Stabilizing Pearlite In Gray Cast Iron.

This article examines five alloying elements as possible pearlite stabilizers to promote improved properties in gray cast iron production.

In casting production, elements added to gray iron can be classified into four principal categories--graphitizers, carbide stabilizers, pearlite stabilizers and pearlite refiners. The elements carbon (C), silicon (Si), aluminum, titanium, copper (Cu) and nickel (Ni) tend to promote the formation of graphite during solidification and therefore would be considered graphitizers. All elements are not of equal potency, however, as Cu is 0.05% as effective as Si. Ni and Cu play a dual role and promote more and finer pearlite (their primary role) and therefore also would be classified as pearlite stabilizers, like tin (Sn), antimony (Sb), manganese (Mn), molybdenum (Mo), chromium (Cr), vanadium (V) and niobium (Nb) [columbium (Cb)] are classified as carbide stabilizers since they retard graphite precipitation and increase the tendency to form iron carbides. Some of these carbide stabilizers such as Mo play a dual role and act as pearlite refiners, with Mo being a pearlite refiner up to 0.8% and thereafter acting as a carbide stabilizer.

Specific alloys are deliberately added to molten gray iron to form, refine and stabilize its pearlite. A fine pearlite, which is a lamellar matrix structure composed of alternating plates of ferrite and iron carbide, enhances the cast iron hardness and strength properties.

Depending on the cooling rate from austenite, either coarse or fine pearlite can be formed. A slow cooling rate favors a coarse pearlite structure, and a fast cooling rate favors a fine pearlite structure. For example, cooling from a high shakeout temperature such as 1700F (927C) potentially produces harder and finer pearlite than cooling fast from a low shakeout temperature such as 1400F (760C).

Certain alloying elements can cause the formation of fine pearlite during slow cooling from the austenitic region. Cr (0.1-0.6%) and V (0.1-0.4%) are two commonly used, effective pearlite formers. Mo (0.1-0.8%) and Ni (0.1-1.5%) are commonly used pearlite stabilizers. Since certain alloys are effective in promoting the formation of pearlite and others are more effective in refining and stabilizing pearlite, it follows that an optimum alloy system will comprise a combination of alloying elements and shakeout temperature.

Much information has been publicized about the effect of Cr, V, Mo and Ni as alloying elements. To assist metalcasters in better understanding the role of Sb, Cu, Mn, Nb (Cb) and Sn in gray iron production, this article discusses the findings of a literature search and how these elements stabilize the pearlite of gray iron.

Mn in Gray Cast Iron

The principal role and need for Mn in gray cast iron is to form the small, harmless slate-colored manganese sulfide (MnS) inclusions and tie up the S. The MnS inclusions are randomly scattered throughout the metal matrix and help maximize machinability (tool life). The presence of Mn suppresses the formation of iron sulfide (FeS), which has a lower melting point than MnS [2180F (1193C) for FeS vs. 2948F (1620C) for MnS]. The prevention of FeS eliminates embrittlement caused by iron sulfide, which forms as a grain or eutectic cell boundary phase and causes brittleness, hot shortness and residual stress in the casting.

Once the S is tied up, the general perception of the role of Mn in gray iron is that of a pearlite stabilizer or promoter like the role Mn plays in steel. Mn is intentionally present in most grades of gray iron and is a residual constituent of virtually all other types of cast iron. Mn is present in most charge materials, and its use in steels as an alloying constituent has increased. Mn has a negligible solid solution strengthening effect in austenite and only a moderate effect in ferrite. Mn strongly retards the transformation of austenite to ferrite and promotes deep hardening in the heat-treatable steels and cast irons. Mn also lowers the transformation temperature and the eutectoid carbon concentration. Mn is the most cost-effective hardenability-intensifier alloying element.

The critical role of S as a graphite flake nucleating and growth player was well-established and documented through work by Alfred Boyles. This work was the beginning of a new understanding of flake graphite size in cast irons and enabled foundry process controls to achieve consistent and reproducible gray cast irons. Today's accepted minimum S content in gray iron typically deemed necessary to achieve a desired graphite flake morphology is 0.06%. Below this level, chill and flake distribution problems are experienced.

Stoichiometrically, 1.7 times as much Mn (atomic weight of 55) will be required to combine with S (atomic weight of 32) to form MnS. However, it has been recognized for many years that an excess of Mn (0.3-0.35%) is required to prevent the formation of FeS and its related problems in gray cast iron.

