Identifying Casting Defects
A faulty casting has arrived at your facility’s door. You’re not exactly sure what’s wrong with it, but from what you’ve heard, you’re pretty sure it’s “porosity.”
You call your metalcaster. You tell the quality control manager you’ve got porosity. She wants to know more.
What you have on your hands is a cavity-type defect. While many kinds of these defects exist, most buyers and designers of castings know them only as porosity. If you could just give the quality control manager a more specific defect name, she’d know its root cause and therefore how to fix it.
Below are descriptions of defect types and their correct terminology.
1. Upon machining, small, narrow cavities appear on your casting faces.
Dispersed Shrinkage—Characteristic of cast iron, these cavities are most often perpendicular to the casting surface, with depths as great as 0.8 in. (2 cm). The casting defect is most commonly caused in iron components by low carbon content or high nitrogen content in the melt.
2. Several castings in your shipment are showing thin bits of metal at the parting line.
Defect: Flash—Projections at the parting line occur when clearance between the top and bottom of the metalcasting mold halves is great enough to allow metal to enter and solidify. The metalcaster must take more care in pattern, mold and coremaking to eliminate flash or remove it in the cleaning room after pouring.
3. One of your iron castings fractures and reveals smooth, slightly curved facets on the fracture face.
Defect: Conchoidal or “Rock Candy” Fracture—This defect is characterized by separation along the grain boundaries of primary crystallization. The resulting configuration is often compared to the appearance of rock candy. The defect is caused in steel castings by elevated aluminum and nitrogen levels.
4. Your casting has smooth-walled, rounded cavities of various sizes clumped together in one area.
Blowholes/Pinholes—The interior walls of blowholes and pinholes can be shiny, more or less oxidized or, in the case of cast iron, covered with a thin layer of graphite. The defects can appear in any region of a casting. They are caused when gas is trapped in the metal during solidification.
5. Your iron casting has folded, shiny films in its walls.
Defect: Lustrous Carbon—These folded or wrinkled films are distinctly outlined and found within the walls of iron castings, causing a linear discontinuity in the structure. Generally, they are seen only upon fracturing a casting. The defects form when materials from mold or core additives and binders volatize, decompose and become entrained in the melt.
6. Upon x-ray, you observe a cavity in the middle of your casting.
Defect: Axial Shrinkage—All metal shrinks as it solidifies. Axial (or centerline) shrinkage, most often plate-like in shape, occurs when the metal at the center of the casting takes longer to freeze than the metal surrounding it. The defect is partly a function of the section thickness designed into the casting, but it also can be influenced by the metalcaster’s pouring temperature, alloy purity, riser use and pouring speed.
7. A protrusion of metal is sticking out of a 90-degree corner of one of your castings.
Defect: Fillet Vein—These types of metallic projections can divide an interior casting angle in half. This defect can occur when too much binder in the sand causes a crevice to form in a mold or core during mold preparation or casting. The metalcaster will reduce or modify its binder usage to alleviate the defect.
8. All your casting dimensions are incorrect in the same proportion.
Defect: Improper Shrinkage Allowance—All casting alloys shrink as they solidify, but each does so at a different rate. This defect can occur when the patternmaker uses a shrink rule (constant) that differs from the actual shrinkage of the alloy used. The pattern will have to be remade to account for this defect.
9. Your casting is essentially complete except for more or less rounded edges and corners.
Defect: Misrun—This defect can occur with the use of any casting alloy, but in the case of iron, the surface is generally shiny and easily cleaned. The problem can come about due to a lack of alloy fluidity, slow mold filling, inadequate venting of the mold and (in permanent molding) low temperatures.
10. Your casting has a partial separation in one of its walls.
Defect: Cold Shut—Cold shuts vary in depth and can extend either partially or all the way through a casting section. This defect may be accompanied by rounded casting edges (also common to misruns, detailed in question 9). Cold shuts generally occur on wide casting surfaces in thin, difficult-to-fill sections, or where two streams of metal converge in the mold during filling.
