A systematic approach to cast iron defect analysis.
Four important considerations are used to evaluate casting defects, including:
* visual assessment,
* metallographic examination,
* chemical analysis,
* occurrence survey.
These techniques generate information throughout a defect investigation. Though this approach may not lead directly to a final conclusion regarding defect origin, it helps narrow the areas of inquiry and can point to solution possibilities.
Visual assessment refers to macroscopic examination of the defect as it appears in the casting. This can be accomplished with the unaided eye or a magnifying glass. During this portion of the investigation, the size, shape, location and internal characteristics of the defect are observed. There are questions that should be asked during this examination. What is the defect size and configuration? Does it appear on the cope or drag surface? Is it in a thin or thick section? is the defect uniformly distributed, or is it isolated near a core or hot spot? Is it near to or away from the ingates? Are foreign material or metal beads present? If the defect is a hole, does it have a shiny or blue-gray lining? Are dendrites in the cavity? Answers to these questions can often distinguish between gas, exogenous inclusion and shrinkage defects, though they are not normally helpful in pinpointing microstructural problems. Exceptions are chill, which appears as a shiny, white area on a fracture surface, and carbon flotation which imparts a black color to the affected area.
Metallographic examination yields additional information about a defect's features and the characteristics of its surrounding iron matrix. The light microscope, scanning electron microscope (SEM) and electron microprobe are valuable in this evaluation. Proper sample preparation is a prerequisite to this step of analysis. Connventional techniques can be used to polish iron samples. Scratches and pulled-out graphite must be avoided during polishing, particularly when trying to assess graphite structure. Free graphite, either flake or nodular, appears gray, not black, when properly polished. Smeared graphite can give the appearance of an irregularly shaped nodule or can mask the presence of Widmanstatten graphite. Carbides and phosphides are detected with etchants such as 2% Nital. The microscope can also be used to distinguish between the sand grains and oxides that often coexist. The light microscope is not useful in detecting oxide materials.
The scanning electron microscope and electron microprobe are used to determine the composition of oxide inclusions. Elements in the oxide, particularly minor elements, act as tracers or fingerprints. They facilitate tracking the oxide to its origin, be it the iron, treatment alloys or refractories. In addition, tools such as these are also helpful in identifying sand grains, determining the elements responsible for carbide stabilization, distinguishing between iron carbide and steadite, and identifying graphite or iron oxide linings in gas holes.
Chemical analyses are important tools in drawing conclusions about defects such as shrinkage cavities, nitrogen fissures, hydrogen pinholes and carbon monoxide blowholes. In all these cases, iron composition can be part of the problem. Chemical analyses should always be included when microstructural problems such as carbides, steadite and poor graphite shape are suspected.
Occurrence surveys help identify conditions that favor defect formation. All stages of the production process should be reviewed. For example, in the melt department, changes noted in charge materials, furnace operation, including repairs and downtime, treatment practice and alloy lot, determine whether ladle conditions influenced the incidence of unsound castings. Were defects found primarily in iron poured from newly relined or repaired ladles, or poured from ladles with a heavy slag build-up? Were defects more prevalent in castings from the last ladle pour or from the first? Was iron held for an extended period of time during mold department downtime then poured cold? Was a heel left in the ladle? Can the incidence of unsound castings be traced to changes in mold and/or core conditions? Changes in gating and/or casting design? Were there variations in shake out time? Was the pouring temperature too high or too low?
The time of the year should not be overlooked either. Are the defects more prevalent in the summer when temperatures and humidity are high? Consider whether defect speak at the beginning or at the end of a shift, or are more prevalent on one shift than on another? Information acquired during this survey can help prevent future problems by creating an awareness of conditions that favor defect formation. These conditions can then be compensated or avoided. Defect Characterization
It is important that a proposed procedure for analysis have practical application. The remainder of this article is devoted to this subject. The four steps outlined above will be referenced in the discussion, and emphasis placed on defect characteristics and causes.
In many cases, defects are not discovered until the castings near the final stages of processing. Machining and shot blasting operations often reveal porosity and other discontinuities occurring below the cast surface. Visual assessment is used to draw preliminary conclusions about the origin of these subsurface and internal defects. The distinction among gas holes, shrinkage cavities and foreign contamination can often be made during this step of the analysis.
Holes caused by evolved gas tend to be round, except for those caused by nitrogen which are often irregularly shaped, similar to those caused by shrinkage. Gas Defects
If the defect is round and confined to the cope surface of the casting, it is most likely a carbon monoxide blowhole. These holes can also be found on an internal surface below a core. Exuded metal beads are sometimes found in the voids which have a blue-gray lining. The microscope is used to confirm these visual observations since, in gray iron, this defect has three very distinct microstructural features as shown in Fig. 1. Included are: the blowhole caused by carbon monoxide evolution, ladle slag, and a heavy concentration of manganese sulfide inclusions in the matrix near the defect. Manganese sulfides have also been observed in the slag. Areas away from the defect do not exhibit this concentration of sulfides.
