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

Richard Gundlach

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.

Background of the 2013 Study
The original experimental work focused on Class 35B iron cast in sections up to 3-inches. For the study, a 9,000-lb. master heat was produced by Bremen Castings Inc. (Bremen, Indiana). The manganese level of the heat was periodically increased to obtain alloys at three manganese concentrations (0.3%, 0.5% and 0.8%). The sulfur concentration was adjusted in a transfer ladle such that split heats were poured with varying sulfur levels ranging from 0.01 up to 0.15%. All alloys and cast sections were fully pearlitic. Table 1 lists the compositions for the three base alloys.

The hardness and tensile strength were determined in all 24 alloys. (Table 2 lists the compositions for the test bars displaying maximum and minimum strength in each Mn series. The study showed that at optimum manganese and sulfur levels, the strength in B bars can be 7-11 ksi higher than in poorly balanced chemistries. The highest strengths were found in chemistries containing between 0.026% and 0.044% sulfur. When sulfur was increased above these levels, strength began to decrease. Figure 1 illustrates the variation in strength in four cast sections when varying S in Class 35 iron containing 0.28% Mn.

The variation in Mn and S contents produced significant changes in the properties of the metal and a brief summary of the findings of the previous investigation were as follows:

While Class 35B iron was produced, strengths over 40 ksi were achieved at some manganese and sulfur levels. Strengths of 35 ksi were even achieved in the 3-inch test bar. However, at some Mn and S levels, much lower strengths were observed.

Maximum strength occurred when manganese and sulfur were near the solubility limit of MnS, typically when %Mn x %S ranged from 0.02 to 0.04. Minimum strength occurred at the highest sulfur levels studied (0.13 – 0.15%) regardless of the manganese concentration.

Stoichiometric balancing of manganese and sulfur, using an excess Mn of 0.3%, usually produced irons of inferior strength, particularly at higher sulfur levels.

Chilling tendency was greatest at the very lowest sulfur levels and at the very highest sulfur levels, and it was generally lowest at the sulfur levels that produced maximum strength.

Intercellular carbides were observed at levels of 0.10% S and above in the 0.28% Mn series. At higher manganese levels, no carbides were seen. It was concluded that, at higher sulfur levels, manganese controlled free sulfur content, and that IC carbides were avoided when free sulfur was held below 0.1%.

Spikey graphite was observed in all cast sections and manganese levels at higher sulfur concentrations. Spikey graphite was generally avoided when free sulfur was held below 0.08%.

The tensile strength-to-hardness ratio varied with composition and decreased with increasing sulfur content. The fall in UTS/HB ratio suggests the decrease in strength accompanied a change in graphite structure.

Controlling the sulfur content is paramount when maximizing the strength in gray cast iron.  The research shows Mn is only effective up to the solubility limit of MnS. Further research is needed to better understand the negative effects of sulfur at concentrations beyond the solubility limit of MnS.

New Research Findings
The next study sought to provide a more thorough examination of the 96 metallographic samples. These specimens were used to further evaluate the microstructure and assess the influence of variations in manganese and sulfur. More detail on the experimental procedures and results is available in the original paper, “Influence of Mn and S on the Microstructure of Cast Iron” (18-091).

Based on the findings of the previous study, it was hypothesized the changes in mechanical properties were caused by a change in the graphite structure. Since there was little change in hardness as sulfur was increased, it was concluded that the decrease in strength at higher sulfur levels was due to changes in the graphite structure. The results of the current investigation show that the change in strength is more complex.

Eutectic cell count tended to rise and fall with increasing sulfur content in a manner similar to tensile strength. However, in several instances, the strengths in the C bars, with larger eutectic cells, were greater than the strength in B bars with smaller eutectic cell sizes. Therefore, the fall in strength with increasing sulfur could not be entirely attributed to a decrease in cell count.

It was discovered there was an increase in the amount of type D graphite with increasing sulfur content. The D graphite occurred in the cell boundaries and surrounded large eutectic cells (Figure 2). This phenomenon was particularly visible in the fracture faces of the broken tensile bars, where fracture in the D graphite regions were readily distinguished from fracture through the large eutectic cells (Fig. 3). The type D graphite distributions were even observed in the coarse C bars, where type D graphite could hardly be attributed to severe undercooling ahead of eutectic solidification. This finding suggests type D graphite formed toward the end of solidification, rather than at the beginning of solidification.

It could be argued the reduction in tensile strength associated with increased sulfur contents is due to the formation of D graphite in the cell boundaries. However, the refined graphite flakes associated with type D graphite are only damaging to strength when free ferrite is present. In the present samples, the matrix microstructures remained fully pearlitic in all 24 alloys and in all cast sections. Therefore, the reduced strength is not attributed to the presence of type D graphite in the cell boundaries.

