Controlling cast iron gas defects.
Gas defects can be classified by their gas source, but a literature search reveals the terminology for gas defects is often inconsistent. The term "blowhole" is usually associated with a larger cavity than that described as a "pinhole."
The distinction created by the following terminology seeks to differentiate between the various gas defects.
Reaction Gas Holes: caused by the interaction between carbon (C) dissolved in iron and a nonmetallic inclusion to generate carbon monoxide (CO). Described as an endogenous (internally generated) gas defect, it is usually a blowhole from a reaction with slag carried into the mold. These reaction voids may be located throughout a section of a casting but are normally near the cope surface. They may be large, small or irregular with either a smooth or rough shape.
Mechanical Gas Holes: caused when gas is either entrained during pouring or is forced into the shape from the mold or core after pouring as depicted in the schematic in Fig. 2. They are referred to as exogenous (externally caused) gas holes.
The required pressure is defined by liquid metal head and surface tension and bubble radius. A gas hole forms when the gas pressure, at any point, exceeds the head pressure and is influenced by the mold's ability to release the gases generated in the mold (permeability).
The defect is generally large, but may have a small entry hole. It can be located at the source of the trapped gas but may rise toward the thermal center of the casting, where the occurrence of shrinkage cavities can make differentiation between the two defects difficult.
Solubility Gas Holes: caused when the high gas content of a metal, in excess of the metal's solid solubility, is rejected during solidification. The solubility levels of both |H.sub.2~ and |N.sub.2~ change dramatically.
High |N.sub.2~ contents can give rise to blowhole or fissure defects like those shown in Fig. 3(a), (b) and (c). In gray iron, they occur most commonly in thick section, low carbon equivalent (CE) cupola-melted iron. The severity of the defect increases above 130 ppm in light section and 80 ppm in heavy section. Ductile cast iron normally does not have this type of defect.
Pinholes can be caused in ductile cast iron by |H.sub.2~, |N.sub.2~ and CO, but |N.sub.2~-related pinholes are usually tied to mold or core binders.
Solubility gas holes that may contain |H.sub.2~, |N.sub.2~ or CO are generally small, irregular and occur at or just below the casting's surface.
High levels of |N.sub.2~ or |H.sub.2~ can cause rough holes, or fissures. They may occur anywhere in the casting and can aggravate solidification shrinkage at the thermal center of the casting.
During the solidification of most simple iron alloys, there is enrichment of the alloying element due to the greater solubility of the element in the liquid than in the solid metal. The difference in solubilities, determined from the slopes of a solidus-to-liquidus line on a phase diagram, is expressed by the partition ratio KS/L, which has a value less than 1 for most simple alloys. The KS/L value for |N.sub.2~ in high C iron alloys is 1.9% and 2.2% for stable and metastable eutectic solidification. That is, there is temporarily greater solubility of |N.sub.2~ in austenite than in the liquid iron.
However, the solubility of both |N.sub.2~ and |H.sub.2~ in the iron decreases with increasing C content and the solubility of |N.sub.2~ is further reduced by raising the silicon (Si) content.
The thermodynamic pressure of |N.sub.2~ can increase because of the influence of the increasing C content in the liquid during solidification. The pressure may exceed 1 atmosphere, causing the evolution of |N.sub.2~ pinholes.
In the equation for total thermodynamic pressure of all the gases,
|P.sub.T~=K|H.sup.2~ f|H.sup.2~|(%H).sup.2~ + K|N.sup.2~ f|N.sup.2~|(%N).sup.2~
Ki is the equilibrium constant for the gas reaction and fi is the activity coefficient. If the total pressure (|P.sup.T~) exceeds 1 atmosphere, a pinhole may result.
Even at the small value of 4 ppm, |H.sub.2~ is equivalent to 0.4 of an atmosphere and calculations show that the combination of 4 ppm |H.sub.2~ and 80 ppm |N.sub.2~ may have a gas pressure exceeding 1 atmosphere, sufficient to cause a pinhole.
Slag-Related Gas Defects: often caused by the presence of ladle surface slags resulting from oxidation. Gas hole defects are usually revealed during machining as sub-surface defect voids. The associated microstructure around these gas holes shows a characteristic segregation of MnS and a complex crystalline slag.
The conditions that encourage this type of defect are high S, high Mn and a low pouring temperature. This combination allows MnS to precipitate and segregate by flotation into the slag layer. Dissolution in the iron-manganese-silicate slag increases slag fluidity, allowing intimate reactive contact with eutectic graphite to generate CO.
High pouring temperatures and adjusting Mn and S contents to lower levels can alleviate these gas defects. Avoid heels and loss of temperature control that is aggravated by a heel.