11. Your casting has been stored for some time, and when you pull it out for assembly, you notice it has bent out of specification.
Defect: Warped Casting—Distortion due to warpage can occur over time in a casting that partially or completely liberates residual stresses. Common practice in iron casting is normalizing heat treatment to remove residual stress. In aluminum casting, a straightening between quench and aging might be required.
12. Your iron casting has branched grooves of various lengths with smooth bottoms and edges.
Defect: Buckle—Occurring in all ferrous alloys and sometimes in copper-base castings, the defect is caused by the expansion of silica sand. The defect distinguishes itself from a scab (see question 18) in that it does not allow penetration of the metal into the adjacent cavity below.
13. Very small grooves (less than 0.5 in.) on the surface of your casting are almost covered by a folded edge.
Defect: Rat Tail—This shallow defect occurs in ferrous and nonferrous green sand castings. Rat tails most often extend from the area where the metalcaster gates the casting. Rat tails may be accompanied by other projection-like defects. Metalcasters can alleviate this defect by altering their sand mixture.
14. Your iron casting has spherical particles coated with oxide inside it. The particles are the same chemical composition as the base metal.
Defect: Cold Shot (Shot Metal)—Not to be confused with a cold shut, this defect occurs when small droplets of metal fall into a metalcasting mold, solidify and fail to remelt when the remaining metal is introduced to the mold. The defect is caused primarily by faulty pouring practices, but it also can be influenced by misplaced runners and risers. Metalcasters can stop the defect from occurring by improving pouring conditions and protecting the mold openings against metal splashing.
15. Small, gray-green, superficial cavities in the form of droplets or shallow spots appear on your iron castings.
Defect: Slag Inclusions—A reaction between the mold and ferrous metals can cause the formation of a low-melting slag, which can adhere to the casting surface. When the inclusions are dislodged during shot-blasting, a rounded cavity is left behind. The defect is especially common in steels with high chromium contents. The metalcaster will reduce pouring temperatures and cool the castings in a reducing atmosphere to correct the problem.
16. Irregular projections crop up on one side of a vertical casting surface near the parting line.
Defect: Ramoff/Ramaway—This defect is characterized by a thickening of the casting in the vicinity of the parting line or an increase in dimension of a surface parallel to the parting line. It is caused by improper mold creation (ramming), which has in turn caused the sand to separate from certain vertical walls of the pattern.
17. Plate-like metallic projections with rough surfaces jut up parallel to the casting surface.
Defect: Kish Graphite Inclusions—This ferrous casting defect appears as coarse (not smooth) porosity, filled with graphite. It generally becomes visible upon casting machining. The defect is caused by an excessive carbon equivalent in the melt, slow cooling or great differences in section thickness. A redesign on the part of the casting end-user may be in order to address this defect.
18. Your iron casting shows local accumulations of coarse graphite. The graphite has moved into the shrinkage cavities.
Expansion Scab—Another defect caused by the expansion of molding or core sand, expansion scabs can occur in ferrous or copper-based castings. The thin metallic projections with sharp edges are generally parallel to the surface of the casting and have very rough surfaces. They are usually attached to the casting at only a few points and are otherwise loose.
19. Waves of fold markings without discontinuities appear on your casting.
Defect: Seams or Scars—This defect, which generally occurs on horizontal or convex surfaces of thin castings, distinguishes itself from a rat tail in that the two edges of each individual groove are at the same level. The defect may appear in conjunction with kish graphite (detailed in question 18). Sand is not the cause of this defect. Rather, it is metallurgical.
20. Lines of extra metal that look like veins appear on your casting surface.
Defect: Veining—This defect occurs when cracks appear on a sand mold due to sand contraction, which is caused by heat. The metalcaster must regulate its sand composition and heating to keep veining from occurring.