Approaching austempered ductile iron properties by controlled cooling in the foundry

Abstract

In a general sense austempered ductile iron (ADI) is a new class of ductile iron that is twice as tough as standard grades of ductile iron at the same strength level. ADI has excellent toughness and wear resistance and has been shown to have superior fatigue properties as well. The unique microstructure developed by austempering ductile iron provides these remarkable engineering properties. This heat treatment is an extra step in the manufacturing process that adds direct and indirect costs. The current capacity of heat-treatment facilities capable of austempering is limited. There is interest in achieving austempered ductile iron (ADI) properties in the foundry, but the cost of adding the elaborate heat-treating facilities required for austempering is high. Acicular irons, where-by high strengths are achieved by transformation on continuous cooling of alloyed irons through the bainitic transformation region, provide one alternative; however, the high ductility and toughness associated with ADI are generally absent in acicular irons. Another alternative is to employ controlled cooling that closely approximates the thermal schedule for austempering. Controlled cooling can be applied either following a hot shakeout or normalizing treatment. This has been done in a laboratory environment. The results of this study will be discussed, and suggestions for pursuit of this technology are given in view of current and emerging foundry processing technologies.