Gross Blowholes: caused by low-permeability molding sand and encouraged by high mold hardness. High moisture content in the mold, high gas-content corebinders or underbaked cores are likely causes of the gross blowhole, especially if vents in cores and molds become blocked by metal penetration.
Severe casting defects caused by excessive gas can create an appearance in gray iron that would be more appropriate for a rimming steel. The wormy appearance and exfoliation (scaliness) of the riser are attributable to excessive |N.sub.2~ introduced by an unsuitable carbonaceous material. Although the high |N.sub.2~ could be balanced by suitable levels of Ti or Al, avoiding the |N.sub.2~ source is the ultimate change required.
Pinhole Defect: pinholes in gray iron dependent on the surface tension of the molten iron poured into a green sand mold. The |H.sub.2~ pinhole, characteristically, has a graphite lining and a C-free layer at its perimeter. The surface tension is influenced by the presence of surface-active elements.
Aspiration pinholes are related to the |O.sub.2~ available from entrained air during turbulent mold filling. They tend to be round in shape, light colored, mildly oxidized and located near the ingate. Because mold filling turbulence also encourages slag-type pinholes, the two types are often found in the same casting.
In severe cases of aspiration pinholes, metal shot may also develop and be present on the casting surface. The shot surface is oxidized and reacts to form a gas bubble. Fracturing the casting will reveal the gas hole, but the shot may be lost.
Oxidized pinholes are observed in low CE iron, typically white iron for malleablizing. They can appear on the cope surface or vertical faces and may be subsurface, round or elongated, and usually distributed unevenly on the surface or in the casting. The holes are filled or lined with an oxide phase and C in the adjacent iron may be depleted.
Evolution-type pinholes in malleable iron occur throughout the section or just under the surface. They are silvery, rounded holes with no graphite film and are caused by |H.sub.2~ and encouraged by an Al addition to inhibit reaction-type pinholes. The influence of Al on |H.sub.2~ pickup is aggravated by higher Mn and S levels in malleable iron.
White, abrasion-resistant irons with particularly low C and Si contents can be subject to oxidized holes that tend to be deeper and more elongated. Low pouring temperatures foster this type of defect and holes don't contain a heavy oxide layer as evident in malleable iron. The cause is thought to be due to a reaction between C and |O.sub.2~ in iron that forms and traps bubbles.
Malleable Iron: The types of gas porosity in malleable iron that most closely resemble those in gray iron are larger, further below the surface and less frequent. There are three porosity types evolving from reaction, aspiration and evolution.
Reaction between C in the iron and |O.sub.2~ from iron oxide in the carry-over slag or in the sand system may cause clusters of pinholes (especially in thicker sections) that are usually elongated and darkened by oxidation. The tendency for the iron oxide-rich slag to occur in the production of white iron is greater below 2600-25300F.
The reaction-type pinhole is often intermixed with the slag that developed the CO. The addition of 0.02% Al has been recommended as a test to eliminate this type of pinhole, but it also can introduce brightly surfaced |H.sub.2~ pinholes.
It is recommended that the iron oxide content of the slag, during a white iron melt, be kept low, but other factors are believed to be part of the problem. Increased metal temperatures can reduce the defect, although the defect is found in thick sections and where the mold is hottest (the oxide is maintained in a liquid condition and more reactive at the hot spots). A low CE or high S will promote more severe defects in malleable iron.
The graphite film found in pinholes suggests that the gas involved is inert or a reducer. Magnified graphite evidences a hexagonal pattern or concentric rings. The correlation between graphite-lined pinholes and hydrogen pickup from a metal/mold interface is well established.
To understand the role of the gray iron melt condition at the mold/metal interface as a cause of pinholes, measurements of the melt's surface tension properties were made to established the influence of different levels of Al, S, Ti and Te.
* Aluminum: At low Al levels, iron has a high surface tension, which reduces as the Al content increases. Increasing the Al level above 0.2% produces a surface tension higher than the original value recorded at low Al content.
* Sulfur: As the S content increases, the surface tension decreases, and above 0.15% S, pinholes form. Corresponding pinholes formed by decreasing the surface tension by raising S would not have a graphite lining nor be surrounded by a graphite-free layer.
* Titanium: The surface tension value decreased at all levels of Ti and, at Ti levels between 0.08% and 0.36%, the irons contained pinholes.
* Tellurium: The surface tension decreased at levels of Te up to 0.075% and pinholes formed above 0.01%.
The correlation between pinhole occurrence and surface tension is good for gray irons because low surface tension allows easier bubble formation and promotes the occurrence of pinholes.
A similar relationship exists for white irons, but the surface tension threshold value is lower; the ability to produce a casting free from pinholes at a lower surface tension may be associated with the more rapid solidification of white iron.