A complex sulfide may be formed, (FeMn)S, which requires Mn in excess of the stoichiometric value. Actually, MnS and FeS exhibit mutual solubility so that the compound always contains varying amounts of Fe or Mn. It is more complicated because there also is mutual solubility of S and oxygen (O) so that the compound that forms always is [Fe.sub.x][Mn.sub.y][S.sub.v] [O.sub.w] where x, y, v and w will change with local changes in iron composition. One rarely finds free S in cast irons, but may encounter low Mn/S ratios as noted above. The optimum Mn/S ratio appears to be 1.7 x %S + 0.3-0.35% Mn, but this may vary according to section size, foundry processes, etc.

Excess free S in gray iron increases chill depth rapidly, decreases fluidity and decreases the effectiveness of inoculation. These effects are eliminated largely by the addition of enough Mn to combine all the S as MnS. Under proper foundry operating conditions, S does not seem to exert excessive harmful effects on gray cast iron in amounts up to 0.18%, provided Mn is present in amounts sufficient to prevent chill. In the absence of enough Mn, S has a marked stabilizing action on the cementite and promotes carbides (chill).

The optimum excess Mn for a particular foundry will vary according to residual alloy content levels and base S, but probably will fall between 0.2-0.5% excess Mn for the greatest ease of control. Any foundry trying to establish the optimum amount of excess Mn for their particular base iron can make 0.1% Mn increment adjustments for an extended period (about a month) and identify the best combination of strength and hardness (tensile/Brinell ratio) for their operation and product needs. Mn reduces the ratio by increasing eutectic graphite, which can be seen in the Fe-C phase diagram. Low Mn concentrations can result in carbides, chill, finer graphite, more pearlite matrix and more erratic casting properties. Too much excess Mn can cause graphite coarsening, pearlite refinement, and undesirable grain boundary conditions, resulting in a loss in strength and a decrease in iron density.

Final Mn contents greater than 0.7% in gray iron require correspondingly low S contents and higher pouring temperatures to avoid slag reaction gas holes (subsurface blow holes) caused by MnS interactions at the cope surface of the casting. With a specific gravity of 4, these MnS particles float to the surface and act as sites for mold! metal interface reactions. Sub-surface blow holes are seldom seen at S contents less than 0.15%. The presence and control of both Sand Mn are absolutely essential in the production of desirable microstructures and mechanical properties, optimum strengths and avoidance of casting defects.

For practical purposes, in normal gray iron castings with S ranging from 0.05-0.15%, Mn does not materially affect the strength and hardness of gray iron in amounts under 0.65%. To help prevent carb ides from forming in gray iron castings under 1 in. in thickness, Mn used in excess of the amount required to completely tie up the S should be used along with small additions of Ni and/or Cu. Ladle additions of Mn have been reported to contribute to pin hole porosity in thin sections (under0.375-in. thickness) in irons of 4% carbon equivalent and above. This apparently is due to the high affinity of Mn for hydrogen (H), and the H often is not released before the castings solidify. For this reason, if Mn is to be used as an alloying element in gray iron, it is recommended that it be added to the furnace at least 10 mm prior to tapping.

In past AFS research, it was shown that the ultimate tensile strength decreased with increasing Mn concentration (0.39-0.97%) in all section sizes of A, B and C test bars. Tensile-to-Brinell ratios were higher at the lowest Mn level (0.39%) where there might have been excess S. The density of the iron decreased slightly with Mn concentration. The chill depth minimum was at 0.7 1% Mn, suggesting the best balance with the S and the best cell-nucleating conditions for the Mn concentrations tested. The bulk hardness and matrix hardness both decreased slightly with increasing Mn concentration. In general, the Brinell hardness in both A and B bars increased with increasing S concentrations (0.03, 0.09 and 0.16%) at a constant excess Mn of 0.3%. Pearlite hardness was relatively constant for all irons. The 2-in.-diameter bar's bulk hardness was lowest, most likely due to the coarser graphite.

Mn is properly classified as a carbide stabilizer since it retards graphite precipitation and increases the tendency to form iron carbides. Excess Mn should not be reduced to a level at which the tensile strength-to-Brinell ratio decreases (hardness increases with no increase in tensile strength) and formation of FeS causes carbides and shows shortness or embrittlement problems. Conversely, Mn over and above the "typical" 0.3% excess may be useful for mild strengthening, but only to the point at which tensile-to-Brinell ratios begin to drop. If pearlite formation and stabilization are required, one must look at using some combination of Cu and/or Ni with small amounts of Sn or Sb to achieve desired product characteristics.

