Critical Temperature Range in Standard and Ni-bearing Spheroidal Graphite Cast Irons
Describing the conditions for reaustenitization of spheroidal graphite cast irons is of interest for their heat-treatment after casting, e.g. for manufacturing austempered ductile irons. Differential thermal analysis has been used to characterize the direct eutectoid transformation and the reverse transformation, i.e. the reaustenitization. This has been applied to a standard and a Ni-bearing alloy, with a ferritic matrix for the former, both a ferritic and a pearlitic matrix for the latter. The results are discussed in relation with the stable and metastable three phase fields. While earlier description of the direct eutectoid transformation is confirmed, the one for reverse eutectoid has been found more complex and is amended.
Effects of high silicon contents on graphite morphology and room temperature mechanical properties of as-cast ferritic ductile cast irons. Part I – Microstructure
Studying a series of near eutectic spheroidal graphite cast irons with various amounts of silicon up to 9.12wt% confirmed that: 1. silicon stabilizes ferrite to such an extent that ferrite is the Fe-rich phase appearing during solidification at the highest silicon contents; 2. silicon triggers graphite degeneracy such as chunky graphite. As well-known, cerium and magnesium do also increase the risk of chunky graphite formation while antimony counteracts cerium. Based on the metallographic observations of the present work, an index is proposed to evaluate the risk of chunky graphite appearance from the silicon, magnesium, cerium and antimony contents. Above a critical value of this index, the risk for chunky graphite formation increases steadily. Using data from previous studies, it is further demonstrated that the critical value decreases with increase in casting modulus as expected. The evolution of mechanical properties of the prepared cast irons will be presented in a second part of this study.
Neutron Residual Stress Measurements on Rail Sections for Different Production Conditions
Rail sectioning with subsequent neutron diffraction experiments has been used to assess residual stresses in the rails. In this study we present the results of neutron stress measurements performed at the NIST Center for Neutron Research (NCNR) on rail sections from rails that were produced under various conditions. Specifically, these are air-cooled, air-cooled and roller-straightened, head-hardened and head-hardened and roller-straightened. More significantly, a head-hardened and roller-straightened rail was also studied after service to elucidate evolution of the service-induced residual stresses. In the latter case both a transverse-cut slice and the central region of a 0.53 m long piece were studied. Measurements on this piece are the first in which triaxial stresses have been determined for an intact rail. Neutron strain measurements with 3×3×3 mm3 spatial resolution were successfully employed for transversally cut slices to verify the difference in the stress state depending on the production process. Although examination of slices allows determination of only two-dimensional stresses in the plane of the slice, additional measurements on obliquely-cut slices, which were also carried out, and utilization of FEM gives the possibility of reconstructing the full triaxial stress distribution. Together, these approaches provide a better understanding of rail fabrication and the possibility of improving the durability and safety of rails in the future.Copyright © 2004 by ASME