Application of a coloration etching method to the study of microstructures in primary and eutectic solidification in cast irons

Abstract

Cast iron is a very old material with a his- tory dating back several thousand years [1, 2], and it is still widely used around the world today. It is essentially that an Fe- C-Si alloy always contains the minor ele- ments S and P and, often, alloying ele- ments such as Mn and Cr. The metallurgy of cast iron is extremely complex because a wide variety of factors influence its phase transformation. Like other metallic mate- rials with a eutectic, cast iron undergoes primary- and eutectic-phase solidification. Each of the processes is comprised of two stages, nucleation and growth, which can be controlled through alloy composition and solidification conditions, such as in- oculation [3-5] and casting methods [6]. A tremendous amount of research has been performed on every aspect of cast iron during the last century. Among dif- ferent techniques used in the research and production of cast iron, metallography plays perhaps the most important role in the control of product properties and qual- ity as well as in the analysis of failure. The basic microstructural constituents in cast iron are graphite, cementite, austenite, fer- rite, pearlite, martensite, etc. Detailed analysis of the microstructure usually re- quires a fully polished specimen and the use of an etching method. There are some common etchants for ferrous alloys that can be applied for different purposes. For example, A1-7 is used for macroetching, Nital is used for general purpose etching, and Stead's reagent is used for revealing eutectic cells in gray iron and persulfate for darkening the matrix to outline carbides [7]. The overall microstructure in cast iron can be very complex, with many different structures mingled together. For example, when a hypoeutectic gray iron sample is quenched into cold water during the eu- tectic reaction, the resulting microstructure is a combination of primary austenite den- drites, which transform into martensite during quenching, and a eutectic product, eutectic austenite plus graphite, which is surrounded by quenched metastable eu- tectic product, eutectic austenite plus iron carbide. It is very difficult to differentiate between the various structures using the common etching methods. In particular, it is difficult to distinguish the primary aus- tenite from the eutectic austenite. Thus, there is a need to develop better etching methods. Ruff and Wallace [8] used a metallo- graphic approach to examine microstruc- tures of cast iron in a study of the influence of different graphite types and austenitic dendrites on the mechanical properties of gray cast iron. The techniques included ex- amining unetched samples to evaluate graphite morphology, heat treating sam- ples to reveal the structure of the primary austenite dendrites, using a paint counting system to measure the relative amounts of pearlite, ferrite and iron carbide, and ap- plying Stead's reagent to reveal the eutectic cell structure. 329