Sb in Gray Cast Iron

Sb is a potent and cost-effective, but also controversial, pearlite stabilizer in gray irons. Its effects are similar to those of Sn, but it is generally about 2-4 times as effective (and less costly), depending on the base iron to which it is added. It is particularly effective in thin sections where some pearlite of Types B, D or E graphite flakes almost are inevitable. Sb also is useful in preventing pearlite breakdown at high temperatures. The literature contains some contradictory remarks regarding Sb, stating that it generally increases hardness while sometimes increasing and, at other times, decreasing tensile and transverse strength.

Work by Pelleg from 1962 involved Sb additions of 0.005-1% to a Class 35B iron, which was almost fully pearlite, except for some ferrite at the cast surface. The results of these additions were a continual increase in hardness, related decreases in tensile, transverse strengths and deflection, and a reduction in tensile/Brinell ratio. Pelleg concluded that "practical application of...Sb-treated castings is doubtful due to the other detrimental effects of Sb on mechanical properties." Sb recoveries were only 40% in this work, which raises some question about the accuracy of reported Sb levels.

In contrast to these findings, work by Klaban demonstrated that Sb levels of 0.03-0.06% in irons are effective in increasing both hardness and tensile strength of irons that are less than fully pearlite. However, this did not work for levels beyond 0.16% Sb. He did note and agree with Pelleg that additions beyond those needed to ensure a fully pearlite structure result in no further increase and possibly a reduction in tensile strength, assuming a maximum of 0.1% Sb. The hardening effect of high Sb levels is caused by the precipitation of an Sb-rich compound (antimonide) in the eutectic cell boundaries.

Klaban's work also noted that "by addition of Sb to cast iron, it is almost certain that one will prevent precipitation of ferrite in connection with super-cooled forms of graphite as opposed to alloying with Sn." He also observed that "Sb partially suppressed the super-cooled forms of graphite (Types B, D and E) and promoted more Type A flakes."

A drop-off in impact strength occurs as hardness increases similarly to the effect on Sn. For this reason, levels of Sb beyond those needed to ensure a fully pearlite matrix should be avoided except in special applications in which a fully pearlite matrix is critical.

The effect of Sb additions is primarily determined by the type of iron to which it is added. When added to a ferrite-containing iron, it is likely that hardness and tensile strength both will rise, but when added to a fully pearlite iron, the hardness likely will rise while tensile strength may remain fairly constant or even decrease and the tensile/Brinell ratio will decrease.

When utilized effectively, typical amounts of Sb additions are from 0.02-0.06% in the final iron. Sb usually is added in transfer or pouring ladles as the pure metal, typically in the form of high-purity (lead-free) shot. Typical recovery with additions to molten iron should be 90-100% while estimated recoveries or re-melting are 40% in cupola melting and 80% in electric furnace melting.

One of the problems with Sb is the lack of good spectrometric standards and the fact that accurate analysis ultimately requires the use of good wet chemistry methods, which are time-consuming and costly. Any foundry seeking to take advantage of the benefits of Sb can produce its own in-house spectrometer standards. Related to these matters is the problem of alloy recovery in the iron. In other words, how can we reliably calculate recovery without accurate analyses?

Perhaps more important is that a number of iron foundrymen have learned to put this knowledge into practice and to effectively utilize Sb, typically at 0.02-0.06%, to ensure fully pearlitic structures in critical castings. Core and mold washes containing Sb also have been used to minimize or prevent ferrite skin on some gray iron castings. The optimum additions for each type of casting must be determined by good experimental practices via the use of reliable hardness and strength tests as well as exacting metallographic practices and followup with the machine shop.

Sb is particularly useful when small amounts of ferrite remain in critical locations, especially after other, more conventional alloy additions such as Cu, Cr and Sn have failed to eliminate ferrite. It also seems to work better than other alloys when Si levels tend to run on the high side; although in such cases, the best first step is to reduce Si levels, if possible. Sb can be used alone or in combination with other alloys, depending on the specific casting requirements.

In applications in which gray iron castings are exposed to high temperatures [up to 1300F (704C)], the presence of 0.03-0.08% Sb provides a significant increase in structural and dimensional stability compared to other alloys or combinations of alloys including Sn. Even higher levels can be used in special cases in which the stability factor is more important than impact properties. This is because the pearlite promoted by Sb additions is extremely stable and difficult to break down at such temperatures.