The average flake size was observed to decrease with increasing sulfur content (Fig. 4). This could mean the decrease in flake size is the result of an increase in the incidence of type D graphite with sulfur content. That is, the increase in the amount of D graphite flakes in the cell boundaries increased the flake count and caused the average flake size to decrease.

As the sulfur content increased, spikey graphite developed in the eutectic cell boundaries. The presence of spikey graphite might by itself be responsible for the decrease in tensile strength. Figure 5 showed the incidence of spikey graphite was greater in the low-Mn (0.28%) series, and this is attributed to a higher free sulfur level resulting from a lower Mn content (Fig. 6). Since sulfur has such a pronounced effect on growth rate of the graphite phase, spikey graphite could be caused by high free sulfur content.

Evidence of Sulfur Segregation
As the sulfur content increased, intercellular carbides also formed in the eutectic cell boundaries of some alloys. The intercellular carbides were only observed in the 0.28% Mn series, and they are attributed to the particularly high free sulfur levels in the low manganese series. At higher manganese contents, no intercellular carbides (IC) were observed.

The fact that the spikey graphite and carbides are confined to the cell boundaries suggests sulfur is segregating to the remaining liquid as the eutectic cells are growing. Even globules of MnS were found in the cell boundaries (Figs. 7-8). One might also argue that the formation of type D graphite around the eutectic cells in the higher sulfur alloys occurs as a result of segregation of sulfur to the remaining liquid phase during solidification. The partition coefficient, k, for sulfur is commonly reported to be 0.02, indicating sulfur strongly segregates to the liquid phase during solidification.

Micro-hardness testing of the pearlite matrix revealed the hardness of the pearlite varies significantly within each test bar. Many samples displayed a wide range in hardness. Very high pearlite hardness was observed even in the 3-inch bars with hardness values as high as 398 HK. The higher pearlite hardness readings in some regions of the microstructure suggest that that pearlitic constituent has a high combined carbon content.

Pearlite hardness is a function of carbon content and the fineness of the lamellar spacing. The lamellar spacing is a function of the transformation temperature. Due to recoalescense produced by the heat of formation, pearlitic transformation in the test bar castings is expected to occur at a relatively constant temperature and, therefore, the pearlite hardness was expected to be uniform. And yet, there was a significant variation in hardness in each sample as noted by the high standard deviations observed. One would expect to find higher hardness in the cell boundaries, but the locations of the micro-hardness readings were random and could not be specifically located in the cell boundary or cell center. The wide variation in matrix hardness further supports the hypothesis that sulfur is segregating during eutectic solidification and influencing the combined carbon content of the pearlite matrix in the last metal to freeze. Past research indicated that as sulfur increases (and the Mn:S ratio decreases) the combined carbon content in cast iron increases.

Evidence of Embrittlement
Probably the most compelling data generated in this study were the stress-strain properties of the alloys. The reduction in strength associated with increasing sulfur was associated with a reduction in the elongation at fracture. That is, premature fracture was observed in the stress-strain plots (Figs. 9-11). Premature fracture in the tensile test was observed in every manganese series and in every section size. The data in Table 3 shows the elongation at fracture decreased on average 37% and was as much as 53%. The greatest reduction in fracture elongation occurred in the 0.28% Mn series. Once again, it is notable the series also exhibited the highest free sulfur contents.

Scanning electron microscopy on some of the fractured tensile bars revealed changes in the fracture mode in samples of high and low strength. As strength and elongation decreased, the fracture mode changed from ductile tearing to transgranular cleavage, a brittle fracture mode. With transgranular cleavage fracture, there is very limited plastic deformation occurring during crack propagation. It is proposed that the reduction in strength with increasing sulfur was caused by an embrittlement of the alloy. The embrittlement may well be due to the presence of spikey graphite and IC carbides in the cell boundaries. Clearly, the transgranular cleavage fracture mode is associated with the pearlitic matrix; and the harder the pearlite, the more likely the fracture mode will be by transgranular cleavage.   

The original hypothesis was that changes in manganese and sulfur produced changes in the graphite structure, which in turn produced a reduction in strength. The findings of this investigation indicate the reduction in strength is due to an embrittlement phenomenon. It now appears that the degradation in strength with increasing sulfur may be more a function of high free sulfur contents and sulfur segregation during solidification, rather than to a modification of the graphite structure. The roles of MnS precipitation and the influence of MnS inclusions on the microstructure are less clear.  

Click here to see this story as it appears in the November 2018 issue of Modern Casting