Finding the Right Balance of Manganese and Sulfur to Increase Iron Strength


microstructure

Microstructure in B bar with an elevated S concentration (0.15%) in the 0.78% Mn series illustrating large eutectic cells surrounded by type D graphite. Original magnification = 50X

A research study funded and published in 2013 by AFS and the Iron Division demonstrated that, through balancing manganese and sulfur according to the solubility limit of MnS at the eutectic temperature, the strength of gray cast iron can be optimized. Solubility limit occurs when %Mn x %S equals 0.02 to 0.04. Based on a literature review and experimental work, it was possible to define what levels of manganese and sulfur might produce the best properties with regard to strength.

At the same time, the 2013 study raised some questions about the Mn-S relationship in gray cast iron:

  • Are the variations in strength due to variations in microstructure?
  • How do variations in Mn and S affect the microstructure, particularly the graphite structure, when balancing Mn and S for optimum strength?
  • Did pearlite hardness contribute to the variation in strength?
  • Does optimum balancing of Mn and S provide similar strengthening at all CE values (other than 4.0)
  • Can Mn-S balancing be used to make gray iron more competitive with CGI?
  • Does balancing for optimum strength cause problems with machinability?
  • Does proper balancing of Mn and S reduce the need for additional alloying?
  • Does the inoculation response change when optimum balancing is employed? Consider evaluating various inoculants at lower %Mn x %S, where proeutectic MnS precipitation has been inhibited.

The study was continued, and new research focused on a more thorough evaluation of the microstructure in the 96 metallographic specimens from the original study. Additional mechanical testing was performed and fractographic examinations of the fractured tensile bars were also conducted.

The analysis consisted of:

  • Further study of the graphite structure.
  • More complete analysis of eutectic cell count in all cast sections.
  • Determine matrix (pearlite) hardness and its influence on variation in tensile strength.
  • Investigate the formation of type D graphite at cell boundaries with increasing S.
  • Investigate the tendency for spikey graphite and intercellular carbides in all cast sections.
  • Conduct tensile testing of selected materials from the B, C and 3-inch bars, particularly those with the highest and lowest strengths in a series. Tests included precision stress-strain curves.

The test data were evaluated with the intent of finding correlations among the properties, microstructure and composition. Specifically, there was interest in determining what causes the reduction in strength with increasing sulfur content. It was anticipated that fractographic studies might reveal the mechanism(s) causing reduced strength.

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