Indiscriminate use of this alloying element, however, is discouraged and could lead to excessive hardness, low strength and machining problems.

Regarding the environmental aspects of adding Sb to molten cast iron, it is strongly recommended to review MSDS sheets and discussions with plant safety personnel and suppliers. However, some personnel monitoring in production foundries using typical Sb additions have indicated no problems.

Cu in Gray Cast Iron

Cu is said to have a multitude of effects when added to gray cast iron. Those most commonly discussed are as a graphitizer (with 0.2% the strength of Si) and a pearlite promoter, refiner and stabilizer. It is this effect on pearlite, along with Cu's solid solution strengthening of ferrite, that allows for the increase in hardness and mechanical properties, such as ultimate tensile strength. This increase in hardness and tensile strength is more pronounced when starting with ferritic or ferritic/pearlitic base iron and shows the greatest improvement with high C and low Si-base irons. Because of its graphitizing effect, several observers have noted Cu's ability to reduce chill. This chill reduction, most useful in thin sections, along with its ability to combat ferrite in thicker sections, allows Cu to promote better microstructure uniformity in castings that contain varying section sizes.

Good-to-improved machinability has been noted and is related most likely (in part) to the improved uniformity of casting microstructure. The maximum solubility of Cu in a typical gray iron composition has been listed as 3.5%, however, few observers worked at this level. A more common level was in the 1-2% range, with useful improvement seen at levels between normal base up to the 1-2% range. One researcher noted no real improvement beyond 0.4%, but admittedly was dealing with an already fully pearlitic base iron. This same researcher also showed little-to-no chill reduction from Cu.

Another researcher's work equated a 1% Cu level to be equal to that of 0.1% Sn, a common maximum level listed for that element. Many researchers promote using Cu in combination with other alloys such as Sn, Cr, Mo, Ni and V, creating a synergistic effect of the various alloying combination that is over and above the additive effects of the individual alloys. Researchers also point out the positive effects of an element like Cu to balance the negative effect (carbide formation, for example) of an element like Cr. Other effects seen from Cu include refinement of graphite and improved corrosion resistance.

Nb (Cb) in Gray Cast Iron

The effects of Nb (Cb) are of general metallurgical interest and becoming of greater interest due to the increased amount of high-strength, low-alloy (HSLA) steels in the secondary metals/scrap markets. HSLA steel is the most probable source of Nb in the foundry melt as it is starting to be used in auto bodies, structural steels, rail steel, railway tank cars, cold-forming strip, offshore structures, oil and gas pipeline, and shipbuilding. Nb is Atomic No. 41 and is in Group Va in the transition metals section of the periodic table. V also is in the group, which would suggest that their effects in gray iron may be similar.

The literature is full of contradictory statements, but the following four statements generally are agreed upon in regard to the effects of Nb in gray cast iron:

* Nb does not melt [the melting point of ferroniobium (FeNb) is near 2960F (1627C)]. Rather, it dissolves, which requires time and temperature;

* Nb tends to improve gray iron mechanical properties (up to 0.3 wt%). This results from a reduction in the cell size and correspondingly blunt graphite flake size (0.03-0.07wt% Nb);

* Nb decreases the tendency to produce chill carbides due to the reduction in under-cooling and the increase in cell count;

* Nb is a mild pearlite stabilizer and refiner.

Sn in Gray Cast Iron

Sn in gray cast iron is a pearlite stabilizer. The mechanism by which Sn stabilizes pearlite is achieved by blocking diffusion of C from the matrix to graphite flakes during the eutectoid reaction. Sn additions from 0.02-0.1% have been advocated to achieve the pearlite stabilizing effect in gray cast iron. As a consequence of Sn addition, the tensile strength of gray cast iron can be increased directly as a function of the addition. This mechanical property increase has been attributed to an ever-decreasing quantity of ferrite. Consequently, as the quantity of Sn additions increase, the pearlite content increases and tensile strength increases. Three researchers have indicated that Sn additions higher than 0.1% cause the iron to become embrittled. One researcher indicated that mechanical properties increased as a function of Sn addition up to a level of 0.14% in gray cast iron. The same researcher, however, also indicated that Sn additions in ductile cast iron cause tensile strength to increase up to 0.1% an d then decrease with further additions. Another researcher indicated that Sn adversely affects soundness of gray cast iron.