Low pouring temperature is the primary cause of gas defects. Often, only a few castings, those poured from the end of a ladle, will be affected by this problem. This is important information for the occurrence survey. As the metal temperature decreases, there is an increase in the amount of ladle slag that forms on the surface of the iron and in the amount of manganese sulfides that precipitate from the melt. The manganese sulfides, because they are lighter than iron, float to the surface where they mix with the ladle slag, making the slag more fluid and more difficult to remove. As a result, there is a greater chance for the slag to be introduced into the mold to react with the graphite that precipitates during the eutectic reaction. This results in carbon monoxide gas which in turn forms blowholes.
Iron composition can influence gas defects, indicating the importance of chemical analysis. High levels of manganese and/or sulfur increase the amount of slag and manganese sulfide that forms. By keeping manganese and sulfur contents to 0.65% and 0.12% maximum, respectively, the chances of getting into this problem can be reduced.
Carbon monoxide blowhole defects in ductile iron exhibit a spherical or tear drop shape. They are also the result of an oxide-graphite reaction. The oxide, which is a combination of ladle slag and treatment dross, is often found to cause this. Good iron cleaning practices and properly designed gating systems help minimize this problem.
Small round or pear-shaped holes 3 mm (1/8 in.) or less in diameter are evenly distributed just below the mold-metal interface are hydrogen pinholes. They may have a shiny appearance caused by the presence of a continuous graphite lining. If the castings have been heat treated, this lining could be replaced by iron oxide. Nonmetallic inclusions are not present in the voids, but some can contain a small head of metal. Under the microscope, the matrix adjacent to the hole is usually free from graphite flakes as shown in Fig. 2. A ferrite ring is often present along the defect perimeter. In ductile iron, vermicular or even flake graphite may form near the holes. Thin sections are more susceptible to this defect than thick sections.
A number of mold and metal factors that function in combination have been associated with hydrogen pinhole formation. Therefore, it is important that the mold and metal conditions be constantly monitored. Aluminum levels as low as 0.005% in gray iron and 0.01 % in ductile iron have been found to encourage dissociation of water vapor, thus, increasing the hydrogen content of the metal. Aluminum sources include steel scrap and ferroalloys used in iron treatment. Excessive moisture in the mold, coatings, furnace or ladle refractories and alloys are all sources of hydrogen. The magnesium used in ductile iron can encourage the metal to absorb more hydrogen causing a source of moisture.
A shrinkage cavity or a nitrogen fissure appears as an irregularly shaped hole. This distinction is particularly difficult to make in heavy section castings in which nitrogen segregates to the hot spot of the casting, the most likely location for shrinkage. A clear, bright hole with a discontinuous graphite lining is often found to be a nitrogen fissure. The graphite lining may not always be present. Compacted graphite in the vicinity of the defect supports this conclusion.
Chemical analysis can also be used to confirm that nitrogen is responsible for gas defects. Nitrogen has a greater solubility in liquid iron than in solid iron. Therefore, during solidification, nitrogen gas is released. A high residual nitrogen in the casting suggests that the liquid iron contained a higher than normal level. Residual nitrogen levels of 20-80 ppm are typical for gray iron. Heavy section castings can exhibit fissures at levels of 80 ppm. Light section castings may not be affected until levels reach 130 ppm. Levels of nitrogen that may not normally cause fissure defects can become dangerous if hydrogen is also present.
Irons made with a high proportion of steel scrap (50% or greater) are more likely to have this problem. Recarburizing materials containing nitrogenous compounds, molds and cores produced with high nitrogen content resins, as well as mold and core coatings containing carbonaceous and resin components, are all nitrogen sources.
The effect of nitrogen in gray iron can often be neutralized with the addition of 0.02-0.03% titanium. In ductile iron, titanium is not a suitable choice since, in the absence of cerium, it has an adverse affect on nodule shape. Shrinkage
The presence of tree-like structures, known as dendrites, in an irregularly shaped cavity is a good indication of shrinkage. Shrinkage can occur as open or closed holes that can be isolated or interconnected. An example is shown in Fig. 3. Shrinkage is most often found in the heaviest section of a casting which is the last area to solidify. It also appears at other hot spots such as adjacent to ingates and feeders or in regions with varying section size.
