How do manganese, sulfur levels affect gray iron properties?

Inside This Story

* Investigations have sought to determine how the presence of sulfur and manganese affect gray iron properties.

* Uncertainties exist to exact roles the two elements play in gray iron.

* Detailed is an analysis of how the concentration of sulfur and manganese in gray iron at solidification relates to gray iron microstructure and mechanical properties.

For years, sulfur and manganese have been known to play significant roles in gray iron chemistry. A number of studies attempting to explain the effects of these two elements in terms of manganese level, sulfur level or manganese to sulfur ratio in the iron have led to some confusion as to the exact effect these two elements have on the metallurgy of gray iron.

The two elements' effects on gray iron's microstructure and properties often have been attributed to the presence (or absence) of manganese sulfide formation. Manganese and sulfur, when dissolved as a solution in liquid iron at a particular temperature, are in equilibrium with manganese sulfide as follows (equation 1):

Mn (in soln) + S (in soln) = MnS

A similar expression (equation 2) describes the equilibrium between iron and sulfur:

Fe (liq) + S (in soln) = FeS

Combining equations 1 and 2 algebraically results in a common expression (equation 3):

Mn (in soln) + FeS = Fe (liq) + MnS

The stoichiometric amount of manganese needed to simply combine with the sulfur is 1.7 (%S). This means the ratio of manganese to sulfur in a manganese sulfide particle is 1.7. To assure that all sulfur present in gray iron forms manganese sulfide, another formula (equation 4) has evolved throughout the industry: Mn% = 1.7 (S%) + 0.3%

Although equation 4 has been used to avoid unwanted matrix structures, graphite forms and chilling tendencies, it does not define the optimum combinations of manganese and sulfur. It has been noted that strength can be increased by adjusting manganese and sulfur levels to decrease the manganese to sulfur ratios, provided the levels are optimized to avoid unwanted microstructures.

Further, equation 4 defines a relationship ensuring all sulfur is present as manganese sulfide, but it does not describe the conditions under which manganese sulfide forms in gray iron. Thus, investigations were performed using a solubility product approach to evaluate how the effects of sulfur and manganese on gray iron properties are related to the dissolved amounts of both elements remaining in the molten iron when solidification begins.

This paper discusses how this approach can help determine:

* the formation of manganese sulfide in gray iron;

* the effect of manganese and sulfur on the mechanical properties of gray iron.

Applying Solubility Data

It has been proposed that a principal factor influencing the properties of gray iron is the free sulfur Present in the molten iron bath. Free sulfur (or free manganese) is the amount present in uncombined form at the onset of solidification. This can be defined by using the solubility product data. The solubility of manganese sulfide in liquid will vary with composition and temperature (equation 5):

Log10 (%Mn x %S) = -1,920/T (T = Kelvin)

In an analysis of past investigations, tensile strength was plotted as a function of the difference between the actual sulfur and the equilibrium amount of sulfur needed for MnS precipitation before solidification. The tensile strengths of groups of irons were compared to the difference between the actual sulfur in the iron and various equilibrium values at each liquidus point. The equilibrium values of sulfur for each group were calculated from equation 5 for various manganese levels. A negative value indicated the equilibrium sulfur (determined by a particular level of manganese) was greater than the actual sulfur level. This meant that all of the sulfur would be in solution when solidification began.

As manganese was added, the equilibrium level of sulfur decreased and the difference between the actual sulfur and the equilibrium values was reduced, as well. At some point, sufficient manganese was present to initiate MnS precipitation, and the difference value became zero and later a positive value.

A correlation was determined to exist between mechanical properties and the formation of MnS (Fig. 1). If MnS precipitation occurred before solidification, tensile strength would be reduced; however, maximum tensile strength would be obtained if MnS precipitation was avoided prior to the onset of solidification.

[FIGURE 1 OMITTED]

Analyzing Castings

The investigations sought to apply these findings to actual castings. Therefore, a series of heats of gray iron containing varying amounts of sulfur and manganese were cast as step blocks at several metalcasting facilities. Each block was examined for its mechanical properties and microstructure.

The measured tensile strengths were found to depend not only on step thickness but also on the composition of each of the irons. To evaluate the effects of sulfur and manganese alone on tensile strength, a mathematical procedure was employed to adjust the measured tensile strengths to compensate for variability due to differences in the amounts of other elements. This adjustment calculated values of tensile strength for each of the step blocks using a previously established equation, which relates tensile strength to composition and section size.

These results then were "normalized" to strengths calculated from the composition of one of the metalcasting firms that took part in this study.

