Alloy steels containing 0.020% sulfur or more and greater than 20 ppm oxygen are being calcium treated to improve machinability and enhance sulfide shape control. Although from a process control perspective, the success of calcium treating can be measured by the bulk calcium content of the steel, the chemical composition of the steel does not indicate how sulfide and oxide inclusions have been modified by calcium treating. Hence, to characterize an inclusion distribution in a heat of steel, quantitative metallographic techniques are required.
In an aluminum-killed, calcium-treated steel, the oxide inclusions are generally complex calcium-aluminates. Ideally, when a steel is calcium treated, as solidification progresses, the sulfides precipitate from the liquid steel on the solid oxides in the liquid. The net result is a (Ca,Mn)S inclusion that contains an encapsulated oxide. Some sulfides precipitate on fragmented dendrites in the liquid and do not contain oxide particles, hence free oxide inclusions are found in the steel.
Thus, there are four types of inclusions that may be present in calcium-treated steels: 1. Sulfides containing oxides; 2. Free sulfides, i.e. those, without oxides; 3. Free oxides; and 4. Encapsulated oxides.
In this paper, typical inclusion populations from a conventional and calcium-treated AISI 8620 steel are evaluated. Image analysis techniques and custom software that have been developed to separate the four types of inclusions based on reflectivity, “gray level”, are illustrated. Typical inclusion chemical analyses from both steel types are presented. The differences between such parameters as mean length, mean area, and mean roundness between the two steels as well as the four types of inclusions found in the calcium-treated steels are discussed. The changes in the inclusion length and area distributions are considered. The benefits of this methodology to properly characterize inclusion distributions in calcium-treated steels are discussed.