Carbon equivalent should be selected to meet the requirements of the section size being poured. Metal contraction can be minimized by avoiding excessively high pouring temperatures. Molds, particularly green sand molds, need to be rigid enough to withstand the volume expansion caused by graphite precipitation during the eutectic reaction. (Over-inoculation aggravates this expansion and should be avoided.) A soft mold can enlarge during solidification. Exogenous inclusions
Inclusions in cast iron can be either indigenous or exogenous. Indigenous describes the oxide and sulfide inclusions that precipitate from solution during solidification. Exogenous inclusions originate from sources external to the melt. They include: ladle and ferroalloy production slag, dross from treatment alloys such as nodulizers and inoculants, refractories and undissolved alloy particles. Visual examination may be sufficient to separate exogenous inclusion defects from gas holes or shrinkage cavities. it does not, however, allow complete characterization of exogenous inclusions. The light microscope can be used to distinguish among oxide, sand and undissolved alloy particles. Other techniques, such as scanning electron microscope analyses, are needed to fully characterize the oxide. Minor or trace elements in the oxide are identified and subsequently traced to a particular foundry material. An occurrence survey is helpful in recording the origin of these defects for future reference.
Ladle slag is a common cause of gray iron casting scrap. Under the microscope, ladle slag appears as a gray oxide which is often complex in nature. Ladle slag is primarily a combination of iron and manganese silicate. Manganese sulfides have also been identified in the slag, as have rosettes of cristobalite, a silica phase that commonly forms in metal silicates. The amount of slag formed increases as metal temperature decreases. An increase in slag can also be observed if the iron heel remains in the ladle. Ladle slag is often found intermixed with sand grains and, as previously discussed, can cause carbon monoxide blowhole defects.
Nodulizing alloys used in the production of ductile iron castings may contain a small quantity of slag formed during the ferroalloy smelting operation. Defects arising from this slag often appear on the cope surface of a ductile iron casting, particularly one produced by the in mold process. This slag possesses a gray . matrix" which is a calcium, aluminum, magnesium silicate. Angular particles of silicon carbide present in the silicate matrix confirm that the slag is from ferroalloy production rather than from the treatment process. (See Fig. 4.) Use of clean alloys minimizes the occurrence of this defect. Properly designed gating systems and ceramic filters assist in the removal of this and other foreign materials.
Dross can be found alone or in combination with ladle slag. it is a result of alloy additions to both gray and ductile iron. Many of the elements present in commercial alloys, such as inoculants and nodulizers, have a limited solubility in liquid iron and combine readily with oxygen to form dross on the surface of the iron. Examples are barium, calcium, cerium, magnesium and strontium. The greater the amount of these elements in the alloy, or the greater the alloy addition level, the greater the chance of having exogenous inclusion problems. Dross is an expected by-product of alloy use, but the amount generated can be controlled by using proper addition levels and clean alloys. Skimming of ladles and properly designed gating systems helps reduce the occurrence of defects caused by dross.
Sand is the most easily recognized item in the refractory category. Its shape, shown in Fig. 5, and optical properties under polarized light tend to set it apart from other oxides. The main sources are the molds and cores used in casting production. Erosion of furnace and ladle linings, as well as of mold and/or core coatings, contribute to exogenous inclusion defects as does chipping of filters and failure to remove slag coagulants.
Undissolved pieces of alloy appear as shiny spots on the surface of the casting. An example of an undissolved piece of ferrosilicon is shown in Fig. 6. These inclusions are hard and have a negative affection machinability. Areas in the matrix adjacent to these particles may exhibit iron carbides. Alloy characteristics, addition methods and metal temperature influence dissolution, and, therefore, are important in preventing this problem. Graphite Morphology
The free carbon in gray cast iron appears as flakes, and has a round or nodular morphology in ductile iron. The size, shape, amount and distribution of the flakes and nodules influence the mechanical and physical properties of the iron. The combination of Type A graphite and a pearlite matrix is believed to be necessary to optimize the tensile properties of gray iron. Small, well rounded nodules are preferred to large, irregular nodules in ductile iron. Matrix structure in ductile iron varies according to the grade of iron being produced.
As undercooled graphite (Types B and D) appear in a gray iron's matrix, there is a tendency to form more ferrite. This is shown in Fig. 7. The formation of undercooled graphite flakes lowers the carbon content of the corresponding matrix making it more difficult to stabilize the pearlite. The presence of ferrite, not the undercooled graphite itself, is believed to cause a reduction in the iron's tensile strength. Conditions that favor the formation of undercooled graphite include:
* rapid cooling often found in thin
sections, corners and near the surface
of a casting;
* absence of or insufficient inoculation;
* low sulfur content;
* high superheating temperatures;
* extended holding times;
* the presence of titanium.