The appearance of pinholing in ductile cast iron is not directly linked to the presence of Mg, but small amounts of Al will promote pinholes in the presence of Mg and particularly if Ti is also present. The addition of other elements, such as bismuth (Bi) and calcium (Ca), reportedly suppress pinholes in ductile cast iron. But higher levels of the two promoted carbide formation, indicating that solidification mechanism changes are responsible in part for avoiding pinholes.
Effect of Gases on Microstructure
Malleable Iron: High levels of soluble N and H tend to stabilize carbides, reduce mottling and retard first- and second-stage graphitization. Soluble N levels in excess of 100 ppm causes problems and above 140 ppm, these problems are severe. Higher N and H levels allow the production of heavier section white iron castings with higher C and Si contents.
Gray Iron: Increasing the N content from 50 to 175 ppm in gray iron stabilizes the pearlite formation by completely suppressing ferrite. Nitrogen also is a strong carbide stabilizer that, at high levels, leads to a white iron structure.
In thicker section castings, increasing N from 35 to 150 ppm produces graphite flakes that are shorter and thicker and similar to a compacted graphite iron. The end effect of increased N in both heavy and lighter sections is to increase strength, even though there is no visible graphite shape change in smaller castings. The strength increase is attributable to reducing the free ferrite content. Neutralizing the N content with a nitride former, such as Ti, eliminates the effects of N on promoting pearlite or a graphite shape change and the strength increase is lost.
Ductile Iron: The level of soluble N in ductile irons is lower because of the agitation as Mg is added to the molten base iron. No effect of different N levels has been identified with the graphite nodule formation, but a stabilizing effect has been reported (at higher N levels) for the pearlite and cementite contents of ductile iron. It also promotes compacted graphite iron structures common to Ti, Zr and Al additions.
1. R. Monroe, "Gas Holes in Iron and Steel Castings," Steel Founders Research Journal, N3, pp. 5-12, (1983).
2. R. Fruehan, "Gases in Metals," ASM Handbook, pp. 82-87.
3. A. Kagawa, T. Okamoto, Trans. Japan. Inst. Met., Vol. 22, No. 2, p. 137, (1981).
4. H. Davison, F. Chen, J. Keverian, "Solution to a Nitrogen Porosity Problem in Gray Iron Castings," AFS Transactions., Vol. 85, p. 528, (1958).
5. J. Wallace, B. Hernandez, "Mechanisms of Pinhole Formation in Gray Iron," AFS Transactions. pp. 335-348, (1979).
6. J. Dawson, L. Smith, "Gases in Cast Iron with Special Reference to the Pickup of Hydrogen in Sand Molds," AFS Transactions, p. 17, (1958).
7. R. Naik and J. Wallace, "Surface Tension--Nucleation Relations in Cast Iron Pin hole Formation," AFS Transactions., p. 367 (1980).
8. R. Heine, "Observations on Pinhole Defects in White Iron Castings", AFS Transactions., Vol. 66, p. 31, (1958).
9. Barton, Donald, Unpublished work, BCIRA.
10. E. Haack, "What Causes Pinhole Porosity in Gray Iron", Foundry, June, Vol. 89, pp. 80-83 (1961).
11. R. Heine, Modern Castings, February, Vol. 33, pp. 31-35 (1958).
12. Barton, Donald, Unpublished work, BCIRA.
13. International Atlas of Casting Defects
B111 Blowholes, Pinholes B112 Blowholes near inserts, chills B113 Slag Blowholes
14. G. Strong, "A Literature Survey on Nitrogen in Malleable Iron," AFS Transactions, Vol. 77, pp. 29-36, (1977).
15. J. Dawson, L. Smith, B. Bach, "Some Effects of Nitrogen in Cast Iron," BCIRA Journal of R & D, June, Vol. 4, No. 12, pp. 540-552, (1953).
16. F. Mountford, "The Influence of Nitrogen on the Strength, Soundness and Structure of Gray Cast Iron," British Foundryman, April, Vol. 59, pp. 141-151, (1966).
17. S. Moritia, N. Inovama, "Behavior of Nitrogen in Cast Iron," AFS Cast Metals Journal, Sept., pp. 109-115, (1969).
18. J. Dawson, L. Smith, "Gases in Cast Iron with Special Reference to Pickup of Hydrogen in Sand Molds," Modern Castings, February, Vol. 33, pp. 39-52, (1958).
Table 1. Investigative route foundries should follow to identify gas porosity defects.
Within casting body
b) Top face
Any cast face
Cope or drag
Size |is less than~5 mm, smooth, irregular, spherical or fissure
Dull (oxidized) Shiny - metallic Shiny - graphitic
Low-powered binoculars bench microscope or metallograph scanning electron microscope
Avoid destroying evidence by shotblasting Preserve cavity linings by filling with mounting medium prior to polishing Avoid contamination from polishing medium/ water Prepare plane through unexposed portion of cavity
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