The Role of Silicon in the Solidification of High-Cr Cast Irons
- PDF / 2,471,411 Bytes
- 17 Pages / 593.972 x 792 pts Page_size
- 12 Downloads / 178 Views
N
HIGH-CHROMIUM (Hi-Cr) irons are ferrous alloys with chromium contents between 11 and 30 pct and carbon between 1.8 and 3.6 pct. Mo, Mn, Cu, and Ni sometimes are added as additional alloying elements. The typical microstructure consists of hard primary and/ or eutectic carbides (M7C3) in a matrix of austenite.[1–3] The presence of these hard carbides results in excellent abrasion resistance and, consequently, these alloys are commonly used where stability in an aggressive environment is a principal requirement, including mining and mineral processing, cement production, and pulp, paper, and steel manufacturing industries.[4] Both eutectic carbides and the matrix contribute to the wear resistance and toughness. The eutectic carbides have a hexagonal crystal structure and solidify to form colonies of bars or blade laths (eutectic ‘‘grains’’). Once solidified, the carbide structure and morphology are relatively immune to subsequent modification through heat treatment. In contrast, the as-cast austenitic matrix is readily heat treated to form a mixture of martensite/ austenite with small precipitated secondary carbides (SC) within the prior austenite dendrites.[5–7] A. BEDOLLA-JACUINDE and I. MEJI´A, Researchers, are with the Instituto de Investigaciones Metalu´rgicas, Universidad Michoacana De San Nicola´s De Hidalgo, Mexico, Mexico. Contact e-mail: [email protected] M.W. RAINFORTH, Professor, is with the Department of Engineering Materials, The University of Sheffield, Sheffield, U.K. Manuscript submitted April 8, 2012. Article published online September 26, 2012 856—VOLUME 44A, FEBRUARY 2013
The microstructure of these irons can be adapted to a remarkable range of abrasive wear applications. Variations in composition give carbide variations in the range of 10 to 45 pct; as-cast austenitic Hi-Cr irons have good abrasion resistance; however, optimum resistance to abrasion and fracture under conditions of severe repeated impact is obtained in irons heat treated to a martensitic condition.[8–10] It has been recognized that a possible strategy for improving the toughness of white iron alloys as well as the wear resistance under sliding conditions involves the refinement of the eutectic carbide structure by producing finer, more globular carbides.[11–13] Consequently, rapid cooling[14,15] or lower superheat[16] has been used as a means of increasing nucleation and hindering carbide growth, producing finer carbides, while slow cooling results in larger and coarser eutectics.[17–19] Alloying additions have also been used to modify the eutectic carbide structure,[5,20–35] although with only limited success. The aim of the present research is to analyze the effects of the systematic additions of silicon to a 17 pct Cr Hi-Cr iron cast into metallic molds. The literature on the effect of this element on the structure of the Hi-Cr irons is contradictory, and not well understood. For example, Shen and Zhou[22] report a refinement of carbides and a change in morphology from long plates to an isolated chunky form when increasing the silicon c
Data Loading...