A more dramatic decrease in tensile, impact and modulus of elasticity properties occurs when gray iron is contaminated with elements such as bismuth, antimony or lead. In thin section castings, contamination causes the formation of long, interconnected flakes known as mesh graphite. In medium to heavy section castings, sooty, spikey or Widmanstatten graphite is found. These graphite types can be difficult to see under the microscope at low magnifications. Magnifications of 40OX are often required to observe the secondary graphite growth on the primary graphite flakes. Because of this detection problem, contamination of gray iron may not be recognized until premature, catastrophic failure occurs.
Heavy section castings are more susceptible to failure associated with contamination because their long solidification times favor element segregation. Lead levels as low as 0.004% can reduce the tensile strength of these castings by 5O%. It is suspected that hydrogen enhances the effect of lead. Castings poured in green sand molds, and those poured from a freshly lined furnace or ladle are more likely to exhibit this problem. Lead will volatilize during melting and holding. Dynamic bath conditions can cause up to 50% of the lead to be lost in one hour. Very little burning off of the lead, however, occurs under quiescent conditions. The airborne lead that is evolved during melting, if not properly ventilated, poses problems for those in the workplace.
In order to avoid the environmental and metallurgical problems associated with lead, it is best to keep it, as well as other tramp elements, out of the furnace. To do this, the sources of lead need to be recognized. Included are: free machining steels, terne plate, painted components, vitreous enamel scrap and certain nonferrous scrap. Another often overlooked source is the copper or tin, used to promote pearlite, which can contain small amounts of lead.
Tramp elements can also create problems in ductile iron. Increasing amounts can have a negative affect on nodularity. Nodule shape can change from round, to compacted and, in the extreme case, to flake in the presence of tramp elements. In ductile iron, however, cerium can be used to neutralize their affects. This is why most magnesium ferrosilicon nodulizers contain cerium. High sulfur levels, with no magnesium compensation, can also cause poor nodularity. Sulfur should be kept below 0.01 5%, preferably around 0.01 %. Magnesium residuals that are too low or high have been associated with irregularly shaped nodules. Low magnesium residuals cannot bring about the desired rounding affect. At high levels, the carbide stabilizing aspect of magnesium is observed. This is accompanied by a deterioration in nodule shape. Inadequate post inoculation and extended holding of the iron are other factors that decrease nodularity and nodule count. Irons produced from high purity charge materials that are treated with ceriumbearing alloys may also exhibit poor nodularity in heavy sections.
Carbon flotation and chunky graphite are two other microstructural abnormalities that are common in ductile iron. Their occurrence is favored in castings having a section size in excess of two inches where solidification times are long. Both are composition related. Reductions in an iron's mechanical properties occurs if either condition is present.
Chunky graphite can be found at the thermal centers of high carbon equivalent ductile irons, particularly those with high silicon contents. Its presence can cause a 50% reduction in elongation. Reductions in tensile and impact strength can also occur. It has been observed that this problem can be minimized if nodule counts are kept above 60 nodules/mM2. This suggests that good inoculation practice is one factor that can be used to prevent chunky graphite formation. Excessive total carbon and silicon levels should be avoided as well as tramp elements such as lead, tellerium, titanium, bismuth, antimony and arsenic. High purity charges treated with cerium or mischmetal bearing nodulizers often contain chunky graphite. In this case, a tramp element, such as antimony, is deliberately added to the iron to restore nodularity. Chills have also been used as a means of avoiding this problem.
Strongly hypereutectic irons, those with carbon equivalents greater than 4.5 and total carbon contents in excess of 3.7%, are susceptible to carbon flotation. The slow freezing rates of heavy section castings provide time for the nodules, which are less dense than the iron, to float. This leads to dark edges on the cope or upper surfaces of the castings. Dark edges may also appear on the underside of internal cored surfaces. Variations in the appearance of machined surfaces may signal that carbon flotation has occurred. Exploded graphite nodules are often observed within the zone of carbon flotation along with sulfides and oxides of magnesium. Reductions in tensile strength, elongation and impact values accompany this phenomenon. Offset yield strengths and hardness, however, do not seem to be significantly affected. Graphite flotation increases with increasing carbon equivalent, particularly with an increase in the carbon content. Increases in pouring temperature aggravate this condition. Summary
Defects are an undesirable part of the cast iron production process, and their characterization and minimization present a challenge to the foundryman. A systematic approach is helpful in reducing the difficulty of this task. Techniques such as macroscopic and microscopic inspection, scanning electron microscope evaluation and chemical analysis are useful in defect characterization. An occurrence survey helps pinpoint conditions that favor defect formation. It is only after a defect has been identified, and its likely source determined, that the appropriate steps can be taken toward its elimination.
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|Author:||Hornung, Mary Jane|
|Date:||Apr 1, 1990|
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