture mechanics analysis indicated that this was because the fatigue crack growth rate for SG iron was three times as high as that for steel.     

Vermicular graphite cast iron—current state of the art

Vermicular graphite cast iron is a new addition to the family of cast irons. Various methods for producing vermicular graphite cast iron are briefly discussed in this paper. The mechanical and physical properties of cast irons with vermicular graphite have been found to be intermediate between those of gray and ductile irons. Other properties such as casting characteristics, scaling resistance, damping capacity and machinability have been compared with those of gray and ductile irons. Probable applications of vermicular graphite cast irons are suggested.     

Influence of chunky graphite on mechanical and fatigue properties of heavy-section cast iron

This research work aimed to find out the influences of the different amounts of chunky graphite on mechanical and fatigue properties of GJS 800 ductile cast iron. Chunky graphite has been a problem of heavy section thick-walled ductile cast iron components. Chunky graphite is branched and interconnected as a network within eutectic cells and has been observed to form in thermal centres of heavy ductile cast iron sections during solidification. This research work proved that chunky graphite in the microstructure decreases the ultimate tensile strength, the elongation to fracture and fatigue life significantly, but does not influence on the yield stress of ductile cast iron GJS 800 substantially. Low nodular count and nodularity rate also decrease the fatigue life of ductile cast iron, and the difference of fatigue life of specimens containing chunky graphite or having low nodular count and nodularity rate is not large. Influence of the amount of chunky graphite on fatigue life increases until 20% chunky graphite content, and above that the fatigue life does not decrease substantially.     

High temperature decomposition of austempered microstructures in spheroidal graphite cast iron

Abstract

Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. After tempering for 2 h at 570°C all austempered microstructures consisted offerrite and cementite, but cementite was most finely distributed in the material that had been austempered at 300°C, and coarsest in that austempered at 440°C. It is concluded that if SG cast iron is to be nitrided conventionally at temperatures >500°C, then prior austempering to obtain controlled microstructures is of limited value.

MST/3106     

Nuclei for heterogeneous formation of graphite spheroids in ductile cast iron

Abstract

The crystal structure and composition of nuclei for graphite spheroids in a ductile cast iron containing small amounts of magnesium and traces of aluminium have been studied in a transmission electron microscope equipped with EDX and parallel electron energy loss spectrometer and in an electron microprobe analyser. The particles were identified as Al–Mg–Si nitrides, having a trigonal superlattice crystal structure derived from a hexagonal AlN type fundamental cell. The superlattice can be indexed according to a hexagonal Bravais lattice, with parameters a=0.544 nm and c=0.482 nm. The parameters of the fundamental cell are a _f=0.314 nm and c _f=0.482 nm, deviating only 1–3% from the parameters of hexagonal AlN. Based on the compositional analysis, the chemical formula of the nitride particles is suggested to be AlMg_2.5Si_2.5N₆.     

Structure and growth of platelets in graphite spherulites in cast iron

The structure and orientation of graphite platelets in graphite spherulites (GS) in cast irons modified by either cerium or magnesium plus cerium were investigated by TEM and micro-diffraction. The platelets have a rhombohedral structure with their [001] directions nearly parallel to the radius of the GS, but each platelet is twisted about 2 ° over a 1 m length. Randomly orientated graphite is located in the interplatelet areas, in which most of the graphite has a hexagonal structure rather than rhombohedral structure. Based on the crystallographic characterization of the graphite and the cerium and magnesium elemental distribution in the GS, the model for the growth of GS as originally proposed by Double and Hellawell is re-examined.     

Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate

The hot deformation behavior of spheroidal graphite cast iron (SGCI) was investigated quantitatively from 600 °C to 950 °C at high strain rate of 10 s⁻¹ by compression tests on a Gleeble-1500 simulator. The results show that the peak strain increases gradually with increasing deformation temperatures in the range of 600–800 °C and decreases when the temperature is raised to 800 °C and above. The optimum deformation temperature range is determined at 700–900 °C. The graphite particles become spindles or flakes after deformation, even some graphite collapse in the compressed specimens with about 0.7 peak strains. The graphite area fraction decreases as the temperature increases, at the same time, the high peak strain promotes the dissolving of carbon.     

Initial stage of isothermal decomposition of austenite to ferrite and graphite in spheroidal graphite cast iron

Abstract

The isothermal decomposition of austenite to ferrite and graphite in a spheroidal graphite cast iron was investigated by dilatometry, metallography, and image analysis. The results gave quantitative, although indirect, information on the kinetics of carbon transfer from austenite to graphite, as well as on the kinetics of the austenite to ferrite and graphite transformation. The relationship between the kinetics and the graphite nodule count was highlighted. The incubation period before ferrite growth was found to be associated with a global contraction of the material, in spite of the related increase in the graphite volume fraction. Calculations showed good agreement with experimental results, except for the rate of carbon transfer, which was found to be much slower experimentally than predicted by calculations based on the volume diffusion of carbon. Possible reasons for this discrepancy are discussed.     

激光模具表面抗裂仿生单元体模型

目的针对企业模具寿命短、资源浪费、生产成本上升的问题,建立模具表面激光熔凝抗裂仿生单元体模型。方法在研究了植物叶片抗开裂生物原型结构的基础上,提炼针对模具的抗裂要素,形成单体模型,与实际影响模具循环寿命的因素相结合,分析寿命影响因素,然后基于获取到的数学模型构型,对激光熔凝单元体参数进行正交试验设计与优化。结果通过优化单元体的微观组织结构分析,在模具易出现疲劳裂纹位置,进行激光局部熔凝仿生抗开裂单元体制备,熔凝抗开裂单元体起到良好模具裂纹阻断作用,提高模具使用寿命50%以上。结论为提高模具寿命提供了一种新的方法。