By comparing the data from the current investigation (involving "normal" production iron) with previously investigated results, it was confirmed that a relationship exists between tensile strength and the formation of MnS. However, all of the data from this investigation was obtained when the sulfur differential was near zero and when tensile strength decreased rapidly as the sulfur differential increased. Thus, an expanded analysis was carried out to confirm in more detail the observed relationship between tensile strength and the formation of MnS.

This study also showed that similar conditions existed when the actual manganese level was compared to the calculated equilibrium manganese level. As the manganese differential went from negative to positive values, strength decreased.

Maximizing Tensile Strength

These investigations can help optimize manganese and sulfur levels to obtain maximum tensile strength in gray cast iron. Maximum tensile strengths were found to be obtained when MnS precipitation is suppressed to as low a temperature as possible before solidification. The data shows the reduction in tensile strength due to MnS precipitation varied with the carbon equivalent, and ranged from 46-80 MPa (6.6-11.6 ksi). An estimate from the data for the current investigation indicates that a reduction in tensile strength of up to 50 MPa (7.2 ksi) could be anticipated due to MnS precipitation if it occurs before solidification begins.

The ideal situation would be to suppress MnS precipitation to below the liquidus temperature, which is largely dependent on carbon equivalent. This can be estimated from Fig. 2. This temperature then can be used in a series of calculations in equation 5 to develop a curve that shows combinations of manganese and sulfur that are in equilibrium at the onset of solidification.

[FIGURE 2 OMITTED]

If a manganese content is selected for a given gray iron composition, the sulfur level, which is in equilibrium with this manganese level, can be obtained from that curve. A sulfur level slightly less than this value should maximize tensile strength because MnS precipitation would not occur. Conversely, if average sulfur content is known, a manganese level could be obtained, which would avoid MnS precipitation.

This principal can be applied to an example of gray iron with a carbon equivalent of 4.1 and sulfur at 0.09%. From Fig. 2, the liquidus temperature of 2,147F (1,175C) is obtained. Using equation 5, the solubility product curve for the liquidus (Fig. 3) can be obtained. With a sulfur level of 0.09%, equilibrium is established with a manganese level of 0.52%. If sulfur is held constant at 0.09%, strength will be maximized if manganese is below 0.52%. In turn, at manganese content of 0.52%, reducing sulfur below 0.09% will maximize strength. Under these conditions, MnS precipitation will occur at a temperature below the liquidus. This temperature can be calculated from equation 5 using a manganese level and a sulfur level slightly below 0.52% and 0.09%.

[FIGURE 3 OMITTED]

The temperature at the end of the eutectic freezing also can be considered a lower limit to MnS precipitation. Figure 3 also depicts a MnS solubility product curve for the temperature at the end of eutectic freezing, 1,978F (1,081C). Here, 0.09% sulfur is in equilibrium with 0.43% manganese, and 0.52% manganese is in equilibrium with 0.074% sulfur. A range of manganese and sulfur levels can be identified between the two temperatures used to define region A. Combinations of manganese and sulfur levels within this region would maximize strength. Contrarily, combinations of manganese and sulfur levels extending into region B would result in MnS precipitation above the liquidus temperature and reduction in strength.

Figure 4 shows MnS solubility product curves for the liquidus temperature for each of several carbon equivalents. An evaluation of these curves (using the procedure outlined with Fig. 3) Fig. 4 can be used to determine manganese and sulfur levels that avoid precipitation of MnS before solidification. Because liquidus temperature decreases with increasing carbon equivalent, the maximum manganese level to avoid MnS precipitation above the liquidus decreases at any given sulfur level. For example, the manganese level in equilibrium with 0.08% sulfur at a carbon equivalent of 3.7 is 0.66%. At a carbon equivalent of 4.3, the manganese level in equilibrium with 0.08% sulfur is reduced to 0.54%.

[FIGURE 4 OMITTED]

About the Authors

George All. Goodrich is the president and Thomas G. Oakwood is a consultant and former senior metallurgical engineer at Professional Metallurgical Services, Buchanan, Mich. Richard B. Gundlach is the vice president of sales and marketing at Climax Research Services, Wixom, Mich.

For More Information

"Relationship Between Tensile Properties and Matrix Microstructure in Austempered Gray Iron," K. B. Rundman, J.R. Parolini and D.J. Moore, 2005 AFS Transactions, Paper No. 05-145.

"Composition, Section Size and Microstructural Effects on the Tensile Properties of Pearlitic Gray Cast Iron," C.E. Bates, J.R. Tucker and K.S. Starrett, AFS Research Report No. 5, September 1991.