Roberto Enrique Boeri

Revealing the solidification structure of nodular iron

The effectiveness of several metallographic techniques used to reveal the solidification structure of nodular iron has been evaluated by quantitative metallography. A colour metallography reagent, ...

Solidification macrostructure of spheroidal graphite cast iron

Methods to reveal the macrostructure of spheroidal graphite cast iron (SGI) are not available. This has caused a number of controversies involving the solidification mechanism of this metallic alloy. The objective of the present work is to examine the use of a novel technique (DAAS) reported recently, to reveal the solidification macrostructure of SGI made using conventional melting and pouring practices, and cast in sand moulds. The results show that the DAAS technique successfully reveals the macrostructure. The basic features of the solidification macrostructure of SGI are similar to those of most metallic alloys. Nevertheless, some particular features have become evident. The grain size of SGI is considerably larger than previously estimated. The grain size of hypereutectic SGI is consistently smaller than that of hypo and eutectic irons cast in similar conditions. The observation of the actual solidification macrostructure of SGI led the authors to propose a description of the solidification mechanism that takes into account the actual size and morphology of the solidification units. The new DAAS technique is a very valuable tool which can be used to investigate a number of issues about SGI solidification still under discussion.

Influence of the solidification microstructure on the mechanical properties of ductile iron

The increase in the nodule count in ductile iron is customarily used as a means to refine the solidification structure. The solidification structure can be better described by the identification of the solidification units. This study investigates the correlation of the nodule count and the cell count. The solidification microstructure has been revealed by color metallography and characterized by the cell count. The results show that both the nodule count and the cell count decrease as the section size increases. Nevertheless, no direct correlation has been found between the refinement of the nodule count and of the solidification structure. The experiments suggest that the nucleation of graphite and austenite are independent. The drop in mechanical properties caused by the increase in section size appears to be related to the presence of unwanted phases at the cell boundaries, and has a weak correlation with the coarsening of the solidification microstructure for castings with “clean” cell boundaries.

Influence of the inoculation process, the chemical composition and the cooling rate, on the solidification macro and microstructure of ductile iron

The solidification structure of any casting has a marked influence on its final microstructure and properties. Nevertheless, the correlation between solidification structure, microstructure and properties has not been clearly established for ductile irons. The objective of this investigation is to study the influence of the inoculation process, the chemical composition and the cooling rate on the solidification macro and microstructure of ductile irons cast in sand moulds. The results show that primary and eutectic austenite always grow dendritically, even for hypereutectic alloys. The solidification macrostructure is characterised by the grain size and morphology of the austenite, revealed by using a special technique developed earlier by the authors. The solidification microstructure is characterised by a parameter related with the location of the microsegregated last to freeze regions. Changes in the inoculation process do not cause significant modifications in the degree of the refinement of the solidification macro and microstructures. The carbon equivalent affects the grain size. The cooling rate, on the other hand, has a weak effect on the grain size and morphology, but strongly affects the dendrite arm spacing of the austenite.

Gating system design to cast thin wall ductile iron plates

Abstract The present study describes the development of a horizontal and a vertical mould designed to produce thin ductile iron plates of 1·5–4 mm thickness. Both moulds used a non-pressurised configuration. The horizontal mould design was based on empirical rules, while the vertical mould was based on numerical simulations of fluid flow and thermal field, carried out using the software NovaFlow&Solid. The aim of both designs was to produce sound ductile iron plates that were large enough to make tensile test samples. The results show that the adoption of empirical rules and a non-pressurised configuration were not enough to guarantee proper filling. An optimised design was obtained through the use of mathematical modelling.

Solid state transformation kinetics of high nodule count ductile Iron

This work focuses on the study of the kinetics of solid state transformations of thin wall ductile iron (TWDI). The aim is to identify the effect of the nodule count and the matrix homogeneity on the rate of transformation. The study is carried out on samples taken from sand cast plates of thickness ranging from 1.5 to 4mm, which show nodule counts up to 1700 per square millimetre. The results are compared to those obtained on samples taken from 12.5 mm thick Y-blocks, having approximately 300 nodules per square millimetre. Kinetics of ferritising, austenitising, austempering and cementite dissolution are studied. The results show that solid state transformations involving carbon transport from the matrix to the nodules or vice versa, such as subcritical ferritising and austenitising, show faster kinetics as the nodule count increases. The kinetics of the austempering transformation, which does not involve changes in the carbon content of the matrix, is not affected by the nodule count. The dissolution of the cementite present in unalloyed TWDI, also takes place at a very fast rate, ranging between 3 and 5 % per minute.

Corrosion mechanisms in ADI parts

The objective of this research is to study the influence of microstructure and grinding on the corrosion resistance of ADI in salt water. Immersion and electrochemical tests were performed on samples austempered at two different temperatures and ground. The results indicate that the dissolution rate increases as the nodule count does, and it is also affected by the microstructure and by the surface changes introduced during grinding. A porous layer is formed on ADI-corroded surface due to the selective dissolution of ferrite, and a preferential dissolution of the matrix around graphite is noticeable. Ground surfaces are less corrosion resistant than polished ones.

Review of the regulations for the use of stainless steels for orthopedic implants in Argentina

Motivated by the relatively high rate of failure of orthopedic implants in Argentina, the authors review the current normative regulating the use of stainless steels in the fabrication of these metallic parts in the country, and compare it with the regulations currently in use in other countries. The analysis shows that several standards in effect in the country do not comply with broadly recognized international standards. This situation is aggravated by a recent revision of the normative that failed to improve the quality standards to reach levels similar to those applied in developed countries or even in MERCOSUR associates. The national organization in charge of implant certification in Argentina, complying with the law, accepts the applicability of IRAM standards to certify stainless steels implants. In the opinion of the authors, the current practice used to certify implants does not guarantee the structural stability and biocompatibility of the devices, increasing the risk of failure in service, and escalating the cost of the public health care system.