Influence of Mn and S on the Properties of Cast Iron Part III—Testing and Analysis

Abstract

This work is the culmination of a literature review and foundry experiments designed to re-examine the roles of Mn and S on the strength of gray iron.

A review of the literature showed that there are two principal regions in gray iron chemistries. A region where Mn and S levels are below the solubility limit of MnS at the solidification temperature, and a region above the solubility limit. Twenty-four heats were produced in cast sections up to 3 inches to investigate the influence of S on strength at three Mn levels, including low S concentrations, where S is fully soluble and no MnS precipitation occurs before the start of eutectic solidification. At each Mn level, S was progressively increased until MnS precipitated in the melt prior to reaching the eutectic temperature.

Master heats were produced at three Mn levels (0.3%, 0.5% and 0.8%). Sulfur was varied from 0.01% to 0.15%. The base chemistry was otherwise typical of Class 35 gray iron at a carbon equivalent (CE) value of 3.9 to 4.0 and sufficient alloying with Cu and Sn to assure a fully pearlitic structure. Tensile strength, hardness, chill width, and thermal arrests were determined.

Tensile strength first increased with sulfur, reaching a maximum strength level, and then decreased with further increases in sulfur. Maximum strength coincided with the solubility limit of MnS inclusions. Strengths reached 42 ksi in the 1.2 inch B test bar, and 36 ksi in the 3 inch diameter test bar. For each Mn series, the differences in strength between maximum and minimum values were 44% and 38% for the B test bars and 3 inch bars, respectively. Variations in chilling tendency were modest, except at the extreme lowest and highest S levels.